A Multiscale Nonlocal Progressive Damage Model for Composite Materials

In this paper, the advantages of a nonlocal progressive damage formulation are described and demonstrated. An approximation of the nonlocal formulation was implemented coupled with the MAT162 composite damage model as a User defined material model in the LS DYNA environment. A comparison of the local model and the nonlocal model is simulated for an 8-ply laminate under tension is carried for increasing mesh densities. The results show the regularization achieved by nonlocal models by providing mesh independent results

Progressive Failure Analysis of composite Materials using the Puck Failure Criteria

Fiber reinforced composites have been used in various engineering structures and applications especially in naval, automotive, aeronautical and sports industries. These composite materials generally exhibit brittle damage behavior. The anisotropy in the material and different kinds of failure mechanisms make it difficult to accurately characterize the behavior of composite materials. The present work aims to verify and apply the Puck Failure Criteria using the commercially available finite element package ABAQUS by writing a user-material subroutine in FORTRAN. The model is implemented with different post failure degradation schemes. In the present work, the progressive failure on composite materials in analyzed using the Puck failure criteria to detect damage initiation. The ABAQUS user defined material subroutine UMAT was developed to apply the failure criteria and degradation models. The progressive failure analysis of a single lamina of a composite material is carried out on an open hole specimen under uniaxial tension. A partial discount method and a gradual stiffness degradation method is implemented and the results using these degradation models are compared. The damage initiation and progression obtained from the proposed model is compared with the observed experimental results and the digital image correlation data. This model was then used for the progressive failure analysis of a composite laminate with a central hole loaded in inplane tension with different stacking sequences and compared with the results obtained from literature. From the results, it can be seen that the Puck failure hypothesis is a robust and versatile criteria which can be used for the progressive failure analysis of continuous fiber unidirectional composite laminates

Studies of Damage Tolerance in Automated Fiber Placement Based Heterogeneous Meso-Architectured Carbon/ Epoxy Composite Laminates.

Traditional unidirectional (UD) carbon fiber-reinforced polymer matrix composites manufactured via automated fiber placement (AFP) are widely used in aerospace structures. While heterogeneous at the micro-scale, these materials are homogeneous at the meso-scale (ply-scale). The major limitation is its limited toughness, poor damage tolerance/impact resistance capability and the inability to sufficiently redistribute stresses, resulting in strength-toughness tradeoffs, making them susceptible to impact damage. From previous studies, there is an understanding that fiber architecture has a first order effect on the damage tolerance. Introducing new discontinuous/heterogeneous meso-scale architectures, through an optimal merger of material and structure, can result in unprecedented property improvements due to multiple interacting deformation mechanisms as demonstrated in biological/biomimetic architected materials. Interestingly, controlling/inducing multiple deformation/failure mechanisms via spatial distribution of in-plane anisotropy (i.e., heterogeneity in fiber orientations) and boundaries / interfaces is largely unexplored. In recent years, a specialized AFP layup procedure was introduced to mimic a woven-like architecture using UD prepreg tows. In this modified procedure, tows are intentionally skipped during placement leaving gaps. These gaps are filled in the subsequent passes to produce pseudo-woven meso-architectured composites (MAC). The unique architecture is macroscopically heterogeneous with discontinuous and spatially varying fiber orientations both in-plane and through thickness resulting in multiple interfaces and an expanded design space. This dissertation explores the incorporation of MAC laminates manufactured using AFP to enhance the damage tolerance of unidirectional carbon fiber reinforced polymer composites. An extensive experimental study has been carried out on hybrid carbon fiber reinforced laminates (consisting of MAC and traditional UD sub-laminates) compared against a baseline traditional unidirectional quasi-isotropic control laminate under high velocity impacts of 250-400ft/s, low velocity impacts of 15-55 J, open hole tension and open hole compression. C-scans and digital image correlation techniques are employed to better understand the experimental response. The hybrid MAC laminates demonstrate a significant 45% reduction in back-face surface damage, 19.5% less back face deflection and an 18% increase in penetration velocity when compared to quasi-isotropic control laminate under high velocity impacts. Under a low velocity impact, the hybrid laminate configurations exhibit increased damage resistance with up to 37% higher critical delamination load and increased damage tolerance with 26% higher residual compressive strength after an impact of 55 J compared to control laminate. Additionally, the hybrid MAC laminates demonstrate a 7% increase in OHT strength and up to a 16% reduction in strains near the open hole while the OHC response is found to be similar. To better understand the operating mechanisms in MACs, a python script is developed to mimic the AFP layup procedure to determine the spatial variations in the fiber angle and stacking sequence and determine the representative unit cell. Furthermore, finite element models are developed to better understand the damage propagation mechanisms in these complex heterogeneous materials. The studies demonstrate that improved impact performance in MACs is due to crack deflection, damage diffusion and stress redistribution mechanisms induced by the heterogeneous composite architecture and provides insights into the fundamental deformation and failure mechanisms during impact onto these complex

Mode-I behavior of adhesively bonded composite joints at high loading rates

This study investigates the high loading rate behavior of adhesively bonded carbon/epoxy composite joints under mode I loading. A computationally guided experimental setup is developed to study the mode-I behavior of composite joints in the range of quasi-static to high loading rates. A double cantilevered beam specimen with wedge insert type loading setup is used to conduct quasi-static and dynamic experiments. For the dynamic loading, a modified split Hopkinson bar is used to load the sample at high rates. The local deformation field is measured using high Spatio-temporal resolution digital image correlation (DIC). From the experiments, the mode-I energy release rate is calculated from the load, crack extension and crack root rotation data measured using load cell and DIC. A decrease in the initiation fracture toughness with increase in loading rate was observed which is attributed to the strain rate dependent behavior of the epoxy-based film adhesive. For both quasi-static and high loading rates, a mixed adhesive-cohesive failure is observed from the fracture surface analysis

Mode-I behavior of adhesively bonded composite joints at high loading rates

This study investigates the high loading rate behavior of adhesively bonded carbon/epoxy composite joints under mode I loading. A computationally guided experimental setup is developed to study the mode-I behavior of composite joints in the range of quasi-static to high loading rates. A double cantilevered beam specimen with wedge insert type loading setup is used to conduct quasi-static and dynamic experiments. For the dynamic loading, a modified split Hopkinson bar is used to load the sample at high rates. The local deformation field is measured using high Spatio-temporal resolution digital image correlation (DIC). From the experiments, the mode-I energy release rate is calculated from the load, crack extension and crack root rotation data measured using load cell and DIC. A decrease in the initiation fracture toughness with increase in loading rate was observed which is attributed to the strain rate dependent behavior of the epoxy-based film adhesive. For both quasi-static and high loading rates, a mixed adhesive-cohesive failure is observed from the fracture surface analysis