IMPLICATIONS OF SEM FRACTOGRAPHY

The more detailed SEM fractographic observations are critical in determining which one of the two conflicting failure theories is the most likely. The plaintiff is claiming that fatigue cracks existed at the time of the rebuild and should have been detected and the components replaced. Failure was due to a fatigue of sufficient length that the fracture toughness of the saddle clamp brackets was exceeded under 'normal' landing loads. On the other hand, the defendant claims that these brackets would have lasted until the next scheduled maintenance inspection, and that failure on the day was due to abnormally high landing loads, caused by pilot error. The means to distinguish between these conflicting claims lies in the fracture surface, which contains a full record of all fracture events, provided that this can be deciphered. No less an figure than George Irwin has, apparently, referred to fractography as 'consulting the expert'.

The optical fractograph of Figure 6 indicates the operation of two different mechanisms of fracture below the fatigue crack. The lower region has the smooth shiny appearance often found on shear fractures, but it is not clear why there should be a shear fracture in this position, as once fast fracture initiates a crack generally runs right across a component. Furthermore, shear generally occurs on planes orientated at 45o to the direction of applied load, whereas in this case, the three regions lie in a single plane orientated at 90o to the applied load.

Comparison between Figures 8 and 9 indicates that the corrosion pitting is more extensive near the initiation sites of the fatigue cracks, than near the tip of the crack. This provides strong support that the crack initiated a 'long time' ago, i.e. probably well before the rebuild. Compare the microscopic appearance of the middle band and lower fracture bands with each other, and with the fatigue regions. Then consider which of the buttons below is correct.
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Figure 6
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Figure 8 Fatigue region near the crack initiation site
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Figure 9 Fatigue region near the end of the crack
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Figure 10 Dark middle band
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Figure 11 Shiny Lower band

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