Session: 12-10-03: General: Mechanics of Solids, Structures and Fluids

Paper Number: 112520

112520 - In-Situ Damage Progression Observations in Cross-Ply CFRP Composite Beams Under Low-Velocity Impact and Quasi-Static Indentation Loading 

Composite materials have been favored in modern aerospace, renewable energy, and transportation engineering applications, particularly for having exceptionally high strength-to-weight ratios. Still, their relatively weak interfacial characteristics make them vulnerable to out-of-plane loadings such as low-velocity impact (LVI) events. LVI events may cause internal failures in the form of matrix cracking and delamination that lead to a considerable loss in strength and stiffness of the structures. Therefore, understanding the physics behind the impact-induced damage mechanisms remains vital to design damage-tolerant composite structures, especially for loadings that cause barely visible damage on the outer surfaces. In this work, we have conducted in-situ LVI experiments on cross-ply CFRP beams having stacking layups [0₄/90₄/0₂]s and [(0₂/90₂)₂/0₂]s by utilizing high-speed photography to observe the whole sequence of damage progression in conjunction with digital image correlation (DIC) method for full-field strain measurements. Since the global behavior and damage characteristics of CFRP laminates are similar under LVI and quasi-static indentation (QSI) loading, we have performed QSI experiments in addition to LVI to reduce the difficulties in monitoring the damage progression in the short period of impact loading [1]. QSI experiments allow us to have magnified in-situ observations on one free edge of the beam with a traveling digital microscope, while a camera is employed on the opposite edge to either observe damage sequence or obtain full-field strain distributions using the DIC method. After the tests, the post-mortem damage examinations for all specimens are utilized with the microscope. In both LVI and QSI experiments, we captured the matrix crack initiation followed by the delamination damage at the adjacent interfaces of plies with different fiber orientations. Diagonally oriented matrix cracks are first observed inside the lower plies in both layups, yet the differences in the damage form arising from the ply clustering approaches of tested specimens are addressed. After matrix cracks trigger the delamination damage and delamination completes its dynamic propagation, additional matrix cracks are initiated in the upper 90° plies successively. This matrix cracking-induced delamination sequence continues to occur in the upper plies depending on the transferred energy in LVI or applied displacement in QSI experiments. Moreover, employing a digital microscope during QSI experiments led to the observation of macroscopically undetectable matrix cracks that have no considerable effect on the global response of the beam. Several of these matrix cracks developed before the delamination damage, yet it is observed that the delamination initiated at the previously formed crack. The remaining cracks in the immediate vicinity disappeared as the compressive stress waves emanating from the delamination led to crack closure. The contribution of this work arises from the fact that experiments are conducted on beam specimens under line-loading setup configuration, which was constructed in Bozkurt and Coker’s study [2], that allowed us to make in-situ observations rather than a sequential application of displacement increments in literature where specimens are loaded/unloaded in each damage monitoring intervals. The continuous monitoring of the micro-scale damages in conjunction with the DIC analyses illuminates the details of matrix damage mechanisms seen in cross-ply laminates under LVI loading.

Presenting Author: Onur Ali Batmaz Middle East Technical University

Presenting Author Biography: Onur Ali Batmaz is currently a Master of Science student in the Aerospace Engineering department at Middle East Technical University (METU). He obtained his bachelor's degree from the same department in 2019. After completing his undergraduate studies, he worked as a design engineer in a leading aerospace company in Turkey for one and a half years. Subsequently, he returned to METU to pursue a master of science degree, where his research primarily focuses on the experimental and numerical investigation of impact-induced damage mechanics in composite structures under the supervision of Prof. Dr. Demirkan Coker.

Authors:

Onur Ali Batmaz Middle East Technical University
Demirkan Coker Middle East Technical University