Evaluation of woven fabric composites for automotive applications

This paper summarizes the results of a recent study [1] funded by the National Science Foundation to investigate the feasibility of woven fabric composite materials for automotive applications. By identifying the advantages and limitations associated with woven fabric composites and by comparison with current automotive materials, the potential for successful application of these materials is investigated. In particular, strength, fatigue, moldability, and cost effectiveness have been identified as critical indicators of the potential for these materials in automotive applications. The results of an experimental evaluation of the static and fatigue properties of woven composites and comparable unidirectional tape composite laminates are discussed. An analytic model designed to quantify the effect of fabric weave configuration on relative conformability to complex geometries is also presented. Preliminary component designs utilizing woven fabric composites are considered in terms of potential weight savings, potential fabrication methods, and projected cost effectiveness. Finally, the key factors impeding the successful implementation of these materials in particular automotive structural applications are identified and reviewed.     

碳纤维编织复合材料板材热冲压成形的实验和模拟研究(英文)

提出一种新型的复合材料成形工艺,即热冲压成形,来直接成形复合材料。为了研究复合材料板的成形行为,分析了成形温度对零件的影响,进行了热弯曲和热拉深实验。实验结果表明,编织复合材料板的锁止角为30°,在成形过程中,变形载荷一般小于5 N,并且变形载荷随着温度的升高而降低。成形碳纤维复合材料板的最佳温度是170°C。采用有限元分析软件ABAQUS对模具的温度场分布和复合材料板的变形进行了数值模拟。为了研究碳纤维在成形过程中的运动,采用两节点的三维Truss单元T2D3对纤维进行网格剖分,模拟结果与试验结果相吻合。     

Simulation and experimental study on thermal deep drawing of carbon fiber woven composites

Carbon fiber woven composites are composed of carbon fiber woven and resin matrix. To reduce the manufacture cost, thermal stamping, a new forming technology, was proposed and investigated to fabricate composite part. The mechanical properties of carbon fiber have great influence on the deformation of carbon fiber composites. In this study, shear angle–displacement curves and shear load–shear angle curves were obtained from picture frame test. Thermal deep drawing experiments and simulation were conducted, and the shear load–displacement curves under different forming temperatures and shear angle–displacement curves were obtained. The results show the compression and shear between fiber bundles are the main deformation mechanism of carbon fiber woven composite. The maximum shear angle for the composites in this study is 33°. In the drawing process, the forming temperature affects the drawing force, which drops rapidly with the increasing temperature. The suitable forming temperature in deep drawing of the carbon fiber woven composite is approximately 170 °C.     

Effect of EDT surface texturing on tribological behavior of aluminum sheet

Surface texturing, which fabricates micro dimples or micro channels on the surface of parts, is a growing technique for improving tribological characteristics of materials. Currently, electrical discharge texturing (EDT) technique, one surface texturing method suitable for mass production, is being used to texture aluminum sheets for the applications in automotive industry. It has been widely accepted in industry that EDT improves the forming behavior of aluminum sheets due to better friction behavior. However, how the textures on the surface of sheet metal change the friction behavior has not been investigated. In this paper, the influence of EDT on the friction behavior of aluminum automotive sheet at different contact pressures and sliding speeds is investigated based on both experimental studies and numerical simulations. To fully investigate the tribological behaviors, a flat-on-flat friction test device was built and a numerical code based on mixed lubrication theory was developed. It was found that EDT texturing can reduce the friction coefficient of contacting pair at high contact pressure, however, increase friction coefficient at low contact pressure. Numerical simulations confirmed this finding. Furthermore, the model provides valuable information for the prediction of friction behavior of EDT sheets and helps to optimize processing parameters for various forming processes using EDT aluminum sheets.     

Influence of punch sequence and prediction of wrinkling in textile forming with a multi-punch tool

Liquid composite moulding (LCM) processes show a high potential in automated, large scale production of continuous fibre-reinforced plastics (FRP). One of the most challenging steps is the forming of the two-dimensional textile material into a complex, three-dimensional fibre structure. In this paper, a multi-punch forming process is presented. The upper mould of a generic part geometry is divided into 15 independently controllable punches. Depending on the different punch sequences, draping effects as well as defects related to wrinkling and shearing of the textile material are investigated. It has been shown that the sequence of the punches has a significant influence on the final preform quality. To predict the resulting regions of wrinkling and shearing, a finite-element based simulation model is set up. Forming tests and simulations with different punch-sequences are then performed and evaluated for validation purposes. To make a statement about the global preform quality, different objective functions regarding wrinkling are presented and analysed.     

DEVELOPMENT OF BRAIDED－PULTRUSION PROCESS AND STRUCTURE－PROPERTY RELATIONSHIPS FOR TUBULAR COMPOSITES

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On the gas pressure forming of aluminium foam sandwich panels: Experiments and numerical simulations

Forming of light-weight highly stiff aluminium foam sandwich (AFS) panels into complex 3D components would mark a development in the manufacturing of these materials. In this work, gas pressure forming of AFS panels is investigated experimentally and using numerical simulations. Deformation behaviour of AFS panels is studied during high-temperature uniaxial tension and compression, and constitutive models are developed and incorporated into FE simulations of the gas pressure forming process. Simulation results and experimental observations show reasonable agreement and demonstrate the possibility of forming AFS panels to significant deformations while maintaining considerable core porosity.     

Effect of material scatter on the plastic behavior and stretchability in sheet metal forming

Robust design of forming processes is gaining attention throughout the industry. To analyze the robustness of a sheet metal forming process using finite element (FE) simulations, an accurate input in terms of parameter scatter is required. This paper presents a pragmatic, accurate and economic approach for measuring and modeling one of the main inputs, i.e. material properties and its associated scattering.For the purpose of this research, samples of 41 coils of a forming steel DX54D+Z (EN 10327:2004) from multiple casts have been collected. Fully determining the stochastic material behavior to the required accuracy for modeling in FE simulations would require many mechanical experiments. Instead, the present work combines mechanical testing and texture analysis to limit the required effort. Moreover, use is made of the correlations between the material parameters to efficiently model the material property scatter for use in the numerical robustness analysis. The proposed approach is validated by the forming of a series of cup products using the collected material. The observed experimental scatter can be reproduced efficiently using FE simulations, demonstrating the potential of the modeling approach and robustness analysis in general.     

On the evaluation of superplastic characteristics using the finite element method

In recent years, there has been a considerable interest in the application of superplastic forming in the aircraft and automotive industries to realize complex parts. This requires a detailed design of the technological process in order to better exploit its peculiar potentialities. Nowadays, the finite element method represents the mainly used technique to plan the sheet metal forming processes whose simulation requires determination of material constants for superplastic materials. The aim of the present work is aimed to show a simple method to characterize superplastic materials. In this study, constant pressure free bulging for superplastic materials is analysed by finite element method and expressions to determine the constants of the superplastic materials have been proposed. In order to support the proposed method, numerical and experimental activities have been carried out which show the good reliability of values obtained from proposed expressions.     

Evaluation of effective material properties of composite materials using special FEM

In the present paper, a novel method using the special 2D finite element method (FEM) and the concept of equivalent homogeneous materials has been employed to evaluate the effective material properties of composites. Special 2D finite elements containing an internal defect or reinforcement have been well developed in order to greatly simplify the numerical modeling of composites. It can assure the high precision especially in the vicinity of defects or reinforcements in composite materials. Some numerical examples will be provided to demonstrate the validity and versatility of the proposed method by comparing the existing results from other literatures.     

Simulations of harmonic and pulsed ultrasonic polar scans

Numerous experiments [1], [2], [3], [4] and [5] have shown that ultrasonic polar scans are a promising tool for non-destructive characterization of fiber reinforced composites. However, because of the requirement to invert experimental data for extracting quantitative information [5], numerical simulations are mandatory. Such simulations have been developed before for single layered fiber reinforced composites. Nevertheless, since the vast majority of composites are multi-layered, the development of extended numerical models is needed. Such model is presented; together with a presentation of numerical simulations of ultrasonic polar scans for multi-layered composites. It is also shown that the polar scan of a fabric reinforced composite is quite different from a polar scan of (0/90°)-stacked unidirectional layers.Furthermore, the difference between a polar scan for an incident harmonic wave and for an incident pulse is shown.     

Electrical-assisted embossing process for fabrication of micro-channels on 316L stainless steel plate

The electroplastic effect of metallic materials induced by high density current can dramatically reduce the flow stress, which is beneficial to the forming process of less formable metal. In this study, the electrical-assisted embossing process was proposed to fabricate micro channels on metal workpieces. Experiments and numerical simulations were conducted to demonstrate the feasibility and advantages of the proposed process. An experimental process system was established and the electrical-assisted embossing process experiments for micro channels were performed. Consequently, the numerical model of the embossing process was verified by the results of the process experiments. The influence of process parameters on the embossing process was studied. Both the experiments and the numerical simulations showed that the stress in the workpiece was reduced and the depth of the fabricated channel feature was increased in the embossing process assisted by high-density electric current.     

动车组司机室外板多点拉形工艺研究

介绍了高速动车组铝合金车体司机室外板拉伸成形新工艺,研究了拉形设备和多点模具相结合的多点拉形技术.对典型零件多点拉形成形过程进行了数值模拟研究,并进行了工艺试验.结果表明,多点拉形技术能够解决新型结构司机室铝合金外板的成形问题,实现外板工件的柔性、数字化制造,解决新型结构司机室的国产化问题.     

Process Design for Hybrid Sheet Metal Components

The global trends towards improving fuel efficiency and reducing CO_2 emissions are the key drivers for lightweight solutions. In sheet metal processing, this can be achieved by the use of materials with a supreme strength-toweight and stiffness-to-weight ratio. Besides monolithic materials such as high-strength or light metals, in particular metal–plastic composite sheets are able to provide outstanding mechanical properties. Thus, the adaption of conventional, wellestablished forming methods for the processing of hybrid sheet metals is a current challenge for the sheet metal working industry. In this work, the planning phase for a conventional sheet metal forming process is studied aiming at the forming of metal–plastic composite sheets. The single process steps like material characterization, FE analysis, tool design and development of robust process parameters are studied in detail and adapted to the specific properties of metal–plastic composites. In material characterization, the model of the hybrid laminate needs to represent not only the mechanical properties of the individual combined materials, but also needs to reflect the behaviour of the interface zone between them.Based on experience, there is a strong dependency on temperature as well as strain rate. While monolithic materials show a moderate anisotropic behaviour, loads on laminates in different directions generate different strain states and completely different failure modes. During the FE analysis, thermo-mechanic and thermo-dynamic effects influence the temperature distribution within tool and work pieces and subsequently the forming behaviour. During try out and production phase,those additional influencing factors are limiting the process window even more and therefore need to be considered for the design of a robust forming process. A roadmap for sheet metal forming adjusted to metal–plastic composites is presented in this paper.     

Cold forming processes: some examples of predictions and design optimization using numerical simulations

Predicting the behaviour of steel during a deformation process, and then under service conditions, is one of the main challenges in cold forming. The design of optimized forging schedules, by means of classical trial+errors procedures, has become increasingly heavy in terms of time and cost in a competitive environment. Simultaneously, the improvement of steel qualities requires the microstructure, constitutive behaviour and deformability to be known a priori regarding a targeted application. During the last few years, numerical simulations have become a very efficient tool to reach these goals.

In this paper, we give examples of innovating forging sequences developed by numerical simulations, including the investigation of damage in tools and forged parts. In case of specific processes with very determined geometry — such as wire drawing — we show how systematic numerical studies may lead to predictive models of force, local strains and residual stress…

However, reliable predictions from numerical simulations require reliable input data, including constitutive laws, friction conditions and propensity to ductile damage. These data must be characterized under realistic sollicitations. Typical cold forging loadings are indeed very severe: local strains up to 600%, strain rates locally greater than 1000 s⁻¹, and subsequently, plastic heating over 500°C.

To characterize the constitutive behaviour, the standard upset test between grooved dies is used along with an original methodology to derive the strain hardening curve from the experimental force-displacement recording. Tool elastic deformations, specimen strain heterogeneity… are taken into account. This enables a precise determination of the strain hardening curve up to about 100% of strain. The extrapolation of the flow stress to greater deformations is then very easy and reliable. Such a test can be performed under quasi-static and isothermal conditions (0.1 s⁻¹ but also adiabatic and rapid conditions (up to 10 s⁻¹). This procedure was adapted to a Pellini hammer, which enables very simple characterization at 800 s⁻¹. The comparison of all these flow curves lead to the formulation of an original constitutive model, which accounts for the effects of plastic heating, strain rate, dynamic aging…

In order to predict ductile fracture during the forging process, the most classical criteria were tested over a wide range of experimental loading conditions. None of them were general enough to solve all the cold forming problems. On the other hand, mesoscopic models describing the deformation of the metal matrix around inclusions or second phases have proved to be in good agreement with the various experimental observations. An original plasticity criterium, based on the recent works in porous plasticity theory, has been developed and already displays promising capabilities. Simple experimental procedures enable a reliable classification of steel qualities, heat treatments… in term of forgeability.

Finally, the friction problem is treated using different methodologies based on the forming process considered. For forging operations, a fine analysis of the force-displacement curves in direct extrusion stages may lead to a precise measurement of the friction coefficient under pressures from about 200 MPa up to 1000 MPa and for sliding rates between 1 and 100 mm/s. For wire drawing process, a model relying on an analytical approach using a significantly improved slice method has been developed: the comparison of the experimental drawing force and the predicted one gives the friction coefficient in industrial processing conditions (speed up to 5 m/s).     

Self-piercing riveting process: An experimental and numerical investigation

The self-piercing riveting is a young technology for joining sheet metals in automotive structures. In this paper, the riveting process has been simulated numerically using the finite element code LS-Dyna. A 2D axisymmetric model was generated including two sheets to be joined, rivet and tools. The rivet and tools geometries are based on the Böllhoff standards. An implicit solution technique with r-adaptivity has been used. The advantages and the limits of using r-adaptivity in this class of metal forming process are discussed. In addition, parametric studies on important parameters for the forming process, i.e. friction, mesh size and failure criteria are presented. In order to validate the numerical simulation of the riveting process, a new test device was developed in order to record the force applied on the rivet during the riveting process. An extensive experimental program on specimens made of aluminium alloy 6060 in two different tempers, T4 and T6, has been used as a database for the validation of the numerical simulations. The results show the capability to simulate the riveting process for different combination of plate material and rivet geometries.     

Application of central composite design for optimization of two-stage forming process using ultra-thin ferritic stainless steel

Two-stage forming process for manufacturing micro-channels of bipolar plate as a component of a proton exchange membrane fuel cell was optimized. The sheet materials were ultra-thin ferritic stainless steel (FSS) sheets with thicknesses of 0.1 and 0.075 mm. For the successful micro-channel forming in the two-stage forming approach, three process variables during the first stage were selected: punch radius, die radius, and forming depth. In this study, the effect of the three process variables on the formability of ultra-thin FSSs was investigated by finite element (FE) simulations, experiments, and central composite design (CCD) method. The optimum forming process designed by the CCD showed good agreement with those by experiments and FE simulations. The newly adopted optimization tool, CCD, was found to be very useful for optimization of process parameters in the multi-step sheet metal forming processes.     

New solutions for the manufacturing of spacer preforms for thermoplastic textile-reinforced lightweight structures

The novel woven spacer fabrics consist of upper and lower layers which are connected through internal crosslinks. The aim is to weave spacer fabrics with woven crosslinks in a single production step as near net shape sandwich preforms. By this way reproducible and automated manufacturing of sandwich preforms for composites are realized. The paper deals at first with the conception of a new weaving and take-up technology of complete spacer fabrics without subsequent textile assembly processes. Afterwards, the special technology is described, and finally the paper deals with a simulation model for the prediction of dynamic warp thread forces in order to minimize fiber damage during weaving process.     

Influence of weld geometry and mismatch on formability of aluminium tailor welded blanks: numerical and experimental analysis

Abstract

The forming behaviour of tailor welded blanks (TWBs) has been widely studied since its development. In the numerical simulation studies, the TWBs are modelled as blanks composed of two different materials, and often, the presence of the weld bead is neglected in its finite element discretisation. In the present work, the influence of the weld bead shape on the formability of friction stir welded TWBs, is analysed. Several finite element meshes were constructed in order to represent different weld bead geometries and numerical simulations of the cylindrical cup drawing were performed. Strong influence of the weld bead shape on the formability of the TWBs was observed when the weld was in overmatch relatively to the base material, and little influence when the weld was in undermatch condition. Comparison of the numerical results with experimental ones shows that the numerical analysis is able to preview the formability of the TWBs.     

Forming Limit Diagram Prediction of Tailor-Welded Blank Using Experimental and Numerical Methods

The forming limit diagram (FLD) is a useful method for characterizing the formability of sheet metals. In this article, different numerical models were used to investigate the FLD of tailor-welded blank (TWB). TWBs were CO₂ laser-welded samples of interstitial-free (IF) steel sheets with difference in thickness. The results of the numerical models were compared with the experimental FLD as well as with the empirical model proposed by the North American Deep Drawing Research Group. The emphasis of this investigation is to determine the performance of these different approaches in predicting the FLD. These numerical models for FLD are: second derivative of thinning (SDT), effective strain rate (ESR), major strain rate (MSR), thickness strain rate (TSR), and thickness gradient (TG). Results of this research show necking will be happened, when the value of MSR, TSR, ESR criteria is maximum, TG????0.78 and SDT criterion has the first peak in forming process time. The value of dome height of TWB samples at failure was predicted based on the numerical models for samples with different widths. These numerical predictions were compared with the experimental results. The SDT model indicates a better agreement with experimental results in prediction of both the FLD and the limit dome height (LDH) in comparison to the other numerical models. Both numerical and experimental results show that minimum of LDH is happened in plane strain condition.