Multiaxial Fatigue Crack Propagation of an Edge Crack in a Cylindrical Specimen Undergoing Combined Tension-Torsion Loading

A three-dimensional crack propagation simulation of a hollow cylinder undergoing coupled traction and torsion loading conditions is performed by the Dual Boundary Element Method (DBEM). The maximum tension load and torque are equal to 40 kN and 250 Nm respectively. Specimens, made of Al alloys B95AT and D16T, have been experimentally tested with in-phase constant amplitude loads. The Stress Intensity Factors (SIFs) along the front of an initial part through crack, initiated from the external surface of the hollow cylinder, are calculated by the J-integral approach. The crack path is evaluated by using the Minimum Strain Energy Density (MSED) criterion whereas the Parisâ law, calibrated for the material under analysis, is used to calculate crack growth rates. A cross comparison between DBEM and experimental results is presented, showing a good agreement in terms of crack growth rates and paths

Thermal–Mechanical FEM Analyses of a Liquid Rocket Engines Thrust Chamber

The Italian Ministry of University and Research (MIUR) funded the HYPROB Program to develop regeneratively cooled liquid rocket engines. In this type of engine, liquid propellant oxygen–methane is used, allowing us to reach very good performances in terms of high vacuum specific impulse and high thrust-to-weight ratio. The present study focused on the HYPROB final ground demonstrator, which will be able to produce a 30 kN thrust in flight conditions. In order to achieve such a thrust level, very high chamber pressures (up to 50 bar) and consequently high thermal fluxes and gradients are expected inside the thrust chamber. Very complex and high-fidelity numerical FEM models were adopted here to accurately simulate the thermal–mechanical behavior of the thrust chamber cooling channels, accounting for plasticity, creep, and low-cycle fatigue (LCF) phenomena. The aim of the current work was to investigate the main failure phenomena that could occur during the thrust chamber’s service life. Results demonstrated that LCF is the main cause of failure. The corresponding number of loading cycles to failure were calculated accordingly

LCF assessment on heat shield components of nuclear fusion experiment "Wendelstein 7-X" by critical plane criteria

Abstract The Wendelstein 7-X modular advanced stellarator has started operations at the Max Planck Institute for Plasma Physics in Greifswald, Germany, in 2016. In the first phase, the machine operated restricting the plasma pulses to low power and short lengths. Plans to achieve actively cooled components are scheduled to start in 2020 when the machine will operate in steady-state at full power. FEM simulations for steady-state operations revealed high plastic strains at several locations, for most of all the rigidly supported Plasma Facing Components; therefore, there is the risk of a premature fatigue failure before the end of the scheduled operations of the machine. The aim of this study is to analyse, by means of the commercial code ABAQUS, the behavior of such critical components estimating, eventually, their fatigue life by means of the commercial code fe-safe

Mechanical behavior of chemically-treated hemp fibers reinforced composites subjected to moisture absorption

Natural Fibers Reinforced Composites (NFRC) are finding much interest as substitutes for glass- or carbon-reinforced composites thanks to their lightness, easy handling, processing and recyclability. However, their polarity makes them incompatible with hydrophobic thermoplastic matrices, leading to extended moisture adsorption which causes the debonding between fibers and matrix, affecting, thus, the mechanical properties of NFRCs. In the present work, NFRCs were manufactured using hemp fibers previously chemically treated with NaOH alkali solutions or (3-Glycidyloxypropyl) trimethoxysilane (GPTMS) solutions of various concentrations. To assess the effectiveness of the used chemical treatments in hindering the moisture adsorption and the entailed mechanical failure of the NFRCs, untreated and treated hemp fibers based composites were subjected to moisture adsorption test and then to tensile testing as a function of the chemical treatment, temperature and concentration of reagents. The results show that the treatments with 5 wt% of both NaOH and GPTMS are the most effective, reducing composites' water uptake from 7.74% to 6.46% and 5.58% respectively at room temperature, and from 9.67% to 8.19% and 8.13% respectively at 50 °C. Moreover, the comparison between the mechanical testing results carried out before and after the moisture adsorption test, shows that the water uptake induces mainly a stiffness decrease (about 50% when alkali treatments were used and about 60% using silane treatment), while not significantly affect the loading capability of the composites regardless of chemical treatment. However, the specimen obtained using 5 wt% GPTMS is more effective in the prevent the failure of the composite induced by water uptake

FEM Substructuring for the Vibrational Characterization of a Petrol Engine

In this work the vibrational behavior of a 4-cylinder, 4-stroke, petrol engine has been simulated by leveraging on a reduced modelling strategy, based on the Component Mode Synthesis (CMS), adopted to reduce the size of the full FEM model of the engine. The FEM model of the engine, comprising all of its sub-components, has been preliminary characterized from the vibrational standpoint; subsequently, the CMS has been adopted in order to reduce the FEM model size. Frequency Response Function (FRF) analyses have been used to identify the resonant frequencies and mode shapes of the different FEM models, and the so-obtained results have been compared showing a very good agreement. The reduced model has been able to reproduce with a high accuracy the vibration response at the engine mounts. The adopted reduced modelling strategy turned out to be effective in lowering the computational burden, keeping, at the same time, an accurate replication of the engine vibrational behavior. Runtimes have been significantly reduced from 24 hours for the full FEM model to less than 2 hours for the reduced model

Passive noise control oriented design of aircraft headrests

Two Passive Noise Control (PNC) concepts were numerically evaluated in terms of their impact on the Sound Pressure Level (SPL) perceived by passengers of an aircraft flight. A concept was based on the shape optimization of the headrests, whereas the second one was based on the adoption of a high absorbing material, i.e. a nanofiber textile, to improve the acoustic performances of the headrests. To this aim, an aircraft seat was modelled with the Boundary Element Method (BEM) and loaded with a spherical distribution of monopole sources surrounding the seat. Different configurations of headrest shape and covering textiles were then compared in terms of the SPL calculated at passengers’ ears. The work shows how an acoustic-oriented design of the aircraft headrests could achieve an average SPL reduction for passengers up to 3 dBA

Neural networks for fatigue crack propagation predictions in real-time under uncertainty

Crack propagation analyses are fundamental for all mechanical structures for which safety must be guaranteed, e. g. as for the aviation and aerospace fields. The estimation of life for structures in presence of defects is a process inevitably affected by numerous and unavoidable uncertainty and variability sources, whose effects need to be quantified to avoid unexpected failures or excessive conservativism. In this work, residual fatigue life prediction models have been created through neural networks for the purpose of performing probabilistic life predictions of damaged structures in real-time and under stochastically varying input parameters. In detail, five different neural network architectures have been compared in terms of accuracy, computational runtimes and minimum number of samples needed for training, so to determine the ideal architecture with the strongest generalization power. The networks have been trained, validated and tested by using the fatigue life predictions computed by means of simulations developed with FEM and Monte Carlo methods. A real-world case study has been presented to show how the proposed approach can deliver accurate life predictions even when input data are uncertain and highly variable. Results demonstrated that the “H1-L1” neural network has been the best model, achieving an accuracy (Mean Square Error) of 4.8e-7 on the test dataset, and the best and the most stable results when decreasing the amount of data. Additionally, since requiring only very few parameters, its potential applicability for Structural Health Monitoring purposes in small cost-effective GPU devices resulted to be attractive

DBEM crack propagation for nonlinear fracture problems

A three-dimensional crack propagation simulation is performed by the Dual Boundary Element Method (DBEM). The Stress Intensity Factors (SIFs) along the front of a semi elliptical crack, initiated from the external surface of a hollow axle, are calculated for bending and press fit loading separately and for a combination of them. In correspondence of the latter loading condition, a crack propagation is also simulated, with the crack growth rates calculated using the NASGRO3 formula, calibrated for the material under analysis (steel ASTM A469). The J-integral and COD approaches are selected for SIFs calculation in DBEM environment, where the crack path is assessed by the minimum strain energy density criterion (MSED). In correspondence of the initial crack scenario, SIFs along the crack front are also calculated by the Finite Element (FE) code ZENCRACK, using COD, in order to provide, by a cross comparison with DBEM, an assessment on the level of accuracy obtained. Due to the symmetry of the bending problem a pure mode I crack propagation is realised with no kinking of the propagating crack whereas for press fit loading the crack propagation becomes mixed mode. The crack growth analysis is nonlinear because of normal gap elements used to model the press fit condition with added friction, and is developed in an iterative-incremental procedure. From the analysis of the SIFs results related to the initial cracked configuration, it is possible to assess the impact of the press fit condition when superimposed to the bending load case

Control of positive end-expiratory pressure (PEEP) for small animal ventilators

<p>Abstract</p> <p>Background</p> <p>The positive end-expiratory pressure (PEEP) for the mechanical ventilation of small animals is frequently obtained with water seals or by using ventilators developed for human use. An alternative mechanism is the use of an on-off expiratory valve closing at the moment when the alveolar pressure is equal to the target PEEP. In this paper, a novel PEEP controller (PEEP-new) and the PEEP system of a commercial small-animal ventilator, both based on switching an on-off valve, are evaluated.</p> <p>Methods</p> <p>The proposed PEEP controller is a discrete integrator monitoring the error between the target PEEP and the airways opening pressure prior to the onset of an inspiratory cycle. In vitro as well as in vivo experiments with rats were carried out and the PEEP accuracy, settling time and under/overshoot were considered as a measure of performance.</p> <p>Results</p> <p>The commercial PEEP controller did not pass the tests since it ignores the airways resistive pressure drop, resulting in a PEEP 5 cmH₂O greater than the target in most conditions. The PEEP-new presented steady-state errors smaller than 0.5 cmH₂O, with settling times below 10 s and under/overshoot smaller than 2 cmH₂O.</p> <p>Conclusion</p> <p>The PEEP-new presented acceptable performance, considering accuracy and temporal response. This novel PEEP generator may prove useful in many applications for small animal ventilators.</p

In-Network Outlier Detection in Wireless Sensor Networks

To address the problem of unsupervised outlier detection in wireless sensor networks, we develop an approach that (1) is flexible with respect to the outlier definition, (2) computes the result in-network to reduce both bandwidth and energy usage,(3) only uses single hop communication thus permitting very simple node failure detection and message reliability assurance mechanisms (e.g., carrier-sense), and (4) seamlessly accommodates dynamic updates to data. We examine performance using simulation with real sensor data streams. Our results demonstrate that our approach is accurate and imposes a reasonable communication load and level of power consumption.Comment: Extended version of a paper appearing in the Int'l Conference on Distributed Computing Systems 200