Die Science: Splitting or cracking?

I conduct and attend many technical conferences about die design techniques and metal forming methods. During these conferences, the topic of metal failure almost always comes up. Terms such as splitting , cracking , ripping , tearing , necking , laminations , and elongations often are the main points of discussion.

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.

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. For example, I have has tooling guys ask, “My forming punches are galling. 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.

The same basic concept apple to forming failures. For instance, there is a big difference between a crack and a split. 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. The failure mechanism is different. The failure mechanism is different, as is the corrective action.

Necking

Necking is a type of tensile failure, meaning that it occurs as a result of overstitching the metal. 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 usually reveals itself as a horizontal curved line in the metal somewhere just bellow a punch radius (see Figure 1 ). Look for necking around the tops of deep-drawn parts.

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.

It’s important to note that the metal can stretch even more after necking has occurred and before a full rupture is visible. Many material suppliers include this extra stretching that occurs after necking is their total elongation metal report. 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.

One of the most common corrective actions of necking is to increase the size of the punch radius. 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. Changes in lubricant also can help, as using a thicker metal or one with greater elongations and n values.

Splitting

Splitting, often referred to as tearing or ripping , is an open void that results from too much localized stretching (see Figure 2 ). It occurs after the onset of necking—the metal first necks, and if more stretching occurs, splitting will result.

Splitting, which is the most common type of failure that occurs on drawing and stretching dies, is an unacceptable metal forming failure. 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. Because splitting is a tensile failure, like necking, the same corrective actions are used to solve the problem.

Cracking

Cracking is completely different from splitting, and the corrective action is dramatically different than that used to correct splitting and necking failures. Cracking results from excessive compressive cold working or strain hardening in a localized area.

Cracking failures used to be found mostly in stainless steel parts made with deep drawing. However, with the increased usage of stronger, lighter-gauge materials, they are becoming more common. If you stamp parts for the automotive industry, chances are you will experience cracking at some point.

Cracking commonly occurs as a vertical open failure (see Figure 3 ). Unlike tensile failures, cracking results from too much localized compression. For this reason, the metal at the failure will be thicker than the metal’s original thickness. These types of cracks are commonly referred to as compression cracks .

I’m inclined to argue that cracks do not occur in tension. Cold working, not the crack, occurred during compression. Nevertheless, the term compression crack lives on, and I won’t try to change it.

A few common solutions for cracking are:

• Change the metal to a lower tensile strength.

• Anneal the metal between between the drawing stations.

• Change the feature causing the localized compression.

• Change the process from simple flanging to stretch forming or drawing.

• Reduce the amount of metal flow into the failure area.

Know Your Failure

It is very important to be able to identify each type of failure before you attempt to correct it. Trying to correct a crack the same way you would correct a split will result in bigger failures. A crack is a crack and a split is a split, but a crack is never a split!

Also keep in mind that the corrective actions for each of these failures can go way beyond the few common remedies outlined in this article.

Until next time… Best of luck!