I've also found absolutely nothing through dozens of inspections
see
http://www.pprune.org/engineers-tech...detection.html.
Maybe there IS evidence but not where people are looking!
if ash was encountered in low concentrations it would only cause greater wear on engines
errrr....
Don't forget the potential impact of ash accumulation in the turbine hot-section cooling channels. More to do with overheating than just 'wear'!
Are filters (oil filters from engines, air filters from engines and other parts of the a/c, ...) being sent for chemical / microscopic analysis?