Igor Simonovski

Damping identification using a continuous wavelet transform: application to real data

A continuous wavelet transform (CWT) based on the Gabor wavelet function is used to identify the damping of a multi-degree-of-freedom system. The common procedures are already known, especially the identification with a Morlet CWT. This study gives special attention to the following: a description of the instantaneous noise, the edge-effect of the CWT, the frequency-shift of the CWT, the bandwidth of the wavelet function and the selection of the parameter σ of the Gabor wavelet function of the CWT. The procedures are demonstrated using several numerical examples and on signals acquired from the lateral vibration of a uniform beam. The study demonstrates the advantages of using the amplitude and phase methods, both of which provide information about the instantaneous noise. The procedures presented are appropriate for automating the identification process.

The norms and variances of the Gabor, Morlet and general harmonic wavelet functions

Abstract This paper deals with certain properties of the continuous wavelet transform and wavelet functions. The norms and the spreads in time and frequency of the common Gabor and Morlet wavelet functions are presented. It is shown that the norm of the Morlet wavelet function does not satisfy the normalization condition and that the normalized Morlet wavelet function is identical to the Gabor wavelet function with the parameter σ=1. The general harmonic wavelet function is developed using frequency modulation of the Hanning and Hamming window functions. Several properties of the general harmonic wavelet function are also presented and compared to the Gabor wavelet function. The time and frequency spreads of the general harmonic wavelet function are only slightly higher than the time and frequency spreads of the Gabor wavelet function. However, the general harmonic wavelet function is simpler to use than the Gabor wavelet function. In addition, the general harmonic wavelet function can be constructed in such a way that the zero average condition is truly satisfied. The average value of the Gabor wavelet function can approach a value of zero but it cannot reach it. When calculating the continuous wavelet transform, errors occur at the start- and the end-time indexes. This is called the edge effect and is caused by the fact that the wavelet transform is calculated from a signal of finite length. In this paper, we propose a method that uses signal mirroring to reduce the errors caused by the edge effect. The success of the proposed method is demonstrated by using a simulated signal.

Fault Detection in DC Electro Motors Using the Continuous Wavelet Transform

Two time–frequency methods were used to detect typical faults in DC electro motors: the windowed Fourier transform and the continuous wavelet transform. Four groups containing three electro motors each were manufactured with typical faults and examined. These faults included a bearing fault, an increased unbalance, a fragmented brush and a fragmented collector. The velocity of the vibrations at selected points on the electro motors was measured with a laser probe. The parameters of both transforms were selected in order to make both methods comparable. Because of the poor frequency variance, the windowed Fourier transform was, in this case, proven to be inferior to the continuous wavelet transform. Therefore, the continuous wavelet transform was chosen as the primary tool for fault detection.Three criteria were found that successfully discriminated between the typical faults. These were the highest magnitude level, the frequency of the first and second harmonics and the time period between the magnitude pulses in the third (highest) frequency region. If the maximum magnitude levels versus the period of the pulses in the third frequency range are plotted, four distinct regions corresponding to four different faults are obtained. Since the regions do not overlap, linear classifiers can be used with the presented criteria.

Towards Modelling Intergranular Stress-Corrosion Cracks Using Experimentally Obtained Grain Topologies

Predicting the effects of material aging in view of development of intergranular damage is of particular importance in a number of nuclear installations and especially in structural integrity assessments of critical components in energy generating power plants. Since the damage is initialized on small length scales, detailed multiscale models should be employed to tackle the problem. However, the complexity of such models is high due to the need of incorporating microstructural features. In line of this the research group from Jozef Stefan Institute and The University of Manchester joined forces and knowledge in development of such detailed multiscale models. The basic idea was to pair the knowledge of advanced experimental techniques of The University of Manchester group with the knowledge of advanced microstructure modelling techniques of the group at Jozef Stefan Institute. The presented paper proposes a novel approach for intergranular crack modelling whereby a state-of-the-art X-ray diffraction contrast tomography technique is used to obtain 3D topologies and crystallographic orientations of individual grains in a stainless steel wire and intergranular stress corrosion cracks. As measured topologies and orientations of individual grains are then reconstructed within a finite element model and coupled with advanced constitutive material behaviour: anisotropic elasticity and crystal plasticity. Due to the extreme complexity of grain topologies, transferring this information into the finite element model presents a challenging task. The feasibility of the proposed approach is presented. Difficulties in building a finite element model are discussed. Preliminary results of the analyses are also given.Copyright

The Influence of the Grain Structure Size on Microstructurally Short Cracks

The dominant processes in the initialization and propagation of microstructurally short cracks include microstructural features such as crystallographic orientations of grains, grain boundaries, inclusions, voids, material phases, etc. The influence of the microstructural features is expected to vanish with distance from the crack tip. Also, the influence of the nearby microstructural features is expected to be smaller for a long than for a small crack. Finally, a crack of sufficient length can be modeled using classical fracture mechanic methods. In this paper the approach to estimate the crack length with vanishing influence from the microstructural feature is proposed. To achieve this, a model containing a large number of randomly sized, shaped, and oriented grains is employed. The random grain structure is modeled using a Voronoi tessellation. A series of cracks of lengths from about 1 to 7 grain lengths is inserted into the model, extending from a grain at the surface toward the interior of the model. The crack tip opening displacements are estimated and statistically analyzed for a series of random crystallographic orientation sets assigned to the grains adjacent to the crack. Anisotropic elasticity and crystal plasticity constitutive models are employed at the grain size scale. It is shown that the standard deviation of the crack tip opening displacement decreases from about 20% for a short surface crack embedded within a single grain to about 7% for a surface crack extending through seven grains. From the engineering point of view, a crack extending through less than about ten grain sizes is therefore considered to strongly depend on the neighboring microstructural features.

Bispectral analysis of the cutting process

The dynamics of three different cutting regimes were compared using the bicoherence estimates. For two regimes corresponding to cutting with strong and weak chatter, several significant interactions between spectral components are found, suggesting the presence of quadratic type nonlinearities in the dynamics of chatter. By contrast, the dynamics of the chatter-free cutting as the third regime, reflect almost no such interactions. The bicoherence analysis on simultaneously measured time series of three orthogonal components of the main cutting force showed that practically the same information is contained in all three of them.

Finite element models of progressive failure of grain boundaries

Integranular cracking is among the most notable material degradation processes in operating nuclear power plants. Examples include intergranular stress corrosion cracking of austenitic steels and nickel based alloys and irradiation assisted stress corrosion cracking of stainless steel baffle bolts. Recent advances in computational methods and computational resources facilitated development of multiscale computational methods with increasing degrees of realism, stemming mostly from the explicit treatment of randomly shaped and oriented anisotropic grains. Such methods can simulate the polycrystalline aggregates of a few hundred or even thousand grains and are receiving increasing support from the non-destructive experimental techniques such as for example X-ray diffraction contrast tomography. Further development of increasingly realistic multiscale simulation methods requires resolution of some modeling issues. These include reliable, accurate and numerically efficient modeling of the progressive damage along the grain boundaries. Critical appraisal of a number of possible approaches to model the progressive damage along the grain boundaries represents the core of the proposed paper.Copyright

Some Useful Tests for the Finite Element Meshes of Polycrystals With Explicit Account of the Grains and Grain Boundaries

A growing number of computational material science and computational mechanics research is currently devoted to the explicit modeling of microstructures at various length and time scales. The finite element models of grains and grain boundaries in polycrystals include discretization of the grain interior. In addition, grain boundaries are explicitly discretized as cohesive zones with appropriate damage properties to facilitate the simulation of intergranular cracking. Such finite element models may easily involve hundreds of grains and millions of finite elements. They may also be combined with advanced lattice orientation dependent constitutive models, such as for example anisotropic elasticity and crystal plasticity. The complexity of the model, including the random lattice orientations, may therefore represent a serious difficulty in detecting possible issues in the finite element model and the interpretation of the results. A number of self-consistency model-checks are therefore needed to verify the model. Two tests are proposed and demonstrated in the paper. The first is aiming at the assessment of the finite element mesh quality within the grains in terms of the results. The second is primarily aiming at the verification of the consistent modeling of the cohesive layer at the grain boundaries. In addition, some useful information about the finite element mesh quality in terms of results is also given.Copyright

Evolution of Crystal Orientations in Plastically Deformed Steels: Role of Constitutive Models Used in Finite Element Simulations

Stainless steel is a commonly used material in safety-important components of nuclear power plants. In order to study degradation mechanisms in stainless steels, like crack initiation and propagation, it is important to characterize the degree of plastic strain on microstructural level. One way to estimate local plastic strain is by measuring local crystal orientations of the scanned surfaces: the electron backscatter diffraction (EBSD) measurements on stainless steel revealed a strong correlation between the spread of crystal orientations within the individual grains and the imposed macroscopic plastic strain. Similar behavior was also reproduced by finite element simulations where stainless steel was modeled by an anisotropic elasto-plastic constitutive model. In that model the anisotropic Hill’s plasticity function for yield criteria was used and calibrated against the EBSD measurements and macroscopic tensile curve.In this work the Hill’s phenomenological model is upgraded to a more sophisticated crystal plasticity model where plastic deformation is assumed to be a sum of crystalline slips in all activated slip systems. The hardening laws of Peirce, Asaro and Needleman and of Bassani and Wu are applied in crystal plasticity theory and implemented numerically within the user subroutine in ABAQUS. The corresponding material parameters are taken from literature for 316L stainless steel. Finite element simulations are conducted on the analytical Voronoi tessellation with 100 grains and initial random crystallographic orientations. From the simulations, crystal and modified crystal deformation parameters are calculated, which quantify mean and median spread of crystal orientations within individual grains with respect to central grain orientation. The results are compared to EBSD measurements and previous simulations performed with Hill’s plasticity model.© 2013 ASME

Stratified Flows in Pipes: A Parametric Study Towards Fatigue Relevance

Stratified flows may form in pipelines under certain conditions and could lead to increased fatigue loading that was only marginally accounted for during the design phase of the second generation of nuclear power plants designed in accordance with the ASME Boiler and Pressure Vessel Code. Extension of operational license would require explicit account for fatigue loads imposed by stratified flows. This typically involves rather complex state-of-the-art computational technology, which may in some cases be combined with measurements of the temperatures at the outside surfaces of pipes, which comprise pressure boundary of the reactor coolant. A parametric study using detailed finite element analysis of the entire span of the pipe has been performed to quantify the possible range of fatigue loads and fatigue usage factors. The example taken was a typical pressurized water reactor pressurizer surge line containing stratified flow of cold and hot water. The investigated parameters include the film coefficients governing the heat transfer from the both fluids to the pipe wall and the velocity of the interface between the cold and hot water. The main results include the expected ranges of fatigue loading given the range of investigated parameters. It is clearly shown that the choice of the film coefficients is essential to arrive at reliable fatigue estimate. Additionally, predictions of readings provided by hypothetical thermocouples at the pipe outer surface are provided. Some of their limitations are identified and discussed.Copyright