Chapter 3: Failure

Question 1

Mechanical failure

Answer 1

Unwanted loss of load carrying capacity of a material caused by fatigue, creep, impact or corrosion

Question 2

Physical failure causes (3)

Answer 2

Improper material choice/processing, inadequate design, misuse

Question 3

Grain boundaries

Answer 3

Narrow region with a higher energy between atoms that doesn’t fit perfectly together

Question 4

Stress concentration

Answer 4

Magnified stresses around discontinuities. Ductile materials fail in a brittle manner when present.

Question 5

Stress concentration formula

Answer 5

σmax = σ + 2σsqrt(a/pt)

Question 6

Stress intensity factor

Answer 6

How fast stress multiplies at a crack

Question 7

Stress intensity factor formula

Answer 7

Kt = σmax/σ = 1+2sqrt(a/pt)

Question 8

Stress concentrations in ceramics formula

Answer 8

σc = sqrt((2Eγ) / (pi*a))

Question 9

Fracture toughness/critical stress intensity formula

Answer 9

Kc = γσsqrt(pi*a)

Question 10

Charpy testing

Answer 10

Hitting a notched specimen with a hammer at different temperatures to determine it’s energy to fracture and tendency to become brittle.

Question 11

Temperature’s relation to type of fracture

Answer 11

Ductile fracture happens at higher temperatures, brittle fracture happens at low temperatures

Question 12

Fatigue

Answer 12

Cracking of materials due to cyclic stress.

Question 13

Mean stress formula

Answer 13

σm = (σmax + σmin)/2

Question 14

Stress amplitude formula

Answer 14

σa = (σmax - σmin)/2

Question 15

Stress-failure curve relationship

Answer 15

Plot of mean stress to number of cycles to failure. As σm increases, the curve shifts down and left.

Question 16

BCC fatigue limit

Answer 16

Defined

Question 17

FCC fatigue limit

Answer 17

Undefined

Question 18

Failure surfaces, beachmarks, striations

Answer 18

Visible markings where crack propagation stopped long enough for oxidation to occur. Initiated via stress concentrations and progress by stress reversals that cause the crack tip to move forward.

Question 19

Ways to decrease fatigue (3)

Answer 19

Lowering the mean stress, altering the surface via compression or polishing, reducing exposure to corrosive environments

Question 20

Paris Law

Answer 20

Rate of growth of a crack

Question 21

Paris law formula

Answer 21

da/dN = C(ΔK)^m

Question 22

Creep

Answer 22

Failure of metal below the yield strength due to slow continuous deformation. Needs time, temperature, stress.

Question 23

Creep Temperature

Answer 23

0.33 < T/Tmelt

Question 24

Creep test

Answer 24

Constant load used to mesure strain

Question 25

Primary creep

Answer 25

Work hardening occurs, decreasing slope and strain rate

Question 26

Secondary creep

Answer 26

Work hardening cancelled out by recovery, constant slope and strain

Question 27

Secondary creep formulae

Answer 27

dε/dt = Ae^(-Q/RT) = Bσ^7

Question 28

Tertiary creep

Answer 28

Failure occurs, slope and strain increase

Question 29

Relationship between creep temperature and stress

Answer 29

As temperature increases, stress increases