G. Demelio

A review of analytical aspects of fretting fatigue, with extension to damage parameters, and application to dovetail joints

Abstract Recent advances by the authors in analytical methods for the analysis of plane fretting fatigue (FF) contact problems are described, and new consequences for FF damage are derived. Constant normal load and oscillating tangential load (the celebrated Cattaneo–Mindlin case) are considered with in-phase oscillating moderate bulk stresses, for an arbitrary spline rotated geometry and, in particular, the flat punch with rounded corners in view of application to the dovetail joints. Extremely simple, new results are found for initiation parameters such as tangential microslip and frictional energy, which have been used under certain conditions as threshold parameters for FF. Finally, it is shown that for an “almost flat” geometry, the surface damage parameters decrease, but the tensile stress concentration increases, although it becomes more localized, suggesting that for cracks eventually initiated, the likelihood of self-arrest is higher than in the equivalent Hertzian case with same loads. This seems to interpret recent experiments, although it is not clear whether the optimal geometry in terms of FF life is the perfectly flat one, or an intermediate one.

An experimental investigation of static and fatigue behaviour of sandwich composite panels joined by fasteners

Abstract An experimental investigation has been carried out to estimate the static and fatigue behaviour of specimens made up of steel plates and sandwich composite panels joined together by either blind or mechanical lock fasteners. A preliminary study was carried out in order to analyse the drilling operation of sandwich panels to determine the best values of parameters to use for fastener installation. A first set of pull-out and shear static tests was performed in 1992, using sandwich panels composed of a nomex honeycomb core between two laminates of glass/graphite/kevlar fibres in epoxy matrix. The investigation was completed in 1998. It consisted of performing a set of pull-out and shear fatigue tests on joints with blind fasteners, and of performing a new set of static tests on identical specimens with the same loading conditions as in 1992 so as to evaluate the possible ageing effect on mechanical proprieties of sandwich panels tested.

Adhesion of Elastic Thin Films: Double Peeling of Tapes Versus Axisymmetric Peeling of Membranes

The mechanism of detachment of thin films from a flat smooth rigid substrate is investigated. In particular, analytical solutions in closed form are proposed for the double peeling of an elastic tape as well as for the axisymmetric peeling of a membrane. We show that in the case of double peeling of an endless elastic tape, a critical value of the pull-off force is found, above which the tape is completely detached from the substrate. In particular, as the detachment process advances, the peeling angle is stabilized on a limiting value, which only depends on the geometry of the tape, its elastic modulus and on the interfacial energy

Transient analysis of frictionally excited thermoelastic instability in multi-disk clutches and brakes

\Updelta\gamma

On non-symmetrical plane contacts

Δγ. This predicted behavior agrees with the “theory of multiple peeling” and clarifies some aspects of this theory. Moreover, it is also corroborated by experimental results (work in progress) we are carrying out on a standard adhesive tape adhered to a smooth flat poly(methyl methacrylate) surface. In the case of the axisymmetric adhering membrane, a different behavior is observed. In such case, the system is always stable, and the detached area monotonically increases with the peeling force, i.e., the elastic membrane can sustain in principle any applied force. Results are validated by a fully numerical analysis performed with the aid of a finite element commercial software.

The effect of material properties on the thermoelastic stability of sliding systems

Abstract Presented in this paper is a set of modern tools for the design of gearing: kinematic optimisation (minimisation and balancing of specific sliding), static stress analysis (to minimise stress concentrations) and crack propagation studies (to estimate fatigue life under a pre-existing defect). All three aspects are integrated in a software package developed by the authors. In particular, Boundary Element (BE) and Finite Element (FE) grids are automatically generated corresponding to gears manufactured by means of user defined tools with known shape and cutting parameters. BE models are used for a complete and automatic subcritical propagation analysis of cracks. FE models are used mostly for cases without crack propagation but requiring a greater versatility. Tests conducted on cases found in the literature demonstrate the accuracy of the methods used and the effects of rack shift factors and of rim thickness are studied in example cases. It is found that the fatigue life depends significantly on the cracking path mode, which in turn is particularly sensitive to the rim thickness in gears manufactured on thin hollow shafts as are typical in aeronautical applications. Further, the rack shift factors significantly change the stress concentrations (and therefore the maximum torque transmittable, in general in a beneficial manner). However, for designs with same concentration factor the fatigue life is considerably different, and in particular is lower on gears with a low number of teeth. This clearly indicates that the use of a complete crack propagation analysis from the early stages of the design process is highly recommended.

Adhesive contact of the Weierstrass profile

A 2D multilayered model has been considered to estimate the transient evolution of temperature and pressure perturbations in multi-disk clutches and brakes during operation. The model proposed by Decuzzi et al. [1] has been modified here to estimate the variation of b–perturbation growth rate—with V—relative sliding speed. It has been verified that the perturbation with the lowest critical speed has also the highest growth rate, and that low frequency perturbations are less critical than high frequency perturbations, at fixed critical speed. Therefore, when comparing perturbations with identical critical speed, those with higher wave numbers are responsible for more intense thermomechanical damages. Also, for perturbations with wave number smaller than the critical mcr, the temperature increases with m; vice versa for perturbations with wave number larger than mcr the temperature decreases with m. A reduction in thickness ratio a1/a2 between friction and metal disks has the effect of increasing the temperature and growth rate. An approximate formula for the temperature variation with time has been derived for a linearly decreasing engagement speed.

The sliding contact of a rigid wavy surface with a viscoelastic half-space

This paper provides a detailed analysis of the dynamic response of a resistive microswitch. The analysis has been carried out by modeling the microswitch as a cantilever beam, according to the Euler-Bernoulli theory, and considering the damping interaction of the moving beam with the surrounding fluid. Attention has been given to the bouncing of the beam tip on the substrate upon actuation. A general adhesive-repulsive force has been applied at the tip of the beam to model its interaction with the substrate, where the attractive contribution is described by a van der Waals-like term and the repulsive contribution by a classical linear elastic springlike term. The resulting problem has been solved using a second-order-accurate finite difference scheme. It is shown that by tuning the adhesive interaction at the tip/substrate interface the number and amplitude of the bounces can be significantly reduced in favor of the system reliability and performance. Also design maps have been proposed to estimate the ac...