A ductile failure is indicated if there is considerable stretching and thinning of the material on each side of the break and the edges of the break are ragged in appearance. First, examine the break and try to determine if the tensile failure was a ductile or brittle failure. Cracking or Splitting on Outside Bend Radius The extrados fractured in a ductility tensile failure.Ģ. For especially small D of bends and high-wall-factor tubes, a closed-pitch mandrel may be required. These usually require ball links on the end of the mandrel. Small D of bends and thin-wall tubes require more support around the bend. A mandrel wears with use, and as it wears, the tube may show signs of excessive flattening in the bend Generally, the gap between the tube ID and the mandrel OD is about 25 percent of the tube wall thickness-when the mandrel is new. During bending, the mandrel is held stationary inside the tube with a mandrel rod, and the tube is "drawn over" the mandrel.īecause the mandrel fits snugly inside the tube, it supports the tube outside radius and prevents it from collapsing. The other, other fix: Generally, the tube OD, wall thickness, and bend CLR are selected in the design stage, so your only alternative is to support the tube with additional tooling, usually an internal mandrel.Ī mandrel, in its simplest form, is a solid piece of metal with a round end that fits snugly inside the tube. 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.
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