MAI utilizes variety of analytical tools and techniques to identify fatigue fractures and their root cause. These include macroscopic examination, microstructural analysis, hardness testing, chemical analysis, microprobe chemical analysis and scanning electron microscopy (SEM).
There are three stages in the life of a fatigue failure:
- Crack Growth (propagation)
- Final Fracture
These stages are illustrated in the SEM image of a fractured rectangular section wire above. The Initiation is indicated by the large red arrow at the lower left. The area of progressive Crack Growth extends from this arrow to the line indicated by the three smaller red arrows. Final Fracture, the point at which the remaining intact cross section of the wire could not sustain the next cyclic stress load – “the straw that breaks the camel’s back” – and complete fracture occurrs, is the light area above the three arrows. This fracture is an example of bending fatigue (in one direction) initiating from a single point of origin.
The fractured crane lifting hook shown above is an example of reverse bending fatigue (back and forth in opposing directions). In this case, the major bending stress was applied from the side of the fracture oriented to the bottom in this photo, and a lower magnitude cyclic stress was applied from the top. The darker gray area indicates final fracture in a single stress cycle. The thin horizontal band at mid fracture indicates a significant “jump” in the fracture progression that occurred in the cycle proceeding final fracture which almost, but not quite, resulted in complete fracture. This fatigue fracture initiated from multiple origins. Multiple origins are indicated by the steps, or “ratchet marks”, at the outer diameter of the fracture indicated by the arrows. Ratchet marks occur when multiple cracks initiate at slightly different planes on a component’s surface. As these multiple cracks progress into the component, they eventually join into a single fracture plane as show above.
Ratchet marks resulting from multiple fatigue origin locations are shown at high magnification in these images taken on our Scanning Electron Microscope (SEM). Fatigue cracking penetrated only a short distance into this automotive suspension component before it failed completely in a single load cycle. As a result, the multiple origin fatigue cracks never progressed far enough to coalesce into a single fracture plane. Several of the individual origin sites are indicated by arrows. Ratchet marks are not exclusive to fatigue fractures. Other fracture modes can also produce these macro-features.
The diagonal bands exhibited by the fatigue fracture of this compressor connecting rod are commonly called “arrest lines”. These indicate a change in the frequency of cyclic stresses, such as “stop-start” sequences, changes in RPM, or variations in load. The initiation site at the crankshaft journal bore (arrow) is heavily damaged. This is not uncommon in fatigue failures. As the first location to crack, the initiation site is exposed to potential relative movement of the two sides of the crack during propagation up to the point of final fracture.. This presents a significant challenge to the analyst in determining the root cause of fatigue cracking.
As with all failure analyses, the analyst must provide specific answers to three critical questions when evaluating a fatigue failure. They are:
- How did it fail?
- Why did it fail?
- What will prevent future failures?
The accurate identification of the failure mode – How? – is a critical step in the failure analysis sequence. But that identification is only the first objective. Why the failure occurred and how to achieve prevention must now be addressed to complete a root cause failure analysis.
In Failure Analysis of Fatigue – Part 7 we will discuss other examples of fatigue failures and microscopic features that are characteristic of fatigue.