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need to understand the
mechanical properties of the body tissues
what is the force applied to deform a structure called?
stress
what is the resulting deformation of a stress fracture called?
a strain
tension is
pulling ends apart
compression is
pushing together
shear is
perpendicular
how materials change with
age
how materials react to different
force applications
how materials react to a
withdrawl of stress
stress curve is the
force applied perpendicular to a surface to deform a structure
the stress curve force is per
unit area
stress is measured in
N/m2 or pascals (Pa)
↑ force =
↑ stress
↑ area =
↓ stress
stress =
force ÷ area
a strain curve is a
deformation caused by applied stress
strain =
the change of the length ÷ the original length
L =
resting length
elastic =
plastic basically
elastic modules (k) =
slope of the stress/strain curve (stiffness of a material)
yield =
the point where the slope decreases
on average who has a thinner stress-strain curve?
females
what is the elastic region?
the period before the yield
what happens in the elastic region?
structure will return to original length without damage
elastic modules
change shape more easily
what is the plastic region?
the region after the yield point
what happens in the plastic region?
materials are permanently damaged
what is failure?
the force beyond plastic region, where stress falls to zero (whole thing falls apart)
what is failure strength?
the maximum stress before failure
what is the stress in failure strength?
a strain of failure strength
what is failure strain?
peak strain reached prior to failure
what is the safety factor?
the typical stress on structure ( 5-10x )
what is residual strain?
the difference between original length and length resulting from stress into the plastic region
plastic flow --->
the changed shape
residual strain --->
the difference between the original length and the new length
stored mechanical energy is
the area under the stress-strain curve
ME =
1/2 (stress*strain)
o' (with the little line on the top right) is
stress
cursive E is
strain
when applied force is removed
stored energy is released
stored mechanical energy example is
like a spring, rubber band, or even a trampoline
what are the five types of materials?
elastic
viscoelastic
brittle
stiff
compliant
in the elastic region there is a
linear relationship between stress and strain
what happens to the energy stored in the elastic region?
it is fully recovered
what is viscoelastic?
the non-linear (viscous) relationship between stress and strain
the elastic modules (slope) varies
depending on region
what is the loading rate?
how quickly you pull on it
magnitude of stress (in viscoelastic) is
dependent on loading rate
hysteresis is
the energy lost in elastic material (what we loose)
viscoelastic example is
a tendon or ligament
brittle material has the
highest elastic modulus (slope)
what material stores less energy than compliant?
brittle
stiff material has a
high elastic modulus (slope)
compliant material has a
low elastic modulus (slope)
what material stores more energy than stiff material?
compliant