Tooling failure analysis can be very challenging. Frequently, we have to deal with complex loading and multiple simultaneous damage processes. Because the tools are so hard, fracture surface features are often very faint. The clues we get from our visual inspection, always one of the key early steps in a failure analysis, are thus often suggestive rather than compelling. They hint at a damage sequence rather than definitively revealing one.
The pictured cylindrical object, the lower portion of a punch from a stamping press, broke off during production stamping at a deep-draw house. The mating fragment was not provided and neither was a print, so I might have a clue about any lubrication holes present or other features that might locally increase the stress. The cup-shaped piece at the bottom, the workpiece that was eventually to be sold to my client’s customer, has been squeezed from two mutually perpendicular directions (which was probably not intentional) and has resulted in an asymmetric part.
What can we tell from what we see here?
- Figure 1 shows fairly distinct curved lines on the fracture surface, highlighted by the rows of red arrows. These series of concentric arcs are known as beach marks. They indicate fatigue, or progressive crack growth. Once the crack initiates, it may grow a little more each time the press forces the punch into a workpiece or each time the workpiece is stripped from the punch in preparation for the next hit. Beach marks indicate the position and shape of the crack tip at some specific point in time prior to complete fragmentation. The uniformity of the color is consistent with the uniformity of the force of the punch and the environment in which it was used as it is pushed into the flat blank to create the cup. Sadly, the general area from which the crack seems to be spreading, shown by the black x at the top of the left image, is missing. That means that there is no way to confirm or disprove that there was a reason for the crack to have initiated in the suspect position.
- Now look at the image on the right side of Figure 1. It gives us a clearer view of the shape of the crack. The black arrows highlight the “swoop” profile, where the crack is almost transverse (perpendicular to the cylindrical axis) at the left and virtually parallel to the axis on the right. The classical swoop-shaped crack is created in wrought materials by a bending load, initiating in a transverse orientation at a corrosion pit, a wear mark, a scratch or simply the highest stress location due to design.
As the crack propagates deeper into the load-bearing cross section, the crack eventually finds it easier to grow along the “fiber” of the material (similar to what we see when splitting wooden logs). Eventually, the crack turns 90 degrees. So, could this crack actually have started on the cylindrical surface at the lower (blue) x? Axial compression of tall, skinny columns creates bending, so this is not completely impossible. (See Euler’s Buckling on Wikipedia: https://en.wikipedia.org/wiki/Buckling.) Is there anything else we can see that can help us decide?