Materials Chapter 2

What is the effect of gage length on ductility? Give an example of a practical application for this effect.

As gage length increases, ductility decreases. So in applications where a large workpiece is being deformed, the ductility standards from tensile tests should not be used.

What is the relationship between strain rate sensitivity and processing temperature of metals?

As processing temperature increases the strain rate sensitivity of a material also increases (increase in slope on graph) resulting in higher material strength

Using words and a sketch, explain the effect of heat on materials toughness.

As temperature increases, Ductility also increases but UTS decreases. So since Toughness is the area under the stress-strain curve it can either increase or decrease depending on the material.

Define "hardness". Why are there different types of hardness tests available in the industry?

Hardness is a material property which describes its resistance to scratching/wear. There exists a wide range of tests due to the wide variety of material shapes, sizes, strengths, etc.

Using words and a sketch, explain plane-strain conditions in a compression test.

If the width of the workpiece (w) is significantly higher than its thickness (h) and the length of the deformation zone (b), it is assumed that there will be insignificant change in the width under loading conditions; hence there will be changes in two dimension only (length and thickness); i.e. 2-D deformation. This is referred to as "plane strain conditions"

Using a sketch, explain the assumption of plane-strain conditions and how it is used in the design of materials processing systems.

It is assumed that there is no change in width throughout the processing (i.e. a 2-D problem)

What is meant by plane strain conditions and how is it accounted for in the yield strength?

Plane stress means that the deformation occurs in two dimensions only. it is accounted for in yield strength by the following equation:

Using words and a sketch, describe the loading and unloading behavior of metals acting in an ideal rigid-perfectly plastic behavior.

Rigid, perfectly plastic: the material resists any form of deformation until the stress reaches the yield strength of the material (Y), at which the material starts to deform plastically (i.e., permanent deformation) at the same stress level (Y) until it fails. There is no strain hardening during plastic deformation (n=0). If the load is released at any point after it starts deformation and before it fails, the material maintains its current shape under loading.

Which of the five idealized stress-strain behaviors has a strength coefficient that is equal to the yield strength and a strain-hardening coefficient that is equal to zero? (sketch the stress-strain curve for this behavior)

Rigid, perfectly plastic: the material resists any form of deformation until the stress reaches the yield strength of the material (Y), at which the material starts to deform plastically (i.e., permanent deformation) at the same stress level (Y) until it fails. There is no strain hardening during plastic deformation (n=0). If the load is released at any point after it starts deformation and before it fails, the material maintains its current shape under loading.

What is meant by strain rate sensitivity? How does it affect the analysis of metal forming processes?

Strain Rate Sensitivity describes the increase in strength due to the speed of deformation. This affects the analysis of metal forming processes because it is usually ideal to produce faster however since this increases the strength the force needed to process the material is also increased. A modified version of the Flow Rule is used for this analysis.

What is the significance of the strain hardening coefficient (n) in materials processing?

Strain hardening coefficient (n) is a material property which describes the sensitivity of the material to work hardening. The higher the value of n the higher the sensitivity of the material to deformation due to strain hardening. Strain hardening is the strengthening of the material due to deformation. This is important in deforming processes (such as forging and rolling) where the material becomes stronger during the process (for example between passes).

Using words and a sketch, discuss the effect of strain rate on the tensile strength of metals at various operating temperatures. (start by defining strain rate sensitivity and then discuss its effects)

Strain rate sensitivity (m) is similar to strain hardening coefficient (n) in that it is a material property. Strain rate sensitivity (m) describes the sensitivity (i.e. increase) of the material's strength to increasing the rate of deformation (i.e. strain rate). Note that as the temperature increases, the slope (figure 2.10) increases. Thus, tensile strength becomes more and more sensitive to strain rate as temperature increases.

What is the effect of strain rate sensitivity? Give an example of a practical application for this effect.

Strain rate sensitivity is a mechanical property that increases the strength of a material when its rate of deformation is increased. For example: In metal forming, especially at high temperatures, higher forces are needed to deform a material at higher speeds.

What is the difference between strength and toughness of engineering materials?

Strength (Yield strength) is the stress required for a material to begin plastic deformation. Toughness is the total amount of energy absorbed before failure. Found by the area under the entire stress-strain curve.

What is meant by "specific cutting energy"?

The Amount of energy needed to remove 1 Unit Volume of material per Unit Time

What is the significance of Modulus of Resilience in material processing?

The Modulus of Resilience is the amount of energy needed to start plastic deformation. It can be measured as the area under the linear part of the curve. In materials processing this is the energy that must be applied through heat or force to begin shaping a material plastically such as in rolling.

What is the difference between ultimate strength and toughness?

The ultimate strength is the maximum stress a material can withstand before beginning to fail while toughness is the total energy a material can withstand before fracture

What is the difference between Toughness and Modulus of Resilience for metals?

Toughness is the total energy a material can absorb before total failure (area under entire curve) Modulus of Resilience is the energy a material can absorb before beginning to deform plastically (area under linear part of curve)

Discontinuous chips form at "very low or very high speeds". Explain.

Very low speeds > Low shear energy (heat) > Low temp > Less Ductility. Very high speeds > Low Time for heat transfer > Less Ductility.

Using a neat sketch, explain in words the Baushinger effect on metals subjected to tension followed by compression.

Yield strength in compression is lower than yield strength in tension if tension is applied first.