An Investigation of Cure Induced Stresses in Low Cure Temperature Thermoset Polymer Composites

Abstract

Low temperature thermal curing resins (such as LTM-45 ELD and X343-59) are being considered by the aerospace industry for developing prototype parts. By curing at low temperatures, the parts can be cured outside of the autoclave. Therefore, expensive autoclave hardened tooling is not required. Tooling materials such as wood and foam can be used instead. This will dramatically reduce the cost of producing prototype parts. The effect of the standard cure cycles on the cure induced stresses in composites containing such resins is not characterized yet. Most of the cure induced stresses are developed after the polymer reaches the gel point. Any subsequent polymer volume change produces fiber stresses. Stress relaxation acts to reduce stresses developed during cure. Relaxation time increases significantly after the polymer is gelled and thus stress relaxation becomes less effective to reduce cure induced fiber stresses. A closed loop feedback control system has been developed to monitor the cure induced fiber stresses and to determine cure cycles with less residual stresses. In this system, a polymer is cured along a given length of a pretensioned fiber. The cure induced stresses are determined by measuring the fiber tension change. This method was applied to LTM-45 ELD and X343-59 resins during standard two stage cure cycles and during a typical autoclave cure cycle used for 3501-6 epoxy. The cure induced fiber stresses were monitored during these cycles. In addition, the feedback control was used to determine modified cure cycles to reduce these stresses.