How can we tell if it is a fatigue crack if the most well-known feature of a fatigue crack is not present? Figure 2 shows a tie rod from an injection-molding press. While there are a few faint and indistinct beach marks, we have another indicator: ratchet marks. Ratchet marks are (generally) small steps along the part surface. Ratchet marks tell us that there were multiple crack initiations on both sides of the ratchet. Higher stress levels and sharp stress concentrations often result in a series of ratchet marks.

Figure 3 shows another shaft. When viewed directly, the fracture surface is very smooth, almost featureless. The overview image at left already has the contrast enhanced. The detail view at right has the contrast enhanced more to show some slight variations in color. Very smooth fatigue cracks are generally associated with low stress.

If the stress is so low that the fatigue crack kept propagating when there was barely any material left “holding on,” why did the crack start in the first place? There might have been a shock load that was not big enough to create a complete separation. There might have been some damage to the surface, as previously noted. In fact, we see some severe damage to the surface. But the area where the crack appears to have started, left and right of the one obvious ratchet mark, looks pretty clean. A mystery!