Ch 7 Flashcards by KELSEY SWEENEY

When a metal is exposed to mechanical forces, what parameters are used to express;

*force
*magnitude
*degree of deformation

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What is the distinction between elastic and plastic deformation?

elastic: non-permanent; occurs at low levels of stress
stress-strain behavior is linear

plastic: permanent; occurs at higher levels of stress
stress-strain behavior is nonlinear

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How are the following mechanical characteristics of metals measured?

*stiffness
*strength
*ductility
*hardness

*stiffness: material’s resistance to elastic deformation
*strength: a material’s resistance to plastic deformation
*ductility: amount of plastic deformation at failure
*hardness: resistance to localized surface deformation

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What parameters are used to quantify these properties?

*stiffness
*strength
*ductility
*hardness

*stiffness: elastic (young’s) modulus
*strength: yield and tensile strength
*ductility: amount of plastic deformation at failure
*hardness: rockwell, brinell hardnesses

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Define engineering stress

Force/area

*area does not change

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Define strain

response to stress

how much the thing stretched compared to how long it was initially

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What are the units of strain?

unit less, dimensionless

length/length
width/width

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How do stress and strain relate?

atomic bonds act like springs during elastic deformation

Spring constant, slope intercept form and stress strain relationship are all linear
F = kx
y = mx
stress = E strain

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Describe the Modulus of Elasticity

*what does it describe
*how does it relate to bonding
*how can we use the bond energy graphs to provide context

measures the stiffness or strength of bond

EM depends on interatomic bonding forces

EM proportional to slope of interatomic force

stress affects bond length causing bonds to ‘rock’ between the two paths. Ultimately, return to equilibrium

strongly bonded - larger E
weakly bonded - smaller E

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How does the strength of the bonds in metals compare with the strength of the bonds in ceramics and polymers?

Ceramics - ionic or covalent = LARGE EM
Metals - metallic bonding = MIDDLE-LARGE EM
Polymers - covalent and weak secondary forces = SMALL EM

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What does Poisson’s Ratio tell us?

when a material is strained in one direction, it also deforms/changes in another direction

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Give relative values for
ceramics
metals
polymers
for Poisson’s Ratio. What does this tell us about the bonds in these materials?

*relatively equal stretching in both directions
ceramics v = 0.25
metals v = 0.33
polymers v = 0.40
*relatively larger stretching in one direction compared to another direction

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Why is there a negative relationship between strains in two directions for Poisson’s direction.

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Draw the atomic configurations before, during, after load application for elastic regime

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Draw the atomic configurations before during after load application for plastic regime

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If there is a change in length after unload, this is due to what?

Plastic

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Describe what is going on in the plastic region (in regards to bonds) for a metal.

During plastic deformation, bonds are stretched to the point where they snap. This is gradual
Metallic bonds can reform if slid over by one unit/one level. The result is that bonds are formed to different atoms

Draw a stress-strain curve for a metal

What is yield strength?

stress at which noticeable plastic deformation has occurred

ep = 0.002

Describe the differences between ceramic and metal stress strain curves

ceramics are more brittle than metals.
Covalent: highly directional with few close packed planes, few slip systems
ionic: few slip systems, resistance to motion of ions of like charge past one another

therefore, ceramics don’t really have a yield strength. ceramics go elastic to broken. once stretched too far they shatter

What is the tensile strength?

maximum possible stress you can apply to a material
after which, necking develops

What is ductility

Amount of plastic (ONLY PLASTIC, IGNORE ELASTIC) deformation at failure

How is ductility measured?

also how is it measured on the stress strain curve?

ductility is the amount of plastic deformation at failure

measure initial length
measure length after failure after unloading

percent elongation

What is resilience?

amount of energy a material can take before permanently deforming

amount of energy a material can absorb during elasitc deformation

What is toughness?

Amount of energy absorbed before fracture area under the stress-strain curve

Toughness for ceramics metals polymers

small ceramics polymers larger metals

Identify the points on a stress strain curve Plastic Regime Elastic Regime Yield Point Tensile Strength Ductility Resilience Toughness

Relative stress strain curves for Ceramics Metals Polymers

What is engineering stress?

Measure the area once and assume the area never changes. This is not representative of reality. Area actually decreases.

draw the stress strain curves for the following. brittle polymer plastic elastomer

Compare the relative magnitude of stress and strain for ; metals polymers ceramics

Describe the influence of T on Thermoplastics

decreasing temp increases elastic modulus increases TS decreses %EL increases brittleness

Describe the influence of increasing strain rates (speed of force) on Thermoplastics

same as decreasing T increases elastic modulus increases TS decreses %EL increases brittleness

How are ductility and temperature related?

as temperature increases, less mechanical energy is needed to break bonds increase in temp increases ductility

Draw an elastomere stress strain curve and describe the parts

shallow region: can adapt to large strains by adopting different configurations with minimal puling on the bonds (basically unravel to adopt a longer length until they cannot unravel any longer) GREATER ENTROPY steep region:pulling against strong covalent bonds LESS ENTROPY

How would the stress strain curve look like for the following polymers linear branched cross linked networked

BRITTLE POLYMERS: cross-linked and network polymers plastic polymers: behave like metals

Describe how a tensile test works What is a tensile test used for?

stretch material tells us about elastic regions doesn't tell us about plastic regions or elastic regions for brittle materials

describe how a 3 point bending test works what does it tell us about?

good for brittle materials, tells us about the elastic modulus

describe how a hardness test works

resistance to plastic deformation on the surface (scratch or dent)

Safety!