Was the stress-testing done in a fatigue tester, or did it use steady-state loading? In either case, were all the loads considered? Things like the torsional loading of prerotation, possible hard flapping loads from poor takeoff technique and 2/rev loads attributable to the semi-rigid rotor?
Titanium, thankfully, has a more or less fixed endurance (fatigue) limit, as does steel. But this limit is roughly half of its one-time strength A part that passes a steady-load test can break at lower loads if they take the form of vibrations.
Compare the typical bolted Erector-set-style folding or two-piece masts on amateur-designed gyros with the curvaceous, tapered and bonded "strap packs" on professionally-designed helo rotorblades. The latter are configured to avoid the stress concentrations that simply are inherent in lap joints held together with big bolts through drilled holes.