It’s well known that the long term consequences of living under chronic stress are harsh, taking a toll on our health and overall well-being. When it comes to the harmful effects of stress, metal is no different: too much repeated wear and tension will lead to fatigue, and eventually to a breaking point. And much like our bodies, a metal’s breakdown won’t necessarily occur from one major incident. Often the cracking and weakening that leads to metal fatigue will be gradual, going unnoticed until it’s too late.
In the development of metal fatigue, there are three stages:
Stage 1: Crack Initiation
Fatigue cracks will almost always begin around a stress riser point, an area of the metal where the stress becomes concentrated. The stress riser’s susceptibility to cracking may stem from irregularities in the material itself, flaws in the metal part’s construction, or from vulnerabilities caused by damage such as scuffs and scratches. But whatever the reason for a stress riser, its weakness means that micro-cracks in the metal are likely to begin there. The edges of a metal part can be stress risers, since they’re most likely to be receive the brunt of an impact and be damaged. Weld joints are often the culprit as well, since the join of the weld may be weaker than the surrounding material.
Stage 2: Crack Propagation
Repeated stresses will cause micro-cracks, and if they continue, those faults within the metal will begin to grow. Typically crack propagation will be the longest-running stage of metal fatigue, worsening slowly over time with steady repetition. However, it can be accelerated by other factors. Increased moisture collecting on the surface from weather, vibration from moving parts, and greater stress placed on the metal will all increase the likelihood of crack growth. The danger is that these accelerants are obvious pitfalls leading to metal failure, but metal fatigue is often much more subtle and easy to miss. Because of this, a lack of maintenance which allows the unchecked growth of micro-cracks can lead to a sudden and unpredictable failure.
Stage 3: Final Catastrophic Failure
If metal fatigue is allowed to progress to this stage, it will result in a final failure event: the metal will fracture. Depending on the material’s thickness, metal composition, and applied stress, the fracture will be either ductile or brittle. In a ductile fracture, the metal is deformed by being bent out of shape. The edges of the fracture may crumple and bend, but not necessarily break all the way through. When it comes to a brittle fracture, it’s exactly as it sounds: the metal shatters or snaps. In a typical case of brittle fracture metal fatigue, the pieces will snap completely apart. There will be little to no plastic deformation (a change in shape) in the metal as compared to a ductile fracture. Instead, you’ll see smooth snapped edges where the break occurs. In either case, the final stage of metal fatigue is often sudden and without obvious warning signs.