Automotive Structural Parts Research Articles

On the use of modal works of cutting forces to optimize machining conditions in the presence of vibrations

The use of Virtual Machining models may be a valuable approach in the designing stage of a machining operation as long as the models are sufficiently accurate. When vibration risks are suspected, stability analysis approaches to predict regenerative chatter phenomena are generally used. However, although these approaches, when applicable, allow efficient numerical optimization of the process around an operating point, they often require other strong assumptions such as neglecting transient phenomena or oversimplifying kinematics. On the other hand, time domain approaches with detailed matter removal modelling allow to monitor the continuous evolution of cutting conditions and represent various phenomena that the models can reproduce (regenerative chatter, forced vibrations, non-linear behaviours). The amount of data produced is, however, considerable and often costly to analyse. It may therefore be interesting to deduce, from these data, scalar indicators allowing easier and more relevant analysis of the simulation results.In this work, the modal work of the cutting forces upon the workpiece vibratory displacements is proposed as an indicator to discriminate different tool paths. A one degree of freedom theoretical problem and a face milling operation on extruded aluminum profiles extracted from automotive structural part are used to explain and show the relevance of such indicator.

Influence of pre-strain on tensile response of extra deep drawing (EDD) steel under varying strain rates and crash performance

This study explores the impact of pre-strain on the uniaxial tensile stress-strain behavior of extra deep drawing (EDD) steel at varying strain rates. The EDD steel blanks were subjected to a uniaxial pre-strain of 15 % in either the transverse or rolling direction. Subsequently, uniaxial tensile tests were conducted at different strain rates, both orthogonal and parallel to the initial pre-straining direction. The study also investigates the alterations in yield stress (YS), ultimate tensile strength (UTS), uniform elongation (UEL), total elongation (TEL), and strain energy absorption (SED, represented by the area under the tensile stress-strain curve) at diverse strain rates after pre-straining. The experimental findings indicate that pre-straining is advantageous for enhancing YS, UTS, and SED. However, this improvement comes at the expense of UEL or TEL. A square crush tube, which shares similarities with automotive components, was examined to assess the crash performance of several automotive structural parts during crash events. A validated constitutive model was used to predict the square tube's energy absorption and crush performance under high-velocity conditions for different pre-straining scenarios.

Manufacturing constrained shape optimisation of variable width flat web formed channels

The trend in automotive manufacturing towards lower volumes and an increased number of car variants combined with the need for forming higher strength metals to reduce weight has led to the implementation of alternative and flexible manufacturing methods. These have new manufacturing constraints compared to conventional stamping that change the part shapes that can be formed. This requires new methods for part shape optimisation. This study proposes a novel parametrisation for shape design that allows: 1) implementation of a gradient-based optimisation approach; and 2) taking manufacturing constraints into account. Our novel parameterisation can describe most long automotive structural parts using only a small number of design variables. The parts are described using multiple series of straight and curved connected profiles. We have uniquely conducted a detailed sensitivity analysis on the profiles to determine analytical solutions for the first order derivatives of the design variables with respect to the surface area/mass of a generic part. The profiles are also used to determine the final manufacturing strains in a part based on ideal forming. These ideal manufacturing strains can be compared to manufacturing process strain limits to determine the potential manufacturability of the part. The proposed parametrisation is applied to optimise a variable width channel formed by flexible roll forming. The channel is optimised to maximise the stiffness while maintaining both mass and manufacturability. In detail, the effectiveness and the general applicability of the established parametrisation technique and shape optimisation platform are demonstrated using three case studies of a flexible roll formed automotive S-rail channel part subjected to compression and bending loads. Furthermore, the manufacturability of the optimised structure is demonstrated by a forming model of the flexible roll forming process, where the model has been previously validated against experimental data. These examples show that the presented parametrisation and the associated shape optimiser can be successfully applied to increase part stiffness while reducing weight and maintaining manufacturability. The range of problems analysed demonstrates the flexibility and capability of the newly developed optimisation platform.

Development of High-Ductility and Low-Hot-Tearing-Susceptibility Non-heat Treatment Al–Mg–Mn-Based Die Casting Alloy for Automotive Structural Parts

Development of High-Ductility and Low-Hot-Tearing-Susceptibility Non-heat Treatment Al–Mg–Mn-Based Die Casting Alloy for Automotive Structural Parts

The fatigue fracture mechanism of Al7075 aluminum alloy clinching joints

Lightweight aluminum alloys are widely used in automotive structural parts. Clinching (press-joining) is one of the most promising alternatives to welding for joining aluminum alloys, and as the failure of the joint is mainly caused by fatigue, the fatigue properties of the clinched joints directly affect the reliability and safety of the connected specimens. To gain a deeper understanding of this phenomenon, the static mechanical and fatigue properties of Al7075 aluminum alloy single-lap clinched joints were studied using static strength and tensile-tensile fatigue tests. The F–N curves of the joints were fitted using a straight-line and three-parameter power function method. The fractured samples and wear scars were analyzed by scanning electron microscopy and energy-dispersive spectroscopy. The results showed that the neck of the clinched joint and the upper sheet near the joint experienced the highest degree of fatigue fracture, the fretting wear debris consisted mainly of alumina and metallic aluminum, and the fretting wear was of a mode that differed from the two modes identified in the literature that we have dubbed “upper sheet inter-sheet region fretting wear.”

Finite element implementation of hydrostatic pressure-sensitive plasticity and its application to distortional hardening model and sheet metal forming simulations

• The strength-differential (SD) effect of AHSS with ultimate tensile strength exceeding 1 GPa is modeled by a hydrostatic pressure-dependent distortional plasticity theory. • The finite element implementation of the pressure-dependent plasticity theory is proposed in the manuscript. The implemented theory is validated with the comparison study with the associated stand-alone code and experimental measurements. • Both the SD- and strain patch change effects of TIRP1180 are reproduced in finite element simulation by embedding the distortional plasticity model, HAH 20 , in the pressure-dependent yield condition. The effectiveness and applicability of the FE-implemented pressure-dependent distortional plasticity model in sheet metal forming simulations is validated through the springback prediction studies. In this work, the hydrostatic pressure-dependency of the distortional plasticity model HAH 20 was implemented using a finite element (FE) code to account for the strength-differential (SD) effect that has been observed in advanced high-strength steel sheets. To this end, a fully-implicit stress update algorithm formulation was introduced for the pressure-dependent plasticity theory. The implementation was validated by comparing the FE prediction of the material behavior during a number of tests with those of a stand-alone code of the constitutive description as well as with experimental data. In order to investigate the SD effect on the springback simulation results, the U-draw bending test was analyzed within this FE framework. Furthermore, in order to assess the effectiveness and stability of the formulation for a large-scale example, forming simulations of an automotive structural part were conducted. In addition to the SD effect, strain path changes and geometrical aspects were also investigated in this example. It was shown that the distortional plasticity-based pressure-dependent yield criterion well describes the asymmetric SD behavior in sheet metal forming simulations.

Strain rate-dependent crash simulation of woven glass fabric thermoplastic composites

Woven fabric thermoplastic composites possess high specific strength and stiffness along with thermoformability. To utilize the full potential of these materials to achieve better crash-safe designs in automotive structural parts, their crash behavior must be predicted accurately. For reliable crash simulations, strain rate-dependent material data and equally capable material modeling are required. In this study, quasi-static and high-speed tests are carried out to measure tensile and in-plane shear properties. A strain rate-dependent continuum damage mechanics model is formulated to describe the deformation and damage behavior of woven glass fabric composites. Tensile and in-plane shear tests on a lab scale are used to calibrate the material parameters of the model. The model was implemented as a user-defined material subroutine (VUMAT) for Abaqus. Experimental results from coupon tests were used to verify the results of a single-element simulation. Finally, a structural level dynamic crash test of a u-profile on a drop tower was used to validate the predictions of the user material model.

Study of Effect of Varying Clearances on the Springback of Advanced High Strength Steel Sheets

The springback of metal sheets shows a significant effect on the forming results of automotive structural parts. The components of new vehicles often have complex shapes, for which more precise forming procedures are required in order to achieve their desired geometries. Such springback occurrence is highly critical in the case of advanced high strength (AHS) steels. In this work, a V-shape stamping test was carried out for the AHS steel sheets grade 980 with an initial thickness of 1 mm. In parallel, the corresponding finite element (FE) simulations were conducted. Hereby, the Yoshida-Uemori (Y-U) kinematic hardening model was applied for describing the plastic deformation and elastic recovery of material. The parameters of the Y-U model were obtained from a tension-compression test and afterwards verified by using the 1-element model. The predicted bend angles of the formed samples fairly agreed with the experimentally measured results. Furthermore, the effect of defined die clearance at the corner of the formed sample on the magnitude of springback was numerically studied. It was found that the reduction of clearance of 10% led to obviously decreased shape deviations in the V-shape forming test.

高強度鋼板中の水素拡散挙動に及ぼす張出し加工の影響

The hydrogen diffusion behavior in a tempered martensitic steel sheet with 1-GPa grade tensile strength was investigated utilizing a newly developed hydrogen visualization technique using an Ir complex, whose color changes from yellow to orange due to its reaction with hydrogen. The hydrogen permeation through the steel sheet could be monitored from the color change of the Ir complex. Furthermore, the breakthrough time of hydrogen through the specimen could be qualitatively evaluated from the changes in the brightness of the Ir complex. This hydrogen visualization technique was also applied to a steel sheet stretch-formed using a hemisphere punch to simulate press-forming of automotive structural parts. The hydrogen breakthrough time around the top of the specimen was long and decreased with increasing the distance from the top. According to the plastic strain distribution of the specimen calculated by the finite element method, the hydrogen breakthrough time increased with increasing plastic strain. It was considered that the introduction of plastic strain decreased the hydrogen diffusion coefficient due to the introduction of dislocations acting as hydrogen trap sites, resulting in the increase of hydrogen breakthrough time.

Experimental and numerical study of springback effect of advanced high strength steel in a V-shape bending

Advanced high strength (AHS) steel sheets are increasingly used for the production of various automotive structural parts. The components of new lightweight vehicles have very complex Shapes, for which more precise forming procedures are required in order to achieve a desired geometry. Hereby, springback occurrences of such AHS parts are often the most critical concern. In this work, it aimed to investigate springback effects of the AHS steel grade 980 in a V-shape bending process. Experimental bending test and its corresponding FE simulations were conducted. The Hill’48 and Barlat89 yield criteria, and the Yoshida-Uemori (Y-U) kinematic hardening model were applied. The yield function parameters were obtained from the tensile tests of samples in various orientations. The Y-U model parameters were determined from a cyclic tension-compression test and were afterwards calibrated with the 1-element simulation. The resulted bend angles measured from the experiment and predicted by FE simulations using the different models were compared and evaluated. For the bending in this work, the Hill’48 and Barlat89 models showed the predictive errors of springback angle 1% and 2% higher than the Y-U model, respectively. The accuracy of springback prediction could be improved by the Y-U model using C1 and C2 parameters around 1%. In addition, effects of sheet thickness and punch radius on the springback were afterwards studied and discussed by using the Y-U model.

Study of flow-induced fiber in-plane deformation during high pressure resin transfer molding

High Pressure Resin Transfer Molding (HP-RTM) is a new variant of composite Resin Transfer Molding (RTM) process that enables a short cycle time and a high composite strength to weight ratio, thus presents a great potential for fabricating automotive structural parts. Due to the high injection pressure, fiber-tow washout is becoming one of the major defects which impact the properties of composite materials. To predict and mitigate the fiber-tow washout problem, approaches of both experimental process optimization and computational prediction are essential. In this paper, an experimental study of fiber-tow washout is undertaken to determine the flow injection limits beyond which the preform deformation can be observed at various fiber volume fractions. A feasibility map is developed for a specific fabric and resin combination. It provides a means to determine the injection rates and fiber volume fractions to fabricate a quality part with minimal in-plane fiber washout due to the hydrodynamically flow-induced force during the HP-RTM process.

Calibration of a strain path change model for a dual phase steel

Dual phase steels are largely used to form automotive structural parts by deep drawing involving complex loading paths, which influence the material mechanical behavior and formability, as well as subsequent service life. The aim of this paper is to investigate the calibration of a strain path change model from an experimental database involving simple shear tests. The enhanced Homogeneous Anisotropic Hardening (e-HAH) is an advanced constitutive model that can take account of the strain path change influence on the material behavior by using a stress-based indicator. The performance of such a model depends highly on the material parameter identification and the experimental database. The mechanical behavior of a dual phase steel DP600 is first characterized under different linear strain paths, uniaxial and biaxial tension and simple shear, in order to quantify the initial anisotropy. Then, the influence of strain path changes on the material behavior is investigated during sequences involving simple shear, uniaxial tension and compression. Finally, the parameters of e-HAH model are identified and the influence of the experimental database on the optimised parameters is highlighted with the use of different strain paths and different strain ranges.

Modeling Shear Behavior of Woven Fabric Thermoplastic Composites for Crash Simulations

Woven fabric thermoplastic composites possess high specific strength and stiffness along with thermoformability. To utilize the full potential of these materials to achieve better crash-safe designs in automotive structural parts, the measurement of non-linear shear behavior and its material modeling for FEM simulations is required. The standard testing method was used to measure the pure shear behavior of woven fabric composites. These results were compared with the shear behavior of material in the presence of normal stresses along the fiber direction. Tensile and compression cyclic testing of ± 45° laminate were carried out to measure the stiffness degradation and hardening of the material in the presence of tensile normal and compression normal stress. A methodology is proposed for taking into account the differences in shear behavior under different loading directions in an FEM simulation. Based on the experimental evidence, improvements in the mathematical description of plasticity and damage in continuum damage mechanics models are proposed. The model was implemented as a user-defined material subroutine (VUMAT) for Abaqus. The experimental results from coupon tests were used to verify the results of a single element simulation. Finally, a three-point bending test was used to validate the predictions of the user material model.

Abnormal mechanical response in a silicon bearing medium carbon low alloy steel following quenching and bainitic holding versus quenching and partitioning treatment

Multiphase advanced high strength low alloy steels having a refined microstructure of hard bainite and/or martensite laths, interspersed with finely divided retained austenite films, are the potential candidate materials for futuristic industrial applications, such as automotive industry, structural parts, crankshafts and shafts, and powertrain components. In this study, a silicon-bearing, medium-carbon low alloy steel grade of DIN 1.5025 was subjected to both quenching and bainitic holding (Q&B), as well as quenching and partitioning (Q&P) isothermal heat treatments at temperatures closely above and below the martensite start temperature (Ms), respectively. The mechanical response of heat treated samples was evaluated by conducting hardness and tensile tests, corroborated by XRD analysis and metallography using both scanning electron microscopy combined with electron backscatter diffraction as well as transmission electron microscopy. The experimental results showed that a number of Q&B heat treated samples displayed a superior combination of high strength levels with good ductility and work hardening capacity in comparison to that of Q&P ones, akin to the requirements for third generation high strength multiphase steels. It was also found that the superior mechanical response of Q&B heat treated samples was rationalized in respect of the formation of tough strong multiphase microstructures involving a fine mixture of hard bainite and/or martensite laths along with thin films of mostly interlath retained austenite that imparted superior combination of ductility and tensile strength through TRIP phenomenon. Moreover, limited tensile test results conducted in this study, supported by XRD and electron microscopy analyses, suggested that the TRIP response was quite effective in the Q&B multiphase samples containing more than 18 vol.% of retained austenite.

Effect of Hexamethyldisilazane-Modified Nano Fumed Silica on the Properties of Epoxy/Carboxyl-Terminated Poly(butadiene-co-acrylonitrile) Blend: A New Hybrid Approach

In this study, we have developed a hybrid system from epoxy/carboxyl-terminated poly(butadiene-co-acrylonitrile) (CTBN) and hexamethyldisilazane-modified nano fumed silica (HMDS NS). We aim to disclose the knowledge on how modified nano fumed silica influences the morphology and impacts properties of epoxy and epoxy-CTBN blend systems without significantly compromising the other desirable mechanical and thermal properties. High surface area and increased hydrophobicity of HMDS NS cause homogeneous dispersion in the hydrophobic polymer matrix. The results confirmed that the improvement in impact and tensile properties of epoxy thermoset depends mainly on the optimum concentration of CTBN and HMDS NS. The thermomechanical analysis showed that HMDS NS acts as a plasticizer and it increased the mobility of CTBN chain and created flexible regions in system. The current research on the hybrid system from the epoxy/CTBN/HMDS NS approach has the potential for the development of improved structural adhesives, automotive structural part, and electronic encapsulate.

Baking Effect on Desorption of Diffusible Hydrogen and Hydrogen Embrittlement on Hot-Stamped Boron Martensitic Steel

Recently, hot stamping technology has been increasingly used in automotive structural parts with ultrahigh strength to meet the standards of both high fuel efficiency and crashworthiness. However, one issue of concern regarding these martensitic steels, which are fabricated using a hot stamping procedure, is that the steel is highly vulnerable to hydrogen delayed cracking caused by the diffusible hydrogen flow through the surface reaction of the coating in a furnace atmosphere. One way to make progress in understanding hydrogen delayed fractures is to elucidate an interaction for desorption with diffusible hydrogen behavior. The role of diffusible hydrogen on delayed fractures was studied for different baking times and temperatures in a range of automotive processes for hot-stamped martensitic steel with aluminum- and silicon-coated surfaces. It was clear that the release of diffusible hydrogen is effective at higher temperatures and longer times, making the steel less susceptible to hydrogen delayed fractures. Using thermal desorption spectroscopy, the phenomenon of the hydrogen delayed fracture was attributed to reversible hydrogen in microstructure sites with low trapping energy.

Characterisation and Comparison of Process Chains for Producing Automotive Structural Parts from 7xxx Aluminium Sheets

Due to their high specific strength, EN AW-7xxx aluminium alloys are promising materials for reducing the weight of automotive structural parts. However, their formability at room temperature is poor due to pronounced natural ageing. Therefore, we investigated hot stamping and W-temper forming for EN AW-7075 and a modified variant of EN AW-7021. For hot stamping of the modified EN AW-7021, a low-temperature stabilisation heat treatment (pre-aging at 80 °C for 1 h) was incorporated into the process chain design to inhibit natural ageing after forming. The process chains were compared with respect to dimensional accuracy, mechanical properties, microstructure, precipitation status (assessed by differential scanning calorimetry) and crashworthiness. It was found that hot stamping is suitable to form failure-free parts with good dimensional accuracy for both alloys while W-temper forming suffers from springback. Within a time-span of 21 days after forming, hardness values of hot stamped and stabilised parts did not increase significantly. Compared to non-stabilised parts, stabilised parts also showed significantly improved folding behaviour in quasi-static compression testing and absorbed approximately 15% more energy.

Predictive modelling of longitudinal bow in Chain-die formed AHSS profiles and its experimental verification

Chain-die forming, as a newly developed technology, is considered as an alternative method to fabricate AHSS automotive structural parts. The longitudinal bow is one of the most general geometric deviations of cold-formed materials. This paper utilizes analytical modelling, finite element analysis and experimental verification to disclose the mechanism of the formation of the longitudinal bow of Chain-die forming of AHSS profiles. To improve accuracy, the Chaboche hardening model is adopted in the simulation work to characterize the Bauschinger effect of the AHSS products. A detailed comparison of the mechanisms of the formation of the longitudinal bow of Chain-die-formed and roll-formed AHSS profiles is carried out to emphasize their differences. The analytical model indicates that, in contrast to roll-formed profiles, longitudinal bow in Chain-die-formed profiles is primarily caused by the elastic recovery of the accumulated incremental plastic deformation in the bend corner. It can be understood as a follow-up phenomenon accompanied by springback. With a certain material, its severity is related to the thickness, the bend corner radius and the second moment of area of a product. This is consistent with the simulation and experimental results. Finally, a comprehensive numerical investigation is used to quantify the effect of critical parameters on the longitudinal bow.

Manufacturing and Characterization of Multilayered 7000-Series Aluminum with Improved Corrosion Behavior Processed via Accumulative Roll Bonding

High-strength aluminum alloys are promising materials for automotive structural parts since they offer a great potential to reduce vehicle weight and greenhouse gas emissions. For this reason, 7000-series aluminum alloys recently became object of many investigations. However, one major drawback of these alloys is their poor corrosion behavior, which is a crucial aspect and currently limits its application in automotive industry. One approach to influence this factor, without changing the alloying system, is the accumulative roll bonding (ARB) process. Within this contribution, a multilayered material consisting of discrete layers of AA7075 and AA1050 will be manufactured via ARB. The influence of the ARB process on the resulting mechanical properties of the multilayered material will be evaluated applying tensile tests and it will be compared with the initial condition of both single layered materials. Furthermore, the effect of the combination of these two aluminum materials on the corrosion behavior will be investigated

Investigation of the fibre type influence on the energy density of the induction heating process through a semi-analytic method

The study investigated the induction welding of carbon fibre-reinforced Polyamide 66 with different types of carbon fibres and fabrics. The major scope of the study was the plastification of the thermoplastic matrix as the first process step of the continuous induction welding. A new semi-analytic method was created to simulate the dependence of the plastification parameters (induction coil, power, weld velocity and coil-workpiece distance) on the fibre and fabric. Numerical and physical methods of Duhovic [1] and Velthuis [2] were used to investigate the complete process of plastification and consolidation. This kind of simulation uses the principle of the finite element method (FEM) [1]. The calculated results are strongly influenced on the one hand by the simplification of the thermal conductivity of the fibre and the matrix in the composite material (constant shape value, constant beam value) and on the other hand by the parameterization of the heat source.The semi-analytic method uses the radial basic function and the physical correlation between the thermal losses and the induced eddy currents at the electric conductive carbon fibers for the calculations. The calibration of the tool was done by testing a series of plastification parameters for different kinds of fibre and fabrics. In-situ temperature measurements through thermocouples and pyrometer were carried out for the melt zone in addition to destructive testing of the resulting joint. Yarlagadda [3] and Ahmed [4] have investigated the induction heating of carbon fibres and have shown three kinds of heating mechanisms: Joule loss/fibre heating, junction heating/dielectric hysteresis heating and junction heating/contact resistance heating. The simulation model investigated the amount of heating based on these three mechanisms.The semi-analytic model was created to use it for weldability check of material combinations and the pre-parameter setup for induction welding at a fully automated production system of composite automotive structural parts [5].