Manufacturing Process Chapter 2 Review

Impact Test

A typical impact test for determining impact properties of materials consists of placing a notched specimen in an impact tester and breaking the specimen with a swinging pendulum

Work, Heat , and Temperature

Almost all the mechanical work in plastic deformation is converted into heat. However, this conversion is not complete, because a portion of this work is stored within the deformed material as elastic energy, known as stored energy. This is generally 5-10% of the total energy input; in some alloys, however, it may be as high as 30%. In a simple frictionless deformation process, and assuming that the work is completely converted into heat, the theoretical temperature rise, T, in the workpiece is given by the following picture, where u is the specific energy (work of deformation per unit volume), ρ is the density, and c is the specific heat of the material.

Yielding

Area in stress-strain curve where a constant stress results in an increasing strain (the same amount of stress being applied continues to increase the strain); this occurs above the elastic limit/yield stress and will cause the material to deform permanently

Rockwell Hardness Test

Based on the depth of penetration instead of the diameter of the indentation. The indenter is pressed on onto the surface, first with a minor load and then the major load. The difference in the depths of penetration is a measure of the hardness of the material.

Ductile Fracture

Characterized by plastic deformation proceeding failure. Most metals and alloys neck down to a finite are and then fail. Ductile fracture generally takes place along planes on which the shear stress is max. Inclusions, which are impurities in the metal, have an important influence on ductile fracture and, consequently, on the workability of metals. Voids and porosity can also develop during processing of metals.

Creep Curve

Consists of a primary, secondary, and tertiary stages. The point at which the testing specimen fails by necking and fracture is called rupture. Creep rate increase with specimen temperature and applied load.

Engineering Stress/Nominal Stress

Defined as the ration of applied load, P, to the original cross-sectional area, A of the test specimen

Strength of Material

Depends on its ability to sustain a load without undo deformation or failure; it is inherent to the material itself

Fatigue Failure

Failure that occurs at a stress level below that at which failure would occur due to a static load. Caused by cracks that grow with cyclic stresses until the material fractures at a critical crack length. These cracks propagate through the material Fatigue failure is responsible for majority of failures in mechanical components.

Relationship between UTS and Brinell Hardness

For a load of 3000 kg in the Brinell Hardness test, the following relationships give estimates to the UTS of the material:

Determining the Yield Stress Point of a Stress-Strain Curve

For soft and ductile materials, the yield stress point is usually defined by drawing a line with the same slope as the linear elastic curve but that is offset by a strain of 0.002, or 0.2% elongation. The yield stress is then defined as the stress where this offset line intersects the stress-strain curve

Hardness

Generally defined as resistance to permanent indentation. It is not a fundamental property as resistance to indentation depends on shape of the indenter and the load applied. Hardness commonly gives general indication of the strength of the material and its resistance to scratching and wear. Multiple hardness tests have been developed.

S-N Curves

Graphs that are created from fatigue tests in which various stress amplitudes and the number of cycles it takes to cause total failure are recorded. They are based on complete reversal of the stress - that is, maximum tension, then maximum compression, then maximum tension, and so on—such as that imposed by bending a rectangular eraser. For these graphs, any point under the plotted line will not result in failure. Any point on or above the line will result in failure.

Engineering Strain

L is the instantaneous length of the specimen, Lo is the original length

Modulus of Toughness

Largest amount of strain energy per unit volume a material can absorb before fracture. It is equivalent to the entire area under the stress-strain curve

Modulus of Resilience

Largest amount of strain energy per unit volume the material can absorb without permanent damage (measured up to proportional limit). Equivalent to triangular area under elastic region

Impact/Dynamic Loading

Loading that occurs when a load is put on a material in a short time span. Tests usually involve a swinging pendulum that measures how much energy is required to break the specimen. This energy is known as the impact toughness of the material.

Zone of Deformation

Location where the indentation should be done. Generally, the location should be at least two diameters of the indentation from the edge of the specimen, and the thickness of the specimen should be at least 10 times the depth of penetration of the indenter. Also, successive indentations on the same surface of the workpiece should be far enough apart so as not to interfere with each other.

Residual Stresses

May develop when workpieces are subjected to plastic deformation that is not uniform throughout the part; these are stresses that remain within a part after it has been formed and all the external forces are removed. The removal of a layer of material from a bar that has residual stresses will disturb the equilibrium of the residual stresses. The bar will acquire a new radius of curvature in order to balance the internal forces, which results in warping.

Strain Hardening

More stress is required to continue increasing the strain of the material. This continues until the ultimate tensile strength.

Tension Test

Most commonly used method for determining the mechanical properties of materials, such as strength, ductility, toughness, elastic modulus, and strain-hardening capability.

Engineering Stress-Strain Curve (Not to Scale)

Note that the the area in which elastic deformation occurs is much smaller. Engineering stress-strain curves are based on original area

Scaled Engineering Stress-Strain Curve

Notice in this graph how the area for elastic deformation is extremely small

Brittle Fracture

Occurs with little or no gross plastic deformation. In tension, fracture takes place along the crystallographic plane (cleavage plane) on which the normal tensile stress is maximum. Face centered cubic metals usually do not fail by brittle fracture, whereas body-cented cubic and some hexagonal lose-packed metals fail by cleavage. In general, low temperature and a high rate of deformation promote brittle fracture.

Creep

Permanent deformation of a component under a static load maintained for a period of time. This occurs in metal and some non-mental materials. For metals and alloys, creep of any significance occurs at elevated temperatures.

Proportional Limit

Point at which the deformation is no longer directly proportional to the applied force. Hooke's Law no longer applies.

Brinell Hardness Test

Results in Brinells Hardness Number (HB), defined as the ratio of the applied load, P, to the curved surface area of the indentation; the harder the material tested, the smaller the impression. Typical loads are 1500 or 3000 kilograms, and its uses a 10-mm steel or tungsten-carbide ball

Vickers Hardness Test

Results in Vickers Hardness Number (HV); it uses a pyramid shaped diamond instead of a steel or tungsten-carbide ball, and its load ranges from 1 to 120 kg.

Strain Energy Density

Strain Energy per unit volume. It indicates the work required to plastically deform a unit volume of the material to that strain.

Hooke's Law

Stress is equal to the strain of a material multiplied by the material's modulus of elasticity. The modulus of elasticity is the slope of the linear part of the elastic region.

Cyclic Stresses

Stress that is cause by fluctuating mechanic loads, such as: 1. Stress on gear teeth or reciprocating sliders 2. Stress by rotating machine elements under constant bending stresses, as is commonly encountered in shafts 3. Stress by thermal stresses, as when a room temperature die comes into repeated contact with hot workpieces, and then begins to cool between successive contacts

Stiffness

The ability of a material to resist plastic deformation; represented by the modulus of elasticity, which is the linear slope of the elastic portion of the curve

Strain Energy

The energy stored in a material due to its deformation

Ductility

The extent of plastic deformation that the material undergoes prior to fracture. Measured in either total elongation or reduction of area, which are both percentages.

Elastic Region

The linear and non-linear portion of any given stress/strain curve that occurs before the yield stress; if the stress is removed in this section, the material will return to its original shape.

Necking

The material begins to have a decreasing area in a certain part of it, usually in the middle of the length of the material. This continues until the fracture stress

Ultimate Tensile Strength (UTS)

The maximum engineering stress a material can handle until it begins to decrease. Uniform elongation ends after this point, and necking begins. Before this value, the elongation of the material is uniform

Endurance Limit/Fatigue Limit

The maximum stress to which the material can be subjected without fatigue failure, regardless of the number of cycles.

Test Specimen

The specific material being used to complete any form of material testing

Yield Stress

The stress at which a material begins to undergo plastic deformation.

Fracture stress

The stress at which a material breaks.

True Stress-Strain Curve

True stress is based on actual area corresponding to current P. True stress continually increases all the way to fracture. True strain is the sum of the incremental elongation divided by the current gauge length at load P.

Creep Test

Typically consists of subjecting a specimen to a constant tensile load at elevated temperature and measuring the changes in length at various time increments.

Failure

When a material fails to support a load. There are two general types of failure: 1. Fracture: Cracking that is internal or external. Subclassified into two general categories: ductile and brittle 2. Buckling