I have searched everywhere for the stiffness matrix of GFRP, but there isn't a single paper or site that experimented the full matrix. Is there any sort of database for the stiffness matrix of many composite materials?
The material has to be tested for each application as the fibre density, weave and number of layers all have effects on the stiffness.
This means even each batch may have a variation so exact values may not be easily available.
Some companies are very "expert" in this as they are using carbon fibre sheets in extreme situations and have found what works / what does not and also about what can be repaired and how. Airbus comes to mind.
When using a material like a steel, then the composition etc can be agreed and the performance specified - so leading to steels that are easier to machine or behave better in some loading situations.