113 research outputs found
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
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
Numerical modeling and testing of mechanical behavior of AM Titanium alloy bracket for aerospace applications
A key issue in designing a new product made through the Additive Manufacturing (AM) is the prediction of mechanical properties of material. Several experimental results show that AM-based products are often affected by widespread porosity, low density regions within their volume and anisotropy. Those effects are due to the manufacturing process, despite of efforts spent to improve the process parameters. This paper presents the numerical modelling of a geometrically complex structural bracket for aerospace application, which was re-designed through a topological optimization and produced in Ti-6Al-4V by means of the AM. The design activity herein described required to resort to a suitable model of constitutive properties of material by facing the problem of a large number of porosity/low density areas, as detected by a tomographic analysis of the mechanical component. According to some references an equivalent isotropic and homogeneous model of material was applied. Nevertheless the limitations of that approach were investigated through a validation of the numerical model and a testing activity. It was demonstrated that the Finite Element model based upon the assumptions of homogeneous and isotropic material might be effective in predicting the material and component strength, at least in static design, but even in case of design against fatigue, provided that a suitable experimental characterization of material was performed. The procedure of optimization was then assessed and compared to some preliminary tests performed on the real component, thus providing a preliminary good practice to the industrial partner involved in this research activity
- …