Evaluation of machining damage around drilled holes in a CFRP by fiber residual stresses measured using micro-Raman spectroscopy

Drilling used to assemble carbon fiber reinforced plastic (CFRP) parts is employed widely in industries. With drilling of CFRPs microscopic damage like residual stress or interfacial debonding between fiber and matrix is accompanied prior to macroscopic damage like delamination or chipping. Although not only macroscopic damage but also microscopic damage is closely related with overall performance of composites, there is no effective method to evaluate microscopic damage. In the present study, quantitative evaluation of drilling-induced damage was attempted by measuring details of residual stresses in fibers for a unidirectional CFRP. Stress distributions along fiber located at the drilled-hole periphery were evaluated in mu m spatial resolution by means of micro-Raman spectroscopy. At first, to clarify the dependence of drilling effect on fiber orientation and fiber location, residual stresses in fibers orientated at angles of 0, 90 and 180 to cutting-edge were measured both on drill-entry and drill-exit side surfaces. As the result residual stresses in fibers caused by drilling were all compressive and showed considerable dependence on fiber orientation to cutting-edge of the drill. Residual stress in fibers at 90 arose firstly even in low feed speed. Difference was also observed between stress distributions of fibers on drill-entry side and those on drill-exit side. Next in order to decide the highest speed drilling without damage, interfacial stress distributions were monitored with increasing feed speed. Experiments of the same feed rate with different rotational speed were also conducted to examine the effect of rotational speed. Damage at the interface, i.e., interfacial debonding occurred between 30 mm/min to 150 mm/min feed. And higher rotational speed resulted in smaller residual stress even in the same feed rate a rotation. Such results show that evaluation using micro-Raman spectroscopy is well applicable to prove details of drilling-induced damage quantitatively.