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”
