Alireza Forghani

A structural modelling framework for prediction of damage development and failure of composite laminates

This article presents the latest developments of a constitutive modelling framework, CODAM (COmposite DAmage Model), for predicting the non-linear in-plane response of composite laminates using continuum damage mechanics. The methodology is best suited for non-linear structural analysis of large-scale laminated composites whose boundaries do not interfere/interact with the damage zone that develops and grows within the structure. The new development presented here, CODAM2, addresses the deficiencies in both the numerical and material objectivity of the original version of CODAM. While the previous CODAM formulation was essentially a local smeared crack model that was augmented with crack band scaling to overcome one aspect of the numerical objectivity, namely the mesh-sensitivity, CODAM2 introduces a non-local regularisation scheme to alleviate both the spurious mesh dependency and mesh orientation problems that plague all local strain-softening models. Two of the 13 test cases, provided in the third-world wide failure exercise, which were related to the in-plane tensile and compressive loading of open hole specimens, were used in order to demonstrate the effectiveness of CODAM2 in predicting the damage development and the corresponding overall response in such structural loading configurations.

Computational Modeling of Damage Development in Composite Laminates Subjected to Transverse Dynamic Loading

This paper presents a robust computational model for the response of composite laminates to high intensity transverse dynamic loading emanating from local impact by a projectile and distributed pressure pulse due to a blast. Delaminations are modeled using a cohesive type tie-break interface introduced between sublaminates while intralaminar damage mechanisms within the sublaminates are captured in a smeared manner using a strain-softening plastic-damage model. In the latter case, a nonlocal regularization scheme is used to address the spurious mesh dependency and mesh-orientation problems that occur with all local strain-softening type constitutive models. The results for the predicted damage patterns using the nonlocal approach are encouraging and qualitatively agree with the experimental observations. The predictive performance of the proposed numerical model is assessed through comparisons with available instrumented impact test results on a class of carbon-fiber reinforced polymer (CFRP) composite laminates. Force-time histories and other derived cross-plots such as the force versus projectile displacement and progression of projectile energy loss as a function of time are compared with available experimental results to demonstrate the efficacy of the model in capturing the details of the dynamic response. Another case study involving the blast loading of CFRP composite laminates is used to further highlight the capability of the proposed model in simulating the global structural response of composite laminates subjected to distributed pressure pulses.

Progressive Damage Modeling of Composite Materials Under Both Tensile and Compressive Loading Regimes

A constitutive model is presented for the complete in-plane response of composite materials within the framework of a previously developed continuum damage mechanics model, CODAM. While the previous CODAM formulation was primarily developed to simulate the progression of damage under tensile loading, the proposed extension is guided by a mechanical analogue model that accounts for the initiation and propagation of damage mechanisms under both tension and compression. Calibration of the tensile damage parameters of the model using the over-height compact tension test (OCT) is presented. Simulations of notched panels under quasi-static in-plane tension and compression loading are used to demonstrate the effectiveness of the model in predicting the load-displacement response as well as the overall damage zone size. Finally, limitations of local smeared crack models are discussed and the preliminary results of a non-local approach to simulating damage progression that overcomes such limitations are presented.

Multiscale characterization and representation of composite materials during processing

Given the importance of residual stresses and dimensional changes in composites manufacturing, process simulation has been the focus of many studies in recent years. Consequently, various constitutive models and simulation approaches have been developed and implemented for composites process simulation. In this paper, various constitutive models, ranging from elastic to nonlinear viscoelastic; and simulation approaches ranging from separated flow/solid phases to multiscale integrated phases are presented and their applicability for process simulation is discussed. Attention has been paid to practical aspects of the problem where the complexity of the model coupled with the complexity and size scaling of the structure increases the characterization and simulation costs. Two specific approaches and their application are presented in detail: the pseudo-viscoelastic cure hardening instantaneously linear elastic (CHILE) and linear viscoelastic (VE). It is shown that CHILE can predict the residual stress formation in simple cure cycles such as the one-hold cycle for HEXCEL AS4/8552 where the material does not devitrify during processing. It is also shown that using this simple approach, the cure cycle can be modified to lower the residual stress level and therefore increase the mechanical performance of the composite laminate. For a more complex cure cycle where the material is devitrified during a post-cure, it is shown that a more complex model such as VE is required. This article is part of the themed issue ‘Multiscale modelling of the structural integrity of composite materials’.

An overview of continuum damage models used to simulate intralaminar failure mechanisms in advanced composite materials

The literature abounds with models for progressive damage and fracture behaviour of composite materials at various scales of resolution. We present a brief review of these models with an emphasis on continuum approaches used for modelling the intralaminar damage mechanisms in laminated composites. We also address challenges involved in experimental calibration and numerical implementation of such models.

Simulating Prepreg Tack in AFP Process

Wrinkles, puckers and fiber bridging are among the major issues encountered in Automated Fiber Placement (AFP) process. These defects are all different manifestations of fiber misalignment. Residual stresses introduced in the tow during the deposition stage by the AFP head are the main driver for these defects. Tack between the deposited tape and the substrate is the resisting force against formation of such defects. Tack is a very complex phenomenon that is influenced by a variety of process parameters including temperature, head pressure and speed, as well as prepreg aging, moisture content and surface condition. A physics-based modelling framework for simulation of tack is developed in this study that allows for prediction of tack response. A model is also developed to simulate the deposition of the prepreg tape by the AFP head. It is shown that the model is capable of predicting AFP-induced puckers.

Simulation of Gas and Resin Transport Mechanisms in Manufacturing Process of Composite Structures and Their Effect on Porosity

Porosity, as a manufacturing process-induced defect, highly affects the mechanical properties of cured composites. There are several contributing factors to the formation of porosity in composite structures, most notably entrapped air, bag leaks, and volatiles or off gassing. These sources highlight the multi-scale and multi-physics nature of the formation of porosity which make it such a challenging problem to solve. Understanding how these mechanisms contribute to the formation of porosity becomes a key tool in minimizing their negative effect. Experimental evidences show that local resin pressure history plays a major role in void growth during a cure cycle. Local reductions in resin pressure due to factors such as geometric features or cure shrinkage can lead to growth of existing voids and increase the likelihood of porosity becoming locked into the final part structure. Prediction of local resin pressure has been the focus of this approach to modeling the cure of composite structures. This study is focused on developing an efficient FE modeling framework to simulate the resin pressure and porosity (bubble) formation in a composite part. The case study presented here features a laminate with ply drop-off subjected to compaction pressure via a caul plate.