Load and Material Properties

load

force or forces applied to a structure

who can apply loads to the tissues of the body

gravity and other external forces → tissues respond to the loading or lack of loading that may modify structure, composition, and function o the tissues

deformation

changes in length, shape, etc when a load is applied onto a tissue → shortens and bulges

load-deformation curve: toe-region

initial amount of load needed to take up the slack of the tissue

load-deformation curve: elastic region

able to return to original state following formation → the more you stretch, the longer it gets temporarily

load-deformation curve: yield point

• end of elastic region

• will not return to original state immediately after load is removed

• may return to original state with time

load deformation curve: plastic region

loading great enough that permanent deformation occurs after the load is removed —> microfailure, could be useful for lengthening tissues

load-deformation curve: ultimate failure point

• when loading in plastic region continues creating overt failure

• rupture: within structure of connective tissue

• avulsion: at tendon/ligament attachment to bone

• fracture: within bony tissue

load-deformation curve general

• used to determine strength and stiffness of whole structure of various sizes, shape, and material composition

• examines elasticity, plasticity, ultimate strength and stiffness of a material

• amount of energy storage before failure

stress

• measure of a load in an object

• expressed in terms of force per unit

• calculated → can’t be measured directly

strain

• percent change in length or cross section of material

• calculated → can’t be measured directly

stress-strain curve

• amount of deformation of a sample vs the load per unit area of the sample

• examines material properties of substances including tissues

strength of a material

• load sustained before failure

• deformation sustained before failure

• energy it can store before failure

• increased load = increased strength

stiffness

• resistance offered to external loads by a specimen or structure as it deforms

• indicated by slope of load-deformation curve in elastic region

• increase load to deform a stiffer tissue

modulus of elasticity (Young’s modulus)

• value obtained by dividing the stress at any point in the elastic region of the stress-strain curve by the strain at that point

• helps to examine stiffness of a materal

• higher stiffness, lower compliance and vice versa

Young’s modulus with bone, metals cement

metal > bone > cement

type of stress and strain that develop in human tissues depend on:

• the material

• type of load applied

• the point at which the load is applied

• the direction and magnitude of the load

• the rate of loading

• duration of loading

what can stress-strain curves be used to compare

• strength properties of materials

• the same tissue under different conditions (ligaments before and after immobilization)

viscosity

• material’s resistance to flow

• high viscosity → high resistance to deformation

• low viscosity → low resistance to deformation

• viscosity is diminished as loads are slowly applied and increased as loads are rapidly applied

viscoelastic materials

• connective tissues

• elasticity + viscosity = properties of both

• time, rate, and history dependent behavior

creep

constant low loading over extended period that produces a slow deformation of soft tissues

• takes longer to occur with rapid loading

• ex) stretching a shortened tissue → therapist applies a constant force and the tissue gradually elongates

stress-relaxation

• ability of tissues to need gradually less stress to maintain the same elongation over time

• you can stretch a ligament with a certain amount of force and then slowly decrease the force but the ligament remains deformed

• applies to muscle stretching, joint mobilization, static progressive splint, etc

hysteresis

• loss of energy demonstrated by a viscoelastic material

• difference between energy expended when loaded and energy regained when unloaded

• seen by path of load-deformation curve

  • no change in path = no hysteresis

  • change in path = hysteresis

strain-rate sensitivity

• most tissues behave differently if loaded rapidly or slowly

• when the load is applied rapidly, the tissue is stiffer and a larger peak fore can be applied to the tissue than if the load was applied slowly → subsequent stress-relaxation will be larger than if the load was applied slowly

isotropic

properties do not vary regardless of where force is applied → ex) glass

anisotropic

behavior exhibited by a structure whose strength and elasticity vary when loaded in different directions → ex) most of human tissues like bones