Biomechanics Test 5

Front 1

MECHANICAL STRESS

Back 1

- internal force/ cross sectional area
- stress is an internal force holding the object in place

Front 2

TENSION

Back 2

- pull force
- the state of an object that results from forces pulling on it
- example of a tensile stress

Front 3

TENSILE STRESS

Back 3

- the axial or normal stress that occurs at the analysis plane as a result of a force or load that tends to pull apart the molecules that bond the object together at the plane.
- When the bone's axis is transverse and the pone is being pulled apart the stress acts away from the analysis plane making it tensile stress.
- very large tensile loads may sprain or rupture ligaments and tendons, or tear muscles and cartilage, or break bones.

Front 4

COMPRESSION

Back 4

- the push force
- an axial stress that results when a load pushes molecules more tightly together.
- deforms by shortening in the direction of the external forces

Front 5

SHEAR STRESS

Back 5

- a transverse stress that acts parallel to the analysis plane as a result of forces acting parallel to the plane
= to the force at the analysis plane divided by the cross sectional area of the object
- represented by symbol tau
- scissors
- In body cause blisters or joint dislocations
- changes the orientation of the sides of the object

Front 6

BENDING LOAD

Back 6

- produces more than one stress at the analysis plane.
- When a pencil is bent by pushing away with your thumbs and pulling it towards you with your fingers it will break. The top will break first because the pencil can withstand compression more than it can withstand the tensile force. This is because the fingers applying the tensile force are further from the axis and create more torque.
- An object with greater depth (more cross sectional area farther from its neutral axis) is able to withstand greater bendinglads because it has a larger moment arm.
- torque is produced by one force and has to be counteracted by another force. T = F x r
- curving occurs
- toes of the foot will break before the ankle does. This is because the depth of the ankle is greater than the toes which creates a greater moment arm. The torque is able to overcome the pressure better.

Front 7

CURVING

Back 7

- An object subjected to a bending load will deform by curving. The tension side of the object will elongate, whereas the compression side will compact and shorten, causing the object to bend.

Front 8

STRAIN

Back 8

- quantification of the deformation of material
- result of an external force causing a deformation which overcomes the stress force.
- On x-axis of stress strain curve because it is the dependent variable

Front 9

LINEAR STRAIN

Back 9

- results in change of objects length
= change in length/ original length

Front 10

SHEAR STRAIN

Back 10

- occurs with a change in orientation of adjacent molecules as a result of these molecules slipping past each other
- Poisson's Ratio

Front 11

POISSON'S RATIO

Back 11

- a specific ratio of strain in the axial direction to strain in the transverse direction exists for each different type of material.
- When a rubber band is stretched its length is longer but there is also an effect on its width. It becomes narrower because the molecules are slipping past
- Intervertebral discs are loaded with compression which pushes the discs down and they bulge out. Under extreme pressure they will burst.

Front 12

ELASTIC MODULUS

Back 12

- Young Modulus
= change in stress/change in strain
- The stiffer the object the larger the elastic modulus

Front 13

YIELD POINT

Back 13

- elastic limit
- coincides with the proportional limit which is the end of the linear elastic range of the curve
- Below the point the material is elastic, but above the point it is plastic

Front 14

STIFF OBJECT

Back 14

- plastic
- The steeper the slope in the stress-strain curve the greater the stiffness
- when an object is elongated the tensile stress increases

Front 15

PLIANT OBJECT

Back 15

- elastic
- The less steep the slope in the stress-strain curve the greater the elasticity.

Front 16

YIELD STRENGTH IN ELASTIC MATERIALS

Back 16

- stress at the elastic limit of a materials stress strain curve
- the object fights to maintain its own shape so when the strain begins to be stronger than the stress is overcome and deformation occurs
- Past this point the material fails to regain its strength

Front 17

ULTIMATE STRENGTH IN ELASTIC MATERIALS

Back 17

- maximum stress a material can withstand
- measure of material's strength. Past this point and the stress decreases

Front 18

FAILURE STRAIN IN ELASTIC MATERIALS

Back 18

- strain exhibited by a material when breakage occurs
- materials with large failure strains are ductile materials (can be changed into new forms)

Front 19

BRITTLE

Back 19

- material with a moderate stress-strain curve

Front 20

TOUGHNESS

Back 20

- ability of a material to absorb energy before failing
- tissues are soft and ductile
- bones are stiff and ductile

Front 21

BONES

Back 21

- the quicker load is applied the stronger the bone
- strongest in compression and weakest in shear

Front 22

TENDONS

Back 22

- stronger when force is aligned axially (parallel to fibers)
- fibers run parallel
- they are less elastic (more plastic) than ligaments which makes them hard to break.
- A serious injury when tendons break

Front 23

LIGAMENTS

Back 23

- Fibers criss cross and mesh together
- More pliable (elastic) than tendon.
- Easy to damage but damage isn't very serious

Front 24

MUSCLE

Back 24

- stiffness varies as a function of the number of active contractile elements.
- produces tension within itself and the structures to which it attaches
- failure strain is much larger due to the ability of the contractile filaments to slide past each other as the muscle increases in length.