Proposal of a stress-based isothermal LCF life model for Aluminium alloy cylinder heads

The paper presents and discusses the calibration procedure and the results related to a new easy-to-use low cycle fatigue (LCF) life prediction empirical stress-based model. The model was applied to a commercial Aluminium alloy diesel engine cylinder head. The material characterization was carried out on commercial cylinder heads made by primary AlSi9Cu1, investigating the mechanical properties on sets of specimens obtained from layers positioned at different distances from the gas face of the cylinder heads. The results of mechanical characterization and LCF model calibration parameters are presented for each layer. The material characterization was carried out at room temperature to assess the procedure and validate the model. The life assessment performance of the model was compared with the corresponding Basquin-Manson-Coffin model. The model prediction fitted the experimental data trend with a determination coefficient ranging from 0.75 to 0.98, which is globally higher with respect to the parameter fitting obtained with the Basquin-Manson-Coffin calibration. Furthermore, all life forecasts are close to the experimental results with a variance lower than 55%. A future development of the research work with further material characterization at different temperature will allow to validate and discuss the temperature dependence of the model parameters and to investigate its thermo-Mechanical Fatigue (TMF) life assessment performance

Loudness calculation procedure to study electronic steering column lock noise measurement

In the automotive field, the customer requirements for low interior noise and pleasant sound quality inside a vehicle are getting higher and higher. Various national and international regulatory authorities established and reviewed vehicle interior noise for the past years. Besides, lots of studies have shown that vehicle noise can influence the driver's perceptions and also his or her driving capabilities. To succeed in this scenario, all manufacturers are investing in technology and research in order to improve their component performance. Working on the noise source so as to reduce the seriousness of these noise problems can be really effective. However, various engineering techniques are available to deal with noise and sound-measuring instruments, and systems can help to identify the nature of the problem and they can also be helpful in determining the right procedure to analyse the noise problem. This work proposes a procedure to evaluate the noise originated from an electronic steering column lock device which is used to lock and unlock the steering wheel according to European safety requirements. In particular, different standards and requirements for loudness evaluation have been discussed and the formulation of a straightforward procedure, which can be used to evaluate the loudness according to costumer's requirements, is defined

Experimental characterization of a Si-Mo-Cr ductile cast iron

High temperature-resistant ductile cast irons behaviour is highly interesting for the manufacture of components, such as exhaust manifolds for automotive applications. In the present paper the temperature-dependent static, high cycle and low cycle fatigue behaviour of a heat-resistant Si-Mo-Cr ductile cast iron (Fe-2.4C-4.6Si-0.7Mo-1.2Cr) is investigated. Tensile and high cycle fatigue properties, in terms of elastic modulus, yield stress, elongation at break, fatigue limits, and the stress-life Basquin’s curve parameters have been determined at room temperature, 160 °C, 500 °C and 800 °C, thus covering the usual temperature range to which actual components, obtained with this kind of material, are subjected. The alloy showed good monotonic properties at low temperature, but showed to be fragile during fatigue tests, due to the high Silicon content in the alloy. At 500 °C mechanical properties are still good, with a 40% decrease with respect to 160 °C, and ductility is increased. The last temperature level of 800 °C has caused a noticeable drop of the cast iron strength, due to softening and oxidation effects

Some comments on mechanical fatigue characterization of steel rails in Standards

Current Standards and recommendations on characterization of steel materials for rail production define tests for material supplying. As reported in technical literature, fatigue is the phenomenon which represent one of the main cause of rail damage and failure. Experimental testing of fatigue characterization according to Standards on different samples and with different surface roughness values, satisfying the Standard requirements, are performed. The results are then presented and discussed. Some nomenclature ambiguities are pointed out, which can lead to different loading conditions for fatigue testing

Surface factor assessment in HCF for steels by means of empirical and non destructive techniques

The fatigue limit value in steels is strongly influenced by many factors, among them the surface finish. In particular, the fatigue limit decreases with increasing the surface roughness, referring to standard grinded specimen.Technical literature provides an empirical correction factor, named surface factor, to be used if surface roughness is different from standard specimen conditions. This factor is traditionally lower than 1 and it reduces the fatigue limit value corresponding to the material in standard conditions. This coefficient may be obtained from literature graphs and it can be identified by means of two parameters: materials ultimate tensile strength and surface finish Ra.Aim of the present paper is to evaluate the effectiveness of fast procedures to assess the surface factor. The reference is the Murakami model, which estimates the fatigue limit by means of roughness parameters other than Ra.In the present paper the fatigue limit estimations related to specimens with sanded Rahave been obtained by utilizing empirical destructive and nondestructive methods and then have been compared each other.Experimental testing was carried out on a structural steel specimens by means of axial alternate fatigue testing with two different surface roughness.The results obtained referring to Murakami model have been compared with those obtained by means of both thermographic and Staircase method.The Murakami model results to be easy to use and non destructive.The corresponding fatigue limit estimations match with the thermographic ones above all when surface roughness is elevated

Mechanical behavior of macadamia nutshells

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Geodesic domes for planetary exploration

Venus and the Ocean Worlds are emerging areas of interest for space exploration, as they can potentially host, or have hosted, conditions compatible with life. Landers and probes for in-situ exploration, however, must deal with very high external pressure, due to the environmental conditions, often resulting in thick and heavy structures. Robust, reinforced shell structures can provide a lightweight solution for the primary structure. In this frame, the isogrid layout is already a standard in aerospace, especially for flat panels or cylindrical shells. In this paper, isogrid-stiffened hemispherical shells, or "geodesic domes", are described, focusing on the case of a concept of a Venus lander. Early design methods for both plain and geodesic domes subjected to external pressure are presented, providing design equations. Additive Manufacturing is identified as the key technology for fabricating metallic geodesic domes, due to the complexity of the internal features.Moreover, it allows to fabricate ports and integrated thermostructural systems in the same process, potentially resulting in improved performance or cost and schedule savings

Advanced Life Assessment Methods for Gas Turbine Engine Components

In combustion systems for aircraft applications, liners represent an interesting challenge from the engineering point of view regarding the state of stress, including high temperatures (up to 1500°C) varying over time, high thermal gradients, creep related phenomena, mechanical fatigue and vibrations. As a matter of fact, under the imposed thermo-mechanical loading conditions, some sections of the liner can creep; the consequent residual stresses at low temperatures can cause plastic deformations. For these reasons, during engine operations, the material behaviour can be hardly non-linear and the simulation results to be time expensive. Aim of this paper is to select and implement some advanced material life assessment methods to gas turbine engine components such as combustor liners. Uniaxial damage models for Low Cycle Fatigue (LCF), based on Coffin-Manson, Neu-Sehitoglu and Chaboche works, have been implemented in Matlab®. In particular, experimental LCF and TMF results for full size specimens are compared to calibrate these models and to assess TMF life of specimens. Results obtained in different testing conditions have been used for validation. In particular, each model needs specific parameter calibrations to characterize the investigated materials; these parameters and their relation with temperature variation have been experimentally obtained by testing standard specimens

Multiaxial damage assessment and life estimation: Application to an automotive exhaust manifold

AbstractSome mechanical components are subjected to thermo-mechanical fatigue, which occurs when both thermal and mechanical loads vary with time. Due to the complexity of the components geometry, stresses and strains field becomes multiaxial, worsening the fatigue resistance. In this paper several damage models are applied and compared on a case study, an automotive exhaust manifold simulacrum replying the material and the geometrical features of the commercial component. A complete thermo-structural FE analysis has been run and results have been post-processed by means of a numerical code implementing several multiaxial damage models available in literature and based both on a critical plane approach (Kandil-Brown-Miller, Fatemi-Socie) and strain-based models (Von Mises, ASME Code and Sonsino-Grubisic). The model calibration has been carried out by means of literature experimental data referred to commercial exhaust manifolds of similar geometry and material