Tissue Mechanics I

Question 1

injury occurs when an imposed ______ exceeds the ____ (load-carrying ability) of a tissue

Answer 1

• load; tolerance

Question 2

structural v. material properties

Answer 2

structural: load-deformation relationships of like tissues (force stretch graph)
material: stress-strain relationships of different tissues (strain stress graph)

Question 3

compressive v. tensile v shear stress

Answer 3

compressive = push closer together
tensile = pull them further apart
shear = slide one layer across another

Question 4

symbol for stress

units for stress

Answer 4

sigma

Pascal Pa = 1 N/m squared

Question 5

define stress

Answer 5

internal resistance to an external load

Question 6

axial stress =

Answer 6

compressive or tensile sigma = F/A

Question 7

shear stress

Answer 7

parallel or tangential forces theta = F/A

Question 8

change in shape or deformation

Answer 8

strain (epsilon)

Question 9

equation for strain

Answer 9

epsilon = change in dimension / original dimension

- strain is a ratio and has no actual units

Question 10

what are the three types of strain?

Answer 10

• correspond to three types of stress
• push together molecules = compressive
• separation of molecules = tensile
• sliding of molecules layer on layer = shear

Question 11

proportional change in dimension

Answer 11

strain

Question 12

_______ is what is done to an object, ______ is how the object responds

Answer 12

stress; strain

stress and strain are proportional to each other

Question 13

how do you measure the stiffness/compliance of a material?

Answer 13

stress-strain ratio
E = sigma (stress)/epsilon(strain)
high ratio = stiff; low ratio = compliant

Question 14

Hooke’s law

Answer 14

• stress and strain are linearly related (elastic region)

- only true for biological tissues when the magnitude of the stretch is relatively small

Question 15

ability of a material to resume its original size and shape upon removal of applied loads

Answer 15

extensibility and elasticity

- no known material elastic at all stresses

Question 16

when a long bone bends, where is the compressive stress and where is the tensile stress?

Answer 16

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

Question 17

where is the failure with 3 point bending?

Answer 17

failure at middle (ski boot fracture)

Question 18

where is the failure with 4 point bending?

Answer 18

at the weakest point between the two inside forces

Question 19

twisting action applied to a structure

Answer 19

torsion

Question 20

_____ radius of shaft, greater resistance to torsion

Answer 20

larger

Question 21

why do spiral fractures occur with torsion?

Answer 21

• in addition to shear stress, normal stress (tensile and compressive) are produced in a helical path

Question 22

absence of any plastic deformation prior to failure

Answer 22

brittleness

Question 23

measure of energy absorbed by a material and returned when load is removed

Answer 23

resilience

materials that quickly return to their original shape are called resilient

Question 24

property of material enabling it to endure high-impact or shock loads; ability to absorb energy during plastic deformation; measure of capacity of a material to sustain permanent deformation

Answer 24

toughness

“how much energy can be absorbed before failure/break”

Question 25

what provides viscoelasticity to biological material?

Answer 25

fluid component

Question 26

how does viscoelasticity affect strain and stiffness?

Answer 26

increase in strain rate produces and increase in the stiffness of the material

Question 27

pure elastic v. viscoelastic tissue

Answer 27

elastic: all energy returned, no loss
viscoelastic: lose energy due to heat, energy is not returned immediately
hysteresis: area representing energy lost

Question 28

loading and unloading path is different resulting in a loss of energy

Answer 28

hysteresis

Question 29

slow, progressive deformation of a material under constant stress

Answer 29

creep

if load is released at a later time, the strain will exhibit recovery or progressive decrease of deformation

Question 30

gradual decrease of stress when material is held at constant strain

Answer 30

stress relaxation

Question 31

strain and stress: the longer it takes for a load to be applied …

Answer 31

more strain, less stress

Question 32

strain and stress: the faster it takes for a load to be applied …

Answer 32

less strain, more stress