4: Tissue Mechanics I - Vardaxis Flashcards
injury occurs when an imposed ______ exceeds the ____ (load-carrying ability) of a tissue
- load; tolerance
structural v. material properties
structural: load-deformation relationships of like tissues (force stretch graph)
material: stress-strain relationships of different tissues (strain stress graph)
compressive v. tensile v shear stress
compressive = push closer together tensile = pull them further apart shear = slide one layer across another
symbol for stress
units for stress
sigma
Pascal Pa = 1 N/m squared
define stress
internal resistance to an external load
axial stress =
compressive or tensile
sigma = F/A
shear stress
parallel or tangential forces
theta = F/A
change in shape or deformation
strain (epsilon)
equation for strain
epsilon = change in dimension / original
dimension
- strain is a ration and has no actual units
what are the three types of strain?
- correspond to three types of stress
- push together molecules = compressive
- separation of molecules = tensile
- sliding of molecules layer on layer = shear
proportional change in dimension
strain
_______ is what is done to an object, ______ is how the object responds
stress; strain
stress and strain are proportional to each other
how do you measure the stiffness/compliance of a material?
stress-strain ratio
E = sigma (stress)/epsilon(strain)
high ratio = stiff; low ratio = compliant
Hooke’s law
- stress and strain are linearly related (elastic region)
- only true for biological tissues when the magnitude of the stretch is relatively small
ability of a material to resume its origianl size and shape upon removal of applied laods
extensibility and elasticity
- no known material elastic at all stresses
when a long bone bends, where is the compressive stress and where is the tensile stress?
compressive - inner (concave side)
tensile - outer (convex) side
max stresses near the edge of material, least stress near the neutral axis (median line of bone)
- whole of bent area is subject to shear stress
where is the failure with 3 point bending?
failure at middle (ski boot fracture)
where is the failure with 4 point bending?
at the weakest point between the two inside forces
twisting action applied to a structure
torsion
_____ radius of shaft, greater resistance to torsion
larger
why do spiral fractures occur with torsion?
- in addition to shear stress, normal stress (tensile and compressive) are produced in a helical path
absence of any plastic deformation prior to failure
brittleness
measure of energy absorbed by a material and returned whern load is removed
resilience
materials that quickly return to their original shape are called resilient
property of material enabling it to endure high-impact or shock loads; ability to absorb energy during plastic deformation; measure of capapcity of a material to sustain permanent deformatmion
toughness
“how much energy can be absorbed before failure/break”
what provides viscoelasticity to biological material?
fluid component
how does viscoelasticity affect strain and stiffness?
increase in strain rate produces and increase in the stiffness of the material
pure elastic v. viscoelastic tissue
elastic: all energy returned, no loss
viscoelastic: lose energy due to heat, energy is not returned immediately
hysteresis: area reperesenting energy lsot
loading and unloading path is different resulting in a loss of energy
hysteresis
slow, progressive deformation of a material under constant stress
creep
if load is released at a later time, the strain will exhibit recovery or progressive decrease of deformation
gradual decrease of stress when material is held at constant strain
stress relaxation
strain and stress: the longer it takes for a load to be applied …
more strain, less stress
strain and stress: the faster it takes for a load to be applied …
less strain, more stress
