Mechanical properties of materials

Question 1

Engineering Stress

Answer 1

The stress on an object is define as the ratio between the force (F) and the original cross-sectional area (A0). The stress is typically expressed in units of MPa

Question 2

What are the four principal ways a force may be applied?

Answer 2

There are four principal ways in which a load may be applied, namely: tension, compression, torsional and shear.

Question 3

Engineering Strain

Answer 3

Strain (ε) is the ratio of elongation to original length, where l0 is the initial length, li is the instantaneous length and ∆l is the change in length.

Question 4

What are the two main types of strain?

Answer 4

Elastic and Plastic deformation.

Question 5

Young’s Modulus

Answer 5

The Young’s modulus (E) is a measure of stiffness of an elastic material and is defined as the ratio between the tensile stress (σ) and the strain (ε).

Question 6

Poisson’s Ratio

Answer 6

The Poisson’s ratio (ν) is the negative ratio of the transverse to axial strain.

Question 7

What is the theoretical Poisson’s Ratio for an isotropic material.

Answer 7

Poisson’s ratio for isotropic materials should be 0.25

Question 8

Shear Stress

Answer 8

The shear stress on an object is define as the ratio between the shear force (F) and the original cross-sectional area (A0)

Question 9

Shear Modulus

Answer 9

The shear modulus (G) is defined as the ratio of shear stress (τ) to shear strain (γ). For isotropic materials this can be related to the Young’s modulus and Poisson’s ratio.

Question 10

What is Tensile Strength?

Answer 10

The tensile strength TS (MPa) is the stress at the maximum on the engineering stress–strain curve

Question 11

True Stress

Answer 11

True stress σT is defined as the load F divided by the instantaneous cross-sectional area Al over which deformation is occurring (i.e., the neck, past the tensile point)

Question 12

What are the two most common types of hardness test.

Answer 12

There are different types of indentation test of which the Brinell (BH) and Vickers(VH) hardness tests are the most common.

Question 13

What is the difference between the Brinell (BH) and Vickers (VH) hardness test.

Answer 13

Brinell test uses a spherical indenter, Vickers uses a diamond shaped indenter

Question 14

What are materials with a negative poisons ratio called?

Answer 14

Auxetics, a type of mechanical metamaterial

Question 15

Under what conditions can you relate the shear modulus to the Young’s modulus according to the equation E = 2G(1 + ν)

Answer 15

When a material is isotropic