Thermal Effects on the Bearing Behavior of Composite Joints

Thermal effects on the pin-bearing behavior of an IM7/PETI5 composite laminate are studied comprehensively. A hypothesis presents factors influencing a change in pin-bearing strength with a change in temperature for a given joint design. The factors include the change in the state of residual cure stress, the material properties, and the fastener fit with a change in temperature. Experiments are conducted to determine necessary lamina and laminate material property data for the IM7/PETI5 being utilized in this study. Lamina material properties are determined between the assumed stress free temperature of 460F down to –200F. Laminate strength properties are determined for several lay-ups at the operating temperatures of 350F, 70F, and –200F. A three-dimensional finite element analysis model of a composite laminate subject to compressive loading is developed. Both the resin rich layer located between lamina and the thermal residual stresses present in the laminate due to curing are determined to influence the state of stress significantly. For the laminate modeled, the effect of modeling temperature dependent material properties and nonlinear stress-strain behavior was found to be negligible. Simply using the material properties measured at the operating temperature of interest was sufficient for predicting stresses accurately in a linear analysis for the current problem. Pin-bearing tests of several lay-ups were conducted to develop an understanding on the effect of temperature changes on the pin-bearing behavior of the material. As expected, for all lay-ups that failed in the bearing mode, pinbearing strength decreased with an increase in temperature. Micrographs of failed specimens revealed severe damage in the outermost two plies with shear cracks emanating from that region. A computational study investigating the factors influencing pin-bearing strength was performed. A finite element model was developed and used to determine the residual thermal cure stresses in the laminate containing a hole. Very high interlaminar stress concentrations were observed two elements away from the hole boundary at all three operating temperatures. The interlaminar stresses quickly reduced to zero within one laminate thickness from the hole boundary. Changes in fastener fit with a change in temperature were predicted to be insignificant on influencing changes in pin-bearing strength. The pin-bearing problem was modeled assuming a rigid frictionless pin and restraining only radial displacements at the hole boundary. A uniform negative pressure load was then applied to the straight end of the model. A solution, where thermal residual stresses were combined with the state of stress due to pin-bearing loads was evaluated. The presence of thermal residual stresses intensified the interlaminar stresses predicted at the hole boundary in the pin-bearing problem. This dissertation shows that changes in material properties drives pin-bearing strength degradation with increasing temperature. The thermal residual cure stresses affect the extent that the pin-bearing strength changes with temperature, where they can only lessen the strength degradation with increasing temperature.