Art Hedrick
Owner

Resolving Splitting in Stretch Flanges and Cracking in Compression Flanges­—Part One

Splitting and cracking in a formed metal flange is one of the more common part defects faced by metal stampers. A split or a crack renders the part basically useless, and in most cases the part ends up being discarded as scrap. Here we’ll discuss tension and compression failures and explain a few of the potential causes and solutions.

Defining Two Flange Types

Flanging simply can be defined as bending on a curved axis. The two basic styles of flanges (Fig. 1): tension (stretch flange) and compression (shrink flange). During the forming of a tension flange, the blank edge distance increases, which results in the sheet metal being stretched into a greater length of line. This results from the concaved shape of the bend. Naturally, as the sheet stretches it becomes thinner. If it thins too much it will fail and split. We easily can identify a split because there will be thinning at the point of failure. The split typically originates at the blank edge where the greatest amount of stretching has occurred.

During the forming of a compression flange, the length of line decreases due to convex curvature of the part. This essentially means that the metal is being squeezed together rather than being stretched, which may cause wrinkles or, in many cases, thickening of the sheet. Failures occurring in a compression flange are cracks. In other words, splits occur due to excess localized tension and cracks result from excess localized compression. We can identify cracks because there typically is no thinning of the sheet at the origin of the defect (Fig. 2). 

Strain Basics

Mitigating the failure—a split or crack—requires understanding the effects of strain, simply defined as the permanent deformation of the sheet metal. There are two primary types of strain: tension strain and compression strain. In other words, we can do only two things to a metal sheet: stretch it (tensional strain) or squeeze it (compressive strain).  

Every time we deform a piece of sheet metal beyond a mechanical property known as its yield point, where we’ve made a permanent shape change, the sheet will work harden. This work hardening, also referred to as strain hardening, varies in severity based on the type of metal alloy. Grades used in structural safety stampings for motor vehicles, for example, work harden extensively, while others may work harden less severely.