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Most numerical approaches for coupling fluid flow with porous media flow rely either on Stokes equations in the fluid part of the domain and Darcy's law in the porous part, or on Brinkman's equation. In both cases, difficulties arise at the boundary between the two parts because the equations used in the porous part are not of Stokes type. In this paper, an alternative to Darcy's law is proposed for modeling flows in porous media. This alternative relies on equations of Stokes type where the permeability tensor is replaced by force and stress derivative tensors. Numerical procedures are presented to compute these tensors from simulations at pore scale. Simulations in domains containing both fluid and porous parts are finally conducted simply assuming continuity of velocity and pressure and hence without imposing any condition at the boundary between the two parts. Results show that the proposed method is accurate and hence a promising alternative to Darcy's law for problems involving both fluid and porous parts. • Alternative to Darcy's law relying on equations of Stokes type for modeling flows in porous media. • Permeability tensor replaced by force and stress derivatives tensors computed from simulations at pore scale. • Flows in domains containing both fluid and porous parts modeled without any boundary conditions between the two parts. [ABSTRACT FROM AUTHOR]

We performed an experimental study to manufacture flax fiber textile reinforced thermoplastic composites by direct impregnation method in a closed mold. To reduce the process time, we adopted compression resin transfer molding resin because of its short flow path during the highly viscous resin impregnation process. Fiber preheating process was introduced before the impregnation process in order to avoid the resin cooling down when contacting the flax fibers. Optimal process temperature conditions such as mold temperature, resin temperature and fiber temperature were experimentally determined while minimizing the effect of thermal degradation of flax fiber and obtaining good impregnation quality. In particular, a sequential pressure cycle was proposed to improve the impregnation quality and to obtain the uniform fiber volume fraction in the final part. The impregnation quality was assessed by the density measurement and by the micrographic image analysis. The mechanical properties of the final parts were comparable with the literature values obtained by the compression molding of matrix-fiber premixed materials such as commingle yarns or film stacking, whereas the process cycle time of was much shorter than the other conventional methods in the literature. [ABSTRACT FROM AUTHOR]

This work deals with the development and investigation of self-reinforced polylactic acid (SRPLA) composites manufactured from two different nonwoven preforms with two different bicomponent filament architectures. The SRPLA nonwovens prepared via spun bonding technology have two different bicomponent filament architectures, that is, core-sheath, and islands in the sea. For the core-sheath filaments, spunbound nonwovens are compared with the dry-laid SRPLA nonwovens. The SRPLA nonwoven preforms are compression molded at three different mold temperatures to fabricate composite plates. The optimum mold temperature is identified based on the flexural properties and residual void volume fraction of composites. The flexural modulus and strength are about 38% and 50% higher than those of the pure PolyLactic Acid (PLA) matrix, respectively. The influence of bicomponent filament architecture on the mechanical performance of composites is negligible, whereas the nonwoven architecture leads to significant differences both in the residual void volume fraction and in the mechanical properties of composites. Finally, the mechanical properties of the SRPLA composites are compared with those of the conventional composites to prove their applicability to semi-structural parts. [ABSTRACT FROM AUTHOR]

The most limiting aspect in liquid composite molding processes is the porosity issue. A successful manufacturing design depends mainly on the accurate prediction of the dynamics of air bubbles while processing composite structures. Especially, while simulating with a constant injection flow rate, where the part is supposed to exhibit a uniform porosity distribution according to the capillary number theory as proposed in the literature, it is not true in real practice. With this motivation, we present here a new approach, based on intra and intertow flows competition, to quantify the porosity of resin transfer molded composite parts. Combining the numerical modeling of the resin transfer molding (RTM) process based on the control volume finite element and volume of fluid methods on one hand, and the pore doublet model (PDM) on the other hand, air bubble contents at dual scale of pores are modeled in the case of constant pressure and constant injection flow rate. The results of the developed modeling permit a detailed spatiotemporal description of the created compressed and transported micro and macro air bubbles, besides fibrous fabric saturation. Consequently, taking into account the compression and transport phenomena, the porosity distribution is no longer uniform in the part, but higher in the downstream than in the upstream. Furthermore, the void transport phenomenon is also considered using the PDM concept. To validate the proposed approach, the numerical and experimental data are compared. [ABSTRACT FROM AUTHOR]

A numerical analysis of the influence of race tracking on dry spots formation and the accuracy of permeability measurement during the resin-transfer-molding process is presented. In the numerical simulation of the mold-filling process, defects are randomly generated, and their effect is assessed by a Monte Carlo simulation method. The effect of race tracking on the unsaturated permeability measurement and dry spots formation is investigated on flat plates. It is observed that the race-tracking defects located near the injection gate increase up to 40% of the value of the measured unsaturated permeability. The race-tracking defects located near the air vents are more likely to generate dry spots, whereas those near the injection gates have a less significant influence on dry spots generation. Depending on vent location, it has for instance been shown that the dry spot area can increase by a factor of 30. Dry spots may be mitigated by placing an air vent at a suitable location based on the numerical analysis results. Moreover, those results may be helpful to determine optimal sensor locations for the on-line control of mold-filling processes. Finally, the approach is successfully applied to a complex geometry. [ABSTRACT FROM AUTHOR]

The presence of the tufts has direct influence on the permeability of textile preforms. In this paper, new insights are presented into the mechanisms responsible for changes to the in-plane permeability of tufted preforms according to three different tufting patterns relative to flow direction with a constant tufting distance. The saturated permeabilities are measured based on the rectilinear flow in a tool constituted by a steel base, a transparent PMMA top, and steel supports. Beforehand, the permeability measurements of textile preform composed of eight layers of plain weave glass fabric without tufting are conducted at three different cavity heights. The results show that the in-plane permeability values for the tufted preforms are lower than the permeability values of the preform without tufting. The paper reveals the significant influence of the tufting patterns on the permeability values for three different fibre volume fractions. [ABSTRACT FROM AUTHOR]

Numerical artifacts in the form of spurious oscillations are among the critical issues of Fast Fourier Transfer (FFT) methods for solving multiphase elastic problems such as numerical homogenization, in spite of their computational simplicity and efficiency. In the first part of the present work, it is shown that the irregular discretization of the interface due to the use of a voxel-based discretization is the dominant cause of oscillations. The second part of the present work focuses on numerical artifacts reduction schemes, and in particular special treatments for dealing with the irregular discretization of the interface such as the composite voxel method and neighbor averaging methods. An improved composite voxel method by using the level-set technique is proposed, which alleviates the implementation difficulty of the composite voxel method. This improved method is particularly relevant for non-parametrized interface representations such as those obtained from three-dimensional experimental images. [ABSTRACT FROM AUTHOR]

This paper focuses on the numerical implementation of phase-field models of fracture using the Fast Fourier Transform (FFT) based numerical method. Recent studies on phase-field models focus on the discussions of the choice of the value of regularization length proposed to smear the discontinuity of the sharp crack. Some studies argue that it should be considered as a material property because it has a significant impact on the mechanical behavior of a material in some phase-field models, for instance, in the model proposed by Miehe. However, our results in this study for heterogeneous materials have shown that the choice of regularization length not only affects the macroscopic mechanical behavior but also the local crack propagation patterns. As a result, it can be challenging to select an appropriate value that produces both accurate macroscopic responses and local crack patterns for certain phase-field models, such as Miehe's model. Thus, the phase-field model proposed by Wu, which has been proven to reduce the length sensitivity for homogeneous material, has been successfully implemented in an FFT-based solver with the application of Newton–Krylov algorithm. The length sensitivity of Wu's phase-field model for heterogeneous materials is also investigated in this paper. Our tests show that Wu's phase-field model has partial length sensitivity for heterogeneous materials. However, the main reason for this sensitivity is that one phase enters the damage zone of other phase, which is different from Miehe's model. As a result, a set of criteria for accurately choosing the regularization value has been established for Wu's model in this work. Meanwhile, it has also been found that Wu's model may be more suitable than Miehe's model for brittle failure due to the introduction of an elastic stage. • Phase-field modeling of the fracture using the Fast Fourier Transform-based method. • Newton–Krylov algorithm to solve a phase-field problem in its complete formulation. • Investigation of the regularization length sensitivity of the complete formulation. • Investigation of regularization length and its effects on the local crack patterns. [ABSTRACT FROM AUTHOR]

Abstract The variational phase-field method is an attractive non-local approach of modeling fracture in heterogeneous materials. However, these materials usually require a fine mesh to resolve the fracture process zone. Consequently, the standard finite element solver becomes cumbersome due to the large number of elements in applications with highly heterogeneous materials. Motivated by this limitation, an algorithm based on FFT methods has been introduced in this paper to solve the phase-field model of brittle fracture. Relying on a staggered update scheme, the proposed algorithm solves the fracture problem and mechanical problem separately, both using the FFT technique. It inherits the advantages of classical FFT methods in terms of simplicity of mesh generation and parallel implementation. Introduced within a FFT-based code "AMITEX", it takes the advantage of massively parallel capabilities associated with a distributed memory implementation. The characteristics of the proposed method are analyzed in a single edge notched specimen benchmark. Representative numerical examples demonstrate that the proposed FFT solver is capable of predicting different crack modes and complex crack configuration, such as crack interaction, branching and coalescence. Finally, a model of an idealized continuous fiber composite with void involving over 32 million voxels is solved, illustrating the potential of the FFT solver in large-scale problems. [ABSTRACT FROM AUTHOR]

We propose a new model for the capillary rise of liquid in flax fibers whose diameter is changed by liquid absorption. Liquid absorption into the flax fibers is taken into account in a new modified Washburn equation by considering the mass of the liquid absorbed inside the fibers as well as that imbibed between the fibers. The change of permeability and hydraulic radius of pores in a fibrous medium due to the fiber swelling is modeled by a statistical approach considering a non-uniform distribution of flax fiber diameter. By comparisons between capillary rise test results and modeling results, we prove the validity of the proposed modified Washburn model to take into account the effects from fiber swelling and liquid absorption on the decrease of capillary rise velocity. The experimental observation of long-term capillary rise tests show that the swelling behavior of the fibers highly packed in a closed volume and its influence on the capillary wicking are different from those of an individual single fiber in a free space. The current approach was useful to characterize the swelling of fibers highly packed in a closed volume and its influence of the long-term behavior of capillary wicking. [ABSTRACT FROM AUTHOR]

This paper focuses on the numerical implementation of phase-field models of fracture using the Fast Fourier Transform based numerical method. Recent developments in that field rely on the separate solution of a coupled problem where the mechanical equilibrium problem is solved first, and then the phase-field evolution equation. The latter involves a diffusion term which has been simplified in previous works relying on the Fast Fourier Transform based numerical method. This simplification has theoretically no effect for homogeneous materials, but might influence predictions significantly for heterogeneous materials where fracture properties vary between the different components. In this paper, the influence of this simplification is assessed and a complete formulation is proposed as well as a novel implementation of this formulation using the Fast Fourier Transform based numerical method. The assessment relies on simulations with a material containing two components, one of them being defined as unbreakable by using higher fracture properties. Using the simplified formulation, the presence of an artificial diffusion of damage between the two components is evidenced, and non-zero damage values are observed in the unbreakable component. Although the complete formulation leads to an increase of the number of iterations to solve the phase-field evolution equation, it suppresses completely the diffusion of damage towards the unbreakable component. The two formulations, in fact, lead to identical results when the fracture properties are homogeneous, but the results diverge both in terms of local fracture patterns and global stress–strain relations when the fracture properties contrast increases. This difference is also more pronounced when the regularization length introduced by the phase-field model increases. • Phase-field modeling of fracture using the Fast Fourier Transform based numerical method. • Development of a new complete formulation of the phase-field equation. • Comparison with a simplified formulation previously proposed in the literature. • Complete formulation shown to eliminate unphysical damage diffusion between phases. [ABSTRACT FROM AUTHOR]

• Ten flax varieties cultivated in two Belgian regions in two years were investigated. • Accumulated temperature at germination and emergence affects the flax fiber yield. • Precipitation during early phase of growth influences tensile strength of fibers. • A couple of flax varieties with good tensile properties were identified. • Pulling date and retting degree are key factors affecting the tensile properties. Flax (Linum Usitatissimum L.) fiber composites serve as sustainable replacement for glass fiber composites. Given that these composites are based on natural sources, their performance is susceptible to variation induced by several factors. Hence, ten flax fiber varieties cultivated in two different regions of Belgium in 2017 and 2018 are compared in terms of their yield and their tensile strength. Statistical analyses are carried out to determine the varietal difference in terms of tensile strength of bundles and of composites. The mechanical tests of flax fibers are carried out in two ways, i.e. dry fiber bundle tests and impregnated fiber bundle tests (IFBT). The results from the two consecutive growth years show their significant influence on the yield of the flax fibers, which are highly dependent on the weather conditions, in particular the precipitation during the growing season. The influence of the date of pulling and the retting degree on the yield of flax fibers and the tensile strength of fiber bundles is also investigated. The early pulling and lower retting degree result in a detrimental effect on the fiber strength whereas the late pulling shows little to no effect on the fiber strength. The difference of flax fiber strength among ten flax varieties over two years can be easily identified using IFBT and a couple of flax varieties exhibit a good tensile strength with low variation. This methodology can be used to select good flax varieties which have high mechanical properties with a small variation, which are suitable for a fiber reinforcement of high performance natural fiber reinforced composites. [ABSTRACT FROM AUTHOR]