MECHANICAL PROPERTIES OF MATERIALS Flashcards
• isotropy:
A homogenous material that looks the same in every direction and exhibits material properties that are not orientation-dependent, e.g. a rubber ball
• anisotropy:
opposite of isotrophy
• tension:
two forces pull on the bones
• compression:
two forces push on the bones
• shear:
to forces on an object moving in opposite directions
• torsion:
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.
• bending:
2 upwards and one downward force
- bones are hollow to resist bending
• stress:
Force (N) / Area (m2) = Stress
units = N.m-2, pascals, Pa
• strain:
Extension (m) / Orig. length (m) = Strain
no units, or %
• modulus of elasticity:
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.
• stiffness:
N/m
• compliance:
how many mm a material will deform under a particular force
m/N
is the inverse of stiffness
• elasticity:
the ability of a solid to recover its shape when the deforming forces are removed.
• viscoelasticity:
materials for which the relationship between stress and strain depends on time.
• creep:
the continued deformation of a material over time as the material is subjected to a constant load.
relaxation:
the reduction of stress within a material over time as the material is sub- jected to a constant deformation.
• mechanotransduction
cells convert mechanical stimulus into electrochemical activity.
plays a role in bone processes such as physical adaptation, pathological fracture healing, and therapeutic distraction osteogenesis
Elastic limit/yield point
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.
yield point
limit of elastic behaviour
permanent deformation
ligament/muscle strain or bone fracture
elasticity - tendon
When crimp is ‘straightened’ tendons behave elastically until failure.
crimp
The crimp pattern =wavy appearance of collagen fibers in dense regular connective tissue
tendon failure
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
Some phenomena in viscoelastic materials are:
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
