Closed crack imaging using time reversal method based on fundamental and second harmonic scattering

A recent variant of time reversal imaging is employed for reconstructing images of a closed crack, based on both the fundamental and the second harmonic components of the longitudinal scattered field due to an incident longitudinal wave. The scattered field data are generated by a finite element model that includes unilateral contact with Coulomb friction between the crack faces to account for the Contact Acoustic Nonlinearity. The closure state of the crack is controlled by specifying a pre-stress between the crack faces. The knowledge of the scattered field at the fundamental (incident) frequency and the second harmonic frequency for multiple incident angles provides the required inputs for the imaging algorithm. It is shown that the image reconstructed from the fundamental harmonic closely matches the image that is obtained from scattering data in the absence of contact, although contact between the crack faces reduces the amplitude of the scattered field in the former case. The fundamental harmonic image is shown to provide very accurate estimates of crack length for low to moderate levels of pre-stress. The second harmonic image is also shown to provide acceptable estimates of crack length and the image is shown to correlate with the source location of second harmonic along the crack, which becomes increasingly localized near the crack tips for decreasing levels of pre-stress. The influence of the number of sensors on the image quality is also discussed in order to identify the minimum sensors number requirement. Finally, multiple frequency imaging is performed over a fixed bandwidth to assess the potential improvement of the imaging algorithm when considering broadband information

Demodulation technique to identify nonlinear characteristics of vibro-acoustic NDT measurements

A novel approach for nonlinear ultrasonics is presented which uses the demodulated response of a nonlinear vibro-acoustic signal. The demodulation process is similar to the quadrature demodulation process used in telecommunications. It is shown that the demodulation method provides an alternative approach for the analysis of nonlinear behavior compared to the analysis of frequency spectra or other techniques. The proposed method suggests that a representative reconstruction of the time-domain nonlinearity behavior of the system can be obtained. The paper provides a theoretical background to the technique as well as experimental validation. The proposed method is demonstrated for the analysis of four different types of contact acoustic nonlinear interfaces. It is shown that the demodulated waveforms are consistent with the expected mechanical behavior of the different interfaces. The results suggest that this method can provide more insight into the nonlinear behavior of a system compared to conventional spectral amplitude analysis techniques

Numerical and experimental study of the nonlinear interaction between a shear wave and a frictional interface

The nonlinear interaction of shear waves with a frictional interface are presented and modeled using simple Coulomb friction. Analytical and finite difference implementations are proposed with both in agreement and showing a unique trend in terms of the generated nonlinearity. A dimensionless parameter ξξ is proposed to uniquely quantify the nonlinearity produced. The trends produced in the numerical study are then validated with good agreement experimentally. This is carried out loading an interface between two steel blocks and exciting this interface with different amplitude normal incidence shear waves. The experimental results are in good agreement with the numerical results, suggesting the simple friction model does a reasonable job of capturing the fundamental physics. The resulting approach offers a potential way to characterize a contacting interface; however, the difficulty in activating that interface may ultimately limit its applicability

Numerical study of nonlinear interaction between a crack and elastic waves under an oblique incidence

A Finite Element (FE) model is proposed to study the interaction between in-plane elastic waves and a crack of different orientations. The crack is modeled by an interface of unilateral contact with Coulombs friction. These contact laws are modified to take into account a pre-stress σ0 that closes the crack. Using the FE model, it is possible to obtain the contact stresses during wave propagation. These contact stresses provide a better understanding of the coupling between the normal and tangential behavior under oblique incidence, and explain the generation of higher harmonics. This new approach is used to analyze the evolution of the higher harmonics obtained as a function of the angle of incidence, and also as a function of the excitation level. The pre-stress condition is a governing parameter that directly changes the nonlinear phenomenon at work at the interface and therefore the harmonic generation. The diffracted fields obtained by the nonlinear and linear models are also compared

Application of the noncollinear mixing method to an interface of contact

The non-collinear mixing technique is applied on a contacting interface with friction. Two shear waves are generated with an oblique incidence on the interface of contact. The nonlinear effect of contact is activated and leads to the interaction of the incident waves, which results in the scattering of a longitudinal wave of twice the input frequency. The method is applied experimentally on an interface made of two aluminum solids. The amplitude of the longitudinal wave is obtained as a function of the compressional stress applied on this interface. A Finite Element model is proposed to study this method. The interface is modeled by unilateral contact law with Coulomb's friction. This modeling enables to reproduce the generation of the longitudinal wave. The numerical results regarding to the amplitude of the longitudinal wave qualitatively agree with the experimental results

2D finite element modeling of the non-collinear mixing method for detection and characterization of closed cracks

The non-collinear mixing technique is applied for detection and characterization of closed cracks. The method is based on the nonlinear interaction of two shear waves generated with an oblique incidence, which leads to the scattering of a longitudinal wave. A Finite Element model is used to demonstrate its application to a closed crack. Contact acoustic nonlinearity is modeled using unilateral contact law with Coulomb׳s friction. The method is shown to be effective and promising when applied to a closed crack. Scattering of the longitudinal wave also enables us to image the crack, giving its position and size

Nonlinear interaction of ultrasonic waves with a crack of different orientations

The nonlinear interaction of shear waves with a crack of different orientations is presented. A Finite Element model is proposed to obtain the evolution of the generated nonlinearity when the orientation of the crack changes. The crack is modeled by an interface of unilateral contact with Coulomb's friction. These contact laws are modified in order to take into account a pre-stress s0 that closes the crack. Finally, the evolution of the higher harmonics is obtained and is related to the nonlinear contact behavior of the crack

Rational Design of Ultrasensitive Pressure Sensors by Tailoring Microscopic Features

Wearable sensors are increasingly used in a wide range of applications such as tactile sensors and artificial skins for soft robotics, monitoring human motions for wellbeing and sports performance, and pressure control of compression garments for wound healing. In this work, an ultrasensitive resistive pressure sensor based on conductive polydimethylsiloxane (PDMS) thin films with different microstructures is presented. These microscopic features include micropyramids, micro-semispheres, and micro-semicylinders which are created by soft lithography replication of 3D printing templates. To enable piezoresistivity, a thin layer of carbon nanofibers (CNFs) is spray-coated on the textured PDMS film. The resistance changes of the three microstructure designs under compression loading show that the micro-semicylinder-based sensor has the highest sensitivity of −3.6 kPa−1. Finite element modeling reveals that among the three designs, the micro-semicylinders show the largest change in contact area under the same pressure, consistent with the experimental results that the largest resistance change under the same pressure. This sensor is capable of detecting pressure as low as 1.0 Pa. This 3D printing technology is a promising fabrication technique to design microstructured piezoresistive layers, paving the way to tailor sensor performance by engineering their microstructures and to produce ultrasensitive pressure sensors at low cost