Skew rolling is the process for reducing the diameter of a round billet. In this process, a heated round billet is rotated and advanced by rolls as the roll axes are skewed counterclockwise to the pass line. Depending on the rolling conditions, internal fracture may occur in the rolled material. This phenomenon also occurs in cross rolling to form axially symmetric parts. The mechanism of internal fracture has been researched since the 20th century. On the basis of such studies, various theories on its mechanism have been proposed. However, since the theories are based on qualitative experimental results that have not been quantitatively evaluated, the mechanism of internal fracture has not been fully clarified. To investigate the internal fracture initiation and propagation, we conducted hot rolling experiments and quantitatively evaluated the stress and strain history of the rolled material by elasto-plastic finite element analysis. The results show that the internal fracture arises on the internal surface in which shear stress acts owing to the combined effect of tensile and shear stress. In this paper, a new ductile fracture criterion is discussed to quantitatively predict the occurrence of internal fracture in skew rolling.
Shot peening has been used to improve the fatigue strength of metal structural components because it can produce large compressive residual stress near the surface. The distribution of compressive residual stress is strongly dependent on shot peening conditions such as the material of the shot, the shot diameter, the shot velocity, the incident angle, and the peening time (coverage). In this study, a finite element model including multiple shots and the specimen was developed in order to investigate the influence of the incident angle on the compressive residual stress and the depth of its location from the surface in high-strength aluminum alloy. The residual stress and its distribution were analyzed when multiple shots were randomly impacted on the surface of the specimen. The analyzed residual stress distribution using the kinematic material model agreed very well with experimental ones at the incident angles of 90° and 30°. It can be concluded that the depth of the compressive residual stress is proportional to the velocity component perpendicular to the surface.
In this study, the anisotropic plastic properties of perforated sheet metal with regular staggered holes were analyzed by the homogenization method. A hexagonal unit cell with periodic boundary conditions was selected for the analysis. To validate the analysis results, comparisons were made with the experimental results obtained from a uniaxial tensile test. The analyzed stress-strain curves obtained from directional tensile tests match the corresponding experimental stress-strain curves. The in-plane variations of the analyzed r-values also agree qualitatively with the experimental results. In the biaxial tensile test, the macro yield surface was evaluated as an equal plastic work contour in the proportional stress path. Obtained results indicate differential (anisotropic) hardening and slight tension-compression asymmetry of the subsequent macro yield surface. In the case of the proportional loading path, the associated flow rule (AFR) for the initial and subsequent yield surfaces is applicable. However, regarding the non-proportional loading path, the AFR may not be applicable in certain cases because the distribution patterns of the strain rate around the holes depend on the history of local work hardening.
Recently, tube hydroforming is appreciated for its contribution to improving fuel economy and crash safety. For further complex shapes, sheet hydroforming has some advantages because sheet blank shape can be designed freely. In this paper, the basic forming characteristics of punchless sheet hydroforming are investigated on the basis of the results of experiments and finite element (FE) analysis. The main conclusions are as follows. (1) In hydroforming, panel dent resistance is improved compared with that in stamping, because larger strain can be induced and work hardening occurs in the punch bottom area in hydroforming. (2) Fractures and wrinkles occur in the stamping drawing test. On the other hand, wrinkles do not occur in the hydroforming drawing test, because, in hydroforming, circumferential stress at the lower area of the side wall shifts in the tensile direction compared with that in stamping and sheets are supported by internal pressure, which suppresses the out-of-plane deformation. (3) The formable range of the drawing test in hydroforming is greater than that in stamping, since relatively large strain can be induced in the entire specimen in the equibiaxial strain state in hydroforming.