Week 3: Failure Flashcards
Fatigue
CL - cool looks
S - say
YS - you’re sick
A type of failure due to cyclic loading at stresses below yield strength.
Creep
D - dingo
SS - sold sweet
T - things
T - today
Permanent plastic deformation under static stress at elevated temperatures for long periods of time.
S-N Curve
A representation of the stress to the number of cycles of a material.
From this, we can interpret the material’s endurance limit (m=0), fatigue life and fatigue strength.
Why does fatigue occur at stresses below the yield strength?
Because the material is being elastically deformed constantly and so it becomes weaker with time and the number of cycles on the material.
Eventually this leads to plastic deformation.
What are the 3 stages to fatigue failure?
- Crack formation
- Crack growth
- Crack propagation & fracture
How can we improve fatigue life?
TS - today some
VS - vigilantes stole
NC - nick’s cookies
- Increase the tensile strength (takes more stress to break)
- Change the variation in stress (lower the max and min values of stress on the fatigue cyclic stress curve)
- Lower the number of cycles
Fatigue Life
The number of cycles a material is expected to fail at a specified stress level.
Fatigue Strength
The stress where fracture occurs after a specified number of cycles.
Endurance/Fatigue Limit
If the level of stress is kept below the value of the fatigue limit, the material is guaranteed to not fail due to fatigue.
Miner’s Rule
Considers that fatigue loads are often not constant. Finds the damage on the material to consider when it will fail.
Found by number of cycles at a specified stress / number of cycles to fail at the same stress
(this makes use of the cyclic stress and S-N curves respectively and uses the same value for stress)
What happens when the value for Miner’s rule is greater than 1?
Then the material is expected to fail.
Values < 1 predict that failure won’t occur.
What are ways to avoid creep?
MT - might try
E - eating
GS - gummy snakes
- Materials with high melting temperatures
- High modulus of elasticity (E)
- Large grain sizes
What are some materials resilient to creep?
SS - somewhere some
RM - random man
S - sleeps
Stainless steels, refractory metals and super-alloys.
Fracture
Where a material is separated into 2 or more parts due to an applied stress
Tensile Strength
The maximum value of stress on the stress-strain curve
