Chapter 6 (lecture 5)

Compliance

- property of a material of undergoing elastic deformation when subjected to an applied force -reciprocal to stiffness - increasing stiffness results in a decrease to this - The greater the strain produced by a given amount of stress, the greater the __________ in the material. - The yield point of the load-deformation curve of a material is usually signified by a marked increase in __________ (decreased in stiffness).

Fatigue to failure Load to failure

2 ways a material fails

strain rate dependency

3words The mechanical characteristics of a viscoelastic material depending of the rate of strain. -higher (1)______ (2)____ = smaller degree to which creep and stress relaxation can occur during deformation = increased stiffness, strength and toughness of the material. - increased toughness of a viscoelastic material in response to an increased strain rate enables it to absorb more energy - ^______ ____s may allow the body to dissipate large amounts of energy as heat, however if it is dissipated in the form of failure, damage to the musculoskeletal system may occur - if a bone fails under low ______ ____ =clean break, little damage to surrounding tissues - if bone fails under high ______ ____ = bone may shatter into smaller pieces, causing greater damage to the bone and surrounding tissues

Work

A force does ____ when it moves its joint of application in the direction of the force Force * Distance - unit of ____ is in jouls (J) -when a muscle contracts isometrically, it expends energy but does no work -when a muscle contracts concentrically (shortens) it expends energy in doing positive work (work done BY the muscle) (pulls skeletal attachments closer together) -when a muscle contracts eccentrically (lengthens) it expends energy in doing negative work (work doneONthe muscle) As musculotendinous unit lengthens in an eccentric action, the musculotendinous unit absorbs energy in the form of strain energy

Elasticity

Ability for a material to return their original dimensions (when a load is removed), after that material has already been deformed in response to a load - restores its original size and shape IMMEDIATELY in response to unloading -- most materials ended to respond __________ in response to high rates of loading and viscoelastically in response to low-moderate rates of loading. - usually represented in mechanical models by a spring

Load to failure

Absolute force needed to apply to break an object - continuous application of force until the material breaks -example) continously adding weight to a bone until it breaks

Deform

All materials ______ to a certain extent - Greater load = greater ______ation

Toughness

Amount of energy absorbed by the material prior to failure - area under the load-deformation curve represents the strain energy absorbed by the material. - synonymous with strong - greater amount of energy absorbed, the greater _________ of the material - a material that is strong and ductile is _____

great

Bone changes over time and with disease. subtle changes in density can result in _____ changes in strength and elastic modulus - changes in tissue correspond to changes in how that tissue responds to stress

(1) Bone (2) Bone (3) ligament (4) Ligament

Bone vs ligament: (tendons and cartilage respond to loading in a similar manner to ligament (1) which is stiffer and stronger? (2) which tends to fail suddenly? (3) which exhibits progressive, albeit rapid, failure following the proportional limit? (4) which has a characteristic "toe" region in which a relatively small increase in stress results in a relatively large increase in strain during the first 1% of strain

heat light sound electricity Chemical energy (various forms of)Mechanical energy

Different forms of energy (6) total energy in the universe is constant; cannot be created or destroyed, can only be transformed from one form to another All interactions in nature are the result of transformation of energy from one form to another

Kinetic energy

Energy possessed by a body due to its speed of movement - an object of mass (m) and speed of movement (v) has - = m*v^2/2 - another form of mechanical energy

Stress relaxation

If a viscoelastic material is deformed (within elastic range) and then held in the deformed position, the stress experienced by the material decreases asymptotically with time until a point at which no further decrease in stress occurs - If the load is then removed, the material gradually restores is original dimension -If the load is tensile, the tension stress experienced by the material decreases with time until a point where no further decrease in tension stress occurs, returning to original dimension after load has been removed

Lengthen Shorten

In response to a tension load, a material tends to (1)_________ in the direction of the tension load. In response to a compressive load, a material tends to (2) _______ in the direction of the compression load

Relative quantities

Most tissues in the body are made up of the same materials, the differences in the material comes from the ________ __________ of the material. These ________ __________ of the materials determine how that tissue will respond to stresses.

Failure point

Point of strain at which a material completely fail - stress at this point is the ultimate stress of the material

(ANSWER)(1) Stiffness (2) Strain

Repeated microfracture and healing of cancellous bone may result in progressive increase in (1)__________, IN TURN increasing the (2)______ on the subchondral bone and articular cartilage. - This results in decreased shock-absorbing capacity of joint - Shock absorbing capacity of healthy joint is greater than that of degenerated joints

Fatigue to failure

Repeated, below threshold loading that results in failure due to fatigue. - reduces the amount of force required to reach failure -example)Repeatedly bending a paper clip

Young modulus (of elasticity)

The gradient of the stress-strain curve in the hookean region - indicates the amount of stress needed to produce 100% strain - most materials fail long before 100% strain but it provides a standard measure of stiffness for comparing different materials - larger the _____ _______, stiffer the material

Proportional limit

Upper limit of the hookean region - in some materials this point is = to yield point - in Most materials this point occurs before the yield point

Creep

When a viscoelastic material is subjected to a constant (tensile, compressive or shear) load (lower than yield stress) the material deforms asymptotically with time - it gradually deforms at a progressively decreasing rate until a point at which further deformation ceases - If the load is then removed, the material gradually restores its original dimension

Strain anisotropy

When the young modulus of a particular material is different in all three forms of loading (compressive, tensile, shear), that material exhibits ______ __________. - tissues that respond differently depending on the direction of stress -example) bone, stiffer during compression than Tension. Stiffer in tension than under shear

Strain isotropy

When the young modulus of a particular material is the same in all three forms of loading (compressive, tensile, shear), that material exhibits ______ ________.

Brittle

a material that fails after relatively little strain, the more _______ the material. -_______ materials have small plastic regions -opposite of ductile (osteoporotic bone is more brittle than normal bone)

Ductile

a material that only fails after a relatively large amount of strain, the more _______ the material is. -_______ materials have large plastic regions -opposite of brittle

Ultimate stress (strength)

amount of stress at the failure point

Compressive strain

decrease in length of the material as a proportion of the length of the material when it is not loaded

Strain

deformatioas a proportion of the dimensions of the material when unloaded - denoted by ε - no units, since it is the ratio of the two lengths -Chapter1) Deformation of an object that occurs in response to a load (tension strain in tendons)

Strain energy

energy can be dissipated as heat in the musculotendinous unit and subsequently in the rest of the body and the surrounding air. - faster the changeover, the smaller the proportion of ______ ______ dissipated as heat and, consequently, the greater the proportion available to contribute to the subsequent movement -example) landing: if landing is prefect, all strain energy is lost in heat, if landing is immediately followed by a rebound, some of the strain energy in the musculotendinous units may be recycled in the subsequent movement in the form of work

Mechanical energy

energy that can do work -example) strain energy

Tension strain

increase in length of the material as a proportion of the length of the material when it is not loaded

Hookean region

linear region of the stress-strain curve - All of part of the stress-strain curve within the elastic range is linear (the increase in strain is directly proportional to the increase in stress). - Any material that behaves this way is said to obey Hooke's Law - Steeper the slope = stiffer the object - Less steep slope = more flexible object

Stress

load per unit cross-sectional area - denoted by σ - measured in units of force/unit cross-sectional area N/cm^3 -Chapter1) Resistance of the intermolecular bonds of an object to the strain cause by a load

Viscosity

measure of the resistance of liquid or semi liquid substance to shear deformation in response to a shear load - greater the _________, the greater the resistance - shear stress is equal to the shear force/ the area (SS=F/A) -_________ = shear stress/shear stress rate

Yield point

point at which the material starts to tear or fracture

Viscoelastic (ity)

quality of a material that deforms GRADUALLY in response to loading and GRADUALLY restores its original size and shape when unloaded - most materials ended to respond elastically in response to high rates of loading and _________ in response to low-moderate rates of loading. -response of a _____________ material is always time dependent - usually represented in mechanical models by a hydraulic piston - rate at which a force is applied -the way material reacts to force will be different if the rate at which the force is applied is slow of fast -example) Bone: slow=flexible, fast=stiff

Fragility

refers to a lower level of toughness -fragile material absorbs relatively little strain energy prior to failure (osteoporotic bone is more fragile than normal bone)

Potential energy

stored energy, that, given appropriate conditions, ma be used to do work -example) strain energy stored energy in the muscle _________ ______ in the form of complex chemical substances (adenosine triphosphate and creatine phosphate)

Stiffness

the resistance of the solid material to deformation by a load - the gradient of the stress-strain curve (young modulus of elasticity) in the hookean region reflects this property of the material - The greater the stress required to produce a given amount of strain the greater the _________ of the material

Toughness modulus

the strain energy absorbed per unit volume of the material prior to failure - represented by the area under the stress-strain curve of the material - used for the purpose of comparing the toughness of different materials

Plastic range

the strain range between the yield point and the failure point - if the strain on the material is within this range, the material will deform physically (permanent damage to a degree corresponding to the amount of strain)

Elastic range

the strain range between zero strain and the strain at the yield point - if the strain on the material is within this range, the material will not be damaged and will remain perfectly elastic - any strain beyond the yield point results in the material being deformed physically (lose some elasticity and not return to original shape)