Ralf Ossenbrink

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.

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.

Hydrogen Charging of High Strength Steel Specimens and Physical Simulation of Cold Cracking under Laser Beam Welding Conditions

Cold cracks occur during the cooling down of welded joint at low temperatures or later at room temperature after the end of welding. It is associated with the formation of brittle microstructures as martensite in the presence of diffusible hydrogen as well as of tension stresses. By using an enhanced Simulation-und Testing Center Gleeble 3500, a procedure for physical simulation of cold cracking under laser beam welding conditions is suggested. The approach reproduces combinations of the cold crack main parameters, a brittle microstructure, tension stress and high local hydrogen concentration under welding conditions which induce a cold crack. A specimen geometry and technique were developed to enable the gaseous hydrogen charging from pure hydrogen atmosphere. The amount of charged hydrogen can be adjusted through varying the charging parameters like temperature, gas pressure and charging time. The hydrogen charging technique and the cold crack testing procedure were proven with high strength steel specimens.

Electrochemical Corrosion Characteristic of Structured Low Carbon Steel DC04 and Stainless Steel 304 Sheets in Sodium Chloride Solution

In this work structured sheet metals were investigated using electrochemical measurement techniques. The main purpose is obtaining fundamental information about the corrosion resistance of structured sheet metals in comparison to smooth sheet metals as well determination of the influence of the structuring process on the corrosion properties. The corrosion resistance of structured sheet metals is affected by manufacturing process. One of the main influence factors is the change of the surface roughness. In this study the low carbon steel DC04 and the stainless steel 304 (X5CrNi18-10) were investi-gated. The electrochemical tests were carried out in 3%-NaCl solution. Potentiodynamic linear polarization was used to determine such electrochemical characteristics as the free corrosion potential, the corrosion current, the pitting and protection potential. Furthermore, the corrosion rate was calculated for smooth and structured sheet metals of the low carbon steel DC04. For the stainless steel 304 the pitting density was estimated. The surface roughness was measured for both materials. The electrochemical corrosion tests show a small difference in the corrosion behaviour of structured and smooth sheet metals. Structured sheet metals have a lower corrosion resistance than smooth sheet metals. The steel DC04 shows the worst corrosion properties at the structure location “negative” in comparison to the structure location “positive”. The corrosion resistance of the stainless steel 304 is better at the structure location “negative” than at the structure location “positive”. Moreover, the results show the correlation between the surface roughness and the corrosion resistance for structured sheet metals.