Hybrid structures to reduce the geometrical unconformities on thin composite parts during cure process

During the manufacturing process of polymeric composite materials, geometric unconformities may arise due to the onset of residual stresses, that are due to the chemical shrinkage of the matrix, to the polymerization process and to the difference in value between the CTE of matrix and that of fibres during cooling. The behaviour of thin non-planar form laminates (L, T, U shapes) is not the same of flat ones: even if the laminate presents a symmetrical and balanced structure, the condition of stresses and deformations is more complex and causes geometric deviations. The solution currently used to solve this problem is the equipment compensation. In this paper a new methodology to reduce the unconformities of thin composite parts is presented. It consists in developing a hybrid structure that does not require a forming mould as the traditional monolithic structures. In particular, the innovative hybrid structure consists of prepreg laminate stratified around an aluminium core, which serves as mould. In this way a new L-shape laminate is developed too. Experimental tests were carried out to validate the proposed methodology. The obtained results demonstrated that the proposed approach may allow to obtain values of the span angle very near to the nominal ones. Moreover, in this case a compensation of residual deformation is very easier, since only the bending angle of the metal sheet needs to be changed, in a very simple way