Sandwich structure shows high vulnerability at low speed impact, such as hail impact. Several non-destructive health monitoring methods exist, but not many practical methods can detect damage in real time. In this experiment, time domain reflectometry, TDR, is used to detect impact damages on sandwich structure. Time domain reflectometer sends a pulse of signal that travels through electrical pathway and measures reflection that caused by any impedance change. TDR typically used for detecting faulty wire. As long as signal can pass through and electrical pathway is not discontinued, then damages along the pathways can be detected in real-time. This application has practical real-time measurement capability.

In this experiment, specially designed sandwich structure, which is composited of fiber glass faceplate and aluminum honeycomb core, is prepared to detect low speed impact test. Faceplates of sandwich structure have embedded sensor, series of very thin copper lines or electrical pathways, which were carved out of copper plated layer on faceplate. Sandwich structure is damaged by drop tower with 3, 6, 9 and 20 J of impact energy to simulate hail damage. Time domain reflectometer is able to detect any impedance change on sandwich structure.

TDR were able to detect damage location within 7% of error when distance between damages was normalized to total length. Degree of impact was identifiable, as long as impacted area on sensor was similar. If impact energy were similar, full damage or partial damage relative to sensor were also identified. Within one sensor, varying impact energies were applied and all impact energy and locations were identified.