Fracture Mechanics & Microscopic Analysis of Fatigue

Metallurgical Consulting was called in to inspect numerous failures that were occurring in one of six nominally identical stainless steel exhaust stacks on six identical diesel generators. A number of other metallurgical services had attempted to determine the cause of the failures with little success.

Dr. Clarke, a specialist in the field, was asked to intervene and try to estimate the fatigue stresses, which initiated at the welds (see Figure 1). Fatigue fractures usually have a very characteristic appearance and can be readily identified as such. Detailed stress information can also be obtained from fatigue fractures in many cases using expertise in electron microscopy, fracture mechanics and stress analysis. All require an in-depth knowledge of the nature of the applied stress spectrum.

Dr. Clarke quickly determined that welding was not the problem, as some worse looking welds failed to initiate fatigue. Figures 2 and 3 show examples of the individual fatigue striations which resulted from individual stress cycles that were very close to the crack origin. Numerous scanning electron microscope images such as these were taken and the striation spacing measured. Published data on crack growth rates versus stress intensity factor provided a basis to calculate the applied stress range using fracture mechanics. Resulting stress estimates from the striation spacing measurements and an independent stress estimate for initiation of fatigue crack growth were in agreement, however, the stress estimates were about 1000 times higher than any design estimates.

Subsequent work by others revealed that two turbo chargers were out of sync and the one stack that was failing was oval in cross section, while the others were circular. These two findings could account for harmonic induced stresses on the order of the fatigue striation stress estimates.

In a similar project, fatigue was observed at fastener holes in a pressurized cargo aircraft fuselage. Figure 4 shows the fatigue striations .006" from the fastener hole wall. Striation measurements over a distance from the origin enabled creation of a calculated crack growth curve which could be used to set inspection intervals. Stress estimates from striation measurements were in accord with both strain gauge and finite element analysis.

To learn more about fracture mechanics and microscopic analysis of fatigue, contact Metallurgical Consultants directly.

Images From The Metallurgical Services Provided This Microscopic Analysis of Fatigue

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