Both of these approaches will strengthen the section modulus of the tube, which, in effect, increases the support against the stress. The other fix: You also can improve the cross-sectional shape support by increasing the wall thickness or decreasing the tube OD. D is defined as the bend centerline radius (CLR) divided by the tube OD. The strain on the outside fibers of a 3D bend is less than the strain on a 2D bend, so linear stress is lower. The fix: You can reduce the stress by using a larger bend centerline radius. When the flattening becomes excessive, it is a defect. When this happens, the tube flattens on the outside radius (see lead image). There comes a point at which the stress on the outside radius overcomes the ability of the tube's cross-sectional shape to support it. When the stress on the outside radius overcomes the ability of the tube's cross-sectional shape to support it, the tube flattens on the outside radius.
Part I discusses surface defects Part II covers other defects, such as wall thinning, ovality, buckling, and fractures. Editor's note: This is the second part of a two-part article that examines tube bending defects, possible causes, and suggested remedies.
