Abstract The limited capability to predict material failure in composite materials and specifically in wavy composite layers has led to high margins of safety for the design of composite structures. Thus, the full lightweight potential of this class of materials is left unused. To understand the complex failure behavior of composite materials containing out-of-plane fiber waviness under compressive and tensile loading, a non-linear 2D material model was implemented in ABAQUS and validated with extensive experimental test data from compression and tensile tests. Each test was recorded by a stereo camera system for digital image correlation to resolve damage initiation and propagation in detail. This study has shown excellent agreement of numerical simulations with experimental data. In a virtual testing approach various parameters, i.e. amplitude, wavelength and laminate thickness, have been studied. It was found that the failure mode changed from delamination to kink shear band formation with increasing laminate thickness. The wavelength has shown minor influences compared to amplitude and laminate thickness.

Abstract The texture evolution of copper tube manufactured by floating plug drawing process was investigated by Electron Backscatter Diffraction techniques and Visco-Plastic Self-consistent (VPSC) modelling method. The results obtained from experimentation indicate that the initial textures of copper tube transformed sharply after the first pass of the drawing process, and the final textures in the tube consist of Goss {110} <001> and P {110} <111> components. The VPSC model was established in order to study the texture transformation and the activity of slip systems of the tube during the drawing process. Two types of transformation paths were proposed to explain the mechanism of the texture evolution, which has a good agreement with the VPSC simulation results.

Processing tubes from poly (l-lactic acid) (PLLA) by stretch blow moulding (SBM) is used in the manufacture of bioresorbable vascular scaffolds (BVS) to improve their mechanical performance. To better understand this processing technique, a novel experimental setup by free stretch blow inside a water bath was developed to visualise the tube forming process and analyse the deformation behaviour. PLLA tubes were heated, stretched and blown with no mould present inside a temperature-controlled water bath whilst recording the processing parameters (axial force, inflation pressure). The onset of pressure activation relative to the axial stretch was controlled deliberately to produce a simultaneous (SIM) or sequential (SEQ) mode of deformation. Real-time images of the tube during forming were captured using high speed cameras and the surface strain of the patterned tube was extracted using digital image correlation (DIC). The deformation characteristics of PLLA tubes in SBM was quantified by analysis of shape evolution, strain history and stress-strain relationship.

Abstract Incremental sheet-bulk metal forming (iSBMF) enables the manufacture of functional lightweight components featuring a load-adapted shape with a high material efficiency. The flexibility of the incremental forming process allows for the modification of the strain path through the adjustment of the tool motion while maintaining the final product geometry. These modifications generate both a different strain hardening and damage evolution. In this paper, a numerical and experimental investigation of the different strain paths is carried out to identify their impact on the resulting load capacity of gears. In experiments on the quasistatic load capacity of the gears it is validated that forming of gears with a strain path showing a reduced damage potential leads to a 50% higher load capacity compared to the most unfavorable strain path. Moreover, all investigated load paths present load changes that have to be taken into account in numerical modeling of iSBMF processes. Therefore, a new approach for a material characterization under multiple load changes and high effective plastic strain is tested. Compared to numerical modeling with a characterized monotonically flow curve, this approach decreases the deviation force prediction by around 80% without increasing the calculation time.

The paper presents the analysis of hot deformation behavior of Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) alloy using the theory of dynamic material modeling (DMM) based on hot compression tests performed to a total true strain of 1 at the strain rates from 0.01 to 100 s−1 and at the temperatures within the range between 800 and 1100 °C. The processing maps according to the Prasad’s criterion were developed. The analysis of the processing maps allowed for the placement of domains describing the areas of potentially favorable combinations of hot deformation parameters. The microstructure observations of the investigated alloy specimens after hot deformation in stability and instability areas were conducted. The optimal processing parameters for numerical modeling of Ti-6246 alloy forging were selected based on processing maps. After complex analysis of the obtained results, microstructural observations and numerical modeling of forging of selected part, the forging tests of Ti-6246 alloy were realized. The obtained product quality assessment was carried out by computed tomography non-destructive testing.

Abstract It is worthy to investigate how it will affect the parameters calibration using an average stress state variable before the practical application of a ductile fracture criterion. In order to study this problem, ten notch specimens of AA7075-T6 sheet were designed to implement the tension tests and a parallel simulation of each test was run to obtain the variation of related variables such as stress triaxiality, Lode parameter and fracture strain. The Lou-Huh criterion was selected to research the prediction error through the difference between the damage value calculated by integral expression and that calculated by analytical expression. Based on the error analysis method, a clear answer was given on how to choose the tension tests of notch specimens in the calibrating process of fracture parameters. The studying results show that the stress state variation of notch specimens has a significant influence on the calibration result. It turns out that how to choose specimens from the ten notch specimens to calibrate the fracture parameters also has big influence on the result. Therefore, it is necessary to conduct an error analysis after the calibration of fracture parameters. Based on the error analysis results, the fracture parameters of AA7075-T6 sheet were optimized and its forming limit diagram (FLD) was deduced based on the optimized parameters. The predictive result of FLD is safe compared with the experimental forming limit results.

In the original publication of this paper, an incorrect image of Figure 3 was used in error.

Abstract Large components, for instance in the energy industry, mining and heavy machinery, are produced from high weight cast as ingots followed by open-die forging. Besides achieving a certain final geometry and microstructure, one of the main objectives during the forging process is the elimination of casting defects, like voids from the solidification shrinkage. This process is divided in the two stages of void closure and void healing. During the healing of the closed void a solid bond is established at high temperature. In literature void closure in open die forging is thoroughly investigated. Concerning the healing by solid bond generation there are only few studies related to voids in open die forging but there is substantial literature related to bond formation in roll hot bonding and diffusion bonding. Most of this work however determines the bond strength after cooling to room temperature. Concerning future appropriate modelling of the closure and healing process in open die forging, it is important to decide, whether a bond, which was established in one forging stroke, would be strong enough to withstand the following strokes. As a first step in this direction, this paper experimentally examines the bond strength directly after bond formation under conditions typical for open die forging strokes. The results quantitatively confirm the expected influence of forming temperature, surface enlargement, holding time and oxide films.

Additive manufacturing is the more and more considered in industry, however efficient simulation tools able to perform accurate predictions are still quite limited. The main difficulties for an efficient simulation are related to the multiple scales, the multiple and complex physics involved, as well as the strong dependency on the process trajectory. This paper aims at proposing a simplified parametric modeling and its subsequent parametric solution for evaluating parametrically the manufactured part distortion. The involved parameter are the ones parametrizing the process trajectories, the thermal shrinkage intensity and anisotropy (the former depending on several material and process parameters and the last directly depending on the process trajectory) and the deposited layers. The resulting simulation tool allows evaluating in real-time the impact of the parameters just referred on the part distortion, and proceed to the required geometrical compensation.

Abstract A numerical methodology is proposed to predict void content and evolution during autoclave processing of thermoset prepregs. Starting with the initial prepreg void content, the void evolution model implements mechanisms for void compaction under the effect of the applied pressure, including Ideal Gas law compaction, and squeeze flow for single curvature geometries. Pressure variability in the prepreg stack due to interactions between applied autoclave pressure and anisotropic material response are considered and implemented. A parametric study is conducted to investigate the role of material anisotropy, initial void content, and applied autoclave pressure on void evolution during consolidation of prepregs on a tool with single curvatures. The ability of the model to predict pressure gradient through the thickness of the laminate and its impact on void evolution is discussed.

Abstract In bubble assisted vacuum thermoforming, measuring pressure-induced mechanical strains through the stereo-digital image correlation (stereo-DIC) technique while shaping thermoplastic sheet requires consideration of an appropriate reference state of surface deformations. However, when the stereoscopic measurements can be only performed after the heating step, the correlation problem should be well-posed otherwise the reliability of results is limited. This study focuses on stretching by bubble inflation processes following thermal warpage and sagging of initially flat sheets. For this purpose, an experimental rig is instrumented to heat high impact polystyrene (HIPS) sheets and to perform synchronized pressure and stereoscopic measurements during 1.5 s stretching. A two-step method is introduced to separate mechanical strains which are affected by the uncontrollable change of initial conditions from the global stereo-DIC strains. The first step relies on amplification of damped oscillations at the initiation of the inflation process due to sagging. Out-of-plane displacements confirm the existence of a temperature-dependent characteristic time that marks the transition from the sagged to the strained surface shapes. The second step uses these characteristic times to objectively shift the reference of image-correlation computations. To evaluate the effectiveness of the suggested method, inaccuracy levels of global strains are evaluated at a fixed pressure level under different thermal conditions. It is shown that inaccuracy levels are the highest when stereo-DIC measurements followed warpage and they decrease with amplification of sagging. The developed approach extends the use of in-situ stereo-DIC measurements when changes of initial conditions are uncontrollable and thermal strains cannot be measured.

Abstract To simulate the behaviour of textiles, three major characteristics are important, kinematic fibre interaction, shear behaviour, and thickness changes of the fabric caused by shearing. Instead of anisotropic continuum mechanical models normally used, a macroscopic finite element coupled with an internal unit cell, made of beam elements is proposed here. The beam elements represent the yarns. The method is generalized for unit cells with parallelogram shaped unit cell geometries. The coupled unit cell model can improve finite element simulations, in terms of calculation time and modelling effort, because the major characteristics named before can be described in detail by the unit cell without using full-scale models.

Hemming is the main assembly process used for auto-body panels. Surface defects resulting from the hemming operation may significantly influence the appearance aesthetics of an automobile. The formation of surface defects can be forestalled, and the process can be optimized and compensated by the numerical simulation method during the process design stage. In this study, the hemming assembly of a fuel tank cap is chosen as the research object, and the specimen is made of DX54D + Z deep drawing steel sheet. Numerical simulation and forming experiments have been carried out. It was found that the surface defect index can be expressed as the weighted sum of the design variable effects by simulation and orthogonal design. In addition, the magnitude of each effect can be ranked from the most significant to the least significant. Using this information helps to obtain an optimal hemming process plan. However, process parameter optimization cannot completely eliminate surface defects. The surface compensation of tool geometry based on the continuity of curvature was used to eliminate the surface lows and the hemming test results indicated that the obtained component was of the required quality.

Bioresorbable Vascular Scaffolds (BVS) manufactured from poly (l-lactic acid) (PLLA) offer an alternative to metal scaffolds for the treatment of coronary heart disease. One of the key steps in the manufacture of these scaffolds is the stretch blow moulding process where the PLLA is biaxially stretched above glass transition temperature (Tg), inducing biaxial orientation and thus increasing ductility, strength and stiffness. To optimise the manufacture and performance of these scaffolds it is important to understand the influence of temperature and strain rate on the constitutive behaviour of PLLA in the blow moulding process. Experiments have been performed on samples of PLLA on a custom built biaxial stretch testing machine to replicate conditions typically experienced during blow moulding i.e. in a temperature range from 70 °C to 100 °C and at strain rates of 1 s−1, 4 s−1 and 16 s−1 respectively. The data is subsequently used to calibrate a nonlinear viscoelastic material model to represent the deformation behaviour of PLLA in the blow moulding process. The results highlight the significance of temperature and strain rate on the yielding and strain hardening behaviour of PLLA and the ability of the selected model to capture it.

Abstract Springback simulation of stamped sheet metal components using finite element method depends on the accuracy of appropriate material models and consideration of appropriate experimental strategies. In this work, tension-compression tests with different strategies, e.g. tension-compression, compression-tension up to various strain levels and multicycle compression-tension tests were conducted to determine parameters of the Yoshida-Uemori (Y-U) nonlinear dynamic hardening model using optimization analysis software LS-OPT. Finite element simulations with LS-DYNA were performed to predict springback behavior of both the advanced high strength steel MP980 (a 980 MPa grade multiphase steel) and aluminum alloy 6022-T4, which was then compared to measurements of stamped U-channel specimens. Results suggest that although the various tension-compression testing strategies can significantly affect the determined values of Yoshida-Uemori model parameters, springback prediction accuracy with this model does not depend on the associated variation of model parameters, at least for the two-dimensional sidewall curl of a U-channel shape. For materials (e.g. MP980) exhibiting a clear Bauschinger effect but insignificant texture anisotropy, the selection of suitable yield criteria (e.g. Hill48), the consideration of elastic modulus degradation combined with the Y-U model can obviously increase the accuracy of springback prediction. In contrast, materials (e.g. AA6022-T4) that exhibit little Bauschinger effect but have significant texture anisotropy, the use of a yield criterion that accounts for anisotropy (e.g. YLD2000-2D) is more important for improving the accuracy of springback prediction.

Abstract To this day, the design of preforms for hot forging processes is still a manual trial and error process and therefore time consuming. Furthermore, its quality vastly depends on the engineer’s experience. At the same time, the preform is the most influencing stage for the final forging result. To overcome the dependency on the engineer’s experience and time-consuming optimization processes this paper presents and evaluates a preform optimization by an algorithm for cross wedge rolled preforms. This algorithm takes the mass distribution of the final part, the preform volume, the shape complexity, the appearance of folds in the final part and the occurring amount of flash into account. This forms a multi-criteria optimization problem resulting in large search spaces. Therefore, an evolutionary algorithm is introduced. The developed algorithm is tested with the help of a connecting rod to estimate the influence of the algorithm parameters. It is found that the developed algorithm is capable of creating a suitable preform for the given criteria in less than a minute. Furthermore, two of the five given algorithm parameters, the selection pressure und the population size, have significant influence on the optimization duration and quality.

Forming nickel-based superalloy aero-engine components is a challenging process, largely because of the risk of high degree of springback and issues with formability. In the forming tests conducted on alloy 718 at room temperature, open fractures are observed in the drawbead regions, which are not predicted while evaluating the formability using the traditional forming-limit diagram (FLD). This highlights the importance of an accurate prediction of failure during forming as, in some cases, may severely influence the springback and thereby the accuracy of the predicted shape distortions, leading the final shape of the formed component out of tolerance. In this study, the generalised incremental stress-state dependent damage model (GISSMO) is coupled with the isotropic von Mises and the anisotropic Barlat Yld2000-2D yield criteria to predict the material failure in the forming simulations conducted on alloy 718 using LS-DYNA. Their effect on the predicted effective plastic strains and shape deviations is discussed. The failure and instability strains needed to calibrate the GISSMO are directly obtained from digital image correlation (DIC) measurements in four different specimen geometries i.e. tensile, plane strain, shear, and biaxial. The damage distribution over the drawbeads is measured using a non-linear acoustic technique for validation purposes. The numerical simulations accurately predict failure at the same regions as those observed in the experimental forming tests. The expected distribution of the damage over the drawbeads is in accordance with the experimental measurements. The results highlight the potential of considering DIC to calibrate the GISSMO in combination with an anisotropic material model for forming simulations in alloy 718.

In the present paper, finite element simulations are used to gain a better understanding of the setting process of a blind rivet nut. A blind rivet nut is a mechanical fastener capable of clinching materials whilst providing a threaded solution without the need for thread forming. The technique relies on plastic deformation introduced by axial compression of the rivet nut in such a way that a counter head is formed on the opposite side of the work piece. For certain applications, stresses in the plate material induced by the setting process are detrimental for the fastener’s integrity. Hence an improved design of the fastener is desired. To embark on such a redesign, an appropriate numerical model to reveal the influence of several parameters is indispensable. In this work, a strategy is presented to simulate the setting process involving large plastic strains and contact pressures. An FE based inverse method was used to identify the local plastic material properties of the blind rivet nut. The forming simulation was validated in terms of predicted shape of the rivet nut and the evolution of the setting force. A quasi-static FE model using the shape and solution variables of the deformed rivet nut was used to reproduce the torque-to-turn resistance as a function of the setting force. The strategy was successfully applied on two blind rivet nuts, different in geometry and base material. Finally, three industrial case studies confirmed the viability of the model.

This paper presents an experimental study of the effect of mesoscopic buckles defect and reinforcement shear, which result from forming, on the low velocity impact behavior of a composite laminate. The material studied is a glass/polyester composite with three layers of mat and one layer of taffeta fabric. To assess the properties induced on the final composite, plates with different amplitudes of calibrated defects and deformations were manufactured. First, the healthy material, which serves as a reference, was subjected to three levels of impact energy to observe the evolution of its behavior and damage mechanisms. Results of the impact tests and observations performed on the materials with calibrated defects identified a negative effect of buckling on elastic parameters and revealed greater damage relative to the healthy material. The reinforcement shear had a beneficial effect on the impact properties of the laminate, which was attributed to the increase in local fiber density.

The possibility to study and clearly define the physics phenomena that occur during the machining process of various metal materials is becoming one of the interpretative keys to quantify a product’s quality and life cycle performance. An accurate understanding of the surface integrity can be achieved through the knowledge of the fundamental details about the mechanical response and the behaviour of the affected material layers caused by thermo-mechanical loads induced by machining operations. Therefore, this set of information can help the designer to produce parts with superior quality. The aim of this work is to study the surface layer states in terms of metallurgical and mechanical properties of aluminium alloy 7075 in a machined and a sereverely plastic deformed by the Equal Channel Angular Pressing (ECAP) process. The outcomes provided by the experimental measurements permitted to find possible links regarding the microstructural and hardness changes observed between the machined surface layer and the region of material deformed by ECAP. Finally, this scientific investigation aims to establish the basis for an innovative method to study and quantify the metallurgical phenomena that occur beneath the machined surface of bulk metal materials.

Abstract Formability of a continuous fiber-reinforced material is known to be influenced by its intraply shear behavior. This study investigates a 2 × 2 twill weave carbon fabric and the corresponding vinyl-based thermoset prepreg developed for press-cured structural parts. Intraply shear tests of bias-extension and picture-frame were conducted for a range of industrial-relevant processing conditions of temperature and shear rate. The dry fabric was characterized similar to the prepreg to isolate the influence of semi-cured resin on the woven prepreg fabric formability in shear. The shear deformation behavior of the prepreg, usually dependent on the fabric architecture, is found to be controlled by the state of the resin. The results clearly show the significance of the choice of process parameters on the prepreg shear behavior. It is demonstrated that preheating the prepreg to temperatures considerably lower than required to initiate cure can make the shear formability of the woven prepreg equivalent to the constituent (dry) reinforcement fabric. The actual shear angle measurement during the bias-extension tests demonstrates the level of inter-tow slippage for the prepreg fabric at relatively elevated temperatures. The comparison of normalized shear data from the two test methods helps to determine the improved procedure for prepreg fabric testing.

Abstract Dimensional analysis is performed as a method to evaluate the characteristics of superplastic forming processes. The analysis is focused on the forming time results from superplastic free bulge tests so that an estimator for the forming time is obtained based on dimensionless parameters. The dimensional analysis is performed by applying the normalisation to the dynamic equations and their corresponding boundary conditions, from which five dimensionless parameters are obtained. Particular conditions of the tests allow to reduce the parameters to two. This preliminary study of the applicability of the dimensional analysis on superplastic forming processes will guide for further steps in which this technique may help during the initial stages of the process layout.

Abstract In the last decades, new flexible manufacturing processes have been developed to face the demands, by many industrial fields, for highly customized complex functional parts. The peculiar design of these components often overcomes conventional sheet metal and bulk metal forming processes capabilities. In order to face this issue, new hybrid techniques, capable of exploit key advantages of different processes, have to be developed. In this study, a method to obtain sheet-bulk joints, based on the Linear Friction Welding process, is proposed. The feasibility of the technique was investigated through an experimental campaign carried out with varying pressure and oscillation frequency using AA6082-T6 aluminum alloy. The main mechanical and metallurgical properties of the produced joints, including typical material flow defects, were highlighted. It was found that sound hybrid sheet-bulk joints can be produced by the proposed approach. Finally, it was highlighted how the height of the weld center zone plays a key role on the mechanical properties of the produced joints.

Abstract Continuous-bending-under-tension (CBT) has been conceived as a mechanical test or a forming process imparting cyclic bending during stretching of a metallic sheet or strip in order to increase its elongation to fracture (ETF) relative to simple tension (ST). In a recent work, over five times improved ETF by CBT over ST has been reported for a dual-phase (DP) steel DP 1180. This paper evaluates the behavior in CBT of three additional automotive advanced high strength steels (AHSS), DP 590, DP 780, and DP 980. In doing so, the process parameter space defined in terms of crosshead velocity applying the tensile force, and roller depth imposing the amount of bending to the specimen has been explored to maximize the ETF of these materials. The studied steel sheets had different thicknesses, in addition to intrinsically containing different fractions of ferrite and martensite phases. After establishing the optimal process parameters, significant improvements in ETF are achieved for all studied steels. Based on comprehensive data, it is found that lower martensitic content moderately improves ETF, while increasing of the sheet thickness rapidly deteriorates ETF, under CBT. The behavior in tension of sheets that were subjected to CBT processing under the established optimal process condition is investigated to determine enhancement in strength and any residual ductility of the materials. In addition to testing, a combination of electron microscopy along with electron-backscattered diffraction and neutron diffraction is employed in order to assess the initial microstructure, evolution of crystallographic texture, and fracture mechanisms for the studied steels. Texture evolution in CBT forms a more pronounced {011} fiber along the stretching direction than in ST, revealing that the deformation in CBT could extend to greater strain levels than those reached at the fracture location in ST. Fractured surfaces after CBT are found to consist of fine ductile dimples, while those after ST consist of coarser dimples and some content of brittle flat martensitic regions.

Abstract To investigate the effects of forming parameters on the thermal rheological behavior and microstructure evolution of a 30CrMnSiNi2A ultra-strength steel during thermal plastic deformation, thermal compression tests were carried out under different forming temperatures and strain rates. The microstructure of 30CrMnSiNi2A steel suffered from thermal compression and rapid cooling was composed of lath-shaped martensite, plate-shaped martensite, and retained austenite. The sizes, morphologies, and proportions of those ferrous phases in compressed 30CrMnSiNi2A steel specimens were affected significantly by forming temperatures and strain rates. The effects of forming temperature and strain rate on austenitization and dynamic recrystallization of 30CrMnSiNi2A steel were investigated. Based on the experimental results, the constitutive equation and processing map of 30CrMnSiNi2A steel were established. The unsafe areas of 30CrMnSiNi2A steel were not only distributed in the regions with lower temperature and lower strain rate, but also distributed in the regions with higher temperature and higher strain rate. When thermal processing was conducted at 1050 °C with a strain rate of 0.1 s−1, ideal thermal plastic formability was exhibited by 30CrMnSiNi2A steel.

Abstract Cold pilgering process is a seamless tube forming technology, in which the metal is subjected to a series of small incremental deformations. It has been extensively applied for the manufacturing of cladding tube in nuclear reactors, due to high dimensional accuracy and favorable texture evolution. Notwithstanding, the pilgered tubes showed a kind of shear cracks similar to those obtained during low-cycle shear fatigue experiments. To confirm it, a novel low-cycle shear fatigue experiment was designed, in which the shear stress amplitude was obtained from a tracking point in the inner surface of the pilgered tube through finite element model (FEM). Additionally, a theoretical model was established based on the experimental results to predict fatigue life during the cold pilgering process. Through this model, the effects of Q value, side relief, and friction coefficient on the safety factor during cold pilgering were investigated. The results showed that with the increase of Q value, and the decrease of the difference between the friction coefficients of the inner and outer surfaces, the safety of cold pilgering tube could be improved by decreasing the tendency of shear cracks.

Abstract Die swelling is one of the paramount factors that impresses dimensional accuracy and quality of functional parts in Fused Deposition Modeling (FDM), one of the most famous Additive Manufacturing (AM) processes. Die swelling is considered a critical phenomenon in polymer extrusion process affected by melt flow rate, extruder temperature and its geometry. In this research, the ABS melt polymer behavior in the extrusion process of FDM and the die swell of extruded polymer have been investigated by empirical experiments and simulated by the Finite Element Method (FEM) using rheological properties of ABS. The internal geometry of the nozzle is investigated via analytical simulation and finite element analysis to obtain pressure and velocity distributions of the material inside the extruder as well as the die swell of extruded filament. Additionally, experiments were carried out to validate the analytical and numerical simulations. The results showed that higher melt temperature and lower material flow rate result in less pressure drop inside the nozzle and less swelling of the extruded filament.

The panel production of small batch sizes for the hull of large ships requires a stable and flexible forming process, which is momentarily manually controlled by a system operator. The manual forming press control includes the metal sheet handling above the forming tool for defining the contact point and engagement depth of the sword and subjective monitoring of the forming degree by using the light gap check method. For objectifying the process monitoring and reducing the dependency on the experience of the system operator an automated solution is needed. Within the automated process control the metal sheet deformation behavior has to be predicted in real-time during the forming process. To achieve this, the deformation prognosis for the ship panel’s production is handled inside the described work. Based on a state of art analysis a geometrical approach to describe the metal sheet deformation behavior is developed for the multi-step forming process by three-point-bending. The related geometrical parameters are predicted using a new type of prediction method by means of an artificial neural network. This prediction method requires the network definition and extensive experimental investigations for training the artificial neural network.

Hydroforming is a relatively new metal forming process with many advantages over traditional cold forming processes including the ability to create more complicated components with fewer operations. For certain geometries, hydroforming technology permits the creation of parts that are lighter weight, have stiffer properties, are cheaper to produce and can be manufactured from fewer blanks which produces less material waste. This paper provides a detailed survey of the hydroforming literature of both established and emerging processes in a single taxonomy. Recently reported innovations in hydroforming processes (which are incorporated in the taxonomy) are also detailed and classified in terms of “technology readiness level”. The paper concludes with a discussion on the future of hydroforming including the current state of the art techniques, the research directions, and the process advantages to make predictions about emerging hydroforming technologies.

The paper proposes a novel approach to model the in-plane resin flow in deformable thin-walled fiber preforms for liquid composite molding processes. By ignoring the through-thickness flow in large scale thin-walled components, the 3-D resin flow is simplified to an in-plane flow inside the preform by a specialized divergence theorem. Shell kinematics are used to describe the fiber preform deformation, and the compressible flow is modeled in the context of the free surface flow in porous media. For simplicity and efficiency, the normal stretch, which is driven by the internal fluid and applied external pressure, represents the fiber preform expansion and compression. As compared with full 3-D models, the proposed shell model significantly reduces the problem size, while it still represents the primary physical phenomena during the process. The effects of neglecting the through-thickness flow are illustrated in a numerical example that compares the flow for a set of preforms with different thickness. The model is demonstrated from the numerical example of the mold filling in a doubly curved thin-walled fiber preform. Due to the applied vacuum and the consequent resin flow motion, the relevant deformation of the preform is observed.

The precise determination of the forming limit strains of a sheet metal is necessary in order to accurately predict the onset of failure in sheet metal forming processes. The method used to detect the onset of necking (i.e. the necking criterion) is a most important factor in formability analysis as it significantly affects the forming limit strains. Many different necking criteria have been developed, which take advantage of the flexibility and high resolution of digital image correlation (DIC) strain measurements. Several time-dependent and time/geometry-dependent necking criteria were carefully investigated in order to evaluate their ability to reliably and consistently detect the onset of necking of TRIP780 sheet specimens that were stretch-formed over a hemispherical punch. It was found that the geometry-dependent, surface slope criterion was the most robust and consistent criterion of those evaluated.

For affordable high-volume manufacture of sandwich panels with complex curvature and varying thickness, fabric skins and a core structure are simultaneously press-formed using a set of matched tools. A finite-element-based process simulation was developed, which takes into account shearing of the reinforcement skins, multi-axial deformation of the core structure, and friction at the interfaces. Meso-scale sandwich models, based on measured properties of the honeycomb cell walls, indicate that panels deform primarily in bending if out-of-plane movement of the core is unconstrained, while local through-thickness crushing of the core is more important in the presence of stronger constraints. As computational costs for meso-scale models are high, a complementary macro-scale model was developed for simulation of larger components. This is based on experimentally determined homogenised properties of the honeycomb core. The macro-scale model was employed to analyse forming of a generic component. Simulations predicted the poor localised conformity of the sandwich to the tool, as observed on a physical component. It was also predicted accurately that fibre shear angles in the skins are below the critical angle for onset of fabric wrinkling.

Anisotropic composite cylinders and pressure vessels have been widely employed in automotive, aerospace, chemical and other engineering areas due to high strength/stiffness-to-weight ratio, exceptional corrosion resistance, and superb thermal performance. Pipes, fuel tanks, chemical containers, rocket motor cases and aircraft and ship elements are a few examples of structural application of fiber reinforced composites (FRCs) for pressure vessels/pipes. Since the performance of composite materials replies on the tensile and compressive strengths of the fiber directions, the optimum design of composite laminates with varying fiber orientations is desired to minimize the damage of the structure. In this study, a complete mathematical 3D elasticity solution was developed, which can accurately compute stresses of a thick multilayered anisotropic fiber reinforced pressure vessel under force and pressure loadings. A rotational variable is introduced in the formalism to treat torsional loading in addition to force and pressure loadings. Then, the three-dimensional Tsai-Wu criterion is used based on the analytical solution to predict the failure. Finally, a global optimization algorithm is used to find the optimum fiber orientations and their best combination through the thickness direction.

Stringer sheet forming enables an efficient production of branched sheet metal structures. Compared to conventional sheet metal components, stringer sheets show a significant increase in stiffness and therefore offer new possibilities for lightweight design. A challenge in stringer sheet forming is the failure due to instability, which appears in the buckling of the stringer in concave curvatures. The prediction of this failure mode is so far only possible by complex numerical simulations. This work introduces an analytical model for the prediction of the buckling failure during forming of concave stringer sheet curvatures under different process boundary conditions. It is derived from Kirchhoff’s plate theory. A detailed sensitivity analysis of all influencing parameters is shown and extends the process understanding. The model is validated by means of a 4-point bending test and a stamping process. It can be used for a conservative estimation of the buckling failure limit in stringer sheet forming.

In addition to strain hardening and residual stress, damage influences the product performance of forward rod extruded parts. Damage is usually neglected and difficult to quantify. The evolution of ductile damage in metal forming is closely correlated to the load path. An experimental approach using automated energy dispersive X-ray spectroscopy (EDX) particle analysis in scanning electron microscopy (SEM) is used to successfully quantify the void area fraction and obtain information on ductile damage. The method is performed on forward rod extruded 16MnCrS5 workpieces with varying extrusion strains and shoulder opening angles (and thus varying underlying load paths). The quantified damage is directly correlated to the load path, which can be described by the stress triaxiality evolution during forming. Density measurements were performed to further validate the results. By observing the change of strain-weighted stress triaxiality and maximum stress triaxiality, it is shown, that the maximum stress triaxiality is the decisive parameter enabling void nucleation.

This article describes the problems involved in modelling material cracking in skew rolling processes. The use of the popular damage criteria is impossible because of the lack of a calibration test that would make it possible to determine the critical value of material damage under conditions similar to those found in skew rolling. To fill this gap, a test called channel-die rotational compression was proposed. It consisted of rolling a disk-shaped specimen in a cavity created by two channels of cooperating tools (flat dies), which had heights smaller than the diameter of the specimen. When the rolling path was sufficiently long, a crack formed in the axial zone of the specimen. In this test, modelling using the finite element method made it possible to determine the critical values of material damage. As an illustration, the test was used to determine the critical damage value when conducting a rotational compression process on 50HS steel (1.5026) specimens formed in the temperature range of 950–1200 °C. The analysis was conducted using the Cockcroft–Latham damage criterion.

The majority of high-performance composite parts are nowadays designed using advanced numerical simulations that are able to accurately predict a part’s strength and deformation, providing that the internal ply architecture and exact fibre orientation are known with sufficient accuracy. However, most parts have some deviation of the fibre orientation from the ‘as-designed’ geometry, leading to the simulation overestimating the component’s strength. Up until recently, the advancement of the process simulation tools has not been sufficient to allow knowledge of this fibre deviation before any part has been manufactured, thus leading to overly conservative designs and costly experimental optimisation of the manufacturing process to reduce fibre path defects. This results in additional cost, waste of material and increased fuel consumption (due to the unnecessary weight of the components). This paper shows how state-of-the-art composite manufacturing simulations of the autoclave consolidation process can predict and help to mitigate against out-of-plane wrinkle formation in components made from toughened UD prepregs and thus raise confidence in failure analyses predictions. The industry relevant case of a stepped laminate is used as an example. Model predictions for the internal ply geometries are quantitatively compared to micrograph images of real samples. It is then shown how the input of the simulated ply architecture helps improving the accuracy of the failure simulations.

Nowadays, the functional integration of workpieces challenges existing forming processes. The combination of established forming processes – like sheet metal and bulk forming – offers the possibility to counter this issue. The application of bulk forming operations on sheet metal semi-finished products, also called sheet-bulk metal forming (SBMF), is an innovative approach. The potential of SBMF cannot be fully exploited, as there are no recommendations in terms of workpiece design and layout influence on the process result. Therefore, this paper focuses on the analysis of semi-finished products and component design parameters on resulting part and process properties in two extrusion processes in SBMF. The investigation is based on a combined numerical and experimental approach. It is shown that the investigated design parameters, in addition to the achievable dimensional accuracy, substantially influence the occurring tool loads as well as the required process forces.

The use of composite materials reinforced by flax fibres has been increasing steadily over the last 20 years. These fibres show attractive mechanical properties but also some particularities (naturally limited length, presence of a lumen, fibres grouped in bundles in the plant, complex surface properties and composition). An analysis of the available literature indicates that the quality of the composite materials studied is not always optimal (high porosity, incomplete impregnation, heterogeneous microstructure, variable fibre orientation). This paper reviews published data on the specific nature of flax fibres with respect to manufacturing of biocomposites (defined here as polymers reinforced by natural fibres). All the important steps in the process which influence final properties are analyzed, including the plant development, retting, fibre extraction, fibre treatment, preform preparation, available manufacturing processes, the impregnation step, fibre cell wall changes during processing and fibre/matrix adhesion.

Finite element simulation has become an important tool of process and production design in various fields, especially in the automotive industry. The calculation of forming processes in the early concept phase of new cars allows virtual adaptions, which can reduce costs of later phases in the product development significantly. Therefore, the precise characterization and modelling of the material behavior is necessary to ensure a robust and reliable numerical process design. The mechanical properties of numerous materials are highly influenced by the rolling or extrusion direction in the production process. This necessitates the characterization of materials in different loading directions. However, depending on the dimensional aspects of the semi-finished product, the manufacturing of specimens can be challenging or even impossible. Thus, in this investigation, an innovative, indirect approach for the identification of the Lankford coefficient in transversal direction is presented. Based on numerical and experimental data of layer compression tests the Lankford coefficient is determined by inverse modelling of the resulting specimen contour. Due to the characteristics of the layer compression test, it can even be used for semi-finished products with small transversal dimensions like extruded profiles. The presented methodology is on the one hand verified by conventional uniaxial tensile tests for aluminum as well as steel blank material. On the other hand it is used to determine Lankford coefficients for an aluminum extrusion hollow profile and the inversely identified material model is validated by comparison of strain distributions of experimental and numerical square tube bending tests.

In-plane shear is considered as the main deformation mechanism during the forming of fabrics on double curved geometries. Non-Crimp Fabrics (NCFs) are more and more used in the industry thanks to their high mechanical performances. The uniaxial bias extension (UBE) test is commonly used for characterizing the in-plane shear behavior of fabrics. However, presence of slippages calls the reliability of this test into question for NCF material. These slippages lead to a macroscopic kinematic which does not respect the fundamental hypotheses of UBE test theory. The variety of NCF architectures is usually pointed while the lack of standardized experimental methods is seldom discussed. The first section of this paper presents a two-way approach to detect slippage on an NCF. This approach is based on two kinematical descriptions of the UBE test. The first one assumes a pure shear behavior whereas the second one assumes a simple shear behavior. These behaviors correspond respectively to the rotation of fibers and to the slippage of fibers from a macroscopic point of view. In the second section, the two-way approach is used to analyze experimental UBE tests. This investigation highlights the influence of the sample width on the deformation mode during a UBE test. More precisely, it is shown that increasing the sample width of NCF specimens improves the UBE test reliability.

Tubular structures find wide application in the automotive context. In particular, rectangular cross-section tubes are used to fabricate structural frames via different techniques, such as Three-Roll-Push-Bending with the addition of twisting component (TRPBT) and the Rotary Draw Bending (RDB). However, whether the accumulated plastic strains, hardening and residual stresses influence the load capacity of the tubular component is still unclear. This paper is intended to shed light on this issue. The load capacity of a tubular mock-up obtained by sequential combination of TRPBT and RDB has been empirically assessed by a destructive compression test. A finite element (FE) model has been devised and validated to analyse the manufacturing processes. This work puts in light the need to correctly model the compliance of the tool set-up for Roll Bending in the numerical calculations. The final shape of the mock-up obtained by FE analysis is the input of the numerical simulation of the compression test. The present modelling has shown clearly that the global resistance of a tubular component is sensitive to plastic strains, hardening and residual stresses resulting from the previous forming processes. Taking into account these three factors greatly improves the capability of the FE to model the mechanical response of the structural part.

Manufacturing lightweight plastic parts with high productivity while maintaining a high level of quality and excellent reproducibility of cellular structure reduces the amount of raw material needed while improving the carbon balance thanks to the bio-sourced origin of the polymer and the decrease of the transported mass. In this study, structural modifications of PBS were carried out in order to control the foaming mechanism in each phase of cell formation (gas dissolution, cell nucleation, cell growth and cell stabilization). Cell morphology has been improved by modifying the molecular architecture (ramified/branched, semi-reticulate structures), promoting nucleation (decrease of surface tension leading to a decrease in Gibbs’s energy barrier), or by adjusting the extensional viscosity or Newtonien viscosity of the material. The resulting formulation exhibits a decrease of more than 80% in cell size and a cell density multiplied by 450 regarding the linear structured injection moulding PBS reference FZ71 (Mitsubishi Chemical Corporation (MCC), Japan) noted here L-PBS.

Significant progress has been achieved on modeling the influence of plastic anisotropy on forming of polycrystalline metal sheets. In contrast, the effect of crystal orientation on forming of single-crystal sheets has been largely unexplored. In this paper, using a recently developed single crystal criterion, it is shown that the single-crystal orientation has a very strong influence on forming. Results of F.E. simulations of cup drawing and hole expansion are reported. The same set of values of the anisotropy coefficients, which correspond to Al single crystal (with 5% Cu) is used in all simulations. It is predicted that for the {100}〈001〉 orientation four ears develop whereas for the \( \left\{111\right\}\left\langle 1\overline{1}0\right\rangle \) and \( \left\{122\right\}\left\langle 1\overline{1}0\right\rangle \) crystal orientations six, and eight ears form. Moreover, correlations between the location of the ears and the variation of the Lankford coefficients in the plane of the respective single-crystal sheets are established. F.E. analysis of hole expansion also show a strong influence of crystal orientation on the distribution of thickness strains and strain localization.

The granular-media-based thin-wall elbow push-bending process involves filling a tube with granular media and pushing the tube into a die to bend a tubular blank into an elbow shape. By means of the mechanical characteristics of granular filler, an elbow tube with t/D < 0.01 (the ratio of wall thickness to outer diameter) and R/D < 1.5 (the ratio of bending radius to outer diameter) can be formed. To investigate the interaction between thin-wall elbow and granular filler, A 3D FEM-DEM coupling numerical model is developed, which takes into account both the deformation behavior of tubular blank (continuum, finite element method FEM) and mechanical characteristics of granular filler (discrete media, discrete element method DEM). By means of the coupling model, the key forming parameters of an elbow tube such as forming force, wall thickness distribution, wrinkling are simulated and compared to experimental results.

Open-die forging is an incremental forming process, which is mainly used for the production of large parts with high requirements regarding the mechanical properties and reliability of the forged parts. Finite element analysis (FEA) is able to simulate open die forging sequences. It is therefore very suitable to confirm, whether a selected schedule will be successful in terms of reaching the desired geometry and internal product quality. However, it is comparably slow and therefore not suitable for early process design, when out of an almost infinite number of potential sequences of strokes, an appropriate pass schedule needs to be designed. This is today usually achieved by pass schedule planning software, which takes into account volume constancy, empirical spread behavior and average temperature evolution. However, they do not account for product quality characteristics like microstructure and voids closure. In this paper recently developed fast models, which are able to calculate the temperature, equivalent strain and microstructure evolution along the core fibre of a forged workpiece are coupled with an optimization algorithm to allow automatic pass schedule layout and optimization. Different cost functions are evaluated regarding their impact on the resulting properties of the workpiece. The results indicate that for an overall optimization of open-die forging processes different phenomena and influencing parameters need to be considered, since all of these parameters have a significant influence on the resulting properties such as equivalent strain, temperature and grain size of the ingot.

The new idea is to produce specimens by forward rod extrusion, where in the core of the extrudate a deviatoric tension-loading is present, which is superposed by an adjustable hydrostatic pressure. Various damage levels are hence possible in the extrudate. Conducting tensile and upsetting tests with the pre-strained specimens both the influence of a load reversal as well as the material weakening through ductile damage on the resulting flow curve is explored. Not only can the results be utilized to identify flow curves of materials up to high strains (ε > 1.7), but also to get new insights into the plastic material behaviour, which can be used for generating or adapting new damage models as well as kinematic hardening models under cold forging conditions. The proposed method was first assessed by means of analytical and numerical methods and then validated experimentally, by the example of the typical cold forging steel 16MnCrS5.

The use of natural fibre reinforced composites such as flax fibre / polypropylene is in a constant expansion particularly in automotive and marine industries due to their good mechanical properties, low density and thus, lightness, low environmental impact, low cost, recyclability, renewable properties of flax fibres and a minimum energy intake during their process. One of the major challenges of thermoplastic composites for the automotive industry is to manufacture finished parts in a single processing step within a minimum amount of time. For this purpose, a stamping airflow device was specifically developed. It is designed to produce woven comingled composite parts from comingled woven fabrics such as flax/polypropylene in only 200 s. Firstly, preliminary tests and the optimization of processing parameters were performed. Then, a quasi-static mechanical characterization of the formed parts was realized. By using criteria based on mechanical properties, the optimal process parameters such as the level of pressure, temperature, holding time and cooling rate so that to obtain the lowest voids rate were determined. Finally, a comparison of the mechanical properties of parts obtained from using the new manufacturing process and a conventional thermo-compression process is presented to demonstrate the interest and the level of performance achieved by this original and fast manufacturing device.

Carbon fiber reinforced plastics (CFRP) is widely used in the aerospace and automotive industries due to their light weight and high strength. The effective joining method of CFRP is bonding, and the surface state of CFRP is the main reason that affects the shear strength of the bonded joints. In order to improve the surface state of CFRP and shear strength, this paper systematically studied the effects of different power (80 W, 120 W, 180 W) laser treatment on the surface characteristics and bonding performance of CFRP and compared with the mechanical treatment (alcohol ultrasonic cleaned, sandpaper sanding). Four different adhesives were used in the adhesive joints, which were performed lap-shear tests. The SEM, XPS and contact angle measuring instrument were used to analyze the changes of physicochemical properties of CFRP after different surface treatment. The results showed that compared with alcohol cleaned and sandpaper sanding, 120 W laser treatment greatly improved the surface roughness of CFRP and formed micron-sized gaps. The water contact angle was reduced from 85° to 59.5°, and the surface energy increased from 25.6 mJ/m2 to 62.7 mJ/m2. The lap-shear results showed that the shear strength of the epoxy joints was improved after laser treatment, and the failure type was changed from interface failure to substrate failure. The polyurethane joint shear strength was not obviously improved and the combination failure of interface and cohesion was transformed into a pure interface failure after surface treatment.