Experimental and numerical characterization method for forming behavior of thermoplastics reinforced with woven fabrics

The automotive and aviation industry has to achieve significant weight reduction in order to fulfil legal obligations. This leads to an increasing use of new materials or new material combinations like fibre-reinforced plastics (FRP) as they provide a high lightweight potential due to the combination of low density and high tensile strength. Meanwhile pre-impregnated sheets with a thermoplastic matrix reinforced with woven carbon fibres are commercially available. This has led in a significant cost reduction and hence, the FRP have become affordable for large scale production. The material properties, in particular the forming and failure behaviour of the FRP, differ strongly from that of conventional metal materials like steel or aluminium. Therefore, new material characterisation techniques, investigation methods as well as numerical models are required. The main focus of this paper lies on the development of a non-orthogonal material model for the FRP, its implementation in a commercial FE-software as well as on the use of a combined experimental-numerical procedure for material characterisation. Since the properties of these materials are strongly temperature dependent, the forming process of reinforced thermoplastics is typically carried out at elevated temperatures. Thus, temperature sensitivity has to be taken into account during experimental testing as well as in the model approach. The model parameterisation is carried out based on an iterative numerical optimization procedure. For this purpose, the experimentally obtained results are investigated by means of digital image correlation and linked with the numerical model in combination with an automated optimization process

Fracture Characterisation and Modelling of AHSS Using Acoustic Emission Analysis for Deep Drawing †

Driven by high energy prices, AHSS are still gaining importance in the automotive industry regarding electric vehicles and their battery range. Simulation-based design of forming processes can contribute to exploiting their potential for lightweight design. Fracture models are frequently used to predict the material’s failure and are often parametrised using different tensile tests with optical measurements. Hereby, the fracture is determined by a surface crack. However, for many steels, the fracture initiation already occurs inside the specimen prior to a crack on the surface. This leads to inaccuracies and more imprecise fracture models. Using a method that detects the fracture initiation within the specimen, such as acoustic emission analysis, has a high potential to improve the modelling accuracy. In the presented paper, tests for fracture characterisation with two AHSS were performed for a wide range of stress states and measured with a conventional optical as well as a new acoustical measurement system. The tests were analysed regarding the fracture initiation using both measurement systems. Numerical models of the tests were created, and the EMC fracture model was parametrised based on the two evaluation areas: a surface crack as usual and a fracture from the inside as a novelty. The two fracture models were used in a deep drawing simulation for analysis, comparison and validation with deep drawing experiments. It was shown that the evaluation area for the fracture initiation had a significant impact on the fracture model. Hence, the failure prediction of the EMC fracture model from the acoustic evaluation method showed a higher agreement in the numerical simulations with the experiments than the model from the optical evaluation

Failure Modelling of CP800 Using Acoustic Emission Analysis

Advanced high-strength steels (AHHS) are widely used in many production lines of car components. For efficient design of the forming processes, numerical methods are frequently applied in the automotive industry. To model the forming processes realistically, exact material data and analytical models are required. With respect to failure modelling, the accurate determination of failure onset continues to be a challenge. In this article, the complex phase (CP) steel CP800 is characterised for its failure characteristics using tensile tests with butterfly specimens. The material failure was determined by three evaluation methods: mechanically by a sudden drop in the forming force, optically by a crack appearing on the specimen surface, and acoustically by burst signals. As to be expected, the mechanical evaluation method determined material failure the latest, while the optical and acoustical methods showed similar values. Numerical models of the butterfly tests were created using boundary conditions determined by each evaluation method. A comparison of the experiments, regarding the forming force and the distribution of the equivalent plastic strain, showed sufficient agreement. Based on the numerical models, the characteristic stress states of each test were evaluated, which showed similar values for the mechanical and optical evaluation method. The characteristic stress states derived from the acoustical evaluation method were shifted to higher triaxialities, compared to the other methods. Matching the point in time of material failure, the equivalent plastic strain at failure was highest for the mechanical evaluation method, with lower values for the other two methods. Furter, three Johnson–Cook (JC) failure models were parametrised and subsequently compared. The major difference was in the slope of the failure models, of which the optical evaluation method showed the lowest slope. The reasons for the differences are the different stress states and the different equivalent plastic strains due to different evaluation areas

Fracture modelling of magnesium sheet alloy AZ31 for deep drawing processes at elevated temperatures

Today, the reduction of CO2 emissions is essential to meet global climate requirements. In this context, a reduction in vehicle weight is the most efficient way to reduce the fuel consumption of a passenger car. Magnesium combines relatively high strength with low weight and is therefore an interesting construction material for lightweight solutions. In numerical process design, it is essential to be aware of the forming capacity of a material. The common method to describe the failure behaviour is the use of forming limit curve (FLC). Stress-based models offer the advantage of a strain path consideration and an extension in the area of shearing and compression. In this paper a stress-based damage model, Modified Mohr-Coulomb (MMC), was parameterized by IFUM Butterfly-Tests for an AZ31 magnesium sheet alloy under consideration of elevated process temperatures. For this purpose, the tests were carried out at different stress states and temperatures using a specially designed testing device. In addition, forming limit curves were determined by Nakajima tests. Finally, both methods, MMC and FLC, were compared to an experimental deep-drawing test. This comparison showed that the MMC Model achieved significantly better results regarding the fracture prediction in this application case

Material Characterization and Modeling for Finite Element Simulation of Press Hardening with AISI 420C

The process of press hardening is gaining importance in view of the increasing demand for weight reduction combined with higher crash safety in cars. An alternative to the established manganese-boron steel 22MnB5 is hot-formed martensitic chromium steels such as AISI 420C. Strengths of 1850 MPa and elongations of 12% are possible, exceeding those of 22MnB5. In industrial manufacturing, FE-simulation is commonly used in order to design car body parts cost-efficiently. Therefore, the characterization and the modeling of AISI 420C regarding flow stress, phase transformations as well as failure behavior are presented in this paper. Temperature-depended flow curves are determined, showing the low flow stress and hardening behavior at temperatures around 1000 °C. Cooling experiments are carried out, and a continuous cooling diagram is generated. Observed phases are martensite and retained austenite for industrial relevant cooling rates above 10 K/s. In addition, tests to investigate temperature-dependent forming limit curves are performed. As expected, the highest forming limit is reached at 1050 °C and decreases with falling temperature. Finally, a simulation model of a press-hardening process chain is set up based on the material behavior characterized earlier and compared to experimental values. The forming force, phase transformation and forming limit could be calculated with good agreement to the experiment

Methodology to Investigate the Transformation Plasticity for Numerical Modelling of Hot Forging Processes

Hot forging is a complex process involving the mutual influence of numerous thermo-mechanical-metallurgical material phenomena. In particular, the strains of transformation-induced plasticity (TRIP) have a significant influence on the distortions and residual stresses of the components. The TRIP strains refer to the anisotropic strains depending on the orientation and significance of the stress conditions during cooling superimposed to the phase transformation. With the use of numerical models, the impact of this effect can be investigated in order to ensure the production of high quality components. However, an experimental determination of the characteristic values of TRIP is challenging, which is why only few corresponding data are available in the literature. Therefore, this paper presents an experimental and numerical methodology as well as the results of studies on the interaction between stresses and phase transformations in the materials AISI 4140 and AISI 52100. The investigations of the TRIP strains are carried out using hollow specimens, which are thermo-mechanically treated in the physical forming simulator Gleeble 3800-GTC. The specimens are austenitised, quenched to test temperature and held there while diffusion controlled phase transformation takes place. The extent of TRIP as a result of different superimposed tensile or compressive loads is determined by means of dilatometry. In addition, the extent of TRIP for diffusionless martensitic phase transformations was investigated by continuous cooling tests under tensile and compressive loads. It was found that the transformation plasticity varies depending on the material, the phase type, the temperature and the tensile or compressive stresses. Subsequently, simulations of the physical experiments using the FE software Simufact. Forming verified the determined phase specific values of TRIP

Experimental investigations on the interactions between the process parameters of hot forming and the resulting residual stresses in the component

In metal forming, the arising residual stresses influence the material behaviour during manufacturing as well as the performance of the final component. In the past, the focus of forming process design was on minimising or eliminating residual stresses. However, residual stresses can also serve to improve the properties of the components through targeted use, for example with regard to distortions or wear behaviour. For this purpose, knowledge of the interactions between the process parameters of the hot forming process and the resulting residual stresses in the final component is required. In this work, the influences of the process parameters are analysed by means of a reference process of hot forming. In this process, cylindrical specimens with eccentric holes are hot-formed, which leads to an inhomogeneous stress distribution in the material as it occurs in an industrial hot forming process. In the reference process, forming temperature, cooling strategy, forming speed, degree of deformation and steel alloys are varied. It is observed that both, process parameters and material properties, have a significant influence on the resulting residual stresses. Mainly responsible for these phenomena are microstructural effects in the material. As a result of forming at temperatures between 1000 °C and 1200 °C, static and dynamic recrystallisation processes occur, which affect the austenite grain size. The austenite grain size as well as the cooling strategy have a significant influence on the microstructure transformation behaviour, which has a decisive effect on the resulting residual stresses. In addition, the cooling strategy determines whether a diffusion-free phase transformation or a diffusion-controlled phase transformation occurs. At high cooling rates, diffusion-free transformation of the austenitic into the martensitic phase takes place, which leads to severe stresses in the crystal lattice. During diffusion-controlled phase transformation, which occurs during air cooling, comparatively lower residual stresses in the range of zero can be observed

Numerical Investigations on Stresses and Temperature Development of Tool Dies During Hot Forging

Hot-forming tools are subjected to high thermal and mechanical stresses during their application. Therefore, a suitable design of the tool die is important to ensure a long tool life. For this purpose, numerical simulations can be used to calculate the occurring stresses and the temperature development in the tools during the course of a stroke or over several forging cycles. The aim of this research is to investigate the effect of different radii on the resulting stresses in the lower die of the forming tools. Furthermore, the temperature evolution over several cycles is analysed to determine their effect on the temperature. When investigating the stress, it was found that a larger radius leads to a reduction in stresses. In addition, it could be numerically proven that the base temperature of the die levels off after a certain number of cycles. These findings will be used in further research dealing with the service life calculation of dies subjected to thermo-mechanical alternating stresses

Functionalisation of the Boundary Layer by Deformation-Induced Martensite on Bearing Rings by means of Bulk Metal Forming Processes

During cold forming of metastable austenitic steels, a strength-increasing phase transformation induced by externally superimposed stresses occurs in addition to strain hardening. The effect of deformation-induced martensite formation has so far not been utilized industrially in the area of bulk forming, but could be suitable for the production of highly-loaded components in oxidative atmospheres. The aim of this study is the analysis of local phase transformations in metastable austenitic steels in the boundary layer of bulk formed components. For this purpose, the relationship between the process conditions occurring during bulk metal forming and the resulting martensitic phase fraction was determined. Cylinder compression tests are carried out in which the influence of various process parameters can be investigated. These include forming temperature, true plastic strain and forming speed. In a quantitative measurement by means of a magnetic induction process, local martensite formation is determined and hardness measurements are carried out. The recorded flow stress curves are implemented in a numerical simulation. Furthermore, the influence of different tool surface topographies on the contact conditions of the workpiece-tool system is characterized by means of ring compression tests. With the numerical simulations and experimentally obtained results, a surface hardening process for bearing rings is designed. The relationship between local true plastic strain and deformation-induced martensite development is explained by material flow simulations, taking into account the process route for manufacturing the bearing ring and the varying friction factors

Multi-Layer Wear and Tool Life Calculation for Forging Applications Considering Dynamical Hardness Modeling and Nitrided Layer Degradation

As one of the oldest shaping manufacturing processes, forging and especially hot forging is characterized by extreme loads on the tool. The thermal load in particular is able to cause constant changes in the hardness of the surface layer, which in turn has a decisive influence on the numerical estimation of wear. Thus, also during numerical wear, modeling hardness changes need to be taken into account. Within the scope of this paper, a new implementation of a numerical wear model is presented, which, in addition to dynamic hardness models for the base material, can also take into account the properties of a nitride wear protection layer as a function of the wear depth. After a functional representation, the new model is applied to the wear calculation of a multi-stage industrial hot forging process. The applicability of the new implementation is validated by the evaluation of the occurring hardness, wear depths and the locally associated removal of the wear protection layer. Consecutively, a tool life calculation module based on the calculated wear depth is implemented and demonstrated. In general, a good agreement of the results is achieved, making the model suitable for detailed 2D as well as large 3D Finite Element calculations