1,720,999 research outputs found
Model for phase transition during heat treatment in eutectoid steel
No full textSteel heat treatments are very important industrial processes which involve microsctructural transformations. In the shop practice the design of these processes is conducted on empirical basis using the so called Bain curves, which cannot describe the phenomenon in its globality. Here, a two dimensional model for an eutectoid steel is proposed which takes into account either diffusive or displacive transformations. The first law of the thermodynamics as well as the dynamic equilibrium equation and an evolution equation for the diffusive phase transformations are involved in the model, together with a description of the internal energy. © AES-Advanced Engineering Solutions
A theoretical study of the parameters affecting the power delivery efficiency of an agricultural tractor
Full text linkTractive efficiency is of major concern to agricultural tractors manufacturers, end-users and to society as well, both for economical and environmental reasons. In this article, a traction model of a whole vehicle is developed which accounts for the special features of a MFWD agricultural tractor.The aim of the article was to identify the key design parameters affecting the power delivery efficiency of an agricultural tractor and to quantify their effect on the tractive performance. To this end, numerical simulations were performed varying several tractor design parameters. The results of the simulations were then analysed using a gradient-based method which allowed to identify the most influential design parameters. A regression surface for the estimation of the tractive efficiency as a function of the relevant tractor design parameters was used to approximate the results of the numerical simulations and a quantitative relation to calculate the optimal mass distribution in terms of power delivery efficiency is proposed.Within the range of variation of the design parameters explored in this study, the maximum power delivery efficiency was found for a tractor having equal kinetic rolling radii of front and rear tyres, no lead of the front wheels and the centre of mass shifted towards the front axle. However, if the front tyres kinetic rolling radius becomes smaller than that of rear tyres and if there is lead of the front wheels, the tractor centre of mass has to be shifted towards the rear axle to attain the maximum overall traction efficiency. (C) 2019 IAgrE. Published by Elsevier Ltd. All rights reserved
Mechanical characterization of straw bales for use in construction
No full textFrom the point of view of environmental awareness, construction sustainability is a theme that is gaining considerable attention. This has brought to a re-discovery of the use of straw as a construction material, a technology that has been widely used in the past centuries. Straw is a natural material having excellent insulating capabilities, considerable mechanical properties when packed in bales and is relatively low cost (it is indeed a by-product of farming activity). Two are the techniques adopted in straw bale construction: post-and-beam, where the load is sustained by a wooden frame and straw bales are used merely as infill material; and load-bearing, where the entire load is sustained by the plastered straw bale walls. Especially regarding the latter technique, there is a need for a deeper understanding of the mechanical properties of straw bales. The characterization of the material is made difficult by the variability brought about by the variety of plants from which straw can be obtained, unpredictability of the baling process, moisture content in the bale and other parameters. Moreover, there is not a standard regulating the productive chain of straw bales to be used for construction. In this paper, a literature review on the tests performed on single unplastered straw bales is conducted and, based on this, some ideas are proposed for the development of a model describing the mechanical behavior of straw bales
Optimal criteria for durability test of stepped transmissions of agricultural tractors
No full textTractor manufacturers are constantly trying to reduce the time-to-market and production costs of their products. To achieve these goals, precise knowledge of the service demands, and reliable accelerated tests are necessary. To design an accelerated test, detailed knowledge of the service loads throughout the machine life is demanded. The measurement and the analysis of service loads is a non-trivial task, because loads are influenced by many factors: machine configuration; working conditions; and driver usage. Monitoring loads in all these conditions can be very expensive, especially if carried out with sample of machines equipped with non-embedded sensors. In this paper, a methodology to analyse loads on tractors transmissions acquired with embedded sensors is proposed and an optimal test schedule was calculated. The operating parameters of the transmissions of a fleet of 44 tractors were monitored over a year of usage. From the acquired data, load amplitude and frequency on each transmission component were calculated, and pseudo-damages under different failure modes were computed. Pseudo-damages were then statistically analysed, and a severe-damage profile was estimated. Using an optimisation technique, an accelerated test schedule is determined. The analysis showed that the test schedule should be set to maximise the torque applied to each component. Moreover, not all the gear ratios need necessarily to be tested to test all the gear-wheels in the transmission. The calculation of the test schedule with optimisation solvers permits to quickly design tests for transmissions
Investigation over the parameters affecting the mechanical behaviour of small prismatic straw bales for use in construction
No full textStraw bale construction is a building technique offering many advantages: it provides excellent hygro-thermal insulation; it ensures good performance against earthquakes; and it is sustainable. From a mechanical point of view, straw bales can act as a surviving cell for the building in case of calamity. For this reason, measuring the mechanical properties of straw bales is important for the comprehension of the behaviour of straw bale buildings. Despite the fact that the use of straw bales in construction has been constantly increasing since the ‘80s, the behaviour and the performance of straw bale buildings have not been fully assessed so far. Using a recently developed test methodology (Maraldi et al., 2016), an analysis of the performance of small prismatic straw bales under monotonic compressive loads has been conducted. Force-displacement curves obtained from the tests have been fitted with a two-parameter rheological model to extract bales performance indicators. Results show that bales laid flat are stiffer than bales laid on-edge. However, if the influence of bales initial geometry is disregarded by considering stress and strain instead of force and displacement, no difference in the elastic modulus between flat and on-edge orientation appears. Furthermore, bales stiffness and elastic modulus are dependent on the initial density of the bale. It has been also found that straw bales exhibit a typical deformation pattern which depends on the baling process and that the Poisson’s ratio does not remain constant along the longitudinal direction during loading, whereas it is null along the transverse direction
Method for the characterisation of the mechanical behaviour of straw bales
No full textThe use of straw bale construction is strongly on the rise. Despite the need for a deep understanding of the mechanical behaviour of straw bales, there is little research on the testing of single unplastered straw bales and a standard test method does not exist. In this paper, a method able to evaluate the mechanical behaviour of single straw bales is proposed. Force and displacement of the bale in all the three directions was measured in real time without stopping the test; this allowed to best deal with the time-dependent nature of the mechanical behaviour of the bales to be. The test apparatus included a hydraulic press for loading plus digital cameras and a 3D laser scanner for measuring the lateral displacement of the bale. The method was validated by testing six rice bales (three bales laid flat and three on-edge). Results showed that there is no significant difference in the elastic modulus between flat and on-edge orientations. For on-edge bales, string burst was observed, whereas for flat bales no string failure occurred. By using digital image correlation it was observed that straw bales exhibit a typical deformation pattern which is due to the baling process. Measurements also showed that the Poisson's ratio does not remain constant along the longitudinal direction during loading and it is null along the transverse direction. The proposed method can be implemented to evaluate the influence of a variety of parameters and loading conditions on straw bales mechanical response
Grading bamboo through four-point bending tests. A report on six species of Italian bamboo
No full textThe adoption of natural construction materials can help mitigate the environmental impact of the construction industry by reducing the amount of energy required and emissions. Among the natural materials, bamboo is a remarkable option, combining high mechanical resistance with a low specific weight and a very high growth speed; however, its application is limited by the absence of a suitable grading method. This paper aims at filling this lack by providing a strength-based grading methodology relying on a four-point bending test. Experimental results were processed considering bamboo both as a homogeneous material and as a nonhomogeneous material and using the ANOVA test. Results show that statistical groups can be defined which are not based on the bamboo species, but on the values of the mechanical properties, hence supporting the appropriateness of strength classes. The bending tests used in this paper could be a crucial test for bamboo grading: it is easier to perform than a tensile test and it gives complete information on the change of material properties along the culm wall thickness. The study focuses on six different species of bamboo grown in Italy, for which little data is present in the literature
Mechanical Devices for Mass Distribution Adjustment: Are They Really Convenient?
Full text linkSince the introduction of four-wheel drive (4WD) and especially front wheel assist (FWA),
many studies have been conducted on the optimal weight distribution between tractor front and rear axles because this influences traction efficiency. The aim of this paper is to evaluate the traction and efficiency advantages in the adoption of mechanical ballast position adjustment devices. The tested device is an extendable ballast holder mounted on the front three-point hitch of the tractor, able to displace the ballast up to 1 m away from its original position. An estimation of the fuel consumption during ploughing with the extendable ballast holder in different configurations was performed.
Tractive performance was evaluated through drawbar tests, performed on loam soil with a 4WD tractor having a maximum engine power of 191 kW and a ballasted mass of 9590 kg. Results show that changing the tractor weight distribution over the range allowed by the extendable ballast holder produces limited effects in terms of tractive performance and fuel saving. The adoption of such devices is thus ineffective if other fundamental factors such as tyre pressure, choice of the front-to-rear wheel combination and lead of the front wheels are not considered during tractor setup
Analysis of the parameters affecting the mechanical behaviour of straw bales under compression
No full textStraw bale construction is a building technique that has excellent thermal performance and limited impact on the environment. Improving knowledge of the mechanical properties of straw bales is important to understand the behaviour of straw bale buildings, especially in case of natural calamities, where straw bales may carry mechanical load and act as a âsurviving cellâ. The results of extensive tests conducted on bales of different materials are presented. The influence of the material, bale density, bale orientation, baling process and loading rate on the mechanical properties of straw bales was investigated. Force-displacement curves obtained from monotonic compression tests were analysed and relationships between the mechanical properties of straw bales and their geometry and density were determined. Continuous measurements of bales lateral displacement allowed Poisson's ratio to be calculated and, using a simple model, the strain to which bale strings are subjected during loading was estimated. Young's modulus was shown to mainly depend on the square of the density, while no influence of the loading rate and of strings pre-tension was observed. The Poisson's ratio did not remain constant during loading and it exhibited a different trend depending on the orientation of the bales. Moreover, it was observed that for flat bales a rearrangement of the straw fibres during loading occured and the maximum strings strain remained limited. Strings strain reached higher values for on-edge bales instead, and strings bursts occured more frequently
Time-dependent mechanical properties of straw bales for use in construction
No full textStraw bale construction is a building technique with low environmental impact that has been gaining popularity over the last few decades. The aim of the paper is to contribute to the assessment of the potential of straw bales as a building material by measuring their dynamic mechanical properties and by modelling their response in time to mechanical load. To this end, relaxation tests, creep tests and tests with cyclic loads were performed on small prismatic straw bales at different loading levels. Data were fitted using different models: power models for cyclic load data and stretched exponential models for creep and relaxation data. Results show that bales exhibit a viscous-type response; the models used for data fitting predict that straw bales eventually settle to an asymptotic value of displacement in case of creep and of force in case of relaxation. Under cyclic loading, straw bales can dissipate energy; such capability, as well as the elastic performance of straw bales, decreases as cyclic loading continues, but can be retrieved after some time of resting from loading. A comparison between the creep performance of straw bales and that of conventional building materials showed that, in this respect, straw bales perform on average in a similar manner to masonry brickwork
- …
