Fatigue FEA
Fatigue FEA post-processes stress results to estimate damage from repeated loading. For a time-varying load, linear FEA unit-load cases can be scaled and superposed to produce a stress history at each location. The history is reduced to stress amplitudes and mean stresses; variable-amplitude loading normally requires cycle counting, such as rainflow counting. High-cycle fatigue with mainly elastic strain is evaluated with stress-life (S-N) data, while low-cycle fatigue with significant plastic strain is evaluated with strain-life data. Nonzero mean tensile stress may be corrected with a relation such as Goodman’s equation. At notches, the FEA stress interpretation must match the material curve: a modeled notch radius gives a local stress, whereas a sharp corner may produce a mesh-dependent singularity unless a hot-spot stress definition is used. Damage from different load blocks is commonly accumulated with Miner’s rule.
Knowledge Check
1.Why are unit-load FEA cases often used when constructing a fatigue stress history?
2.At a node, the maximum stress is 400 MPa and the minimum stress is 100 MPa. What is the stress amplitude?
3.For the same cycle, with maximum stress 400 MPa and minimum stress 100 MPa, what is the mean stress?
4.A notch has an elastic stress concentration factor Kₜ = 1.5, and the nominal alternating stress is 80 MPa. What local alternating stress should be used in a simple elastic notch calculation?
5.A component experiences 2,000 cycles in a block where N = 10,000 cycles, followed by 1,000 cycles where N = 5,000 cycles. What Miner damage is accumulated?
6.A fatigue evaluation gives σₐ = 150 MPa and tensile mean stress σₘ = 100 MPa. Using Goodman’s correction with Sᵤ = 600 MPa, what is the equivalent zero-mean alternating stress?
7.What is the appropriate FEA treatment of stress at a mathematically sharp reentrant corner?
8.Which fatigue method is most appropriate when a component experiences significant plastic strain each cycle?
9.Why is rainflow counting commonly used for a variable-amplitude FEA stress history?
10.An S-N curve was generated using polished specimens. What should be done when applying it to a machined component?