MAI utilizes a variety of analytical tools and techniques to identify fatigue fractures and their root cause. These include stereo microscopy, high magnification optical microscopy and scanning electron microscopy (SEM).
Fatigue fractures exhibit distinctive features, called striations, when viewed at high magnification using a scanning electron microscope. Striations appear as relatively evenly spaced parallel lines. Each striation is actually a microscopic crack that results from a single load, or stress, cycle on the effected part. Repetition of these cycles produces an advancing progression of microscopic cracks as shown in the SEM image above (1) of a fatigue fracture in a hydraulic valve body. This process of repeated cracking is characterized by the term, “fatigue crack propagation”.
The appearance, or morphology, of fatigue fracture striations varies depending on the magnitude and frequency of the applied load and the physical characteristics of the affected component such as its hardness, microstructure and the chemical composition of the alloy. These SEM images illustrate fatigue striations in an aluminum valve body (2) and an alloy steel high pressure hydraulic cylinder (3).
In some cases, the root cause of a fatigue failure can only be determined by an analysis of the internal characteristics of a component at the crack location. In this example (4), a metallographic cross section through a fatigue crack revealed decarburization (dark phase at arrow) at the surface of a steering arm due to faulty heat treating. This carbon depleted layer has significantly reduced hardness and strength, as well as residual tensile stress, conditions highly conducive to fatigue crack initiation.
Other types of internal defects which act as initiation sites for fatigue are sometimes apparent on the fracture surface. Examination of this brake return spring by SEM (5) revealed fracture features which radiate from a single initiation point. Viewed at higher magnification (6), this initiation point exhibits a void containing a non-metallic inclusion which acted as a stress concentration.
An effective failure analyses must provide specific answers to three critical questions when evaluating a fatigue failure. They are: 1. how did it fail? 2. Why did it fail? and 3. What will prevent future failures? The accurate identification of the failure mode – How? – is the critical first in identifying “Why”. With these two questions resolved, the objective of the failure analysis can now be accomplished; a specific plan of action to prevent future failures.