The influence of machining processes on strain distribution and progressive failure characteristics when producing holes in CFRP

Machining strategy can significantly influence surface integrity when producing holes in CFRP laminate, which has drastic impact on mechanical behavior and service life of related structures. Therefore, it is vital to understand the effect of defects caused by machining on cracking/splitting initiation, propagation and final failure modes. In this paper, the influence of four different machining processes used to produce 6.0 mm diameter holes in CFRP was investigated in relation to mechanical behaviour under tensile loading. The processes involved were high power fiber laser cutting, abrasive waterjet machining, conventional drilling and ultraviolet laser multiple-pass machining. The complex strain distributions around the holes were analyzed using digital image correlation (DIC) method, while crack propagation was studied using high speed imaging technique. The correlation between initial surface integrity and mechanical behavior was developed. Stress concentrations and machining defects around holes accelerated damage accumulation with final failure modes highly related to fiber orientations rather than initial surface damage.