Chapter 3: Failure Flashcards
Mechanical failure
Unwanted loss of load carrying capacity of a material caused by fatigue, creep, impact or corrosion
Physical failure causes (3)
Improper material choice/processing, inadequate design, misuse
Grain boundaries
Narrow region with a higher energy between atoms that doesn’t fit perfectly together
Stress concentration
Magnified stresses around discontinuities. Ductile materials fail in a brittle manner when present.
Stress concentration formula
σmax = σ + 2σsqrt(a/pt)
Stress intensity factor
How fast stress multiplies at a crack
Stress intensity factor formula
Kt = σmax/σ = 1+2sqrt(a/pt)
Stress concentrations in ceramics formula
σc = sqrt((2Eγ) / (pi*a))
Fracture toughness/critical stress intensity formula
Kc = γσsqrt(pi*a)
Charpy testing
Hitting a notched specimen with a hammer at different temperatures to determine it’s energy to fracture and tendency to become brittle.
Temperature’s relation to type of fracture
Ductile fracture happens at higher temperatures, brittle fracture happens at low temperatures
Fatigue
Cracking of materials due to cyclic stress.
Mean stress formula
σm = (σmax + σmin)/2
Stress amplitude formula
σa = (σmax - σmin)/2
Stress-failure curve relationship
Plot of mean stress to number of cycles to failure. As σm increases, the curve shifts down and left.
BCC fatigue limit
Defined
FCC fatigue limit
Undefined
Failure surfaces, beachmarks, striations
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.
Ways to decrease fatigue (3)
Lowering the mean stress, altering the surface via compression or polishing, reducing exposure to corrosive environments
Paris Law
Rate of growth of a crack
Paris law formula
da/dN = C(ΔK)^m
Creep
Failure of metal below the yield strength due to slow continuous deformation. Needs time, temperature, stress.
Creep Temperature
0.33 < T/Tmelt
Creep test
Constant load used to mesure strain
Primary creep
Work hardening occurs, decreasing slope and strain rate
Secondary creep
Work hardening cancelled out by recovery, constant slope and strain
Secondary creep formulae
dε/dt = Ae^(-Q/RT) = Bσ^7
Tertiary creep
Failure occurs, slope and strain increase
Relationship between creep temperature and stress
As temperature increases, stress increases
