Manufacturing Processes Chapter 3

If stress values were measured during a tensile test, which of the following would have the higher value: (a) engineering stress or (b) true stress?

(b).

The shear strength of a metal is usually (a) greater than or (b) less than its tensile strength?

(b).

What is the difference between engineering stress and true stress in a tensile test?

Engineering stress divides the load (force) on the test specimen by the original area; while true stress divides the load by the instantaneous area which decreases as the specimen stretches.

How is shear strength S related to tensile strength TS, on average?

S = 0.7 TS, on average.

Which of the following are the three basic types of static stresses to which a material can be subjected (three correct answers): (a) compression, (b) hardness, (c) reduction in area, (d) shear, (e) tensile, (f) true stress, and (f) yield?

(a), (d), and (e).

If strain measurements were made during a tensile test, which of the following would have the higher value: (a) engineering strain, or (b) true strain?

(a).

Most hardness tests involve pressing a hard object into the surface of a test specimen and measuring the indentation (or its effect) that results: (a) true or (b) false?

(a).

Which one of the following materials has the highest hardness: (a) alumina ceramic, (b) gray cast iron, (c) hardened tool steel, (d) high carbon steel, or (e) polystyrene?

(a).

Which one of the following types of stress strain relationship best describes the behavior of metals at temperatures above their respective recrystallization points: (a) elastic and perfectly plastic, (b) elastic and strain hardening, (c) perfectly elastic, or (d) none of the above?

(a).

Which one of the following materials has the highest modulus of elasticity: (a) aluminum, (b) diamond, (c) steel, (d) titanium, or (e) tungsten?

(b).

Which one of the following types of stress strain relationship best describes the behavior of most metals at room temperature: (a) elastic and perfectly plastic, (b) elastic and strain hardening, (c) perfectly elastic, or (d) none of the above?

(b).

The plastic region of the stress-strain curve for a metal is characterized by a proportional relationship between stress and strain: (a) true or (b) false?

(b). It is the elastic region that is characterized by a proportional relationship between stress and strain. The plastic region is characterized by a power function - the flow curve.

Viscosity can be defined as the ease with which a fluid flows: (a) true or (b) false?

(b). Viscosity is the resistance to flow.

Which one of the following is the correct definition of ultimate tensile strength, as derived from the results of a tensile test on a metal specimen: (a) the stress encountered when the stress-strain curve transforms from elastic to plastic behavior, (b) the maximum load divided by the final area of the specimen, (c) the maximum load divided by the original area of the specimen, or (d) the stress observed when the specimen finally fails?

(c).

Which one of the following types of stress strain relationship best describes the behavior of brittle materials such as ceramics and thermosetting plastics: (a) elastic and perfectly plastic, (b) elastic and strain hardening, (c) perfectly elastic, or (d) none of the above?

(c).

What is the defining characteristic of a Newtonian fluid?

A Newtonian fluid is one for which viscosity is a constant property at a given temperature. Most liquids (water, oils) are Newtonian fluids.

Tensile testing is not appropriate for hard brittle materials such as ceramics. What is the test commonly used to determine the strength properties of such materials?

A three-point bending test is commonly used to test the strength of brittle materials. The test provides a measure called the transverse rupture strength for these materials.

What is the complicating factor that occurs in a compression test?

Barreling of the test specimen due to friction at the interfaces with the testing machine platens.

Why cannot a direct conversion be made between the ductility measures of elongation and reduction in area using the assumption of constant volume?

Because of necking that occurs in the test specimen.

Why are different hardness tests and scales required?

Different hardness tests and scales are required because different materials possess widely differing hardnesses. A test whose measuring range is suited to very hard materials is not sensitive for testing very soft materials.

How is the shear modulus of elasticity G related to the tensile modulus of elasticity E, on average?

G = 0.4 E, on average.

What is hardness, and how is it generally tested?

Hardness is defined as the resistance to indentation of a material. It is tested by pressing a hard object (sphere, diamond point) into the test material and measuring the size (depth, area) of the indentation.

State Hooke's law.

Hooke's Law defines the stress-strain relationship for an elastic material: σ = Eε, where E = a constant of proportionality called the modulus of elasticity.

How does the change in cross-sectional area of a test specimen in a compression test differ from its counterpart in a tensile test specimen?

In a compression test, the specimen cross-sectional area increases as the test progresses; while in a tensile test, the cross-sectional area decreases.

Define the recrystallization temperature for a metal.

The recrystallization temperature is the temperature at which a metal recrystallizes (forms new grains) rather than work hardens when deformed.

Define tensile strength of a material.

The tensile strength is the maximum load experienced during the tensile test divided by the original area.

Define yield strength of a material.

The yield strength is the stress at which the material begins to plastically deform. It is usually measured as the 0.2% offset value, which is the point where the stress-strain curve for the material intersects a line that is parallel to the straight-line portion of the curve but offset from it by 0.2%.

What is the dilemma between design and manufacturing in terms of mechanical properties?

To achieve design function and quality, the material must be strong; for ease of manufacturing, the material should not be strong, in general.

What is viscoelasticity, as a material property?

Viscoelasticity refers to the property most commonly exhibited by polymers that defines the strain of the material as a function of stress and temperature over time. It is a combination of viscosity and elasticity.

Define viscosity of a fluid.

Viscosity is the resistance to flow of a fluid material; the thicker the fluid, the greater the viscosity.

In what case does the strength coefficient have the same value as the yield strength?

When the material is perfectly plastic and does not strain harden.

What is work hardening?

Work hardening, also called strain hardening, is the increase in strength that occurs in metals when they are strained.

What are the three types of static stresses to which materials are subjected?

tensile, compressive, and shear.