Materials- Mechanical Properties

Question 1

Describe elastic deformation

Answer 1

When a load is applied to any material, the atomic bonds will stretch. When the load is removed, the atoms return to their original positions (before the load was applied) so the deformation is not permanent.

Question 2

Describe plastic deformation

Answer 2

When the load applied to a material is increased, eventually the atoms will slip past each other. When the load is removed, the atoms remain in their new positions and the deformation is permanent.

Question 3

What is engineering stress?

Answer 3

The instantaneous load applied perpendicular to the specimen cross section per unit cross-sectional area before the load was applied.

Question 4

What is engineering strain?

Answer 4

The instantaneous length of the specimen when a load is being applied minus the original length before any load is applied. All divided by the original length.

Question 5

Describe the tensile test

Answer 5

A rectangular “dogbone” sample of the material is deformed, usually to fracture, with a gradually increasing tensile load that is applied uniaxially along the long axis of a specimen. The specimen is mounted by its ends into the holding grips of the testing grips of the machine which elongates it at a constant rate. The bottom holding grip is fixed to the base.

Question 6

Why is a “dogbone” shaped specimen used in a tensile test?

Answer 6

So that during testing, deformation is confined to the narrow centre region (uniform CSA across its length) and also to reduce the likelihood of fracture at the ends of the specimen.

Question 7

What is Hooke’s law?

Answer 7

For small loads and deformations the stress applied to a metal (σ) is directly proportional to the strain it causes in the metal (ε). The constant of proportionality is the material’s Young’s modulus or modulus of elasticity (E).

Question 8

What is the Poisson’s ratio formula?

Answer 8

ν=-(εy)/(εx)
Minus sign is because εy and εx are in opposite directions (one is compression the other extension)
εy is transverse strain (perpendicular to applied force)
εx is longitudinal strain (parallel to applied force)
Most metals have value between 0.3 and 0.35
Only works for isotopic material

Question 9

What is an isotropic material?

Answer 9

One that has identical values of a property in all crystallographic directions so will deform in the same way for each direction of force. Polycrystalline materials can often be considered isotropic.

Question 10

Describe the engineering stress against strain curve for the tensile test

Answer 10

Straight diagonal line up from origin up to elastic limit. Then starts to curve and then curves back down slightly until the curve stops when the material has fractured at the failure strength. The top of the curve is the ultimate tensile strength (UTS). Offset yield strength just to right of elastic limit. Elastic region under straight line. Then yielding. Then after UTS there is necking.

Question 11

What is yielding?

Answer 11

The onset of plastic deformation

Question 12

What is the offset yield strength of a material?

Answer 12

The stress required to produce a very slight but specified amount of plastic strain (normally 0.2%).

Question 13

What is the ultimate tensile strength of a material?

Answer 13

The maximum engineering stress in tension that may be sustained without fracture

Question 14

How does the stress strain curve vary for materials of different strengths and stiffnesses?

Answer 14

A stronger material reaches a greater UTS. A stiffer material has a steeper gradient in the elastic region than a compliant material.

Question 15

What is true stress?

Answer 15

The force applied over instantaneous CSA

Question 16

What is true strain?

Answer 16

The change in length over instantaneous length. Equals ln(lf/l0) where lf is final length and l0 is original length. Doesn’t work for necking.

Question 17

What is shear stress (τ)?

Answer 17

F over A0
Where F is the load or force imposed parallel to the upper and lower faces (so parallel to the CSA) which each have an area of A0.

Question 18

What is shear strain (γ)?

Answer 18

The tangent of the strain angle
Tan(φ)
The strain angle is the angle from the original side face (perpendicular to force applied) to the new side face. For small angles γ=φ.

Question 19

What is the shear modulus of a material?

Answer 19

Shear stress (τ) over shear strain (γ)

Question 20

Describe a material’s bulk modulus (K) and how to calculate it

Answer 20

Relates the volume change of an object to an isostatic stress (equal pressure on all sides). Describes a material’s resistance to isostatic pressure.
Equal to -V x dP/dV
Can be used in the relation P = -K x ΔV/Vi
V is volume. Vi is initial volume. P is isostatic pressure.

Question 21

What does the strength of a material relate to?

Answer 21

It is a measure of its ability to withstand stress. Several characteristic strengths like UTS and yield strength.

Question 22

What is ductility?

Answer 22

A measure of the degree of plastic deformation that has been sustained by a material at fracture.

Question 23

What is the difference between a ductile material and a brittle material?

Answer 23

A ductile material can undergo a high degree of plastic deformation and they yield before fracture. A brittle material experiences very little or no plastic deformation upon fracture which is generally at the elastic limit.

Question 24

What does malleability relate to?

Answer 24

A material’s ability to deform under compressive stress.

Question 25

How can ductility be quantified?

Answer 25

The percentage elongation is change in length over initial length times 100.
The percentage reduction in area is initial CSA minus final CSA over initial CSA times 100.

Question 26

Describe plastic flow and what it needs for it to happen

Answer 26

Where a material undergoes a rearrangement of its internal molecular or microscopic structure when atoms are being moved to new equilibrium positions. It requires a mechanism for molecular mobility (in crystalline materials, this arises from dislocation motion).

Question 27

Why is knowledge of ductility important?

Answer 27

It indicates to a designer the degree to which a structure will deform plastically before fracture. It specifies the degree of allowable deformation during fabrication operations.

Question 28

What is hardness?

Answer 28

A measure of a material’s resistance to localised plastic deformation (eg a small dent or a scratch)

Question 29

Why are hardness tests performed more frequently than any other mechanical test?

Answer 29

They are simple and inexpensive (no special specimen needs to be prepared and apparatus relatively inexpensive). The test is non-destructive. Other mechanical properties may be estimated from hardness data like tensile strength.

Question 30

Describe the Vickers hardness test and how the results are used

Answer 30

A pyramidal diamond or hardened steel ball is pressed into the surface of the material (at a specific load) which leaves a tiny permanent indent (meaning plasticity has occurred). The two diagonal lengths are measured in mm and averaged to get d. A measure of the material’s resistance to this plasticity is given by:
Hv=1.854F/d^2 units kgf/mm^2
Hv means hardness using Vickers test
F is applied load in kgf. kgf is kilogram force = 9.81N
Generally used for softer materials

Question 31

Describe the Brinell hardness test and how the results are used

Answer 31

A hardened steel or tungsten carbide spherical indented is forced into the surface of the sample. It creates a circular impression in the sample. A measure of the material’s hardness is given by:
Hb=2F/πD(D-rt(D^2-d^)) units kgf/mm^2
Hb means hardness using Brinell test
D is diameter of indenter in mm
d is diameter of impression in mm
F is applied force in kgf
Used for harder materials

Question 32

How are Brinell hardness test results presented?

Answer 32

Hardness number calculated next to HBW (using tungsten carbide) or HBS (using hardened steel) next to ball diameter in mm. Then / force used in kgf / duration of load application in s.
E.g 360 HBW 10/3000/15

Question 33

What does toughness relate to?

Answer 33

The material’s resistance to fracture. Usually expressed as the energy a material can absorb before fracture. It is an indication of the amount of stress required to propagate a pre-existing flaw.

Question 34

What is fracture toughness?

Answer 34

A property indicative of a material’s resistance to fracture when a crack (or other stress-concentrating defect) is present.

Question 35

What does the stiffness of a material depend on in terms of its atoms?

Answer 35

The resistance to separation of adjacent atoms so the strength of the interatomic bonding forces

Question 36

Examples of flaws in materials

Answer 36

Cracks, voids, inclusions (other material in it), weld defects, or some combination of these.

Question 37

Ways of measuring toughness

Answer 37

Charpy impact test determines the amount of energy required to break a material. Area under stress-strain curve for the tensile test gives the material toughness.

Question 38

How does toughness depend on strength and ductility?

Answer 38

Toughness increases with both strength and ductility. However an increase in strength of a material often decreases ductility so a stronger material can be less tough than a weaker more ductile material.