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### Research Papers

ASME J Nondestructive Evaluation. 2017;1(2):021001-021001-6. doi:10.1115/1.4038030.

In recent years, there is a much interest in developing of nondestructive testing (NDT) systems using the pulse-echo laser ultrasonics. The key idea is to combine a low-power and short-pulsewidth laser excitation with a continuous sensing laser; and use a scanning mechanism, such as five degrees-of-freedom (5DOF)-axis robot, laser mirror scanner, or motorized linear translation or rotation scanner stage, to scan the combined beam on the structure. In order to optimize the parameters of the excitation laser, a realistic theoretical model of the epicenter displacement in thermo-elastic regime is needed. This paper revisits and revises the study of Spicer and Hurley (1996, “Epicentral and Near Epicenter Surface Displacements on Pulsed Laser Irradiated Metallic Surfaces,” Appl. Phys. Lett., 68(25), pp. 3561–3563) on thermo-elastic model of epicenter displacement with two new contributions: first, we revised Spicer’s model to take into account the optical penetration effect, which was neglected in Spicer’s model; and second, the revised model was used to investigate the effect of laser rise time and beam size to the epicenter displacement. We showed that a pulse laser with short rise time generates an equivalent surface displacement with a pulse laser with long rise time, except a “spike” at the beginning of the epicenter waveform; also when the laser beam size increases, the epicenter displacement decreases. These two conclusions were then validated by experiments.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2017;1(2):021002-021002-11. doi:10.1115/1.4038116.

This paper addresses the predictive simulation of acoustic emission (AE) guided waves that appear due to sudden energy release during incremental crack propagation. The Helmholtz decomposition approach is applied to the inhomogeneous elastodynamic Navier–Lame equations for both the displacement field and body forces. For the displacement field, we use the usual decomposition in terms of unknown scalar and vector potentials, $Φ$ and $H$. For the body forces, we hypothesize that they can also be expressed in terms of excitation scalar and vector potentials, $A*$ and $B*$. It is shown that these excitation potentials can be traced to the energy released during an incremental crack propagation. Thus, the inhomogeneous Navier–Lame equation has been transformed into a system of inhomogeneous wave equations in terms of known excitation potentials $A*$ and $B*$ and unknown potentials $Φ$ and $H$. The solution is readily obtained through direct and inverse Fourier transforms and application of the residue theorem. A numerical study of the one-dimensional (1D) AE guided wave propagation in a 6 mm thick 304-stainless steel plate is conducted. A Gaussian pulse is used to model the growth of the excitation potentials during the AE event; as a result, the actual excitation potential follows the error function variation in the time domain. The numerical studies show that the peak amplitude of A0 signal is higher than the peak amplitude of S0 signal, and the peak amplitude of bulk wave is not significant compared to S0 and A0 peak amplitudes. In addition, the effects of the source depth, higher propagating modes, and propagating distance on guided waves are also investigated.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2017;1(2):021003-021003-8. doi:10.1115/1.4038577.

Composite materials are becoming ever more popular in an increasing number of applications. This because of their many advantages, amongst others the possibility to create a new material of given characteristics in a quite simple way by changing either the type of matrix, or reinforcement, and/or rearranging the reinforcement in a different way. Of course, once a new material is created, it is necessary to characterize it to verify its suitability for a specific exploitation. In this context, infrared thermography (IRT) represents a viable means since it is noncontact, nonintrusive, and can be used either for nondestructive evaluation to detect manufacturing defects, or fatigue-induced degradation, or else for monitoring the inline response to applied loads. In this work, IRT is used to investigate different types of composite materials, which involve carbon fibers embedded in a thermoset matrix and either glass or jute fibers embedded in a thermoplastic matrix, which may be neat, or modified by the addition of a percentage of a specific compatibilizing agent. IRT is used with a twofold function. First, for nondestructive evaluation, with the lock-in technique, before and after loading to either assure absence of manufacturing defects, or discover the damage caused by the loads. Second, for visualization of thermal effects, which develop when the material is subjected to impact. The obtained results show that it is possible to follow inline what happens to the material (bending, delamination, and eventual failure) under impact and get information, which may be valuable to deepen the complex impact damaging mechanisms of composites.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2017;1(2):021004-021004-12. doi:10.1115/1.4038630.

A major challenge in structural health monitoring (SHM) is to accurately identify both the location and severity of damage using the dynamic response information acquired. While in theory the vibration-based and impedance-based methods may facilitate damage identification with the assistance of a credible baseline finite element model, the response information is generally limited, and the measurements may be heterogeneous, making an inverse analysis using sensitivity matrix difficult. Aiming at fundamental advancement, in this research we cast the damage identification problem into an optimization problem where possible changes of finite element properties due to damage occurrence are treated as unknowns. We employ the multiple damage location assurance criterion (MDLAC), which characterizes the relation between measurements and predictions (under sampled elemental property changes), as the vector-form objective function. We then develop an enhanced, multi-objective version of the dividing rectangles (DIRECT) approach to solve the optimization problem. The underlying idea of the multi-objective DIRECT approach is to branch and bound the unknown parametric space to converge to a set of optimal solutions. A new sampling scheme is established, which significantly increases the efficiency in minimizing the error between measurements and predictions. The enhanced DIRECT algorithm is particularly suited to solving for unknowns that are sparse, as in practical situations structural damage affects only a small region. A number of test cases using vibration response information are executed to demonstrate the effectiveness of the new approach.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;1(2):021005-021005-14. doi:10.1115/1.4038731.

This investigation determined the effect of specimen out-of-plane movement on the accuracy of strain measurement made applying two-dimensional (2D) and three-dimensional (3D) measurement approaches using the representative, state-of-the-art digital image correlation (DIC)-based tool ARAMIS. DIC techniques can be used in structural health monitoring (SHM) by measuring structural strains and correlating them to structural damage. This study was motivated by initially undetected damage at low strains in connections of a real-world bridge, whose detection would have prevented its propagation, resulting in lower repair costs. This study builds upon an initial investigation that concluded that out-of-plane specimen movement results in noise in DIC-based strain measurements. The effect of specimen out-of-plane displacement on the accuracy of strain measurements using the 2D and 3D measurement techniques was determined over a range of strain values and specimen out-of-plane displacements. Based upon the results of this study, the 2D system could measure strains as camera focus was being lost, and the effect of the loss of focus became apparent at 1.0 mm beam out-of-plane displacement while measuring strain of the order of magnitude of approximately 0.12%. The corresponding results for the 3D system demonstrate that the beam out-of-plane displacement begins to affect the accuracy of the strain measurements at approximately 0.025% strain for all magnitudes of out-of-plane displacement, and the 3D ARAMIS system can make accurate strain measurements at up to 2.5 mm amplitude at this strain. Finally, based upon the magnitudes of strain and out-of-plane displacement amplitudes that typically occur in real steel bridges, it is advisable to use the 3D system for SHM of stiff structures instead of the 2D system.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;1(2):021006-021006-8. doi:10.1115/1.4038722.

Ultrasonic guided waves (GWs) are being extensively investigated and applied to nondestructive evaluation and structural health monitoring. Guided waves are, under most circumstances, excited in a frequency range up to several hundred kilohertz or megahertz for detecting defect/damage effectively. In this regard, numerical simulation using finite element analysis (FEA) offers a powerful tool to study the interaction between wave and defect/damage. Nevertheless, the simulation, based on linear/quadratic interpolation, may be inaccurate to depict the complex wave mode shape. Moreover, the mass lumping technique used in FEA for diagonalizing mass matrix in the explicit time integration may also undermine the calculation accuracy. In recognition of this, a time domain spectral element method (SEM)—a high-order FEA with Gauss–Lobatto–Legendre (GLL) node distribution and Lobatto quadrature algorithm—is studied to accurately model wave propagation. To start with, a simplified two-dimensional (2D) plane strain model of Lamb wave propagation is developed using SEM. The group velocity of the fundamental antisymmetric mode ($A0$) is extracted as indicator of accuracy, where SEM exhibits a trend of quick convergence rate and high calculation accuracy (0.03% error). A benchmark study of calculation accuracy and efficiency using SEM is accomplished. To further extend SEM-based simulation to interpret wave propagation in structures of complex geometry, a three-dimensional (3D) SEM model with arbitrary in-plane geometry is developed. Three-dimensional numerical simulation is conducted in which the scattering of $A0$ mode by a through hole is interrogated, showing a good match with experimental and analytical results.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;1(2):021007-021007-11. doi:10.1115/1.4038862.

This paper focuses on the development and validation of a robust framework for surface crack detection and assessment in steel pipes based on measured vibration responses collected using a network of piezoelectric (PZT) wafers. The pipe structure considered in this study contained multiple progressive cracks occurring at different locations and with various orientations (along the circumference or length). The fusion of data collected from multiple PZT wafers was investigated based on two approaches: (a) combining the raw data from all sensors before establishing a statistical model for damage classification and (b) combining the features from each sensor after applying a multiclass support vector machine recursive feature elimination (MCSVM-RFE), for dimensionality reduction, and taking the union of discriminative features among the different sources of data. A MCSVM learning algorithm was employed to train the data and generate a statistical classifier. The dataset consisted of ten classes, consisting of nine damage cases and the healthy state. The accuracy of the prediction based on the two fusion approaches resulted in a high accuracy, exceeding 95%, but the number of features needed to enrich the accuracy (95%) differed between the two approaches. Furthermore, the performance and the precision in the prediction of the classifier were evaluated when the data from only a single sensor was used compared with the combined data from all the sensors within the network. Very promising results in the classification of damage were obtained, based on the case study that included multiple damage scenarios with different lengths and orientations.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;1(2):021008-021008-8. doi:10.1115/1.4039230.

An in-process cure monitoring technique based on “guided wave” concept for carbon fiber reinforced polymer (CFRP) composites was developed. Key parameters including physical properties (viscosity and degree of cure) and state transitions (gelation and vitrification) during the cure cycle were clearly identified experimentally from the amplitude and group velocity of guided waves, validated via the semi-empirical cure process modeling software RAVEN. Using the newly developed cure monitoring system, an array of high-temperature piezoelectric transducers acting as an actuator and sensors were employed to excite and sense guided wave signals, in terms of voltage, through unidirectional composite panels fabricated from Hexcel® IM7/8552 prepreg during cure in an oven. Average normalized peak voltage, which pertains to the wave amplitude, was selected as a metric to describe the guided waves phenomena throughout the entire cure cycle. During the transition from rubbery to glassy state, the group velocity of the guided waves was investigated for connection with degree of cure, Tg, and mechanical properties. This work demonstrated the feasibility of in-process cure monitoring and continued progress toward a closed-loop process control to maximize composite part quality and consistency.

Commentary by Dr. Valentin Fuster
ASME J Nondestructive Evaluation. 2018;1(2):021009-021009-5. doi:10.1115/1.4039229.

Adhesive bonding, an effective joining technique for platelike structures in aircraft and automobiles, requires postbond inspection preferably with noncontact and single-sided access. The present study discusses the application of an imaging technique with a scanning laser source (SLS) to evaluate adhesive bonds in a platelike structure. When a laser Doppler vibrometer (LDV) is used as a receiver, the SLS technique realizes noncontact measurements with single-sided access. The imaging experiments that used narrowband burst waves and broadband chirp waves indicated that the imaging technique is appropriately applied to adhesive bonds and that the use of broadband chirp waves provides clearer images and reduces spurious images due to resonance. Furthermore, images of adhesive bonds were clearly obtained for a complex plate structure that consisted of a top-hat section and a flat plate, and this demonstrates that the imaging technique can be widely applied to evaluate various adhesive bonds.

Commentary by Dr. Valentin Fuster

### Technical Brief

ASME J Nondestructive Evaluation. 2018;1(2):024501-024501-3. doi:10.1115/1.4039477.

The purpose of this brief note is to present an alternative way of deriving the orthogonality relations for wave modes, by approaching the reciprocity relationship in direct notation, with the tools provided by tensor algebra and analysis. In this way, the classical result of elastodynamics is obtained through the instruments of continuum mechanics.

Topics: Waves , Tensors
Commentary by Dr. Valentin Fuster