MAE 3324 Ch 6 Flashcards
What factors should be considered when choosing a material?
Nature of the applied load (cyclic vs constant), duration, environmental conditions (sea spray–> corrosion, temp–> Thermal expansion coeficient) etc.
Ref: Pg 143
What are the three principal ways that a load can be applied?
- Compression
- Tension
- Shear
Ref: Pg 144
True or False
A stress-strain test is destructive? (e.g. permanent deformation takes place and usually fracture)
True
Ref: Pg 145
Define Engineering stress and give the SI and BG units.
Stress = Force/Area before deformation
Pa or psi
Ref: Pg146
Define Engineering strain and give the units.
Strain = change in length/ origional length
Unitless
Ref: Pg 146
True or False
Sometimes strain value is shown as a percentage.
True
Ref: Pg 146
True or False
In a compression test, a compressive force is considered to be positive.
False, compressive forces are negative by convention.
Ref: Pg 147
True or False
In a compression test, a compressive strain is considered to be negative.
True
Ref: Pg 147
Define “Pure shear,” and give the units.
Shear = Force/Area [Pa, or psi]
Note: The force acts parallel to the cross-sectional area.
Ref: Pg 147
Define Shear strain and give the units.
Shear strain is the tangent of the strain angle. [rad]
Ref: Pg 147
Define Hooks Law.
Hook’s law states that stress is equal to the product of strain and the modulus of elasticity E (also known as Young’s Modulus).
Ref: Pg 148
Define “Elastic Deformation.”
Deformation in which stress and strain are proportional. A plot of stress vs strain should show a linear relation.
Ref: pg 148
True or False
On a stress-strain graph, in the elastic deformation region, the slope of the cure is the modulus of elasticity.
True
Ref: Pg 148
True or False
The greater the value for the modulus of elasticity (E) the stiffer the material (resistance to elastic deformation).
True
Ref: Pg 148
True or False
Elastic deformation is permanent.
False, it is non-permanent.
Ref: Pg 149
For materials that demonstrate non-linear behavior in the elastic region what is used to find the modulus of elasticity?
The tangent (secant) modulus is used.
Ref: Pg 149
Describe elastic strain on an atomic scale.
Macroscopic elastic strain is manifested as small changes in the interatomic spacing and the stretching of interatomic bonds.
Extra Notes: As a consequence, the magnitude of the modulus of elasticity is a measure of the resistance to the separation of adjacent atoms (interatomic bonding forces)
Ref: Pg 149
What is the relationship between the modulus of elasticity and the force-separation curve?
The modulus of elasticity is proportional to the slope of the interatomic force-separation curve at the equilibrium spacing.
Ref: Pg 149
For a weekly bonded material the force-separation curve will have a ____ slope and a ____ modulus of elasticity.
Shallow
Low
Ref: Pg 149
For a strongly bonded material, the force-separation curve will have a ____ slope and a ____ modulus of elasticity.
Steep
High
Ref: Pg 149
True or False
The values of the modulus of elasticity for ceramic material are about the same as for metals.
True
Ref: Pg 150
True or False
The values for the modulus of elasticity for polymers are higher than for metals.
False, they are lower for polymers.
Ref: Pg 150
As temperature increases, the modulus of elasticity ____.
Decreases.
Ref: Pg 150
Shear stress and strain are related through ____. What are the units?
The shear modulus of elasticity (G) which also has the units of pascals.
Ref: Pg 150
How is G found experimentally?
By measuring the slope of the shear stress vs shear strain curve in the elastic region.
Ref: Pg 150
What is anelasticity?
The time-dependent behavior of materials under loads. For example, when a material is deformed elastically it takes a certain time period for it to return to its original state.
Ref: Pg 151
True or False
In metals, the anelastic component can often be ignored because it is so small.
True
Ref: Pg 151
What is Poisson’s ratio?
The ratio of the lateral and axial strains on a body.
v = -1* ex/ez
Ref: Pg 151
What is the relationship between shear (G) and elastic modulI (E)? (give the actual equation)
E = 2G(1 + v)
Ref: Pg 152
For most metallic materials, elastic deformation persists only to strain of about ____.
0.005
What is plastic deformation?
Plastic deformation occurs when strains exceed the elastic region and permanent deformation occurs. At the start of the plastic region hooks law is no longer necessarily valid and stress is no longer directly proportional to strain.
Ref: Pg 154
Describe plastic deformation from the perspective of atomic bonds.
Plastic deformation corresponds to the breaking of bonds with original atomic neighbors and then reforming of bonds with new neighbors as large numbers of atoms or molecules move relative to one another. Upon removal of this stress, they do not return to their original positions.
Ref: Pg 154
For crystalline solids, plastic deformation is accomplished by means of a process called _____.
Slip; which involves the motion of dislocations discussed in Chapter 7.2
Ref: Pg 154
Plastic deformation in non-crystalline solids is accomplished by ____.
A viscous flow mechanism discussed in section 12.10
Ref: Pg 154
What is the “proportional limit” with respect to materials?
The point at which a material goes from elastic to plastic deformation.
Ref: Pg 154
What does the yield strength measure?
In a metal, the yield strength is a measure of its resistance to plastic deformation.
Ref: Pg 155
What is the tensile strength?
The maximum on the stress-strain curve.
Ref: Pg 155
Where does fracture ultimately occur?
At the necking point.
Ref: Pg 156
Define “ductility.”
It is a measure of the degree of plastic deformation that has been sustained at fracture.
Ref: Pg 158
A metal that experiences very little or no plastic deformation upon fracture is termed _____.
Brittle.
Ref: Pg 158
What is “resilience” with respect to materials?
The capacity of a material to absorb energy when it is deformed elastically and then upon unloading to have this energy recovered.
Ref: Pg 159
What are the two main characteristics of resilient materials?
High yield strengths and low modulus of elasticities (used in springs).
Ref: Pg 160
What is toughness with respect to materials?
The ability of a material to resist fracture when a crack or other defect is present.
Ref: Pg 160
What is true stress?
The force divided by the instantaneous cross-sectional area.
Ref: Pg 161
What is true strain? (Give the equation)
e_t = ln( li / lo )
