I am attempting to simulate the cool down from cure of a bistable unsymmetric composite laminate. Due to the unsymmetric layup, a mismatch in thermal expansion through the thickness causes residual thermal stresses to develop in the laminate leading to curved deformation of the laminate and bistability.

It is a square laminate of 200 mm edge length with a layup of [0/90]T. I have set up two models; the first modelling the laminate as a surface that is meshed with S4R elements (global element size of 2 mm) and giving the surface a conventional shell composite layup as shown in the first attached image (total laminate thickness = 0.37 mm). The third ply of the laminate is a resin rich layer that forms on the side of the laminate that is not in contact with the tool plate during vacuum bag processing of the laminate in real life, and aids convergence to one of the two stable states of the laminate in FE simulations. The laminate is clamped at its centre node and a temperature change of -158 K is applied to the whole laminate to simulate cool down from a cure temperature of 180 C to room temperature 22 C. The second model models the laminate as a solid volume (thickness = 0.37 mm) and uses a solid composite layup as shown in the second attached image. The volume is meshed with C3D20R solid elements and has the same mesh density as the shell model, i.e. 2 mm global element size. The same boundary conditions and temperature change are applied as in the shell model.

The out-of-plane (U3) displacements are in relatively good agreement, as shown in the third attached image. The issue I have encountered is with the stresses reported in both models being quite different. The first image of the stresses shows the S11 stresses for the bottom and top surfaces of the laminate, with the left side stress contours being for the shell model, and right side stress contours being for the solid model. The second image shows the S22 stresses.

Although the form of the stress contours are similar, there are obvious disparities between the values of stresses reported between the two models, especially for the top surface where stresses are more than double in the solid model when compared to the shell model. I'm unsure as to the cause of the differences and I would like to request advice from the community as to potential causes of the disparities in the stresses. Thank you for your time.