PhD defense of Mr. Vishwas Divse on Progressive Damage Modeling of Fiber Reinforced Plastic Composites including Drilling-induced Damage 

Abstract:
Fiber-reinforced plastic (FRP) composite laminates are extensively used in aerospace structures for their high specific strength and stiffness. However, the presence of holes for assembly and drilling-induced damage significantly weakens the laminates, leading to premature failure due to stress concentration. Designers and engineers currently lack a comprehensive understanding of damage propagation and limiting strength in FRP laminates with holes and drilling-induced damage. As a result, components made of FRP laminates are often over-designed. To address this issue, the development of reliable progressive damage models (PDMs) that accurately capture damage propagation in FRP laminates has become a critical research focus for the past three decades.

This study focuses on the development of mesoscale progressive damage models (PDMs) based on continuum damage mechanics, incorporating well-established failure criteria such as 2D Hashin, 3D Hashin, Puck, and LaRC05. These PDMs take into account various damage modes, including matrix cracking, matrix crushing, ber breakage, and fiber kinking. Additionally, they incorporate shear non-linearity, in-situ strengths, and mixed mode fracture and employ a numerical search algorithm to determine the kink-band plane and matrix fracture angles. The PDMs were implemented within the Abaqus/explicit finite element framework using the VUMAT subroutines written in FORTRAN. Specifically, finite element method (FEM) based models were employed to de ne pre-existing ber or matrix damage in the subroutines. The PDMs were first verified with single-element and mesh dependency tests. Furthermore, FEM-based models were developed to simulate open-hole tension (OHT) and open-hole compression (OHC) tests that aim at predicting damage propagation and the corresponding limiting strength of the FRP laminates. Lastly, both FEM-based simulations and experiments were conducted to analyze drilling-induced damage and its impact on the OHT strength of FRP laminates.

The study reveals that the stress concentration factor (SCF) substantially increases with an increase in hole size. Interestingly, laminates can be designed with SCF (<3) lower than an open hole in a finite isotropic plate by suitable stacking of on- and off-axis plies. When subjected to tension, an open hole lamina always fails due to matrix cracking along the ber direction. However, laminates demonstrate complex failure interactions involving multiple damage mechanisms. During OHT tests, ply-blocked laminates show around 30% higher strength and fracture strain than non-ply-blocked laminates due to delayed damage propagation. The ply-blocked laminates display reduced sensitivity to hole size, resulting in a 14.3% reduction in OHT strength when the hole size increases from 6 to 9 mm, compared to a 21.14% reduction in the non-ply-blocked laminates. In OHC tests, ber splitting and kinking initiate at regions of maximum in-plane shear stress and propagate due to longitudinal compression and in-plane shear stress. The ultimate failure of 0 degree dominant and quasi-isotropic layup is governed by ber kinking in 0 degree plies, while matrix cracking and delamination in 45 degrees plies contribute to the nal failure of shear dominant layups. Notably, the quasi-isotropic layup was observed to be more sensitive to hole (or notch) size than the other laminates.

Furthermore, it was found that drilling-induced damage increases with feed rate and drill-bit size, while cutting speed has a slight mitigating effect. Drilling-induced damage predominantly propagates along the ber orientation of the exit ply in the laminate. The digital image correlation (DIC) analysis shows heightened strain concentration at the location of drilling-induced damage, resulting in accelerated damage propagation and reduced overall laminate strength. This leads to up to a 15% decrease in open-hole tension (OHT) strength and a 7-11% decrease in open-hole compression (OHC) strength. Drilled laminates experience a significant strength reduction of up to 50% compared to plain laminates due to the combined effect of the hole and drilling-induced damage. Therefore, designing FRP laminates with holes and pre-existing damage requires careful consideration. The proposed models show predictions within a 10-15% deviation from experimental results for OHT tests, OHC tests, and drilling. These models hold the potential for solving engineering problems, especially when extended to account for cyclic loading scenarios.

External Examiner: Prof. Naresh Bhatnagar, IIT Delhi
Internal Examiner: Prof. Dnyanesh Pawaskar 
Chairperson: Prof. Krishna Kaliappan, Chemistry 
 
Supervisor: Prof. Deepak Marla
Co-Supervisor: Prof. Suhas Joshi