Effect of Ply Thickness on Dynamic Damage Progression in Cross-ply Laminates Under Low-velocity Impact

  • Batmaz, Onur Ali (Middle East Technical University Aerospace En)
  • Coker, Demirkan (Middle East Technical University Aerospace En)

In session: - Impact

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Composite materials have been favoured 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 study, we have conducted in-situ LVI experiments on cross-ply CFRP beams having stacking layups [04/904/02]s and [02/902/02/902/02]s using the experiment configuration constructed in [1]. Despite both layups having an equal number of 0° and 90° plies, they differ in terms of how these plies are clustered within the laminate. In LVI experiments, the progression of damage is observed through high-speed photography. In addition to LVI, quasi-static indentation (QSI) experiments are performed to reduce the difficulties in monitoring the damage progression in the short period of impact loading. QSI experiments allow us to have magnified in-situ observations on the visible edge of the beam with a traveling digital microscope. Moreover, these experiments are modelled numerically using the finite element method in the ABAQUS/Explicit platform. To account for fiber breakage and matrix cracking, we employed a three-dimensional continuum damage mechanics approach, implemented through a VUMAT subroutine, that incorporates the maximum stress and LaRC05 damage initiation criteria with a linear softening damage progression. In LVI experiments, we have captured the whole damage progression sequence of matrix crack initiation followed by the delamination propagation, and the different damage mechanisms of the tested layups. QSI experiments revealed the occurrence of diagonally oriented matrix cracks within the lower 90° plies in both layups before delamination initiates. Numerical simulations of these experiments are in agreement with results in terms of failure patterns and sequences. The comparison between numerical simulations and experimental observations allowed us to investigate the influence of ply clustering on the LVI-induced damage mechanisms.