Vesselin Michailov

Experimental Investigation and Analytical Calculation of the Bending Force for Air Bending of Structured Sheet Metals

The increasing interest in structured sheet metals for lightweight constructions and automotive can be seen in recent years. The driving force of this trend is higher stiffness of structured sheet metals in comparison to smooth sheet metals. The structured sheet metal is a sheet metal with a periodical three-dimensional geometry, which is manufactured by hydroforming process. The improved properties of this sheet metal allow the weight reduction of car components and lightweight structures. The purpose of this study is the determination of the force requirements by air bending of structured sheet metal and an analysis of influence factors on the bending force. Moreover an improvement of an analytical calculation of the maximal force for air bending of structured sheet metals is presented. In this work the steels DC04, DX56D-Z and X5CrNi18-10 were investigated. The results have shown that the bending position and the structure location have a big influence on the bending force. All investigated materials have similar behaviour. The largest and smallest bending force can be seen in the bending positions III and II respectively. At the structure location “negative” the maximal bending force is smaller than at the structure location “positive”. The results of the different calculation methods were compared to the experiments. The developed analytical approach provides more precise results than conventional method. In contrast to existing analytical calculation methods it takes into account the influence of the structure location and bending position of structured sheet metals on the bending force

Experimental Characterisation of Structured Sheet Metal

Structured sheet metals with regular bumps offer higher bending stiffness compared to flat sheet metals. The application of structured sheet metals requires new investigations regarding their strength and deformation behaviour. Standardised testing methods like the tensile test considering defined specimen geometry and measuring methods exist. Those methods, however, have been developed for plain sheets and cannot be directly transferred to structured sheet metals. The assessment of the strength and deformation behaviour of structured sheet metals needs adapted specimen geometry and measuring methods. In this paper the adaption of the standardised tensile test in accordance with DIN EN ISO 6892-1 to the specific characteristics of structured sheet metals is introduced. In order to determine the appropriate specimen geometry their dimensions were methodically varied and the influence of the structure position on the strength and the deformation behaviour was identified. The analysis of the local strain behaviour was carried out by 3D displacement measurement with the ARAMIS-system. For the derivation of the material properties different analysing methods were developed. The test results were compared to those of flat sheet metals.

Analytical and Numerical Calculation of the Force and Power Requirements for Air Bending of Structured Sheet Metals

Structured sheet metals with regular bumps offer higher stiffness compared to smooth sheet metals. They can be produced by a hydroforming process. The application of the structured sheet metals, however, is inhibited by the lack of knowledge for the subsequent processing steps. In this paper, the force and power requirements for air bending of structured sheet metals are calculated with a Finite Element Simulation (FE) and an analytical approach. In the first step, the hydroforming manufacturing process of the structured sheet metals is simulated in order to predict the exact geometry and the change in the material properties. Following, air bending simulations have been done taking into account the results of the hydroforming simulation. The FE-Simulations have been carried out with the software package LS-DYNA. The simulation models are validated with the optical displacement measuring system ARGUS and by a series of bending tests. For the analytical calculation the model based on the bending theory is adapted by simplifying the cross section of the structured sheet metals. The results of the FE-Simulation, the analytic calculation and the experiments are compared. The advantages and disadvantages as well as the application areas of the considered methods are indicated.

Physical and Numerical Simulation of the Heat-Affected Zone of Multi-Pass Welds

The paper presents a numerical and experimental approach for the quantification of the thermo-mechanical properties in multi-pass welds heat affected zone (HAZ) of low alloy steel S355J2+N. First, the characteristic temperature cycles for multi-pass welds were identified by FE temperature field simulations of welding. Based on the identified temperature cycles, the microstructure in the HAZ has been physically simulated with the simulation and testing system Gleeble 3500 to investigate the influence of multi thermal exposure on the thermo-mechanical properties. Thus, the thermo-mechanical material properties including thermal strain and temperature dependent stress strain behaviour as function of peak temperatures and cooling rates have been determined. These material properties were used to calibrate a developed model for numerical prediction of the material properties of multi-pass weld HAZ.

Corrosion Behavior of Brazed Zinc-Coated Structured Sheet Metal

Arc brazing has, in comparison to arc welding, the advantage of less heat input while joining galvanized sheet metals. The evaporation of zinc is reduced in the areas adjacent to the joint and improved corrosion protection is achieved. In the automotive industry, lightweight design is a key technology against the background of the weight and environment protection. Structured sheet metals have higher stiffness compared to typical automobile sheet metals and therefore they can play an important role in lightweight structures. In the present paper, three arc brazing variants of galvanized structured sheet metals were validated in terms of the corrosion behavior. The standard gas metal arc brazing, the pulsed arc brazing, and the cold metal transfer (CMT®) in combination with a pulsed cycle were investigated. In experimental climate change tests, the influence of the brazing processes on the corrosion behavior of galvanized structured sheet metals was investigated. After that, the corrosion behavior of brazed structured and flat sheet metals was compared. Because of the selected lap joint, the valuation of damage between sheet metals was conducted. The pulsed CMT brazing has been derived from the results as the best brazing method for the joining process of galvanized structured sheet metals.

Neutron diffraction studies of laser welding residual stresses

The residual stress and microstrain distribution induced by laser beam welding of the low-alloyed C45 steel plate was investigated using high-resolution time-of-flight (TOF) neutron diffraction. The neutron diffraction experiments were performed on FSD diffractometer at the IBR-2 pulsed reactor in FLNP JINR (Dubna, Russia). The experiments have shown that the residual stress distribution across weld seam exhibit typical alternating sign character as it was observed in our previous studies. The residual stress level is varying in the range from -60 MPa to 450 MPa. At the same time, the microstrain level exhibits sharp maxima at weld seam position with maximal level of ∼4.8·10-3. The obtained experimental results are in good agreement with FEM calculations according to the STAAZ model. The provided numerical model validated with measured data enables to study the influence of different conditions and process parameters on the development of residual welding stresses.

Particularities of testing structured sheet metals in 3-point bending tests

Abstract Thin sheet metals from deep drawing steel DC04 are very often used in the production of car body and case parts. Quality improvement of sheet metal components by new constructive solutions (structuring) as well as adapted joining technology is going on. Structured sheet metals differ from each other by their high bending stiffness. At the same time, they show certain anisotropy due to the structure. Therefore a typical testing method of structured semi-finished parts (single sheet metals, sandwiches) is the bending test. The literature review revealed that in many studies no special demands on tests of structured materials were made. This concerns particularly the structure arrangement, structure direction and structure location of the specimen relative to the mandrel position during bending tests, i. e., the direction of the fixed load relative to the structure. The aim of this study was to determine the influence of the test specification on flexural behavior. In the present paper, honeycomb-structured sheet metals were examined using 3-point bending tests. Bending stiffness and lightweight potential were calculated with respect to the location of load application and compared for different structure arrangements, directions and locations. The influence of the anisotropy on flexural behavior of the honeycomb-patterned sheet metals was moderate.

Modelling the Local Microstructure Properties due to Multi-Pass Welding

The paper presents an advanced simulation approach developed for considering the local microstructure properties variation due to multiple heat treatment. The model describes the resulting microstructure as a function of the peak temperature, austenisation time, cooling time and takes into account the microstructure formed after each thermal cycle. The model is calibrated with experimental material data obtained by repeated thermal loads. It is qualified to calculate the hardness and local microstructure properties in the HAZ of multi-pass welds. Thermo-mechanical simulation of the residual welding stresses and distortions in multi-pass welded joint is performed and validated by measurements.

Analytical-Numerical Modeling Approach for Calculation of the Structural Distortions after Welding and Thermal Straightening

The analytic-numerical hybrid model for calculating welding distortions in large welded structures is presented. Objective of the analytical model is the calculation of the plastic strains and their distribution after welding and thermal straightening process. The consideration of the essential physical relations is put into discussion. Afterwards the obtained plastic strains by the analytical calculation are loaded on an elastic FE-model of the structure and the distortions of the whole structure are predicted. The consideration of welding and thermal straightening scenarios and the assembling stages is done by taking into account the intermediate variation of the strain state at every processing step. The model is intended to be used for solving industrial tasks, i.e. intending acceptable precision and calculation time as well as low simulation costs. The application of the model is demonstrated on structures with many welds and straightening spots.

Strength Calculation of Stiffened Structures Taking Into Consideration Realistic Weld Imperfections

The topic of this article is the application of an analytical numerical hybrid model for a realistic prediction of imperfections induced by welds. At the beginning, the analytical model, its physical basis as well as the physical interrelationships are explained. This is followed by the explanation of the coupling procedure between the analytical model and the numerical calculation. Afterwards, the coupled hybrid model is applied on the investigated stiffened curved structure for the determination of the weld imperfections. An ultimate load analysis gives information about the load carrying behavior under axial loading. The results are compared against the traditional approach using eigenmode-based imperfections. The comparison underlines the potential additional utilization of load bearing capacity by this new approach. Introduction The strength calculation of stiffened plates by the finite element method (FEM) has been part of the state of the art for a long time. Geometrical nonlinearities as well as the nonlinear material behavior are considered within the calculation. To simplify, both types of imperfections, geometrical and structural ones, are mostly combined in these strength calculations being considered as equivalent geometrical imperfections. Values for standard cases are included in EN 1993-1-5 in case of plated structures or slightly curved panels [1]. Because there are no specific rules for considering imperfections and its scaling in a load capacity calculation of stiffened curved panels they may be also assumed according to EN 1993-1-5 [2,3] as a first approach. However, it remains unclear to some extent how accurate these geometrical imperfections represent the actual residual stresses and deformations caused by welds, especially for more complex cases. The significance of numerical load capacity calculations could be increased enormously if these imperfections were known more exactly and could be considered directly during the computation. Nowadays, the residual stresses and deformations can be determined by means of a thermomechanical FE simulation achieving quite realistic values. However, relevant structures and weld length are very large what leads to enormous calculation time and a huge demand of storage capacity [4]. Simplified numerical approaches are available and able to remedy this situation. However, the application of these models partly demands more expertise than a conventional thermomechanical FE calculation [5] or the simplifications are so extensive that the weld imperfections calculated by the approach partially lose their validity [6]. In order to be able to take weld distortions and residual stresses directly into account in a load capacity calculation, Residual Stresses 2018 – ECRS-10 Materials Research Forum LLC Materials Research Proceedings 6 (2018) 245-250 doi: http://dx.doi.org/10.21741/9781945291890-39 246 fast but still sufficiently accurate procedures that, at the same time, are easy in their application are essential. The Coupled Analytical Numerical Hybrid Model The basic idea of the coupled analytical numerical hybrid model [7] is the linking of the major advantages of both, analytical and numerical procedures. On the one hand, the matchless very short calculation time of the analytical shrinkage force model and its simple application, and on the other hand the possibility to conduct a FE simulation to calculate stresses and distortions at any location of complex welded structures. According to this, all the determining factors on quality and quantity of weld imperfections are passed to an analytical calculation program, capturing the mathematical approach of the shrinkage force model. The output is a mechanical load and the point of action in longitudinal and transversal direction, equivalent to the heat effect of welding. The loads are then applied to the FE model of the weld structure and the distortions and stresses are calculated by a nonlinear elastic calculation. The influence of the weld sequence on the arising weld imperfections is captured by a back coupling. The numerically calculated stresses in the regarded weld caused from a previous weld are submitted to the analytical calculation. The results of the application of the hybrid model are subsequently superposed with additional fabrication tolerances followed by the load capacity calculation, Fig. 1. Figure 1: Scheme of the Load Capacity Calculation Taking Into Consideration Realistic Weld Imperfections. Weld imperfections depend significantly on the maximum temperatures that every point perpendicular to the weld direction is exposed to and the stiffness of the structure. Equations for the calculation of the maximum temperatures were derived by Rykalin [8] constituting the basis of the shrinkage force model. For calculating a force alongside the weld, Okerblom [9] considered the border case of a line source in a rigid thin plate and integrated the thermal strains over the zone of plastic deformations: