Effect of dwell time on the properties of composite materials

Material manufacturers often specify maximum duration a prepreg is allowed to dwell in an autoclave at the higher isothermal cure temperatures preceding which, their acceptance is usually questioned, due to possible structural damage owing to the overexposure. As the cure specifications for the material are usually designed considering ideal temperature conditions, it may indeed not be satisfied in real manufacturing situations. The discrepancies in the leading and lagging thermocouples, presence of different tools in the autoclave for example may lead to over exposure and may be rejected in the first instance according to specifications. In such examples, not only the productivity is hindered but also the cost of production increases dramatically. Therefore, in view of improving productivity, investigations have been carried out on composite parts manufactured using Cycom 977-2 prepregs. Test coupons were manufactured using standard 120 minutes and two extended postcure cycles, 200 and 300 minutes. ILSS tests were conducted to examine any change in laminate shear strength, DMA was used to measure the variation in T_g, DSC was used to determine the extent of cure and flatwise tensile tests were conducted to determine any change in the core-to-facings bonding, with respect to postcure durations. The results from ILSS show negligible changes in the laminate shear strength (average 74 MPa), exhibiting standard deviation of 2 MPa. DMA test results show 6-10°C shift in Tg when the postcure duration is changed from standard 120 minutes to 300 minutes, flatwise tensile tests reveal consistent core failure exhibiting strengths of 2.5 ±0.2 MPa for all the samples and the DSC confirms a consistent cure of greater than 97% for all the specimens. In conclusion, there is negligible changes in mechanical and material properties of the parts that are subjected to extended postcure, upto 90 minutes from the maximum allowable 210 minutes.