Flat Composite Panel Impact Testing and Characterization by Ultrasonic Non-Destructive Evaluation

Low velocity impact damage to laminated composite structures can cause a range of damage modes. Of interest is small damage composed of delamination (i.e. separation of plies) that is less than 50 mm in size, with barely visible impact-side damage. Formation of actual impact-induced damage, mainly multiple planes of delamination that are interconnected via matrix cracks, is the main goal of this study, which supports a larger project where injection repair of the damage is being investigated. Three 25-ply composite panel types having varying percentages of 0° fiber content have been low velocity impacted at 10, 15, and 20 Joule energy levels. 81 total impact studies were performed using a pendulum impact system with the composite panels clamped on all edges. Multiplanar internal delamination regions with barely visible impact damage on the impacted surface were created. In addition to measuring the force history during the impacts event, the internal damage is investigated and characterized using ultrasonic pulse-echo scanning methods. The perimeter of damaged regions is determined using gel-coupled hand-mapped A-scan methods, while an extensive investigation of the multiple planes of delamination damage is investigated using C-scanning methods where the specimen is immersed in de-ionized water. Results show that the panel type with the least amount of 0° plies has the largest internal delamination damage, but this panel type also requires the highest impact force for damage to occur. In addition to impact-side C-scans, nine specimens are back side C-scanned to investigate the pulse-echo shadowing effect which commonly hides lower-depth delaminations