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 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

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

Process analyses of friction drilling using the Smoothed Particle Galerkin method

As a cost-effective hole production technique, friction drilling is widely used in industrial and automotive manufacturing. Compared with the traditional bolted connection, it enables the fastening of thin metal sheets and thin-walled tubular profiles. Friction drilling results in higher thread length and joint strength, thus better fulfilling the demand for lightweight structures. However, in the numerical simulation of friction drilling, the traditional finite element method encounters difficulties caused by the extreme deformation and complex failure of the material. A large number of elements are usually deleted due to the failure criterion, which significantly reduces the solution accuracy. The development of meshless methods over the past 20 years has alleviated this problem. Especially the Smoothed Particle Galerkin (SPG) method proposed in recent years and incorporating a bond-based failure mechanism has been shown to be advantageous in material separation simulations. It does not require element removal and can continuously evolve each particle's information such as strain and stress after the material failure. Therefore, the SPG method was used in this research for the simulation of frictional drilling of HX220 sheet metal. First the particle distance and the friction coefficient were varied to investigate the applicability of the SPG method to the friction drilling process. Predicted and experimental results were compared and found to be in high agreement. Furthermore, the influence of input parameters, such as sheet thickness, feed rate and rotational speed, on axial force as well as torque of the tool and the surface temperature of the workpiece during friction drilling was investigated numerically

Extension of the Conventional Press Hardening Process by Local Material Influence to Improve Joining Ability

Press hardened structural components are a key factor in lightweight car design and thus in reducing vehicle mass while increasing crash safety. The use of quenched 22MnB5 (Usibor 1500) has been established in hot sheet forming for the production of safety-relevant car body components. In order to expand the field of application for press-hardened components, a process-reliable joining technique is essential. Ultrahigh-strength components can be joined with other parts in car bodies using the resistance spot welding process. Here, challenges like uneven welding lens formations with an incorrect connection in multi-sheet joints arise. Mechanical joining processes, for example self-pierce riveting, can only be used to a limited extent due to the high hardness of the hardened parts. For this purpose, an annealing treatment is often carried out in order to reduce the strength of the material after press hardening. Another possibility to create softened areas is the introduction of local deformation in the austenitic material. The phase areas in the continuous cooling transformation diagram are shifted to shorter cooling times, which enables the development of deformation-induced ferrite. The local thinning and softening improves joinability by means of mechanical and thermal joining processes but increases the forming force. Therefore, in this study the process of hot forming and local deformation is first pre-estimated using numerical simulation. The required force for the deformation and an optimal positioning, as well as the possible number of deformation punches, are investigated. Furthermore, the first experimental results of the feasibility of locally thinned and softened sheets are presented. In addition, joining tests by resistance spot welding and self-pierce riveting are carried out on the generated specimen to illustrate the practical effectiveness of the local thinning and the use of deformation-induced ferrite for critical joints

Investigation of the Hardness Development of Molybdenum Coatings under Thermal and Tribological Loading

The increasing global demand for innovative and environmentally friendly lubricants can be met through the use of solid lubricants. By switching from conventional lubricants such as various oils or grease to solid lubricants, new scopes of application can also be opened up. The main requirements for solid lubricants are a reduction in the coefficient of friction (CoF) and an increase in wear resistance. Due to the favourable material properties, molybdenum (Mo) coatings fulfil the tribological requirements and are therefore promising solid lubricants which can be applied via physical vapour deposition (PVD). In this work, the impact of substrate temperature on the hot hardness of deposited Mo coatings was determined. The specimen with the highest hot hardness was then tribologically examined both at the micro and nano level. Through an analysis of the wear tracks by means of nanoindentation and scanning electron microscopy (SEM), it was possible to detect the influence of the tribological load separately from that of the thermal loads. The results showed that the tribological load influenced the Mo coating by significantly increasing its hardness. This was achieved due to the work hardening of the Mo layer leading to an increase in the wear resistance of the coating

Improved failure characterisation of high-strength steel using a butterfly test rig with rotation control

A forming limit diagram is the standard method to describe the forming capacity of sheet materials. It predicts failure due to necking by limiting major and minor strains. For failure due to fracture, the fracture forming limit diagram is used, but fracture caused by plastic deformation at a shear-dominated stress state cannot be predicted with a conventional fracture forming limit diagram. Therefore, stress-based failure models are used as an alternative. These models are describing the fracture of sheet materials based on the failure strain and the stress state. Material-specific parameters must be determined, but a standardised procedure for the calibration of stress-based failure models is currently not established. Most test procedures show non-constant stress paths and varying stress states in the crack initiation area, which leads to uncertainties and inaccuracies for modelling. Therefore, a new test methodology was invented at the IFUM: a prior presented butterfly test rig was extended to enable an online rotation to adapt the loading angle while testing. First, butterfly tests with CP800 were performed for three fixed loading conditions. The tests were modelled numerically with boundary conditions corresponding to the tests. Based on the numerical results, the stress state as well as failure strain were identified and the stress state deviations were calculated. Afterwards, the necessary angular displacements to compensate the stress state deviations for the adaptive test rig were iteratively determined with numerical simulations using an automatised Python script. Finally, the butterfly tests were performed experimentally with the determined adaptive loading angles to identify the specimen failure and compared to the simulations for validation

Modelling failure of joining zones during forming of hybrid parts

Combining diverse materials enables the use of the positive properties of the individual material in one component. Hybrid material combinations therefore offer great potential for meeting the increasing demand on highly loaded components. The use of hybrid pre-joined semi-finished products simplifies joining processes through the use of simple geometries. However, the use of pre-joined hybrid semi-finished products also results in new challenges for the following process chain. For example, the materials steel and aluminium may form brittle intermetallic phases in the joining zone, which can be damaged in the following forming process under the effect of thermo-mechanical loads and thus lead to a weak point in the final part. Due to their small thickness as well as their position in the component, the analysis of the joining zone is only possible by complex destructive testing methods. FE simulation therefore offers an efficient way to analyse the development of damage in the process design and to reduce damage by process modifications. Therefore, within this study a damage model based on cohesive zone elements is implemented in the FE software MSC Marc 2018 and calibrated using experimental local tensile tests performed under process relevant conditions. Introductio

Evaluating material failure of AHSS using acoustic emission analysis

Driven by high energy prices and strict legal requirements on CO2 emissions, high-strength sheet steel materials are increasingly gaining importance in the automotive industry regarding electric vehicles and their battery range. Simulation-based design of forming processes can contribute to exploiting their high potential for lightweight design. However, previous studies show that numerical simulation with conventional forming limit curves does not always provide adequate prediction quality. Failure models that take the stress state into account represent an alternative prediction method for the shear-dominated failure, that frequently occur in high-strength steels during forming. The failure behaviour of the sheet materials can be determined by different specimen geometries for a wide range of stress states and by using an optical measurement system to record the local strain on the surface of the specimen at the location of failure. However, for many high-strength steels, critical damage or failure initiation already occurs inside the specimen. Therefore, a method is needed that allows detection of failure initiation at an early stage before the crack becomes visible on the surface of the specimen. One possible method is the use of acoustic emission analysis. By coupling it with an imaging technique, the critical strains leading to failure initiation inside the specimen can be determined. In the presented paper, butterfly tests are performed for a wide range of stress states and measured with an optical as well as an acoustical measurement system. The tests are analysed regarding the failure initiation using a mechanical, optical as well as acoustical evaluation method and compared with each other

Finite Element and Finite Volume Modelling of Friction Drilling HSLA Steel under Experimental Comparison

Friction drilling is a widely used process to produce bushings in sheet materials, which are processed further by thread forming to create a connection port. Previous studies focused on the process parameters and did not pay detailed attention to the material flow of the bushing. In order to describe the material behaviour during a friction drilling process realistically, a detailed material characterisation was carried out. Temperature, strain rate, and rolling direction dependent tensile tests were performed. The results were used to parametrise the Johnson–Cook hardening and failure model. With the material data, numerical models of the friction drilling were created using the finite element method in 3D as well as 2D, and the finite volume method in 3D. Furthermore, friction drilling tests were carried out and analysed. The experimental results were compared with the numerical findings to evaluate which modelling method could describe the friction drilling process best. Highest imaging quality to reality was shown by the finite volume method in comparison to the experiments regarding the material flow and the geometry of the bushing