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This paper discusses an experimental program that addressed the effect of low-velocity impact damage on the fatigue behavior of two laminates, fabricated from AS/3501-6 graphite/epoxy unidirectional tapes, with lay-ups representative of high-performance fighter aircraft wing skin lay-ups. Prior to testing, specimens from these laminates were subjected to one of two types of low-velocity impact damage. The first type was created using a blunt-tipped (1.6-cm tip diameter) impactor, and the impact energy was chosen to induce internal damage, approximately 5 cm in diameter, with no visible sign of damage on the outer surfaces. The second type of damage was created using a sharp (tetrahedral-tipped) impactor, with visible signs of damage on the impacted and opposite surfaces. Subsequent to introducing low-velocity impact damage, test specimens were subjected to static and constant amplitude fatigue loading. In the presence of compressive loads, lateral knife-edge supports were placed near the outer edge, for specimen stability, permitting unconstrained delamination growth elsewhere. Static tension and compression tests were conducted initially, and significant strength reductions due to the impact damages were measured. Fatigue load amplitudes were chosen to be fractions, S, of the static strengths. The behavior of impact-damaged specimens was investigated under tension-tension (R ≈ 0), tension-compression (R = −1), and compression-compression (R → −∞) fatigue loads. During fatigue, the growth in the impact damage was monitored by means of ultrasonic pulse-echo records. Fatigue life data for the various laminate-damage-loading combinations were obtained in the form of S-N curves. Interesting damage growth patterns were observed, and the criticality of each type of impact damage on the fatigue behavior of the tested laminates was assessed.
composites, graphite/epoxy, impact damage, tension fatigue, compression fatigue, reversed loading, damage tolerance
Engineering specialist, Structural Mechanics Research, Northrop Corp., Aircraft Division, Hawthorne, CA