ABSTRACT
Alumina-based ceramic hip endoprosthesis heads have excellent tribological properties, such as low wear rates. However, stress peaks can occur at the point of contact with the prosthesis stem, increasing the probability of fracture. This risk should be minimized, especially for younger and active patients. Metal elevations at the stem taper after revision surgery without removal of a well-fixed stem are also known to increase the risk of fracture. A solution that also eliminates the need for an adapter sleeve could be a fixed titanium insert in the ceramic ball head, which would be suitable as a damping element to reduce the occurrence of stress peaks. A viable method for producing such a permanent titanium-ceramic joint is brazing. Therefore, a brazing method was developed for coaxial samples, and two modifications were made to the ceramic surface to braze a joint that could withstand high cyclic loading. This cyclic loading was applied in multiple amplitude tests in a self-developed test setup, followed by fractographic studies. Computed tomography and microstructural analyses-such as energy dispersive X-ray spectroscopy-were also used to characterize the process-structure-property relationships. It was found that the cyclic loading capacity can be significantly increased by modification of the surface structure of the ceramic.
ABSTRACT
Disc springs are conical annular discs, which are characterized by a high spring force with a small spring travel and good space utilization. In operation, they must meet high demands on the stability of the spring characteristic and the fatigue strength. Under loading, tensile stresses occur which limit the possible applications of disc springs. Compressive stresses can be generated in the stressed areas by means of shot-peening in order to extend the operating limits for a given yield and fatigue strength. Since the spring geometry and characteristics change during shot-peening, the design of the shot-peening treatment is iterative and cumbersome. The present research proposes an incremental forming process for forming and integrated targeted adjustment of residual stresses in disc springs from metastable austenitic stainless steel (MASS), to achieve improved spring properties and high cyclic strength. The main mechanism of residual stress generation is the transformation of metastable austenite into martensite under the action of the forming tool. Different experimental characterization techniques like the hole drilling method, X-ray diffraction, disc compression tests, optical microscopy and cyclic tests are used to correlate the residual stresses and disc spring properties. A numerical model is developed for simulating the martensite transformation in disc springs manufacturing. The results prove that incremental forming enables process-integrated engineering of the desired compressive residual stresses, entailing a higher spring force of metastable austenitic disc springs in comparison to conventional disc springs. Due to martensite formation, the generated residual stresses are stable under cyclic loading, which is not the case for conventionally manufactured springs.
ABSTRACT
As brazed stainless steel components in service often have to withstand cyclic loads in corrosive environments, the corrosion fatigue properties of brazed joints have to be characterised. Application-relevant corrosion fatigue tests in corrosive media are extremely rare for brazed joints and cyclic deformation curves are barely investigated. In this study, fatigue tests of brazed AISI 304L/BAu-4 joints were performed in air and synthetic exhaust gas condensate K2.2 according to VDA 230-214. The fatigue behaviour of the brazed joints was compared to properties of the austenitic base material. Strain, electrical, magnetic, temperature and electrochemical measurement techniques were applied within fatigue and corrosion fatigue tests to characterise the cyclic deformation and damage behaviour of the brazed joints. It was found that the fatigue strength of 397 MPa at 2 × 106 cycles was reduced down to 51% due to the superimposed corrosive loading. Divergent microstructure-related damage mechanisms were identified for corrosion fatigue loadings and fatigue loadings of specimens in the as-received and pre-corroded conditions. The investigations demonstrate the important role of corrosive environments for the mechanical performance of brazed stainless steel joints.
