LECTURE #20 + 21 - mechanics of biological materials Flashcards
external (mechanical) loads (forces) deform internal structures in a addition to producing movement … in what parts of the body ?
- ligament
- tendon
- cartilage
- bone
- muscle
what are the two types of mechanical load types ?
uniaxial and combined
what are some examples of uniaxial mechanical load types ?
compression, tension and shear
what are some examples of combines mechanical load types ?
bending and torsion
FILL IN THE BLANK
mechanical loads create mechanical _____
stress
what is stress ?
force (internal) per unit area over which the force acts
what are the units of stress ?
N/m^2 or pascal
what are the three principal types of mechanical stress ?
tension, compression and shear
what is tension ?
pulling or stretching force directly axially through a body
what is compression ?
pressing or squeezing force directly axially through a body
what is shear ?
force directed parallel to a surface (causes a sliding of surfaces in opposite directions)
what are combined loads ?
more common during function activities (multi axial)
what are some causes of combined loads ?
- multiple stresses at analysis plane
- different magnitudes of stress at analysis plane
what are the two types of combines loads ?
bending and torsion
what is torsion ?
fancy word for twisting
what is bending ?
loads applied to structure causing it to bend about a neutral axis
what is three point bending ?
- compression and tension developed on opposite sides of axis
- shear developed along axis
what is cantilever bending ?
is the bending or flexing of bones when subjected to off-center forces, similar to how a diving board bends under weight
what’s torsion ?
load producing twisting of a body around its longitudinal axis
where does shear stress occur in torsion ?
parallel to axis (greatest magnitude)
where does compression and tension occur in torsion ?
occur diagonal to axis
if you have an increase in distance from axis, what does that mean for magnitude ?
increase in magnitude
what is strain ?
defamation of material with external forces
TRUE OR FALSE
excessive deformation (strain) causes injury ?
TRUE
whats happening to our tendons during strain ?
there stretched and lengthen more than it should / can
whats the difference between stress and strain ?
- mechanical stresses causes strain
- strain is the actual response
what is linear strain due to ?
compressive or tensile loads
what is a load - deformation curve ?
is a graph that shows how a material deforms under the influence of an applied load. It typically illustrates the relationship between the amount of force applied (load) and the resulting deformation or displacement of the material
what are some factors of strength (resistance to deformation/failure) in regards to load - deformation curve :
- yield strength
- ultimate strength
- failure strength
- total deformation before failure point
- energy stored before failure point
what is a limitation for the load - deformation curve ?
can’t compare materials of different size and shape
what is a fix for the load - deformation curve ?
stress - strain curve
what are some functions of bone biomechanics ?
- protect vital organs
- support soft tissue
- produce RBC’s
reservoir for minerals - provide attachment site for skeletal muscle
- act as a system of machines to receive muscle torques and make moment possible
what are some factors influencing mechanical properties of bone biomechanics ?
- structure
- geometry
- mode of loading
- rate of loading
- frequency of loading
- muscle activity
- age
what are the 4 major structural components of bone
- collagen
- mineral
- ground substance
- water
define collagen’s structure in bone :
- 25-30% dry weight
- tensile strength
- provides some flexibility : however, relatively little
define mineral’s structure in bone :
- 60-70% dry weight
- calcium and phosphate
- compressive strength
define ground substance’s structure in bone :
- 5% dry weight
- gel like substance surrounding collagen fibers
- compressive strength
define water’s structure in bone :
- 20-25%
define the geometry of bone biomechanics :
- cross sectional area is proportional to ultimate failure point
- increased area = increased ultimate strength
- distribution of bone tissue around its neutral axis
what does “polar moment of inertia” mean ?
ability to resist twisting (torsion)
what does it mean for polar moment of inertia if mass distribution away from neutral axis ?
increased polar moment of inertia
what does it mean for polar moment of inertia if mass distributed close to neutral axis ?
decreased polar moment of inertia
mode of loading : what does anisotropic mean ?
stiffness & strebt depend upon mode of loading
when is the failure point highest during ?
compression (193 MPa)
when is the failure point second highest during ?
tension (133 MPa)
when is the failure point lowest during ?
shear (68 MPa)
what is the order of strangest to weakest in regards to load of loading for bone biomechanics ?
compression > tension > shear (aka shear break more common then a bone breaking due to compression)
what is bending ?
loads applied to structure causing it to vend about a neutral axis
where do compression and tension develop on a bone ?
on opposite sides of axis
where does shear develop on a bone ?
developed along the axis
what are the two major types of bending we talk about ?
three point bending and cantilever bending
what are the factors influencing mechanical properties of bone biomechanics ?
- structure
- geometry
- mode of loading
- rate of loading
- frequency of loading
- muscle activity
- age
what does viscoelastic mean ?
stiffness and strength depend upon see of applied load
when do we see a high loading rate ?
- increased stiffness
- increased ultimate failure point
- increased energy storage prior to failure
TRUE OR FALSE
loading rate influences fracture patters and sift tissue damage
TRUE
if we see decrease loading rate what does that mean ?
cracking (no fragments), little or no soft tissue damage
if we see increased loading rate what does that mean ?
comminuted (fragments displaced), extensive soft tissue damage
what term is used to describe the following : materials may behave in both elastic and viscous manners with different rates of loading
viscoelasticity
for rate-sensitive loading, higher loading rate = increased or decreased stiffness ?
increased stiffness
for rate-sensitive loading, lower loading rate = increased or decreased stiffness ?
decreased stiffness
what is bone fatigue ?
wearing of bone during repeated landing
what are some factors that may lead to bone fatigue ?
- magnitude of load
- number of load applications
- number of load applications in a given time
for fatigue fractures (stress factors), high load = how many repetitions ?
high load = few repetitions
for fatigue fractures (stress factors), low load = how many repetitions ?
many repetitions
TRUE OR FALSE
muscles may produce tensile or compressive stresses on bone
TRUE
TRUE OR FALSE
we see progressive decrease in bone density and strength with age
TRUE
what factors do we see in bones as we age ?
- decrease bone density
- decrease bone mass
- decrease bone size
- decrease stiffness
- decreased strength
what does bone remodelling mean ?
altering size, shape and structure based on the imposed mechanical demands
what is wolff’s law ?
bone tissue is gained or lost depending upon the level of stress sustained
if we see an increase in mechanical stress, do we see an increase or decrease in bone tissue production ?
increase in bone tissue production
if we see a decrease in mechanical stress, do we see an increase or decrease in bone tissue production ?
dec erase in bone tissue production
