Finite Element Analysis and Experiments for Predicting Fatigue and Rolling Contact Fatigue Behavior of Spur Gears

This paper presents the Finite Element (FE) analyses carried out with the aim to predict the tooth root fatigue and Rolling Contact Fatigue (RCF) behavior of spur gears, in terms of crack propagation maximum number of cycles. The combination of different materials, i.e. steel and titanium, and surface treatments, i.e. case-hardening and application of surface layers by Physical Vapor Deposition (PVD), are investigated. The residual stresses induced by the deposition of the coating are modelled. The stress intensity is described by linear elastic relations based on the crack tip opening displacement and the crack propagation in the case-hardened spur gears is described with the help of mathematical models. Experiments are carried out to evaluate tooth damage under RCF for different treated gears. The best solutions in terms of bulk material – treatment combination among the ones investigated are identified, also highlighting innovative possibilities which can guarantee appreciable performance

Numerical and theoretical models to predict fatigue life in aggressive environments from experimental data

Corrosion fatigue produce sensible effects in the fracture mechanics of structural materials. Aggressive environments in presence of dynamic fatigue load are indeed responsible of multiple effects, regarding crack nucleation and propagation rates. Considering Ti-6Al-4V in air, inert paraffin oil and 3.5 wt.% NaCl mixture, environmental effects are sensible in terms of acceleration of Fatigue Crack Growth Rate – i.e. da/dN vs. stress intensity factor ΔK. Several literature studies dealt with the topic in the past years. However, research has been focused mainly on the FCGR description, and the prediction of number of cycles to failure in aggressive environments is not addressed. In the presented poster, a methodology to obtain a numeric model which reconstruct da/dN vs ΔK from experimental results, including crack length and applied stress, is presented and compared against literature data. Results are related to R = 0.1 axial test involving smooth and notched flat dogbone specimens, with varying notch radius. The proposed model is used to reconstruct the number of cycles to failure of the tested specimens, resulting in a satisfactory correlation with experimental data. Comparison with other literature models highlights the necessity to develop a proper numerical model with each test case. Please click Additional Files below to see the full abstract

study on a new mobile anti terror barrier

Abstract The vehicle-ramming terror attacks in Berlin, Barcelona, London and Nice highlighted our vulnerability: all of us could be wounded or killed during a walk in a crowded place, it is sufficient a car, a van or a truck. The authors of this paper designed a planter full of water and mainly made of steel and cast iron. For this reason, this device serves as both mobile anti-terror barrier and street furniture. This barrier can stop a 3500 kg vehicle running at 64 km/h and the system itself in less than five meters as demonstrated by the experimental crash test. Starting from these considerations, a simplified mathematical model of the impact was developed and a finite element model was calibrated. The first one points out the main features needed by the obstacle; the second one is a good base for further analyses

A numerical model to assess the role of crack-tip hydrostatic stress and plastic deformation in Environmental Assisted Fatigue Cracking

To better understand the mechanics of environmentally assisted cracking, and particularly hydrogen embrittlement, a correct description of the hydrostatic stress field is indispensable. The concentration of hydrogen in the proximity of the crack tip is indeed dependent of the hydrostatic stress effect on the microstructural lattice of the material. The overall parameters of the hydrostatic stress, including peak value, its location, gradient and distribution size are fundamental to assess the effect on hydrogen distribution near the crack tip. H concentration show indeed an exponential dependence on hydrostatic stress, so that even a moderate increase in the applied stress is traduced in a dramatic rise of hydrogen concentration. In order to investigate the mechanism beneath hydrogen embrittlement, a proper modelling of the hydrostatic stress is mandatory. Moreover, strain hardening might significantly interfere with the actual stress distribution, thus becoming a relevant parameter. The stress and strain field is also deeply affected by the presence of sharp notches. In the present work, a detailed Finite Element model, based on actual fatigue testing of notched Ti-6Al-4V specimens is proposed, providing a deeply informative tool to assess the hydrostatic stress and the plastic strain in the proximity of the crack tip. The data from the model are compared to available results and experiences in literature. Please click Additional Files below to see the full abstract

Effects of FOD on fatigue strength of 7075-T6 hourglass specimens

The term "Foreign Object Damage (FOD)" refers to the damage associated with the impact of particles on aircraft engine components that causes changes in fatigue strength as a result of induced stress concentrations, residual stresses and microstructural changes. This paper presents the Finite Element (FE) models created to study the stress field induced by the impacts of a steel sphere on a 7075-T6 hourglass specimen. The impacts occur at the minimum cross section, in the normal and oblique directions. The results reveal that in the event of an oblique impact the axial tensile stresses are greater and closer to the crater created by the impact. The superposition of the residual axial stresses to the bending stresses by fatigue test allows to identify the areas in which the maximum stresses are reached and therefore crack initiation is expected

Transition from small to large cracks in Ti-6Al-4V specimens

Light alloys allow us to have light components with interesting mechanical features. Ti-6Al-4V is a bimodal titanium alloy employed in aerospace, automotive, maritime and biomedical applications. This alloy has also a good corrosion strength which can be reduced by damages on the passivating surface layer. These damages can be due to an inadequate adhesion of the surface layer, variable loads and interactions with aggressive media [1,2]. For these reasons, quasi-static and fatigue tests on Ti-6Al-4V specimens in different inert, aggressive and very aggressive environments were carried out in the past. Air, air + beeswax coating, paraffin oil, 3.5wt.% NaCl-water solution and water-methanol solutions with several percentages were investigated in order to evaluate the chemical and mechanical forces of the corrosion fatigue phenomena [3]. Stress corrosion cracking (SCC) tests on a low strength steel [4] showed that the threshold of dJ/dt decreases with decreasing deformation rate and that the electrochemical energy contribution on the crack growth is independent from the displacement rate but dependent from the electrochemical conditions at the crack tip. As stated in [5] the crack size effect must be considered because small cracks have very high growth rate. Slow strain rate tensile and low-amplitude cyclic tests on micro-notched high strength low alloy steel specimens showed that crack growth strongly depends on the notch-tip plastic zone and hydrogen activity itself. High cycle fatigue tests with different notch shapes showed in [7] that the maximum stress and gradient increase with decreasing defect size. The stress state at the notch root is a function of the geometry. In [7] an elasto-plastic FE modelling with a multiaxial fatigue criterion and a correction for the stress gradient is also shown. Tanaka et al. [8] applied the fracture mechanics approach to fatigue crack initiation for also small notch-tip radius. In [9] micro-notched 316L steel specimens in a chloride medium gave the crack propagation rate in function of the global loading. A FE model was also developed in [9]. Micro-notched Ti-6Al-4V specimens were machined to get various notch lengths (up to about 100 ) & tested under static & cyclic loads to obtain fracture properties, incubation times. The notches were made using Electro Discharge Machining (EDM) in order to reach various values of stress concentration factor ( ) without notch tip plasticity. The tests were carried out on a testing machine previously designed by the Structural Mechanics Laboratory (SM-Lab) of the University of Bergamo and now modified so that bigger specimens can be used and easier setting can be reached. During the tests an axial load was applied with a fixed increment of it every fixed time. The machine has specific grips in order to avoid unwanted bending moment on the specimens. Test results are plotted on Kitagawa diagram to analyze the role of environment on static and cyclic applied loads. At the threshold i.e. at the endurance limit (horizontal line in Kitagawa diagram) we calculated: . [1] Brown B.F. 1972: 147–244, Naval Research Laboratory, Washington, D.C. [2] Lee E.U., Vasudevan A.K., Sadananda K. International Journal of Fatigue 2005; 27: 1597-1607. [3] Baragetti S., Villa F. Fracture and Structural Integrity 30 (2014), 84-94. [4] Sarioğlu, F. and Doruk, M. Conf.Proc., Advances in Fracture Research (ICF-7), Houston, Texas, USA, ed. K. Salama, 1989, 1, pp. 259–65. [5] Gangloff RP. Met Trans A 1985;16A (5):953-69. [6] Bosch, C., Delafosse, D., Longaygue, X. European Corrosion Congress 2010 (Eurocorr2010), Sep 2010, Moscou, Russia. 2, pp.1558-1570, 2010. [7] Franck Morel, Anne Morel, Yves Nadot. International Journal of Fatigue,Elsevier, 2009, 31 (2), pp.263-275. [8] Tanaka K., Nakai, Y., Kawashima, R. Engineering Fracture Mechanics 18, 5, pp.1011-1023,1983. Tinnes, J.P., Delafosse, D., Bosch, C., Raquet, O., Santarini, G. Euroorr 2004

Effects of Geometrical Clearances, Supports Friction, and Wear Rings on Hydraulic Actuators Bending Behavior

Hydraulic actuators are commonly adopted in machines and structures to provide translating forces with significant magnitudes. Although their application dates back to the industrial revolution, their bending behavior under compression is typically addressed by simple Euler’s instability analysis on the rod, neglecting effects such as the cylinder inertia and stiffness, the presence of contact elements in the cylinder-rod junction and on the piston, geometrical misalignments and imperfections, and friction moments at the support. Such simplifications lead to unjustified reduced critical load calculations on the component. In the present paper, a complete mathematical formulation, which accounts for such effects, is presented and validated against experimental data. A numerical sensitivity analysis is conducted, to assess the contributions of initial rectilinear imperfections, wear rings stiffness and dimension, and supports friction on the actuator’s limit buckling load and bending behavior under compression. Results are presented, including the effect of the cited parameters on the buckling load, providing a reliable tool for the mechanical designer. In particular, an optimum position for the wear ring distance is found. Moreover, increased wear ring stiffness and reduced imperfections increase the buckling load and reduce the bending stresses before the critical load

Preface

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