Development of spring-in angle during cure of a thermosetting composite

The development of spring-in angle during cure of AS4/8552 thermosetting composite is investigated by using a cure quench technique. C-shaped preforms are cured on the inner wall of an aluminium tube. The cure is interrupted at various points during the Manufacturer's Recommended Cure Cycle (MRCC) by quenching the tool tube into water. The diameters of the specimens cured in this way are measured and the spring-in angles for a 90° arc of the specimens are calculated. The test data show that the samples quenched at earlier stages of cure exhibit a larger spring-in and the spring-in angle reduces as the specimen is further cured.

A cure kinetics simulation is performed to understand the development of cure throughout the MRCC. It has been found that the vitrification of the specimen occurs approximately 45 min after the start of the 180 °C dwell period, and the specimens quenched before vitrification are observed to have larger spring-in.

An explanation of this observation is the fact that, before vitrification the specimen is in the rubbery state during the temperature range between the cure temperature and the instantaneous glass transition temperature, and in this state it has a larger thermal expansion coefficient compared to that in the glassy state, causing more contraction in the through-the-thickness direction, hence more spring-in.

The cure quench experiment provides an insight into the relative importance of the thermal contraction above and below the glass transition temperature and the cure shrinkage.