The fatigue properties of new potential metallic implant materials are one of the key factors for a successful introduction of new alloy generations in biomedical applications. For osteosynthesis in the special case of systemically weakened bones revision surgery has to be avoided. As a consequence, implants have to be designed in order to minimize “stress-shielding” while providing high fatigue strength. Regarding these requirements, the investigated binary ß-based Ti-40(wt%)Nb alloy is very promising due to its exceptionally low Young’s modulus of 60 GPa adjusted on the basis of a specific thermomechanical processing. Moreover, it shows excellent corrosion resistance without toxic ion release.
To evaluate the fatigue strength of the new material this work focuses on cyclic mechanical testing of miniature fatigue samples and real osteosynthesis plates (OSP) with a laser processed surface condition in tension-compression mode (R=-1). Reference tests with equal test parameters were executed on commercially pure titanium (ASTM Grade 4) and Ti-45(wt%)Nb. Results show that the newly developed alloy does not yet reach the fatigue strength of the commercially available alternative. Detailed analysis of the laser cut surfaces before and after cyclic loading as well as fractography with digital microscopy, SEM and FIB revealed crack initiation always on surface notches caused by the laser processing. Results of OSP show even further decreased fatigue strength due to an overlapping influence of geometrical notches and multi-axial-loading in the smallest cross sections. Future investigations have to reveal whether a reduction of the notch sensitivity and an optimized surface treatment for the new Ti-40Nb alloy will result in a pronounced increase of the cyclic strength.
This work was funded by the German Research Foundation (DFG) under SFB/Transregio 79 as part of sub-project M1.