Mitigation of machining damage on delamination of fiber reinforced composite tensile coupons

Abstract

The accurate development of material allowables is a fundamental aspect of any successful engineering design. Successful testing and development of material properties will lead to accurate analysis of a structure, and predictable behavior of the end design. However, in some cases (especially for laminated composite materials) the overall behavior of a structure cannot be predicted by small scale testing. One of the leading causes of this is that edge effects have a larger influence on small specimens, leading to under-prediction of overall material behavior. An experimental study was conducted to investigate the influence of machining induced surface damage, and the viability of multiple techniques to mitigate that damage in order to produce coupon specimen results that are more representative of the behavior of a bulk laminate. Coupon extraction was conducted utilizing a rotary diamond saw. Two edge treatments were tested (application of a commercially available cyano-acrolate adhesive, and a commercially available epoxy resin). A pair of laminates was tested; each was designed to be susceptible to delamination at a known position. Tensile testing showed that application of a cyano-acrolate edge treatment improves the performance of narrow 30 degree specimens by a margin of 11%. A statistically significant difference was not observed in 45 degree laminates, epoxy coated specimens, or wide specimens. There was a statistically significant performance difference between narrow and wide specimens, including edge treated specimens. A 6% performance gap remained between narrow cyanoacrolate coated specimens, when compared against wide specimens. This indicates that edge stresses are only partially mitigated by the edge treatment applied to the coupon.