Fracture and Failure Flashcards
What are the 2 steps in a fracture?
Crack Formation and crack propagation
Ductile and brittle describe either end of a spectrum, but what is the spectrum a measure of?
The ability for a material to undergo plastic deformation before a fracture.
Facts of ductile fracture
Most metals; extensive plastic deformation; the crack is stable (it stills until further force is applied.)
Facts of brittle fracture
Ceramics, ice, cold metals; little plastic deformation; the crack is unstable (will propagate further at same force.)
In most applications which is preferred?
Ductile
What shape would expect from either end of a very ductile fracture? Which materials would this include?
Tapered to a point; soft metals such as gold, glass at high temperature (See lecture 7 slide number 5 for a picture)
What shape would you expect from a brittle fracture? Which material is this failure typical of?
Flat ends with no tapering; cold metals and ceramics (See lecture 7 slide number 5 for a picture)
What shape would you expect from a moderately ductile fracture? Which material is this failure typical of?
Tapered near the fracture but a perpendicular rough tear; typical for ductile metals (See lecture 7 slide number 5 for a picture)
What are the steps in a ductile fracture failure?
Necking - Void Nucleation - Void Coalescence - Crack Propogation - Separation
What are the steps in a brittle fracture failure?
Crack propagates quickly; nearly perpendicular to direction of stress; with no appreciable plastic deformation.
What are the two types of brittle fracture?
Transgranular and Intergranular, respectively, fracture passes through grains and frack propagates along boundaries.
What causes stress-concentration?
A reduction in cross-sectional area due to strain causes increasing stress.
What is a stress raiser?
A stress-concentration. Commonly a crack or pore.
The max stress at a stress raiser is:
2 x sigma_0 x (a/p_t)^1/2 a is the half-length of the crack, and p_t is the radius at the crack tip. (Think this is only for cylinders???)
Alloys have higher or lower ductile-to-brittle transition temperatures than their pure metal alternatives?
Higher
What is the result of fatigue?
Failure can occur at loads considerably lower than the yield strength of the material under a static load.
What causes fatigue?
Fluctuating/cyclic stresses
What are the stages of a fatigue failure?
Crack initiation, incremental crack propagation, final catastrophic failure
What are the three types of cyclic stress?
Reversed stress cycle: periodic and symmetrical about zero stress. Repeated stress cycle: periodic and asymmetrical about zero stress. Random stress cycle: random fluctuation.
What is the fatigue limit?
The maximum stress below which the material will not fail, irrelevant of the number of stress cycles it experiences.
Fatigue strength:
Increase load at which fracture occurs after a specific number of cycles.
Fatigue life:
Number of cycles to fail at a specified stress level.
Crack propagation:
Stage 1: Initial slow propagation along crystal planes involving just a few grains. Stage 2: Faster propagation perpendicular to the applied stress. Crack eventually reaches critical dimension and propagates very rapidly.
Explain N_f = N_i + N_p
N_f = N_i + N_p The total number of cycles until failure are counted as the number of cycles as the crack initiates and the number of cycles the crack propagates.
