I asked this a while ago, but decided to dig a little deeper before asking again. When using the Abaqus JH1/JH2/JHB Johnson-Holquist models for brittle materials, the FS (Failure strain) element removal threshold value is extremely important to ensure the simulation matches experiments, yet in practically all of the studies I have looked into, none of them have reported this FS value despite its major significance.
In the Abaqus documentation, FS was given as 0.2 for SiC. Subsequent studies on alumina have claimed that this FS value needed to be adjusted for accuracy, but then never reported it (e.g. Liu et al. 2015; Khan et al., 2021; Guo et al. 2020; Goda & Girardot 2021).
If anyone can shed some light about what this FS value is meant to be and how to obtain it experimentally or analytically, I would love to understand it better. Thank you!
Hi Zherui,
FS is a parameter that provides flexibility on correlating numerical with experimental results. I reproduced the formulation of plastic strain in the attached file, as one can see the FS value depends on the values of D_1 and D_2 and the negative hydrostatic pressure strength, T.
If one is confident with the remaining parameter values, then an acceptable strategy for deciding on FS value would be to minimise the error between experimental and numerical results based on multiple references. In the case of impact, the ballistic limit, projectile velocity-time profile and residual projectile velocity for instance.
Please consider [1] for a comprehensive analysis of the effects of FS.
For HDCs, FS=0.6 was previously used yielding satisfactory results for the ballistic limit and residual velocity and FS=1.5 for DoP.
In addition, FS=3.0 was used in [2] to validate Johnson and Holmquist [3] cases,
successfully reproducing the JH results.
Hope my answer helped.
Regards,
Theo
Theodosios Stergiou thanks for your comprehensive answer! I agree with you. Several papers have discussed this as a simulation parameter for element deletion to match experiments, but it appears that for Abaqus, setting FS = 1.5 - 3.0 (LS-DYNA's typical values) makes the ceramic behave like a ductile metal.
I think my added issue with this FS parameter is that it is so seldom reported for Abaqus simulations in existing studies. Since it is technically a simulation parameter, I personally feel like it needs to be reported for accurate reproduction. LS-DYNA studies report this FS value more frequently than Abaqus studies, it appears.
As mentioned in various studies the easiest way is to use the experimental values to fine tune the value of FS. Its a common practice not to mention a particular value as it varies with many parameters and its empirical in nature. It will confuse others if a particular value is mentioned while using Abaqus.
Although, recently an article presented a parametric study for FS.
Article FE coupled to SPH numerical model for the simulation of high...
I hope it will help you
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