Theoretical and experimental analysis of roll forming

Abstract

Roll forming has been applied successfully in many industries. Despite this, it is still one of the least understood metal forming processes. This is because during roll forming the deformed sheet adopts a very complex 3D-shape which is not easy to analyse. In this thesis some new concepts and techniques are proposed which lead to a considerable improvement in the modelling of this process. It has been demonstrated that if the pass length between two roll stations is sufficiently long, then in addition to the springback region, three deformation regions exist for each pass: Region I where the bend angle does not change, Region II where the bend angle changes but the strip does not contact the roll and Region III where the bend angle changes as a function of the roll geometry. While two of these regions had been known previously, this thesis identifies the third region where the sheet is in contact with the rolls. The whole deformation process can be defined by the generation of a bend angle curve, which describes the bend angle distribution along the pass. With the help of this curve, the deformed 3D-surface can be defined and analysed. Important information such as the strain distribution can be extracted from this curve. The geometric restriction from the rolls in region III is studied in det. :1 in this research, this restriction is shown to have significant influence on the bend angle curve, and three modes of constraint are established according to the relative flange length to roll diameters during the roll forming of a channel section. Theoretical results indicate that when the forming angle is close to 90°, horizontal rolls provide more severe geometric constraint than side rolls. Four fundamental deformation types have been identified for roll forming. These e the longitudinal stretching, shearing, transverse bending and out-of-plane bending. Expressions for the corresponding strains and energy terms have also been developed. Each deformation mode was found to be associated with a particular function of the bend angle and of the geometry of the section; these functions are simple geometric properties of the section, such as polar second moment of area. Classification of these modes and their relation with section properties that are familiar to engineers but had not been previously identified greatly improves the determination of the forming severity in roll forming. The influence of various forming parameters on the longitudinal strain distribution has been investigated when roll forming channel section. These parameters include the flange length, strip thickness, forming angle, forming angle increment, roll diameter, and strain hardening coefficient of the material. To verify the theoretical results comprehensive strain measurement experiments have been performed for ch el profile and other sections. The characteristics of the theoretical strain distribution and its variation with fo mg parameters are generally consistent with those of the experimental results, but there have been poor quantitative agreements between the two; the predicted strain values are substantially higher for the longitudinal membrane strain while lower for the out-of-plane bending strain. The discrepancies are examined in the thesis.