In recent years, the application of tubular materials has been expanding, especially in the automotive industry, but a method of measuring the r-value, an important property for the secondary processing of tubular materials, had not been established. Therefore, we investigated a method of accurately measuring the axial and circumferential r-values of tube materials by both FEM and experiments. The axial r-value was calculated from the circumferential and thickness direction strains after a tensile test with an arc-shaped specimen to obtain a highly accurate r-value. A highly accurate r-value can also be calculated from the average of the strains obtained from tensile tests with strain gauges attached to the inside and outside surfaces of the arc-shaped tensile specimen. However, a correction factor must be applied. The circumferential r-value can also be calculated from the average of the strains obtained from tensile tests with strain gauges attached to the inside and outside surfaces of a ring tensile specimen. However, a correction factor must again be applied.

In this paper, we propose rotary tube flaring with a relieved punch to achieve high formability and productivity. The punch, which has grooves as relief, is pressed into a rotated tube to flare the tube end. The effects of the contact area between the punch and the tube and feed rate on formability were investigated by experiments and finite element analysis. The contact area was controlled by changing the relief size of the punch, and the contact angle was defined as the size of the contact area. As a result, the flaring limit of the proposed method was higher than that of conventional press forming, which was obtained by using the relieved punch with an optimum contact angle. The maximum flaring ratio was 1.12 in the proposed method. When the contact angle was very small, cracks easily occurred at the flared tube end. When the contact angle was very large, buckling easily occurred. When the feed rate was increased, the tube wall thickness increased at the flared portion, and buckling easily occurred owing to the large axial load.

The press bending of a tube using movable rolls is a process in which the distance between the rolls at the support points is small in the initial stage of bending, and the rolls move outward along the punch as the punch is lowered, bending the tube. In this process, the rolls always require a force in the inward direction during the bending process, and this load is called the required roll load. Although this process has a buckling resistance superior to that of conventional 3-point bending, there is a slight dimensional error in the bent radius and slight flattening occurs at the center of the bend after processing. The effects of various factors on the required roll load, the slight dimensional error in the bent radius, and the flattening ratio were investigated using the finite element method. The following conclusions were obtained. The required roll load increases with decreasing n-value, increasing wall thickness, and decreasing friction coefficient μ. Although the effects of n and r-values on the slight dimensional error and flatness after bending are small, the slight dimensional error and flatness increase with decreasing friction coefficient μ. However, the effect is smaller than that in conventional 3-point bending.