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The effects of strain rate and friction on the formability of the steel sheet are studied according to the three major deformation modes that are widely used in the industry. Experiments for circular cup stretch, plane strain stretch, and circular cup drawing are carried out at two speeds of 3 SPM and 30 SPM for SGACUD 0.7 t and DP 590 1.0 t steel sheets. The corresponding finite element simulations are carried out. A constitutive equation, which is a function of strain and strain rate, is used to account for the effect of strain rate on flow stress in the finite element analysis. The formability is evaluated using the polar effective plastic strain (PEPS) forming limit diagram. An experimental equation that takes the sliding velocity into account is utilized in this study. In conclusion, the effects of strain rate and friction are clearly understood; finite element analysis by using the constitutive equation to consider strain rate, and also the friction coefficient model according to the sliding velocity, can be effectively used to predict the formability as forming speed changes for the three deformation modes. [ABSTRACT FROM AUTHOR]

In this paper, a new general finite element (FE) method for predicting shearing, blanking, and piercing of ductile or forgeable metals is presented, emphasizing usability and conformality of the target shear-separated material in the context of automatic multi-stage metal forming. The approach is based on an element deletion/splitting element degradation model and sheared surface quality control; the former ensures the generality of the approach, whereas the latter aims to promote usability of the sheared or blanked material. The proposed approach is compatible with the traditional theory of ductile fracture, and material separation can be verified experimentally. The combined sheared surface quality control and element degradation model is applied at the precise time that material separation takes place. In the sheared surface quality control scheme, free nodes that should disappear after shearing/piercing instead move to positions nearest the fractured surface for conformality of the FE mesh. The shearing simulation is finalized by remeshing, followed by the application of a "free node algorithm" for surface quality control. This new method overcomes the numerical issues that can arise after deleting the severely damaged elements or split edges observed with conventional methods. The proposed scheme was used to simulate a three-dimensional round-bar shearing process for the production of short billets. The simulation and experimental results were compared. The results showed that our approach can easily, systematically, and precisely simulate metal forming processes involving finely sheared or blanked materials under a compressive stress state. [ABSTRACT FROM AUTHOR]

Traditional fatigue fracture theory and practice focus principally on structural design. It is thus too conservative and inappropriate when used to predict the high-cycle fatigue life of dies used for metal forming, especially cold forging. We propose a novel mean stress correction model and diagram to predict the high-cycle fatigue lives of cold forging dies, which focuses on the upper part of the equivalent fatigue strength curve. Considering the features of die materials characterized by high yield strength and low ductility, a straight line is assumed for the tensile yield line. To the contrary, a general curve is used to represent the fatigue strength. They are interpolated, based on the distance ratio, when finding an appropriate equivalent fatigue strength curve at the mean stress and stress amplitude between the line and curve. The approach is applied to a well-defined literature example to verify its validity and shed light on the characteristics of die fatigue life. The approach is also applied to practical forging and useful qualitative results are obtained. [ABSTRACT FROM AUTHOR]

Incremental sheet metal forming can manufacture various sheet metal products without a dedicated punch and die set. In this study, we developed a two-stage incremental forming process to decrease shape errors in the conventional incremental forming process. The forming process was classified into the first single point incremental forming (1st SPIF) process for forming a product and the counter single point incremental forming (counter SPIF) process to decrease shape error. The counter SPIF gives bending deformation in the opposite direction. Furthermore, the counter SPIF compensates for shape errors, such as section deflection, skirt spring-back, final forming height, and round. The tool path of the counter SPIF has been optimized through a relatively simple optimization method by modifying the tool path of the previous step. The tool path of the 1st SPIF depends on the geometry of the product. An experiment was performed to form a circular cup shape to verify the proposed tool path of the 1st and counter SPIF. The result confirmed that the shape error decreased when compared to the conventional SPIF. For the application, the ship-hull geometry was adopted. Experimental results demonstrated the feasibility of the two-stage incremental forming process. [ABSTRACT FROM AUTHOR]