Properties of materials

Question 1

What sort of stress is usually applied when stress-strain graphs are plotted?

Answer 1

Tensile

Question 2

What is anisotropy?

Answer 2

When the mechanical properties of a material differ depending on the direction of application of force

Question 3

Examples of anisotropic and isotropic materials?

Answer 3

Isotropic - Metals, PMMA (cement), Ultra-high molecular weight polyethylene (UHMWPE)(but only initially)

Anisotropic - Bone, tendon, all biological materials

Question 4

Difference in stress/strain vs load/deformation

Answer 4

Stress/strain is applied to a standardised piece of material

Load/deformation applies to an actual object and is object specific

Question 5

What is yield stress?

Answer 5

The stress at which behaviour changes from elastic to plastic

Question 6

What is ultimate stress?

Answer 6

The stress at which a material ruptures

Question 7

Difference between strength and toughness?

Answer 7

Strength is the load per unit area before failure

Toughness is the energy absorbed before rupture

Question 8

Brittle vs ductile?

Answer 8

Brittle materials don’t deform before failure. Their pieces can be fitted together after failure

Ductile materials undergo deformation before failure

Question 9

What is fatigue toughness?

Answer 9

The work done to failure after repeated loading

Question 10

Some new ceramics are being created e.g. BIOLOX delta which are combined by transformational toughening. How does this help?

Answer 10

Combination of smaller and larger microstructures increases the toughness and so material is less prone to fracture

Question 11

What is visco-elasticity?

Examples of some viscoelastic materials?

Answer 11

It describes the behaviour of materials whose mechanical properties are time or rate dependent.

E.g. Polyethylene, Articular cartilage, Bone, Biological materials in general

Question 12

Visco-elastic properties include hysteresis, creep, and stress relaxation. What is hysteresis?

Answer 12

The phenomenon in which the value of a physical property lags behind changes in the effect causing it

Question 13

In a hysteresis graph, what does the area between the loading and unloading curves represent?

Answer 13

Energy - usually in the form of heat

Question 14

What is creep?

Answer 14

Deformation that occurs under a constant load

It doesn’t reverse once the load is removed

Question 15

What is stress relaxation?

Answer 15

Stress required to maintain a constant deformation - it decreases with time

Question 16

Brittle vs fatigue vs creep fractures?

Answer 16

Brittle - single load that exceeds ultimate stress
Fatigue - repeated loads above an endurance limit
Creep - constant load below ultimate stress but above a threshold

Question 17

What is the plot of a shear stress vs shear rate graph of viscosity of Newtonian fluids?

Answer 17

It is linear and passes through the origin

Question 18

There are time-dependent and non-time-dependent viscosity Newtonian fluids. What time dependent viscosity types are there?

Answer 18

Rheopectic - viscosity increases with stress over time (constant shear) e.g. some lubricants

Thixotropic - viscosity decreases with stress over time (constant shear) e.g. landslide mud, synovial fluid

Question 19

What non-time-dependent fluid types are there?

Answer 19

Shear thickening - viscosity increases with increased stress e.g. corn starch + water

Shear thinning - viscosity decreases with increased stress e.g. joint fluid, paint, ketchup

NB: some shear thinning materials’ behaviour is also time-dependent - if it is not then the term “pseudoplastic” is used

Question 20

What is Bingham Plastics?

Answer 20

The minimal critical stress required for strain to occur

Question 21

What is the biphasic nature of articular cartilage?

Answer 21

Solid phase - stress in solid matrix
Fluid phase - hydrodynamic pressure
Permeability decreases non-linearly under compression
Initial load is taken by the fluid phase
Load is gradually transferred to the solid phase