Enrica Riva

Contact fatigue failure analysis of shot-peened gears

Abstract The paper includes the results of experimental investigations and numerical analysis performed with the aim of studying contact fatigue damage in carburized and shot-peened gearwheels used in gearpumps. On the basis of SEM observations, a model for the evolution of the damage was developed, supposing that the residual stresses induced by shot-peening are mainly effective in stopping microcrack propagation but not in preventing microcrack initiation. In the light of this hypothesis, and following experimental measurements of residual stress, an approach is proposed to evaluate the effect of shot-peening parameters on the performance. The development of this approach required the execution of a numerical analysis of the engagement cycle taking into account the presence of residual stress and microcracks: the results allow one to determine, at least in comparative terms, the parameters that give the best strength improvement against contact fatigue.

Mechanical behaviour prediction in plain weave composites

The correct application of woven fabric composites requires an accurate knowledge of their mechanical behaviour, in terms of both global stiffness and strength. The aim of this paper is the development of a finite element (FE) approach to the description of the mechanical response starting from the actual mesostructure of the material. Experimental observations identified the unit cell on the basis of symmetry considerations. Different stacking configurations of the unit cell were considered. A global-local procedure was used to reduce the computational effort without losing computational accuracy. A damage model of the material under tensile load was defined and implemented in the FE code. A comparison with experimental stress-strain curves validates the approach proposed.

High Temperature Fatigue Strength and Quantitative Metallography of an Eutectic Al-Si Alloy for Piston Application

Eutectic Al-Si alloys are typically used for the production of internal combustion engine pistons. A high-cycle, high-temperature fatigue characterization of AlSi12 alloy performed using specimens extracted from actual pistons is presented and discussed. Fatigue strength at 107 cycles were obtained at test temperatures of 250 °C, 300 °C and 350 °C. The fatigue strength reduction was quantified. The micro structural features were quantified by quantitative metallography and fatigue fracture surfaces inspected to identify the initiation causes.

Comparison of analytical and multibody dynamic approaches in the study of a V6 engine piston

Two methods for the modelling and analysis of the piston/cylinder interaction are compared in this paper with reference to a V6 turbocharged gasoline engine piston. The first method is a traditional analytical method based on the equation of motion derived for the axial movement of the piston, written according to the kinematics of the crank mechanism. The second method is based on a multibody analysis, performed with the software AVL Excite™ Power Unit after a modal reduction of the system. The lateral force between the piston and the cylinder calculated using the analytical method is compared to the one calculated with a simplified multibody model. The results are well correlated, but in order to identify the piston secondary motion, an advanced multibody model is necessary since the consideration of the hydrodynamic effects, the flexibility of the bodies and the real profiles of the contacting surfaces is crucial. With regard to the latter aspect, a finite element model of the system under thermal boundary conditions has been implemented to evaluate the radial deformation and proper constraints have been considered to take into account the engine block stiffness. The obtained results are important because they have an impact on the radial clearance between the piston and the liner. When the multibody model takes into account these advanced effects, the piston slap events are clearly identified after the top dead centre and corresponding secondary peaks in the lateral force acting on the thrust side are calculated. At the end, a sensitivity analysis to the change of the piston-pin offset is also performed. Apart from the expected result of a lower lateral force peak when the offset is taken towards the anti-thrust side, some interesting differences are detected about the slapping behaviour. These events are clearly recognisable only if the offset is taken towards the thrust side because of the unfavourable piston rotation just prior to their occurrences. The pronounced anti-clockwise rotation of the piston prior to the lateral force peak occurrence in case of anti-thrust side offset, on the other hand, does not allow for a proper piston slap, even if the lateral acceleration is higher due to the centre of mass position with respect to the piston pin axis.

Influence of Microstructure and Defect Population on the Fatigue Performance of Cast A356-T6 Automotive Components

In the paper fatigue specimens are extracted from different regions of cast aluminum cylinder heads produced by two foundries. A high strength region and a low strength region were identified within the cylinder head and the A356-T6 material locally characterized in terms of microstructure and defect population. High cycle fatigue testing according to a reduced staircase method was performed to determine the local fatigue strength at 107 cycles in the cylinder heads of the two foundries. The implications of the experimental observations are discussed.