MSE 2001 Chapter 9

• What is the ductile-to-brittle transition?

In some material, mainly steels, ductility can decrease very sharply with temperature, so ductile materials becomes brittle - known as the ductile brittle transition. The standard test is

• Do all materials have a well-defined endurance limit? If not how is this problem overcome?

No, some materials such as nylon, aluminum, copper, and other FCC metals, do not exhibit a well-defined endurance limit. The S-N curve continues to slope downward (see Figure 9.5-3). An operational endurance limit is defined for such materials as the stress amplitude corresponding to 10^7 cycles to failure. Loading frequency is important in determining the fatigue behavior only when time-dependent effects are significant.

• Compare the Charpy response over a temperature range for an FCC alloy, BCC alloy high-strength alloys, and brittle ceramics.

Now if we look at all the different materials that we have studied with respect to metallic materials, what we'll see is the only material type that has a ductile to brittle transition temperature are the body centered cubic alloys. And in the case of BCC materials, at high temperatures, where it's possible to have more slips systems activated, we can have ductile behavior, and we can have larger amounts of energy absorbed in the specimen. On the other hand, as the temperature drops, we see this low impact energy and more brittle failure. Now when we compare the materials like face centered cubic materials or high strength alloys in terms of their relative positions, that's not the important issue. What is the important issue is, the fact that FCC materials regardless of what their strength is, those materials are going to be independent of temperature. So their behavior is essentially independent of temperature. On the other hand, when we look at ceramics, the material is going to behave essentially the same all the way across the different temperature ranges. The only material that has that transition are the materials that are fabricated from body center cubic alloys.

Types of Mechanical Tests

Smooth Specimens - Tensile - Four Point Bend Test Hardness Notched Specimens Fatigue Time-dependent deformation Fracture Mechanics

• Define shear modulus

Sometimes referred to as Modulus of Rigidity is the ratio of shear stress to shear strain. The higher the value of shear modulus, the more rigid the material

• Explain the origin of the upper and lower yield points in a carbon steel.

The drop in stress is caused by the sudden mobility of dislocations as they are released from the strain fields associated with interstitial atoms (i.e., C in steel). • In several FCC metals, such as copper and aluminum, the yield point is not well defined • Some materials, including carbon steels, exhibit a complex yield behavior, as shown in Figure 9.2-9b. The transition from elastic to plastic deformation occurs abruptly and is accompanied by a reduction in stress. With continued deformation, the stress level remains constant, then begins to rise. The drop in stress is caused by the sudden mobility of dislocations as they are released from the strain fields associated with interstitial atoms (i.e., C in steel). The yield strength is defined by the lowest stress at which plastic deformation occurs and is identified as the lower yield point. The upper yield point characterizes the stress at which plastic deformation first begins.

• What is Poisson's ratio?

When an axial force is applied to a bar, the bar not only elongates but shortens in the other two orthogonal directions. Poisson's ratio (v) is the ratio of lateral strain to axial strain. minus sign needed to obtain a positive value.

Striations

are parallel ridges that are seen on the surfaces of many fatigue failures at high magnifications. In some cases one striation forms for each fatigue cycle.

• What is a three (four) point bend test? What materials are usually evaluated using this technique and why?

because of the importance of how failures occur in ceramic materials, we often test them in a setup that's referred to as a four-point bend test. And when those stationary points that allow the sample to rest on them. When there is a force applied downward, what we see is that prismatic beam turns out to have a state of compression on the top surface and a state of tension on the bottom surface. Consequently, what we see then in that particular case is we can focus our attention on the bottom side of that material where the material typically fails intention before compression.

Larsen-Miller parameter

is a parameter employed in the analysis of creep. It may be used at a given stress to determine how long the part will last at a given temperature or to determine the maximum allowable temperature for fixed time duration.

Creep

is a process in which a material elongates with time under an applied load.

Ductile

is a term used to describe materials that are able to absorb energy by "bending" rather than by fracture (breaking into pieces) when subjected to external loads. For example, the metal or plastic bumper on a car is ductile, while the glass in a car window is not. More precisely, ductile materials exhibit a high failure strain.

Brittle

is a term used to describe materials that are unable to absorb energy by "bending" but instead fracture (break into pieces) when subjected to external loads. For example, the glass in a car window is brittle, while the metal or plastic bumper is not. More precisely, brittle materials exhibit a low failure strain.

Plastic Deformation

is deformation that is permanent, for example, a metal part that has been permanently bent is said to have undergone plastic deformation.

Elastic Deformation

is deformation that is recoverable when a load is removed. This means that the part will return to its original size when the load is removed. Deformation of a rubber band is typically elastic.

Strain Hardening

is hardening that occurs as a result of deforming a metal. During strain hardening, dislocations are generated and the high dislocation density makes it difficult for other dislocations to move.

Hardness

is the ability of a material to resist penetration. A material is said to be hard if large forces are required to cause a permanent indentation mark.

Engineering Strain (E)

is the change in length of a specimen or component divided by the original length.

True stress (Qt)

is the force divided by the instantaneous area normal to the applied force.

Stress (Q)

is the load divided by the original cross-sectional area normal to the applied load.

Ultimate tensile strength (Quts)

is the maximum value of the engineering stress in a tensile test.

True strain (Et)

is the natural logarithm of the instantaneous length divided by the original length. Numerically, it is essentially equivalent to the engineering strain for strains up to about 0.1.

Poisson's ratio (v)

is the negative ratio of the transverse strain divided by the longitudinal strain in a tensile test.

Fracture Toughness

is the property of a material that has to do with its ability to absorb energy before fracturing. If a material can absorb much energy it is said to have high fracture toughness. It is also common to refer to the value of the stress intensity parameter at which fracture occurs as the fracture toughness.

Stress relaxation

is the reduction in stress that occurs when a component is subjected to a constant value of strain. Typically polymers and metals at relatively high temperatures exhibit stress relaxation.

Shear modulus (G)

is the slope of the shear stress versus shear strain curve in the elastic region.

Unit cell

is the smallest representation of a material. In crystals the unit cell is the smallest patterned collection of atoms or ions that repeats in space.

Uniform strain (Eu)

is the strain in a specimen that occurs before reaching the ultimate tensile strength. Deformation up to this point is uniformly distributed throughout the gage section so the strain is also uniform.

0.2% offset yield strength

is the stress at which a line starting at 0.2% on the strain axis and drawn parallel to the initial elastic portion of a stress-strain curve intersects the stress-strain curve. This is the conventional yield strength for materials that do not exhibit a "sharp" transition from elastic to plastic behavior.

Elastic Limit

is the stress beyond which there is permanent deformation. Below the elastic limit all the deformation is recovered when the load is removed. elastomer A rubber that has been imparted a memory, usually by crosslinking or incorporation of a hard segment, so that up to about 600% extension the polymer recovers substantially completely. Only polymers can be elastomers.

Engineering Strain at Fracture (Ef)

is the value of the strain when failure occurs.

Endurance limit Se

is the value of the stress amplitude in fatigue below which failure will not occur regardless of the number of repeated load applications.

• How does the presence of a notch affect the S-N fatigue performance of a material?

limit plasticity and lead to mechanical behavior that is more brittle than would be apparent from a tensile test. Therefore, an alternative test method is required. The Charpy impact test meets these needs . The back-and-forth slip during cycling causes intrusions and extrusions that result in the formation of a notch within the slip band, as shown in Figure 9.5-10a and b. This notch is the nucleus of the fatigue crack, which grows during subsequent cycling and eventually causes catastrophic fracture.

Percent reduction in area (%RA)

refers to the change in area divided by the original area (expressed in percent) of a tensile specimen that has been fractured.

Flow Stress

refers to the stress required to continue plastic deformation. The stress at which plastic deformation first occurs is a specific value of the flow stress and is referred to as the yield stress.

Ductile-to-brittle transition temperature (DBTT)

refers to the temperature at which 50% of the fracture surface in a Charpy specimen shows crystallographic facets. Alternatively, it is the temperature corresponding to the midpoint between the lower and upper shelf energies in a Charpy test.

• Describe the three stages of creep. Sketch the strain versus time plot at a particular stress level. How is the strain-time response affected by increasing the stress or increasing the temperature?

second part, this is a linear region. material is well behaved and can very easily be modeled. So we're in this linear behavior and the strain rate is constant. Now when we get into that final stage where fracture is going to occur. So, increasing the temperature is going to shift this curve to much faster creep times and we'll see how this all goes through. With respect, focusing on the three different stages.

• Describe a Charpy Impact Test. What data does it provide?

• A pendulum is used to hit a material sample with a v-notch cut out. These trials are measured over various temperatures and the energy used to break the sample is recorded. It is used to measure Impact energy for studying the toughness of a material.

• Describe the microstructural mechanisms that promote creep.

• Creep behavior is extremely sensitive to the microstructure of the material, to its prior processing and mechanical history, and to composition. It is thus an important property that can be usefully manipulated through judicious choices of composition and processing history.

• What is a hardness test? What is it used for? What are its limitations?

• In a hardness test a load is placed on an indenter (a pointed probe), which is driven into the surface of the test material. The degree to which the indenter penetrates the sample is a measure of the material's ability to resist plastic deformation. Since hardness testing is essentially nondestructive and no special specimens are required, it is a comparatively inexpensive quality assurance test and an indicator of material condition. Another advantage of hardness testing is that properties such as ultimate tensile strength, wear resistance due to friction, and resistance to fatigue (a failure mechanism described in Section 9.5) can all be accurately predicted from hardness data.

• Describe an S-N fatigue test.

• The basis of the Stress-Life method is the Wohler S-N diagram, shown schematically for two materials in Figure 1. The S-N diagram plots nominal stress amplitude S versus cycles to failure N. There are numerous testing procedures to generate the required data for a proper S-N diagram. S-N test data are usually displayed on a log-log plot, with the actual S-N line representing the mean of the data from several tests

• How does the presence a corrosive media affect the S-N response?

• These advances have led to a more accurate characterization of fatigue behavior. Also, tests can be conducted under more realistic service conditions, such as at elevated temperature or under corrosive environments. • In aggressive environments or at elevated temperatures, both the stress-strain behavior and the fracture of materials become time-dependent. For example, during cyclic loading at elevated temperature or in a corrosive environment, the frequency of loading becomes important in determining the number of fatigue cycles to failure. Some polymers and low-melting-point metals, such as lead, can exhibit time-dependent deformation at room temperature. In such instances, the loading rate must be taken into account in order to accurately describe the mechanical response. For example, the influence of loading rate on the stress-strain behavior of polycarbonate is shown in Figure 9.6-1a.

• What are the drawbacks to smooth specimen fatigue and how are they overcome by the fracture mechanics approach to fatigue?

• You don't know where the crack will happen so energy tests are used with a notch in the specimen so that crack will occur at a certain region • Smooth specimen fatigue behavior that included both low stress cycling (HCF) and low cycle fatigue (LCF) were presented.