Materials Science Exam 2

Poisson's Ratio

(v = -El/Ea) For elastic deformation, the negative ratio of lateral and axial strains that result from an applied axial stress.

Shear

A force applied so as to cause or tend to cause two adjacent parts of the same body to slide relative to each other, in a direction parallel o their plane of contact.

Annealing

A generic term used to denote a heat treatment wherein the microstructure and, consequently, the properties of a material are altered. Frequently refers to a heat treatment whereby a previously cold-worked metal is softened by allowing it to recrystallize.

Ductility

A measure of a materials ability to undergo appreciable plastic deformation before fracture; it may be expressed as percent elongation or percent reduction from a tensile test

Toughness

A measure of the amount of energy absorbed by a material as it fractures. Indicated by the total area under the material's tensile stress-strain curve.

Impact Energy

A measure of the energy absorbed during the fracture of a specimen of standard dimensions and geometry when subjected to very rapid (impact) loading. Charpy and Izod impact tests are used to measure this parameter, which is important in assessing the ductile-to-brittle transition behavior of a material.

Ductile Fracture

A mode of fracture that is attended by extensive gross plastic deformation.

Stress Raiser

A small flaw or a structural discontinuity at which an applied tensile stress will be amplified and from which cracks may propagate.

Safe Stress

A stress used for design purposes; for ductile metals, it is the yield strength divided by a factor of safety

Fracture Mechanics

A technique of fracture analysis used to determine the stress level at which preexisting cracks of known size will propagate, leading to fracture.

Corrosion Fatigue

A type of failure that results from the simultaneous action of a cyclic stress and chemical attack.

Thermal Fatigue

A type of fatigue failure wherein the cyclic stresses are introduced by fluctuating thermal stresses.

Elastic Deformation

Deformation that is nonpermanent, that is, totally recovered upon release of an applied stress

Plastic Deformation

Deformation that is permanent or nonrecoverable after release of the applied load. It is accompanied by permanent atomic displacements.

Fatigue

Failure at relatively low stress levels of structures that are subjected to fluctuating and cyclic stresses.

Recrystallization Temperature

For a particular alloy, the minimum temperature at which complete recrystallization will occur within approximately one hour.

Intergranular Fracture

Fracture of polycrystalline materials by crack propagation along grain boundaries.

Transgranular Fracture

Fracture of polycrystalline materials by crack propagation through the grains.

Brittle Fracture

Fracture that occurs by rapid crack propagation and without appreciable macroscopic deformation.

Solid-Solution Strengthening

Hardening and strengthening of metals that result from alloying in which a solid solution is formed. The presence of impurity atoms restricts dislocation mobility.

Case Hardening

Hardening of the outer surface of a steel component by a carburizing or nitriding process; used to improve wear and fatigue resistance.

Rockwell Hardness Test

Most commonly used method to test for hardness.

Elastic Recovery

Nonpermanent deformation that is recovered or regained upon the release of a mechanical stress

Izod Test

One of two tests that may be used to measure the impact energy of a standard notched specimen. An impact blow is imparted to the specimen by a weighted pendulum.(single shear)

Charpy Test

One of two tests that may be used to measure the impact energy or notch toughness of a standard notched specimen. An impact blow is imparted to the specimen by means of a weighted pendulum. (double shear)

Slip

Plastic deformation as the result of dislocation motion; also, the shear displacement of two adjacent planes of atoms.

Design Stress

Product of the calculated stress level and a design factor. Used to protect against unanticipated failure.

Resilience

The capacity of a material to absorb energy when it is elastically deformed.

Engineering Strain

The chance in gauge length of a specimen in the direction of an applied stress divided by its original gauge length. (delta/L)

Slip System

The combination of a crystallographic plane and, within that plane, a crystallographic direction along which slip occurs.

Recrystallization

The formation of a new set of strain-free grains within a previously cold-worked material; normally an annealing heat treatment is necessary.

Grain Growth

The increase in average grain size of a polycrystalline material; for most metals, an elevated-temperature heat treatment is necessary.

Strain Hardening

The increase in hardness and strength of a ductile metal as it is plastically deformed below its recrystallization temperature.

True Stress

The instantaneous applied load divided by the instantaneous cross sectional area of a specimen.

Engineering Stress

The instantaneous load applied to a specimen divided by its cross sectional area before any deformation

Tensile Strength

The maximum engineering stress, in tension, that may be sustained without fracture. Often termed ultimate strength.

Hardness

The measure of a materials resistance to deformation by surface indentation or by abrasion.

True Strain

The natural logarithm of the ratio of instantaneous gauge length to the original gauge length of a specimen being deformed by a uniaxial force.

Yielding

The onset of plastic deformation

Cold Working

The plastic deformation of a metal at a temperature below that at which it recrystallizes

Proportional Limit

The point on a stress-strain curve at which the straight line proportionality between stress and strain ceases.

Anelasticity

The property of a solid in which deformation depends on the time rate of change of stress as well as on the stress itself.

Modulus of Elasticity

The ratio of stress to strain when deformation is totally elastic; also a measure of the stiffness of a material.

Recovery

The relief of some of the internal strain energy of a previously cold-worked metal, usually by heat treatment.

Yield Strength

The stress required to produce a very slight yet speficied amount of plastic strain; a strain offset of .002 is commonly used.

Creep

The time-dependent permanent deformation that occurs under stress; for most materials it is important only at elevated temperatures

Fatigue Life

The total number of stress cycles that will cause a fatigue failure at some specified stress amplitude.

Ductile to Brittle Transition

The transition from ductile to brittle behavior with a decrease in temperature exhibited by BCC allows; the temperature range over which the transition occurs is determined by the Charpy and Izod impact tests.