Dimpling process in cold roll metal forming by finite element modelling and experimental validation

The dimpling process is a novel cold-roll forming process that involves dimpling of a rolled flat strip prior to the roll forming operation. This is a process undertaken to enhance the material properties and subsequent products’ structural performance while maintaining a minimum strip thickness. In order to understand the complex and interrelated nonlinear changes in contact, geometry and material properties that occur in the process, it is necessary to accurately simulate the process and validate through physical tests. In this paper, 3D non-linear finite element analysis was employed to simulate the dimpling process and mechanical testing of the subsequent dimpled sheets, in which the dimple geometry and material properties data were directly transferred from the dimpling process. Physical measurements, tensile and bending tests on dimpled sheet steel were conducted to evaluate the simulation results. Simulation of the dimpling process identified the amount of non-uniform plastic strain introduced and the manner in which this was distributed through the sheet. The plastic strain resulted in strain hardening which could correlate to the increase in the strength of the dimpled steel when compared to plain steel originating from the same coil material. A parametric study revealed that the amount of plastic strain depends upon on the process parameters such as friction and overlapping gap between the two forming rolls. The results derived from simulations of the tensile and bending tests were in good agreement with the experimental ones. The validation indicates that the finite element analysis was able to successfully simulate the dimpling process and mechanical properties of the subsequent dimpled steel products

Change of the yield stress in roll formed ERW pipes considering the Bauschinger effect

ERW pipes formed with the roll forming process show a yield stress distribution along the circumferential direction and their quality is strongly influenced by the magnitude and by the distributions of the yield stress. In addition to that, strips are subjected to cyclic loading during roll forming process. Since ERW pipes are firstly roll formed, welded and then sized, in order to develop an enhanced predicting method for the calculation of the ERW pipe yield stress, the same process flow has been also applied to authors\u2019 numerical simulations. The Yoshida-Uemori kinematic hardening model has been applied considering several subdivision of the strain range, and different parameters, aiming to find the best correlation between the estimated Bauschinger effect and the one measured in the relevant cyclic loading experiment. The comparisons between estimated and experimentally-measured values of the thickness distribution, and of the locally-measured yield stress, prove both reliability and accuracy of the adopted process chain analysis. The growth of the sizing effect ratio has shown to cause the increase of the yield stress, which becomes more uniform along the circumferential direction

Finite element analysis of residual stresses in large cold-rolled thread

As a subproject of the DFG Research Training Group Graduiertenkolleg 2075 Modelling the constitutive evolution of building materials and structures with respect to aging (GRK 2075), the fatigue behaviour of rolled-after heat-treatment and hot-dipped galvanized HT-bolts is examined. Bolted joints are one of the most frequently used joining connections in mechanical and civil engineering. HT-bolts with large diameters (M 30 up to M 72) are widely used in wind power plants both on-shore and off-shore. During the cold-rolling process, compressive residual stresses are generated at the root of the thread in combination with strain hardening. Therefore, bolts rolled-after heat-treatment under tensile loading have a much longer fatigue life than rolled-before heat-treatment ones. In order to calculate the fatigue life of rolled-after heat-treatment bolts with a notch-strain concept, it is necessary to calculate the residual stresses and the material state from the forming process. A simulation of the forming process of the M 48 thread will be presented. It is investigated if the residual stress and material state from the forming process can be captured with the transient cyclic material model of Chaboche et al.[1] implemented in the commercial FE program Abaqus. The forming process is simulated with remeshing and Mesh-to-Meshsolution-Mapping [3] on an axisymmetric model. The result will be evaluated in terms of geometry, stress and strain state. The determined stress state is compared to measured residual stresses in Unglaub [2] and Fares [4] The simulation of the forming process gives a good coincidence with the thread geometry in practice. The residual stress path corresponds qualitatively to the measured residual stresses. The plastic equivalent strain in the thread root is overestimated because of the chosen material model and the axisymmetric modeling

Implicit finite element study of non-steady effects in cold roll forming

The ability of ABAQUS Standard to obtain a non-steady implicit solution to the problem of cold roll forming a channel section is investigated. A solution can be found with careful selection of parameters, but solutions are unacceptably slow for commercial use. The implicit solutions show buckling on the first pass that does not develop into an edge wave, in contrast to a published explicit solution. Faster solutions to steady rolling can be obtained using ALE models that permit convection of stress in the direction of rolling

Theoretical extension of elastic-perfectly plastic deformation length in roll forming of a channel section

In this paper, the Young’s modulus and the yield strength of the strip are considered in order to modify the deformation length analysis proposed by Bhattacharyya et al. New analytical equations are developed assuming an elastic-perfectly plastic material behaviour and the deformation length analysed for the simple case of roll forming a U-channel; the analytical results are verified by comparison with experimental data found in the literature. The proposed elastic-plastic deformation length is shorter than Bhattacharyya’s which is rigid-perfectly plastic. It is observed that the influence of elastic properties on the deformation length is not as significant as the plastic properties; however, the authors believe that the elastic effects become more important under conditions where a major area of the strip is under elastic deformation such as when the flange length is long

A Comparative Evaluation of Predictive Models of the Flat Rolling Process

The predictive abilities of several mathematical models of the cold, flat rolling process are tested by comparing their predictions to experimental measurements. The models include an empirical model, a one-dimensional model, a finite element model and an upper bound model. The coefficient of friction and the friction factor are first determined by the inverse approach, using the model deemed to be the most comprehensive. The effects of including or excluding an account of roll flattening, using elastic-plastic or rigid-plastic strips, and constant or velocity dependent coefficients of friction or friction factors are examined

Development of innovative manufacturing technology : chain-die forming for ultra-high strength steels

In recent years, there is high demand for automotive manufacturers to design fuel-efficient vehicles without compromising passenger safety. One potential solution is to construct lighter vehicles using materials such as Ultra High Strength Steel (UHSS). However, its high-strength properties result in the difficulty to form desired profiles using traditional sheet metal forming processes such as Cold Roll Forming. To overcome this problem, a potential solution of Chain-die Forming (CDF) was recently developed. The basic principle of the CDF is to fully combine roll forming and bending processes, and its primary advantage was the elongated deformation length which significantly increases effective roll radius. In this study, three case studies were conducted to develop a reliable finite element-based numerical model of CDF with Advanced High Strength Steel (AHSS). These simulations demonstrated the effectiveness of this forming process while capturing the mechanical behaviours of AHSS. The numerical modelling and simulations served as Computer Aided Engineering (CAE) tools which determined tool geometries and to control unwanted springback. The experimental work conducted on novel 2nd Generation Chain-die forming machine. An automotive martensitic steel DOCOL 1400M from SAAB was adopted for this research as a typical AHSS material, which has a yielding strength of 1,150 MPa and a tensile strength of 1400-1600 MPa. According to the numerical simulations and experimental results obtained, CDF can be considered as an affordable, sustainable and environmentally friendly manufacturing process

Influence of the cold work effects in perforated rack columns under pure compression load

This paper analyses the influence of residual stresses and strain hardening due to the cold rollforming process on the load carrying capacity of perforated rack columns. First of all, a residual strain distribution of the manufacturing process is obtained via a finite element analysis and then it is introduced in the model as initial state to carry out a nonlinear buckling analysis. Two different methodologies to introduce the residual strain pattern are presented in order to reproduce the local stress effects because of perforations. Moreover, the changes of the mechanical properties of the material by strain hardening are evaluated through experimental tests and its influence on the rack column behaviour is investigated. The results obtained agree well with experimental tests and show a method to predict the load carrying capacity of perforated rack column taking into account the effects of the cold roll-forming process.Postprint (author’s final draft

Micro-Indentation based study on steel sheet degradation through forming and flattening : toward a predictive model to assess cold recyclability

Sheet metal forming has always been an important sector of the metal industry thanks to work hardening, however complex set of deformation entails evolution of damage in the material through the forming stages. With an outlook to cold recycling of sheet metals, this paper focuses on experimental quantification of damage and degradation in load carrying capacity due to forming process. Assuming that cold recycling of sheet metals involves an intermediate flattening prior to secondary forming process, the adverse effects of flattening on material was also investigated. An industrial cold roll forming process was taken as case study. Experimental investigation using microhardness mapping on the cross-section of fold zones, in conjunction with 3D global-local FE modelling were the basis of through-thickness damage analysis. Taking advantage of strain-hardness correlation a new method was established to extrapolate hardness for ductile damage characterization. Investigation on sensitivity of the measured microhardness to crystallographic texture and sample surface preparation backed the experimental results. The results particularly outlined the progressive decrease in load carrying capacity of material after forming and after flattening. The possibility of a secondary manufacturing process is discussed