Abstract

The relevance of fiber-reinforced polymer composite materials to aerospace has increased to the point that they are used for high-load-bearing applications. For example, thick flex beam composite parts could be used in rotorcraft industries to manufacture helicopter’s main rotor yoke. Manufacturing thick thermosetting laminates inside an autoclave is usually accompanied with an exothermic reaction and temperature overshoot during the curing process. This can cause process-induced stresses to build-up. To measure these internal stresses, the numerical and experimental approaches were studied. 80 layer unidirectional and cross-ply glass/epoxy laminates were manufactured by hand lay-up. The temperature distribution during curing process inside the autoclave was measured through placing five thermocouples at every 20 layers inside the laminate. The finite difference formulation of the one-dimensional transient heat transfer including internal heat generation was developed to simulate the temperatures and degrees of cure. The calculated temperature profile was compared with the experimental result and it was found that the numerical values agreed well with the measured temperatures through the thickness. Afterwards, the evolution of residual stresses as the direct consequence of the complex gradients of temperature and degree of cure across the thickness were calculated incrementally.
To experimentally quantify the residual stresses in thick composite laminates, deep hole drilling technique was proposed. This method relies on the measurement of strain relaxation resulting from material removal. First, a small hole was drilled in a plate consisting of 80 layer glass/epoxy laminate. The diameters of the hole were measured. Then, the changes in the hole diameters due to the stress relaxation by trepanning a core from around the hole were re-measured. Finite element analyses were then carried out to provide distortion coefficients for far field applied stresses. Residual stresses were be determined through the combination of numerical and experimental approach.