Residual stresses influence on the fatigue strength of structural components

Several production processes, both conventional and innovative, may result in residual stresses arising in critical areas of a component. The main issues include high distortion, reduced fatigue life, fracturing or delamination. In this context, standard fatigue design codes traditionally consider residual stresses through conservative assumptions, leading to either sub-optimal design or unexpected failures. Recently, innovative computational techniques have been developed to address residual stresses in a more comprehensive way. As a result, a more effective material utilisation and a more accurate fatigue life assessment can be achieved. The present work examines the influence of residual stresses on the fatigue endurance of S355JR structural steel components. Both welded and notched components were analysed, carrying out numerical and experimental analyses. In the case of welded components, residual stresses resulting from the welding process were numerically evaluated by means of an uncoupled thermal-structural simulation, while for notched specimens a preload causing limited yielding was used to induce a local residual stress field comparable to that obtained for welded specimens nearby the critical locations. Even if he work is still in progress, tests carried out with different specimens under different loading conditions allowed to understand the effect of residual stresses on the fatigue life

Effects of coating on the fatigue endurance of FDM lattice structures

Additive Manufacturing techniques, such as Fused Deposition Modeling (FDM), are widely used to produce lattice structures with complex unit cell geometries. These structures can be designed to meet specific requirements in a wide range of application fields, ranging from biomedical to mechanical sectors. The mechanical behavior of these structures is often impaired by a low surface quality. However, the mechanical strength of polymer lattice structures can be significantly improved with the use of post-processing treatments. Coating post-processing is one of the treatments that showed the best results. Nevertheless, research interests are often targeted at studying the static mechanical properties rather than the fatigue behavior of polymer components. In this work, the effect of a polymeric coating on the fatigue life of Polylactic acid (PLA) lattice structures, produced by FDM, was investigated. Specimens have been designed to enable the application of both tensile and compressive loads. Preliminary tensile tests were carried out to assess the static strength of the specimen before the fatigue tests. Experimental fatigue tests were performed with varying testing frequencies and displacements. The results evidenced differences in the behavior of coated and non-coated components when subjected to different testing frequencies and loading conditions. The polymeric coating produced an increase in fatigue endurance across different testing frequencies over a particular displacement range

Rapid and accurate fatigue assessment by an efficient critical plane algorithm: application to a FSAE car rear upright

The topic of material fatigue is widely discussed and researched in both scientific and industrial communities. Fatigue damage remains a significant issue for both metallic and non-metallic components, leading to unforeseen failures of in-service parts. Critical plane methods are particularly recommended in case of multiaxial fatigue assessment and have gained relevance as they allow for the identification of the component's critical location and early crack propagation. However, the standard method for calculating critical plane factors is time-consuming, utilizing nested for/end loops and, for that, is mainly applied in a research context, or when critical regions are already known. In many cases, the critical area of a component cannot be identified due to complex geometries and loads or time constraints. This becomes particularly relevant after topological optimization of components and, more generally, in lightweight design. An efficient algorithm for critical plane factors evaluation have been recently proposed by the authors. The algorithm applies to all critical plane factors that require the maximization of a specific parameter based on stress and strain components or a combination of them. The methodology is based on tensor invariants and coordinates transformation law. This paper presents and validate the proposed methodology through an automotive case study: the new algorithm was tested on a rear upright of a FSAE car, having complex geometry, subjected to non-proportional loading conditions. The efficient algorithm showed a significant reduction in computation time compared to the (blind search-for) standard plane scanning method, without any loss in solution accuracy

Frequency analysis of random fatigue: setup for an experimental study

The frequency-domain approach to fatigue life estimation in random loading has been largely investigated due to its computational advantages, and several methods for the frequency translation of the most common time-domain methods have been proposed. Between the most known frequency methods there are the Bendat's Method, valid for narrow-band signals, and the Dirlik's formula, which is considered the best result for wide-band signals. However, the great part of the frequency methods takes the rainflow count as a reference time-domain method and uses the rainflow damage computation as the exact value to emulate. Therefore, very few experimental data for fatigue life of mechanical components subject to random loads are available in the literature. This work presents the setup for a series of experimental tests for specimens subjected to random loads, aiming at achieving experimental data to compare with the results provided by frequency methods. After a brief description of the materials used for the setup, the two-step test concept is described: firstly, the specimen will be subjected to random loads obtained by a certain PSD for an amount of time which should nominally cause a 30% of damage; then, the fatigue test will be ended on a resonance testing machine to compute the actual residual fatigue life of the specimen; this two-step testing also allows to reduce the time requested for the tests. The test bench developed for the experimental investigation is described in the paper, together with the results of some preliminary tests, aimed at verifying the feasibility of the conceived procedure

A magnetorheological clutch for efficient automotive auxiliary device actuation

In this paper the results of a project funded by Regione Toscana aimed at reducing the powerabsorption of auxiliary devices in vehicles are presented. In particular the design, testing and application of amagnetorheological clutch (MR) is proposed, aimed at disengaging the vacuum pump, which draws in air fromthe power-brake booster chamber, in order to reduce the device power absorption.Several clutch preliminary studies done to choose the clutch geometry and the magnetic field supply areillustrated. The final choice consisted in an MR clutch with permanent magnet, which satisfied size, torque andfail-safe specifications. The clutch characteristics, in terms of torque versus slip, were obtained experimentallyfor three different clutch prototypes on an ad-hoc developed test bench.As result of a preliminary simulation, a comparison between the power absorption of a current productionvacuum pump, an innovative vacuum pump and both vacuum pumps coupled with the MR clutch is presented.The New European Driving Cycle is considered for simulating the vacuum pump operation both in urban andhighway driving. Results show that the use of the innovative vacuum pump reduces the device consumption ofabout 35%, whereas the use of MR clutch coupled with the innovative vacuum pump reduces it up to about44% in urban driving and 50% in highway driving

Numerical-experimental characterization of the dynamic behavior of PCB for the fatigue analysis of PCBa

In today's highly digitized and mechatronics-based world, the need for reliable and cost-effective electronic components has become essential. The reliability of these components is not only based on their electrical and circuit aspects but also on their structural properties. This paper presents a study carried out on two-layer Printed Circuit Boards (PCBs) of rectangular shape, which are representative of many industrial applications. The aim of this study is to compare different numerical models, developed in Ansys Workbench and in a FEM software specifically designed for circuit boards, with experimental tests to determine the most interesting ones for further studies on Printed Circuit Board Assemblies (PCBAs). The comparison includes both static and dynamic behaviors, tested through isostatic bending tests and dynamic analyses with a shaker and a fiber optic laser. The models developed are capable of reproducing statics and dynamics of PCBs with varying degrees of accuracy and numerical complexity. However, increasing the details of the models does not always correspond to an increase in accuracy in reproducing the dynamic behavior. Prior to the experimental dynamic analysis, the influence of constraints’ modeling strategies and damping on the first eigenmode was studied, and the results were used to set up tests and simulations to achieve more consistent results. Future work will extend the dynamic characterization to PCBAs by populating the studied PCBs with components, and continue with the study of predictive models for their structural reliability

On the use of shape memory alloys for deployable passive heat radiators in space satellites

The present work presents a multifunctional structure for space engineering application part of the TOPDESS project, funded by ESA. The main aim of the project is the design of a thermal control device able to deploy through passive actuation. A combined device has been designed, made up of a Pulsating Heat Pipe (PHP) foldable heat exchanger and Shape Memory Alloy (SMA) wire. The deployment of the SMA wire is conceived to be controlled by thermal contact with the heat source and by conduction along the wire. Since the heat sources are lumped and the wire is subject to convection, a temperature gradient develops along the wire. A monodimensional mode able to predict the behavior of an SMA wire subjected to a spatial temperature gradient, is presented in this paper. The results show that the system can carry out folding and unfolding cycles with rotation angles greater than 80° only if the wire is subjected to uniform temperature distribution; in the case of temperature gradient, the achievable rotation angle is about 20°. The analysis states the feasibility of the actuation system, highlighting the critical technological aspects, to lay the groundwork for the future development of the whole system