Piaras Kelly

A viscoelastic model for the compaction of fibrous materials

Fibrous materials experience compression in many important industrial and technical applications. They are known to undergo a viscoelastic response in such circumstances, exhibiting phenomena such as dependence on compaction velocity, stress relaxation and stress–strain hysteresis. In this paper, a model has been developed for the stress in compacting fibrous materials. The model is based on the multiplicative decomposition of the stress into a function of the strain and a second function of the strain‐rate. The model is applicable to that class of materials whose stress–strain responses at different compaction velocities can be collapsed onto a single master curve when the stress is normalised appropriately. The model parameters can be determined using a least‐squares fitting to a select number of test data. The model has been tested for two materials of different architectures over a range of compaction speeds and maximum volume fractions; the match to experimental data is excellent.

Experimental investigation into the post-filling stage of the resin infusion process

The resin infusion process has developed as a low-cost method to produce large composite parts in low to medium quantities. Although the process is conceptually simple, the effects of many of the processing parameters on the post-filling stage of the process are not well understood. Most manufacturers tend to develop their approach to infusion process through trial and error, and then adhere to their ‘secret recipe’ without knowledge of the effect of each parameter. This paper describes an experimental investigation of the controllable process parameters and their effect on the final laminate composition, by monitoring local fluid pressure and full field laminate thickness data through the filling and post-filling stages. From the understanding of the effect of each parameter, guidelines are drawn to help manufacturers to optimise their process. The effect of using a ‘brake’ between the part and the vent are evaluated, and the benefits of turning the inlet into a vent at the onset of post-filling are highlighted together with methods of gaining some control on the final laminate fibre volume fraction.

Simulation and experimental validation of force controlled compression resin transfer molding

The simulation of composite manufacturing processes is a great aid to obtaining efficient production and high-quality parts. The mold and process design must allow for fast filling times as well as dry-spot free parts. Besides an accurate simulation of the resin flow through the reinforcement, the compaction response of the preform is also needed. The stress response of the textile to compaction has an influence on the local and global forces exerted on tooling. The numerical prediction of the clamping force helps to trade-off fast production times against affordable machinery. This article describes the accurate simulation of force controlled Resin Transfer Molding (RTM) and Compression RTM, and compares results of simulations with experimental data. A parametric study is performed in order to minimize the simulation time without compromising the accuracy of the results. The controlled force algorithms have been implemented within SimLCM, a code under development at the University of Auckland to address the liquid composite molding (LCM) family of manufacturing processes. With these new tools, the trade-off between production process time and equipment cost can be considered, and optimal process design solutions found.

Three-dimensional cracks with Dugdale-type plastic zones

A general method for the determination of three-dimensional crack-front plastic zones is presented. The crack-model utilised is of the Dugdale-type, that is a plastic zone is assumed to spread in front of the crack, in the plane of the crack. The solution method relies on the eigenstrain approach to crack-problem solving. Using this approach, the elastic and plastic regions of the crack are first discretised into triangular elements and the stress arising over the crack due to a constant or linear crack-surface displacement in each element is obtained. The stresses are given in terms of hyper-singular integrals, which may be solved numerically. The boundary conditions in the elastic portion (traction-free) and in the plastic zones (Tresca or some other yield condition) are encompassed in an object function in such a way that the boundary conditions are satisfied if and only if the object function is zero. The resulting quadratic programming problem is solved numerically, and hence the crack displacements, and other important quantities, are obtained. Results agree well with some known analytic results regarding the penny shaped crack in tension.

Transverse compression properties of composite reinforcements

Abstract: This chapter discusses transverse compression, one of the dominant deformation modes arising in fibrous reinforcement materials during composites forming and manufacture. After a brief review of the experimental procedure to determine fabric compressibility, the standard compaction curve is introduced, together with associated numerical models. The chapter then discusses the distinctive inelastic properties and response of fibrous materials, including their viscoelasticity and plasticity. The concept of locked energy is introduced, and modelling strategies for its incorporation within a general thermomechanical framework of reinforcement compression is discussed. The chapter ends with a short review of current challenges within transverse compression and possible future trends.

Automated tool to determine geometric measurements of woven textiles using digital image analysis techniques

An automated geometric characterization tool for woven textiles is presented. This tool is used to obtain the in-plane geometry of woven textiles in a fast and efficient method. These measurements can be used to provide a better understanding of the textile geometry and can be used in further applications such as quality assurance and textile modeling for prediction of material properties. Unique to this tool is that the use of morphological operations has been avoided, resulting in a significantly more robust image analysis procedure, enabling the analysis of a greater range of textiles with reduced user input. This procedure is coupled with a simple image acquisition technique. The image of the textile is captured using a standard office scanner, eliminating the need for a complex setup or specialized equipment. The tool’s capability is demonstrated for a variety of textiles; an in-depth description of the techniques used is also given. The resulting geometric information obtained using the tool highlights the importance of analyzing the textile sample of interest: significant geometric variations were captured in each sample, which are not ordinarily communicated through the material data sheets provided by manufacturers.

Predicting permeability based on flow simulations and textile modelling techniques: Comparison with experimental values and verification of FlowTex solver using Ansys CFX

This paper compares the predicted permeability values obtained from conducting simulations with experimental results from the second permeability benchmark exercise. An automated tool, which has been developed for this purpose and is presented here, carries out flow simulations on WiseTex generated meshes using Ansys CFX. Different meshing methods are explored, and the effects of different boundary conditions, number of layers used to model the preform and the incorporation of nesting are examined. The predicted permeability values of the textile model closest to the real preform structure were higher by a factor of approximately two compared to the experimental values. In the second part of this work, the permeability values obtained using Ansys CFX and FlowTex solvers are compared and the ability of both solvers to capture variations in unit cell geometric parameters is demonstrated. A close agreement between the two solvers was found.

Efficient experimental characterisation of the permeability of fibrous textiles

Two experimental set-ups used to characterise the in-plane and through-thickness permeabilities of reinforcing textiles have been developed and are presented. Both the experimental testing and data processing techniques used have been selected to ensure that the characterisation is completed in an efficient and robust method, increasing the repeatability of tests while minimising user induced errors as well as the time and resources needed. A number of key results and outputs obtained are presented from tests carried out on a plain woven reinforcing textile with a range of number of layers and at different fibre volume fractions.

A rate-independent thermomechanical constitutive model for fiber reinforcements

In liquid composite molding (LCM) processes, the constitutive behavior of fibrous reinforcements has a strong bearing on the choice of manufacturing parameters and final part properties. In many LCM processes, the fibrous preform is subjected to loading and unloading, the latter occurring during both filling and postfilling phases of the manufacturing process. Fiber reinforcements display inelastic behavior with rate-dependent and rate-independent components and this must be modeled accurately over several load–unload cycles in order to accurately simulate such processes. An important feature of the material behavior is its unchanging response to successive load cycles once a large number of load cycles has been applied. Inelastic effects such as fiber–fiber frictional sliding occur during loading as well as unloading and the inelastic deformation remaining after successive cycles appears unchanged. The model presented is developed within a thermomechanical framework and reproduces such behavior using a single internal variable to account for inelasticity. It is compared to cyclic loading experiments on three fiber reinforcements of different architecture, showing that the model is effective in capturing the repeatable compaction behavior of debulked preforms and serves as a starting point for the incorporation of effects such as cyclic softening and rate effects through additional internal variables.

Application of a Complete Tooling Force Analysis for Simulation of Liquid Composite Moulding Processes

The term Liquid Composite Moulding (LCM) encompasses a growing list of composite manufacturing processes. The focus of this paper is prediction of tooling forces for Resin Transfer Moulding (RTM). Previous experimental work has demonstrated the influence of reinforcement compaction behaviour, which is strongly non-elastic. A viscoelastic compaction model has been developed which addresses both dry and wet response, and is implemented in RTM simulations of simple flat parts. Non-planar geometries introduce a tangential stress acting on mould surfaces, due to shear of the reinforcement. The tooling force analysis is extended to complex parts using an existing RTM filling simulation, LIMS, which has been developed at the University of Delaware.