Mechanical Properties of Materials

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; whereas true stress divides the load by the instantaneous area which decreases as the specimen stretches.

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 that might be considered analogous to necking in a tensile test?

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

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.

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

S = 0.7 TS, on average.

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

tensile, compressive, and shear