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An internal project at BAM (Bundesanstalt für Materialforschung und -prüfung) pursue the goal to develop methods and models to investigate ageing of CFRP (carbon fiber reinforced plastic) composites pressure vessels. One main challenge is the application of appropriate testing methods for detecting the influences of pressure cycles and creep behavior on the material. This contribution presents results we achieved using conventional eddy current testing (ET) and high frequency ET. Before investigating pressure vessels, we studied test samples with artifcial defects. Here, we could show that also conventional ET with low frequencies are useable for CFRP samples which have at bottom holes of different size and depth. On the basis of the received results we deployed conventional ET besides high-frequency applications in order to investigate pressure vessels of different material combinations like steel-CFRP,
Aluminum-CFRP and synthetics-CFRP. The thickness of the CFRP varies from 4 mm up to 8 mm. Here, we could detect both defects in the metal liner and in the CFRP.
The use of composite pressure vessels instead of conventional vessels made of steel or aluminum grew strongly over the last decade. The reason for this trend is the tremendous weight saving in case of composite vessels. However, the long-time behavior is not fully understood for filling and discharging cycles and creep strength and their influence on the CFRP coating (carbon fiber reinforced plastics) and the internal liner (steel, aluminum, or plastics) [1]. The CFRP ensures the pressure resistance while the inner liner is used as container for liquid or gas. To overcome the missing knowledge of ageing BAM started an internal project to investigate degradation of these material systems [2]. Therefore, applicable testing methods are needed. Normally, high-frequency eddy current testing (HF-ET, f > 10 MHz) is deployed for CFRP due to its low conductivity of the fiber, which is in the order of 0.01 MS/s, and the capacitive coupling between the fibers. Nevertheless, in some cases conventional ET can be applied. We show a concise summary of studies on the application of conventional ET of composite pressure vessels.
The application of composite pressure vessels is of great interest due to tremendous weight saving. Nevertheless, ageing of CFRP (carbon fiber reinforced plastic) composites pressure vessels underlies complex interactions between metallic liner and composite and is not fully understood yet. One main challenge is the application of appropriate testing methods for detecting the influences of pressure cycles and creep behavior on the material. This contribution presents results achieved by using conventional eddy current testing (ET) and high frequency ET. Here, we could show that also conventional ET with low frequencies are useable to investigate pressure vessels of different material combinations like aluminum-CFRP and synthetics-CFRP.