SQUID NDE on Braided Carbon Fiber Reinforced Polymers With Middle-End Fibers Under Step-by-Step Tensile Loading

Three braided carbon fiber reinforced polymer (CFRP) samples with different middle-end fibers were fabricated. Damage mechanisms of the samples under step-by-step tensile loading were investigated using a nondestructive evaluation (NDE) method utilizing a high-temperature superconducting SQUID gradiometer. Commercial carbon fibers UTS50 were used for base braided fabrics, while UTS50, XN05 with higher electric resistivity, and XN60 with lower resistivity were used for the middle-end fibers. In the step-by-step tensile tests, observation of surfaces and measurements of stress-strain curves of the samples were carried out. At certain damage stages, the SQUID NDE method, which visualizes flowing currents in the samples, were applied to detect damage and assess the integrity of the carbon fiber yarns in the samples after removal of the loading. As a result, different damage mechanisms and stress-strain curves were observed and measured. From the results using the SQUID NDE method, it was shown that the current distributions in the samples were determined by the middle-end fibers at respective virgin stages, and the middle-end fibers XN60 were broken in some parts of the sample at its final fracture.