The paper presents a damage characterization framework based on a simulation library and matching pursuit algorithm to estimate damage features in typical aerospace structures. The large damage database is generated by numeric simulation. The recent development of the University of Michigan’s Local Interaction Simulation Approach (UM/LISA) is an ideal tool for generating such a damage database in a very efficient manner. It includes capability for piezoelectric coupled field simulation, non-reflective boundary techniques, and contact penalty method for nonlinear guided wave simulation, and can execute on multiple-GPU platform for fast computation. The selected damage identification problem in the paper is modeled as contact interface and simulated in UM/LISA using the contact penalty method. The process first populates a library of possible damage signals using UM/LISA by varying damage parameters, such as crack length, depth and orientation. The matching pursuit decomposes the damage difference signals into atoms and the atom parameters are used as signal features. Then the algorithm evaluates a matching merit metric and its special distribution provides parametric regions of damage presence. A representative model of fatigue cracks on aluminum plate considering various crack scenarios is investigated to test the effectiveness of the algorithm. The relation between the crack features and signal features provides better understanding of the nonlinear interactions between the guided waves and fatigue cracks. The matching quality plots demonstrate that the framework can provide good estimation of the crack parameters.
A novel d36-type piezoelectric wafer fabricated from lead magnesium niobate-lead titanate (PMN-PT) is explored for the generation of in-plane horizontal shear waves in plate structures. The study focuses on the development of a linear phased array (PA) of PMN-PT wafers to improve the damage detection capabilities of a structural health monitoring (SHM) system. An attractive property of in-plane horizontal shear waves is that they are nondispersive yet sensitive to damage. This study characterizes the directionality of body waves (Lamb and horizontal shear) created by a single PMN-PT wafer bonded to the surface of a metallic plate structure. Second, a linear PA is designed from PMN-PT wafers to steer and focus Lamb and horizontal shear waves in a plate structure. Numerical studies are conducted to explore the capabilities of a PMN-PT-based PA to detect damage in aluminum plates. Numerical simulations are conducted using the Local Interaction Simulation Approach (LISA) implemented on a parallelized graphical processing unit (GPU) for high-speed execution. Numerical studies are further validated using experimental tests conducted with a linear PA. The study confirms the ability of an PMN-PT phased array to accurately detect and localize damage in aluminum plates.
Local interaction simulation approach (LISA) is a highly parallelizable numerical scheme for guided wave simulation in structural health monitoring (SHM). This paper addresses the issue of simulating wave propagation in unbounded domain through the implementation of non-reflective boundary (NRB) in LISA. In this study, two different categories of NRB, i.e., the non-reflective boundary condition (NRBC) and the absorbing boundary layer (ABL), have been investigated in the parallelized LISA scheme. For the implementation of NRBC, a set of general LISA equations considering the effect from boundary stress is obtained first. As a simple example, the Lysmer and Kuhlemeyer (L-K) model is applied here to demonstrate the easiness of NRBC implementation in LISA. As a representative of ABL implementation, the LISA scheme incorporating the absorbing layers with increasing damping (ALID) is also proposed, based on elasto-dynamic equations considering damping effect. Finally, an effective hybrid model combining L-K and ALID methods in LISA is developed, and guidelines for implementing the hybrid model is presented. Case studies on a three-dimensional plate model compares the performance of hybrid method to that of L-K and ALID acting independently. The simulation results demonstrate that best absorbing efficiency is achieved with the hybrid method.
This paper presents a parallelized modeling technique for the efficient simulation of nonlinear ultrasonics introduced by the wave interaction with fatigue cracks. The elastodynamic wave equations with contact effects are formulated using an explicit Local Interaction Simulation Approach (LISA). The LISA formulation is extended to capture the contact-impact phenomena during the wave damage interaction based on the penalty method. A Coulomb friction model is integrated into the computation procedure to capture the stick-slip contact shear motion. The LISA procedure is coded using the Compute Unified Device Architecture (CUDA), which enables the highly parallelized supercomputing on powerful graphic cards. Both the explicit contact formulation and the parallel feature facilitates LISA’s superb computational efficiency over the conventional finite element method (FEM). The theoretical formulations based on the penalty method is introduced and a guideline for the proper choice of the contact stiffness is given. The convergence behavior of the solution under various contact stiffness values is examined. A numerical benchmark problem is used to investigate the new LISA formulation and results are compared with a conventional contact finite element solution. Various nonlinear ultrasonic phenomena are successfully captured using this contact LISA formulation, including the generation of nonlinear higher harmonic responses. Nonlinear mode conversion of guided waves at fatigue cracks is also studied.
This paper presents a hybrid modeling technique for the efficient simulation of guided wave propagation and interaction with damage in composite structures. This hybrid approach uses a local finite element model (FEM) to compute the excitability of guided waves generated by piezoelectric transducers, while the global domain wave propagation, wave-damage interaction, and boundary reflections are modeled with the local interaction simulation approach (LISA).
A small-size multi-physics FEM with non-reflective boundaries (NRB) was built to obtain the excitability information of guided waves generated by the transmitter. Frequency-domain harmonic analysis was carried out to obtain the solution for all the frequencies of interest. Fourier and inverse Fourier transform and frequency domain convolution techniques are used to obtain the time domain 3-D displacement field underneath the transmitter under an arbitrary excitation. This 3-D displacement field is then fed into the highly efficient time domain LISA simulation module to compute guided wave propagation, interaction with damage, and reflections at structural boundaries. The damping effect of composite materials was considered in the modified LISA formulation. The grids for complex structures were generated using commercial FEM preprocessors and converted to LISA connectivity format. Parallelization of the global LISA solution was achieved through Compute Unified Design Architecture (CUDA) running on Graphical Processing Unit (GPU). The multi-physics local FEM can reliably capture the detailed dimensions and local dynamics of the piezoelectric transducers. The global domain LISA can accurately solve the 3-D elastodynamic wave equations in a highly efficient manner. By combining the local FEM with global LISA, the efficient and accurate simulation of guided wave structural health monitoring procedure is achieved. Two numerical case studies are presented: (1) wave propagation in a unidirectional CFRP composite plate; (2) wave propagation in a stiffened cross-ply CFRP plate with delamination.
The local interaction simulation approach (LISA), a finite difference based numerical method, has been proven to be efficient in modeling guided wave (GW) propagation in isotropic and composite laminated structures. Recently, the LISA framework has been augmented to incorporate the piezoelectric material directly in the formulation so to more accurately model the transducer effects in the GW generation. This paper presents a study to assess the importance of the actuation modeling from surface-mounted piezoelectric actuators in LISA. Actuation modeling includes the prescribed displacements (either in plane or out of plane) that are commonly found in the literature, as well as the direct modeling of the piezoelectric material of the actuator with prescribed electric potentials. The study is carried out both for isotropic and composite laminated substrates. Numerical and experimental results are used to characterize the quality of the actuator modeling options.
This paper considers the effects of various damage features on guided wave (GW) propagation in isotropic and composite plates using both the local interaction simulation approach (LISA) and experimental methods. First, through-thickness holes in isotropic plates and graphite-epoxy laminates are simulated to establish LISA's ability to capture the GW scattering effects of various hole sizes and positions. GW generation from piezo-ceramic wafers is modeled using the recently developed LISA hybrid approach. The LISA results for the cross-ply case are compared with experimental measurements to evaluate the quality of the simulation. Next, low-velocity impact damage in composite plates is simulated and experimentally characterized. Barely-visible impact damage from a drop-weight fixture is analyzed using laser vibrometry and surface-mounted sensors to quantify its effect on GW fields. Three different methods of simulating the resulting impact are demonstrated using LISA, and the results are compared with the experiment to evaluate the damage modeling techniques. Results from the through thickness hole study show the effect of sensor position on the strength of damage signals. Impact damage is successfully modeled in LISA using an inverted V-shaped profile with reductions in shear and elastic moduli.
Recently there has been an increased utilization of composite structures in aerospace and other industries due to their superior physical attributes compared to traditional metallic structures. This has spurred the need for structural health monitoring (SHM) systems to support structural integrity. Guided wave (GW) based techniques for health monitoring have shown to be reliable and promising. The local interaction simulation approach (LISA), a finite difference based numerical method, has been proven to be efficient in modeling GW propagation in isotropic and composite plate structures. Piezoelectric actuators are traditionally used to generate GW in structures. In this work, iterative equations which form the basis of the LISA method are derived for a generic orthotropic laminated structure with a piezoelectric actuator on top. The piezoelectric actuator is modeled by considering the coupled electromechanical equations.
Composite structures are being extensively used in modern industrial applications due to their superior physical
attributes, thus necessitating the need for structural health monitoring (SHM) systems to ensure their structural integrety.
Guided wave (GW)-based methods are an obvious choice because of their ability to travel long distances through the
thickness of the composite structures. In this work, local interaction simulation approach (LISA), a finite difference
(FD)-based numerical method, is used to study the GW propagation characteristics in laminated composite plates. The
iterative equations, which form the core of the LISA method, have been derived for orthotropic materials with in-plane
rotation. Simulation results for uni-ply and quasi-isotropic graphite/epoxy laminates are used to demonstrate the
capabilities of the advanced equations.
Composite structures are being extensively used in the modern industries because of their superior strength to weight
ratio, high stiffness, and long fatigue life. The ability to tailor the material properties along different directions also
increases the avenues of composites material application. The ever-increasing demand for composite structures and the
need to ensure the structural integrity necessitates the development of sustainable and efficient structural health
monitoring (SHM) systems. Guided wave (GW) methods offer an attractive solution for SHM due to their tunable
sensitivity to different defects and their ability to interrogate large structural surfaces. Because of the anisotropy present
in the composite materials, the development of the SHM methods is significantly more complex and challenging than in
the case of isotropic materials. This paper presents numerical simulations based on the local interaction simulation
approach (LISA) to characterize the propagation of GW in laminated composite plates.
The increasing use of composite materials in multiple engineering applications has emphasized the need for structural
health monitoring (SHM) technologies capable of detecting, locating, and classifying structural defects in these
materials. Guided wave (GW) methods offer an attractive solution for SHM due to their tunable sensitivity to different
defects and their ability to interrogate large structural surfaces. The complications associated with the material
anisotropy and directionality in composites result in an increased need for accurate and efficient simulation tools to
characterize GW excitation and propagation in these materials. This paper presents a theoretical model based on three-dimensional
elasticity to characterize GW excitation by finite-dimensional transducers in composite laminates. The theory uses an eigenbasis expansion for a bulk transversely isotropic material combined with Fourier transforms, the global matrix approach, and residue theory to find the displacement field excited by an arbitrarily shaped finite-dimensional transducer. Experimental results obtained in a cross-ply composite laminate are used to assess the accuracy of the theoretical solution.
KEYWORDS: Transducers, Structural health monitoring, Sensors, Electrodes, Composites, Manufacturing, Microsoft Foundation Class Library, Damage detection, Semiconducting wafers, Epoxies
Structural health monitoring (SHM) is the component of damage prognosis systems responsible for interrogating a
structure to detect, locate, and identify any damage present. Guided wave (GW) testing methods are attractive
for this application due to the ability of GWs to travel over long distances with little attenuation and their
sensitivity to different damage types. The Composite Long-range Variable-direction Emitting Radar (CLoVER)
transducer is introduced as an alternative concept for efficient damage interrogation and GW excitation in GW-based
SHM systems. This transducer has an overall ring geometry, but is composed of individual wedge-shaped
sectors that can be individually excited to interrogate the structure in a particular direction. Each wedge-shaped
sector is made with piezoelectric fibers embedded in an epoxy matrix surrounded by an interdigitated
electrode pattern. The multiple advantages over alternative transducer concepts are examined. In particular, it
is shown that the geometry of each sector yields actuation amplitudes much larger than those obtained for a ring
configuration under similar electric inputs. The manufacture and characterization procedures of these devices
are presented, and it is shown that their free strain performance is similar to that of conventional piezocomposite
transducers. Experimental studies of damage detection simulating the proposed damage interrogation approach
are also presented.
KEYWORDS: Sensors, Structural health monitoring, Temperature metrology, Actuators, Aluminum, Space operations, Transducers, Data modeling, Epoxies, Lab on a chip
Large thermal variations can cause significant changes in guided-wave (GW) propagation and transduction for
structural health monitoring (SHM). This work focuses on GW SHM using surface-bonded piezoelectric wafer
transducers in metallic plates for the temperature range encountered in internal spacecraft structures (20°C to 150°C).
First, studies done to determine a suitable bonding agent are documented. That was then used in controlled experiments
to examine changes in GW propagation and transduction using PZT-5A piezoelectric wafers under quasi-statically
varying temperature (also from 20°C to 150°C). Modeling efforts to explain the experimentally observed increase in
time-of-flight and change in sensor response amplitude with increasing temperature are detailed. Finally, these results
are used in detection and location of mild and moderate damage using the pulse-echo GW testing approach within the
temperature range.
Signal processing algorithms for guided wave pulse echo-based SHM must be capable of isolating individual reflections from defects in the structure, if any, which could be overlapping and multimodal. In addition, they should be able to estimate the time-frequency centers, the modes and individual energies of the reflections, which would be used to locate and characterize defects. Finally, they should be computationally efficient and amenable to automated processing. This work addresses these issues with a new algorithm employing chirplet matching pursuits followed by a mode correlation check for single point sensors. Its theoretical advantages over conventional time-frequency representations in all aspects are elaborated and these are demonstrated using numerical simulations and experiments in isotropic plate structures. The issue of in-plane triangulation is then discussed and experimental work done to explore this issue is presented. This work concludes with a description of how the algorithm can be extended to composite plate structures.
KEYWORDS: Actuators, Sensors, Structural health monitoring, 3D modeling, Transducers, Fourier transforms, Composites, Microsoft Foundation Class Library, Aerospace engineering, Waveguides
This work addresses the 3-D elasticity modeling of the guided wave (GW) fields excited by piezoelectric actuators in various configurations for isotropic structures. First, a general derivation for the GW field excited by an arbitrary shape, finite dimension, and surface-bonded piezo actuator in isotropic plates is presented. This is then used to generate solutions for the specific cases of ring-shaped and rectangular piezo actuators and rectangular Macro Fiber Composite actuators. An expression for the response of a piezo-sensor in a GW field is developed. Experimental verification supporting the model is provided. Excellent correlation is found between theoretical and experimental results.
Among the various schemes being considered for Structural Health Monitoring (SHM), Lamb-wave testing has shown great promise. While Lamb-wave testing using hand-held transducers for Non Destructive Evaluation (NDE) is a well-established technology, Lamb-wave testing for SHM using surface-bonded/embedded piezos is a relatively new field. Little effort has been made towards a precise characterization of Lamb-wave excitation using piezos and often the various parameters involved are chosen without mathematical foundation. In this work, modeling of transient plane and circular-crested Lamb-wave generation and sensing using surface-bonded piezos in isotropic plates based on the 3-D linear elasticity equations is explored. Equations for the output voltage response of surface-bonded piezo-sensors in Lamb-wave fields are presented and optimization of the actuator/sensor geometry and materials is done based on those. Finally, numerical and experimental results to examine the validity of these models are discussed.
The twist actuation of piezocomposite actuators embedded in a composite wing is numerically investigated. Parametric analysis of the actuation authority is conducted for wing cross sections with double and triple cells, considering different distributions of anisotropic piezocomposite actuators. The variational asymptotic beam cross-sectional (VABS) analysis is used to compute the airfoil stiffness, actuation force and mass properties. As a result, the regions with the highest specific actuation are determined and a cost-effective way of adding active material to the cross section is proposed. Results indicate that 50% of the maximum mass penalty associated with the addition of active plies is responsible for generating approximately 80% of the maximum available induced twist.
Cost effective and reliable damage detection is critical for the utilization of composite materials in structural applications. Non-destructive evaluation techniques (e.g. ultrasound, radiography, infra-red imaging) are available for use during standard repair and maintenance cycles, however by comparison to the techniques used for metals these are relatively expensive and time consuming. This paper presents part of an experimental and analytical survey of candidate methods for the detection of damage in composite materials. The experimental results are presented for the application of modal analysis techniques applied to rectangular laminated graphite/epoxy specimens containing representative damage modes, including delamination, transverse ply cracks and through-holes. Changes in natural frequencies and modes were then found using a scanning laser vibrometer, and 2-D finite element models were created for comparison with the experimental results. The models accurately predicted the response of the specimems at low frequencies, but the local excitation and coalescence of higher frequency modes make mode-dependent damage detection difficult and most likely impractical for structural applications. The frequency response method was found to be reliable for detecting even small amounts of damage in a simple composite structure, however the potentially important information about damage type, size, location and orientation were lost using this method since several combinations of these variables can yield identical response signatures.
A finite-element based analysis for modeling active composite beams with embedded anisotropic actuation is presented. It is derived from three-dimensional electroelasticity, where the original problem is reduced via the variational asymptotic method. The resulting cross-sectional analysis takes into consideration passive and active anisotropic and nonhomogeneous materials, and represents general (thin-walled, thick-walled, solid) cross-sectional geometries. The formulation requires neither the costly use of 3-D finite element discretization nor the loss of accuracy inherent to any simplified representation of the cross section. The developed formulation is numerically implemented in VABS-A, and several numerical and experimental tests cases are used to support validation of the proposed theory. Also, the effect of the presence of a core in originally hallow configurations is presented and counter-intuitive conclusions are discussed. The generality of the method and accuracy of the results increase confidence at the design stage that the active beam structure will perform as expected and, consequently, should lower costs from experimental tests and further adjustments.
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