I'm modelling a fatigue disbond using abaqus. I've understood the disbond growth rate is equal to:

$$\dfrac{\partial a}{\partial N} = c_3 \Delta G^{c_4}$$

I've set up an analysis with MMB specimen. I've set $c_3 = 5.59\times 10^{-20}$ and $c_4 = 4.33$. The strain energy release rate is shown below:

energy release rate

Doing the math myself, I find:

da_dn_expected = c3 * energy_release ^ c4 = 5.59e-20 * 635 ^ 4.33 = 7.645e-8 [m/cycle]

Comparing that to the simulated results:

da_dn_found = element_width / cycles_to_next_node_release = 0.001 / 13713 = 7.29e-8 [m/cycle]

Which is a 5% difference with what I expected to find.

I've run a number of simulations with different mesh sizes as well as different settings and I've found the same difference amoung all of these results for all different crack lengths. I've added a figure below to illustrate:

Error in crack growth rate

Does anybody know what is causing this offset?

  • $\begingroup$ I think it might be that the error is very small and the fact that it's raised to the power of 4.33 that's increasing the error. But then I'm still confused that the error seems so... consistent $\endgroup$
    – Nathan
    Jul 26, 2020 at 15:49


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