Chapter 6 Flashcards
What material is this chapter about?
metals
When can you use a strain test?
If a load is static or changes relatively slowly with time and is applied uniformly over a cross section or surface of a member
When are strain tests most commonly conducted?
Metals at room temperature
What are the three principal ways in which a load may be applied?
Tension, compression, and shear
What is the principal way that strain tests are performed?
Tension
Why was the “dogbone” specimen configuration chosen?
So deformation is confined to the narrow center region and to reduce the likelihood of fracture at the ends of the specimen
What is the tensile testing machine designed to do?
Elongate the specimen at a constant rate and measure the applied load
In order to minimize the geometrical factors in tensile testing (such as area of a specimen) what parameters did they create?
Engineering stress and engineering strain
What is engineering stress’ relationship?
Force/cross-sectional area
What engineering strain’s relationship?
deformation elongation/ original length
In what direction does the specimen contract in a compression test?
Along the direction of the stress
Are compressive strains positive or negative?
Negative
When are compressive tests used?
When a material’s behavior under large and permanent strains is desired or when the material is brittle in tension
What is torsion?
A variation of pure shear in which a structural member is twisted and the torsional forces produce a rotational motion about the longitudinal axis of one end of the member relative to the other end
What are three examples of torsion?
Machine axles, drive shafts, twist drills
Through what relationship are stress and strain proportional?
Hooke’s law
stress= E (strain)
What is an elastic deformation?
A deformation in which stress and strain are proportional
What does the slope of a linear stress v. strain plot correspond to?
The modulus of elasticity
What can you think of the modulus of elasticity as?
Stiffness/ a material’s resistance to elastic deformation/ the resistance of separation of adjacent atoms
- the greater the modulus, the stiffer the material
Is elastic deformation permanent?
No
On an atomic scale, what does macroscopic elastic strain look like?
Small changes in the interatomic spacing and the stretching to interatomic bonds
How does the modulus of elasticity relate between ceramics, metals, and polymers?
Ceramics and metals are about the same and polymers are lower
What is anelasticity?
The time-dependent elastic behavior (time it takes for the material to actually return to normal after deformed)
What is the anelastic component like in metals?
Small and often neglected
What is the anelastic component like in polymers?
In some the magnitude is significantly
Theoretically, what should Poisson’s ration be for isotropic materials?
1/4
What does elastically anisotropic mean?
The elastic behavior varies with crystallographic direction
Is plastic deformation permanent?
yes
From an atomic perspective, what does plastic deformation correspond to?
The breaking of bonds with original atom neighbors and then re-forming of bonds with new neighbors as large numbers of atoms
What is a slip?
The process through which deformation is accomplished for crystalline solids
How does plastic deformation in noncrystalline occur?
viscous flow mechanism
How can you identify where plastic deformation begins on a stress strain curve? What is this point called?
When the line stops being linear
The proportional limit P
What is the magnitude of the yield strength for a metal a measure of?
it’s resistance to plastic deformation
What does the M point in a stress strain graph correspond to?
the maximum stress that can be sustained by a structure in tension
At maximum stress, where is the deformation? What is the called?
deformation is confined at the neck. This is called necking.
What is ductility?
A measure of the degree of plastic deformation that has been sustained at fracture
What is the opposite of ductile?
Brittle
What are the two ways you can numerically express ductility?
Percent elongation
Percent reduction in area
What is fracture strain of a brittle material?
Less than 5%
Do the magnitudes of yield and tensile strengths decline or increase with increasing temperature?
decline with increasing temperature
What is the yield point phenomenon?
When the elastic-plastic transition is very well defined and occurs abruptly
What occurs at the upper yield point?
Plastic deformation is initiated with an apparent decrease in engineering stress
What is the lower yield point?
Where continued deformation fluctuates slightly about some constant stress value
What is the magnitude of the yield strength of a metal a measure of?
It’s resistance to plastic deformation
What is resilience?
the capacity of a material to absorb energy when it is deformed elastically and then to have this energy recovered
What is the modulus of resilience?
Ur, the strain energy per unit volume required to stress a material from an unloaded state up to the point of yielding
What are two characteristics of resilient materials?
high yield strength and low modulus of elasticity
What is fracture toughness?
a property that is indicative of a material’s resistance to fracture when a crack is present
What is notch toughness?
The ability of a material to absorb energy and plastically deform before fracturing.
For a metal to be tough, what two characteristics must it display?
Ductile and strength
Does the decline in the stress necessary to continue deformation past the maximum point M indicate that a metal is becoming weaker?
no. It is increasing in strength. the cross-sectional area is decreasing rapidly within the neck region.
Where is the region of the true stress-strain curve?
From the onset of plastic deformation M to the point of necking
Does necking occur in compression?
No
What is hardness?
a measure of a material’s resistance to localized plastic deformation
What is the most often performed mechanical test? Why?
Hardness
- Simple and inexpensive
- Test is nondestructive
- Tensile strength can be estimated from hardness data
What are the four hardness tests?
Rockwell Hardness
Brinell Hardness
Knoop
Vickers
What are both tensile strength and hardness indicators of?
A metal’s resistance to plastic deformation
What are the two symbols for hardnes?
HB and HRC
How do you calculate the modulus of resiliene?
The area under the elastic portion of the engineering stress-strain curve
What are five factors that can lead to scatter in measured material properties?
- test method
- variations in specimen fabrication procedure
- operator bias
- apparatus calibration
Do materials with large or small elastic moduli deform less?
Large
What does the elastic modulus depend on?
Interatomic bonding forces
What is Poisson’s ratio for metals, ceramics, and polymers?
Vmetals= 0.33
Vceramics= 0.25
Vpolymers = 0.40
What are the 4 elastic constants?
- Young’s Modulus (E)
- Shear Modulus (G)
- Bulk Modulus (K)
- Modulus of Resilience (Ur)
What kind of deformations are in the plastic region?
elastic and plastic
What is the fracture strength?
Strength that the material breaks
Visually, when does the maximum on the stress-strain curve appear for metals?
When it starts necking
What is the toughness for metals, ceramics, and polymers?
Metals- large
ceramics/polymers- small
Is there a lower or higher yield strength for the second deformation?
Higher
What are the two things that a large hardness means?
- High resistance to deformation from compressive loads
- Better wear properties
What is the hardness range for each scale? The useful range?
range: 0-130
useful: 20-100
