J. Albizuri

On the Fatigue Strength of Ball Burnished Mechanical Elements

This work presents the quantification of the improvement in the high-cycle fatigue strength of machine parts manufactured with medium-carbon steels by means of low-plasticity burnishing. A complete experimental study has been performed on specimens: surface integrity, in-depth residual stresses, residual stresses relaxation, alternating bending fatigue tests and fractographic analysis. The tests show that the fatigue strength of ball-burnished components is significantly improved in the range 104 to 106 cycles, and the fatigue limit is increased by 20%. The residual stresses in the surface layer are non-isotropic biaxial compressive stresses varying in time because of stress relaxation. The quantitative data here provided are the modifying coefficient for the Marin equation and a uniaxial effective mean stress for each stress level. For fatigue calculations, this effective constant mean stress is equivalent to the effects produced by ball burnishing, and is an alternative to the calculations based on modifying coefficients.

Influence of vertical misfit in screw fatigue behavior in dental implants: A three-dimensional finite element approach

Misfit is unavoidable in dental implant-supported prostheses due to machining process or inappropriate assembling, and the definition of an admissible misfit is still a controversial issue. This work aims to understand the behavior of the screws in dental implant-supported prostheses to estimate an admissible vertical misfit value in terms of screw fatigue failure. For that purpose, a finite element model of a dental implant-supported prosthesis was created and analyzed. Vertical misfits were introduced in different positions, the lower and upper screws were tightened to the bolting force values recommended by the manufacturer, and two different occlusal loads were analyzed. In addition, two different prosthesis materials were studied. Screw load variations were reported and a fatigue analysis was performed. As a result, it was observed that the screw tightening sequence closed small vertical misfits (equal to or less than 40 µm), whereas larger misfits (more than 40 µm) remained open. If the vertical misfit is closed by the end of the tightening sequence, it may be considered equivalent to the ideal fit situation in regard to screw fatigue failure. The prosthesis material had no significant influence on the fatigue behavior.

Eigendesign in brake carriers

This work presents the application of a first-order perturbation method to evaluate the eigensensitivity of brake carriers when local geometric alterations arise. This method can be applied both to establish dynamic acceptance conditions of manufacturing-related errors (concept of ‘eigenacceptance’) and to assess the variation of eigenfrequencies and eigenmodes when design variations are done (concept of ‘eigendesign’). As an application of the first concept, the adequacy of the proposed method has been tested in the eigensensitivity of the rest of ingate caused by the manufacturing process; and with regard to the eigendesign concept, an easy-to-use methodology for the redesign of brake carriers has been developed and correlated.

Design of an active control of vibration in a centreless grinding machine: theoretical study and experimental implementation

The appearance of vibrations during centreless grinding process is one of the main factors limiting the current requirements of precision and productivity. This paper presents a novel procedure for the design of an active vibration reduction system in a centreless grinding machine using piezoelectric actuators. The different phases involved in the modelling procedure of the machine tool are explained, together with the followed approach to locate the actuators. Taking as reference the finite element model of the machine, the most important mode shapes affecting the chatter behaviour have been considered to obtain a low order state space model. This reduced model has been used to simulate the effectiveness of the control strategy and the results have been compared with those obtained experimentally after the system implementation. A good agreement has been obtained demonstrating that the modelling procedure followed is useful to predict the effectiveness of control systems.

Burnishing of rotatory parts to improve surface quality

In this paper, the use of rolling burnishing process to improve the final quality of railway and automotive workpieces is studied. The results are focused on the improvement of the manufacturing processes of rotary workpieces used in railway and automotion industry, attending to generic target of achieving ‘maximum surface quality with minimal process time’. Burnishing is a finishing operation in which plastic deformation of surface irregularities occurs by applying pressure through a very hard element, a roller or a ceramic ball. This process gives additional advantages to the workpiece such as good surface roughness, increased hardness and high compressive residual stresses. The effect of the initial turning conditions on the final burnishing operation has also been studied. The results show that feeds used in the initial rough turning have little influence in the surface finish of the burnished workpieces. So, the process times of the combined turning and burnishing processes can be reduced, optimizing...