Key Engineering Materials Vols. 611-612

Abstract: We present a simplified numerical method which can be used to predict efficiently the response of long thin plates under effects of residual stresses induced by production process such as rolling or continuous annealing. The principle consists in assuming harmonic buckling mode along the sheet length, and we consider Koiter-Budiansky post-buckling theory to compute the stress-deflection curve. In this way, only the width of the sheet has to be discretized by 1D finite elements. The size and shape of the flatness defects can be predicted efficiently and for a large number of cases. Various types of residual stresses and loadings can be accounted for. In particular, we will see the influence of the global traction on the buckling and post-buckling behavior. The numerical results are compared with experimental data and full numerical computations

Abstract: During forming operations the contact conditions relating to the surface of the tools, the intermediate coolant or lubricant and the hot work piece material are determining the material flow and the resulting wear. The possibilities for an optimization of the tool surfaces are aspects of various scientific activities of IST and IFUM and are subject of this paper. Tools typically made of hot working steel can be treated with different technologies in order to achieve wear resistant properties. First of all, the surface is mainly determined by its topography which is defined by the manufacturing method or applied finishing technology. This paper will give an overview to adjustable surface properties with additional conditioning methods like severe shot peening. The stabilization of the topography is a new approach to enhance the wear resistance of forming tools. Several models like the Abbott-Firestone graph are used in order to obtain suitable describing parameters such as the roughness-parameter s_k. The development of s_k during the running-in stage has a strong influence on the tool life which can be shown. A stabilization of the topographical conditions can enhance the service life of the tools. It will be shown, that this is possible by applying plasma diffusion treatments and thin film coatings by means of vacuum coating technologies.

Abstract: The increasing use of finite element simulation in the field of composite material forming involved in the past few years a large among of research on the constitutive modelling of textile material at the mesoscopic scale (i.e. the scale of individual fibre tow). Up to now, a consensus does not exist on the most appropriate approach. The present contribution aims on the comparison between hypo-and hyper-elastic approaches to describe the mechanical behaviour of a single tow in the framework of nonlinear continuum mechanics. A particular attention is paid to the ease of implementation in standard finite element codes.

Abstract: Thermo-mechanical process simulation of an industrially pultruded rectangular hollow profile is presented. Glass/polyester is used for the continuous filament mat (CFM) and the uni-directional (UD) layers. The process induced residual distortions together with the temperature and degree of cure are predicted using a three dimensional (3D) thermo-chemical model sequentially coupled with a 2D quasi-static generalized plane strain mechanical model. The predicted deformation pattern at the end of the process is found to agree quite well with the one observed for the real pultruded parts in a commercial pultrusion company. In addition, the predicted warpage behaviour is further analysed by adjusting the mandrel length as well as including the mandrel heating. Using the proposed process model, the effect of the mandrel configurations on the quality of the pultrusion is investigated in terms of temperature, degree of cure and distortions.These unwanted residual distortions may lead to not meeting the desired geometrical tolerances e.g. warpage of pultruded window frames and hollow profiles as well as spring-in of L-shaped profiles, etc.

Abstract: In this work, the deformability of a flax fabric adopted as composite reinforcement is experimentally investigated. The fabric (commercialized as FLAXPLY UD 180 by LINEO) is a quasi-unidirectional woven fabric with thin weft yarns connecting thick slightly twisted warp using satin weave interlacing pattern. The study is dedicated to the understanding and measurement of the main deformation mechanisms occurring during forming processes. The deformation during extension is investigated under uniaxial and biaxial loading in the in-plane tow directions (i.e. warp and weft). Particular attention is dedicated to the behavior during shear loading because this is considered the primary deformation mechanism in the reinforcement forming. Uniaxial bias extension and picture frame tests are adopted to measure the shear deformation. The tests are assisted by digital image correlation (DIC) technique to have a continuous measurement of the local deformation in the fabric plane during loading.

Abstract: Resin Infusion (RI) process is one of the common techniques used in the industry for large composite parts production. This technique uses vacuum pressure to drive the resin into a laminate. Preform is laid dry into the mold and the vacuum is applied before the resin is introduced. Once a complete vacuum is achieved, resin is sucked into the laminate via placed tubing. Fig.1 shows a diagram of this process.An appropriate modeling of flow front’s shapes constrained by LRI process during filling can be based on the continuous deformation of the vent oriented flow pattern due to the driving pressure from the inlet. One of the main objectives is that the flow achieves the contour vent uniformly to avoid pressure drop and ensuring complete filling. In LRI, the flow front shape progression is mainly conditioned by the initial arrangement of the injection line allocation and the permeability of the preform that can evolve along the mold

Abstract: In this study, a thermo-mechanical finite element model is developed to predict the spring-in of an industrially pultruded L-shaped profile made of glass/polyester composite. The resin curing kinetics are obtained from the differential scanning calorimetry (DSC) experiments. The development of the resin modulus is derived using the dynamic mechanical analysis (DMA) tests and the effective mechanical properties of the processing composite are calculated using a micromechanical model. The temperature and degree of cure distributions are obtained in a three dimensional (3D) thermo-chemical anlaysis using the finite element method (FEM). The process induced distortions are then calculated using these distributions in a 2D quasi-static mechanical analysis in which generalized plane strain elements are utilized. The predicted spring-in pattern at the end of the process is found to agree quite well with the one observed for the real pultruded parts in a commercial pultrusion company. In addition, the effects of the pulling speed and the part thickness on the spring-in formations are investigated using the proposed numerical simulation tool. It is found that the magnitude of the spring-in increases with an increase in the pulling speed and part thickness.

Abstract: In several fields of engineering the automation of the CFRP production chain is a major issue. In this production chain the forming plays a key role, as the result of the forming influences everything in the chain from the infusion step until the part mechanics. To understand the influence of the material choice onto the forming process is a task followed by many scientists during the last 20 years. Basic tests for shear characterization like Picture Frame Test (PFT) and Bias Extension Test (BiasExt) were developed and used widely. This work deals with the comparison of the BiasExt to a fiber extraction test. The fiber extraction test is developed and used for the characterization of a woven and two non-crimp fabric material. The results are important for the process information and the judgment of primary deformation mechanisms. The tests are simulated for the unidirectional material in a mesoscopic approach and the results are compared in order to judge the capability of the mesoscopic simulation and its residual limitations.

Abstract: 3D woven composite reinforcements preforming simulations are an unavoidable step of composite part processing. The present paper deals with thick composite fabric behavior modelling and issues arising during the numerical simulation of preforming. After the description of the independent deformation modes of initially orthotropic reinforcements, a physically motivated and invariant based hyperelastic strain energy density is introduced. This constitutive law is used to show the limitations of a classical finite element formulation in 3D fabric simulations. Tension locking is highlighted in bias extension tests and a reduced integration hexahedral finite element with specific physical hourglass stabilization is proposed. Instabilities due to the highly anisotropic behavior law, witnessed in bending dominated situations, are exposed and a stabilization procedure is initiated.

Abstract: This experimental study is focused on identification of tribological mechanisms acting during forming of polymer composites. The range of relevant processes includes fibre placement, tape lay-up, moulding, draping, and RTM. Two types of tribological experiments, relying both on simultaneous application of compression and shear loadings, are carried out. Firstly, model macromechanical tests are undertaken on plastic rods of millimetric diameter immersed in a viscous liquid, representing composite fibres and matrix, respectively. By careful simulation of forming conditions, this experiment helps to identify the friction phenomena occurring in real composites. On the other hand, the micromechanics of forming processes is studied through a microscopic experiment on real carbon fabric. This material is clamped between two glass plates and pulled in opposing directions in the plane of the fabric. It is hypothesized that the evolution of contact area due to shearing that can be measured in this experiment is an essential feature of the tribology of forming processes, a topic which hitherto has not been investigated.