Some more tidbits for youse to speculate upon: [Earlier we got off on the wrong foot about 'hollow back to solid blades' - impression corrected above I hope?]
More F-35 Engines Show Signs of Blade Rubbing 14 Jul 2014 Brendan McGarry
"...The incident is the second in less than a year to affect a part of the engine known as the integrally bladed rotor, or IBR. The component contains three sets of fans to help compress air into the engine. A first-stage fan made of hollow titanium blew apart during ground testing in December and is undergoing redesign, while a third-stage fan made of solid titanium disintegrated during the June takeoff.
Officials said the two cases aren't related.
A review of almost 100 engines in the fleet didn't find evidence of the same problem affecting the third-stage fan, though several showed signs of mild rubbing, Bogdan said. The extent of the problem appears to be isolated to that particular aircraft,..."
More F-35 Engines Show Signs of Blade Rubbing | Military.com