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.
What factors affect fatigue life? (Think of the K-values from design)
Magnitude of stress; quality of surface finish; thermal fatigue (creep); corrosion.
Explain creep:
Creep is time-dependant and permanent. It is the deformation of a material when subjected to a constant load at a high temperature > 0.4 of its absolute melting point.
What are the stages of creep?
- Instantaneous deformation, mainly elastic; 2. Primary/ transient creep: strain slows as time increases (a form of work-hardening); 3. Secondary/steady-state creep: Rate of strain v time is steady; 4. Tertiary: Rapidly accelerating strain rate until failure (formation of cracks).
Which creep stage is the most important parameter for long-life applications:
Secondary/steady-state
How is creep minimised?
High melting point and high elastic modulus
Examples of creep resistant materials:
Stainless steel; and cobalt and nickel based alloy
