Manufacturing

Fatigue lifetime can also be affected by changes in the environment. When metals are subjected to corrosion during cyclic loadings, the condition is known as corrosion fatigue, and both specimen lifetime and the endurance limit can be significantly reduced. Moreover, the nature of the environmental attack need not be severe. For some materials, tests conducted in air have been shown to have shorter lifetimes than those run in a vacuum, and further lifetime reductions have been observed with increasing levels of humidity. The test results can also be dependent on the frequency of the loading cycles. For slower frequencies, the environment has a longer time to act between loadings. At high frequencies, the environmental effects may be somewhat masked.The application of test data to actual products, therefore, requires considerable caution. Residual stresses can also alter fatigue behavior. If the specimen surface is in a state of compression, such as that produced from shot peening, carburizing, or burnishing, it is more difficult to initiate a fatigue crack, and lifetime is extended. Conversely, processes that produce residual tension on the surface, such as welding or machining, can significantly reduce the fatigue lifetime of a product. If the magnitude of the load varies during service, the fatigue response can be extremely complex. For example, consider the wing of a commercial airplane. As the wing vibrates during flight, the wing-fuselage joint is subjected to a large number of low-stress loadings. The large number of these load applications may be far less damaging, however, than a few high-stress loadings, like those that occur when the plane contacts the runway during landing. From a different perspective, however, the heavy loads may be sufficient to stretch and blunt a sharp fatigue crack, requiring many additional small-load cycles to "reinitiate" it. Evaluating how materials respond to complex patterns of loading is an area of great importance to design engineers. Since reliable fatigue data may take a considerable time to generate, we may prefer to estimate fatigue behavior from properties that can be determined more quickly. Table 2-3 shows the approximate ratio of the endurance limit to the ultimate tensile strength for several engineering metals. For many steels the endurance limit can be approximated by 0.5 times the ultimate tensile strength as determined by a standard tensile test. For the nonferrous metals, however, the ratio is significantly lower. FATIGUE FAILURES Components that fail as a result of repeated or cyclic loadings are commonly called fatigue failures.These fractures form a major part of a larger group known as progressive fractures. Consider the fracture surface shown in Figure 2-28. The two arrows identify the points of fracture initiation, which often correspond to discontinuities in the form of surface cracks, sharp corners, machining marks, or even "metallurgical notches," such as an abrupt change in metal structure. With each repeated application of load, the stress at the tip of the crack exceeds the strength of the material, and the crack grows a very small amount. Crack growth continues with each successive application of load until the remaining cross section is no longer sufficient to withstand the peak stresses. Sudden overload fracture then occurs through the remainder of the material. The overall fracture surface tends to exhibit two distinct regions: a smooth, relatively flat region where the crack was propagating by cyclic fatigue, and a coarse, ragged region, corresponding to the ductile overload tearing. The smooth areas of the fracture often contain a series of parallel ridges radiating outward from the origin of the crack. These ridges may not be visible under normal examination, however.They may be extremely fine; they may have been obliterated by a rubbing action during the compressive stage of repeated loading; or they may be very few in number if the failure occurred after only a few cycles of loading ("low-cycle fatigue"). Electron microscopy may be required to reveal the ridges, or fatigue striations,

a reading of 60 was obtained.The B and C scales are used more extensively than the others, with B being common for copper and aluminum and C for steels.3 Rockwell tests should not be conducted on thin materials (typically less than 1.5 mm or 1/16 in.), on rough surfaces, or on materials that are not homogeneous, such as gray cast iron. Because of the small size of the indentation, variations in roughness, composition, or structure can greatly influence test results. For thin materials, or where a very shallow indentation is desired (as in the evaluation of surface-hardening treatments such as nitriding or carburizing), the Rockwell superficial hardness test is preferred. Operating on the same Rockwell principle, this test employs smaller major and minor loads (15 or 45 kg and 3 kg, respectively) and uses a more sensitive depth-measuring device. In comparison with the Brinell test, the Rockwell test offers the attractive advantage of direct readings in a single step. Because it requires little (if any) surface preparation and can be conducted quite rapidly (up to 300 tests per hour or 5 per minute), it is often used for quality control purposes, such as determining if an incoming product meets specification, assuring that a heat treatment was performed properly, or simply monitoring the properties of products at various stages of manufacture. It has the additional advantage of producing a small indentation that can be easily concealed on the finished product or easily removed in a later operation. Vickers Hardness Test. The Vickers hardness test is also similar to the Brinell test but uses a 136° square-based diamond pyramid as the indenter and loads between 1 and 120 kg.Like the Brinell value, the Vickers hardness number is also defined as load divided by the surface area of the indentation expressed in units of kilograms per square millimeter.The advantages of the Vickers approach include increased accuracy in determining the diagonal of a square impression as opposed to the diameter of a circle and the assurance that even light loads will produce some plastic deformation.The use of diamond as the indenter material enables the test to evaluate any material and effectively places the hardness of all materials on a single scale. Like the other indentation or penetration methods, the Vickers test has a number of attractive features: (1) it is simple to conduct, (2) little time is involved, (3) little surface preparation is required, (4) the marks are quite small and are easily hidden or removed, (5) the test can be done on location, (6) it is relatively inexpensive, and (7) it provides results that can be used to evaluate material strength or assess product quality. Microhardness Tests. Various microhardness tests have been developed for applications where it is necessary to determine the hardness of a very precise area of material or where the material or modified surface layer is exceptionally thin. These tests might be more appropriately termed microindentation hardness tests, since it is the size of the indentation that is extremely small, not the measured value of hardness. Special machines, such as the one shown in Figure 2-16,

ics begins with the premise that all materials contain flaws or defects of some given size. These may be material defects, such as pores, cracks, or inclusions; manufacturing defects, in the form of machining marks, arc strikes, or contact damage to external surfaces; or design defects, such as abrupt section changes, excessively small fillet radii, and holes. When the specimen is subjected to loads, the applied stresses are amplified or intensified in the vicinity of these defects, potentially causing accelerated failure or failure under unexpected conditions. Fracture mechanics seeks to identify the conditions under which a defect will grow or propagate to failure and, if possible, the rate of crack or defect growth. The methods concentrate on three principal quantities: (1) the size of the largest or most critical flaw, usually denoted as a; (2) the applied stress, denoted by and (3) the fracture toughness, a quantity that describes the resistance of a material to fracture or crack growth, which is usually denoted by K with subscripts to signify the conditions of testing. Equations have been developed that relate these three quantities (at the onset of crack growth or propagation) for various specimen geometries, flaw locations, and flaw orientations. If nondestructive testing or quality control methods have been applied, the size of the largest flaw that could go undetected is often known. By mathematically placing this worst possible flaw in the worst possible location and orientation, and coupling this with the largest applied stress for that location, a designer can determine the value of fracture toughness necessary to prevent that flaw from propagating during service. Specifying any two of the three parameters allows the computation of the third. If the material and stress conditions were defined, the size of the maximum permissible flaw could be computed. Inspection conditions could then be selected to ensure that flaws greater than this magnitude are cause for product rejection. Finally, if a component is found to have a significant flaw and the material is known,the maximum operating stress can be determined that will ensure no further growth of that flaw. In the past, detection of a flaw or defect was usually cause for rejection of the part (detection rejection).With enhanced methods and sensitivities of inspection, almost every product can now be shown to contain flaws. Fracture mechanics comes to the rescue. According to the philosophy of fracture mechanics, each of the flaws or defects in a material can be either dormant or dynamic. Dormant defects are those whose size remains unchanged through the lifetime of the part and are indeed permissible.A major goal of fracture mechanics, therefore, is to define the distinction between dormant and dynamic for the specific conditions of material, part geometry, and applied loading.4 Alternative efforts to prevent material fracture generally involve overdesign, excessive inspection, or the use of premium-quality materials—all of which increase cost and possibly compromise performance. Fracture mechanics can also be applied to fatigue, which has already been cited as causing as much as 90% of all dynamic failures. The standard method of fatigue testing applies cyclic loads to polished, flaw-free specimens, and the reported lifetime consists of both crack initiation and crack propagation. In contrast, fracture mechanics focuses on the growth of an already-existing flaw. Figure 2-38 shows the crack growth rate (change in size per loading cycle denoted as da/dN) plotted as a function of the fracture mechanics parameter, (where increases with an increase in either the flaw size and/or the magnitude of applied stress). Since the fracture mechanics approach begins with an existing flaw, it provides a far more realistic guarantee of minimum service life. Fracture mechanics is a truly integrated blend of design (applied stresses), inspection (flaw-size determination), and materials (fracture toughness).The approach has proven valuable in many areas where fractures could be catastrophic.

(55,000 psi), the specimen will fail after about 100,000 cycles. If the peak stress were further reduced to 350 MPa (51,000 psi), the fatigue lifetime would be extended by an order of magnitude to approximately 1,000,000 cycles. With a further reduction to any value below 340 MPa (49,000 psi), the specimen would not fail by fatigue, regardless of the number of stress application cycles. The stress below which the material will not fail regardless of the number of load cycles is known as the endurance limit or endurance strength, and may be an important criterion in many designs.Above this value, any point on the curve is the fatigue strength, the maximum stress that can be sustained for a specified number of loading cycles. A different number of loading cycles is generally required to determine the endurance limit for different materials. For steels, 10 million cycles are usually sufficient. For several of the nonferrous metals, 500 million cycles may be required. For aluminum, the curve continues to drop such that, if aluminum has an endurance limit, it is at such a low value that a cheaper and much weaker material could be used. In essence, if aluminum is used under realistic stresses and cyclic loading, it will fail by fatigue after a finite lifetime. The fatigue resistance of an actual product is sensitive to a number of additional factors. One of the most important of these is the presence of stress raisers (or stress concentrators),such as sharp corners,small surface cracks,machining marks,or surface gouges.Data for the S-N curves are obtained from polished-surface,"flaw-free" specimens, and the reported lifetime is the cumulative number of cycles required to initiate a fatigue crack and then grow or propagate it to failure. If a part already contains a surface crack or flaw, the number of cycles required for crack initiation can be reduced significantly. In addition, the stress concentrator magnifies the stress experienced at the tip of the crack, accelerating the rate of subsequent crack growth. Great care should be taken to eliminate stress raisers and surface flaws on parts that will be subjected to cyclic loadings.Proper design and good manufacturing practices are often more important than material selection and heat treatment. Operating temperature can also affect the fatigue performance of a material. Figure 2-27 shows S-N curves for Inconel 625 (a high-temperature Ni-Cr-Fe alloy) determined over a range of temperatures. As temperature is increased, the fatigue strength drops significantly. Since most test data are generated at room temperature, caution should be exercised when the product application involves elevated service temperatures.

Caution should also be placed on the use of impact data for design purposes. The test results apply only to standard specimens containing a standard notch. Moreover, the tests evaluate material behavior under very specific conditions. Changes in the form of the notch, minor variations in the overall specimen geometry, or faster or slower rates of loading (speed of the pendulum) can all produce significant changes in the results. Under conditions of sharp notches, wide specimens, and rapid loading, many ductile materials lose their energy-absorbing capability and fail in a brittle manner. [For example, the standard impact test should not be used to evaluate materials for bullet-proof armor, since the velocities of loading are extremely different.] The results of standard tests, however, can be quite valuable in assessing a material's sensitivity to notches and the multiaxial stresses that exist around a notch. Materials whose properties vary with notch geometry are termed notch-sensitive. Good surface finish and the absence of scratches, gouges, and defects in workmanship will be key to satisfactory performance. Materials that are notch-insensitive can often be used with ascast or rough-machined surfaces with no risk of premature failure. Impact testing can also be performed at a variety of temperatures.As will be seen later in this chapter, the evaluation of how fracture resistance changes with temperature can be crucial to success when selecting engineering materials for low-temperature service. The tensile impact test, illustrated schematically in Figure 2-24, eliminates the use of a notched specimen and thereby avoids many of the objections inherent in the Charpy and Izod tests. Turned specimens are subjected to uniaxial impact loadings applied through drop weights, modified pendulums, or variable-speed flywheels. FATIGUE AND THE ENDURANCE LIMIT Materials can also fail by fracture if they are subjected to repeated applications of stress, even though the peak stresses have magnitudes less than the ultimate tensile strength and usually less than the yield strength.This phenomenon, known as fatigue, can result from either the cyclic repetition of a particular loading cycle or entirely random variations in stress. Almost 90% of all metallic fractures are in some degree attributed to fatigue. For experimental simplicity, a periodic, sinusoidal loading is often utilized, and conditions of equal-magnitude tension-compression reversals provide further simplification.These conditions can be achieved by placing a cylindrical specimen in a rotating drive and hanging a weight so as to produce elastic bending along the axis, as shown in Figure 2-25. As a result of the elastic bending, material at the bottom of the specimen is stretched, or loaded in tension, while material on the top surface is compressed. As the specimen turns, the surface of the specimen experiences a sinusoidal application of tension and compression with each rotation. By conducting multiple tests, subjecting identical specimens to different levels of maximum loading, and recording the number of cycles necessary to achieve fracture, curves such as that in Figure 2-26 can be produced. These curves are known as stress versus number of cycles, or S-N, curves. If the material being evaluated in Figure 2-26 were subjected to a standard tensile test, it would require a stress in excess of 480 MPa (70,000 psi) to induce failure. Under cyclic loading with a peak stress of only 380 MPa

Figure 2-32 shows the combined effects of temperature and strain rate (speed of testing) on the ultimate tensile strength of copper. For a given temperature, the rate of deformation can also have a strong influence on mechanical properties. Room-temperature standard-rate tensile test data will be of little value if the application involves a material being hot-rolled at speeds of 1300 m/min (5000 ft/min). The effect of temperature on impact properties became the subject of intense study in the 1940s when the increased use of welded-steel construction led to catastrophic failures of ships and other structures while operating in cold environments. Welding produces a monolithic (single-piece) product where cracks can propagate through a joint and continue on to other sections of the structure! Figure 2-33 shows the effect of decreasing temperature on the impact properties of two low-carbon steels.Although similar in form, the two curves are significantly different.The steel indicated by the solid line becomes brittle (requires very little energy to fracture) at temperatures below 4°C (25°F) while the other steel retains good fracture resistance down to 26°C (15°F). The temperature at which the response goes from high energy absorption to low energy absorption is known as the ductile-to-brittle transition temperature. While all steels tend to exhibit this transition, the temperature at which it occurs varies with carbon content and alloy. Special caution should be taken, therefore, when selecting steels for low-temperature applications.

Figure 2-34 shows the ductile-to-brittle transition temperature for steel salvaged from the Titanic compared to currently used ship plate material. While both are quality materials for their era, the Titanic steel has a much higher transition temperature and is generally more brittle. Recalling that the water temperature at the time the Titanic struck the iceberg was 2°C, the results show that the steel would have been quite brittle. Two curves are provided for each material, reflecting specimens in different orientation with respect to the direction of product rolling. Here we see that processing features can further affect the properties and performance of a material. CREEP Long-term exposure to elevated temperatures can also lead to failure by a phenomenon known as creep. If a tensile-type specimen is subjected to a constant load at elevated temperature, it will elongate continuously until rupture occurs, even though the applied stress is below the yield strength of the material at the temperature of testing.While the rate of elongation is often quite small, creep can be an important consideration when designing equipment such as steam or gas turbines, power plant boilers, and other devices that operate under loads or pressures for long periods of time at high temperature. If a test specimen is subjected to conditions of fixed load and fixed elevated temperature, an elongation-versus-time plot can be generated, similar to the one shown in Figure 2-35. The curve contains three distinct stages: a short-lived initial stage, a rather long second stage where the elongation rate is somewhat linear, and a short-lived third stage leading to fracture. Two significant pieces of engineering data are obtained from this curve: the rate of elongation in the second stage, or creep rate, and the total elapsed

For certain applications, the physical properties of an engineering material may be even more important than the mechanical ones.These include the thermal, electrical, magnetic, and optical characteristics. We have already seen several ways in which the mechanical properties of materials change with variations in temperature. In addition to these effects, there are some truly thermal properties that should be considered. The heat capacity or specific heat of a material is the amount of energy that must be added to or removed from a given mass of material to produce a 1° change in temperature.This property is extremely important in processes such as casting, where heat must be extracted rapidly to promote solidification, or heat treatment, where large quantities of material are heated and cooled. Thermal conductivity measures the rate at which heat can be transported through a material. While this may be tabulated separately in reference texts, it is helpful to remember that for metals, thermal conductivity is directly proportional to electrical conductivity. Metals such as copper, gold, and aluminum that possess good electrical conductivity are also good transporters of thermal energy.Thermal expansion is another important thermal property. Most materials expand upon heating and contract upon cooling, but the amount of expansion or contraction will vary with the material. For components that are machined at room temperature but put in service at elevated temperatures, or castings that solidify at elevated temperatures and then cool, the manufactured dimensions must be adjusted to compensate for the subsequent changes. dega-c02_028-055-hr 1/9/07 3:22 PM Page 52 Electrical conductivity and electrical resistivity may also be important design considerations. These properties will vary not only with the material but also with the temperature and the way the material has been processed. From the standpoint of magnetic response, materials are often classified as diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, and ferrimagnetic. These terms refer to the way in which the material responds to an applied magnetic field. Material properties, such as saturation strength, remanence, and magnetic hardness or softness, describe the strength, duration, and nature of this response. Still other physical properties that may assume importance include weight or density, melting and boiling points, and the various optical properties, such as the ability to transmit, absorb, or reflect light or other electromagnetic radiation. ■ 2.8 TESTING STANDARDS AND CONCERNS When evaluating the mechanical and physical properties of materials, it is important that testing be conducted in a standardized and reproducible manner. ASTM International, formerly the American Society of Testing and Materials, maintains and updates many testing standards, and it is important to become familiar with their contents. For example, ASTM specification E370 describes the "Standard Test Methods and Definitions for Mechanical Testing of Steel Products."Tensile testing is described in specifications E8 and E83, impact testing in E23, creep in E139, and penetration hardness in E10. Other specifications describe fracture mechanics testing as well as the procedures to evaluate corrosion resistance, compressive strength, shear strength, torsional properties, and corrosion-fatigue. In addition, it is important to note not only the material being tested but also the location from which the specimen was taken and its orientation. Rolled sheet, rolled plate, and rolled bars, for example, will have different properties when tested parallel to the direction of rolling (longitudinal) and perpendicular to the rolling direction (transverse). This variation of properties with direction, known as anisotropy, may be crucial to the success or failure of a product.

In most cases, toughness is associated with impact or shock loadings, and the values obtained from high-speed (dynamic) impact tests often fail to correlate with those obtained from the relatively slow-speed (static) tensile test. True Stress-True Strain Curves. The stress-strain curve in Figure 2-6 is a plot of engineering stress, S, versus engineering strain, e, where S is computed as the applied load divided by the original cross-sectional area and e is the elongation, divided by the original gage length, Lo. As the test progresses, the cross section of the test specimen changes continually, first in a uniform manner and then nonuniformly after necking begins. The actual stress should be computed based on the instantaneous cross-sectional area, not the original. Since the area is decreasing, the actual or true stress will be greater than the engineering stress plotted in Figure 2-6.True stress, can be computed by taking simultaneous readings of the load and the minimum specimen diameter.The actual area can then be computed, and true stress can be determined as The determination of true strain is a bit more complex. In place of the change in length divided by the original length that was used to compute engineering strain, true strain is defined as the summation of the incremental strains that occur throughout the test. For a specimen that has been stretched from length to length L, the true, natural, or logarithmic strain would be: The last equality makes use of the relationship for cylindrical specimens that applies only up to the onset of necking. Figure 2-10 depicts the type of curve that results when the data from a uniaxial tensile test are converted to the form of true stress versus true strain. Since the true stress is a measure of the material strength at any point during the test, it will continue to rise even after necking. Data beyond the onset of necking should be used with extreme caution, since the geometry of the neck transforms the stress state from uniaxial tension (stretching in one direction with compensating contractions in the other two) to triaxial tension, in which the material is stretched or restrained in all three directions. Because of the triaxial tension, voids or cracks (Figure 2-11) tend to form in the necked region and serve as a precursor to final fracture. Measurements of the external diameter no longer reflect the true load-bearing area, and the data are further distorted.

Manufacturing has been accurately defined as the activities that are performed in the conversion of "stuff" into "things." Successful products begin with appropriate materials. You wouldn't build an airplane out of lead or an automobile out of concrete—you need to start with the right stuff. But "stuff" rarely comes in the right shape, size, and quantity for the desired use. Parts and components must be produced by subjecting engineering materials to one or more processes (often a series of operations) that alter their shape, their properties, or both. Much of a manufacturing education relates to an understanding of (1) the structure of materials, (2) the properties of materials, (3) the processing of materials, and (4) the performance of materials, and the interrelations between these four factors, as illustrated in Figure 2-1. This chapter will begin to address the properties of engineering materials. Chapters 3 and 4 will discuss the subject of "structure" and begin to provide the whys behind various properties. Chapter 5 introduces the possibility of modifying structure to produce desired properties. Most engineering materials do not have a single set of properties but rather offer a range or spectrum of possibilities.Taking advantage of this range, we might want to intentionally make a material weak and ductile for easy shaping (forming loads are low and tool life is extended) and then, once the shape has been produced, make the material strong for enhanced performance in use. When selecting a material for a product or application, it is important to ensure that its properties will be adequate for the anticipated operating conditions. The various requirements of each part or component must first be estimated or determined. These requirements typically include mechanical characteristics (strength, rigidity, resistance to fracture, the ability to withstand vibrations or impacts) and physical characteristics (weight, electrical properties, appearance) as well as features relating to the service environment (ability to operate under extremes of temperature or to resist corrosion). Candidate materials must possess the desired properties within their range of possibilities. To help evaluate the properties of engineering materials, a variety of standard tests have been developed, and data from these tests have been tabulated and made readily available. Proper use of this data often requires sound engineering judgment. It is important to consider which of the evaluated properties are significant, under what conditions the test values were determined, and what restrictions or limitations should be placed on their use. Only by being familiar with the various test procedures, their

One of the simplest ways to evaluate ductility is to determine the percent elongation of a tensile test specimen at the time of fracture. As shown in Figure 2-9, ductile materials do not elongate uniformly when loaded beyond necking. If the percent change of the entire 8-in. gage length is computed, the elongation is 31%. However, if only the center 2-in. segment is considered, the elongation of that portion is 60%. A valid comparison of material behavior, therefore, requires similar specimens with the same standard gage length. In many cases, material "failure" is defined as the onset of localized deformation or necking. Consider a sheet of metal being formed into an automobile body panel. If we are to ensure uniform strength and corrosion resistance in the final panel, the operation must be performed in such a way as to maintain uniform sheet thickness. For this application, a more meaningful measure of material ductility would be the uniform elongation or the percent elongation prior to the onset of necking. This value can be determined by constructing a line parallel to the elastic portion of the diagram, passing through the point of highest force or stress. The intercept where the line crosses the strain axis denotes the available uniform elongation. Since the additional deformation that occurs after necking is not considered, uniform elongation is always less than the total elongation at fracture (the generally reported elongation value). Another measure of ductility is the percent reduction in area that occurs in the necked region of the specimen. This can be computed as where Ao is the original cross-sectional area and Af is the smallest area in the necked region. Percent reduction in area, therefore, can range from 0% (for a brittle glass specimen that breaks with no change in area) to 100% (for extremely plastic soft bubble gum that pinches down to a point before fracture). When materials fail with little or no ductility, they are said to be brittle. Brittleness, however, is simply the lack of ductility and should not be confused with a lack of strength. Strong materials can be brittle, and brittle materials can be strong. Toughness. Toughness, or modulus of toughness, is the work per unit volume required to fracture a material.The tensile test can provide one measure of this property, since toughness corresponds to the total area under the stress-strain curve from test initiation to fracture, and thereby encompasses both strength and ductility. Caution should be exercised when using toughness data, however, because the work or energy needed to fracture can vary markedly with different conditions of testing. Variations in the temperature or the speed of loading can significantly alter both the stress-strain curve and the toughness

Relationship of Hardness to Tensile Strength. Table 2-2 and Figure 2-19 show a definite relationship between tensile strength and hardness. For plain carbon and lowalloy steels, the tensile strength (in pounds per square inch) can be estimated by multiplying the Brinell hardness number by 500. In this way, an inexpensive and quick hardness test can be used to provide a close approximation of the tensile strength of the steel. For other materials, however, the relationship is different and may even exhibit too much variation to be dependable.The multiplying factor for age-hardened aluminum is about 600, while for soft brass it is around 800. ■ 2.3 DYNAMIC PROPERTIES In many engineering applications, products or components are subjected to various types of dynamic loading. These may include (1) sudden impacts or loads that vary rapidly in magnitude, (2) repeated cycles of loading and unloading, or (3) frequent changes in the mode of loading, such as from tension to compression. To handle these conditions, we must be able to characterize the mechanical properties of engineering materials under dynamic loadings. Most dynamic tests subject standard specimens to a well-controlled set of test conditions. The conditions of actual application, however, rarely duplicate the controlled conditions of a standardized test.While identical tests on different materials can indeed provide a comparison of material behavior, the assumption that similar results can be expected for similar conditions may not always be true. Since dynamic conditions can vary greatly, the quantitative results of standardized tests should be used with extreme caution, and one should always be aware of the test limitations. IMPACT TEST Several tests have been developed to evaluate the toughness or fracture resistance of a material when it is subjected to a rapidly applied load, or impact. Of the tests that have become common, two basic types have emerged: (1) bending impacts, which include the standard Charpy and Izod tests, and (2) tension impacts. The bending impact tests utilize specimens that are supported as beams. In the Charpy test, shown schematically in Figure 2-20, the standard specimen is a square bar containing a V-, keyhole-, or U-shaped notch. The test specimen is positioned horizontally, supported on the ends, and an impact is applied to the center, behind the notch, to complete a three-point bending.The Izod test specimen, while somewhat similar in size

Strain Hardening and the Strain-Hardening Exponent. Figure 2-12 is a true stress-true strain diagram, which has been modified to show how a ductile metal (such as steel) will behave when subjected to slow loading and unloading. Loading and unloading within the elastic region will result in simply cycling up and down the linear portion of the curve between points O and A. However, if the initial loading is carried through point B (in the plastic region), unloading will follow the path BeC, which is approximately parallel to the line OA, and the specimen will exhibit a permanent elongation of the amount OC. Upon reloading from point C, elastic behavior is again observed as the stress follows the line a slightly different path from that of unloading. Point D is now the yield point or yield stress for the material in its partially deformed state. A comparison of points A and D reveals that plastic deformation has made the material stronger. If the test were again interrupted at point E, we would find a new, even higher-yield stress. Thus, within the region of plastic deformation, each of the points along the true stress-true strain curve represents the yield stress for the corresponding value of strain. When metals are plastically deformed, they become harder and stronger, a phenomenon known as strain hardening. If a stress is capable of producing plastic deformation, an even greater stress will be required to continue the flow. In Chapter 3 we will discuss the atomic-scale features that are responsible for this phenomenon. Various materials strain-harden at different rates; that is, for a given amount of deformation different materials will exhibit different increases in strength. One method of describing this behavior is to mathematically fit the plastic region of the true stress-true strain curve to the equation and determine the best-fit value of n, the strain-hardening exponent.2 As shown in Figure 2-13, a material with a high value of n will have a significant increase in material strength with a small amount of deformation.A material with a small n value will show little change in strength with plastic deformation. Damping Capacity. In Figure 2-12 the unloading and reloading of the specimen follow slightly different paths. The area between the two curves is proportional to the amount of energy that is converted from mechanical form to heat and is therefore absorbed by the material.When this area is large, the material is said to exhibit good damping capacity and is able to absorb mechanical vibrations or damp them out quickly. This is an important property in applications such as crankshafts and machinery bases. Gray cast iron is used in many applications because of its high damping capacity. Materials with low damping capacity, such as brass and steel, readily transmit both sound and vibrations. Rate Considerations. The rate or speed at which a tensile test is conducted can have a significant effect on the various properties. Strain rate sensitivity varies widely for engineering materials. Plastics and polymers are very sensitive to testing speed. Steels are also sensitive, but aluminum is rather insensitive. Those materials that are sensitive to speed variations exhibit higher strengths and lower ductility when speed is increased. It is important to recognize that standard testing selects a standard speed, which may or may not correlate with the conditions of product application. COMPRESSION TESTS When a material is subjected to compressive loadings, the relationships between stress and strain are similar to those for a tension test. Up to a certain value of stress, the material behaves elastically. Beyond this value, plastic flow occurs. In general, however, a compression test is more difficult to conduct than a standard tensile test.Test specimens must have larger cross-sectional areas to resist bending or buckling.As deformation proceeds, the material strengthens by strain hardening and the cross section of the specimen increases, combining to produce a substantial increase in required load. Friction between

When the forces that are applied to a material are constant, or nearly so, they are said to be static. Since static loadings are observed in many applications, it is important to characterize the behavior of materials under these conditions. For design engineers, the strength of a material may be of primary concern, along with the amount of elastic stretching or deflection that may be experienced while under load. Manufacturing engineers, looking to shape products, may be more concerned with the ability to mechanically deform the material without fracture. As a result, a number of standardized tests have been developed to evaluate the static properties of engineering materials.Test results can be used to determine if a given material or batch of material has the necessary properties to meet specified requirements. Other tests provide the materials characterization base used for material selection. In all cases, it is important to determine that the service conditions are indeed similar to those of testing. Even when the service conditions differ, the results of standard tests may be helpful in qualitatively rating and comparing various materials. TENSILE TEST The most common of the static tests is the uniaxial tensile test. The test begins with the preparation of a standard specimen with prescribed geometry, like the round and flat specimens described in Figure 2-4. The standard specimens ensure meaningful and re

and appearance, is supported vertically as a cantilever beam and is impacted on the unsupported end, striking from the side of the notch (Figure 2-21). Impact testers, like the one shown in Figure 2-22, supply a predetermined impact energy in the form of a swinging pendulum. After breaking or deforming the specimen, the pendulum continues its upward swing with an energy equal to its original minus that absorbed by the impacted specimen. The loss of energy is measured by the angle that the pendulum attains during its upward swing. The test specimens for bending impacts must be prepared with geometric precision to ensure consistent and reproducible results. Notch profile is extremely critical, for the test measures the energy required to both initiate and propagate a fracture. The effect of notch profile is shown dramatically in Figure 2-23. Here two specimens have been made from the same piece of steel with the same reduced cross-sectional area.The one with the keyhole notch fractures and absorbs only 43 ft-lb of energy, whereas the unnotched specimen resists fracture and absorbs 65 ft-lb during the impact

capabilities, and their limitations can one determine if the resulting data are applicable to a particular problem. METALLIC AND NONMETALLIC MATERIALS While engineering materials are often grouped as metals, ceramics, polymers, and composites, a simpler distinction might be to separate them into metallic and nonmetallic. The common metallic materials include iron, copper, aluminum, magnesium, nickel, titanium, lead, tin, and zinc as well as the alloys of these metals, such as steel, brass, and bronze. They possess the metallic properties of luster, high thermal conductivity, and high electrical conductivity; they are relatively ductile; and some have good magnetic properties. Some common nonmetals are wood, brick, concrete, glass, rubber, and plastics.Their properties vary widely, but they generally tend to be weaker, less ductile, and less dense than the metals, and to have poor electrical and thermal conductivities. Although metals have traditionally been the more important of the two groups, the nonmetallic materials have become increasingly important in modern manufacturing. Advanced ceramics, composite materials, and engineered plastics have emerged in a number of applications. In many cases, metals and nonmetals are viewed as competing materials, with selection being based on how well each is capable of providing the required properties. Where both perform adequately, total cost often becomes the deciding factor, where total cost includes both the cost of the material and the cost of fabricating the desired component. Factors such as product lifetime, environmental impact, energy requirements, and recyclability are also considered. PHYSICAL AND MECHANICAL PROPERTIES A common means of distinguishing one material from another is through their physical properties. These include such features as density (weight); melting point; optical properties (transparency, opaqueness, or color); the thermal properties of specific heat, coefficient of thermal expansion, and thermal conductivity; electrical conductivity; and magnetic properties. In some cases, physical properties are of prime importance when selecting a material, and several will be discussed in more detail near the end of this chapter. More often, however, material selection is dominated by the properties that describe how a material responds to applied loads or forces. These mechanical properties are usually determined by subjecting prepared specimens to standard test conditions. When using test results, however, it is important to remember that they apply only to the specific conditions that were employed. The actual service conditions of engineered products rarely duplicate the conditions of laboratory testing, so considerable caution should be exercised when applying test results. STRESS AND STRAIN When a force or load is applied to a material, it deforms or distorts (becomes strained), and internal reactive forces (stresses) are transmitted through the solid. For example, if a weight, is suspended from a bar of uniform cross section and length L, as in Figure 2-2, the bar will elongate by an amount For a given weight, the magnitude of the elongation, depends on the original length of the bar. The amount of elongation per unit length, expressed as is called the unit strain. Although the ratio is that of a length to another length and is therefore dimensionless, strain is usually expressed in terms of millimeters per meter, inches per inch, or simply as a percentage. Application of the force also produces reactive stresses, which serve to transmit the load through the bar and on to its supports. Stress is defined as the force or load being transmitted divided by the cross-sectional area transmitting the load.Thus, in Figure 2-2, the stress is where A is the cross-sectional area of the supporting bar. Stress is normally expressed in megapascals (in SI units, where a pascal is 1 newton per square meter) or pounds per square inch (in the English system). In Figure 2-2, the weight tends to stretch or lengthen the bar, so the strain is known as a tensile strain and the stress as a tensile stress. Other types of loadings produce other types of stresses and strains (Figure 2-3). Compressive forces tend to shorten the material and produce compressive stresses and strains. Shear stresses and strains result when two forces acting on a body are offset with respect to one another.

deform a 1-inch length to a final length of 1.002 inches (a 0.2% strain). If the applied stresses are then kept below the 0.2% offset yield strength of the material, the user can be guaranteed that any resulting plastic deformation will be less than 0.2% of the original dimension.While 0.2% is a common offset for many mechanical products, applications that cannot tolerate that amount of deformation may specify offset values of 0.1% or even 0.02%. Offset yield strength is determined by drawing a line parallel to the elastic line, but displaced by the offset strain, and reporting the stress where the constructed line intersects the actual stress-strain curve. Figure 2-7 shows the determination of both 0.1% offset and 0.2% offset values, and ,respectively. The intersection values are reproducible and independent of equipment sensitivity. Offset yield values are meaningless unless they are reported in conjunction with the amount of offset strain used in their determination. The 0.2% value is most common and is generally assumed unless another number is specified. As shown in Figure 2-6, the load (or engineering stress) required to produce additional plastic deformation continues to increase.The load that a material or specimen can bear (load-bearing ability) can be computed by multiplying the material strength times its cross-sectional area. During tensile deformation, the specimen is getting longer. The cross-sectional area is decreasing, but the load-bearing ability of the specimen continues to increase! For this to occur, the material must be getting stronger. The mechanism for this phenomenon will be discussed in Chapter 3, where we will learn that the strength of a metal continues to increase with increased deformation. During the plastic deformation portion of a tensile test, the weakest location of the specimen undergoes deformation and becomes stronger. Since it is no longer the weakest location, another location assumes that status and deforms. As a consequence, the specimen strengthens uniformly and maintains its original cylindrical or rectangular geometry. As plastic deformation progresses, however, the additional increments of strength decrease in magnitude, and a point is reached where the decrease in area cancels or dominates the increase in strength. When this occurs, the load-bearing ability peaks, and the force required to continue straining the specimen begins to decrease, as seen in Figure 2-6.The stress at which the load-bearing ability peaks is known as the ultimate strength, tensile strength, or ultimate tensile strength of the material. The weakest location in the test specimen at that time continues to be the weakest location by virtue of the decrease in area, and further deformation becomes localized. This localized reduction in cross-sectional area, known as necking, is shown in Figure 2-8. If the straining is continued, necking becomes intensified and the tensile specimen will ultimately fracture. The stress at which fracture occurs is known as the breaking strength or fracture strength. For ductile materials, necking precedes fracture, and the breaking strength is less than the ultimate tensile strength. For a brittle material, fracture usually terminates the stress-strain curve before necking, and possibly before the onset of plastic flow. Ductility and Brittleness. When evaluating the suitability of a material for certain manufacturing processes or its appropriateness for a given application, the amount of plasticity that precedes fracture, or the ductility, can often be a significant property. For metal deformation processes, the greater the ductility, the more a material can be deformed without fracture. Ductility also plays a key role in toughness, a property that will be described shortly

easy to conduct, and is used extensively on irons and steels. On the negative side, however, the Brinell test has the following limitations: 1. It cannot be used on very hard or very soft materials. 2. The results may not be valid for thin specimens. It is best if the thickness of material is at least 10 times the depth of the indentation. Some standards specify the minimum hardnesses for which the tests on thin specimens will be considered valid. 3. The test is not valid for case-hardened surfaces. 4. The test must be conducted far enough from the edge of the material so that no edge bulging occurs. 5. The substantial indentation may be objectionable on finished parts. 6. The edge or rim of the indentation may not be clearly defined or may be difficult to see. The Rockwell Test. The widely used Rockwell hardness test is similar to the Brinell test, with the hardness value again being determined through an indentation produced under a static load. Figure 2-15a shows the key features of the Rockwell test. A small indenter, either a small-diameter steel ball or a diamond-tipped cone called a brale, is first seated firmly against the material by the application of a 10-kg "minor" load. This causes a slight elastic penetration into the surface and removes the effects of any surface irregularities. The indicator on the screen of the tester, like the one shown in Figure 2-15b, is then set to zero, and a "major" load of 60, 100, or 150 kg is applied to the indenter to produce a deeper penetration (i.e., plastic deformation). When the indicating pointer has come to rest, the major load is removed. With the minor load still applied, the tester now indicates the Rockwell hardness number on either a dial gage or digital display. This number is really an indication of the depth of the plastic or permanent penetration that was produced by the major load, with each unit representing a penetration depth of Different combinations of major loads and indenters are designated by letters and are used for materials with various levels of strength. Table 2-1 provides a partial listing of the Rockwell scales and typical materials for which they are used. Because of the different scales, a Rockwell hardness number must be accompanied by the letter corresponding to the particular combination of load and indenter used in its determination. The notation RC60 (or Rockwell C 60), for example, indicates that a 120° diamond-tipped brale indenter was used in combination with a major load of 150 kg, and

have been constructed for this type of testing.The location for the test is selected under high magnification. A small diamond penetrator is then loaded with a predetermined load ranging from 25 to 3600 g. In the Knoop test, an elongated diamond-shaped indenter (long diagonal seven times the short diagonal) is used and the length of the indentation is measured with the aid of a microscope. Figure 2-17 compares the indenters for the Vickers and Knoop tests, and shows a series of Knoop indentations progressing left-to-right across a surface-hardened steel specimen, from the hardened surface to the unhardened core.The hardness value, known as the Knoop hardness number, is again obtained by dividing the load in kilograms by the projected area of the indentation, expressed in square millimeters. A light-load Vickers test can also be used to determine microhardness. Other Hardness Determinations. When testing soft, elastic materials, such as rubbers and nonrigid plastics, a durometer can be used. This instrument, shown in Figure 2-18, measures the resistance of a material to elastic penetration by a spring-loaded conical steel indenter. No permanent deformation occurs. A similar test, used to evaluate the strength of molding sands used in the foundry industry, will be described in Chapter 14. In the scleroscope test, hardness is measured by the rebound of a small diamondtipped "hammer" that is dropped from a fixed height onto the surface of the material to be tested.This test evaluates the resilience of a material, and the surface on which the test is conducted must have a fairly high polish to yield good results. Because the test is based on resilience, scleroscope hardness numbers should only be used to compare similar materials.A comparison between steel and rubber, for example, would not be valid. Another definition of hardness is the ability of a material to resist being scratched. A crude but useful test that employs this principle is the file test, where one determines if a material can be cut by a simple metalworking file. The test can be either a pass-fail test using a single file or a semiquantitative evaluation using a series of files that have been pretreated to various levels of known hardness. Relationships among the Various Hardness Tests. Since the various hardness tests often evaluate different material phenomena, there are no simple relationships between the different types of hardness numbers. Approximate relationships have been developed, however, by testing the same material on a variety of devices.Table 2-2 presents a correlation of hardness values for plain carbon and low-alloy steels. It may be noted that for Rockwell C numbers above 20, the Brinell values are approximately 10 times the Rockwell number.Also, for Brinell values below 320, the Vickers and Brinell values agree quite closely. Since the relationships among the various tests will differ with material, mechanical processing, and heat treatment, correlations such as Table 2-2 should be used with caution

original cross-sectional area, and the elongation is divided by the original gage length, the size effects are eliminated and the resulting plot becomes known as an engineering stress-engineering strain curve (see Figure 2-6).This is simply a load-elongation plot with the scales of both axes modified to remove the effects of specimen size. In Figure 2-6 it can be noted that the initial response is linear. Up to a certain point, the stress and strain are directly proportional to one another.The stress at which this proportionality ceases is known as the proportional limit. Below this value, the material obeys Hooke's law, which states that the strain is directly proportional to the stress. The proportionality constant, or ratio of stress to strain, is known as Young's modulus or the modulus of elasticity.This is an inherent property of a given material1 and is of considerable engineering importance. As a measure of stiffness, it indicates the ability of a material to resist deflection or stretching when loaded and is commonly designated by the symbol E. Up to a certain stress, if the load is removed, the specimen will return to its original length.The response is elastic or recoverable, like the stretching and relaxation of a rubber band. The uppermost stress for which this behavior is observed is known as the elastic limit. For most materials the elastic limit and proportional limit are almost identical, with the elastic limit being slightly higher. Neither quantity should be assigned great engineering significance, however, because the determined values are often dependent on the sensitivity and precision of the test equipment. The amount of energy that a material can absorb while in the elastic range is called the resilience.The area under a load-elongation curve is the product of a force and a distance, and is therefore a measure of the energy absorbed by the specimen. If the area is determined up to the elastic limit, the absorbed energy will be elastic (or potential) energy and is regained when the specimen is unloaded. If we perform the same calculation on an engineering stress-engineering strain diagram, the area beneath the elastic region corresponds to an energy per unit volume and is known as the modulus of resilience. Elongation beyond the elastic limit becomes unrecoverable and is known as plastic deformation.When the load is removed, only the elastic stretching will be recovered, and the specimen will retain a permanent change in shape. For most components, the onset of plastic flow represents failure, since the part dimensions will now be outside of allowable tolerances. In manufacturing processes where plastic deformation is used to produce a desired shape, the applied stresses must be sufficiently above the elastic limit to induce the required amount of plastic flow. Permanent deformation, therefore, may be either desirable or undesirable, and it is important to determine the conditions where elastic behavior transitions to plastic flow. Whenever the elastic limit is exceeded, increases in strain no longer require proportionate increases in stress. For some materials, a stress value may be reached where additional strain occurs without any further increase in stress.This stress is known as the yield point, or yield-point stress. For low-carbon steels, with curves like that in Figure 2-6, two distinct points are significant. The highest stress preceding extensive strain is known as the upper yield point, and the lower, relatively constant, "run-out" value is known as the lower yield point. The lower value is the one that usually appears in tabulated data. Most materials, however, do not have a well-defined yield point and exhibit stress-strain curves more like that shown in Figure 2-7. For these materials, the elasticto-plastic transition is not distinct, and detection of plastic deformation would be dependent upon machine sensitivity.To solve this dilemma, we elect to define a useful and easily determined property known as the offset yield strength. Offset yield strength does not describe the onset of plastic deformation but instead defines the stress required to produce a given, but tolerable, amount of permanent strain. By setting this strain, or "offset," to 0.2% (a common value), we can determine the stress required to plastically

producible results, and are designed to produce uniform uniaxial tension in the central portion of the specimen while ensuring reduced stresses in the enlarged ends or shoulders that are gripped. Strength Properties. The standard specimen is then loaded in tension in a testing machine like the one shown in Figure 2-5. A force or load, is applied and measured by the testing machine, while the elongation or stretch of a specified length (gage length) is simultaneously monitored. A plot of the coordinated load-elongation data produces a curve similar to that of Figure 2-6. Since the loads will differ for differentsized specimens and the amount of elongation will vary with different gage lengths, it is important to remove these geometric or size effects if we are to produce data that are characteristic of a given material, not a particular specimen. If the load is divided by the

that are characteristic of fatigue failure. Figure 2-29 shows an example of these markings at high magnification. For some fatigue failures, the overload area may exhibit a crystalline appearance, and the failure is sometimes attributed to the metal having "crystallized." As will be noted in Chapter 3, engineering metals are almost always crystalline materials.The final overload fracture simply propagated along the intercrystalline surfaces (grain boundaries) and revealed the already-existing crystalline nature of the material. The conclusion that the material crystallized is totally erroneous, and the term is a definite misnomer. Another common error is to classify all progressive-type failures as fatigue failures. Other progressive failure mechanisms, such as creep failure and stress-corrosion cracking, will also produce the characteristic two-region fracture. In addition, the same mechanism can produce fractures with different appearances depending on the magnitude of the load, type of loading (torsion, bending, or tension), temperature, and operating environment. Correct interpretation of a metal failure generally requires far more information than that acquired by a visual examination of the fracture surface. A final misconception regarding fatigue failures is to assume that the failure is time dependent.The failure of materials under repeated loads below their static strength is primarily a function of the magnitude and number of loading cycles. If the frequency of loading is increased, the time to failure should decrease proportionately. If the time does not change, the failure is dominated by one or more environmental factors, and fatigue is a secondary component. ■ 2.4 TEMPERATURE EFFECTS (BOTH HIGH AND LOW) The test data used in design and engineering decisions should always be obtained under conditions that simulate those of actual service.A number of engineered structures, such as aircraft, space vehicles, gas turbines, and nuclear power plants, are required to operate under temperatures as low as 130°C (200°F) or as high as 1250°C (2300°F). To cover these extremes, the designer must consider both the short- and long-range effects of temperature on the mechanical and physical properties of the material being considered. From a manufacturing viewpoint, the effects of temperature are equally important. Numerous manufacturing processes involve heat, and the elevated temperature and processing may alter the material properties in both favorable and unfavorable ways.A material can often be processed successfully, or economically, only because heating or cooling can be used to change its properties. Elevated temperatures can be quite useful in modifying the strength and ductility of a material. Figure 2-30 summarizes the results of tensile tests conducted over a wide range of temperatures using a medium-carbon steel. Similar effects are presented for magnesium in Figure 2-31. As expected, an increase in temperature will typically induce a decrease in strength and hardness and an increase in elongation. For manufacturing operations such as metalforming, heating to elevated temperature may be extremely attractive because the material is now both weaker and more ductile.

the testing machine surfaces and the ends of the test specimen will alter the results if not properly considered. The type of service for which the material is intended, however, should be the primary factor in determining whether the testing should be performed in tension or compression. HARDNESS TESTING The wear resistance and strength of a material can also be evaluated by assessing its "hardness." Hardness is actually a hard-to-define property of engineering materials, and a number of different tests have been developed using various phenomena. The most common of the hardness tests are based on resistance to permanent deformation in the form of penetration or indentation. Other tests evaluate resistance to scratching, wear resistance, resistance to cutting or drilling, or elastic rebound (energy absorption under impact loading). Since these phenomena are not the same, the results of the various tests often do not correlate with one another. Caution should be exercised to ensure that the selected test clearly evaluates the phenomena of interest. Brinell Hardness Test. The Brinell hardness test was one of the earliest accepted methods of measuring hardness. A tungsten carbide or hardened steel ball 10 mm in diameter is pressed into the flat surface of a material by a standard load of 500 or 3000 kg, and the load is maintained for 10 to 15 seconds to permit the full amount of plastic deformation to occur. The load and ball are then removed, and the diameter of the resulting spherical indentation (usually in the range of 2 to 5 mm) is measured using a special grid or traveling microscope. The Brinell hardness number (BHN) is equal to the load divided by the surface area of the spherical indentation when the units are expressed as kilograms per square millimeter. In actual practice, the Brinell hardness number is determined from tables that correlate the Brinell number with the diameter of the indentation produced under the various loads. Figure 2-14 shows a typical Brinell tester, along with a schematic of the testing procedure. Portable testers are available for use on pieces that are too large to be brought to a benchtop machine. The Brinell test measures hardness over a relatively large area and is somewhat indifferent to small-scale variations in the material structure. It is relatively simple and

time to rupture.These results are unique to the material being tested and the specific conditions of the test. Tests conducted at higher temperatures or with higher applied loads would exhibit higher creep rates and shorter rupture times. When creep behavior is a concern, multiple tests are conducted over a range of temperatures and stresses, and the rupture time data are collected into a single stress- rupture diagram, like the one shown in Figure 2-36. This simple engineering tool provides an overall picture of material performance at elevated temperature. In a similar manner, creep-rate data can also be plotted to show the effects of temperature and stress. Figure 2-37 presents a creep-rate diagram for a high-temperature nickel-based alloy. ■ 2.5 MACHINABILITY, FORMABILITY, AND WELDABILITY While it is common to assume that the various "-ability" terms also refer to specific material properties, they actually refer to the way a material responds to specific processing techniques. As a result, they can be quite nebulous. Machinability, for example, depends not only on the material being machined but also on the specific machining process; the conditions of that process, such as cutting speed; and the aspects of that process that are of greatest interest. Machinability ratings are generally based on relative tool life. In certain applications, however, we may be more interested in how easy a metal is to cut, or how it performs under high-speed machining, and less interested in the tool life or the resulting surface finish. For other applications, surface finish or the formation of fine chips may be the most desirable feature. As a result, the term machinability may mean different things to different people, and it frequently involves multiple properties of a material interacting with the conditions of a process. In a similar manner, malleability, workability, and formability all refer to a material's suitability for plastic deformation processing. Since a material often behaves differently at different temperatures, a material with good "hot formability" may have poor deformation characteristics at room temperature. Furthermore, materials that flow nicely at low deformation speeds may behave in a brittle manner when loaded at rapid rates. Formability, therefore, needs to be evaluated for a specific combination of material, process, and process conditions. The results cannot be extrapolated or transferred to other processes or process conditions. Likewise, the weldability of a material may also depend on the specific welding or joining process and the specific process parameters. ■ 2.6 FRACTURE TOUGHNESS AND THE FRACTURE MECHANICS APPROACH A discussion of the mechanical properties of materials would not be complete without mention of the many tests and design concepts based on the fracture mechanics approach. Instead of treating test specimens as flaw-free materials, fracture mechan

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