MECHANICAL PROPERTIES OF MATERIALS

Question 1

• isotropy:

Answer 1

A homogenous material that looks the same in every direction and exhibits material properties that are not orientation-dependent, e.g. a rubber ball

Question 2

• anisotropy:

Answer 2

opposite of isotrophy

Question 3

• tension:

Answer 3

two forces pull on the bones

Question 4

• compression:

Answer 4

two forces push on the bones

Question 5

• shear:

Answer 5

to forces on an object moving in opposite directions

Question 6

• torsion:

Answer 6

torque, or twisting, is applied to the beam instead of a bending moment
Torsion generates shear stresses in a beam, and the equation to calculate the shear stress is directly analo- gous to the bending equation.

Question 7

• bending:

Answer 7

2 upwards and one downward force

- bones are hollow to resist bending

Question 8

• stress:

Answer 8

Force (N) / Area (m2) = Stress

units = N.m-2, pascals, Pa

Question 9

• strain:

Answer 9

Extension (m) / Orig. length (m) = Strain

no units, or %

Question 10

• modulus of elasticity:

Answer 10

The slope of the straight line in a stress–strain diagram
Indicates either (a) how much a material stretches or strains when it is subjected to a certain stress or (b) how much stress builds up in a material when it is stretched or strained by a certain amount.

Question 11

• stiffness:

Answer 11

N/m

Question 12

• compliance:

Answer 12

how many mm a material will deform under a particular force
m/N
is the inverse of stiffness

Question 13

• elasticity:

Answer 13

the ability of a solid to recover its shape when the deforming forces are removed.

Question 14

• viscoelasticity:

Answer 14

materials for which the relationship between stress and strain depends on time.

Question 15

• creep:

Answer 15

the continued deformation of a material over time as the material is subjected to a constant load.

Question 16

relaxation:

Answer 16

the reduction of stress within a material over time as the material is sub- jected to a constant deformation.

Question 17

• mechanotransduction

Answer 17

cells convert mechanical stimulus into electrochemical activity.
plays a role in bone processes such as physical adaptation, pathological fracture healing, and therapeutic distraction osteogenesis

Question 18

Elastic limit/yield point

Answer 18

The deformation of an elastic material obeys Hooke’s law, which states that deformation is proportional to the applied stress up to a certain point. This point is called the elastic limit. Beyond this point additional stresses will cause permanent deformation.

Question 19

yield point

Answer 19

limit of elastic behaviour

Question 20

permanent deformation

Answer 20

ligament/muscle strain or bone fracture

Question 21

elasticity - tendon

Answer 21

When crimp is ‘straightened’ tendons behave elastically until failure.

Question 22

crimp

Answer 22

The crimp pattern =wavy appearance of collagen fibers in dense regular connective tissue

Question 23

tendon failure

Answer 23

c.10% strain; c.100MPa stress
In life: 2% strain and <15MPa stress is usual
hysteresis for tendon is about 5% so every time a tendon is stretched, 5% of energy lost as friction and internal heat

Question 24

Some phenomena in viscoelastic materials are:

Answer 24

if the stress is held constant, the strain increases with time (creep)
• if strain is held constant, stress decreases with time (relaxation)
• the effective stiffness depends on rate of application of the load - The higher the loading rate (of bone) the stiffer, stronger and more energy stored
• if cyclic loading is applied, hysteresis (a phase lag) occurs, leading to a
dissipation of mechanical energy
• rebound of an object following an impact is less than 100%

Tendon creep

Question 25

bone

Answer 25

bone is better at resisting longitudinal loading cf. other loading directions (bone behaviours differently in longitudinal axis than transverse)
- the behaviour of bone changes based on microstructure

Question 26

ligaments

Answer 26

Ligaments comprise predominantly of collagen, so mechanical properties are often similar to those of tendons. However, there are three major differences
• Ligaments are bone-bone, tendons are muscle-bone (Structural difference)
• Ligaments contain elastin (low modulus, great extensibility, low strength)
• Ligaments have graded failure, tendons tend to snap in a single event
ligaments are shorter and loaded differently compared to tendons

Question 27

cartilage

Answer 27

Cartilage comprises collagen fibrils held in a pressurised fluid-gel matrix. Mechanical behaviour is very complex, but articular cartilage is very resistant to compressive loading and together with synovial fluid has an extremely low coefficient of friction, allowing for smooth movement and little wear and tear (until osteoarthritis).
* if damage cartilage, it becomes an ongoing problem

Question 28

mechanotransduction phases

Answer 28

in the bone includes 4 phases:
• mechanocoupling
biochemical coupling
transmission of the signal from the sensor cell to the effector cell effector cell response

Question 29

plastic region

Answer 29

The region in which the material deforms permanently is called the plastic region.

Question 30

ultimate stress

Answer 30

The largest stress that the material with- stands before it breaks
Pa, pascals

Question 31

elastic region

Answer 31

The region of the stress-strain curve in which the material returns to the undeformed state when applied forces are removed is called the elastic region.