Quiz 4

Question 1

simple fracture

Answer 1

the separation of a body into two or more pieces in response to a static stress

Question 2

ductile failure

Answer 2

-slow crack propagation
-significant plastic deformation
-fails with warning
-one piece
-large deformation

Question 3

brittle fracture

Answer 3

-rapid crack propagation
-little to no plastic deformation
-fails without warning
-clean cut, flat surfaces
-many pieces, small deformations

Question 4

cup and cone fracture

Answer 4

moderately ductile
-stages
necking –> void nucleation –> void growth and coalescence –> crack propagation –> failure

Question 4

brittle fracture surface

Answer 4

1. intergranular crack propagation (between grains)
2. transgranular crack propagation (faster and goes straight through grains)

Question 5

fracture occurs as a result of…

Answer 5

crack propagation

Question 6

stress concentration is higher for…

Answer 6

sharp cracks
-propagate at lower stresses than cracks with blunt tips

Question 7

Criterion for crack propagation

Answer 7

crack propagation and failure occurs when σ_m > σ_c for lowest σ_c
-largest, most highly stressed cracks grow first

Question 8

fracture toughness

Answer 8

K_c, measure of a material’s resistance to brittle fracture when a crack is present
-high K_ic values for ductile materials and low for brittle

Question 9

test conditions promoting brittle fracture of a ductile material

Answer 9

-high strain rate
-deformation at low temperatures
-presence of a notch

Question 10

some BCC metals exhibit…

Answer 10

Ductile-to-Brittle Transition Temperature (DBTT)
-metals with DBTT should only be used at temperatures when ductile

Question 11

fatigue

Answer 11

failure under lengthy period of repeated stress/strain cycling
-can cause part failure even though applied stress σ_max < σ_y
-responsible for ~90% of mechanical engineering failures

Question 12

fatigue life

Answer 12

N_f
-total number of cycles to cause fatigue failure at specified stress amplitude

Question 13

techniques to improve fatigue life

Answer 13

1. Reduce magnitude of mean stress
2. Surface treatments (shot peening, carburizing)
3. Design changes (avoid sharp corners)