Stresses during thermoset cure
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Stresses during thermoset cure Douglas B. Adolf, James E. Martin, Robert S. Chambers, Steven N. Burchett, and Thomas R. Guess Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received 20 March 1997; accepted 5 September 1997)
Production problems attributed to excessive stresses generated during the cure of epoxies led us to develop a formalism to predict these stresses. In our first studies, we developed a fundamental understanding of the complex evolution of viscoelasticity as the cure progresses. We then incorporated these results into a proper tensorial constitutive equation that was integrated into our finite element codes and validated using more complicated geometries, thermal histories, and strain profiles. The formalism was then applied to the original production problem to determine cure schedules that would minimize stress generation during cure. During the pursuit of these activities, several interesting and puzzling phenomena were discovered that have stimulated further investigation.
I. INTRODUCTION
It takes little imagination to envision the numerous practical problems that could potentially be ameliorated by numerical evaluation of stresses during the cure of thermosetting resins. For example, residual stresses during the production of large epoxy composite parts can cause strains that exceed design tolerances. Lack of dimensional stability in multilayer printed wiring boards could preclude small through-hole design features. Minimizing stresses in these applications, or even acquiring the ability to predict the resulting strains, would greatly reduce the time and cost of realizing new designs. At SNL, we became interested in stresses during epoxy cure when production problems were encountered with a new component design. These problems were traced to epoxy cohesive failure during cure and attributed to unacceptably high stresses generated during cure. Our component of interest consists of a metal bottle, filled with electronic components, into which we pour and cure an epoxy. Since our pour hole is narrow and the epoxy adheres to the inside of the metal bottle during cure, thermal or cure shrinkage strains cannot be transferred into shear deformations, but translate directly into high stresses that arise from the extremely low compressibility. Encapsulating this design is problematic, but we thought that a well engineered thermal cure profile might be successful in reducing stresses to acceptable levels during cure. Stresses are generated in a nonisothermal cure from both mismatches in coefficients of thermal expansion (CTE) and from cure shrinkage. By increas530
J. Mater. Res., Vol. 13, No. 3, Mar 1998
ing the temperature during cure, the expansive thermal strain, arising from the large epoxy CTE compared to that of the metal bottle, could, in theory, exactly balance the contractive cure shrinkage strain. To realize this goal, we had to first develop the capability to predict cure stresses quantitatively. Since epoxy cure is exothermic a
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