Application of UNSM (ultrasonic nanocrystal surface modification) technology for increasing fatigue strength of Ti-6Al-4V ELI spine rod and for decreasing 15% of the spine rod diameter

The spine rod in the spinal fixation system should be tough enough to support the dynamic force and flexible also for flexible movement. 5.5 mm ~ 6 mm diameter spine rods made of Ti-6Al-4V ELI are common products. Two possibilities are studied. The one is that while keeping the flexibility unchanged (using 6mm diameter), how much the fatigue strength can be increased. The other is while the flexibility increased more than two times, how much the diameter of spine rod can be reduced. UNSM technology has been applied to increase the fatigue strength of Ti-6Al-4V ELI. The surface roughness, hardness and compressive residual stress are compared before and after UNSM treatment. 6 mm and 5 mm gauge diameter of rotary bending test specimens are made. S-N curves of rotary bending test results are compared before and after UNSM and analyzed with their fracture observation.