Mohamed Taktak

An indirect method for the characterization of locally reacting liners

An indirect technique for educing the homogenized acoustic impedance of a liner mounted on the wall of a barrel is presented. It is based on measurements and computational simulations of the multimodal scattering matrix of this lined duct. Measurements are performed with a multisource method and the use of an anechoic duct termination. The numerical computation of the scattering matrix relies on a finite element model, and assume that the duct is axisymmetric and uniformly covered by a locally reacting material. The impedance is educed by minimizing the difference between the theoretical and experimental acoustic power dissipations, which are deduced from the scattering matrix. The source is an incoming pressure field generated at one end of the duct only and composed of all cut-on modes. This technique was tested on a cylindrical barrel whose wall was partially lined with a realistic, locally reacting material made of honeycomb cells and a perforated facing sheet. Results for the acoustic impedance are compared with those deduced from semiempirical models and the experimental two microphone method. The best agreement with the indirect method is found with the semiempirical impedance results when the difference between the acoustic power dissipated by the actual lined barrel and the reference barrel is chosen as the cost function of the minimizing procedure.

One stage spur gear transmission crankcase diagnosis using the independent components method

The preventive diagnosis of mechanical systems presents a promising tool for increasing the production because it allows the industry to save time and money. In fact, the conventional diagnosis process is based on the FRF method which can be used to determine the inside defects frequencies of the system. Hence, this paper presents a new and a useful idea based on the blind source separation (BSS) which allow determining the wave form and the excitation frequency of internal defects present in a given mechanism by using only the structural vibratory response of the system. Numerical example of a one stage spur gear transmission crankcase including a spur gear pairs, shafts, rolling element bearings and a casing composed by three rigid plates and one flexible plate is presented and analysed in this paper. The separation results show a good agreement between original and estimated sources. This demonstrates the usefulness of the proposed method in the diagnosis process.

Numerical and Experimental Characterization of the Dynamic Properties of Flax Fiber Reinforced Composites

In this paper, the damping properties of flax fiber reinforced composites were investigated. Throughout a series of resonance vibration tests, the natural frequencies and the modal damping were evaluated. A numerical modelling was also produced by using a finite element model to determine the energies dissipated in each layer of the laminate structure in order to calculate the damping factors. The results obtained for the dynamic properties of flax fiber reinforced composites from experimental data and numerical analysis method show close agreement. The effect of fiber orientations on the damping behavior for this material was investigated. Another part of our work was to insert a thin viscoelastic layer within the flax fiber laminate. The interposition of this viscoelastic layer had a significant influence on the vibration behavior, bending stiffness and damping factors.

ESTIMATION OF DYNAMIC SYSTEM'S EXCITATION FORCES BY THE INDEPENDENT COMPONENT ANALYSIS

This paper deals with a numerical investigation for the estimation of dynamic systems excitation sources using the independent component analysis (ICA). In fact, the ICA concept is an important technique of the blind source separation (BSS) method. In this case, only the dynamic responses of a given mechanical system are supposed to be known. Thus, the main difficulty of such problem resides in the existence of any information about the excitation forces. For this purpose, the ICA concept, which consists on optimizing a fourth-order statistical criterion, can be highlighted. Hence, a numerical procedure based on the signal sources independency in the ICA concept is developed. In this work, the analytical or the finite element (FE) dynamic responses are calculated and exploited in order to identify the excitation forces applied on discrete (mass-spring) and continuous (beam) systems. Then, estimated results obtained by the ICA concept are presented and compared to those achieved analytically or by the FE and the modal recombination methods. Since a good agreement is obtained, this approach can be used when the vibratory responses of a dynamic system are obtained through sensors measurements.

A 3D Multiport Scattering Matrix Based-Method for Educing Wall Impedance of Cylindrical Lined Duct Section: Simulation and Error Evaluation

The first step to achieve the development of an original indirect method to educe the wall normalized acoustic impedance of a cylindrical lined duct section which includes frequency and modal content pressure field dependence is introduced. It is based on the minimization of the difference between numerical and experimental acoustic power dissipations deduced from the 3D numerical and experimental scattering matrices of a lined duct element. The work presented in this paper is a step toward conducting experiments with a flow duct facility developed during the European DUCAT program. To validate this eduction technique, a simulation of the experiment is performed for no flow conditions assuming an axi-symmetric wall lined with a locally reacting material whose impedance was measured with the two microphone method (TMM). The simulation conducted for two incident pressure vectors with a Monte Carlos technique also provides an assessment of the uncertainty in three predominant experimental parameters on the scattering matrix coefficients, the acoustic power dissipation, and the educed impedance whose results will be useful during the experiments being conducted.

Investigations on Crack Propagation and Strain Energy Release Rate in Notched Woven-Ply Thermoplastic Laminates at High-Temperature

The strain energy release rate G is of prime importance in composite materials fracture mechanics. In order to experimentally and numerically evaluate this parameter in the case of quasi-isotropic and angle-ply (AP) woven-ply thermoplastic (TP) laminates, single edge notched (SEN) specimens have been subjected to monotonic tensile loading at T > Tg when the toughness and the viscous behavior of the (TP) matrix are exacerbated. From the simulation standpoint, a particular attention was paid to the type of meshing as well as its refinement in the vicinity of the crack tip where the triaxiality rate leads to significant stress concentrations. For this purpose, a linear spectral viscoelastic and a generalized Norton-type viscoplastic models have been used. A comparison between two types of meshing (radiant and concentric) has been conducted. Both types of meshing allow us to define crowns in order to represent the surface of the integration ring around the crack tip. These crowns are necessary to evaluate the strain energy release rate GI in opening mode using Gθ-integral computation. Both overstress and overstrain profiles near the crack tip were investigated and validated using theoretical stress fields derived from the linear elastic fracture mechanics (LEFM) framework and overstrain fields obtained from digital image correlation (DIC) to verify the model’s ability to provide accurate mechanical fields at singularity zones.

Numerical Analysis of a Segmented Wind Turbine Blade Using the Substructure Method

The wind turbine blade segmentation development remains a tough challenge for constructors to reduce the blade manufacturing and transport cost. In this paper, numerical analysis is developed in order to study the dynamic characteristics of a segmented wind turbine blade assembled with a spar. The spar and the blade segments are assimilated to a shell structure with different homogeneous and isotropic materials. Accordingly, the three nodes triangular shell element DKT18 is adopted to model the blade. To reduce the problem size the Craig–Bampton substructure method is applied. This study covers the effects the of twist angle on the blade natural frequencies. To validate the accuracy and reliability of the proposed substructuring approach, numerical results obtained from the present study were compared with those determined by modal analysis using ABAQUS software. Results showed that the blade natural frequencies are not monotonically varying with respect to the twist angle which must be investigated to be well above the wind turbine system frequencies.

Flexural Fatigue of Bio-Based Composites

The paper presents an investigation of the mechanical fatigue behavior of a bio-based composite with an interleaved natural viscoelastic layer. The material used is a unidirectional laminate with different orientations. It is composed of a GreenPoxy resin reinforced with long flax fibers. A viscoelastic layer made of natural rubber has been introduced at the middle layer of the laminate. Different configurations of unidirectional specimens were tested in static tests. The mechanical properties such as the characteristics at failure of the structures with and without natural rubber were determined and compared. The effect of the insertion of the viscoelastic layer in the studied composite on the stiffness, hysteresis loops and loss factors were investigated during cyclic fatigue tests. A comparison of the fatigue behavior of the composite with and without viscoelastic layer was made. The results show that the composite with an interleaved natural viscoelastic layer presents an interesting damping properties considering the high values of the loss factors obtained from cyclic fatigue tests.

Investigation of Parameters Affecting the Acoustic Absorption Coefficient of Industrial Liners

Reducing the noise generated in the industrial environment becomes the main objective of industrial designers in many applications (building, transport industry, etc.). The passive control represents one of the used methods to achieve this objective. It is based on the use of acoustic absorbent materials like porous materials, Helmholtz resonators, or perforated plates, etc. In order to obtain high-quality acoustic absorbers, the acoustic performance of each element is used and sometimes coupled to increase the total acoustic absorption of the liner. In this study, three industrial acoustic liners are studied: porous material, porous material with perforated plate, and perforated plate with air cavity. The liner characteristic impedance is defined using the material and the plate impedances. The porous material impedance is formulated by two models: the Delany–Bazley and the Lafarge–Allard ones. A comparative study is elaborated in order to show the impact of plate and material parameters variation on the acoustic absorption coefficient.

Three Dimensional Simulation of the Flow Effect on the Acoustic Propagation inside Lined Duct

The flow presents an important factor influencing the acoustic propagation inside a duct element. In this paper, a three dimensional numerical method based on the convected Helmholtz equation was used to evaluate this effect in the case of cylindrical wall - lined - wall duct element by simulating the acoustic field inside a duct element presenting an impedance discontinuity. The flow effect was evaluated for several modes and Mach numbers. A comparison between acoustic fields inside the duct for several studied cases showed that the direction and the amplitude of the flow may affect on the attenuation of the acoustic pressure inside lined ducts.