Impact Damage Imaging Using Non-contact ACT/LDV System

Abstract

The aim of this study is to investigate the capability of the zero-lag crosscorrelation (ZLCC) imaging condition of an A0 Lamb wave mode in imaging a barely visible impact damage (BVID) in a carbon fiber reinforced plastic (CFRP) using a fully non-contact Lamb wave based non-destructive inspection (NDI) system. A 16- ply [45/0/-45/90]2S CFRP laminate was impacted at three different locations with different impact energies using a drop ball at three drop heights causing three BVIDs with different dimensions, ~7 mm, ~12 mm and ~17.5 mm in diameter. The A0 Lamb wave mode is generated inside the laminate using a circular air-coupled transducer (ACT) and detected along the damaged region using a laser Doppler vibrometer (LDV). The measured wavefield is then decomposed into a forward and backward propagating wavefields by applying a frequency-wavenumber filtering postprocessing technique. The decomposed wavefields are then cross-correlated in the frequency domain using ZLCC method producing a detailed cumulative damage image. The experimental images highlight the three damaged areas with higher ZLCC values compared to other parts of the inspected areas. However, it was found that depending on the damage size and if delaminations are happening all around the impacted spot, the delamination boundary on the right side of the impact is difficult to detect if the generated wavefield is traveling from left to right which interacts with the delaminations of the left side and vice versa. In this case, the region of interest around one of the damages (12 mm) was scanned twice with the excitation being to the left and then to the right of it and overlapping the images have given a good approximation of the actual damaged area. The experimental investigation has shown a good correlation between the ZLCC imaging condition and C-scan images, which demonstrate a strong capability of guided wave ZLCC imaging condition technique in approximating the location and size of relatively small BVIDs in thin composite structures. The presence of three separate delaminations in a 2.4 mm thick CFRP laminate impacted by 13.56, 19, and 27 Joule drop ball energies were detected in a fully non-contact ultrasonic technique. doi: 10.12783/SHM2015/319