Creep and Fatigue Flashcards
(13 cards)
What causes fatigue?
1) Cyclic stress
2) Tensile stress
3) Plastic stress/deformation
What 3 stages does fatigue happen in?
1) Crack initiation
2) Crack propagation
3) Sudden fracture at remaining cross-section
How do you know a material failed due to fatigue?
Material fails due to cyclic loading at stresses below UTS or the yield point of the material
What are some examples of cyclic stresses?
1) Completely reversed stresses
2) Pulsating stress with only tension
3) Intermittent/irregular tensile stress
What is fatigue strength?
Cyclic stress level that a material fails at
What is endurance limit?
A limiting stress below which a load can be applied an infinite amount of times without causing failure.
How many cycles determine the endurance limit?
10 000 000
Where does fatigue failure usually originate?
Highly stressed regions of the surface. Since the crack progressively propagates characteristics of beach marks can be seen.
Factors that change the surface will have a large effect on fatigue strength. What are some ways the surface of a material can change?
1) Stress concentrations
2) Scratches or surface roughness decreases fatigue strength
3) Metallurgical inclusions that interrupt surface continuity
4) Residual stress patterns improves fatigue strength
5) Corrosive environment caused by the atmosphere
What is creep?
A phenomenon that occurs when materials under stresses exhibit continuous plastic deformation.
What are the 3 stages of creep?
1) Primary creep: rapid strain that decreases over time
2) Secondary creep: a steady state between work hardening and recovery where creep rate changes very little
3) Tertiary creep: exhibits creep at an accelerating rate causing fracture
What are the 2 types of creep tests?
1) Creep rate test: performed to determine creep in the secondary stage
2) Creep rupture test: performed to measure the time to failure
A typical design criteria permits stress levels that will cause a creep rate of ___________ in 10 000 or 100 000 hours.
1%
