Hail Impact Testing of Stitched Carbon Fiber Epoxy Composites Laminates

Abstract

The hail impact response of the composite laminates is crucial in providing design guidance on the minimum allowable gauge thickness of the novel Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) composite panels for next-generation aerospace structures. In this work, we performed simulated hail impact tests for stitched and non-stitched flat carbon fiber epoxy composite laminate panels to study their different damage behaviors. All panels consist of a mid-bay area and four stiffening strips representing the aircraft composite structure stiffened with stringers. The tests were conducted using a gas gun and ice ball projectiles with a diameter of 40 mm. Different hail impact speeds (40 - 77 m/s), and hence different impact energies were imposed through tuning the pressure of the gas gun. After the hail impact tests, both visual observation and ultrasonic C-Scan were conducted to observe the surface damage and internal delamination. It was found that the matrix cracking occurs in every composite panel along the edges of the stiffening strip. In addition, delamination is found for both stitched and non-stitched panels in the mid-bay area and stiffening strips. The delamination area in the mid-bay area for the stitched composite panel is smaller than that for the non-stitched composite panel. The delamination in the stiffening strips could be caused by the stress concentration due to the geometric discontinuity between the skin and the stiffening strips. The observed different damage behaviors provided insights into the delamination arrestment capability of the stitched composites.