Advanced Material Exchange does not allow stress-strain data to be entered for a material where stiffening is present. In other words, the slope of the stress-strain curve must not increase.
Consider the example below where stiffening is present.
This example shows material stiffening at the start of the stress-strain curve. The secant stiffness of the material increases after the first non-zero data point. To resolve this issue you would need to remove or edit the bad data point. This problem can arise when using stress-strain data directly from a mechanical testing device.