Analyzing Material Fatigue
Posted on 04. May, 2010 by Rob in Failure Analysis
A variety of analytical tools and techniques are used 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; 1. Initiation, 2. Crack Growth (propagation), and 3. Final Fracture. These stages are illustrated in the SEM image of a fractured rectangular 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 camels back” – and complete fracture occurred, 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 above is an example of reverse bending fatigue (back and forth in two directions). In this case, the major bending stress was applied from the bottom of the fracture as oriented in this photo, and a minor stress 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 fatigue 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.
Rachet 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 NASCAR Racing 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.
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. This presents a significant challenge to the analyst in determining the root cause of fatigue cracking.
Fatigue fractures exhibit distinct features, called striations, when viewed at high magnification using Scanning Electron Microscopy. Striations appear as relatively evenly spaced parallel lines. Each striation is actually a shallow crack that results from a single load, or stress, cycle. Repetition of these cycles produces an advancing repetition 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”.
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 (left) and an alloy steel high pressure hydraulic cylinder (right).
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.
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.
