Ba Nghiep Nguyen

Fiber Length and Orientation in Long-Fiber Injection-Molded Thermoplastics. Part I: Modeling of Microstructure and Elastic Properties

This article develops a methodology to predict the elastic properties of long-fiber injection-molded thermoplastics (LFTs). The corrected experimental fiber length distribution and the predicted and experimental orientation distributions were used in modeling to compute the elastic properties of the composite. First, from the fiber length distribution (FLD) data in terms of number of fibers versus fiber length, the probability density functions were built and used in the computation. The two-parameter Weibulls distribution was also used to represent the actual FLD. Next, the Mori—Tanaka model that employs the Eshelbys equivalent inclusion method was applied to calculate the stiffness matrix of the aligned fiber composite containing the established FLD. The stiffness of the actual as-formed composite was then determined from the stiffness of the computed aligned fiber composite that was averaged over all possible orientations using the orientation averaging method. The methodology to predict the elastic properties of LFTs was validated via experimental verification of the longitudinal and transverse moduli determined for long glass fiber injection-molded polypropylene specimens. Finally, a sensitivity analysis was conducted to determine the effect of a variation of FLD on the composite elastic properties. Our analysis shows that it is essential to obtain an accurate fiber orientation distribution and a realistic fiber length distribution to accurately predict the composite properties.

A Mechanistic Approach to Damage in Short-Fiber Composites Based on Micromechanical and Continuum Damage Mechanics Descriptions

A micro-macro mechanistic approach to matrix cracking in randomly oriented short-fiber composites is developed in this paper. At the micro-scale, the virgin and reduced elastic properties of the reference aligned fiber composite are determined using micromechanical models [Proc. Roy Soc. Lond. A241 (1957) 376; Acta Metall. 21 (1973) 571; Mech. Mater. 2 (1983) 123], and are then distributed over all possible orientations in order to compute the stiffness of the random fiber composite containing random matrix microcracks. After that the macroscopic response is obtained by means of a continuum damage mechanics formulation, which extends the thermodynamics based approach in [Comp. Sci. Technol. 46 (1993) 29] to randomly oriented short-fiber composites. Damage accumulations leading to initiation and propagation of a macroscopic crack are modeled using a vanishing element technique. The model is validated against the published experimental data and results [Comp. Sci. Technol 55 (1995) 171]. Finally, its practical application is illustrated through the damage analysis of a random glass/epoxy composite plate containing a central hole and under tensile loading.

Prediction of the Elastic—Plastic Stress/Strain Response for Injection-Molded Long-Fiber Thermoplastics

This article proposes a model to predict the elastic—plastic response of injection-molded long-fiber thermoplastics (LFTs). The model accounts for elastic fibers embedded in a thermoplastic resin that exhibits the elastic—plastic behavior obeying the Ramberg—Osgood relation and J-2 deformation theory of plasticity. It also accounts for fiber length and orientation distributions in the composite formed by the injection-molding process. Fiber orientation was predicted using an anisotropic rotary diffusion model recently developed for LFTs. An incremental procedure using Eshelbys equivalent inclusion method and the Mori—Tanaka assumption is applied to compute the overall stress increment resulting from an overall strain increment for an aligned-fiber composite that contains the same fiber volume fraction and length distribution as the actual composite. The incremental response of the latter is then obtained from the solution for the aligned-fiber composite by averaging over all fiber orientations. Failure during incremental loading is predicted using the Van Hattum—Bernado model that is adapted to the composite elastic—plastic behavior. The model is validated against the experimental stress—strain results obtained for long-glass-fiber/polypropylene specimens.

A Mechanistic Approach to Matrix Cracking Coupled with Fiber–Matrix Debonding in Short-Fiber Composites

A micro-macro mechanistic approach to damage in short-fiber composites is developed in this paper. At the microscale, a reference aligned fiber composite is considered for the analysis of the damage mechanisms such as matrix cracking and fiber/matrix debonding using the modified Mori-Tanaka model. The associated damage variables are defined, and the stiffness reduction law dependent on these variables is established. The stiffness of a random fiber composite containing random matrix microcracks and imperfect interfaces is then obtained from that of the reference composite, which is averaged over all possible orientations and weighted by an orientation distribution function. The macroscopic response is determined using a continuum damage mechanics approach and finite element analysis. Final failure resulting from saturation of matrix microcracks, fiber pull-out and breakage is modeled by a vanishing element technique. The model is validated using the experimental data and results found in the literature as well as the results determined for a random chopped fiber glass/vinyl ester system.

An Elastic-plastic Damage Model for Long-fiber Thermoplastics:

This article proposes an elastic-plastic damage model that combines micromechanical modeling with continuum damage mechanics to predict the stress— strain response of injection-molded long-fiber thermoplastics. The model accounts for distributions of orientation and length of elastic fibers embedded in a thermoplastic matrix whose behavior is elastic-plastic and damageable. The elastic-plastic damage behavior of the matrix is described by the modified Ramberg—Osgood relation and the 3D damage model in deformation assuming isotropic hardening. Fiber/matrix debonding is accounted for using a parameter that governs the fiber/matrix interface compliance. A linear relationship between this parameter and the matrix damage variable is assumed. First, the elastic-plastic damage behavior of the reference aligned fiber composite containing the same fiber volume fraction and length distribution as the actual composite is computed using an incremental Eshelby—Mori—Tanaka mean field approach. The incremental response of the latter is then obtained from the solution for the aligned-fiber composite by averaging over all fiber orientations. The model is validated against the experimental stress—strain results obtained for long-glass-fiber/polypropylene specimens.

Analysis of Tube Hydroforming by Means of an Inverse Approach

This paper presents a computational tool for the analysis of freely hydroformed tubes by means of an inverse approach. The formulation of the inverse method developed by Guo et al. is adopted and extended to the tube hydrofoming problems in which the initial geometry is a round tube submitted to hydraulic pressure and axial feed at the tube ends (end-feed). A simple criterion based on a forming limit diagram is used to predict the necking regions in the deformed workpiece. Although the developed computational tool is a stand-alone code, it has been linked to the Marc finite element code for meshing and visualization of results. The application of the inverse approach to tube hydroforming is illustrated through the analyses of the aluminum alloy AA6061-T4 seamless tubes under free hydroforming conditions. The results obtained are in good agreement with those issued from a direct incremental approach. However, the computational time in the inverse procedure is much less than that in the incremental method.

Analysis of Tube Free Hydroforming Using an Inverse Approach With FLD-Based Adjustment of Process Parameters

This paper employs an inverse approach (IA) formulation for the analysis of tubes under free hydroforming conditions. The IA formulation is derived from that of Guo et al. established for flat sheet hydroforming analysis using constant strain triangular membrane elements. At first, an incremental analysis of free hydroforming for a hot-dip galvanized (HG/Z140) DP600 tube is performed using the finite element Marc code. The deformed geometry obtained at the last converged increment is then used as the final configuration in the inverse analysis. This comparative study allows us to assess the predicting capability of the inverse analysis. The results will be compared with the experimental values determined by Asnafi and Skogsgardh. After that, a procedure based on a forming limit diagram (FLD) is proposed to adjust the process parameters such as the axial feed and internal pressure. Finally, the adjustment process is illustrated through a re-analysis of the same tube using the inverse approach

Predictive Engineering Tools for Injection-Molded Long-Carbon-Fiber Thermoplastic Composites

This quarterly report summarizes the status for the project planning to complete all the legal and contract documents required for establishing the subcontracts needed and a Cooperative Research and Development Agreement (CRADA) with Autodesk, Inc., Toyota Motor Engineering and Manufacturing North America (Toyota), and Magna Exterior and Interiors Corporation (Magna). During the second quarter (1/1/2013 to 3/31/2013), all the technical and legal documents for the subcontracts to Purdue University, University of Illinois, and PlastiComp, Inc. were completed. The revised CRADA documents were sent to DOE, Autodesk, Toyota, and Magna for technical and legal reviews. PNNL Legal Services contacted project partners’ Legal counterparts for completing legal documents for the project. A non-disclosure agreement was revised and sent to all the parties for reviews.

- Prediction of damage in a randomly oriented short-fiber composite plate containing a central hole

Publisher Summary This chapter presents a micro–macro mechanistic approach to matrix cracking in randomly oriented short-fiber composites that is used for the damage and failure analysis of a random glass/epoxy plate containing a central hole under tensile loading. At the micro scale, the virgin and reduced elastic properties of the composite are computed using micromechanical models and are then averaged over all possible orientations and weighted by an orientation distribution function. The macroscopic response is performed by means of a continuum-damage mechanics formulation, in which the damage evolution law is obtained using a damage threshold function and the concepts of thermodynamics of continuous media. Damage accumulations leading to initiation and propagation of a macroscopic crack are modeled through a vanishing element technique. The use of short-fiber composites has become of great interest to the automotive sector as a possible means to reduce vehicle weight. The fiber and matrix properties, fiber aspect ratio, and volume fraction, as well as their orientation distribution, strongly influence the mechanical response of these materials. In this approach, the virgin and reduced elastic properties of the composite are computed through micromechanical models.

A multiscale modeling approach to analyze filament-wound composite pressure vessels

A multiscale modeling approach to analyze filament-wound composite pressure vessels is developed in this article. The approach, which extends the Nguyen et al. [Prediction of the elastic-plastic stress/strain response for injection-molded long-fiber thermoplastics. J Compos Mater 2009; 43: 217–246.] model developed for discontinuous fiber composites to continuous fiber ones, spans three modeling scales. The microscale considers the unidirectional elastic fibers embedded in an elastic–plastic matrix obeying the Ramberg–Osgood relation and J2 deformation theory of plasticity. The mesoscale behavior representing the composite lamina is obtained through an incremental Mori–Tanaka [Average stress in matrix and average elastic energy of materials with misfitting inclusions. Acta Metall 1973; 21: 571–574.] type model and the Eshelby [The determination of the elastic field of an ellipsoidal inclusion and related problems. Proc R Soc Lond, Ser A 1957; 241: 376–396.] equivalent inclusion method. The implementation of the micro–meso constitutive relations in the ABAQUS® finite element package (via user subroutines) allows the analysis of a filament-wound composite pressure vessel (macroscale) to be performed. Failure of the composite lamina is predicted by a criterion that accounts for the strengths of the fibers and of the matrix as well as of their interface. The developed approach is validated in the analysis of an aluminum liner – T300 carbon/epoxy pressure vessel to predict the burst pressure. The predictions compare favorably with the numerical and experimental results by Lifshitz and Dayan [Filament-wound pressure vessel with thick metal liner. Compos Struct 1995; 32: 313–323]. The approach will be further demonstrated in the study of the effects of the lamina thickness, helical angle, and fiber–matrix material combination on the burst pressure.