| As an economic profile product, roll-forming product is widely used in transportation, engineering machine, civil construction because of its uniform section, high strength and low consumption of production energy. With more and more application of high-strength steel, residual stresses (RS) is a key element of safety factor and performance of cold-formed sections when it is used as main part in major engineering works. Roll-forming process is very complicated. It is a process of large displacement and finite strain, in which the friction between the strip and rolls is included. Because roll-forming is a process with geometric, material and boundary nonlinearity, its forming rules is hard to control, which in turn make it difficult to study RS of the sections. Because there is no systemic simulation analysis and prediction technique in theory and no RS trade rules and standards in the industry, the application and development of cold-formed sections in major engineering works is delayedIn this paper roll-forming sections with multi-stand were taken as object, and RS produced from the process was studied in detail with finite element method (FEM) simulation and experiment. Based on the analysis on the process of roll-forming and welding, used commercial FEM code ABAQUS to establish the FEM simulation model from witch RS of open and closed sections produced by roll-forming can be predicted. On the ground of a large number of experimental data of RS, the effect of process factors to distribution of RS is studied. Based on above research objectives, the following four works were done mainly:(1) FEM simulation model of roll-forming process for residual stresses predictionThe method of computing algorithm and contact algorithm was analyzed. Taking a real product produced by factory as objective, the method of establishing simulation model was studied. The simulation manner, material model, element type and mesh, simulation algorithm, load and boundary were discussed in detail, especially for the solution of convergence. The reliability of the simulation model is check by the experimental data. Then the analysis of parameters for precision of the model was conducted. At the same time, based on the model for the open sections, the simulation model for closed section was present using continuum shell element in which two steps were introduced, forming and closing. Emphasis was put on the load and boundary condition definition. Taking a real closed-form section product, the FEM simulation for multi-stand roll-forming process was conducted successfully, and the simulation results were proved by the measurement.(2) Experiment and simulation analysis for roll-forming process of open sectionsThe method of X-rays diffraction was studied and the basic formula for inside surface measurement was derived. Based on X-rays diffraction method, inside surface and outside surface longitudinal residual stresses (LRS) and transversal residual stresses (TRS) of a real open section product were measured. Using the models researched above and parameters of product, the RS FEM simulation model for multi-stand roll-forming of this real open section product was established. Through the comparison of dimension measurement and RS experimental data, the reliability of the FEM simulation model based on implicit algorithm studied above was validated. Finally, on the ground of FEM model, the research on the effect of material, process and dimension factors to RS of open sections was conducted in detail.(3) Fully coupled thermal-stress analysis for closed-form sections of roll-formingBased on the research on simulation model for closed-form section, a method of fully coupled thermal-stress analysis using element body hot generated was present. Firstly, the method of simulation algorithm for welding is introduced, and the heat source and heat waste of the simulation were made sure. Then the element type, mesh, heat physics parameters of material and simulation was analyzed in detail. On the ground of the design data of a real product of which RS have been measured, the fully coupled thermal-stress analysis simulation model for multi-stand roll-forming was established. Through the comparison between the experiment and simulation, the reliability of the fully coupled thermal-stress analysis simulation model was validated. The model present an acceptable solution for further research for high-frequency induced welding process. (4) Residual stresses experiment for roll-formed closed-formed sectionsIn the paper a large amount of measurement were conducted for the close-formed sections using X-rays diffraction method. There were totally 13 different specimens, including 2 materials and 2 processes, and 560 points to be measured. All specimens'outside surface RS were measured. Considering the limitation of the equipment's dimension, only inside surface RS of 2 large dimension specimens were measured. At the same time, since through thickness residual stresses is also import for the application of the product, in the experiment machining was used for rough removal and electrolytic polishing was followed. Based on a large number of experimental data, several important factors affecting the distribution of RS, and results showed that the slenderness (b/t) values have an important effect on the distribution of RS.In this study, by absorbing and referring the exist achievements, RS of roll-formed sections were researched and analyzed. Based on the FEM simulation and a large number of experiments, the rule of RS in multi-stand roll-forming is present. The objective of the research is to facilitate the application of roll-formed sections in major engineering works, and to provide the data support for establishment of RS trade rules and standards in the industry. Above all, the research fruits in this dissertation have very important theory value and practice significance.
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