Chapter 6: Mechanical Properties: Part One

Common hardness test

- Brinell test, Rockwell test, Knoop test, and Vickers test - simple & inexpensive - essentially nondestructive - used to estimate other mechanical properties (TS for carbon steels)

Higher temperature (below melting point) tends to make steels more ductile and less strong, likely due to...

- increased mobility of dislocations - exceeding the ductile-brittle transition temperature (DBTT)

Bingham plastic

A Bingham plastic is a viscoplastic material that behaves as a rigid body at low stresses but flows as a viscous fluid at high stress. (e.g., toothpaste)

Strain gage

A device used for measuring strain. A strain gage typically consists of a fine wire embedded in a polymer matrix. The strain gage is bonded to the test specimen and deforms as the specimen deforms. As the wire in the strain gage deforms, its resistance changes. The resistance change is directly proportional to the strain.

Proportional limit

A level of stress above which the relationship between stress and strain is not linear.

Anelastic (viscoelastic) material

A material in which the total strain developed has elastic and viscous components. Part of the total strain recovers similar to elastic strain. Some part, though, recovers over a period of time. Examples of viscoelastic materials include polymer melts and many polymers including Silly Putty. Typically, the term anelastic is used for metallic materials.

Stiffness

A measure of a material's resistance to elastic deformation. Stiffness is the slope of a load-displacement curve and is proportional to the elastic modulus. Stiffness depends on the geometry of the component under consideration, whereas the elastic or Young's modulus is a materials property. The inverse of stiffness is known as compliance. Proportional to Young's modulus.

Hardness

A measure of a material's resistance to localized plastic deformation.

What is a slip system and what role does it play in plastic deformation?

A slip system consists of a slip plane and slip direction, usually close-packed, in which line defects can move. The movement of these defects or dislocations is called slip. Slip is plastic deformation in crystalline solids.

Yield strength

A stress value obtained graphically that describes no more than a specified amount of deformation (usually 0.002). Also known as the offset yield strength. The point where the material transitions from elastic to plastic deformation, accounting for instrument error.

Offset yield strength

A stress value obtained graphically that describes the stress that gives no more than a specified amount of plastic deformation. Most useful for designing components. Also, simply stated as the yield strength.

Glass-transition temperature (Tg)

A temperature below which an otherwise ductile material behaves as if it is brittle. Usually, this temperature is not fixed and is affected by processing of the material.

Offset strain value

A value of strain (e.g., 0.002) used to obtain the offset yield stress.

Viscous material

A viscous material is one in which the strain develops over a period of time and the material does not return to its original shape after the stress is removed.

Yield point phenomenon

An abrupt transition, seen in some materials, from elastic deformations to plastic flow.

Extensometer

An instrument to measure change in length of a tensile specimen, thus allowing calculation of strain. An extensometer is often a clip that attaches to a sample and elastically deforms to measure the length change.

Bend test

Application of a force to a bar that is supported on each end to determine the resistance of the material to a static or slowly applied load. Typically used for brittle materials.

Impact loading

Application of stress at a very high strain rate (~ > 100 s^-1).

Work of fracture

Area under the stress-strain curve, considered as a measure of tensile toughness.

Macrohardness

Bulk hardness of materials measured using loads > 2 N.

Stress relaxation

Decrease in stress for a material held under constant strain as a function of time, which is observed in viscoelastic materials. Stress relaxation is different from time dependent recovery of strain.

Elastic deformation

Deformation of the material that is recovered instantaneously when the applied load is removed.

True strain

Elongation per unit length calculated using the instantaneous dimensions.

Engineering strain

Elongation per unit length calculated using the original dimensions.

Strain

Elongation per unit length.

Impact toughness

Energy absorbed by a material, usually notched, during fracture, under the conditions of the impact test.

FCC metals are often recommended for use at low temperatures, particularly when any sudden loading of the part is expected. Explain.

FCC metals do not normally display a transition temperature; instead the impact energies decrease slowly with decreasing temperature and, in at least some cases (such as some aluminum alloys), the energies even increase at low temperatures. The FCC metals can obtain large ductilities, giving large areas beneath the true stress strain curve.

Stress

Force per unit area over which the force is acting.

Elastic strain

Fully and instantaneously recoverable strain in a material.

Microhardness

Hardness of materials typically measured using loads less than 2 N with a test such as the Knoop (HK).

Nanoindentation

Hardness testing performed at the nanometer length scale. The imposed load and displacement are measured with micro-Newton and sub-nanometer resolution, respectively.

Necking

Local deformation causing a reduction in the cross-sectional area of a tensile specimen. Many ductile materials show this behavior. The engineering stress begins to decrease at the onset of necking.

Materials processing

Manufacturing or fabrication methods used for shaping of materials (e.g., extrusion, forging).

Pseudoplastics (shear thinning)

Materials in which the apparent viscosity decreases with increasing rate of shear.

Shear thinning (pseudoplastics)

Materials in which the apparent viscosity decreases with increasing rate of shear.

Dilatant (shear thickening)

Materials in which the apparent viscosity increases with increasing rate of shear.

Shear thickening (dilatant)

Materials in which the apparent viscosity increases with increasing rate of shear.

Newtonian

Materials in which the shear stress and shear strain rate are linearly related (e.g., light oil or water).

Non-Newtonian

Materials in which the shear stress and shear strain rate are not linearly related; these materials are shear thinning or shear thickening (e.g., polymer melts, slurries, paints, etc.).

Rheopectic behavior

Materials that show shear thickening and also an apparent viscosity that at a constant rate of shear increases with time.

Thixotropic behavior

Materials that show shear thinning and also an apparent viscosity that at a constant rate of shear decreases with time.

Viscosity (η)

Measure of the resistance to flow, defined as the ratio of shear stress to shear strain rate (units Poise or Pa-s).

Impact test

Measures the ability of material to absorb the sudden application of a load without breaking. The Charpy and Izod tests are commonly used impact tests.

Notch sensitivity

Measures the effect of a notch, scratch, or other imperfection on a material's properties such as toughness or fatigue life.

Tensile test

Measures the response of a material to a slowly applied uniaxial force. The yield strength, tensile strength, modulus of elasticity, and ductility are obtained.

Elastomers

Natural or synthetic plastics that are composed of molecules with spring-like coils that lead to large elastic deformations (e.g., natural rubber, silicones).

Plastic deformation or strain

Permanent deformation of a material when a load is applied, then removed.

Kinematic viscosity

Ratio of viscosity and density, often expressed in centiStokes.

Viscoelastic (or anelastic) material

See Anelastic material.

Ultimate tensile strength (UTS)

See Tensile strength.

Strain rate

Shear strain is the ratio of the change in deformation to its original length perpendicular to the axes of the member due to shear stress.

Ductility

The ability of a material to be permanently deformed without breaking when a force is applied.

Engineering stress

The applied load, or force, divided by the original area over which the load acts.

Tensile toughness

The area under the true stress-true strain tensile test curve. It is a measure of the energy required to cause fracture under tensile test conditions.

Impact energy

The energy required to fracture a standard specimen when the load is applied suddenly.

Load

The force applied to a material during testing.

Hooke's law

The linear-relationship between stress and strain in the elastic portion of the stress-strain curve.

True stress

The load divided by the instantaneous area over which the load acts.

Elastic limit

The magnitude of stress at which plastic deformation commences.

Modulus of resilience (Er)

The maximum elastic energy absorbed by a material when a load is applied. Er = (1/2) (Yield Strength) (Strain at Yielding)

Flexural modulus

The modulus of elasticity calculated from the results of a bend test; it is proportional to the slope of the stress-deflection curve.

Poisson's ratio

The negative of the ratio between the lateral and longitudinal strains in the elastic region.

Fracture toughness

The resistance of a material to failure in the presence of a flaw.

Shear modulus (G)

The slope of the linear part of the shear stress-shear strain curve.

Young's modulus (E)

The slope of the linear part of the stress-strain curve in the elastic region, same as modulus of elasticity.

Flexural strength (modulus of rupture)

The stress required to fracture a specimen in a bend test.

Tensile strength

The stress that corresponds to the maximum load in a tensile test.

Ductile to brittle transition temperature (DBTT)

The temperature below which a material behaves in a brittle manner in an impact test; it also depends on the strain rate.

Percent reduction in area

The total percentage permanent decrease in the cross-sectional area of a specimen due to a tensile test. % Reduction in Area = (Ao - Af)/Ao x 100

Percent elongation

The total percentage permanent increase in the length of a specimen due to a tensile test. % Elongation = (lf - lo)/lo x 100

Shear-strain rate

Time derivative of shear strain.

In a tensile test, true stress and engineering stress can both be defined by stress = force / area. What is the difference between them?

True stress uses an instantaneous (cross-section) area; engineering stress uses a constant area.

Apparent viscosity

Viscosity obtained by dividing shear stress by the corresponding value of the shear-strain rate for that stress.

Modulus of elasticity (E)

Young's modulus, or the slope of the linear part of the stress-strain curve in the elastic region. It is a measure of the stiffness of the bonds of a material and is not strongly dependent upon microstructure.