Extracting Damage Mode and Depth in Composite Panels Through Measured Wave Characteristics Using a Novel Mini-Impactor Excitation Source

Abstract

Ultrasonic guided wave (UGW) is an established nondestructive evaluation method capable of assessing a wide area of the structure. A novel composite mini-impactor was used as a broadband excitation source for the UGW damage characterization of composite plates. Unlike the traditional impact hammers for composite structures which have an excitation frequency below 30 kHz, the unidirectional carbon/epoxy mini-impactor can excite between a range of 30-500 kHz. The excitation source was utilized to conduct basic studies on the effect of single-mode damage states such as: delamination, matrix crack, and fiber breakage. The single-mode damage states in AS4/977-3 prepreg composite plates are manufactured during layup by incorporating release film strips for delamination, and perpendicular cuts to create matrix or fiber breaks. Broadband air-coupled transducers (BATS) were used to measure the UGW structural response to the mini-impactor excitation through a pitch-catch setup. Two reception transducers are placed before and after the damage on the wave propagation path to assess the true structural transfer function (STF) through deconvolution between the signal output of both receivers. Comparisons of measured wave characteristics between pristine and damage types are conducted using extraction of damage-indicative signal features and by calculating Difference Signals. Additionally, double-sided UGW interrogations were conducted on the composite plates having non-centered flaws to determine different damage depths. Damage type and depth were distinguished based on STF statistical features and Difference Signals. STF of Damaged Signals farther from the surface of excitation were not indicative of damage but were identifiable through Difference Signals. Signals indicating fiber breakage produce a strong reduction in amplitude when breakage is closer to the surface of excitation.