Fatigue Flashcards
1
Q
What is fatigue, and how do we work out stress range and amplitude?
A
- Fatigue is associated with 90% of matellic failures in serivce.
- In service, components often see fluctuating stresses.
- We have a stress range, sigma r, and we can work out the stress amplitude from this.
- Even though max stress isn’t near yield stress or fracture failure, fatigue may still cause failure eventually.
- A small defect initiates and grows until it reaches ‘a’ critical and sudden fast failure occurs.
2
Q
How is Fatigue characteristics on an S-N curve found?
A
- A smooth polished test pieces are cyclically loaded until fatigue failure occurs.
- Normally a series of tests are done at decreasing amplitudes S.
- The number of loading cycles vs stress amplitude is noted.
- Higher S, the smaller the number of cycles the material is able to sustain.
3
Q
What is the Fatigue Limit?
A
- Seen in iron and titanium alloys.
- Fatigue limit: the largest value of fluctuating stress that will not cause failure for essentially an infinite number of cycles. Extremely good and important.
- For steels, the fatigue limits can range between 35% and 60% of the tensile strength.
4
Q
What is Fatigue life and Fatigue strength? Usually found on an S-N curve.
A
- Fatigue life: the number of cycles to cause failure at a specific stress level
- Fatigue strength (endurance limit): stress level at which failure will occur for some specified number of cycles.
5
Q
What are the three steps in the fatigue process?
A
- Crack initiation: number of cycles for crack to initiate, Ni
- Crack propagation: number of cycles for crack to propergate, Np
- Final fracture
6
Q
What is the equation for working out fatigue life, Nf?
A
Nf = Ni + Np
7
Q
What are the two types of fatigue?
A
- High cycle: low stress and longer Ni
- Low cycle: high stress and Np becomes > than Ni
8
Q
What are some factors that influence fatigue life?
A
- Surface finish: rough surfaces have a multitude of stress concentration points, reduced Ni
- Design: any sharp corners or stress concentration within structures
- Surface treatments: case hardening (putting carbon on the surface of materials to harden up the surface), also shock pealing (putting plastic deformation, work hardening).
9
Q
What is the use of the stress intensitivity factor range, ΔK and its equations? Why do we need to consider damage tolerances in design?
Wha
A
- Designs should take into account the crack growth rate
- The stress intensity factor, delta K, is what drives the crack growth at the crack tip. It describes the stress range found at the crack tip.
- The equation is shown below.
10
Q
How is stress intensitivity factor range, ΔK, and crack growth related?
A
- delta K increases with time as the crack grows in tension.
- If the minimum stress is compressive, Kmin and sigma min are taken to be 0 as we are closing the crack essentially.
- We can see that in periods of tension, the stress increases the length of the crack (a). This increases K as a result. Thus stress intensity factor delta K is increasing.
- The rate delta K increases also depends on the min stress if it is above 0, then we always have tensile stress and the crack widens more and in length grows.
11
Q
How can we understand crack propagation rate? What equation and plot?
A
- If we plot the log of the rate of crack growth and delta K, against one another, we can understand crack propagation from the graph.
- Initially on the graph, we have region Ni, where we don’t have a flaw in the material
- Region 2 we start to get flaws in the material. Taking log of both the crack growth rate vs delta K, we get a linear portion which is described by the Paris Law (equaiton shown below).
- A and m are material constants and we can see the number of cycles before flaws grow enough to reach K1c where it will break.
- Region 3 is unstable crack growth
- We can use the Paris Law to determine the number of cycles for a crack to grow from one length to another length, good for checking safety of flaws and structural lifeinging.
