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[FAQ]

Answer: The difference between EPPL,EQV (current equivalent plastic strain) and NL,EPEQ (accumulated equivalent plastic strain) can be explained with an example: For monotonic, proportional loading, the values of EPPL,EQV and NL,EPEQ will basically be the same. The main difference between the two will be observed for cases involving cyclic loading. Imagine a uniaxial specimen that undergoes cyclic loading such that it has plastic straining in tension and compression. This will basically traverse the cyclic stress-strain history where plastic strain will increase, then decrease to zero in reverse loading, then increase again, etc. This equivalent plastic strain is EPPL,EQV. However, if we look at it from the perspective of the damage of the material, the equivalent plastic strain decreasing gives the wrong impression. Hence, NL,EPEQ always increases, and it represents the accumulation of plastic strains during the entire load history (it is always increasing and positive). Hence, in the case of the uniaxial specimen, after compressing the rod to “recover” the plastic strain, EPPL,EQV would show 0 strain while NL,EPEQ would show 2*max_plastic_strain since NL,EPEQ is accumulated (NL,EPEQ is often used in damage calculations).