Reducing NDT Effort by Coupled Monitoring and Simulation of Liquid Composite Molding Processes

Kurzfassung

Liquid Composite Molding (LCM) is a manufacturing technique which could reduce material and process costs in comparison to the widely used prepreg processes. The raw materials are less expensive and easier to store and geometrical complex parts can be manufactured. But LCM requires more sophisticated process design due to its sensitivity to deviations in the raw materials or process setup. Gaps for example can lead to race tracking which often results in trapped gas inside of the component. Such gas inclusions can lead to porosity or even larger dry spots and thus an unusable part. The DLR Institute of Composite Structures and Adaptive Systems developed flexible low cost ultrasonic sensors for LCM monitoring. The sensors do not require direct contact to the part and therefore do not affect the part surface or mold. Due to enhanced signal analysis not only the flowfront arrival can be detected, but also its flow speed. Further the moments of wetting of the mold surface and the infiltration through the part thickness can be distinguished. The sensor system is also able to perform cure monitoring including the detection of gelation and vitrification points. These sensor inputs are coupled with a fast flow simulation where the model parameters are adapted to match the measurement by a sophisticated optimization algorithm. The most important model parameters are the permeability and gap sizes. By adapting the simulation the pointwise monitoring results are transformed into a 2.5-dimensional representation of the flowfront that enables the prediction of its further course. The results are used to detect possible gas inclusions and areas of high porosity. If none of these defects are detected the NDT process could be omitted. In the case of detected defects only these areas would have to be tested potentially.