A methodology is presented to directly measure the damage properties and strain softening response of laminated composites by conducting over-height compact tension (OCT) and compact compression (CC) ...tests. Through the use of digital image correlation (DIC) technique, and analysis of the measured surface displacement/strain data, the strain-softening response of composites is constructed. This method leads to a direct determination of the Mode I translaminar fracture properties with the assumption that the shear stress is negligible around the damage zone and the crack growth occurs in the symmetric opening mode. Using this methodology, and by correlating the observed failure mechanisms with the strain-softening curves, the interaction of failure mechanisms leading to the final failure and also the distinction between the tensile and compressive failure mechanisms can be studied. The effectiveness of the method in accurate identification of the damage parameters is demonstrated through sectioning and deplying techniques. As a consistency check and further verification of the method, the obtained strain-softening curves are fed into a numerical damage mechanics model and successfully used to simulate the detailed response of the very same OCT and CC specimens from which the strain-softening curves were extracted.
Porosity is a challenge in virtually all composite processes but in particular in low pressure processes such as out of autoclave processing of prepregs, where the maximum pressure is one atmosphere. ...This paper discusses the physics behind important transport phenomena that control porosity and how we can use our understanding of the underlying science to develop strategies to achieve low porosity for these materials and processes in an industrial setting. A three step approach is outlined that addresses and discusses: gas evacuation of trapped air, volatiles and off-gassing, and resin infiltration of evacuated void space.
Resin cure shrinkage and anisotropic thermal expansion cause process induced residual stresses in polymer composites. When relieved, the residual stresses cause reduction in enclosed angles of ...composite laminates; a phenomenon often called spring-in. Spring-in compromises the dimensional fidelity of composite parts and is often accounted for when designing the tool the part is made on. Spring-in is often estimated using past experience or simple analytical formulas that ignores many process parameters affecting the spring-in.
This paper presents an experimental study that shows that spring-in can be strongly affected by a number of factors such as cure cycle, tool surface, part geometry, and lay-up. The paper also shows that by developing material models that accurately represent the stress transfer between the part and the tool at the tool-part interface, and by implementing a large deformation solution technique, the experimental results observed in this study can be predicted using finite element based process models.
Dimensional control of composite components is critical for cost effective manufacturing of large composite aerospace structures. This paper presents an engineering approach to the prediction of ...process-induced deformations of three-dimensional (3D) autoclaved composite components. A 6-step method that uses a two-dimensional (2D) special purpose finite element (FE) based process simulation code and a standard 3D structural FE code is presented. The approach avoids the need to develop a full 3D process model, significantly reducing the computational effort yet retaining much of the detail required for accurate analysis. The methodology is presented together with numerical examples and two case studies demonstrating the validity, utility, and limitations of the approach.
Fiber misalignment in composites in the form of in-plane waviness and out of-plane wrinkling, is a major defect arising from processes such as hot drape forming. This work studies the micro-level ...mechanisms of forming in-plane fiber waviness and out-of-plane wrinkling. An out-of-autoclave unidirectional prepreg system by SOLVAY (CYTEC), CYCOM 5320/T650-35 was used to conduct multiple forming experiments and study the effects of various parameters including forming temperate and lay-up sequence. The effect of partial impregnation, or ‘Engineered Vacuum Channels’, in out-of-autoclave prepreg systems on the intra-ply separation/slippage and consequently fiber misalignment was studied and found to be a significant contributor to micro-level mechanisms. Cross sectioning and microscopy of the parts and examination of the end termination profiles were used to analyze the effect of forming parameters on ply end-shortening and consequently fiber misalignment.
A method to measure the fibre bed compaction curve directly from composite prepreg is presented. The method was used to measure the compaction curve of unidirectional and quasi-isotropic AS4/3501-6 ...carbon–epoxy prepregs. Similar compaction curves were obtained in all cases. The compaction curve obtained was used by a finite element process model, COMPRO, to simulate the uniaxial compaction of 8 and 16 ply laminates at different temperatures. The force–displacement response predicted by the model closely matched the experimental results. The method which can be used on both tape and fabric prepregs, has the major advantages of being a direct measure of the prepreg behaviour, and requires no special preparation of the sample.
This article demonstrates the use of response sensitivities in numerical simulation of composite processing via four different application examples: real-time result validation, model calibration, ...reliability analysis, and optimization. The analyses are carried out with integrated simulation software with new response sensitivity capabilities. Notably, the response sensitivities are computed by the direct differentiation method. This is an efficient and accurate alternative to the finite difference approaches. A brief review of the derivation and implementation of sensitivity equations is provided. The primary objective of this article is to demonstrate and promote a suite of techniques to incorporate uncertainties into the simulation of the composite manufacturing process, facilitated by efficient sensitivity computations.
A computationally efficient modeling approach for the accurate evaluation of process-induced deformations and residual stresses in composite parts is presented. A family of refined one-dimensional ...kinematic models, developed in the framework of the Carrera Unified Formulation, has been used to predict the accurate through-thickness deformation of layered structures during the manufacturing process. The composite material curing phase has been simulated exploiting the capabilities of the software RAVEN. A cure hardening instantaneously linear elastic model has been used. A benchmark based on an L-shaped component has been selected to compare the results obtained using different computational approaches. A closed-form solution, the present refined one-dimensional models and classical solid models, have been considered. The effects of the modeling approach on the prediction of the spring-in angle and on the residual stress field have been evaluated and discussed. The results demonstrate that the use of refined kinematic models can lead to a high-fidelity description of the problem and a quasi-3D accuracy while reducing the computational cost with respect to classical FEM approaches. The through-thickness effects have been predicted with a high level of accuracy and the use of layer-wise models has led to an accurate description of the stress field, including the transverse shear stresses.
The early impact behaviour of single and multi-ply Kevlar
® 129 fabric armour systems is investigated using an explicit finite element code, TEXIM, developed in-house. This numerical model is ...carefully validated using continuous temporal data obtained from an instrumented experimental setup. The model is then used to explore the loss in ballistic efficiency of woven fabric targets, as experienced early in the impact event, when either the number of layers in the panel or the yarn denier is increased.
In this article we derive, implement, and verify equations to compute the sensitivity of responses from numerical simulation of composites manufacturing. The responses considered are part temperature ...and degree of cure, as well as process-induced deformation of the cured part. The `direct differentiation method' (DDM) is used, which entails a one-time investment of effort to differentiate the governing response equations analytically. The implementation of the derivative equations facilitates efficient and accurate computation of response sensitivities in all subsequent analyses. This article extends the DDM methodology developed earlier for mechanical problems. Novel `shape sensitivity' equations and efficient implementation techniques are also included. In order to verify the implementations, the model predictions are compared with those obtained from the less efficient finite difference approach. A comprehensive example is presented where the usefulness and interpretation of response sensitivities are emphasized. It is observed that the responses are particularly sensitive to certain model parameters, for which further data gathering and model improvement efforts should be focused.
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