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OBJECTIVE Perfusion imaging identifies anterior circulation stroke patients who respond favorably to endovascular thrombectomy (ET), but its role in basilar artery occlusion (BAO) is unknown. We hypothesized that BAO patients with limited regions of severe hypoperfusion (time to reach maximum concentration in seconds [Tmax] > 10) would have a favorable response to ET compared to patients with more extensive regions involved. METHODS We performed a multicenter retrospective cohort study of BAO patients with perfusion imaging prior to ET. We prespecified a Critical Area Perfusion Score (CAPS; 0-6 points), which quantified severe hypoperfusion (Tmax > 10) in cerebellum (1 point/hemisphere), pons (2 points), and midbrain and/or thalamus (2 points). Patients were dichotomized into favorable (CAPS ≤ 3) and unfavorable (CAPS > 3) groups. The primary outcome was a favorable functional outcome 90 days after ET (modified Rankin Scale = 0-3). RESULTS One hundred three patients were included. CAPS ≤ 3 patients (87%) had a lower median National Institutes of Health Stroke Scale score (NIHSS; 12.5, interquartile range [IQR] = 7-22) compared to CAPS > 3 patients (13%; 23, IQR = 19-36; p = 0.01). Reperfusion was achieved in 84% of all patients, with no difference between CAPS groups (p = 0.42). Sixty-four percent of reperfused CAPS ≤ 3 patients had a favorable outcome compared to 8% of nonreperfused CAPS ≤ 3 patients (odds ratio [OR] = 21.0, 95% confidence interval [CI] = 2.6-170; p 3 patients had a favorable outcome, regardless of reperfusion. In a multivariate regression analysis, CAPS ≤ 3 was a robust independent predictor of favorable outcome after adjustment for reperfusion, age, and pre-ET NIHSS (OR = 39.25, 95% CI = 1.34->999, p = 0.04). INTERPRETATION BAO patients with limited regions of severe hypoperfusion had a favorable response to reperfusion following ET. However, patients with more extensive regions of hypoperfusion in critical brain regions did not benefit from endovascular reperfusion. ANN NEUROL 2021.

BACKGROUND Acute promyelocytic leukemia (APL) has long been associated with coagulation disorders. The proposed mechanism is a combination of fibrinolysis, proteolysis, platelet dysfunction, thrombocytopenia, and possibly disseminated intravascular coagulation. Hemorrhagic complications are prominent. OBSERVATIONS In this case, a 25-year-old female with newly diagnosed APL developed extensive cerebral venous thrombosis (CVT) and was initiated on a protocol with idarubicin and all-trans retinoic acid. The general recommendation for treating CVT is anticoagulation to stabilize the existing thrombus and prevent propagation. The patient was initiated on a heparin drip, but her clinical course was complicated by subdural hemorrhage (SDH) and epidural hemorrhage in the setting of thrombocytopenia. Anticoagulation was held, and her CVT propagated on follow-up imaging. To restart anticoagulation for CVT with a limited risk of SDH, the authors pursued middle meningeal artery (MMA) embolization. The patient was transitioned to apixaban and discharged to home. LESSONS MMA embolization enables safe anticoagulation in patients with concomitant CVT and SDH. The authors report the complex clinical course and effective management of this rare clinical scenario.

Background and Purpose: The novel coronavirus of 2019 (COVID-19) has been associated with worse outcomes for hospitalized patients across the US and worldwide. Our study investigates the effect of the pandemic on functional outcomes for patients presenting with stroke-like symptoms in two high-volume Tele-Stroke networks. Methods: We performed retrospective chart review of 7689 consecutive patients seen through two Tele-Stroke networks (Medical University of South Carolina and Augusta University) from July 2019 through May 2020. Discharge outcomes were assessed using modified Rankin scale (mRS). We dichotomized patients to preCOVID-19 pandemic (July 2019 to February 2020) and during COVID-19 pandemic (March to May 2020). Results: During COVID, a higher percentage of patients received intravenous tissue plasminogen activator (tPA) (14.2% vs 11.3% before, p=0.001), despite a similar baseline NIHSS. Also, patients during the pandemic had a shorter symptom-onset to hospital door time (288 vs 353, P=0.012). Demographics were similar between groups, other than older age in patients presenting during COVID (66.02 vs 65.15 before, p=0.04) (table 1). During the pandemic, a higher percentage of patients had a poor functional outcome, as defined by mRS 3-6 (35.7% vs 30% before, p=0.012), and a higher percentage died or were discharged to hospice (7.0% vs 5.1%, p=0.000). Conclusions: Despite earlier ED presentation and increased tPA administration, our results demonstrate worse discharge outcomes during the pandemic in two large telestroke networks. Changes in workflow and resulting delays during COVID-19 may explain these findings.

Objective To highlight the occurrence of ischemic stroke after blunt cerebrovascular injuries and discuss the neurologist's role in preventing and managing ischemic strokes in this trauma population. Methods A retrospective chart review was performed and included data from 2016 to 2019 from a Level I trauma center. Demographics, injury mechanism, ischemic stroke occurrence, interventions, and neurology consultations were examined and descriptive statistics were utilized to characterize the nature of ischemic strokes and their management. Results A total of forty patients (81% male, average age 44) presented with blunt cerebrovascular injury, nine of whom later developed ischemic stroke. Eighteen patients had a carotid artery injury with six developing ischemic stroke. Twenty-seven patients had a vertebral artery injury with three developing ischemic stroke. Six of the nine ischemic strokes occurred on hospital day two, whereas neurology was generally consulted on hospital day four. Conclusions A considerable portion of patients may go on to develop ischemic stroke following blunt cerebrovascular injuries. Polytrauma may interfere with prompt diagnosis which may contribute to delayed anti-thrombotic therapy for ischemic stroke prevention. Neurologists have the opportunity to reduce ischemic stroke burden in this trauma population and patients may benefit from earlier neurology consultation.

Introduction: The practice of endovascular therapy has evolved dramatically over the last 10 years with randomized clinical trials investigating the benefit of thrombectomy in select patient populations based on time of presentation, imaging criteria, and procedural technique. We sought to understand the benefit of thrombectomy in patients treated within the context of a clinical trial at a single academic center. Methods: Patient-level data recorded in case forms and core-lab adjudicated data were analyzed from patients enrolled in RCTs investigating the benefit of endovascular thrombectomy over medical management (IMSIII, MR RESCUE, ESCAPE, SWIFT PRIME, and DAWN) between 2007 and 2017 at a single academic referral center. Results: A total of 134 patients (intervention group, n = 81; medical group, n = 53) were identified across five clinical trials (IMSIII, n = 46; MR RESCUE, n = 4; ESCAPE, n = 24; SWIFT PRIME, n = 14; DAWN, n = 46). There were no significant differences between the treatment arm and control arm in terms of age, gender, baseline NIHSS, ASPECTS, and site of occlusion. Rates of good outcome were superior in the intervention group with early neurological recovery (NIHSS of 0-1 or increase NIHSS of 8 points at 24 h) at a higher rate of 49% vs. 17% (p =

This article presents a theoretical and numerical analysis of guided wave released during an acoustic emission event using excitation potentials. Theoretical formulation showed that guided wave generated using excitation potentials follows the Rayleigh–Lamb equation. The numerical studies predict the out-of-plane displacement of acoustic emission guided wave on the plate surface at some distance away from the source. Parameter studies were performed to evaluate the effect of (1) pressure and shear potentials acting alone and in combination, (2) plate thickness, (3) source depth, (4) rise time, and (5) propagating distance away from the source. Numerical results showed that peak amplitude of S0 signal increases with increasing plate thickness, whereas the peak amplitude of A0 signal initially decreases and then increases with increasing plate thickness. Regarding the source depth, it was found that peak amplitude of S0 signal decreases and A0 signal increases with increasing source depth. Peak time showed a notable contribution to the low-frequency component of A0 signal. There were large losses in S0 and A0 peak signal amplitude over the propagation distance from 100 to 500 mm.

We report 2 cases of parent artery occlusion (PAO) for anterior cerebral artery (ACA) fusiform aneurysm embolization after superselective provocative testing was performed to confirm distal territory viability. The first case involves a patient in the second decade of life who presented with subarachnoid hemorrhage and underwent PAO after a balloon test occlusion in the distal ACA revealed no neurophysiology changes. The second case involves another patient in the forth decade of life who presented with an enlarging pseudoaneurysm and underwent PAO after a sodium amobarbital infusion in the distal ACA revealed no clinical change. Both patients tolerated PAO without clinical compromise. PAO after provocative testing may be a safe and effective strategy in the management of fusiform aneurysm treatment. Key Messages: Provocative testing with superselective balloon test occlusion and sodium amobarbital infusion are both viable options for clinical and physiological interrogation of brain tissue prior to parent vessel occlusion. Neurophysiological monitoring may be a useful surrogate for clinical examination after provocative testing, particularly if patients were treated under general anesthesia.

In this article, the effect of the acoustic emission sensor on the acoustic emission waveforms from fatigue crack growth in a thin aerospace specimen is presented. In situ acoustic emission fatigue experiments were performed on the test coupons made of aircraft grade aluminum plate. Commercial Mistras S9225 acoustic emission sensor and piezoelectric wafer active sensor were used to capture the acoustic emission waveforms from the fatigue crack. It has been shown that the piezoelectric wafer active sensor transducer successfully captured the fatigue crack–related acoustic emission waveforms in the thin plate. The piezoelectric wafer active sensor transducer seems to capture more frequency information of the acoustic emission waveform than the conventional acoustic emission sensor in this particular application. We have also shown the evolution of the acoustic emission waveforms as the fatigue crack grows. The signatures of the fatigue crack growth were captured by the evolution of the acoustic emission waveforms. This waveform evolution is highly related to the physical boundary conditions of the cracks as well as the fatigue crack growth mechanism. The fatigue loading and acoustic emission measurement were synchronized using the same acoustic emission instrumentation. This synchronization provided the exact load level when the acoustic emission signals had occurred during the fatigue crack growth.

The aerospace industry continues to increase the use of adhesives for structural bonding due to the increased joint efficiency (reduced weight), even distribution of the load path and decrease in stress concentrations. However, the limited techniques for verifying the strength of adhesive bonds has reduced its use on primary structures and requires an intensive inspection schedule. This paper discusses a potential structural health monitoring (SHM) technique for the detection of disbonds through the in situ inspection of adhesive joints. This is achieved through the use of piezoelectric wafer active sensors (PWAS), thin unobtrusive sensors which are permanently bonded to the aircraft structure. The detection method utilized in this study is the electromechanical impedance spectroscopy, a local vibration method. This method detects disbonds from the change in the mechanical impedance of the structure surrounding the disbond. This paper will discuss how predictive modeling can provide valuable insight into the inspection method, and provide better results than purely empirical methods will provide. A method for identifying the appropriate frequency range and sensor locations is presented, and the method was verified experimentally using a large aluminum test article, and included both pristine and disbond coupons.

The manufacturing process of carbon fiber reinforced polymer (CFRP) composite structures can introduce many characteristic defects and flaws such as fiber misorientation, fiber waviness, and wrinkling. Therefore, it becomes increasingly important to detect the presence of these defects at the earliest stages of development. Eddy current testing (ECT) is a nondestructive inspection (NDI) technique that has been proven quite effective in detection of damage in metallic structures. However, NDI of composite structures has mainly relied on other methods such as ultrasonic testing (UT) and X-ray to name a few and not much on ECT. In this paper, the authors explore the possibility of using ECT in NDI of CFRP composites by conducting simulations and experiments thereafter. This research is based on the fact that the CFRP displays some low-level electrical conductivity due to the inherent conductivity of the carbon fibers. This low-level conductivity may permit eddy current pathways to cause the flow of eddy currents in the CFRP composites that can be exploited for nondestructive damage detection. An invention disclosure describing our high-frequency ECT method has also been processed. First, the multiphysics finite element method (FEM) simulation was used to simulate the detection of various types of manufacturing flaws and operational damage in CFRP composites such as fiber misorientation, waviness, wrinkling, and so on. Thereafter, ECT experiments were conducted on CFRP specimens with various manufacturing flaws using the Eddyfi Reddy eddy current array (ECA) system.

ObjectiveTo evaluate the long-term functional outcome of interhospital transfer of patients with stroke with suspected large vessel occlusion (LVO) using Helicopter Emergency Medical Services (HEMS).MethodsRecords of consecutive patients evaluated through 2 telestroke networks and transferred to thrombectomy-capable stroke centers between March 2017 and March 2018 were reviewed. Inverse probability of treatment weighting (IPTW) using the propensity score was performed to address confounding factors. Multivariate logistic regression analysis with IPTW was used to determine whether HEMS were associated with good long-term functional outcome (modified Rankin scale score ≤ 2).ResultsA total of 199 patients were included; median age was 67 years (interquartile range [IQR] 55–79 years), 90 (45.2%) were female, 120 (60.3%) were white, and 100 (50.3%) were transferred by HEMS. No significant differences between the 2 groups were found in mean age, sex, race, IV tissue plasminogen activator (tPA) receipt, and thrombectomy receipt. The median baseline NIH Stroke Scale score was 14 (IQR 9–18) in the helicopter group vs 11 (IQR 6–18) for patients transferred by ground (p = 0.039). The median transportation time was 60 minutes (IQR 49–70 minutes) by HEMS and 84 minutes (IQR 25–102 minutes) by ground (p < 0.001). After weighting baseline characteristics, the use of HEMS was associated with higher odds of good long-term outcome (OR 4.738, 95% CI 2.15–10.444, p < 0.001) controlling for transportation time, door-in-door-out time, and thrombectomy and tPA receipt. The magnitude of the HEMS effect was larger in thrombectomy patients who had successful recanalization (OR 1.758, 95% CI 1.178–2.512, p = 0.027).ConclusionsHEMS use was associated with better long-term functional outcome in patients with suspected LVO, independently of transportation time.

This paper presents the interaction of Lamb waves with rivet hole cracks from multiple directions of incident using the finite element approach. Lamb waves undergo scattering and mode conversion after interacting with the damage. Shear horizontal waves appear in the scattered waves because of the mode conversion. Instead of analyzing the whole large structure, the local damage area is analyzed using finite element analyses and analytical formulation is used to analyze the whole structure. The scatter fields are described in terms of wave damage interaction coefficients that involve scattering and mode conversion of Lamb waves. Lamb wave mode (S0 and A0) hit the damage from multiple directions and corresponding wave damage interaction coefficients are obtained around the damage. Harmonic analysis has been performed over the fundamental frequency domain and “scatter cubes” of complex-valued wave damage interaction coefficients are formed. The scatter cube provides the information of relative amplitude and phase of scattered waves around the damage that can be used for designing the sensor installation. An application based on real time domain signal has been illustrated for the problem of multiple-rivet-hole cracks using the scatter cubes with the analytical framework.

BackgroundIn acute stroke due to large vessel occlusion, faster reperfusion leads to better outcomes. We analyzed the effect of optimization steps aimed to reduce treatment delays at our center.MethodsConsecutive patients with ischemic stroke treated with endovascular therapy were prospectively analyzed. We divided the patients into pre-optimization (20 April 2012 to 8 October 2013) and post-optimization (9 October 2013 to 29 July 2014) periods. The main interventions included: (1) continuous feedback; (2) standardized immediate emergency department attending to stroke attending communication with interventional team activation for all potential interventions; (3) pre-notification by the emergency medical service; (4) minimizing additional diagnostic testing; (5) direct transport to the CT scanner; (6) transport directly from the CT scanner to the angiography suite. The main metric used to measure improvement was door to groin puncture time (D2P).ResultsWe included a total of 286 patients (178 pre-optimization, 108 post-optimization). There were no significant differences between major baseline characteristics between the groups with the exception of higher median CT Alberta Stroke Program Early CT Score in the pre-optimization group (p=0.01). Median D2P improved from 105 min pre-optimization to 67 min post-optimization (p=0.0002). Rates of good clinical outcomes (modified Rankin Scale 0–2 at 3 months) were similar in both groups, with a trend toward a better outcome in the post-optimization group in a subgroup analysis of patients with anterior circulation occlusion who received intravenous tissue plasminogen activator.ConclusionsThis pilot study demonstrates that D2P times can be significantly reduced with a standardized multidisciplinary approach. There was no significant difference in the rate of 3-month good outcome, which is most likely due to the small sample size and confounding baseline patient characteristics.

In this paper, an attempt has been made to conduct controlled low velocity impact experiments in quasi-isotropic carbon fiber reinforced polymer (CFRP) composites of increasing thicknesses to generate barely visible impact damage (BVID) that creates 1″ impact damage diameter. Such impacted CFRP coupons with controlled damage diameter would serve in the future for validation of structural health monitoring (SHM) and nondestructive evaluation (NDE) damage detection methodologies. Various 2-mm, 4-mm and 6-mm thick quasi-isotropic CFRP composite plates having similar stacking sequence were manufactured in a compression molding machine. These plates were cut into numerous 6″ × 4″ coupons to be impacted in accordance with ASTM D7136 standard on a Dynatup impact testing machine. The goal of the experiments was to identify the combination of impactor mass, energy and momentum at which approximately 1″ impact damage diameter could be produced for test coupons of increasing thicknesses. Interesting observations were made with respect to the size and shape of the impact damage as the thickness of the composite coupons was increased from 2-mm to 6-mm. Modified experiments were also conducted on 6-mm coupons to achieve close to 1″ impact damage diameter. The greatest challenge in conducting these experiments was in the thick 4-mm and 6-mm coupons which did not display a predictable trend. Future work should focus on overcoming this challenge towards achieving a predictable impact damage size methodology.

Structural health monitoring (SHM) is in urgent need and must be integrated into the nuclear-spent fuel storage systems to guarantee the safe operation. The dry cask storage system (DCSS) is such storage facility, which is licensed for temporary storage for nuclear-spent fuel at the independent spent fuel storage installations (ISFSIs) for certain predetermined period of time. Gamma radiation is one of the major radiation sources near DCSS. Therefore, a detailed experimental investigation was completed on the gamma radiation endurance of piezoelectric wafer active sensors (PWAS) transducers for SHM applications to the DCSS system. The irradiation test was done in a Co-60 gamma irradiator. Lead Zirconate Titanate (PZT) and Gallium Orthophosphate (GaPO4) PWAS transducers were exposed to 40.7 kGy gamma radiation. Total radiation dose was achieved in two different radiation dose rates: (a) slower radiation rate at 0.1 kGy/hr for 20 hours (b) accelerated radiation rate at 1.233 kGy/hr for 32 hours. The total cumulative radiation dose of 40.7 kGy is equivalent to 45 years of operation in DCSS system. Electro-mechanical impedance and admittance (EMIA) signatures and electrical capacitance were measured to evaluate the PWAS performance after each gamma radiation exposure. The change in resonance frequency of PZT-PWAS transducer for both in-plane and thickness mode was observed. The GaPO4-PWAS EMIA spectra do not show a significant shift in resonance frequency after gamma irradiation exposure. Radiation endurance of new high-temperature HPZ-HiT PWAS transducer was also evaluated. The HPZ-HiT transducers were exposed to gamma radiation at 1.233 kGy/hr for 160 hours with 80 hours interval. Therefore, the total accumulated gamma radiation dose is 184 kGy. No significant change in impedance spectra was observed due to gamma radiation exposure.

TN32 casks are multi-layer cylindrical structures used for storage of nuclear spent fuel. The National Center for Physical Acoustics at the University of Mississippi has manufactured a scaled down model of the TN32 cask. To identify the most relevant nondestructive evaluation parameters, which will be useful while doing experiments on real TN32 casks, a series of experiments have been conducted on TN32 cask model. This paper discusses the data analysis of the experiments conducted on the cask model and the conclusions based on those experiments. Elastodynamic waves are generated in the cask model by pencil lead break and hammer hit excitation and the waves in the cask at certain locations are sensed using piezoelectric wafer active sensors (PWAS). The waveforms and frequency spectrums of waveforms arriving at PWAS are studied. There are two types of joints on the cask model: structures joined using adhesives and structures joined using press fit. The effects of various joints in the structure on elastodynamic wave propagation are also studied. Pitch catch experiments on the cask was also done using in plane excitation using PWAS. The most sensitive frequency for the cask model was identified from the frequency response spectrum obtained from a wide band chirp excitation. The influence of various joints on the frequency response spectrum is also studied. Analytical modeling of cask geometry for a given excitation is done using Normal Mode Expansion (NME) technique. Prediction of wave propagation through the scaled down model is done based on the theoretical expression derived.

Structural health monitoring (SHM) and nondestructive evaluation (NDE) deals with the nondestructive inspection of defects, corrosion, leaks in engineering structures by using ultrasonic guided waves. In the past, simplistic structures were often considered for analyzing the guided wave interaction with the defects. In this study, we focused on more realistic and relatively complicated structure for detecting any defect by using a non-contact sensing approach. A plate with a stiffener was considered for analyzing the guided wave interactions. Piezoelectric wafer active transducers were used to produce excitation in the structures. The excitation generated the multimodal guided waves (aka Lamb waves) that propagate in the plate with stiffener. The presence of stiffener in the plate generated scattered waves. The direct wave and the additional scattered waves from the stiffener were experimentally recorded and studied. These waves were considered as a pristine case in this research. A fine horizontal semi-circular crack was manufactured by using electric discharge machining in the same stiffener. The presence of crack in the stiffener produces additional scattered waves as well as trapped waves. These scattered waves and trapped wave modes from the cracked stiffener were experimentally measured by using a scanning laser Doppler vibrometer (SLDV). These waves were analyzed and compared with that from the pristine case. The analyses suggested that both size and shape of the horizontal crack may be predicted from the pattern of the scattered waves. Different features (reflection, transmission, and mode-conversion) of the scattered wave signals are analyzed. We found direct transmission feature for incident A0 wave mode and modeconversion feature for incident S0 mode are most suitable for detecting the crack in the stiffener. The reflection feature may give a better idea of sizing the crack.

This article presents a new approach to designing non-reflective boundary (NRB) for inhibiting Lamb wave reflections at structural boundaries. Our NRB approach can be effectively and conveniently implemented in commercial finite element (FE) codes. The paper starts with a review of the state of the art: (a) the absorbing layers by increasing damping (ALID) approach; and (b) the Lysmer–Kuhlemeyer absorbing boundary conditions (LK ABC) approach is briefly presented and its inadequacy for Lamb wave applications is explained. Hence, we propose a modified Lysmer–Kuhlemeyer approach to be used in the NRB design for Lamb wave problems; we call our approach MLK NRB. The implementation of this MLK NRB was realized using the spring–damper elements which are available in most commercial FE codes. Optimized implementation parameters are developed in order to achieve the best performance for Lamb wave absorption. Our MLK NRB approach is compared with the state of the art ALID and LK ABC methods. Our MLK NRB shows better performance than ALID and LK ABC for all Lamb modes in the thin-plate structures considered in our examples. Our MLK NRB approach is also advantageous at low frequencies and at cut-off frequencies, where extremely long wavelengths exist. A comprehensive study with various design parameters and plate thicknesses which illustrates the advantages and limitations of our MLK NRB approach is presented. MLK NRB applications for both transient analysis in time domain and harmonic analysis in frequency domain are illustrated. The article finishes with conclusions and suggestions for future work.

The paper deals with the development of low-cost tools for fast computer simulation of guided wave propagation and diffraction in plate-like structures of variable thickness. It is focused on notched surface irregularities, which are the basic model for corrosion damages. Their detection and identification by means of active ultrasonic structural health monitoring technologies assumes the use of guided waves generated and sensed by piezoelectric wafer active sensors as well as the use of laser Doppler vibrometry for surface wave scanning and visualization. To create a theoretical basis for these technologies, analytically based computer models of various complexity have been developed. The simplest models based on the Euler–Bernoulli beam and Kirchhoff plate equations have exhibited a sufficiently wide frequency range of reasonable coincidence with the results obtained within more complex integral equation based models. Being practically inexpensive, they allow one to carry out a fast parametric analysis revealing characteristic features of wave patterns that can be then made more exact using more complex models. In particular, the effect of resonance wave energy transmission through deep notches has been revealed within the plate model and then validated by the integral equation based calculations and experimental measurements.

BACKGROUND Fluid attenuated inversion recovery (FLAIR) vascular hyperintensity (FVH) is a novel radiographic marker detected in acute ischemic stroke (AIS) patients, which is linked to slow blood flow and potentially salvageable brain tissue. Poor leptomeningeal collateral status in AIS patients with proximal artery occlusion (PAO) is associated with larger final infarct and worse clinical outcomes, which are also affected by severity of white matter hyperintensity (WMH). We sought to evaluate FVH utility as a marker of acute collateral vessel status and its association with WMH burden in AIS patients. METHODS Consecutive AIS patients with PAO on baseline CT angiography (CTA) were retrospectively selected from a prospectively derived database. FVH was graded by its location, degree, and score on admission MRI obtained immediately after intravenous tissue plasminogen activator administration. Leptomeningeal collateral flow grade was ranked on admission CTA. WMH volume (WMHV) was assessed using a validated volumetric protocol. Relationship between FVH, collateral flow grade, and WMHV were analyzed. RESULTS Among 39 patients (mean age 70.5 ± 12.7 years; 56% women, mean National Institutes of Health Stroke Scale score 17.2 (± 4.4)), median WMHV was 6.0 cm3. FVH score and collateral flow grade were significantly correlated (Spearman's ρ = .41, P = .009). In a univariate regression model, FVH degree was inversely associated with WMHV (β = –.33, P = .04). CONCLUSIONS FVH score detected on acute MRI can be used as a surrogate of collateral flow grade in AIS patients. FVH degree is inversely associated with WMHV, possibly signifying diffuse disease of cerebral vasculature in patients with severe leukoaraiosis.

Piezoelectric wafer active sensors (PWAS) are commonly used for detecting Lamb waves for structural health monitoring application. However, in most applications of active sensing, the signals are of high-amplitude and easy to detect. In this article, we have shown a new avenue of using the PWAS transducer for detecting the low-amplitude fatigue-crack related acoustic emission (AE) signals. Multiphysics finite element (FE) simulations were performed with two PWAS transducers bonded to the structure. Various configurations of the sensors were studied by using the simulations. One PWAS was placed near to the fatigue-crack and the other one was placed at a certain distance from the crack. The simulated AE event was generated at the crack tip. The simulation results showed that both PWAS transducers were capable of sensing the AE signals. To validate the multiphysics simulation results, an in-situ AE-fatigue experiment was performed. Two PWAS transducers were bonded to the thin aerospace test coupon. The fatigue crack was generated in the test coupon which had produced low-amplitude acoustic waves. The low-amplitude fatigue-crack related AE signals were successfully captured by the PWAS transducers. The distance effect on the captured AE signals was also studied. It has been shown that some high-frequency contents of the AE signal have developed as they travel away from the crack.

Background and Purpose: Stent retriever thrombectomy and manual aspiration thrombectomy (MAT) have each been shown to lead to high rates of recanalization as single-modality endovascular stroke therapy. We sought to describe the safety and efficacy of a multimodal approach combining these two techniques termed ‘stent retriever-mediated manual aspiration thrombectomy' (SMAT) and compared them to MAT alone. Methods: Retrospective review of a prospectively acquired acute endovascular stroke database. Results: 195 consecutive patients with large-vessel occlusion were identified between July 2013 and April 2015. Occlusion distribution was as follows: 52% middle cerebral artery segment 1 (M1), 6% M2, 29% internal carotid artery, and 13% vertebrobasilar. Median onset to treatment time was 278 min. Intravenous rtPA was administered in 33% of cases, whereas 34% of cases had symptom onset beyond 8 h. Effective recanalization (TICI 2b/3) was achieved in 91% of patients and in 49% of patients, only a single pass was necessary. Median groin puncture to recanalization time was 40 min. Symptomatic intracerebral hemorrhage occurred in 5% of patients. Favorable outcomes defined as a modified Rankin Scale score of 0-2 were noted in 42% of patients. Compared with MAT alone, SMAT achieved a similar rate of effective recanalization (91 vs. 88%, p = n.s.) but was associated with faster access to reperfusion times (49 vs. 77 min, p < 0.00001). Conclusions: SMAT is a safe and efficacious method to achieve rapid revascularization that leads to faster recanalization compared to manual aspiration alone. Future prospective comparisons are necessary to establish the most clinically effective therapy for acute thrombectomy.

There is considerable demand for structural health monitoring (SHM) at locations where there are substantial radiation fields such as nuclear reactor components, dry cask storage canister, irradiated fuel assemblies, etc. Piezoelectric wafer active sensors (PWAS) have been emerged as one of the major SHM sensing technologies. In order to use PWAS to perform SHM in nuclear environment, radiation influence on sensor and sensing capability needs to be investigated to assure the reliability of the PWAS based method. Radiation may cause degradation or even complete failure of sensors. Gamma radiation is one of the major radiation sources near the nuclear source. Therefore, experimental investigation was completed on the gamma radiation endurance of piezoelectric sensors. The irradiation test was done in a Co-60 Gamma Irradiator. Lead Zirconate Titanate (PZT) and Gallium Orthophosphate (GaPO4) PWAS were exposed under gamma radiation at 100 Gy/hr rate for 20 hours. Electro-mechanical (E/M) admittance signatures and electrical capacitance were measured to evaluate the PWAS performance before and after every 4 hours exposure to gamma radiation. PWAS were kept at room temperature for 6 days after each 4 hours radiation exposure to investigate the effect of time on PWAS by gamma radiation. It was found that, PZT-PWAS show variation in resonance frequency for both in plane and thickness mode E/M admittance. Where, the changes in resonance amplitudes are larger for PZT-PWAS. GaPO4-PWAS E/M impedance/admittance spectra don’t show any reasonable change after gamma irradiation. A degradation behavior of electrical properties in the PZT-PWAS was observed. Capacitance value of PZT-PWAS decreases from 3.2 nF to 3.07 nF after exposing to gamma radiation for 20 hours at 100Gy/hour. This degradation behavior of electrical properties may be explained by the pinning of domain walls by some radiation induced effect. GaPO4-PWAS doesn’t show reasonable degradation in electrical properties. GaPO4 has good radiation endurance, although amplitude sensitivity is relatively low.

Acoustic emission (AE) monitoring technique is a well-known approach in the field of NDE/SHM. AE monitoring from the defect formation and failure in the materials were well studied by the researchers. However, conventional AE monitoring techniques are predominantly based on statistical analysis. In this study we focus on understanding the AE waveforms from the fatigue crack growth using physics based approach. The growth of the fatigue crack causes the acoustic emission in the material that propagates in the structure. One of the main challenges of this approach is to develop the physics based understanding of the AE source itself. The acoustic emission happens not only from the crack growth but also from the interaction of the crack lips during fatigue loading of the materials. As the waveforms are generated from the AE event, they propagate and create local vibration modes along the crack faces. Fatigue experiments were performed to generate the fatigue cracks. Several test specimens were used in the fatigue experiments and corresponding AE waveforms were captured. The AE waveforms were analyzed and distinguished into different groups based on the similar nature on both time domain and frequency domain. The experimental results are explained based on the physical observation of the specimen.

Objective Previous studies have suggested that patients with wake-up stroke (WUS) may have superior outcomes compared with patients with a witnessed late time of onset after revascularization. We sought to test this hypothesis in patients with anterior circulation large vessel occlusion stroke (ACLVOS) treated with endovascular therapy beyond 8 h from time last seen well (TLSW). Methods A single center retrospective review of a prospectively acquired database of consecutive patients was performed to identify patients presenting beyond 8 h of TLSW with radiographic evidence of ACLVOS, small core, and large penumbra who subsequently underwent endovascular treatment. Results We identified 206 patients. Patients were divided into two groups: (1) patients with WUS (38%, n=78) and (2) patients with witnessed onset beyond 8 h (62%, n=128). The groups were similar in age, baseline National Institutes of Health Stroke Scale score, TLSW to reperfusion, baseline infarct volume, and rate of successful recanalization. Rates of good outcome (modified Rankin Scale score of 0–2 at 90 days, 43% vs 50%, p=0.3), parenchymal hematoma (9% vs 5.5%, p=0.3), and final infarct volume (75.2 vs 61.4 mL, p=0.6) were comparable. Multivariate analysis identified age (OR=0.95, 95% CI 0.91 to 0.99, p

This article presents the WaveFormRevealer—an analytical framework and predictive tool for the simulation of guided Lamb wave propagation and interaction with damage. The theory of inserting damage effects into the analytical model is addressed, including wave transmission, reflection, mode conversion, and nonlinear higher harmonics. The analytical model is coded into MATLAB, and a graphical user interface (WaveFormRevealer graphical user interface) is developed to obtain real-time predictive waveforms for various combinations of sensors, structural properties, and damage. In this article, the main functions of WaveFormRevealer are introduced. Case studies of selective Lamb mode linear and nonlinear interaction with damage are presented. Experimental verifications are carried out. The article finishes with summary and conclusions followed by recommendations for further work.

Background and purpose Leukoaraiosis (LA) is defined as ischemic white matter lesions associated with increased stroke risk and poor post-stroke outcomes. These lesions are likely the result of diffuse angiopathic changes affecting the cerebral small vessels. We investigated whether pre-existing LA burden is associated with outcomes in patients with large cerebral artery occlusion undergoing intra-arterial therapy (IAT) for acute ischemic stroke (AIS). Methods We analyzed consecutive AIS subjects undergoing IAT from the institutional Get With The Guidelines-Stroke database enrolled between January 1, 2007 and June 30, 2009, who had National Institutes of Health Stroke Scale scores of ≥8, baseline diffusion weighted imaging volume ≤100 mL, and evidence of proximal artery occlusion (PAO) on pre-IAT computed tomography angiography (CTA). LA volume (LAv) was assessed on fluid attenuated inversion recovery MRI using a validated semi-automated protocol. We used CTA for collateral grade, post-IAT angiogram for recanalization status (Thrombolysis in Cerebral Infarction score ≥2b), and the 24 h head CT for symptomatic intracranial hemorrhage. Logistic regression was used to determine independent predictors of 90 day post-stroke good functional outcome (modified Rankin Scale score ≤2) and mortality. Results Increasing LAv independently reduced the odds of good collateral grade (OR 0.85, 95% CI 0.73 to 0.98). Good functional outcome was independently predicted by intravenous tissue plasminogen activator use (OR 12.86, 95% CI 2.20 to 76.28), and recanalization status (OR 6.94, 95% CI 1.56 to 30.86). Mortality was independently associated with recanalization status (OR 0.08, 95% CI 0.01 to 0.51), age (OR 1.08, 95% CI 1.01 to 1.15), and antecedent use of hypoglycemic agents (OR 6.55, 95% CI 1.58 to 54.01). Conclusions Severity of LA is linked to poor collateral grade in AIS patients undergoing IAT for PAO; however, greater LAv appears not to be a contraindication for acute intervention.

Piezoelectric wafer active sensors (PWAS) are lightweight and inexpensive transducers that enable a large class of structural health monitoring (SHM) applications such as: (a) embedded guided wave ultrasonics, i.e., pitch-catch, pulse-echo, phased arrays; (b) high-frequency modal sensing, i.e., electro-mechanical impedance method; and (c) passive detection. The focus of this paper is on the challenges posed by using PWAS transducers in the composite laminate structures as different from the metallic structures on which this methodology was initially developed. After a brief introduction, the paper reviews the PWAS-based SHM principles. It follows with a discussion of guided wave propagation in composites and PWAS tuning effects. Then, the mechanical effect is discussed on the integration of piezoelectric wafer inside the laminate using a compression after impact. Experiments were performed on a glass fibre laminate, employing PWAS to measure the attenuation coefficient. Finally, the paper presents some experimental and multi-physics finite element method (MP-FEM) results on guided wave propagation in composite laminate specimens.

This paper presents numerical and experimental results on the use of guided waves for structural health monitoring (SHM) of crack growth during a fatigue test in a thick steel plate used for civil engineering application. Numerical simulation, analytical modeling, and experimental tests are used to prove that piezoelectric wafer active sensor (PWAS) can perform active SHM using guided wave pitch-catch method and passive SHM using acoustic emission (AE). AE simulation was performed with the multi-physic FEM (MP-FEM) approach. The MP-FEM approach permits that the output variables to be expressed directly in electric terms while the two-ways electromechanical conversion is done internally in the MP-FEM formulation. The AE event was simulated as a pulse of defined duration and amplitude. The electrical signal measured at a PWAS receiver was simulated. Experimental tests were performed with PWAS transducers acting as passive receivers of AE signals. An AE source was simulated using 0.5-mm pencil lead breaks. The PWAS transducers were able to pick up AE signal with good strength. Subsequently, PWAS transducers and traditional AE transducer were applied to a 12.7-mm CT specimen subjected to accelerated fatigue testing. Active sensing in pitch catch mode on the CT specimen was applied between the PWAS transducers pairs. Damage indexes were calculated and correlated with actual crack growth. The paper finishes with conclusions and suggestions for further work.

This paper presents a hybrid finite element and analytical method to predict the 1-D guided wave propagation interaction with damage for nondestructive evaluation (NDE) and structural health monitoring (SHM) application. The finite element mesh is used to describe the region around the damage (defects or flaws). In contrast to other hybrid models developed elsewhere, the interaction between Lamb waves and defects is computed in the time domain using the explicit solver of the commercial finite element method (FEM) software ABAQUS. Analytical methods can perform efficient modeling of wave propagation but are limited to simple geometries. Realistic structures with complicated geometries are usually modeled with the FEM. However, to obtain an accurate wave propagation solution at ultrasonic frequencies is computationally intensive and may become prohibitive for realistic structures. In response to today's complex cases not covered by the simulation tools available, we aim to develop an efficient and accessible tool for SHM applications. This tool will be based on a hybrid coupling between analytical solutions and time domain numerical codes. Lamb wave interaction with a notch is investigated by using this method, and the results obtained are with respect to transmission, reflection and mode conversion. Because of the symmetric mode shape, S0 is more sensitive to the shallow notch than A0. By making use of the fact that the reflection increases with increase in notch depth and mode conversion are maximized when the notch is around half through the thickness of the plate, the reflection and conversion coefficients can be used to characterize the depth of the notch.

This article presents a combined finite element method and analytical process to predict the one-dimensional guided- wave propagation for nondestructive evaluation and structural health monitoring application. Analytical methods can per- form efficient modeling of wave propagation but are limited to simple geometries. In response to today's most complex cases not covered by the simulation tools available, we aim to develop an efficient and accessible tool for structural health monitoring application. This tool will be based on a hybrid coupling between analytical solution and time-domain numerical codes. Using the principle of reciprocity, global analytical calculation is coupled with local finite element method analysis to utilize the advantages of both methods and obtain a rapid and accurate simulation method. The phe- nomenon of interaction between the ultrasonic wave, the defect, and the structure, leading to a complex signature, is efficiently simulated by this hybrid global-local approach and is able to predict the specific response signal actually received by sensor. The finite element mesh is used to describe the region around the defects/flaws. In contrast to other hybrid models already developed, the interaction between Lamb waves and defects is computed in the time domain using the explicit solver of the commercial finite element method software ABAQUS.

Piezoelectric wafer active sensors (PWAS) at high temperature with reliable operation are desired for structural health monitoring. The PWAS may be generalized as an electro-mechanical system because of the incorporation of electro-mechanical coupling. The basic principle of SHM method by PWAS is to monitor variation in the electro-mechanical (E/M) impedance/ admittance signature. The operational temperature range of PWAS can be limited by the sensing capability of the piezoelectric material at elevated temperatures. Therefore stability of PWAS at high-temperature environments is of great interest for SHM. In such cases SHM can be done at room temperature or at relatively lower temperature. However, during service permanently bonded PWAS can be exposed to very high temperature. The traditional PWAS use piezoelectric materials Lead Zirconate Titanate (PZT) that have been attracted by researchers due to its enhanced sensing, actuation or both capabilities. This paper discusses properties relevant to sensor applications, including piezoelectric materials that are commercially available. For temperature dependence study PWAS were exposed to 50°C to 250°C with 50°C interval at around 2°C/min heating rate. E/M impedance/ admittance and different material properties such as, dielectric constant, dielectric loss, mechanical quality factor, P-E hysteresis loop, in plane piezoelectric constant were determined experimentally at room temperature after exposure to high temperature. The variation in E/M impedance and admittance signature and different material properties were obtained at each temperature. The piezoelectric material degradation was also investigated by microstructural and crystallographic study.

In this article, ultrasonic guided wave propagation and interaction with the rivet hole cracks has been formulated using closed-form analytical solution while the local damage interaction, scattering, and mode conversion have been obtained from finite element analysis. The rivet hole cracks (damage) in the plate structure gives rise to the non-axisymmetric scattering of Lamb wave, as well as shear horizontal (SH) wave, although the incident Lamb wave source (primary source) is axisymmetric. The damage in the plate acts as a non-axisymmetric secondary source of Lamb wave and SH wave. The scattering of Lamb and SH waves are captured using wave damage interaction coefficient (WDIC). The scatter cubes of complex-valued WDIC are formed that can describe the 3D interaction (frequency, incident direction, and azimuth direction) of Lamb waves with the damage. The scatter cubes are fed into the exact analytical framework to produce the time domain signal. This analysis enables us to obtain the optimum design parameters for better detection of the cracks in a multiple-rivet-hole problem. The optimum parameters provide the guideline of the design of the sensor installation to obtain the most noticeable signals that represent the presence of cracks in the rivet hole.

The first part of the paper presents recent developments in the field of structural health monitoring (SHM) with special attention on the piezoelectric wafer active sensors (PWAS) technologies utilizing guided waves (GW) as propagating waves (pitch-catch, pulse-echo), standing wave (electromechanical impedance), and phased arrays. The second part of the paper describes the challenges of extending the PWAS GW SHM approach to in-space applications. Three major issues are identified, (a) cryogenic temperatures; (b) high temperatures; and (c) space radiation exposure. Preliminary results in which these three issues were address in a series of carefully conducted experiments are presented and discussed. The third part of the paper discusses a new project that is about to start in collaboration between three Romanian institutes to address the issues and challenging of developing space SHM technologies based on PWAS concepts. The paper finishes with conclusions and suggestions for further work.

This paper addresses some unresolved predictive modeling issues related to the use of piezoelectric-wafer active sensors for ultrasonic structural health monitoring. An exact model for the shear-lag transfer between the piezoelectric-wafer active sensor transducer and the structure in the presence of N generic guided wave modes is derived from first principles using the normal-mode expansion formulation. The resulting integral–differential equation is solved using the variational iteration approach. The resulting solution is used to derive an improved modelforthetuningbetweenpiezoelectric-waferactivesensortransducersandthemultimodalguidedwavesusedin ultrasonic structural-health-monitoring applications. The numerical predictions generated by the improved tuning model are compared with experimental results obtained through pitch–catch experiments between two 7 mm piezoelectric-waferactivesensortransducersplacedona1-mm2024-T3aluminumplate.The10–700kHzfrequency range was explored. It was concluded that the improved model using the exact shear-lag solution matches much better the experimental results than previous models. Further theoretical and experimental work is warranted as a follow-up on the work reported in this paper to study the accuracy and convergence properties of the solution, to explore experimental comparison beyond the A1-mode cutoff frequency, and to extend the approach to layered structures and composite materials.

Recent advancements in sensors and information technologies have resulted in new methods for structural health monitoring (SHM) of the performance and deterioration of structures. The enabling element is the piezoelectric wafer active sensor (PWAS). This paper presents an introduction to PWAS transducers and their applications in Lamb wave-based SHM. We begin by reviewing the fundamentals of piezoelectric intelligent materials. Then, the mechanism of using PWAS transducers as Lamb wave transmitters and receivers is presented. PWAS interact with the host structure through the shear-lag model. Lamb wave mode tuning can be achieved by judicious combination of PWAS dimensions, frequency value, and Lamb mode characteristics. Finally, use of PWAS Lamb wave SHM for damage detection on plate-like aluminum structures is addressed. Examples of using PWAS phased array scanning, quantitative crack detection with array imaging, and quantitative corrosion detection are given.

An experimental evaluation of the structural health monitoring capability of piezoelectric wafer active sensors on composite structures at cryogenic temperatures is presented. The piezoelectric wafer active sensor–based electromechanical impedance and the pitch–catch methods were first qualified for cryogenic temperatures using piezoelectric wafer active sensor–instrumented composite specimens dipped in liquid N2. Subsequently, damage detection experiments were performed on laboratory-scale composite specimens with (a) impact damage and (b) built-in Teflon patches simulating in service delaminations. Finally, a comprehensive damage detection test was performed on a full-scale specimen subjected to pressure and cryogenic temperature cycles. Based on these tests, we conclude that piezoelectric wafer active sensor–based structural health monitoring methods show promise for damage detection in composite materials even in extreme cryogenic conditions. Recommendations for further work are also included.

This paper discusses the topic of how the integrity of safety-critical structural composites can be enhanced by the use of structural health monitoring (SHM) techniques. The paper starts with a presentation of how the certification of flight-critical composite structures can be achieved within the framework of civil aviation safety authority requirements. Typical composites damage mechanisms, which make this process substantially different from that for metallic materials are discussed. The opportunities presented by the use of SHM techniques in future civil aircraft developments are explained. The paper then focuses on active SHM with piezoelectric wafer active sensors (PWAS). After reviewing the PWAS-based SHM options, the paper follows with a discussion of the specifics of guided wave propagation in composites and PWAS-tuning effects. The paper presents a number of experimental results for damage detection in simple flat unidirectional and quasi-isotropic composite specimens. Calibrated through holes of increasing diameter and impact damage of various energies and velocities are considered. The paper ends with conclusions and suggestions for further work.

Monitoring of fatigue cracking in steel bridges is of high interest to many bridge owners and agencies. Due to the variety of deterioration sources and locations of bridge defects, there is currently no single method that can detect and address the potential sources globally. In this paper, we presented a dual mode sensing methodology integrating acoustic emission and ultrasonic wave inspection based on the use of low-profile piezoelectric wafer active sensors (PWAS). After introducing the research background and piezoelectric sensing principles, PWAS crack detection in passive acoustic emission mode is first presented. Their acoustic emission detection capability has been validated through both static and compact tension fatigue tests. With the use of coaxial cable wiring, PWAS AE signal quality has been improved. The active ultrasonic inspection is conducted by the damage index and wave imaging approach. The results in the paper show that such an integration of passive acoustic emission detection with active ultrasonic sensing is a technological leap forward from the current practice of periodic and subjective visual inspection and bridge management based primarily on history of past performance.

Piezoelectric wafer active sensors (PWAS) are lightweight and inexpensive enablers for a large class of structural health monitoring (SHM) applications. This paper presents and discusses the challenges and opportunities related to the use of PWAS in the structures specific to space applications. The challenges posed by space structures are often different from those encountered in conventional structures. After a review of PWAS principles, the paper discusses the multi-physics power and energy transduction between structurally guided waves and PWAS; predictive modeling results using a simplified analytical approach are presented. Experimental results on space-like specimen structures are presented. Survivability of PWAS transducers under cryogenic space-like conditions is experimentally verified. The paper ends with conclusions and suggestions for further work.

This article presents a systematic investigation of power and energy transduction in piezoelectric wafer-active sensors (PWAS) for structural health monitoring (SHM). After a literature review of the state-of-the-art, we developed a predictive modeling of the multi-physics power and energy transduction between structurally guided waves and PWAS in closed-form analytical expressions. The model assumptions include: (a) one-dimensional axial and flexural wave propagations; (b) ideal bonding (pin-force) connection between PWAS and structure; and (c) ideal excitation source at the transmitter PWAS and fully resistive external load at the receiver PWAS. Both wave propagation method for an infinite beam and normal mode expansion method for a finite beam were considered. Frequency response functions were developed for voltage, current, complex power, active power, etc. The power and energy analysis for single PWAS transmitter, single PWAS receiver and a complete PWAS pitch-catch setup were considered. The numerical simulation and graphical charts show the trends in the power and energy flow behavior of PWAS for SHM. The peaks and valleys of power can be exploited for optimum design for PWAS SHM applications.

A time–frequency analysis-based signal processing study for detecting active corrosion in aluminum plate-like structure utilizing the broadband piezoelectric wafer active sensors is presented in this article. Tests were conducted on an aluminum plate with a network of sensors installed on one side of the plate for Lamb wave generation and reception. The corrosion was emulated as material loss of an area of 50 × 38 mm2 on the opposite side of the plate. The corroded area resulted in a thickness loss on the plate and a change in wave propagation as well. The experimental data were first evaluated by a statistical damage index (DI) based on root mean square values and then the Cohen’s class motivated cross-time–frequency analysis. The cross-time–frequency analysis was found more reliable and precise for detecting the corrosion progression when compared to the DI method. Not only can the proposed metric correctly evaluate the phase difference of specific frequency and time, it also carries useful information of phase difference, which is strongly correlated to the physics of corrosion detection using Lamb waves. Novel aspects of this study include a sensing approach that can sense corrosion damage on both external and internal surfaces of a given structure, the employment of effective tuning in corrosion detection, and using cross-time–frequency analysis to quantitatively evaluate thickness loss. Though the corrosion studied herein is an idealized and simplified situation, the subject work on phase difference and cross-time–frequency analysis is useful first-step effort and opens a new way to perform Lamb wave-based corrosion detection. The results presented in this article combine easy-to-examine corrosion assumptions together with low-frequency antisymmetric Lamb wave analysis to provide a stepping stone for more complicated analysis needed for further real life corrosion assessment.

Piezoelectric wafer active sensors (PWAS) are lightweight and inexpensive enablers for a large class of structural health monitoring (SHM) applications such as: (a) embedded guided-wave ultrasonics, i.e., pitch-catch, pulseecho, phased arrays; (b) high-frequency modal sensing, i.e., the electro-mechanical (E/M) impedance method; (c) passive detection (acoustic emission and impact detection). The focus of this paper will be on the challenges and opportunities posed by the composite structures as different from the metallic structures on which this methodology was initially developed. After a brief introduction, the paper discusses damage modes in composites. Then, it reviews the PWAS-based SHM principles. It follows with a discussion of guided wave propagation in composites and PWAS tuning effects. Finally, the paper presents some experimental results with damage detection in composite specimens. The paper ends with conclusions and suggestions for further work

An analytical investigation of the interaction between piezoelectric wafer active sensor (PWAS), guided Lamb waves, and host structure is presented in this paper, supported with application examples. The analytical investigation assumes a PWAS transducer bonded to the upper surface of an isotropic flat plate. Shear lag transfer of tractions and strains is assumed, and an analytical solution using the space-wise Fourier transform is reviewed, closed-form solutions are presented for the case of both ideal bonding (i.e., load transfer mechanism localized at the PWAS boundary) and not ideal bonding (i.e., load transfer mechanism localized close the PWAS boundary). The analytical solutions are used to derive Lamb wave mode tuning curves which indicate that frequencies exist at which the A0 mode or the S0 mode can be either suppressed or enhanced. The paper further shows that the capability to excite only one desired Lamb wave mode is critical for practical structural health monitoring applications such as PWAS phased array technique (e.g., the embedded ultrasonics structural radar, EUSR) and the sparse array imaging. Extensive experimental tests that verify the tuning mechanism and prediction curves are reported. Examples of correctly tuned EUSR images vs. detuned cases illustrate the paramount importance of Lamb wave mode tuning for the success of PWAS based damage detection.

This paper presents gamma radiation effects on resonant and antiresonant characteristics of piezoelectric wafer active sensors (PWAS) for structural health monitoring (SHM) applications to nuclear-spent fuel storage facilities. The irradiation test was done in a Co-60 gamma irradiator. Lead zirconate titanate (PZT) and Gallium Orthophosphate (GaPO4) PWAS transducers were exposed to 225 kGy gamma radiation dose. First, 2 kGy of total radiation dose was achieved with slower radiation rate at 0.1 kGy/h for 20; h then the remaining radiation dose was achieved with accelerated radiation rate at 1.233 kGy/h for 192 h. The total cumulative radiation dose of 225 kGy is equivalent to 256 years of operation in nuclear-spent fuel storage facilities. Electro-mechanical impedance and admittance (EMIA) signatures were measured after each gamma radiation exposure. Radiation-dependent logarithmic sensitivity of PZT-PWAS in-plane and thickness modes resonance frequency (∂(fR)/∂( logeRd)) was estimated as 0.244 kHz and 7.44 kHz, respectively; the logarithmic sensitivity of GaPO4-PWAS in-plane and thickness modes resonance frequency was estimated as 0.0629 kHz and 2.454 kHz, respectively. Therefore, GaPO4-PWAS EMIA spectra show more gamma radiation endurance than PZT-PWAS. Scanning electron microscope (SEM) and X-ray diffraction method (XRD) was used to investigate the microstructure and crystal structure of PWAS transducers. From SEM and XRD results, it can be inferred that there is no significant variation in the morphology, the crystal structure, and grain size before and after the irradiation exposure.

This paper starts a review of the state of the art in structural health monitoring with piezoelectric wafer active sensors and follows with highlighting the limitations of the current approaches which are predominantly experimental. Subsequently, the paper examines the needs for developing a predictive modeling methodology that would allow to perform extensive parameter studies to determine the sensing method’s sensitivity to damage and insensitivity to confounding factors such as environmental changes, vibrations, and structural manufacturing variability. The thesis is made that such a predictive methodology should be multi-scale and multi-domain, thus encompassing the modeling of structure, sensors, electronics, and power management. A few examples of preliminary work on such a structural sensing predictive methodology are given. The paper ends with conclusions and suggestions for further work

We report on the fabrication of ferroelectric BaTiO3 thin films on titanium substrates using pulsed laser deposition and their microstructures and properties. Electron microscopy studies reveal that BaTiO3 films are composed of crystalline assemblage of nanopillars with average cross sections from 100 nm to 200 nm. The BaTiO3 films have good interface structures and strong adhesion with respect to Ti substrates by forming a rutile TiO2 intermediate layer with a gradient microstructure. The room temperature ferroelectric polarization measurements show that the as-deposited BTO films possess nearly the same spontaneous polarization as the bulk BTO ceramics indicating formation of ferroelectric domains in the films. Successful fabrication of such ferroelectric films on Ti has significant importance for the development of new applications such as structural health monitoring spanning from aerospace to civil infrastructure. The work can be extended to integrate other ferroelectric oxide films with various promising properties to monitor the structural health of materials.

This paper presents the perspective of the Structural Mechanics program of the Air Force Office of Scientific Research (AFOSR) on the damage assessment of structures for the period 2006-2009 when the author was serving as Program Manager at AFOSR. It is found that damage assessment of structures plays a very important role in assuring the safety and operational readiness of US Air Force fleet. The current fleet has many aging aircraft, which poses a considerable challenge for the operators and maintainers. The nondestructive evaluation technology is rather mature and able to detect damage with considerable reliability during the periodic maintenance inspections. The emerging structural health monitoring methodology has great potential, because it will use on-board damage detection sensors and systems, will be able to offer on-demand structural health bulletins. Considerable fundamental and applied research is still needed to enable the development, implementation, and dissemination of structural health monitoring technology.