Mechanics of materials

Question 1

define stress

Answer 1

intensity of the force
- how much of the force is distributed over a given area

Question 2

what is axial / normal stress

Answer 2

component of force perpendicular to the area of interest
- can be compressive (-) or tensile (+)

Question 3

what is shear stress

Answer 3

component of force parallel to the area of interest

Question 4

during stress on an inclined plane, how far apart are the max and min normal stresses

Answer 4

always 90deg apart

Question 5

during stress on an inclined plane, where will max shear stress occur

Answer 5

45deg between max and min normal stress

Question 6

what are the principal stresses

Answer 6

max and min normal stresses

Question 7

define strain

Answer 7

change in length divided by the original length
- can be compressive or tensile

Question 8

what is the difference between the axial and shear components of strain

Answer 8

axial - want to change the size of the object
shear - want to change the shape of the object

Question 9

what is poissons ratio

Answer 9

ratio of transverse to axial strain
- material loaded in one direction will undergo strains both parallel and perpendicular to the direction of load

Question 10

what is hooke’s law

Answer 10

linear relationship between stress and strain

Question 11

what does the elastic / youngs modulus depend on

Answer 11

bonding types between atoms and molecules

Question 12

what is yield strength

Answer 12

stress at which the material will no longer go back to its original shape when unloaded
- deviates from linear because the material is being damaged (bonds being broken)

Question 13

what is ultimate strength

Answer 13

max stress a material can withstand before failure

Question 14

what occurs at the point of failure

Answer 14

material fractures or ruptures (unable to withstand or carry any load)
- can be lower than the ultimate strength in a tougher material

Question 15

what is the difference between toughness and strength

Answer 15

stronger material = can experience more stress
tougher material = can experience more deformation and still carry load after yield prior to failure

Question 16

what is toughness

Answer 16

area under the stress strain curve
- total energy a material can absorb prior to failure
- broken into elastic and plastic components

Question 17

what are material properties

Answer 17

calculated from stress strain curve
independent from structure (don’t depend on size and shape)

Question 18

what are structural properties

Answer 18

calculated from area under force displacement curve

Question 19

are material properties specific to loading mode

Answer 19

yes - often don’t behave the same in compression, tension, and shear

Question 20

what are the three different types of materials

Answer 20

isotropic
tranversely isotropic (anisotropic)
orthotropic (anisotropic)

Question 21

what is an isotropic material

Answer 21

doesn’t exist in biomechanics
material properties are independent of loading direction
(always behaves the same)

Question 22

what is a tranversely isotropic material

Answer 22

one direction has material properties different from the other
two directions behave similarily
ex = cortical bone (due to longitudinal osteons) and tendons (direction of fibres)

Question 23

what is an orthotropic material

Answer 23

material properties are different about every axis
behaves differently in every loading direction
ex = trabecular bone and cartilage

Question 24

what is elastic deformation

Answer 24

when the stress applied is < or = to the yield strength
- deformation will be completely recovered when the load is removed

Question 25

what is plastic deformation

Answer 25

when stress applied is beyond the yield strength
- deformation is permanent