Please review this discussion.nPlease review this discussion.nIt includes a video which shows that the negative slope in the test data is due to necking. The Engineering stress-strain curve, which uses the initial cross-sectional area, went negative because the instantaneous cross-sectional area was reduced in the necking region. The True stress-True strain curve might always have remained positive.nThere are material models that can accept a negative slope, but you may not need them. You can simulate necking using bilinear plasticity with a zero tangent modulus and plot force-displacement data that will produce a negative slope even though the stress-strain curve is flat.n