RESUMO
Experimental analyses of depth distributions of phase-specific residual stresses after deep rolling were carried out by means of laboratory X-ray diffraction and neutron diffraction for the two duplex steels X2CrNiMoN22-5-3 and X3CrNiMoN27-5-2, which differ significantly in their ferrite to austenite ratios. The aim of the investigation was to elucidate to which extent comparable results can be achieved with the destructive and the non-destructive approach and how the process induced phase-specific micro residual stresses influence the determination of the phase- and {hkl}-specific reference value d0, required for evaluation of neutron strain scanning experiments. A further focus of the work was the applicability of correction approaches that were developed originally for single-phase materials for accounting for spurious strains during through surface neutron scanning experiments on coarse two-phase materials. The depth distributions of macro residual stresses were separated from the phase-specific micro residual stresses. In this regard, complementary residual stress analysis was carried out by means of incremental hole drilling. The results indicate that meaningful macro residual stress depth distributions can be determined non-destructively by means of neutron diffraction for depths starting at about 150-200 µm. Furthermore, it was shown that the correction of the instrumental surface effects, which are intrinsic for surface neutron strain scanning, through neutron ray-tracing simulation is applicable to multiphase materials and yields reliable results. However, phase-specific micro residual stresses determined by means of neutron diffraction show significant deviations to data determined by means of lab X-ray stress analysis according to the well-known sin2ψ-method.
RESUMO
BACKGROUND: The risk of femoral stem fracture after total hip replacement is low and can often be associated with a specific implant system or other factors that may reduce the fatigue strength. Additionally, damage to a metal component during revision surgery by an electrocautery device may further affect the fatigue behavior. METHODS: Two clinical cases of stem failure after revision of fractured ceramic components are presented; the retrieved components were analyzed for the cause of failure. In vitro cyclic load-to-failure testing of titanium alloy femoral stems after electrocautery application at 2 different locations (at the base and about midway on the femoral neck) was performed using a stepwise increase in load until implant fracture occurred. In addition, a detailed characterization of the local material structure around the electrocautery marks was performed. RESULTS: Superficial discoloration and melting marks were found on the retrieved components, including at the location of crack initiation in the anterolateral region, which may have reduced the fatigue strength of the material. In addition, elemental analysis indicated material transfer from the electrocautery tip. Damage to the surface by the electrocautery device significantly reduced the in vitro load to failure by up to 47% compared with that of undamaged femoral neck specimens. Material analysis revealed a relevant modification in microstructure, with an extension of approximately 2.7 mm and a depth of 550 µm, which could be divided in 3 structural zones. CONCLUSIONS: Intraoperative electrocautery device contact with the implant during surgical revision of a total hip replacement cannot always be avoided. However, on the basis of our findings, the risk of implant failure is increased due to a change in microstructure and a potential reduction of the implant's fatigue strength. Surgeons and manufacturers of electrocautery devices should be aware of this concern. CLINICAL RELEVANCE: During revision surgery, contact between an electrocautery device and the femoral component should be avoided to reduce the chance of subsequent femoral neck fracture.
