Surface Texture and the Stress Concentration Factor for FRP Components with Holes

Abstract

The influence of hole quality on the mechanical behavior of fiber reinforced laminates was studied. Holes were introduced in tensile specimens of a graphite/epoxy (Gr/Ep) laminate using an abrasive waterjet and commercial drills (diamond coated tungsten carbide twist drills or tungsten carbide drill-reamers). The machined surfaces were characterized using contact profilometry and the surface texture was used in estimating the effective stress concentration factor (øK_t). Utilizing the macroscopic stress concentration posed by the hole and øK_t, the total stress concentration was estimated using the principle of superposition. The Gr/Ep specimens were then loaded in tension and acoustic emission was used to monitor the failure process. The apparent stress concentration factor (øK_t(app)) of the tensile specimens was determined from the ratio of tensile strengths of coupons without holes to that of specimens with holes. Based on results from tension tests the øK_t(app) at first fiber failure ranged from 2.50 to 3.40. The stress concentration factors determined from experiments were within 6% of that predicted using superposition and øK_t. Although the hole quality was dependent on the method of machining and drilling, results from this study confirm previous reports that there is no correlation between the surface texture and first fiber failure or ultimate tensile strength of Fiber Reinforced Plastics (FRPs) with open holes. Holes introduced using worn diamond coated twist drills exhibited the lowest surface roughness but resulted in a significant reduction in first fiber failure strength. Results from this study indicate that surface texture and øK_t cannot be used for a reliable estimate of hole quality in FRPs, especially for holes produced with worn cutting tools.

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