Lukasz Pieczonka

Impact damage detection in light composite sandwich panels using piezo-based nonlinear vibro-acoustic modulations

The nonlinear vibro-acoustic modulation technique is used for impact damage detection in light composite sandwich panels. The method utilizes piezo-based low-frequency vibration and high-frequency ultrasonic excitations. The work presented focuses on the analysis of modulation intensity. The results show that the method can be used for impact damage detection reliably separating damage-related from vibro-acoustic modulations from other intrinsic nonlinear modulations.

Nonlinear vibroacoustic wave modulations for structural damage detection: an overview

Abstract. We present an overview of research developments related to the nonlinear vibroacoustic modulation technique used for structural damage detection. The method of interest is based on nonlinear interactions of a low-frequency pumping wave and a high-frequency probing wave. These two waves are introduced to monitored structures simultaneously. Then the presence of damage is exhibited by additional frequency components that result from nonlinear damage-wave interactions. A vast amount of research has been performed in this area over the last two decades. We aim to present the state-of-the-art of these developments. The major focus is on monitoring approaches, modeling aspects, actuation/sensing, signal processing, and application examples.

Numerical simulations for impact damage detection in composites using vibrothermography

Composite materials are widely used in many engineering applications due to their high strength-to-weight ratios. However, it is well known that composites are susceptible to impact damage. Detection of impact damage is an important issue in maintenance of composite structures. Various non-destructive image-based techniques have been developed for damage detection in composite materials. These include vibrothermography that detects surface temperature changes due to heating associated with frictional energy dissipation by damage. In the present paper numerical simulations are used to investigate heat generation in a composite plate with impact damage in order to support damage detection analysis with vibrothermography. Explicit finite elements are used to model ultrasonic wave propagation in the damaged plate. Simulated delamination and cracks induce frictional heating in the plate. Coupled thermo-mechanical simulations are performed in high frequencies using commercial LS-Dyna finite element code. Very good qualitative agreement between measurements and simulations has been obtained. The area of increased temperature corresponds very well with the damaged area in both experiments and simulations. Numerical model has to be further refined in order to quantitatively match the experiments. The main issues of concern are frictional and thermal properties of composites. The final goal of these research efforts is to predict damage detection sensitivity of vibrothermography in real engineering applications based on numerical models.

Assessment of dynamic behaviour of spot welds with uncertain parameters using genetic algorithms application

Since the topics concerning quantifying dynamic behaviour of different mechanical systems are still of engineers’ concern, there is a significant need of developing new numerical approaches dealing with this research area. While discussing those issues, problems of effective modelling of uncertainties can not be neglected. One should know what is the influence of product variability and uncertainty of design parameters on natural frequencies and normal modes of considered systems. In the paper, dynamic behaviour of spot weld joints is studied. This way of metal linking is mostly used in automotive industry and it is very important to find out as much as possible about vibrations of car body in order to constantly improve comfort of travelling [1]. Finite element method is used for spot welds modelling. A several different variants of model displacements are considered. Different model meshes are used in order to quantify the convergences of the solutions. As far as modelling of uncertainties is concerned, a number of varying parameters are taken into account. Thicknesses of joined shell parts as well as material parameters are assumed to be uncertain. Positions of spot welds and their sizes are also not constant and can change their values randomly within specified ranges. There is a quite large set of existing methods which allow to solve dynamic problems and to introduce uncertainties into models. Some of them are being still developed like the transformation method and new approaches appear as combinations of existing ones. In the paper, genetic algorithms application is being tested as the main tool for mentioned research area. It is based on the optimization of global mass and stiffness matrices of mechanical system [2]. The first natural frequency is evaluated and its relationship with the ranges of uncertain parameters is discussed. The reference results are obtained with Monte Carlo Simulation approach, the vertex method, the transformation method and genetic algorithms used directly for optimizing the first natural frequency. Advantages and drawbacks of tested method are presented and conclusions about its applicability for different types of mechanical structures are described.

Efficient swept sine chirp excitation in the non-linear vibro-acoustic wave modulation technique used for damage detection

In this article, the non-linear vibro-acoustic modulation technique is used for structural damage detection. A new experimental configuration and data processing strategy are proposed to improve the damage detection capability of the technique. The swept sine chirp excitation is used for both low-frequency vibration/modal and high-frequency ultrasonic excitations. The adaptive resampling procedure is then applied to extract information about modulation intensity that relates to damage. The proposed method is illustrated using numerical simulations and experimental tests. The latter involves crack detection in an aluminium beam. The results of the proposed method are compared with the classical approach based on single harmonic excitation, demonstrating that similar damage detection information can be extracted. However, the major advantage of the proposed method is simplicity and robustness since no a priori selection of excitation frequencies is needed. As a result, crack detection is more reliable and unambiguous.

Enhanced nonlinear crack-wave interactions for structural damage detection based on Lamb waves

The paper presents a novel damage detection method that combines Lamb wave propagation with nonlinear acoustics. Low-frequency excitation is used to modulate Lamb waves in the presence of fatigue cracks. The work presented shows that the synchronization of the interrogating high-frequency Lamb wave with the low-frequency vibration is a key element of the proposed method. The main advantages of the proposed method are the lack of necessity for baseline measurements representing undamaged condition and lack of sensitivity to temperature variations. Numerical simulations and experimental measurements are performed to demonstrate the application of the proposed method to detect fatigue crack in aluminum beam.

Fatigue crack detection using nonlinear vibro-acoustic cross-modulations based on the Luxemburg-Gorky effect

This paper investigates the nonlinear cross-modulation vibro-acoustic technique for fatigue crack detection in metallic structures. The method is used in an aluminium plate instrumented with low-profile piezoceramic transducers that are used for excitation. Laser vibrometry is used to acquire vibro-acoustic responses. The results demonstrate the modulation transfer from one excitation signal to the other excitation signal in the presence of crack in the plate. The work presented focuses on the analysis of modulation intensities. The paper demonstrates that the method can be used for fatigue crack detection in metallic structures.

Bayesian parameter identification of orthotropic composite materials using Lamb waves dispersion curves measurement

This paper deals with the problem of elastic constant identification in thin plates made of orthotropic composite materials. The approach is based on the analysis of Lamb wave propagation and the related dispersion curves to find the underlying material elastic constants. In the proposed implementation a scanning laser Doppler vibrometer is used to measure Lamb wave dispersion curves. The Local Interaction Simulation Approach is used simultaneously to find a solution to a high-frequency wave propagation problem. The experimental and simulated data are combined in a Bayesian framework for parameter identification which is robust in condition of parameter, modeling and measurement uncertainty. The results are discussed and compared with the results from a deterministic optimization.

Wideband excitation in nonlinear vibro-acoustic modulation for damage detection

The paper discusses the use of wideband excitation in nonlinear vibro-acoustic modulation technique (VAM) used for damage detection. In its original form, two mono-harmonic signals (low and high frequency) are used for excitation. The low frequency excitation is typically selected based on a modal analysis test and high frequency excitation is selected arbitrarily in the ultrasonic frequency range. This paper presents a different approach with use of wideband excitation signals. The proposed approach gives the possibility to simplify the testing procedure by omitting the modal test used to determine the value of low frequency excitation. Simultaneous use of wideband excitation for high frequency solves the ambiguity related to the selection of the frequency of acoustic wave. Broadband excitation signals require, however, more elaborate signal processing methods to determine the intensity of modulation for a given bandwidth. The paper discusses the proposed approach and the related signal processing procedure. Experimental validation of the proposed technique is performed on a laminated composite plate with a barely visible impact damage that was generated in an impact test. Piezoceramic actuators are used for vibration excitation and a scanning laser vibrometer is used for noncontact data acquisition.

Nonlinear Vibro-acoustic Interactions for Crack Detection in Beams

Recent years have shown a growing interest in damage detection methods based on damage-related nonlinearities in structures. In the paper, the numerical simulations of two techniques – classical Higher Harmonic generation and Vibro-Acoustic Modulation - are presented and validated experimentally. Two-dimensional models of beam with non-propagating cracks are investigated. The focus is on bi-linear breathing crack behavior, which was modeled as a contact between crack faces. This phenomenon is known as crack breathing. Numerical simulation results show that nonlinear phenomena investigated are particularly strong in the vicinity of the crack. A number of aluminum beam specimens with different crack lengths and locations are investigated experimentally to validate numerical simulations. Signal responses were measured at several positions using piezoceramic sensors. The results show that nonlinear phenomena investigated depend not only on excitation amplitude – as expected – but also on crack position, sensor location and boundary conditions. doi: 10.12783/SHM2015/181