Ultrasonic Resin Flow and Cure Monitoring

Kurzfassung

The resin injection and cure processes are crucial manufacturing steps for composite materials with high impact on the part quality and properties. But both processes are sensitive to deviations in raw materials and process parameters. By measuring the process evolution with suitable process monitoring sensors and systems, potential variations can be identified, evaluated and if necessary corrective action can be started. Next to process control such sensors can be used to speed up the process design phase. Ultrasound based sensors on one side do not require direct contact to the part offering the advantage that part surface and mold remain unaffected and simplifying the sensor integration. On the other side almost all of the most crucial process parameters can be obtained by signal analysis. The DLR has developed simplified and low cost sensors which enable the integration of a dense sensor network into the mold. The sensors are applied on the outer side of the mould, from where the sound waves propagate through the wall and then interact with the part. By analyzing the pulse-echo signal, where the same sensor acts as sender and receiver, the resin arrival at the interface between mold and part can be detected. Using the transmission mode with a separate receiver in the opposing mold the time where the volume between the sensors is impregnated can be derived. Combining the information the flow front shape and impregnation mechanics can be derived. Not only the arrival time, also the flow speed and even its direction can be derived by further signal processing. The degree of cure can be derived from measuring the sound velocity, which depends on the part’s elastic modulus. Any increase in elasticity hence leads to an increase in sound velocity. For high degrees of cure there is a linear relation to the sound velocity. Also the gelation and vitrification can be extracted from characteristic points in the sound velocity curve progression. Finally the laminate thickness can be derived from the time of flight of the sound signal through the part. This is the most challenging parameter to calculate, as the time of flight is not only depending on the thickness, but the current sound velocity. The latter depends on cure as well as the fiber material and volume content. These have to be compensated by calibration and special sensor arrangements.