If you stamp parts for the automotive industry, chances are you will experience cracking at some point.Ĭracking commonly occurs as a vertical open failure (see Figure 3 ). However, with the increased usage of stronger, lighter-gauge materials, they are becoming more common. Cracking results from excessive compressive cold working or strain hardening in a localized area.Ĭracking failures used to be found mostly in stainless steel parts made with deep drawing. Because splitting is a tensile failure, like necking, the same corrective actions are used to solve the problem.Ĭracking is completely different from splitting, and the corrective action is dramatically different than that used to correct splitting and necking failures. Although it can occur in any direction on a part surface, splitting unusually shows up as a horizontal failure near the punch radius of a deep formed feature, with severe metal thin-out near the fracture.
Splitting, which is the most common type of failure that occurs on drawing and stretching dies, is an unacceptable metal forming failure. It occurs after the onset of necking-the metal first necks, and if more stretching occurs, splitting will result. Splitting, often referred to as tearing or ripping, is an open void that results from too much localized stretching (see Figure 2 ). Changes in lubricant also can help, as using a thicker metal or one with greater elongations and n values. A radius change or blank holder pressure change might help to solve the problem, but not if the necking failure is located in the center of the part. One of the most common corrective actions of necking is to increase the size of the punch radius. However, this is an unfair representation of the metal’s true capability, as technically metal’s true capability, as technically metal fails as soon as necking becomes visible.
Many material suppliers include this extra stretching that occurs after necking is their total elongation metal report.
BUT CRACK SPLITTING FULL
It’s important to note that the metal can stretch even more after necking has occurred and before a full rupture is visible. This failure commonly is viewed as unacceptable, but for parts in which excessive localized thin-out does not affect the structural integrity or function, minor necking sometimes is permitted. Look for necking around the tops of deep-drawn parts. Necking usually reveals itself as a horizontal curved line in the metal somewhere just bellow a punch radius (see Figure 1 ). Commonly referred to in press and die shops as smiles or elongations, necking occurs when the metal has been stretched to its maximum threshold without reaching a see-through rupture. Necking is a type of tensile failure, meaning that it occurs as a result of overstitching the metal. The failure mechanism is different, as is the corrective action. Although both failure types results in an unacceptable part and essentially mean that the part won’t “hold water”, the reasons they occur and their deformation modes are very different. For instance, there is a big difference between a crack and a split. The same basic concept apple to forming failures. What type of tool steel should I be using to prevent this problem?” My typical response is, “What type of galling is occurring? Is it adhesive galling? Abrasive galling? Corrosive galling?” My recommendations will change depending on the type of galling that is occurring. For example, I have has tooling guys ask, “My forming punches are galling. To make a good, data-based decision with respect to solving a metal forming failure, you need to be able to identify the type of failure that is occurring. While many of these terms mean essentially the same thing-splitting, tearing, and ripping all are overstitching failures-it is important to keep in mind that some failures types are significantly different. Terms such as splitting, cracking, ripping, tearing, necking, laminations, and elongations often are the main points of discussion. During these conferences, the topic of metal failure almost always comes up. I conduct and attend many technical conferences about die design techniques and metal forming methods.