Measurement of residual stresses in fiber-reinforced composites by the incremental hole drilling method: effect of the drilling-induced heating.

The incremental hole drilling method is widely used to determine residual stresses in materials. For fiber-reinforced composites, the drilling-induced heat can create measurement errors that are not taken into account in the calculations. Even if the drilling parameters (cutting and feed speeds, type of cutter, etc.) are optimized to provide correct machining results and more reliable residual stress measurement, thermal effects remain relatively poorly understood and analyzed for fiber-reinforced composites. The purpose of this work is to investigate the effects of drilling-induced heat on the local chemical properties of unidirectional carbon/epoxy laminates that have undergone different curing cycles and to discuss the consequences on residual stresses. First, the thermal field generated by drilling was characterized using an infra-red camera. The measurements showed that relatively high temperatures were reached depending on the increment depth. Then, Modulated Differential Scanning Calorimetry before and after drilling were performed to investigate the effects of the produced heat on the chemical properties. The results showed a local increase in the degree of cure and the glass transition temperature (Tg) of the matrix. It is assumed that these mechanisms are responsible for the apparent residual stresses obtained when large increment depths are used during the process. The main conclusion of this study is that the heat generated during the drilling step of the incremental hole drilling method causes a progression of the cure reaction in the vicinity of the hole, leading to Tg-dependent residual stresses which are higher for low-Tg composites.