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Overview of Database Generation for Development of Bolted Composite Joint Fatigue Methods



In anticipation of service life extension for airframes in the United States Air Force (USAF) with composite materials, Air Force Research Laboratory (AFRL) is currently developing various tools for durability and damage tolerance (DADT) assessment of bolted composites joints. Under AFRL’s Composite Airframe Life Extension (CALE) Project 2 program, parametric and progressive damage analysis (PDA) tools were developed for such purpose. As part of the tools development under Project 2, over 150 single shear bearing (SSB), open- and filled-hole tension and compression (OHT/FHT and OHC/FHC) and tension bearing-bypass (BB) specimens were tested in static, constant amplitude (CA) fatigue, and CA block fatigue loading to establish a database for the semi-empirical parametric model while also providing opportunities for iterative maturation of the PDA tools through many prediction/calibration cycles. Stress ratios, R, in the range of -1.0 to 0.1 were used in CA and CA block fatigue tests as one of the goals for the program was to develop tools capable of variable fatigue stress and amplitude. All specimens were fabricated from toughened 350oF-cure carbon/epoxy unidirectional material and consisted of three thicknesses—0.143”, 0.191”, and 0.239”—from [44/44/12] laminate family. The fastener used was a ¼” diameter stainless steel 100o flush head rivet pin that is commonly used in production. The tests were performed at both Northrop Grumman Corporation (NGC) and AFRL. These tests provided data on 1) static response, 2) hole elongation (%e) vs number of cycles (N), 3) joint stiffness (k/ko) vs N, and 4) residual strength and/or S-N with respect to diameter-to-thickness ratio (D/t), R, fastener preload, and fatigue load profile (i.e., CA vs CA block spectrum). This paper will provide an overview of the various test methods, summary of key results from the trial phase of the 150+ tests, and the effects of these test results on the remainder of the program testing approach.

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