SUMMARY

The goal of this academic project was to study the effects of different variables on the damage progression around a central hole in a coupon specimen. The tracked variables included the type of layup, R values, stress levels, and damage mechanisms observed in each specimen. In-situ x-ray of the individual laminates recorded the extent of damage, mostly longitudinal splitting, as a function of the cycle count. The following lay-ups were included: [45/90/-45/0/0/45/0/0/-45/0]s, [±5/65/(±5)/(±5)/-65/±5]s, and [±5/65/(±5)/(±5)/-65/5/65]s. In particular, the objective was to determine the stress levels at which detectable damage starts developing. The researchers chose to apply 50,000 cycles at each stress level. Once damage was detected, the stress level was typically raised 34.5 MPa, and then cycled another 50,000 cycles until damage exceeding 1.27 cm in length was measured. Once the damage exceeded 1.27 cm, cycling continued to 1,000,000 cycles. Upon completion of the fatigue cycling, each specimen’s residual strength was determined. The damage length versus stress level is plotted as a way to compare damage onset stresses and growth as a function of lay-up, stress ratio.