Fatigue damage identification by means of modal parameters

Abstract Object of the present paper is an experimental investigation about high cycles fatigue damage and corresponding variation of modal parameters (internal damping and resonance frequency) in steel specimens. On the basis of a modal approach, a mixed technique has been developed in order to relate fatigue damage and intrinsic properties of the material. The experimental procedure consists of the modal parameters assessment on steel specimens, once before fatigue test beginning, and thus repeatedafter every 105 cycles, both with constant and increasing loading levels. Infrared thermography has been used to emphasize the specimen thermal emission and the related damage phenomena

Analysis of a load application point in spline coupling teeth

The objective of this paper is to investigate the position of the resultant force in involute spline coupling teeth due to the contact pressure distribution for both ideal and misaligned conditions. In general, spline coupling teeth are in contact all along the involute profile and the load is far from uniform along the contact line. Theoretical models available in publications consider the resultant contact force as it is applied at the pitch diameter, and this study aims to evaluate the error introduced within the confines of a common approximation environment. This analysis is carried out through using finite element method (FEM) models, considering spline couplings in both ideal and misaligned conditions. Results show that the differences between the load application diameter and pitch diameter are not very obvious in both ideal and misaligned conditions; however, this approximation becomes more important for the calculation of the tooth stiffness

Identification of contact regimes in mechanical components for the evaluation of fretting damage

In this work the regimes of contact in misaligned crowned splined couplings have been analyzed. Experimental tests have been performed in order to identify if fretting damage on the components appears as fretting wear or fretting fatigue. A significant difference was identified on the surface of specimen by analyzing two different tests; the first test emphasized a fatigue damage and in the second test a wear phenomena has been achieved. Also a good correlation has been obtained by analyzing the fretting map obtained by using the Mindlin's theory and experimental result

Fretting damage parameters in splined couplings

This work focuses on the analysis of the debris found in the lubrication oil produced by the wear abrasion during wear tests conducted on crowned splined couplings. During each test the presence and the dimensions of the debris in the oil have been monitored. Tests have been performed by means of a dedicated splined couplings test rig and they have been performed by imposing an angular misalignment on the axes of the components. Results shows that when these components work in misaligned conditions, the relative motion between engaging teeth brings to the rise of a wear phenomenon called fretting wea

Investigation about crack propagation paths in thin rim gears

Crack propagation in gears is a problem related not only to the life of the components, but also to the concept of failsafe design. Fail safe design means to design a component in order that, if a failure occurs, this may cause a "safe failure". This aspect is very important above all in aerospace industry. As a matter of fact, in aerospace application, the need of reducing weight brings to produce gears with very thick rim and web. Considering thin rim gears, when a crack is nucleated near the tooth root, it may propagate through the tooth (causing the loss of the entire tooth or a portion of it) or the propagation may follow a path across the wheel diameter (causing the projection of big parts of the gear that may break the gearbox and may cause serious damage to the aircraft). The first failure mode is define as "failsafe failure" and the second one as "catastrophic failure" and of course has to be avoided. Designers need to have robust design criteria in order to predict crack propagation paths and to avoid catastrophic failures. In literature, few works are present concerning this topic, in particular related to the effect of geometrical parameters that may affect the crack propagation. In this work a numerical analysis about crack propagation in gears with respect to the backup ratio (ratio between tooth height and rim thickness), initial crack position and shape has been done by means of the Extended FEM (XFEM) technique, realizing 3D models. XFEM 3D is a relatively new technique consisting in enriching traditional finite elements with more complex shape functions; in this way it is possible to propagate crack also between mesh nodes and to have mesh independent results. Aim of this paper is to highlight the crack propagation path in order to give to designers an high confident design criterion, related to the gear geometry. In particular, the effect of both rim thickness and orientation of the initial crack have been considered in order to enrich the literature knowledge. Numerical results obtained in this work have been compared with those found in the literature, showing a very good correlation

Experimental investigation on crack propagation paths in spur gears

Spur gears subjected to bending fatigue may nucleate cracks at the tooth root fillet. In thin rim gears these cracks may propagate in a safe way (through the tooth) or in catastrophic way (through the rim). Crack propagation direction is mainly influenced by both wheel geometry parameters and crack initiation point, as already pointed out by theoretical and numerical results available in literature. Aim of this work is to set up an experimental activity in order to verify the onset of the bending crack and its propagation path in spur gears with different geometries. In particular, a special device connected to a standard fatigue machine was realized to perform bending tests for both standard and thin rim gears. During bending tests, an IR thermocamera was utilized to monitor the surface thermal profile in the tooth root fillet zone

Dynamic additional loads influencing the fatigue life of gears in an electric vehicle transmission

In recent years the implementation of the electric engine in the automotive industries has been increasingly marked. The speed of the electric motors is much higher than the combustion engine ones, bringing transmission gears to be subjected to high dynamic loads. For this reason the dynamic effects on fatigue life of these components have be taken into account in a more careful way respect to what is done with the usual gears. In the present work the overload effects due to both speed and meshing in a gear couple of an electric vehicle transmission have been analyzed. The electric vehicle is designed for urban people mobility and presents all the requirements to be certified as M1 vehicle (a weight less than 600 kg and a maximum speed more than 90 Km/h). To investigate the overload effects of teeth in contact, the reference gear design Standards (ISO 6336) introduce a specific multiplicative factor to the applied load called Internal Dynamic Factor (K v ). Aim of this work is to evaluate how dynamic overloads may influence the fatigue life of the above quoted gears in term of durability. To this goal, Kv values have been calculated by means of the analytical equations (ISO 6336 Methods B and C) and then they have been compared with the results coming from multibody simulations, involving full rigid and rigid-flexible models

recent advances in spline couplings reliability

Abstract Spline couplings are mechanical components widely used to transmit torque between rotating parts. Although they are well known components, critical issues such as wear damage and un haven loads affect them especially when working in misaligned conditions. Wear damage appears on engaging teeth bringing to component failures. The main way to reduce wear damage is to lubricate the engaging teeth or to apply surface coatings. In this work wear damage on spline coupling was investigated and a compound made of grease added with graphene has been tested against standard lubricants in order to find out if it may improve the wear strength of these components. Experimental tests were performed by means of both standard testing machine provided by a particular testing device and dedicated test rig, designed to perform tests with misaligned spline couplings. Preliminary results show that the presence of graphene improves the grease performance, reducing the coefficient of friction (bringing to a reduction of uneven overloads and wear)

Effect of centrifugal load on crack path in thin-rimmed and webbed gears

Thin rimmed and webbed gears are used in particular applications to reduce systems weight. This kind of gears need an accurate and fail safe design. As a matter of fact, a possible failure, due to bending fatigue, consists in crack nucleation and consequent growth, in particular in the tooth root zone. These cracks may propagate through the tooth or through the rim. Crack propagation direction is basically influenced by the wheel geometry parameters, above all the rim thickness. Studies available in literature emphasize three ranges for the backup ratio values, involving different behaviors. These ranges are related to the crack propagation paths; respectively through the tooth, through the rim and in an unforeseeable way. This last uncertainty zone depends on other parameters, related to both geometry and loading conditions. In this work the effect of wheelspeed related to the bending load has been investigated. The investigation has been carried out by means ofnumerical models involving both 2D finite element and extended finite element models (XFEM). Results showsthat both crack initiation point and crack propagation path are strongly influenced by centrifugal load; thiseffect is mainly evident in the uncertainty zone of the backup ratio

Aging characterization of metals for exhaust systems

The mechanical characteristics of four materials used in automotive exhaust systems have been compared after an aging treatment to evaluate the combined effects of thermo-mechanical fatigue and corrosion. For this purpose, an experimental aging procedure has been developed. This procedure is composed of chemical, thermal and mechanical cycles, which are combined and repeated to simulate the actual operating conditions of automotive exhaust systems. Three austenitic steels (AISI 309, AISI 316Ti, and AISI 321) and a nickel-based alloy (Inconel 625) are tested. The results show that Inconel 625 and AISI 309 are less affected by the aging process than the other material