Theoretical and experimental studies on residual stresses in laminated polymer composites

In this research, the residual stresses in thermoset polymer composites are studied. Generally, these stresses are studied from the macroscopic and microscopic points of view. The main factors responsible for the buildup of these stresses in composite materials are the curing process, moisture, and temperature. The stress-free temperature of various samples of a laminated composite is measured. The macroscopic residual stresses due to the curing process of orthotropic thermoset polymer composites are determined analytically by two methods in addition to all experimental one. The classical lamination theory (CLT) is used for the calculation of residual stresses in each layer of laminated composites. It is shown that the capability of the classical lamination theory in the calculation of the residual stresses without considering the temperature dependent properties is very good. However, the shape of the unsymmetrical laminated specimen at room temperature, calculated by the CLT, is not generally accurate. The energy method is used for the modification of the CLT in predicting the room-temperature shape of the unsymmetrical laminated composites. Experimentally, glass/epoxy and carbon/epoxy laminated composites are studied by the hole drilling method. This method is shown to be reliable for an accurate measurement of the residual stresses in the first layer of laminated composites.