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
bone
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
ligaments
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
cartilage
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
mechanotransduction phases
in the bone includes 4 phases:
• mechanocoupling
biochemical coupling
transmission of the signal from the sensor cell to the effector cell effector cell response
plastic region
The region in which the material deforms permanently is called the plastic region.
ultimate stress
The largest stress that the material with- stands before it breaks
Pa, pascals
elastic region
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.
