MEEN 360 Midterm Exam Terms

Engineering Stress

The load applied to a material divided by its original cross-sectional area, representing how much force a material experiences per unit area.

Engineering Strain

the measure of deformation representing the displacement between particles in a material body. It is calculated as the change in length divided by the original length.

Poisson’s Ration

is the ratio of transverse strain to axial strain in a material when it is subjected to axial loads, reflecting the tendency of material to expand in directions perpendicular to the applied load.

Yield Strength

The maximum stress a material can withstand before it begins to deform plastically, indicating the point at which permanent deformation occurs.

Ultimate Yield Strength

the maximum stress a material can withstand before failure occurs, beyond which it cannot return to its original shape.

Stiffness

is the resistance of an elastic body to deformation when subjected to an applied force, often measured as the ratio of applied load to displacement.

Hardness

is a measure of a material's resistance to localized plastic deformation (e.g., indentation or scratching). It indicates the material's ability to withstand wear and abrasion.

Tensile Toughness

is the ability of a material to absorb energy and plastically deform without fracturing, often measured as the area under the stress-strain curve.

Impact Toughness

is the ability of a material to withstand a suddenly applied load or shock without fracturing, usually assessed through impact testing methods.

Fracture Toughness

is a material's ability to resist crack propagation when subjected to stress, often measured by the critical stress intensity factor.

Elastic/Shear Modulus

Slope in linear part on stress strain or shear stress strain curve

Yield Strength

The maximum stress a material can withstand before it begins to deform plastically, indicating the point at which permanent deformation occurs.

Ultimate Tensile Strength

The maximum stress a material can withstand while being stretched or pulled before necking and fracture occur, corresponding to the highest point on the engineering stress-strain curve.

Elastic Deformation

Reversible deformation; returns to original shape when load is removed

Plastic Deformation

A permanent and irreversible change in the shape or dimensions of a material that persists after the applied load is removed.

Strain Hardening

The increase in strength and hardness of a ductile material resulting from plastic deformation, driven by dislocation generation and accumulation within the crystal structure.

Necking

a phenomenon in a tensile test where a material begins to narrow significantly in one localized region as it is stretched.

Breaking

the point when a material separates into two or more pieces because it can no longer withstand the applied load.

Modulus of Resilience

the maximum amount of energy per unit volume a material can absorb without undergoing permanent deformation.

Tensile Toughness

the amount of energy per unit volume a material can absorb before it fractures (breaks).

%Elongation

how much a material's length increases before fracture, expressed as a percentage of its original length. It is a measure of ductility.

Ceramic Properties

• Strength: Often high in compression, but relatively weak in tension compared with their compressive strength.

• Stiffness: Generally high.

• Ductility: Very low at room temperature.

• Toughness: Generally low fracture toughness compared with metals.

• Hardness: Often very high; good wear resistance.

  • Conductivity: Usually poor electrical conductivity, although some ceramics are conductive.

Polymer Properties

• Strength: Often lower than metals and structural ceramics.

• Stiffness: Generally low.

• Ductility: Highly variable; some polymers stretch considerably, while others are brittle.

• Toughness: Varies significantly with polymer type and temperature.

• Hardness: Generally lower than metals and ceramics.

  • Conductivity: Usually poor electrical and thermal conductivity.

Metal Properties

• Strength: Moderate to very high, depending on alloy and processing.

• Stiffness: Generally high.

• Ductility: Often high; can plastically deform before fracture.

• Toughness: Often good because metals can absorb energy through plastic deformation.

• Hardness: Varies widely; can be increased through heat treatment or work hardening.

  • Conductivity: Generally good electrical and thermal conductivity.

Effects of temperature increase on mechanical properties

Yield strength, elastic modulus, tensile strength decrease; ductility increases

Effects of increased strain rate on mechanical properties

Yield strength increases, modulus of elasticity changes slightly, ductility decreases, and tensile strength usually increases

Ductile to Brittle Transition Temperature

the temperature below which certain materials change from behaving in a ductile manner to behaving in a brittle manner.

How does DBTT happen?

As temperature decreases, dislocation motion becomes more difficult in certain metals. The material has a harder time deforming plastically to relieve stress.

Consequently, a crack may propagate before significant plastic deformation can occur, causing brittle fracture.

What kind of materials does DBTT affect?

BCC and FCC metals, ceramics, and polymers

Tensile Test

Measures strength, stiffness, and ductility

Impact Test

Measures impact energy and toughness

Bending Test

Measures flexural strength and stiffness

Hardness test

Measures resistance to deformation

Nanoindentation tests

Measures hardness and stiffness at a tiny scale

How do various factors affect fracture?

Higher temp = more ductile, less brittle; faster loading = more likely to cause brittle fracture; easier to fracture with larger flaw sizes; larger sample size is more likely to contain a crazy flaw; higher ductility = more stretching before breaking; smaller grain size = usually tougher

Ductile Fracture

Lots of deformation before breaking; often forms a cup-and-cone shape.

Brittle Fracture

Little or no deformation; breaks suddenly with a flat surface.

Fatigue Fracture

Caused by repeated loading; cracks grow over time, often with beach marks.

What happens during ductile fracture?

Voids form, grow, and join together.

What happens during brittle fracture?

Bonds break with little plastic deformation.

What happens during fatigue fracture?

A crack starts and slowly grows with repeated loading until the material breaks.

Transgranular Fracture

Crack travels through the grains

Intergranular Fracture

Crack travels along grain boundaries

Ceramics During Fracture

Usually experience brittle fracture

Polymers During Fracture

Depends on temperature, loading rate, chemical bonds, etc

Metals During Fracture

Usually Ductile Fracture

Weibull Distribution

Probability of fracture vs applied stress

Weibull Modulus

Measures fracture strength variability in a material; metals generally have higher Weibull modulus than ceramics

Fatigue

Lowering of strength due to cyclic loading

Creep

Time dependent deformation under a constant load; usually happens at higher temperatures

Thermoplatsics

Linear or branched molecules

– Weak van der Waals between chains

– Exhibit ductile, plastic behavior

– Amorphous or crystalline

– Soften and melt upon heating

– Easily recycled, e.g. polyethylene (PE)

Themosets

Cross-linked or 3D network structure

– Stronger but more brittle than thermoplastics

– Decompose upon heating, as opposed to melt

– Difficult to recycle, e.g., polyurethane (PU)

Elastomers

Coil like molecules

– Linear, branched with occasional cross-links

– Sustain large elastic deformation >200%

– e.g. natural rubber

Degree of Polymerization

Tensile strength, creep resistance, melting

temperature, and impact and wear toughness

generally increase with increasing degree of

polymerization

Effect of Side Groups in Polymerization

Replacing the hydrogen atoms in polymer chains

with others (e.g., Cl, Fl) increases polarity and

leads to inter-chain hydrogen bonding

– Results in higher strengths, stiffness, and

melting temperatures

– Examples include vinyl and vinylidene polymers

Crystallization

Amorphous —> crystalline

polymer

• Higher crystallinity —> density, mechanical

strength, chemical resistance increase

• Deformation (and sometimes temperature or

electric field) can lead to crystallization

(alignment of molecular chains)

Rubbery and Leathery Polymer State

Slightly below Tm (rubbery state):

– Under —> elastic and plastic deformation

• Well below Tm, (leathery behavior)

– Lower temperature —> bonding is stronger (stiffer

polymer)

Glassy Polymer State

T < Tg, linear amorphous

polymer

• Stiff, hard, brittle

Elastic Deformation of Thermoplastics

Mostly non-Newtonian, viscoelastic behavior

– Deformation is rate-dependent

• Two deformation mechanisms for the elastic part

(1) Stretch and distort covalent bonds  elongate chain

elastically

Recovery is instantaneous

(2) Distort segments of

polymer chains

Recovery over a long

period of time

Plastic Deformation of Thermoplastics

Permanent deformation

• Indicated by a stress drop beyond yield point

• Chain will stretch, rotate, slide or disentangle

• Necking – continued

sliding of the chains

in necking region

• When chains are

nearly parallel,

greater stress to

deform

• Stable necking

Stress Relaxation

stress decreases with time at constant strain

Strain Hardening In Cold Working

Increases strength and hardness but decreases ductility because increasing dislocation density makes further dislocation movement more difficult.

Bauschinger Effect

Prior plastic deformation reduces the yield stress when the loading direction is reversed.

Percent Cold Work

Measures how much a material’s cross-sectional area has been reduced during cold working

Sheet Texture in Cold Working

Crystallographic orientation of grains; think of rolling pizza dough

Residual Stresses

Internal stresses that remain without loading; compressive is usually good, tensile not so much; annealing can be used to remove these

Pros of Cold Working

Strengthen metal while producing desired shape

• Excellent dimensional tolerances and surface finishes

• Relatively inexpensive

Cons of Cold Working

Some metals brittle at room temperature (HCP)

• Ductility, electrical conductivity, and corrosion resistance

impaired by cold working

• Cannot use this for materials that will be used at high

temperatures during service

Annealing During Cold Working

Heat, hold and allow microstructures to change, and let that thing slowly cool

Hot Working

Plastically deforming a material above the recrystallization temperature; material does not strengthen during this process

Hot Working Pros

Easier to shape

• Eliminate imperfections (e.g., dislocations)

Hot Working Cons

Anisotropic behavior due to various cool

rates

• Poor surface finish and dimensional

accuracy

Homogenous Nucleation

Cool below freezing temperature – free

energy of solid is smaller than liquid.

• Create more surface between solid and liquid.

Heterogeneous Nucleation

Impurities in the liquid

• Less undercooling is needed

• Big radius of curvature with little solid/liquid

contact area

Inoculation

adding small particles to promote

nucleation → smaller grains and higher

strength

Dispersion Strengthening

adding fine particles

to form a second phase (solid state

transformation) → higher strength

Metallic Glasses

fast cooling to prevent

nucleation of crystalline structure → amorphous

Planar Growth

• Smooth, flat interface.

• Solid grows in a uniform front.

• Occurs when the interface is stable.

  • Less irregular microstructure.

Dendritic Growth

• Tree-like, branching crystals form.

• Occurs when the interface becomes unstable.

• Common in metal alloys.

  • Can cause uneven distribution of alloying elements (segregation).

Columnar Grains During Casting

Anisotropic properties

Equiaxed Grains During Casting

Isotropic properties

Solidification Defects

Shrinkage

• Interdendritic Shrinkage

• Gas porosity

How to prevent solidification defects?

Proper pouring temperature, mold design, feeding systems, gas removal, and controlled cooling help

Alloy

A metal made by taking a pure metal and adding another material to it

Components

The elements which make up an alloy

Composition

The elemental make up, often by weight %, volume %, etc.

Phase

any portion of a system that is physically

homogeneous and bounded by a surface that

separates it from other distinct portions

Solid Solution

is a solid material in which atoms of one element are mixed uniformly into the crystal structure of another element.

Limited Solubility

• Only a certain amount of one element can dissolve in the other.

  • Beyond that limit, a second phase may form.

Unlimited Solubility

• The two elements can dissolve in each other in any proportion in the solid state.

  • They form a single solid-solution phase across the entire composition range.

Hume-Rothery Rules

Size – must be similar with <15% difference in

atomic radius

• Crystal structure – must be the same

• Valence – must be the same (or close)

• Electronegativity – must be approximately the

same

Solid Solution Strengthening

Formed in metals due to increased resistance to dislocation motion

Degree of strengthening depends on two factors…

1. A large difference in atomic sizes results in

increased strength of the alloy

2. The greater the amount of the alloying element

that is added, the greater the strengthening

Solid-Solution Strengthening Effects on Properties

σY, σUTS, H of the alloy

increase compared to pure

metal

• Ductility decreases

compared to pure metal

• Electrical conductivity

decreases compared to

pure metal

• Resistance to creep and

strength at high temperatures increases

compared to pure metal.

Segregation

the nonuniform composition produced by

nonequilibrium solidification

Microsegregation

heating to below the nonequilibrium

solidus for a few hours eliminates most of the

composition differences (‘homogenization’)