Learning Forum

[Duncan Gibbons]

Hi all,

I am trying to run a fatigue crack growth analysis. I am wanting to plot the crack extension against the number of cycles and determine after how many cycles the part fractures (when residual strength is below a set value). I am getting an error "For Cgrow Id 1 and Crack Id 1, the computed number of cycles is less than 1. Under this loading condition, static crack growth is expected. Please check your model, boundary conditions and fatigue parameters.  The crack growth solve is stopped. " when running a fatigue SMART crack growth analysis. When I run a static crack growth analysis on the same part, same loads & constraints, same crack dimensions etc. the results indicate that the crack wont grow and the SIF is well below the critical fracture toughness (by a magnitude of 10). Additionally, the maximum stresses on the part for the same load case are 174 MPa, well below the yield strength of the alloy of 850 MPa.

Does this mean that I have incorrectly input the Paris law constants or is there something else I am missing? I am using reference units of m, N. My Paris law constants are C = 9e-11 and m = 2.8 for a titanium alloy.

[Daniel Shaw]

The message indicates that for current cylic loading and the current model parameters (crack size, SIFs, Paris Law constants, etc.) the crack will not grow.  If you want to propagate the crack, you need to increase the SIFs (probably by increasing the stress).