In Nevil Shute’s 1948 future-thriller No Highway, we meet aerospace engineer Theodore Honey in a state of panic on a transatlantic flight. He claims that metal fatigue is putting the plane in certain peril, but can’t convince anyone of the imminent danger because there’s no obvious evidence of it.
Six years later, this scenario became a real-life tragedy when a de Havilland Comet broke up over the Mediterranean due to undetected metal fatigue in the corners of the aircraft’s windows.
It’s in a frantic effort to avoid such tragedies that aerospace engineers are so meticulous in their parts testing—both during their assembly and while they’re in service.
But this is a troubled and imperfect undertaking. Too often, just the act of testing a part for its robustness compromises precisely that.
Until now, the solution has been to replace parts according to a conservative schedule of their predicted lifespans, which can be wasteful. Another option has been to engage in the practice of non-destructive testing (NDT), where materials are assessed for cracks or other imperfections caused by stress, chemicals, heat, radiation, etc., in a way that doesn’t require their disassembly or destroy their serviceability.
But these methods can be prohibitively expensive and technically challenging. And they demand esoteric expertise for their interpretation.
Enter the novelty of Second Harmonic Generation (SHG), a new spectroscopic method its proponents claim is more sensitive than existing NDT techniques, and could thus give earlier warning of danger.