Residual Stresses in Thermosetting Resins for Polymer Matrix Composites: Modeling and Effects on Long Term Performance

摘要

We extended a previous model due to the PIs for the evolution of the properties of curing thermoset resins to the BMI/SOC resins of interest to the Air Force. The model was used to estimate the cure stresses in this system in an ideally and isotropically constrained system. The results were surprising in the large magnitude of the stresses predicted and showed that judicious selection of polymer materials for low values of the thermal pressure coefficient will reduce thermal stresses by as much as a factor of three. Also, two distinct methods for measuring isotropic residual stresses were developed and demonstrated. A spherical 'bomb' geometry was developed and provided a method of measuring isotropic stresses during cure when stresses remained below approximately 35 MPa. A thick-willed cylindrical geometry combined with lock-in amplifier electronics to excite the strain gages was developed for high stress measurements-pressures as high as 300 MPa were readily achieved, which is not possible with conventional thin-walled cylinders or with the thin-walled sphere just mentioned. More importantly, in the process of developing the thick-walled cylinder method, we demonstrated that the classical thin-walled tube method of constraining resins in an isotropic state of stress does not actually produce the isotropy expected. Measurements tend to be incorrect and are biased by the fact that the stresses are anisotropic. Hence, the thick-walled cylinder is a true improvement in measurement technology. Using this geometry, we performed the entire cure and post-cure under compression and observed over two hundred MPa cure stresses for model thermosetting resinsconsistent with our model predictions. Our work leads us to conclude that reduction of isotropic residual stresses in thermosetting composites must focus on the reduction of thermal stresses through a reduction in the thermal pressure coefficient rather than focusing on reducing cure shrinkage.