INFLUENCE OF CURING PROCESS INTERRUPT ON ILSS OF THIN CARBON EPOXY LAMINATES

Abstract

Autoclave curing process of any composite part lasts for about 5 to 6 hrs during which the composite parts will undergo physical & chemical changes due to the polymerization process. Any interrupts or breakdown during curing process due to mechanical / power / controller failures can affect the properties and in turn results in improper curing of a composite part. Hence there is a prime need to analyze the effects of process interrupt at critical stages of the curing process on part quality and strength. Efforts are made in this paper to study and analyse the effects of curing process interrupts on the properties of Hexcel 913 carbon /epoxy composite laminates of thickness 2 mm at five critical stages (75 0 C, 90 0 C, 110 0 C, 120 0 C, and 135 0 C ) of a cure cycle. The ILSS (Inter laminar Shear Strength) is analyzed as per Dutch Institute for Norms (DIN) standards and the results are compared with the master specimen. The results exhibits low strength at cure temperatures 90, 110 and 120 o C for process interrupt of 60 mins due to significant decrease in temperature during the process interrupt and eventually result in incomplete resin transition at critical Tg points from 90 to 120 deg.C. The laminates cured with process interrupt for 60 mins at 75 o C have reduced 0.5 % of the ILSS properties and between 90 to 120 o C the ILSS has reduced by 4 - 8% of the reference laminate properties. Hence the autoclave break down / power failure for more than 40 mins at 90 o C and 30 mins at 120 o C is not recommended for curing of composite parts of thickness less than 3 mm as the part strength gradually decreases .However these conditions vary for curing thicker composite laminates, since it consists of more layers and will have high thermal gradients between the layers during curing. Mathematical equations are formulated to determine the Avg. ILSS values at particular cure temperature for different process interrupt periods. This helps the designer or the manufacturer to evaluate and analyse the ILSS values due to process interrupts at critical points during curing process.