Joanna Iwaniec

Nonlinear acoustics for fatigue crack detection – experimental investigations of vibro-acoustic wave modulations

Vibro-acoustic nonlinear wave modulations are investigated experimentally in a cracked aluminum plate. The focus is on the effect of low-frequency vibration excitation on modulation intensity and associated nonlinear wave interaction mechanisms. The study reveals that energy dissipation – not opening–closing crack action – is the major mechanism behind nonlinear modulations. The consequence is that relatively weak strain fields can be used for crack detection in metallic structures. A clear link between modulations and thermo-elastic coupling is also demonstrated, providing experimental evidence for the recently proposed non-classical, nonlinear vibro-acoustic wave interaction mechanism.

Output-Only Technique for Parameter Identification of Nonlinear Systems Working under Operational Loads

Vast majority of industrial mechanical systems do not behave in linear way around any operating point and work under non-measurable operational loads. Identification of such systems can’t be carried out by means of the classical nonlinear system identification methods. The paper concerns the method combining restoring force, boundary perturbation and direct parameter estimation techniques that, on the contrary to classical methods, requires neither input measurement nor linear behaviour of the considered system around an operating point. There are also presented results of method application to parameter identification of machine shaft flexible support.

Recurrence Plot and Recurrence Quantification of the Dynamic Properties of Cross-Shaped Laminated Energy Harvester

The paper examines the dynamic properties of bistable cross-shaped laminate plates for broadband energy harvesting applications by converting mechanical vibration energy into the electrical power output. Bistable laminates plates coupled to piezoelectric transducers were excited by application of harmonic excitations and exhibited a range of vibration patterns. The vibration patterns included single-well oscillations and snap-through vibrations of both periodic and chaotic character; such vibration patterns led to a different power output. Classical spectral analysis of measured voltage, displacement and velocity time histories indicated the presence of a variety of nonlinear and chaotic phenomena. As a result, an analysis of the measured displacement and voltage time histories was carried out with the use of the Recurrence Plots and the Recurrence Quantification Analysis methods. The Recurrence Plots method was used for detection of qualitative changes in the dynamic behaviour of the non-linear harvesting system. In order to facilitate interpretation of piezoelectric voltage and laminate displacement, a detailed analysis using Recurrence Plots, Recurrence Quantification Analysis was employed.

Output-Only Identification of Vibratory Machine Suspension Parameters under Exploitational Conditions

The paper concerns model based identification of vibratory machine body suspension system on the basis of dynamic responses measured under exploitational conditions. The research was carried out by means of the restoring force, boundary perturbation and direct parameter estimation techniques which, on the contrary to classical nonlinear system identification methods, requires neither excitation measurements nor linear behaviour of the considered system around an operating point. At the first stage of the research, parameters of the machine body suspension system were identified. Results accuracy was verified by determining percentage relative error of mass estimation with respect to the value calculated based on the machine geometrical and material properties. In the next step, the suspension system was modified by introduction of a nonlinear damping system. Obtained results proved that the assumed identification method is convenient for vibratory machine suspension condition monitoring and determining forces transferred on machine foundations.

Experimental verification of selected methods sensitivity to damage size and location

The main emphasis of the paper is put on the experimental verification and comparison of classical modal analysis techniques and recurrence plots sensitivity to damage size. Identification experiments were carried out for the laboratory object subjected to random and chirp excitations, respectively. In the course of carried out experiments, the process of damage propagation was simulated by the successive drilling into one of the object elements. Measured time histories of system responses were analyzed with the application of the classical modal analysis, recurrence plots (RP), cross recurrence plots (CRP) and joint recurrence plots (JRP) methods. Obtained results proved that the RP, CRP and JRP methods are much more sensitive to changes in dynamical system properties resulting from damage initialization and propagation than classical modal analysis methods and can be successfully applied to damage detection and tracking changes in the system natural frequencies.

Non-Invasive Methods of Delamination Monitoring in Multi-Layer Structures

In the paper there are discussed the most interesting methods dedicated to frescos structural health monitoring. In the further part two methods and devices for fresco delamination detection developed by authors are presented. In order to enable future automation of developed measurement techniques and autonomic detection of fresco structural defects, the novel measures quantifying the probability of critical condition appearance (the lack of adhesive force) were introduced. Finally, the results of measurements of dynamic characteristics of healthy and delaminated structures are presented.

Methods for Modal Model Quality Improvement — Parameter Estimation Process (part II):

This paper concerns quality improvement of modal models estimated by the use of “standard” modal analysis methods. The authors proposed application of the Least Squares Frequency Domain method making use of the Gauss—Newton algorithm to the quality improvement of stabilization diagrams as well as application of the Balanced Realization method to identify and remove spurious poles from stabilization diagrams. Formulated algorithms were implemented in the Matlab environment and verified for data measured on real structures. Created software supports critical stages of modal analysis, which results in quality improvement of estimated modal models and higher credibility of drawn conclusions.

Methods for Modal Model Quality Improvement — Measurement Data Pre-processing for Model Identification Process (part I)

The paper concerns techniques of modal model quality improvement at the stage of data pre-processing for the purposes of identification process. Basic concepts of problem formulation (matrix conditioning) improvement by the use of the Tikhonov regularization method are presented as well as examples of method application to solution estimation of the Fredholm integral equation of the first kind and to transfer function matrix noise reduction. In the second part of the paper the authors proposed the method for transfer function estimation on the basis of data measured by the use of impulse test on big industrial systems for which classical methods fail for the reason of low coherence between exciting force and system response. Two examples of method application are presented.

Transverse vibrations of transmission tower of variable geometrical structure

The research concerns analysis of transverse vibrations of power line transmission tower of variable cross section (changing on its length). Such constructions are subjected simultaneously to internal and external loads, which results in transverse and longitudinal vibrations. These vibrations are described by two partial differential equations of distributed parameters depending on two independent variables. If vibrations are small, the terms connecting equations of transverse and longitudinal vibrations can be neglected as infinitely small with respect to other quantities. Consequently, each vibration can be considered separately.

Application of Recurrence-Based Methods to Heart Work Analysis

Currently, for the purposes of recorded ECG signals interpretation, the classical methods involving analysis of geometrical properties of the recorded waveforms in time domain are used. Such an analysis consists in determining the values of parameters describing the heart rate variability. However, these indicators can not be treated as an infallible criterion for diagnosis and, moreover, the limits of increasing the accuracy of ECG analysis by increasing the accuracy of finding its characteristic points have already been reached. Therefore, in the paper, for the purposes of analysis of registered ECG signals and acoustical recordings of heart work, it is proposed to use the recurrence plots and RQA analysis methods that consist in searching for the recurrence properties of the registered signals. Application of the recurrence-based methods is natural due to the cyclic character of the heart work while providing patterns characteristic for different cardiac dysfunctions supported by objective, quantitative measures will contribute to early, credible and reliable classification of cardiovascular dysfunction.