Key Engineering Materials Vols. 504-506

Abstract: Most of industrial processes (thermoforming, injection moulding...) require the understanding of thermo-mechanical behaviour of polymeric sheets. Furthermore, the mastery of the deformation of the polymers becomes an important stake. In order to improve and complete the understanding of the deformation of thermoplastic polymer materials during their forming processes, the problem of modelling the thermoforming process for viscoelastic sheet under large strains is considered. The first step of the process that consists in heating the sheet using infrared lamps is taken into account by included a temperature field in viscoelastic behaviour laws under integral forms. The finite element simulation of the different steps will be presented

Abstract: The study is focused on the effect of strain rate, temperature and stretch ratio on the room temperature mechanical properties of PET (Polyethylene terephthalate) following biaxial deformation. Specimens were biaxially stretched within a temperature range 80-110°C, a strain rate in the range 1-16/s and stretch ratio in the range 1-2.8. The tensile moduli of the stretched specimens were obtained using tensile testing. Results show that post-stretching room temperature modulus increases with decreasing temperature, increasing strain rate, and stretch ratio.

Abstract: This paper examines critical issues associated with the fabrication and forming of highly-flexible polymeric composites, reinforced with knitted-fabric structures. Knitted-fabric reinforcements have not generally been preferred over more traditional woven reinforcements in high-performance composites, mainly because of their lower stiffness/strength performance when embedded in a rigid, thermosetting matrix material. However, with their unique formability, knitted fabrics promise great potential in applications where large deformation of the structure is desirable; such as energy/impact absorption and forming applications. One very attractive feature of knitted composite materials, is the large displacements that the underlying knitted fabric can potentially undergo before exhibiting a significant increase in stiffness. The unusual extensional behavior of knit fabric is attributed to the fact that the fibers are more-or-less free to slide over each other before the yarns become highly oriented, eventually “locking” in a packed formation. When the loops become highly elongated, the knit fabric achieves its maximum resistance to in-plane deformation, and exhibits a stiffness closely related to the elastic stiffness of the straightened fiber/yarn bundles. The unique formability of knitted fabrics is mainly due to this yarn movement. The highly “stretchable” behavior of knitted textile reinforcement materials can be used to great advantage in thermoforming composite structures. In order to fully utilize the exceptional stretch properties of the knitted-fabric, the matrix material should be able to deform at least as much as the fabric, and the knitted yarn movements need to be restricted by the matrix as little as possible. In this study, a multi-level finite element procedure was developed to analyze and control the deformation characteristics of plain weft knit reinforced composites. A database of mechanical properties for various knit geometries was obtained. Using these results, it is shown that carefully “tailored” knit fabric reinforcement can be used to improve mechanical performance and facilitate polymer forming processes, such as thermoforming. In this study, elastomeric materials such as polyurea and thermoplastic elastomer (TPE) were used to fabricate composites with knitted-fabric. Two different types of arrangements were experimentally studied: knitted fabric embedded in the elastomer and a sandwich of knitted fabric between elastomeric skins. It is shown that by fully utilizing the high stretchability of the knitted fabric reinforcements, attractive material properties can be obtained especially for energy/impact absorption and forming applications. The improvement of thermoforming process stability with the use of carefully tailored knitted fabric reinforcements is also presented.

Abstract: Knowledge of resin chemical shrinkage is crucial for the determination of residual strains, stresses and warpage of composite parts during curing. Shrinkage measurement is more accurate on several millimetre thick samples. However, in that case thermal properties of resin and the strong coupling between thermoset chemical reactions (generally rapid and strongly exothermal) and thermal fields lead to non-negligible thermal and curing gradients in the piece. It is then necessary to take these variations into account to have an accurate description of the shrinkage. In the present study, a home built device "PVT-α" mould is used to measure the volume variation of vinylester resin and associated composites during the curing and then shrinkage is identified by considering these gradients. The results demonstrate that a linear evolution of the shrinkage with conversion degree is a good model to describe the chemical effect on the global volume behavior of the piece. The contribution of chemical and thermal effects on volume curve is quantified.

Abstract: Poly(lactic acid) (PLA) is a biodegradable, thermoplastic, aliphatic polyester derived from renewable resources, which is a potential candidate to replace conventional polymers in food packaging applications.. Other bio-based materials like polyhydroxyalkanoates (in our case PHBV) could be associated with PLA in order to improve its gas barrier properties and/or its mechanical properties. Two processes, classical 3 layer coextrusion and original polymer blending extrusion, have been used in this study to combine PLA and PHBV to obtain films with different PLA-PHBV structures. The aim of this paper is to compare the thermoformability of the different films and to investigate the final structure in relationship with their gas barrier properties. The effect of the thermoforming steep has been studied by separating heating and stretching effects. We show that the blend morphology offers better mechanical and barrier properties than single PLA or 3 layers PLA/PHBV films.

Abstract: The presentation deals with the non linear strongly elastic and viscous behaviour of poly ethylene terephthalate near the glass transition temperature and biaxially stretched at high strain rates representative of the injection stretch blow moulding process. A non linear visco-hyperelastic model inspired from [1] and identified from the experimental results of the equi-biaxial tension test [2], have been developed and presented in [3] is implemented into a finite element code developed with Matlab. The thermal behaviour modelling, identification and simulation has also been managed. First, a numerical simulation of 2D plane stress case has been performed involving 2 fields (global velocity V and elastic Cauchy Green tensor Be). Rectangular finite elements with quadratic and linear interpolations have been employed for velocity and the elastic left Cauchy Green tensor. Second, an axi symmetric formulation involving 4 fields (global velocity V, lagrange multiplier p associated with the global incompressibility condition, and multiplier q associated with the incompressibility of the elastic part) has been performed using rectangular elements. Degree of interpolation have been tested for all possible combinations to test the LBB like condition. Both simulations are compared with equi biaxial or sequential biaxial testing in order to reproduce the strain hardening effect and the self-heating observed. The final goal of this work is to perform the free blowing simulation to compare with experimental data. Therefore, we should solve an iterative procedure for a thermo-mechanical equation. At each time step, a four-field approach is adopted for the mechanical part, and a classical heat transfer equation is discretised for the thermal part.

Abstract: The laminated composites undergo chemical deformation on cross-linking and thermal deformation while cooling during fabrication process. In thin laminates, due to large displacements and complex shape evolution, these deformations can only be explained by using nonlinear strain-displacement relationship. In the present article, the thermal and chemical deformations occurring in carbon/epoxy laminates are calculated together for the first time by considering a non-linear geometrical approach, to understand the evolution of shape and hence residual stresses induced during fabrication process.

Abstract: In the last ten years carbon nanotube composites are in the focus of the researchers. Different composition of carbon nanotubes and polymers were produced by IDMX mixer. In the experiments polypropylene, polycarbonate and ABS polymers were used as matrix materials. Nanotube master batches were used to prepare different compositions. Concentration series were manufactured by the dynamic mixer. The prepared materials were characterised by scanning electron microscopy. The carbon nanotubes can be seen on the fractured surfaces. We did not find any sign of agglomerates in the materials. During the investigation of isothermal and nonisothermal crystallization of polypropylene nanocomposites it was shown that the carbon nanotube has nucleating effect on polypropylene. Mechanical properties were determined. It was found that the mechanical properties of the nanocomposites decreased. The flammability of the composites is significantly smaller than the original polymers. Polypropylene nanotube composites above 4% nanotube content were not dripping. It is most probable due to some kind of network structure of the material. We assume that the carbon nanotube creates a physical network in the polymers.

Abstract: In this paper a first approach is presented to investigate the electrical parameters of the EDM process using an intelligent process monitoring tool to evaluate different technologies and generators. Goal is to characterize every single discharge by extracting relevant data from recorded current and voltage signals using an adequate algorithm. For demonstration purposes a steel material is machined using a corresponding technology recommended by the machine tool supplier as well as a recommended technology for hard metal. In a first step the machining quality is evaluated by the cutting rate and surface roughness which is common practice for EDM. These results are then compared to the data extracted by the process monitoring tool. In the future this tool may be able to accelerate the parameter development for a specific task or material significantly. The effectiveness of the process could be determined directly during the process and no longer indirectly by a quality inspection of the machined part. By investigating the course of each discharge the process parameters can be linked to the actual physical parameters and a direct influence could be identified.

Abstract: Electrical Discharge Machining (EDM) enables an economical production of high performance ceramics with high hardness and toughness in complex shapes and fine structures. Recent research work identified ZTA-TiC composites as one of the most promising ED machinable materials with high strength for injection molds, which significantly reduce abrasive wear. In order to improve the surface finish and processing time of ED-machined ceramics zirconia toughened alumina composites with addition of 20-30 vol.-% titanium carbide as electrically conductive phase were developed. Processing was performed by hot pressing at 50 MPa and 1525°C. Mechanical and electrical properties were investigated. The influence of TiC volume fraction on the surface quality after wire-EDM and the maximum possible sample feed rate was analyzed. The variation of TiC content moderately influences toughness and bending strength while electrical resistivity and indentation modulus strongly depend on the volume fraction of TiC. A competitive feed rate of 2.45 mm/min for samples with a height of 13 mm was obtained. Further improvement of the surface quality can be achieved by increasing the amount of the electrically conductive phase. Additional machining steps proved feasible to decrease significantly the surface roughness and corrugation depth without inducing any defects in bulk material.