In Guided Wave Structural Health Monitoring (GW-SHM), a strong need for reliable and fast simulation tools has been expressed throughout the literature in order to optimize SHM systems or demonstrate performance. Even though guided wave simulations can be conducted with most finite elements software packages, computational and hardware costs are always prohibitive for large simulation campaigns. A novel SHM module has been recently added to the CIVA software and relies on unassembled high order finite elements to overcome these limitations. This paper focuses on the thorough validation of CIVA for SHM to identify the limits of the models. After introducing the key elements of the CIVA SHM solution, a first validation is presented on a stainless steel pipe representative of the oil and gas industry. Second, validation is conducted on a composite panel with and without stiffener representative of some structures in the aerospace industry. Results show an excellent match between the experimental and simulated datasets, but only if the input parameters are fully determined prior to the simulations.

A novel failure model updating methodology is presented in this paper for composite materials. The innovation in the approach presented is found in both the experimental and computational methods used. Specifically, a dominant bottleneck in data-driven failure model development relates to the types of data inputs that could be used for model calibration or updating. To address this issue, nondestructive evaluation data obtained while performing mechanical testing at the laboratory scale are used in this paper to form a damage metric based on a series of processing steps that leverage raw sensing inputs and provide progressive failure curves that are then used to calibrate the damage initiation point computed by full-field three-dimensional finite element simulations of fiber-reinforced composite material that take into account both intra- and interlayer damage. Such curves defined based on nondestructive evaluation data are found to effectively monitor the progressive failure process, and therefore, they could be used as a way to form modeling inputs at different length scales.

Recently published experimental works on remotely bonded fiber Bragg grating (FBG) ultrasound (US) sensors show that they display some unique characteristics that are not observed with directly bonded FBG sensors. These studies suggest that the bonding of the optical fiber strongly influences how the ultrasound waves are coupled from the structure to the FBG sensor. In this paper, the analytical model of the structure-adhesive-optical fiber section, treated as an ultrasound coupler, is derived and analyzed to explain the observed experimental phenomena. The resulting dispersion curve shows that the ultrasound coupler possesses a cutoff frequency, above which a dispersive longitudinal mode exists. The low propagation speed of the dispersive longitudinal mode leads to multiple resonances at and above the cutoff frequency. To characterize the resonant characteristics of the ultrasound coupler, a semi-analytical model is implemented and the scattering parameters (S-parameters) are introduced for broadband time-frequency analysis. The simulation was able to reproduce the experiment observations reported by other researchers. Furthuremore, the behaviors of the remotely bonded FBG sensors can be explained based on its resonant characteristics.

Timber poles are widely used in electricity transmission and telecommunication sectors throughout the world. The stress wave propagation for the condition assessment of timber poles is identified as a promising non-destructive testing (NDT) technique due to its simplicity and cost-effectiveness compared to other traditional methods. In this paper, a novel damage severity evaluation criterion for timber poles is proposed on the basis of short-time wavelet entropy of the reflected stress waves. The stress waves are generated by transverse impacts close to the ground level of the pole. The reflected stress waves are recorded and processed in the time frequency domain using the discrete wavelet transform. The decomposed signal components using discrete wavelet analysis are used to determine the wavelet entropy. The wavelet entropies of intact and damaged poles are compared to obtain the relative wavelet entropy (RWE) for damage severity estimation. Further, a numerical model for an in situ pole system is developed to simulate the transverse stress wave propagation and to evaluate the capability of the proposed defect severity estimation method. The developed numerical model is validated with experimental data from controlled testing and the data from field tests. The validated numerical model is then used to simulate different defect scenarios. The wavelet entropy is sensitive to the damage severity in timber poles and can be used as an effective tool to evaluate the severity of damages.

A validated analytical model of a transmit–receive coil pair situated above two parallel plates, separated by an air gap, was used as the basis for an inversion algorithm (IA) to extract probe liftoff, second layer plate resistivity, and plate-to-plate gap from multi-frequency eddy current data. The IA was tested over a large range of first layer wall thickness (3.80–4.64 mm), second layer plate resistivity (1.7–174 µΩ cm), second layer wall thickness (1.20–4.85 mm), probe liftoff (2.8–7.9 mm), and plate-to-plate gap (0–13.3 mm). At nominal liftoff (2.8 mm), the IA achieved a gap measurement accuracy of ±0.7 mm and was able to return good estimates of the second layer resistivity within ±1 μΩ cm for low resistivity samples, but with decreasing accuracy for higher resistivities. When the gap was fixed, the IA was able to measure changes in probe liftoff (relative to nominal) to an accuracy of ±0.2 mm. The reported accuracy and a demonstration for the ability to accurately estimate parameters outside of the calibration range provide confidence in the potential utility of the algorithm.

Composites are being increasingly used in various industries due to their lower cost and superior mechanical properties over traditional materials. They are nevertheless vulnerable to various defects during manufacturing or usage which can cause failure of critical engineering structures. Hence, there is a growing need for nondestructive evaluation (NDE) of composites to detect such defective structures and avoid significant loss and damages. Microwave NDE has several advantages over other existing NDE techniques for detecting defects or faults in non-conducting composites or dielectrics. One of the primary benefits of microwaves is large probe-standoff distances which allow for rapid scan times. However, the resolution of such far-field microwave sensors is diffraction limited. Metamaterial-based lens, also known as “superlens,” can achieve resolution beyond the diffraction limits due to its unique electromagnetic (EM) properties. This contribution focuses on the physical design of a metamaterial lens. The theory underlying the design of a metamaterial lens is presented followed by simulation and experimental results. This paper also investigates the feasibility of using the metamaterial lens for improving the resolution of microwave imaging in NDE of composites.

This paper presents a modeling and simulation method for studying ultrasonic guided wave propagation in hybrid metal-composites, also known as fiber-metal laminates. The objective is to develop an efficient and versatile modeling tool to aid in the design of cost-effective nondestructive evaluation technologies. The global–local method, which combines finite element discretization and Lamb wave modal expansion is used. An extension to the traditional global–local method is made to couple the source problem with the scattering problem to deal with a surface source generating Lamb waves that interact with defects in multilayered structures. This framework is used to study the sensitivity of different excitation frequencies to ply gap defects of various sizes. The coupled model considers the transducer contact conditions and the ultrasonic system response in the Lamb wave excitation, along with the scattering phenomenon caused by the defects. This combined result is used to define the optimal excitation frequency for the strongest transmission or reflection for a given defect size that can be observed in a physical experiment. Such results can be applied to the design of a damage detection scheme in realistic aerospace structures.

Robust defect detection in the presence of grain noise originating from material microstructures is a challenging yet essential problem in ultrasonic non-destructive evaluation (NDE). In this paper, a novel method is proposed to suppress the gain noise and enhance the defect detection and imaging. The defect echo and grain noise are distinguished through analyzing the spatial location where the echo is originating from. This is achieved by estimation of the angle of arrival (AOA) of the returned echo and evaluation of the likelihood that the echo is reflected from the point where the array is focused or otherwise from the random reflectors like the grain boundaries. The method explicitly addresses the statistical models of the defect echoes and the spatial noise across the array aperture, as well as the correlation between the flaw signal and the interfering echoes; estimates the AOA and the likelihood in a dimension-reduced beam space via a linear transformation; and determines a weighting factor based on the mean likelihood. The factors are then normalized and utilized to correct and weigh the NDE images. Experiments on industrial samples of austenitic stainless steel and INCONEL Alloy 617 are conducted with a 5 MHz transducer array, and the results demonstrate that the grain noise is reduced by about 20 dB while the defect reflection is well retained, thus the great benefits of the method on enhanced defect detection and imaging in ultrasonic NDE are validated.

Active microwave thermography (AMT) is an integrated nondestructive testing (NDT) technique that utilizes a microwave-based thermal excitation and subsequent thermal measurement. AMT has shown potential for applications in the transportation, infrastructure, and aerospace industries. This paper investigates the potential of AMT for detection of defects referred to as flat-bottom holes (FBHs) in composites with high electrical conductivity such as carbon fiber-based composites. Specifically, FBHs of different dimensions machined in a carbon fiber reinforced polymer (CFRP) composite sheet are considered. Simulation and measurement results illustrate the potential for AMT as a NDT tool for inspection of CFRP structures. In addition, a dimensional analysis of detectable defects is provided including a radius-to-depth ratio threshold for successful detection.

Oil and gas pipelines traverse long distances and are often subjected to mechanical forces that result in permanent distortion of its geometric cross section in the form of dents. In order to prioritize the repair of dents in pipelines, dents need to be ranked in order of severity. Numerical modeling via finite element analysis (FEA) to rank the dents based on the accumulated localized strain is one approach that is considered to be computationally demanding. In order to reduce the computation time with minimal effect to the completeness of the strain analysis, an approach to the analytical evaluation of strains in dented pipes based on the geometry of the deformed pipe is presented in this study. This procedure employs the use of B-spline functions, which are equipped with second-order continuity to generate displacement functions, which define the surface of the dent. The strains associated with the deformation can be determined by evaluating the derivatives of the displacement functions. The proposed technique will allow pipeline operators to rapidly determine the severity of a dent with flexibility in the choice of strain measure. The strain distribution predicted using the mathematical model proposed is benchmarked against the strains predicted by nonlinear FEA. A good correlation is observed in the strain contours predicted by the analytical and numerical models in terms of magnitude and location. A direct implication of the observed agreement is the possibility of performing concise strain analysis on dented pipes with algorithms relatively easy to implement and not as computationally demanding as FEA.

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, T g , 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.

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 ( A 0 ) 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 A 0 mode by a through hole is interrogated, showing a good match with experimental and analytical results.

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.