IN-SITU DAMAGE PROGRESSION OBSERVATIONS IN CROSS-PLY CFRP COMPOSITE BEAMS UNDER LOW-VELOCITY IMPACT AND QUASI-STATIC INDENTATION LOADING

In this work, we conducted in-situ low-velocity impact (LVI) and quasi-static indentation (QSI) experiments on CFRP composite beams with two different layup configurations, [0(4)/90(4)/0(2)]s and [0(2)/90(2)/0(2)/90(2)/0(2)]s, to investigate the ply clustering effects on the LVI-induced damage mechanisms. The LVI experiments were conducted utilizing high-speed photography to observe the complete sequence of damage progression. We performed QSI experiments to qualitatively identify the damage mechanisms observed during LVI events since it reduces the complications in monitoring the damage progression in the short period of impact loading. QSI experiments allowed us to obtain magnified in-situ observations on one edge of the beam with a traveling digital microscope, while a high-resolution camera was employed on the opposite edge to measure full-field strain distributions using the digital image correlation (DIC) method. We observed nearly identical damages in terms of form, location, and sequence on the specimens with identical layup configurations in both LVI and QSI experiments. However, damage mechanisms differed in the [0(4)/90(4)/0(2)]s and [0(2)/90(2)/0(2)/90(2)/0(2)]s configurations, with the latter experiencing fiber breakage in addition to matrix cracking and delamination as observed in the former configuration. The magnified in-situ observations allowed us to detect macroscopically undetectable matrix cracks that did not significantly affect the global response of the beam. The continuous monitoring of the micro-scale damages in conjunction with the DIC analyses illuminated the details of damage mechanisms observed in cross-ply laminates under LVI loading.