Influence of consolidating process on the properties of composites from thermosetting carbon fiber reinforced tapes

Carbon fiber reinforced tapes are a feasible option to tailor material properties of composite structures and to reduce waste due to cut-offs. But composite properties are not only influenced by the selected material, but also via the applied curing process. The aim of this experimental investigation is a comparison of the mechanical properties of thermosetting carbon fiber reinforced tapes, which are cured in different curing processes. In the composites industry the autoclave curing process is considered to be the highest quality process for handling preimpregnated semifinished-products, so called prepregs. Alongside the autoclave process there are several alternative routes, named out-of-autoclave-processes, which can also be chosen for prepreg curing. One option is prepreg hot-pressing, a process which is intended for large part quantities and easy process control. The development of fast curing thermosetting resin systems with the possibility of complete curing in the range of minutes introduced snap-cure prepregs, recommended to be used solely in hot presses. For this reason this type of prepreg, in form of unidirectional tapes, was used for the following experiments. Two additional curing processes were investigated for process-comparison, namely oven-curing and in-situ consolidation. The placement of the prepreg tapes for the laminate build-up was done by an automated fiber placement unit followed by the curing processes as mentioned before. During the characterization of the material properties the general accepted statement of the excellent autoclave laminate quality could be confirmed. Strengths of around 930 MPa and even above were achieved with three-point coupon bending tests for laminates cured in the autoclave. The fiber volume fraction was slightly above 58% and the laminate porosity was 0.5%. In comparison oven cured and hot pressed laminates strength values were lower at around 675-775 MPa. Hot pressed laminate porosity content was about 1.3% and the fiber volume fraction of these laminates was 52%. Oven cured laminates showed a higher porosity content above 4% and a fiber volume fraction of 51%. The new approach for prepreg curing, the in-situ consolidation process provided fairly good results which were comparable to those of the oven cured laminates. In this process the unidirectional prepreg tape is directly cured at the surface of a heatable table during automated fiber placement without subsequent curing under a vacuum bag. The positive testing outcome of in-situ consolidated laminates shows a big potential for future research and development of this process.

Automated summary

This experimental investigation compares the mechanical properties of thermosetting carbon fiber reinforced tapes cured in different curing processes. The results show that the autoclave curing process produces laminates with higher strength and lower porosity compared to oven curing and in-situ consolidation processes.