Materials Engineering Lab Exam 1 - UT Austin

A stress-strain line that changes slope gradually indicates what?

Homogeneous plastic deformation throughout the specimen.

Plastic deformation conserves what?

Volume

Plastic deformation is by...

Shear

Uniform plastic deformation leads to a nonlinear increase in the engineering stress. This is referred to as what?

Strain hardening

Where does necking begin?

At the maximum engineering stress

True stress is greatest where?

In the neck

True plastic strain is greatest where?

In the neck.

After necking, where does the strain hardening occur?

In the neck

Can Young's modulus be evaluated from both the engineering stress-strain curve and the true stress-strain curve?

Yes

What is the primary cause of strain hardening?

An increase in dislocation density

How is the strength coefficient and strain-hardening exponent derived?

They cannot be derived; they just fit measured data.

For metals, what is the range of values for the strain-hardening exponent (n)?

From close to 0 to slightly greater than .5

Can true stress be calculated in the neck?

No, the hydrostatic stress component and the developing voids make it impossible to calculate.

What is one measure of ductility?

Reduction in area.

Reduction in area during necking is uniquely related to what?

True strain at fracture

Can a Rockwell hardness test be performed on a rough surface?

No, only smooth, flat surfaces.

What are the 2 types of indenters used in a Rockwell Hardness test?

A tungsten carbide ball and a diamond cone indenter.

What is the hardness and density of the tungsten carbide ball used during the Rockwell hardness test?

Hardness should be no less than 1500 HV10 and the density is 14.8 g/cm^3

For the diamond indenter in the Rockwell harndess test, what is the radius of the tip?

200 micrometers

What is the preliminary test force for the Rockwell hardness test?

10kgf

What happens to the material around a Rockwell indentation?

It is coldworked

How far away should indentations be made when doing a Rockwell hardness test?

3 diameters away from the center of one indentation to the center of another.

Can a Brinell hardness test be performed on a rough surface?

no

What type of tip is used in a Brinell hardness test?

A hardened steel or tungsten carbide ball

What are the units of hardness?

Units of stress

What is the Brinell hardness number proportional to?

Tensile strength

Can a Vickers hardness test be performed on a rough surface?

No

What kind a tip is used in a Vickers harness test?

Diamond pyramid indenter

What are the units of Vickers hardness number?

Units of stress

The mechanical properties of polymers can be understood in terms of what two parameters?

Degree of cross-linking and temperature

The fundamental feature of a polymer is what?

A chain of carbon atoms bound to each other with strong covalent bonds.

Note: These chains are bonded to adjacent chains with weak secondary bonds. Since the secondary bonds are weak, irreversible sliding of molecules relative to each other (plastic deformation), is possible.

What do cross-links prevent?

Plastic deformation

Polymers become stiff and brittle when....

They are highly cross-linked or at low temperatures

Viscoelasticity is described as a combination of what?

Fast elastic deformation with thermally activated viscous flow.

In polymer deformation, the elastic deformation is collective bending of what?

Carbon chain molecules without relative irreversible motion.

In polymer deformation, what is thermally activated viscous flow?

It is the irreversible sliding of long chain molecules.

In polymer deformation, when thermal activation is involved, when does plastic deformation occur?

At small shear stresses when the thermal molecular motions are faster than the net macroscopic displacements

In polymer deformation, what does plastic deformation require?

It requires higher shear stresses when the thermally activated molecular motion is slow relative to the net macroscopic shears

Viscoelasticity is dependent upon what?

Temperature and strain-rate

When does a VE material show creep?

Under a constant load

When does a Ve material show stress relaxation?

Under a constant extension

What happens at high strain rates in semicrystalline polymers?

Thermally activated molecular motion is not sufficiently fast; high stress is necessary to produce plastic flow.

What happens at low strain rates in semicrystalline polymers?

Thermally activated molecular motion is sufficiently fast; low stress can produce plastic flow.

When do elastomers become brittle?

At very low temperatures when thermal energy is not available for intermolecular silding

Can mechanical properties be measured using a tension test?

It is very difficult, if not impossible to determine mechanical properties of many materials using a tension test.

Isotropic brittle materials

Materials whose properties are the same in all directions

Fracture stress is dependent on what?

Size, shape, and orientation of largest flaw. Large flaws give small fracture stresses.

Does compressive loading or tensile loading give higher fracture stress?

Compressive (~10x)

How is elastic strain energy calculated?

Area under the stress strain curve in the elastic region.

Anisotropic brittle materials

Materials whose properties vary with direction

What is toughness in an anisotropic material?

The area under the load-displacement curve

What is fracture toughness

The resistance to crack growth

intergranular cracking

cracks the follow grain boundaries

transgranular cracking

cracks the cut through grains

What is the stress intensity factor K_I?

It is the driving function for crack growth. As K_I increases, the probability of unstable crack growth increases.

When will a crack not grow?

When the fracture toughness is greater than the stress intensity factor

Benefits of an SEM

Better resolution than a light-optical system

Better depth of field

Chemical microanalysis is possible

Disadvantages of an SEM

Electron are scattered by air before they get to the specimen

Specimens must be electrically conductive or coated to prevent charging

Electron beams can damage delicate specimens

True of False. Elastomers exhibit nonlinear elasticity with a large, reversible extensibility.

True

What is hardness?

The resistance to permanent deformation.

True of False. In a 4-point bending the moment is constant over the entire gauge length.

True

What are the units of the stress intensity factor?

Pa*m^.5

What type of crystallinity does HDPE have?

semi-crystalline

Which has greater amounts of branching: LDPE or HDPE?

LDPE has more branching than HDPE

Is yield strength taken from the lower yield point or the higher yield point?

Higher yield point