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Damage Accumulation During Multiple Stress Level Fatigue of Short-Glass-Fiber Reinforced Styrene-Maleic Anhydride

U.S. Army Research Laboratory, AMSRL-WM-MB, Aberdeen Proving Ground, MD 21005, choppel{at}arl.mil

Robert N. Pangborn

Pennsylvania State University, Dept. of Engineering Science and Mechanics, University Park, PA 16802

Robert W. Thomson

U.S. Air Force, Wright Patterson Air Force Base, Dayton, OH

Multi-stress fatigue of short-glass-fiber-reinforced styrene-maleic anhydride (S/MA) composite materials has been studied. Specimens were tested in tension-tension fatigue (R = 0.1) in two patterns: high-stress fatigue followed by low-stress fatigue (high-low) and low stress followed by high stress (low-high). Results were analyzed using the Palmgren-Miner cumulative damage law. High-low fatigue gave Miner’s sums very close to unity, and Miner’s sums below unity were obtained for low-high fatigue. A minimum value below unity for low-high fatigue corresponded to a regime in which the first block of cycling was carried to 15% of the life.

These results were interpreted by evaluating the evolution of damage and the role of plastic deformation in the fatigue behavior. The high-stress fatigue created a more extensive region of crazing in the matrix material. This plastic deformation increased the mechanical clamping force on the short fibers, making the composite more resistant to subsequent low-stress fatigue testing. When specimens were tested in low-stress fatigue first, cracking occurred in the composite without as much plastic deformation. These cracks were extended by subsequent high-stress fatigue, leading to earlier failure of the composite.

Key Words: short fiber composite • fatigue • Miner's rule • life prediction • damage accumulation • thermoplastic

Journal of Thermoplastic Composite Materials, Vol. 14, No. 1, 84-94 (2001)
DOI: 10.1106/WRWF-0CFQ-2HKQ-JG4Y


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