titanium properties
I’ve read in a variety of sources that the S76 (including the A model) does indeed have titanium spares in the rotor blades.
The following information was gleaned from a Titanium Industries website, where an association promotes the industrial use of titanium, and I’m posting this in case the lighting strike had anything to do with the metal fatigue the AAIB detected in the fractured rotor blade. The information on how to machine titanium and especially how to weld titanium was particularly interesting. The physical property discussion will be limited to only those properties that would be relevant in a lightning strike.
Titanium reacts strongly with normal air when heated, so arc welding requires a pure argon “air blanket”. An arc welded titanium structure shows colors if contamination develops. A “silver metallic” color means no contamination, a gold or straw color means some contamination, and a light or dark blue color means high levels of contamination (the blue colors also mean no structural soundness at all). Contamination can produce porosity, low-notch toughness value, and brittleness.
Titanium comes in 3 classes of alloys that include 2 normal lattice matrix classes, and a class with a mix of the 2 normal matrix. The first matrix is “hexagonal closed-packed” (HCP) lattice structure (also know as “alpha phase”). The second matrix is “body centered cubic” (BCC) lattice structure (also known as “beta phase”). Certain alloys stabilize the titanium to one matrix or the other within certain temperature ranges (“alpha phase” class alloys and “beta phase” class alloys) and some alloys stabilize a mixture of both (“alpha-beta phase” class alloys). Various heat treatings and “annealing” also help to stabilize the matrix of the various alloys.
Titanium can undergo a matrix transformation known as an “allotropic transformation”, that can convert the matrix from one lattice form to the other, when heated to certain temperatures (depending on the alloy). If I understand it correctly something like an “allotropic boundary” (my phrase) can form within a structure when titanium is “spot heated”, causing one lattice (or mixture) to exist on one side of the “boundary” and a different lattice to exist on the other side. This can be a source of structural weakness and/or brittleness.
Titanium is also a poor conductor of heat, which must be watched when welding it. This property allows hot spots to develop more easily within a structure.
All of this would be relevant to a lighting strike, because discoloration would indicate that atmospheric gases (namely oxygen and nitrogen) might have contaminated the metal if the lightning strike heated it. Also, the current from a lighting strike could cause local “hot spots” along a high amperage conduction path. If a matrix change occurred due to heating, the conduction path could possibly be a source for fatigue in the metal.
None of this will be relevant though, if a lighting strike had nothing to do with the fatigue that caused the fracture of this particular rotor blade.
Last edited by Flight Safety; 27th Jul 2002 at 04:50.