Pin-contact failure in composite laminates
Abstract
A study of pin-contact failure in composite laminates was investigated. The experimental part of this research involved the use of a contact test in which a steel pin was pressed against the straight edge of a composite laminate. Specimens were formed by AS4/3501-6 graphite/epoxy composite laminates. It was found that failure initiates at the edge near the flat surfaces at 70-82.5% of the ultimate contact load. Generally, this percentage depends on the specimen lay-up and loading condition. As the load increases, the damage propagates steadily at an angle about 30$\sp\circ$-40$\sp\circ$ into the interior of the specimen until it reaches a critical length. Then unstable growth begins and catastrophic failure occurs. The failure mode in the 0$\sp\circ$-plies resembles the kink band observed in compression failure in unidirectional composites. Fiber microbuckling is believed to be the primary failure mode. An approximate two step 3-D finite element stress analysis was performed to understand the failure mechanisms. Stress analysis results show that elasticity is not adequate for predicting the contact strength of the laminate. Thus, plastic behavior of the composite must be accounted for. In addition, the contact friction between the pin and the composite is also important to the modeling. A fiber microbuckling failure model including effects of non-linear matrix stress/strain behavior was developed, and a failure criterion curve was constructed. Based on experimental inspections and numerical results, the critical volume for damage initiation in composite laminates was determined and regarded as a material constant. Stresses within this volume were averaged to predict the onset of failure. Using the average stresses in conjunction with microbuckling criterion, the failure initiation due to pin-contact was successfully predicted. It is also noted that ply orientation and clamping condition of the laminate have significant impact on contact failure behavior. This is due to strong influence from plastic behavior of the composite as well as out-of-plane shear stress, which was ignored by previous researchers.
Degree
Ph.D.
Advisors
Sun, Purdue University.
Subject Area
Aerospace materials|Mechanical engineering
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