1) Do you have any estimate how the thickness of the specimen can impact the design allowables?
> compression strength typically decreases a bit with lower thickness (this is not due to bucking, which must be avoided, but due to reduced local lamina kink band stability)
2) Can we use the values from uniaxial compression test to evaluate strength of 1 ply or 4 ply-based construction?
> data for thicker specimen will probably be a bit higher, see above. Also, uni-directional strength is not really useful, unless you are using that layup in your actual structure.
I tend to say that 1 ply will require flexure test, but I am not sure about 4 plies. Do you have any experience about thickness limit of the skin from which we can safely use values from standard compression test?
> up to 4-6 plies you should use sandwich flexure data, particularly if the skins are cocured to the core
3) Is it true that flexure test is used only to quality control and material selection and not to obtain design data quality?
> laminate flexure tests are sometimes used for QC, but not typically for in-plane strength design data as the results can be unconservative. Sandwich flexure tests are typically not used for QC - too expensive.