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[FAQ]

The failure model that is simple and easy to use is plastic strain failure model. Material will fail when the effective plastic strain reaches its maximum. The other failure model is Johnson-Cook failure model. In Explicit Material library in Engineering Data, we have the material property data of both Johnson-Cook strength and Johnson-Cook failure models for the typical materials such as Copper (CU-OFHC), Steel (Steel 4340), and Iron (Iron-ARMCO). The reference paper of the Johnson-Cook failure model can be found from the material reference section: “Fracture Characteristics of Three Metals subjected to various strains, strain rates, temperatures and pressures” – Johnson GR, Cook WH, J Eng Mech Vol 21, 1985 Basically, authors have done torsion and Hopkins bar impact tests to find these material property values. The Johnson-Cook failure model contains 3 terms with 5 constants. The first term is the effect of the ratio of pressure/von Mises stress, which can be associated with the stress triaxiality. The second term is the effect of the strain rate. The third term is thermal softening. If you are not concerned with thermal softening, you can ignore the 3rd term and only need to find 4 constants. If you are only concerned with the strain rate dependence, you only need to define 1 constant D4 by curve-fitting the failure strain with the non-dimensional strain rate. If you are also concerned with the increase of material failure with increasing ratio of pressure/stress, you need to curve-fit the experimental data to get the constants D1, D2, and D3. D1, D2, and D3 may not necessarily have the same sign. One or two of them could be negative numbers because the increase of the pressure/stress ratio decreases the material failure strain.