Nonlinear Elastic Wave Spectroscopy Methods Research Articles

Non-contact Non-linear Impact Resonance Acoustic Spectroscopy for thermal damage characterization on mortar samples

Abstract The high sensitivity of the nonlinear terms of the elastic response of materials to the incipient appearance of damage has led to the appearance of the Non-linear Elastic Wave Spectroscopy methods (NEWS). Particularly, the Non-linear Impact Resonance Acoustic Spectroscopy (NIRAS) technique detects changes in the resonance of a material (frequency, damping factor, etc.) as a function of the intensity of the impact. Traditionally, mechanical waves have been monitored using an external sensor in contact with the material. Currently, alternative technologies are capable of capturing mechanical waves without direct contact or with embedded sensors in the material itself from its manufacture. This helps the automation process or the continuous monitoring of the material. Mortar is the most used composite material in construction. In this work, thermal damage on mortar samples is characterized by NIRAS tests using different sensing techniques. Accelerometers have been used as the reference technique, while laser interferometer and Fiber Bragg Grating (FBG) have been used as non-contact and embedded sensor techniques respectively. Two levels of damage are presented: sound samples and 400ºC damage. The different techniques offer similar results, showing the capability of the proposed techniques (FBG and laser interferometry) to be an accurate alternative to traditional contact techniques for NIRAS testing.

Localization and classification of scattered nonlinear ultrasonic signatures in bio-mechanical media using time reversal approach.

This paper deals with the time reversal approach along with signal classification using ϕ-divergences in biomedical applications for localization and statistical classification of ultrasonic nonlinearities. The time reversal (TR) approach in combination with nonlinear elastic wave spectroscopy (NEWS) is used to obtain the nonlinear signature of air bubbles with different sizes and ultrasound contrast agents in a liquid. An optimized chirp-coded signal in the range of 0.6-3 MHz is used as a compression coding. The signal classification is performed using the fuzzy classification method and the divergence decision tree algorithm using specific ϕ-divergence spectral measures extracted from the received ultrasonic response containing acoustic nonlinearities. The classification results prove that different types of nonlinearities extracted with classical "pulse inversion" based coding methods can be identified. Simultaneously, the different positions of scattered sources are distinguished by ϕ-divergence methods. The potential of time reversal nonlinear elastic wave spectroscopy methods for understanding of ultrasonic wave propagation in complex media is clearly exhibited.

Comparison of Ultrasonic Methods for Thermally Damaged Concrete Nondestructive Testing

Behaviour of concrete under elevated temperatures is very complex. There is a change of mechanical and physical parameters with temperature. In this paper we study the relations of thermal damage processes in concrete and parameters obtained by different ultrasonic methods. The concrete specimens were heated in programmable laboratory furnace. Selected temperature (200°C, 400°C, 600°C, 800°C, 1000°C and 1200°C) were maintained for 60 minutes. The first ultrasonic measurement technique in this paper was Ultrasonic Pulse Velocity method. The pulse velocity in a concrete depends on its density and its elastic properties. Therefore, it is possible to deduce the quality and the compressive strength of the concrete from the ultrasonic pulse velocity. The second ultrasonic measurement technique in this paper uses broadband pulse-compression signal, with variable amplitude to measure the change of fundamental frequency. This method is based on Nonlinear Elastic Wave Spectroscopy. Nonlinear Elastic Wave Spectroscopy methods takes advantage of the fact, that nonlinearities in material manifest themselves as a resonant frequency shifts and harmonics or dumping coefficients changes. The progress of nondestructive testing parameters was confirmed by results from the destructive tests.

Application of the chaotic cavity transducer concept for imaging in non-reverberant media.

The concept of chaotic cavity transducer utilizes a combination of a piezoelectric ceramic disk glued on a cavity of chaotic shape on the hardware side, with the time reversal (or inverse filter) technique on the software side. Using a two step procedure of direct excitation-reception and time reversed excitation-reception, it is possible to focus energy anywhere inside a non-reverberating sample, thanks to the ergodicity of the chaotic cavity. Moreover, the same basic concept can be used to create a virtual phased array using a single channel device. Both experimental data and simulations will be provided to illustrate the concepts. The goal is to use the chaotic cavity transducer concept to enhance the localization of micro-damage coupled to nonlinear elastic wave spectroscopy methods.

On the use of a chaotic cavity transducer in nonlinear elastic imaging

We discuss the advantages of “chaotic cavity transducer focusing” to enhance the localization of microdamage in conjunction with nonlinear elastic wave spectroscopy methods. Chaotic cavity transducer focusing is defined as the hardware-software combination of a piezoelectric ceramic glued on a cavity of chaotic shape on the one hand with the reciprocal time reversal (or inverse filter) technique on the other hand. Additional optimization through the use of sweeps and inverse filtering techniques are discussed as well. The technique is applied to image a crack in a steel sample.

Understanding the effect of boundary conditions on damage identification process when using non-linear elastic wave spectroscopy methods

This paper presents an experimental campaign aimed at understanding the limitations and capabilities of non-linear elastic wave spectroscopy (NEWS) non-destructive technique (NDT) methods in the presence of variable boundary conditions. In particular, the objective was to understand if the contact between the structures under investigation and the clamps used to hold the structures could generate non-classical non-linear effects that could affect the damage detection process by producing false-positive indications of defects presence. Two different techniques were analysed with varying degree clamping torque. The first approach evaluates the resonance frequency shift as a function of the external load amplitude, and it is called non-linear resonant ultrasound spectroscopy (NRUS). The second method used, called non-linear wave modulation spectroscopy (NWMS), monitors the generation of sidebands and harmonics when the structure is excited by a double tone external load. The results showed that the non-classical hysteretical non-linear effects were dependent on the boundary conditions, highlighted by the presence of resonance shift and harmonics and sidebands in an undamaged sample. This research shows that more work is needed to demonstrate the effectiveness of the methods and the ease of implementation in a structural health monitoring system and further research studies and methodology development are needed to discern non-classical non-linear effects generated by contacts between mating parts (clamps and sample) from that generated due to the presence of damage.