The growing introduction into the industrial world of additive fused deposition technologies, for producing functional components made of thermoplastic materials (such as PLA), pushes designers to ...know which methods for predicting breakage under load can be used to design correctly these elements. In particular, this paper analyses the “brittle fracture” phenomenon in these materials that too often occurs in various engineering sectors. This research article investigates a methodology for predicting fracture loads in additively manufactured PLA material specimens with different topologies of V-notches.
The proposed method represents a direct evolution of the Average Strain Energy Density (ASED) criterion, originally conceived for brittle materials that develop a liner-elastic behavior. This method, in those cases where the material has a non-linear elastic behavior, has to be correct with the concept of equivalent elastic material (EMC), providing in that way, a simple and reliable method for local settlement prediction in PLA specimens. The results obtained demonstrate that, despite the many variables that come into play in the making process of the specimens, the ASED-EMC combined criteria provide accurate predictions. The breaking load was predicted with an average deviation from the experimental fracture loads of less than 12%.
Fatigue life estimation or fatigue damage evaluation of mechanical components under a multiaxial state of stress time history has an important role in virtual design phases but this evaluation is a ...real problem to resolve because this goal is not reached by classical fatigue criterions. Moreover the virtual simulation of components and structures will only be efficient if it uses reliable and quick tools in each step of the analysis. Above all the analytical instruments used in fatigue damage evaluation must guarantee minimal computational effort starting from simulated stresses. In this context the authors chose the uniaxial equivalent stress as the approach useful for this purpose by combining it with a strength curve for the material (i.e.
S–
N curve) and a rule to evaluate damage (i.e. Miner’s rule). So the authors has developed a new equivalent stress that can be used as a uniaxial stress time history suitable for the application of damage evaluation methods. However the uniaxial equivalent process is not accepted by the majority of researchers because a definition in agreement with experimental results has not been found yet. Furthermore some theoretical problems appear when an equivalent stress defined in static conditions is extended to dynamic conditions. In this paper the authors introduce an original form of equivalent uniaxial process starting from an accurate theoretical analysis of octahedral planes. It will be demonstrated that this process can be defined both in the time domain and the frequency domain. Then the agreement between experimental tests found in literature and the expectations of the proposed method will be also shown by a comparison with the best multiaxial criteria.
Dynamic response of vibrating system subjected to non-Gaussian random loads was investigated through a set of numerical simulation on several lumped systems aimed to determine whether and in what ...form the dynamic behaviour of a vibrating system transfers or masks non-Gaussianity features of the input to the output response. Indeed, in several numerical and experimental activities performed on a Y-shaped specimen it was observed how the system response, both in terms of displacement or stress, changed according to an input variation (stationary and non-stationary Gaussian and non-Gaussian load time histories) and according to a change of the system frequency response function. Moreover, it was observed that even if the system was excited in its frequency range, the response remains unchanged and similar to the input in case of non-stationary and non-Gaussian load, removing preliminarily the possibility to use spectral methods for damage evaluation, going necessarily back to a more “expensive” time-domain analysis. Since the system response characteristics may change significantly according to the input excitation features and to the dynamic system parameters allowing, in some cases, the use of spectral techniques for fatigue damage evaluation also in case of non-Gaussian input loads, the aim of this paper is to understand whether and how the dynamic behaviour of a generic mechanical system transforms the non-Gaussian input excitations into a Gaussian response. To this aim several numerical displacement responses of 1-dof lumped systems characterized by different frequency response functions (resonance frequency position and damping) were analysed and investigated for different stationary and non-stationary Gaussian and non-Gaussian excitations. In such a way, it was possible to a-priori establish under what circumstances the frequency-domain approaches can be adopted to compute the fatigue damage of real mechanical systems.
This work is part of a research project funded by the Italian Ministry of the University and Research (MIUR), under the call for "National Interest Research Projects 2015 (PRIN 2015)", titled "Smart ...Optimized Fault Tolerant WIND turbines (SOFTWIND)".
Within this project, the research unit of the University of Perugia (UniPG) aims to develop dynamic modeling and simulation methodologies and fatigue behavior evaluation ones for wind turbine as a whole. The development of these methodologies will be aimed at predicting the life of generic wind turbines, also providing important and fundamental parameters for optimizing their control, aimed at reducing the failures of these machines.
In the present paper, a small turbine, developed at the Department of Engineering of the University of Perugia, will be analyzed. The multibody modeling technique adopted and the experimental activity conducted in the wind tunnel of UniPG, needed for the tuning of the model, will be described.
The analysis of both model behavior and experimental data has allowed for the definition of a robust multibody modeling technique that adopts a freeware code (NREL - FAST), universally considered to be a reference in this field.
The goodness of the model guarantees the capabilities of the simulation environment to analyze the real load scenario and the fatigue behavior of this kind of device.
This work is part of a research activity inserted into “Smart Optimazed Fault Tolerant WIND Turbines (SOFTWIND)” project of PRIN 2015, funded by the Italian Ministry of the University and Research ...(MIUR). The need to define a robust multibody modelling procedure to realistically characterize the dynamical behavior of a generic wind turbine and to have a reduced computational burden has pushed the authors to adopt a freeware software called Nrel-FAST, that is universally considered to be a reference in the field of aeroelastic wind turbine simulations. The lightness of this software is paid in terms of modelling simplicity, which makes the modelling of wind turbines with unconventional support structures (i.e. that con not directly outlined as a fixed-beam) difficult. In this paper, some methodologies to overcome this obstacle are presented, including the use of a more powerful multibody software which, on the other hand, entails higher simulation times. In particular, the authors present a methodology based on structure stiffness-matrix reconstruction that allows, under appropriate hypothesis, to reduce a complex wind turbine support frame to a simple fixed beam so that the simulations can be done directly in FAST environment, with low computational times. The results obtained from these different approaches are compared using as test-case a small wind turbine property of University of Perugia (UniPG).
In the present paper the authors, starting from a previously proposed method for the combination and the synthesis of equivalent load conditions (by only managing PSD representations of the load ...conditions), developed a new approach based on the concept of Fatigue Damage Spectrum and on the system dynamics.
The proposed approach was then validated by a durability test case, in which two different acceleration motion based load conditions, a norm load condition (by using laboratory test) and an operative one (by using acceleration measurements acquired during an experimental activity conducted on a transport vehicle) were compared.
A frequency domain approach to fatigue life evaluation is an important task in the design of mechanical components subject to random loads.
The greatest advantages of this method are the ...computational efficiency and the capability to give a synthetic representation of a random process. The importance of this method is clear, especially when it is used in the postprocessing phase of a numerical analysis, for example Finite Element (FEA) or Multibody (MBS) simulation. The weak step in this procedure is the direct evaluation of the probability density function (pdf) of the cycles amplitude from stress power spectral density (PSD). In fact, the translation to frequency domain of the Rainflow cycle counting is a serious problem but there is a body of important literature concerning this. After reviewing the most important literature from this research field, a new original index to compare the reliability of the frequency domain methods is proposed. This index efficiently achieves this purpose, and a future objective could be to use such index to define a correction factor in order to exactly evaluate the fatigue damage. A very important class of random processes was considered: the bimodal power spectral density (PSD) random processes. The aim of this paper is to identify independent parameters in these processes and through them evaluate the cycles probability density function (pdf) amplitude.
The suspension system of external stores of a military aircraft has reached the fatigue life limit estimated during the design phase, however, some elements suggest that the duration of the structure ...has been largely underestimated. This work aims to re-evaluate the fatigue life of the system and the potential extension of its use following an experimental numerical approach where, starting from a series of experimental flights in which the structure was instrumented, the forces acting on the system were calculated. Therefore, a methodology was developed to limit the damage calculation time using a hybrid approach that exploits the advantages deriving from the low computational burden typical of the methods in the frequency domain coupled with the Rainflow Counting precision. The study, although penalized by a series of conservative hypotheses, allowed to estimate a residual life equal to past life. It also provided important feedback on the field of application of advanced techniques for estimating the fatigue life of aeronautical structures subjected to random loading stories.
The evaluation of fatigue behavior of wind turbines, that is of supporting structures, blades or gear boxes, is always performed off-line, by post processing experimental acquisitions or simulation ...results. Moreover, the evaluation of potentiality of smart controls, that have the aim to avoid failures by reducing loads and consequently fatigue stresses, is performed in the same way.
In this paper is presented a tool that allows to on-line evaluate and foresight fatigue potential damage by simply on time processing reference signals such as tower top acceleration (typical experimental acquisition) or tower base bending moment (typical numerical measure).
This evaluation technique is converted into a well know numerical code, oriented to control systems (Simulink), to be used into multibody simulation by co-simulation approach. This step allowed to verify its capabilities and the possibility to realize its physical prototype and to use its results as input variable for active control strategies oriented to minimize damage.
As test case a standard 5 MW wind turbine and a classical control logic were used.
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