Shell thinning may be important in applications where shell stretching is prominent -- this is NOT the case in the majority of impact simulations while it IS important in metal forming applications.In crash analysis, we recommend be left as zero (no change in thickness) for several reasons.
All associated data will be adjusted automatically.
In other words, tensorial shear strain is half the engineering shear strain.
You can get a time history of maximum in-plane shear strain by plotting histories of eps1 and eps2 on the same plot and then using the "Oper" button to subtract the eps2 curve from the eps1 curve.
A final note -- it is not proper to extrapolate principal strains from integration points to the surface; extrapolation is only appropriate for the six components of the strain tensor.
You can plot the six components of strain in either the global coordinate system (default for d3plot data) or the element local coordinate system (default for elout data; also available from d3plot data by selecting "Local" in the postprocessor).