Impact damage detection and degradation monitoring of wet GFRP composites using noncontact ultrasonics

Two different non‐crimp glass fabrics with a polyester resin were used to produce laminated plates that were subjected to low velocity impact testing using three impact energy levels. The plates were immersed in water at 65°C for up to 24 months. The effectiveness of a traditional water coupled and an air‐coupled ultrasonic C‐Scan system was assessed in terms of damage size evaluation at various time intervals. The conditioned impacted plates were retested statically in compression to determine the residual strength for evaluation of damage tolerance. Weight change measurements revealed an initial increase due to water diffusion, followed by an extended decrease due to matrix dissolution at long‐term immersion times. The use of water coupled pulse‐echo ultrasonics proved ineffective after long‐term water immersion as damaged areas became ultrasound‐invisible. The contrast between impact damaged areas and water diffused areas was restored with the air‐coupled C‐scan. The macroscopic damage size was not affected by the long‐term water immersion and the overall weight change while the residual compression strength was seemed to be dependent on the time of immersion and the size of the pre‐existing impact damage. Calibrating the air‐coupled system to a dry condition specimen, a good qualitative and quantitative indication of the degraded state of water immersed plates was obtained. This monitoring system for the degradation process seems to be very promising. POLYM. COMPOS., 2009.