The Stretch Bending Forming And Spring-back Analysis Of Extrusions

Stretch bending is an important method of bending of extrusions. Stretch bending can decrease the spring-back effectively and improve the forming precision, so the process of stretch bending was widely used in the forming of extrusions in the car and aircraft manufacture. The spring-back control after stretch bending forming is a major concern in improving extrusions parts accuracy. Otherwise stretch bending forming, with different non-linear part's material parameters and complex stress state, friction conditions during forming, is very difficult to predict the spring back. So, experimental trial-and-error method, which results in the long cycles and the high cost, was used to select the process variables and to compensate die profile surface. But the experimental trial-and-error method can't be used to obtain the precision profile surface before single stretch bending forming practice.In this dissertation, at first, the basic concepts of stretch bending and the two urgent key technical problems needed to solve on stretch bending technology were introduced. Five kinds of forming defects which were easily produced during forming process and the current international research status on stretch bending about mechanical properties and cross-section shape,process parameters in experiment and numerical simulation were summed. The initial yield condition and the basic law including the flowing law and the strengthen law in elastic-plastic mechanics were introduced. The incremental stress-strain relations, which provided a theoretical basis for the subsequent mechanical analysis, were derived for isotropic hardening material. Secondly, the engineering mechanical model used for pre-stretch and bending and post-stretch, was established on a series of basic assumptions. In bending stages friction unit model and stress-strain analysis and bending moment was detailed. The analysis of stretch bending spring-back was introduced and the formula for calculating sheet and extrusions spring-back radius and angle was derived. Thirdly, the theory analysis of bending spring-back was combined with the numerical model of parts to form an computation method, the related formula were derived in detail, to modify the die profile curve which is fitted by arc spline. The modified die profile curve was continuous and its curvature was constant in separate segment along the entire profile. This method through the point's density distribution on the die profile could control fitting error of the curvature of the arc spline. Finally, elastic-plastic FEM is used to compute the magnitude of the spring-back of the stretch bending forming of the extrusion, and to evaluate the reasonableness of the die profile compensated by the method formed. This work in the dissertation may provide a rational and useful method for improving the quality of stretch bending forming of extrusions and a useful tool for promoting computer aided design of the tools.