Turbine D - The crack in the Ti spar was detected before it became a serious problem, and the aircraft continued to be flown after installing strain gauges to monitor propagation of the fracture. This tells me that they are doing a thorough job with the test program.
The aluminum die forgings used for the fuselage frames also had some fracture issues. Fixing those problems required some expensive changes to the massive die sets used to produce the aluminum forgings. However, I imagine the Ti wing spars are not produced from die forgings since the cost would be prohibitive. They are likely machined from shapes hammer or roll forged from wrought stock. Thus adding thickness in most areas of the spar to resolve the cracking issue would only require modification of the CNC machining program.
Titanium is very sensitive to surface flaws, which can result in fractures propagating from these defects. So the surface of a machined titanium part subject to high stress or fatigue conditions must be smooth and/or mechanically pre-stressed in compression by shot peening.
Titanium also tends to work harden when machined. So Ti parts that are subject to significant machining and stock removal require periodic stress relief treatments to produce a dimensionally stable finished component.