Materials Exam 1

What Controls How a Part Performs

Geometry

Material Choice

Fabrication

Heat treatment / postprocessing

Mechanical Property

a property describing how a material responds to an applied force

Physical Property

a property that can be measured without applying a mechanical force, such as density or thermal conductivity

static property

a property describing behavior under a constant force

Normal Stress

σ=F/A

Engineering Strain

ε=ΔL/L₀

Proportional Limit

point where the material no longer behaves linearly

elastic limit

point beyond which the material will not return to its original length when the load is removed

young’s modulus

ratio of stress to strain up to the proportional limit; a measure of stiffness

plastic deformation

permanent deformation after the elastic limit is exceeded

springback

difference between strain while force is applied and strain after the force is removed

yield strength

point where increases in strain no longer require proportionate increases in stress

ultimate strength / tensile strength

point where load-bearing ability peaks; after this, the force needed to continue straining decreases

fracture strength / breaking strength

stress associated with fracture

toughness

energy absorbed per unit volume before fracture; represented by the area under the stress-strain curve to fracture

ductility

ability to deform plastically without fracture

brittle

material that tends to crack, break, or shatter after relatively small deformation

Elastic modulus

E=σ/ε

Engineering Percent Elongation

%EL=[(L_f -L₀)/L₀ ] x 100

Poisson’s Ratio

v=-ε_t /ε_ι

fatigue

failure behavior associated with repeated changes in force or loading

fatigue strength

strength associated with a specified number of loading cycles

endurance limit

a limiting fatigue stress shown in the slides for materials such as many steels, below which very long life may be possible

creep

permanent deformation under sustained force at elevated temperature

ductile to brittle transition

temperature-dependent change in which a material can become substantially less tough/brittle at low temperature

DBTT

ductile to brittle transition temperature

polymorphism

ability of a material to exist in more than one structure

allotropy

polymorphism applied to an element

grain

an individual crystal in a polycrystalline material

grain boundary

the interface where neighboring grains meet

nucleation

formation of initial solid particles/crystals during solidification

crystal growth

expansion of crystals outward from nucleated regions

inoculation

adding impurities/particles to improve nucleation rate

coarse grain steel

less toughness; more distortion; better machinability and harden ability

fine grain steel

tougher; more ductile; less distortion; crack resistant

isotropic

properties are approximately the same in different directions

anisotropic

properties depend on direction

phase

a physically and chemically distinct material region with identifiable/uniform structure and chemistry and distinct boundaries

composition

the elements or compounds present in the alloy

equilibrium state

the stable state associated with specified composition, temperature, and pressure in the context of the phase diagram

tie line

a horizontal line connecting phases in equilibrium with each other at a selected temperature

isotherm

another term used for the tie line in the slides

eutectic reaction

a liquid phase transforms into two solid phases at the eutectic condition

eutectic composition

composition at the eutectic point

eutectic temperature

temperature at which the eutectic reaction occurs

eutectic point

the temperature/composition condition at which the eutectic reaction occurs

austenite

the γ shown in the iron-carbon system

ferrite

the α shown in the iron-carbon system

cementite

The iron-carbon associated with Fe₃C

Pearlite

alternating ferrite/cementite microstructure formed from austenite below the eutectoid temperature

hypoeutectoid

composition below the eutectoid composition

eutectoid

composition at the eutectoid composition

hypereutectoid

composition above the eutectoid composition