Damage accumulation during multiple stress level fatigue of short-glass-fiber reinforced styrene-maleic anhydride

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.