Summary and Conclusions
The case for the defendant rests on establishing the premise that aircraft components are designed to be 'defect-tolerant' and hence to sometimes contain cracks. It then has to prove 'beyond reasonable doubt' that, although the service engineer may have been negligent by not detecting the fatigue cracks which were present in the saddle clamp brackets at the time of the re-build, the failure would not have occurred when it did except under circumstances involving a high level of impact load to the undercarriage.
Without the fractographic evidence, demonstrating this case would have been very much more difficult, as it would largely have been based around the results of FE analyses, that rested on assumptions whose veracity cannot easily be proven. On the plaintiff's side, in contrast, they have clear evidence of a fatigue crack which existed at the time of the re-build. In the eyes of a (usually) non-technical judge, this is likely to have been viewed as damning evidence.
The fractographic and fracture mechanics evidence, however, completely change the balance of probabilities. The fractography is, like the fatigue crack, a fact whose interpretation should be amenable to objective analysis. Coupled with the fracture mechanics analysis of fracture load, and the FE results, there are three independent measures that all indicate the same sequence of events. This becomes powerful evidence to support the defendants claim.
To summarise the findings of the fractographic and mechanics investigation:
Fracture mechanics has indicated that the fracture stress on this component was around 660 MPa, and that fracture was very likely to have occurred under conditions representing the boundary between plastic collapse and fast fracture.
The shear failure stress is around 530 MPa, giving a shear load at failure of around 60 kN. This load value was supported by FE analysis and is in accord with the concept of failure at the plastic collapse - fast fracture boundary.
Fractography shows that the fatigue crack extended by fast ductile fracture, which was superseded by shear fracture over about the lower third of the fracture surface. Identification of shear microvoids in the shear region is a crucial part of this evidence.
The conclusion to be drawn is that the undercarriage leg failed as a result of an impact load applied either immediately before landing, or during the landing. The presence of the fatigue crack did not significantly weaken the undercarriage leg relative to its capacity to withstand a 'normal' landing.