Low Cycle Compressive Fatigue Failure of E Glass-Epoxy Composites
MASSACHUSETTS INST OF TECH CAMBRIDGE AEROELASTIC AND STRUCTURES RESEARCH LAB
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Low-cycle fatigue tests with a mean compressive stress were conducted on modified dogbone type specimens of 0 -90 degrees 3M SCOTCHPLY laminates with fiber volume fractions ranging from 45 to 60 percent. The test results indicate that compressive fatigue strength decreases as a linear function of logcycles, and that strain increases linearly up to approximately 60 percent of fatigue life. Beyond this point a significant increase in strain is observed, indicating that accumulation of major damage is concentrated in the last 40 percent of fatigue life. The progress of damage during fatigue was investigated independently by optical microscopy. These observations indicate that the predominant damage effect is debonding of 0 degree fibers parallel to the load axis, followed by fracture of groups of adjacent fibers. Inter-ply delamination is also observed in some cases. Based on the test results and microscope observations, the failure mechanism of progressive debonding leading to local buckling and fracture of the 0 degree fibers is proposed. The final result is an inclined shear fracture when cracks in the damaged axial plies propagate through the transverse plies. The effects of misaligning a small percentage of the axial plies within the laminate, and of a 21-day immersion in sea water were also studied. Compressive fatigue strength is found to decrease due to both of these effects.
- Laminates and Composite Materials