Material Properties

Question 1

What is the definition of load, stress and strain?

Answer 1

Load: force that acts on a body
Stress: intensity of an internal force
Strain: deformity of an object due to force

Question 2

What is the difference btw elastic and plastic deformation?

Answer 2

Elastic deformation returns to its original shape, plastic does not
Elastic deformation is the straight slope portion of stress/strain curve, while plastic deformation begins at the curved portion (yield point)

Question 3

What is yield strength?

Answer 3

Amount of stress necessary to produce a specific amount of permanent deformation

Question 4

What is the difference btw elastic and plastic zones?

Answer 4

Elastic zone is portion of stress/strain curve where object will return to its original shape, plastic zone occurs thereafter and object will be permanently deformed

Hooke’s law: when object is in elastic zone the stress is proportional to the strain

Question 5

What is the Young’s modulus of elasticity?

Answer 5

Measure of the ability of an object to resist deformity (stiffness) in the elastic zone (slope of the stress/strain curve)
Increasing slope indicates increased stiffness

Question 6

What is the difference btw Ultimate (tensile) strength and breaking point?

Answer 6

Ultimate strength is the load at which a material fails; breaking point is the load at which a material will actually break

Question 7

Define a ductile and brittle material?

Answer 7

Brittle: undergos little to no plastic deformation; linear stress/strain until its break point (PMMA, ceramic)
Ductile: will undergo a great deal of plastic deformation prior to breaking (metal)

Question 8

What is a material that exhibits a stress-strain relationship that is dependent on the load and the rate by which the load is applied?

Answer 8

Viscoelastic material (ligaments, bone)

Question 9

Define fatigue failure and endurance limit?

Answer 9

Fatigue failure: failure at a load less than the ultimate tensile strength due to repetitive loading
Endurance limit: maximal stress under which an object is immune to fatigue failure regardless of the number of cycles

Question 10

What is defined as progressive deformation of metal in response to a constant force over an extended period of time?

Answer 10

creep

Question 11

What are three types of corrosion?

Answer 11

Fretting: due to micromotion at contact site btw two materials (head neck junction in THA)
Galvanic: electrochemical breakdown d/t mixture of dissimilar metals (CoCr and stainless)
Crevice: fatigue cracks from differences in oxygen tension (stainless steel most prone)

Question 12

Which component of bone has good tensile strength?

Answer 12

Hydroxyapetite; poor tensile strength

Collagen has good tensile strength and poor compressive strength

Question 13

Bone is biomechanically weakest to resistance of which force?

Answer 13

shear; strongest in compression

Question 14

How does type of force dictate fracture characteristics?

Answer 14

Tensile: transverse
Compressive: comminuted
Bending: butterfly fragment
Torsion: spiral

Question 15

Which material has a Young’s modulus btw cortical and cancellous bone?

Answer 15

PMMA; strongest in compression, weak in shear

Question 16

What portions of PMMA are the stabilizer and initiator?

Answer 16

benzyl perozide (initiator)
hydroquinone (stabilizer)

Question 17

What component of PMMA is radio-opaque?

Answer 17

barium sulfate

Question 18

What does gamma radiation do to polyethylene?

Answer 18

Highly cross links which improves wear characteristics, but decreases mechanical properties

Question 19

Change in mechanical properties as a result of the direction of a load defines what material property?

Answer 19

Anisotropy; isotropy defines a material thats mechanical properties are independent of direction of load (golf ball)