Navid Zobeiry

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.

A Constitutive Model for Progressive Compressive Failure of Composites

A continuum damage mechanics model previously developed to model the tensile response of composites has been further enhanced to simulate their nonlinear constitutive behavior under compressive loading. The refinements were based on the behavior of an analog model, which was constructed to represent the complete force—displacement response of a representative volume element of the material. The updated constitutive model accounts for mechanisms considered to be characteristic of compressive damage growth in laminated composites, such as matrix cracking, fiber kinking, and delamination. The constitutive model was implemented in the commercial explicit finite element code, LS-DYNA, and is used here to predict the quasi-static compressive response of open-hole laminates as well as the dynamic axial crushing of braided composite tubes. It is shown that the model adequately captures: (1) the damage growth and local strain fields in open-hole specimens, and (2) the failure characteristics and energy absorption of braided composite tubes. This investigation demonstrated that the new model is capable of predicting the experimentally observed compressive response in these two rather different applications, and that it potentially offers an effective means of simulating the nonlinear damaging behavior of composite materials under a variety of loading conditions.

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’.

2.3 Autoclave Processing

Autoclave processing is one of the oldest and most widely used processing methods for high-performance composite materials. This chapter addresses important fundamentals of this processing method including autoclave equipment, gas flow and heat transfer, and aspects of autoclave design. Cure and property evolution of thermoset composite materials are discussed including polymerization, cure kinetics, glass transition temperature, liquid, rubbery and glassy behavior of the material, as well as material modeling and process simulation. The chapter ends with examples of how fundamental understanding of gas flow, heat transfer, and material property evolution can be used in the context of a process model to optimize processes with respect to the thermal response. Process simulation is used to evaluate the effect of autoclave equipment, tooling, part and material on thermal lag and exotherms in the process. Defects and mitigation strategies are also discussed.

A comprehensive multi-scale analytical modelling framework for predicting the mechanical properties of strand-based composites

A multi-scale modelling framework was developed for predicting the mechanical properties of strand-based wood composites. This framework is based on closed-form analytical models at three different resolution levels; micro-, meso- and macro-mechanical. A preprocessing step was performed to provide the input data for the three main modelling steps in this framework. Finite element-based mechanical analyses were employed to verify the accuracy of the analytical models developed for the first two steps. The predictive capability of the entire framework was validated using a set of experimental data reported in the literature. Although the methodology presented is general, it has specifically been applied here to predict an important structural property (modulus of elasticity, MOE) of a special strand-based wood composite product, namely, oriented strand board (OSB). The MOE predictions of OSB panels showed reasonable agreement with the available experimental data, thus providing confidence in the practical utility of this easy-to-use and efficient analytical modelling tool for predicting the properties of wood composites employed in structural members.

An infrared thermography-based method for the evaluation of the thermal response of tooling for composites manufacturing

The manufacture of large complex aircraft structures made of advanced composites is done by heating the parts on complex, thermally massive tools using convective heating inside autoclaves. In recent years, numerical simulation of the process has shown great value, but lack of knowledge of the convective heat transfer boundary conditions remains a major obstacle to widespread adoption. An infrared thermography method is presented, suitable for evaluating the thermal response of these processing conditions. The method is based on increasing the emissivity of a tool surface with a painted vacuum bag before thermal imaging. Accurate readings with an average temperature difference of 1.1℃ compared to thermocouple data were achieved. The benefit of the thermography method is the highly detailed surface temperature map. Three tools with very similar geometries but made of Invar, aluminum and carbon fibre composite, respectively, were tested, and results interpreted using analytical solutions for the different tooling feature and convective boundary condition combinations. Analytical simulations, which were validated by comparison to numerical models, explain well the effect of autoclave airflow, tooling material and sub-structure variation on the temperature profiles measured by this infrared thermography method.

Composite material evaluation at cryogenic temperatures for applications in space-based far-infrared astronomical instrumentation

Over half of the light incident on the Earth from the Universe falls within the Far-Infrared (FIR) region of the spectrum. Due to the deleterious effects of the Earths atmosphere and instrument self-emission, astronomical measurements in the FIR require space-borne instrumentation operating at cryogenic temperatures. These instruments place stringent constraints on the mechanical and thermal properties of the support structures at low temperatures. With high stiffness, tensile strength, strength-to-mass ratio, and extremely low thermal conductivity, carbon fibre reinforced polymers (CFRPs) are an important material for aerospace and FIR astronomical applications, however, little is known about their properties at cryogenic temperatures. We have developed a test facility for exploring CFRP properties down to 4 K. We present results from our ongoing study in which we compare and contrast the performance of CFRP samples using different materials, and multiple layup configurations. Current results include an evaluation of a cryostat dedicated for materials testing and a custom cryogenic metrology system, and preliminary cryogenic thermal expansion measurements. The goal of this research is to explore the feasibility of making CFRP-based, lightweight, cryogenic astronomical instruments.