Failure Analysis of Fatigue – Part 7 – Analyzing Fatigue – Microscopic Features

MAI utilizes a variety of analytical tools and techniques to identify fatigue fractures and their root cause on a microscopic scale. These include stereo microscopy, high magnification optical microscopy and scanning electron microscopy (SEM).

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Fatigue fractures exhibit distinct 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 shallow crack that resulted from a single load, or stress, cycle. Repetition of these cycles produces an advancing progression of shallow cracks as shown above in this fatigue fracture in a hydraulic valve body. This process is characterized by the term, “fatigue crack propagation”.

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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 hardness, microstructure and chemical composition of the alloy. These SEM images illustrate fatigue striations in an aluminum valve body (above-top) and an alloy steel high pressure hydraulic cylinder (above-bottom).

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In some cases, the root cause of a fatigue failure can only be discovered by an analysis of internal characteristics of a component at the crack location. In this example, a metallographic cross section revealed decarburization (dark phase at arrow) of 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.

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Other types of internal defects which act as initiation sites for fatigue are apparent on the fracture surface. Examination of this brake return spring by SEM revealed fracture features which radiate from a single initiation point. Viewed at higher magnification, this initiation point exhibits a void containing a non-metallic inclusion which acted as a stress concentration.

As with all failure analyses, the analyst 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 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.