Ch 7 Flashcards
When a metal is exposed to mechanical forces, what parameters are used to express;
*force
*magnitude
*degree of deformation
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
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
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
Define engineering stress
Force/area
*area does not change
Define strain
response to stress
how much the thing stretched compared to how long it was initially
What are the units of strain?
unit less, dimensionless
length/length
width/width
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
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
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
What does Poisson’s Ratio tell us?
when a material is strained in one direction, it also deforms/changes in another direction
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
Why is there a negative relationship between strains in two directions for Poisson’s direction.
Draw the atomic configurations before, during, after load application for elastic regime
Draw the atomic configurations before during after load application for plastic regime
If there is a change in length after unload, this is due to what?
Plastic
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)
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