Damage Accumulation in Graphite/Epoxy Laminates Due to Cyclic Gradient Stress Fields

The progression of damage in gradient stress fields under cyclic loading was studied in simply-supported graphite/epoxy beam-columns. Three layups, [45 ₄ / —45 ₄ / (0/90) ₄ ] _2s , [±45/0/90 ₄ ] _4s , and [(45 ₂ /-45 ₂ /0) ₂ /90 ₅ ] _{2 s} , were chosen to be consistent with data collected in a previous study with static loading. Specimens were loaded statically until predetermined characteristic damage levels were obtained, after which cyclic loading began. At specific intervals, cyclic loading was stopped, a static test measuring the load and the corresponding center deflection was performed, and edge replicas of the specimen sides were taken. The damage of these specimens varied both along the length and through the thickness of the specimens with two general modes of damage present. Damage due to the static loading generally occurred on the tension side of the specimen and was controlled by matrix cracks. Delammations initiated and grew in areas of crack saturation. Damage due to the cyclic loading also occurred on the compression side of the specimens and was controlled by delammations that initiated and grew independently of matrix cracks, leading to sublaminate buckling. Increasing the maximum cyclic load changed the relative growth of the two modes of damage, which changed the final failure mode.