Papers by Author: Piero Morelli

Abstract: The results of an experimental investigation on the shear strength of structural joints are presented and discussed. Joint typologies generally employed in structural frames of industrial warehouses and intermediate floors are taken into consideration. Specimens were supplied by an industrial shelving manufacturer, in two different configurations: the first one characterized by steel pressed geometrical connectors and the second one consisting in bolted fittings to angular welded supporting plates. A specific testing device has been designed in order to transfer axial loading into shear loading applied to a couple of joints in a symmetrical testing configuration. Quasi-static loads were applied with increasingly intensity steps, until the yielding of the material or the overall structure collapse were reached. Failure modes of the tested joints are analyzed and discussed.

Abstract: Telescopic cylinders are generally employed as linear actuators, when the desired moving span is several times the length of the closed device, like in cargo trucks lifting applications, for instance. During the active phase of the actuation, hydraulic power is normally used to feed pressu-rized fluid inside the cylinder, thus providing the progressive extension of the cylinder ele-ments and the required operative axial thrust. In this condition, therefore, cylinders must bear external compressive loadings in an increasingly slenderness configuration, which can give rise to buckling failures. In this study, experimental measurements of the limiting axial loadings of telescopic cylind-ers, in full extended conditions, have been performed both in laboratory and during real oper-ations on the field. The strains of the material in the critical sections and the lateral deflec-tions of the tested structures have been recorded as a function of the applied loads. The re-sults of this investigation are presented and discussed, in order to identify the signals of inci-pient buckling and find out the ultimate load carrying capabilities of this kind of components.

Abstract: This paper describes the experimental results of a set of fatigue tests performed on T300 carbon fiber / epoxy resin laminates cured in autoclave. The effects of anisotropy in laminates lay up have been investigated, as well as the influence of circular hole notches, for both the case of tensile and compressive loading conditions. Due to the high slenderness of specimens, that have the ASTM D3039 standard suggested geometry, a special gripping device has been designed in order to avoid the axial buckling. Fatigue failure probability of the material has been assessed by means of Maximum Likelihood Estimates (MLE) of the experimental data, as a function of the lay-up and load conditions. Eventually, the micrographic analysis of fractured surfaces outlined the combined effects of both the geometry and notches towards the damage behaviour of this kind of material.

Abstract: This work presents the results of a set of endurance tests performed on steel to steel shaft/bushing tribosystems, without re-lubrication after start-up. The experimental procedure was designed in order to provide simulation for the operative conditions of earthmoving machinery. For this purpose, a new testing machine was specifically manufactured so as to accommodate shaft/bushing assemblies up to a nominal coupling diameter of 90 mm, with a maximum radial load capability of 300 kN. The machine was operated in alternating rotation, with a span angle of 180 degrees, at an angular speed of 10 rpm. The effects of contact pressure intensity were investigated, as well as the effects of grease lubrication type and of the geometry of grease breeding grooves that were located on the bushings inner surface. The tribology behaviour of the tested assemblies was quantified by means of the transmitted torque, grease temperature at the surface of contact and the metallographic analysis of wear out surfaces.

Abstract: This paper presents the results of an experimental investigation on the failure behaviour of power screw linear actuators subjected to very high compressive loads. Quasi-static tests performed in laboratory have shown the presence of primary and secondary buckling failure modes. On the one hand the primary buckling is characterized by plane deflection of the inner screw, on the other hand the secondary buckling involves either spatial buckling, forcing the screw to assume a helical shape, or plane buckling of the external arm, in relation to the actual slenderness and the position of the actuator. Non linearities of the axial stiffness have been observed during the proportional phase of loading, as a consequence of the superposition of primary buckling and the lateral constraint effect opposed by the cylindrical case of the actuator to the bending deformation of the screw. Maximum deflections and longitudinal deformations have been measured as a function of the applied compressive load, whose axial and bending components have been calculated. A mathematical model of the elastic loss of stability has been developed, in order to calculate the critical load as a function of the actuator geometry.

Abstract: This work presents the results of an experimental investigation on the effects of thermal ageing over the residual fatigue strength of AA2618-T6511 aluminium alloy. Among others, this kind of light alloy finds practical applications in highly stressed engine components, such as pistons, that are typically subjected to both thermal and fatigue loads. Thermal cycles are responsible for ageing phenomena, that involve the precipitation of silicates, with a corresponding progressive damage of the microstructure and weakening of the mechanical characteristics of the material. Artificial ageing has been reproduced in laboratory by means of thermal cycles controlled in time and temperature. These variables have been correlated to the hardness values measured on the surface of specimens. Bending fatigue tests have been performed on a rotating machine in a temperature controlled environment. The experimental S-N diagram is finally presented, as a function of the tested temperature, in order to provide a design tool for the fatigue life estimation of AA2618 components.

Abstract: This paper presents the results of an experimental investigation on surface contact fatigue of AA6082 aluminium alloy. After testing, microscopy analysis of the specimen contact area shows plastic deformation at the centre and circumferential cracks at the very edge of the print. Major cracks develop at a certain depth under the border of the contact area and propagate beneath the surface, in the direction of both the centre of contact and the lateral free edge of the specimens. No cracks have been observed at the centre of contact, neither on the surface, nor inside the material. Tensile properties of the alloy have been measured and a non linear finite element analysis has been performed in order to calculate the field of deformation and stress in the contact zone. Finally, stress intensities are correlated with the crack initiation points and an interpretation of the propagation paths, in regard to stress distribution, is given.