Jinying Zhu

Leaky Rayleigh and Scholte waves at the fluid–solid interface subjected to transient point loading

The analysis of acoustic waves generated by a transient normal point load applied on a fluid–solid interface is presented. The closed-form exact solution of the wave motion is obtained by using integral transform techniques. The obtained analytical solution provides necessary theoretical background for optimization of fluid-coupled ultrasonic and acoustic wave detection in experiments. Numerical simulation (elastodynamic finite integration technique) is performed to verify the obtained analytical solution. Detailed descriptions of leaky Rayleigh and Scholte wave solutions are presented. A simplified solution to isolate the contributions of leaky Rayleigh and Scholte waves generated by a transient point load is proposed, and closed-form formulations for displacement and stress components are then presented. The simplified solution is compared to the exact solution for two configurations: water/concrete and air/concrete. The excitation effectiveness of leaky Rayleigh waves for the air/concrete configuration...

Nondestructive Bridge Deck Testing with Air-Coupled Impact-Echo and Infrared Thermography

Two different nondestructive test (NDT) methods, air-coupled impact-echo (IE) and infrared (IR) thermography are evaluated on a full-scale simulated reinforced concrete bridge deck containing simulated delamination and cracking defects. The IE data are presented as two-dimensional frequency maps and spectral B-scan lines. The IR data are presented as temperature maps on the concrete surface. The lateral boundaries of the detected delaminations are also indicated in the images. The results obtained from each of the individual NDT methods show reasonably good agreement with most of the actual defects. The advantages and limitations of each method to characterize defects are discussed. The consistency and sensitivity of each method are also investigated. Finally, a simple data fusion technique is proposed to improve effectiveness of the individual test data. The findings from this study demonstrate that the combination of air-coupled IE and IR thermography tests is a practical option for consistent and rapid in situ evaluation of reinforced concrete bridge decks.

Using air-coupled sensors to determine the depth of a surface-breaking crack in concrete

Previous studies showed that the surface wave transmission coefficient across a surface-breaking crack in concrete can be used to estimate the crack depth. However, inconsistencies in the surface wave transmission measurements limit the test accuracy and application of this technique. The inconsistencies come from near-field scattering by the crack tip and inconsistent sensor coupling conditions on rough concrete surfaces. This study first investigates the near-field size based on numerical analyses, and then suggests that reliable surface wave transmission should be measured in the far field. Based on the far-field measurement, the relationship between the surface wave transmission ratio and the normalized crack depth (crack depth/wavelength) is obtained. In the experimental study, the air-coupled sensing method is proposed as a solution to the sensor coupling problem. Owing to the non-contact feature, the air-coupled sensing method not only improves testing speed but also enables more consistent signal measurement. The experimental study using air-coupled sensors shows good agreement with the results of numerical simulation and analytic solution.

Comparison of NDT Methods for Assessment of a Concrete Bridge Deck

The field application of three different nondestructive tests (NDTs)—air-coupled impact echo (IE), infrared (IR) thermography, and sounding (chain drag)—are evaluated in this paper, where an actual in-service concrete bridge deck is tested. Two different contactless IE test equipment sets are deployed as part of an effort to develop new rapid measurement methods. The IE data are presented as two-dimensional frequency maps, and the IR data are presented as temperature maps over the tested area. Sounding (chain-drag) result maps are also presented. For verification of the location of near-surface delamination damage, eight drilled core samples were extracted from the test area. The results obtained from each of the individual NDT methods show reasonably good agreement with the drilled cores in terms of locating near-surface delamination. Finally, the NDT methods are compared across general performance criteria, considering accuracy, testing practicality, and costs. The analysis shows that all of the evaluated NDT methods are comparable, and the chain-drag method is not more accurate and reliable for detection of shallow delamination in the deck.

Monitoring early age property of cement and concrete using piezoceramic bender elements

Ultrasonic waves are commonly used for non-destructive evaluation of concrete structures. For fresh concrete, ultrasonic waves have also been used to monitor concrete setting and strength development at early ages. However, the conventional ultrasonic test set-up typically needs access to the two opposite sides of concrete structures, which is not always possible for in situ field testing. In this paper, embedded piezoceramic bender elements are proposed to measure ultrasonic shear (S) waves in fresh cement paste, mortar and concrete. The shear wave velocities are obtained from B-scan images of a collection of recorded signals over time. Experimental results indicate that the shear wave velocity is closely related to the setting time, and this relationship is independent of air void content and w/c of cement pastes. The low cost bender elements can also be of use to monitor the setting of fresh concrete and the long term evaluation of hardened concrete. (Some figures in this article are in colour only in the electronic version)

Crack Depth Estimation in Concrete Using Energy Transmission of Surface Waves

A promising nondestructive concrete surface-breaking crack depth inspection technique is the self-calibrating surface wave transmission method. In part due to measurement data variability, however, there is difficulty in using the transmission function cut-off frequency to determine crack depth. In this paper, the proposed method for concrete structure crack-depth estimation is the spectral wave-energy transmission method employing the self-calibrating configuration. The authors report of experimental study results concerning concrete slab with varying crack (notch) depths. Validation of the proposed methods effectiveness is performed through conventional time-of-flight and cut-off frequency-based methods comparison. That, for concrete structure in-place depth estimation, there is excellent potential for spectral energy transmission to be a practical and reliable in-lace nondestructive method is shown in the results.

Use of parabolic reflector to amplify in-air signals generated during impact-echo testing.

The impact-echo method is a commonly used nondestructive testing technique for elastic plates in civil engineering. The impact-echo mode corresponds to the frequency at zero group velocity of S(1) Lamb mode. Recent development of the air-coupled impact-echo (ACIE) method introduces the possibility for rapid scanning of large structures and increases the practicality of in situ measurements. However, sensors used in ACIE are susceptible to ambient noise, which complicates in situ ACIE measurements. This letter presents the results of ACIE measurements taken using a parabolic reflector together with standard measurement microphones to increase the signal to noise ratio for ACIE measurements. The signal gain and effects of sensor location with respect to impact location are discussed.

Using piezoelectric sensors for ultrasonic pulse velocity measurements in concrete

The ultrasonic pulse velocity (UPV) test has been a widely used non-destructive testing method for concrete structures. However, the conventional UPV test has limitations in consistency of results and applicability in hard-to-access regions of structures. The authors explore the feasibility of embedded piezoelectric (PZT) sensors for ultrasonic measurements in concrete structures. Two PZT sensors were embedded in a reinforced concrete specimen. One sensor worked as an actuator driven by an ultrasonic pulse-receiver, and another sensor worked as a receiver. A series of ultrasonic tests were conducted to investigate the performance of the embedded sensors in crack-free concrete and concrete specimens having a?surface-breaking crack under various external loadings. Signals measured by the embedded sensors show a broad bandwidth with a centre frequency around 80 kHz, and very good coherence in the frequency range from 30 to 180 kHz. Furthermore, experimental variability in ultrasonic pulse velocity and attenuation is substantially reduced compared to previously reported values from conventional UPV equipment. Findings from this study demonstrate that the embedded sensors have great potential as a low-cost solution for ultrasonic transducers for health monitoring of concrete in structures.

A focused electric spark source for non-contact stress wave excitation in solids

A focused electric spark is used as a non-contact acoustic source to excite stress waves in solids. The source consists of an electric spark source located at the near focus of an ellipsoidal reflector that focuses the acoustic disturbance generated by the spark source to the far focal point. Experimental studies using both contact and non-contact sensors indicate that the source has the capability to excite the Rayleigh surface wave and impact-echo mode (S1-zero-group-velocity Lamb mode) in a 250 mm thick concrete slab and to enable fully air-coupled testing of concrete specimens.

Multisensor data fusion for impact-echo testing of concrete structures

Based on the investigation of spatial variations of impact-echo signals for different source and receiver locations, a simple multisensor data fusion strategy is proposed to increase the accuracy of nondestructive evaluation of concrete structures using the impact-echo test. The data fusion strategy fuses the ratios between spectral amplitudes of the delamination and concrete bottom echo signals (D/B ratios) from multiple source–receiver arrays. The fused D/B ratios demonstrate different characteristics for test locations above the delamination, above the sound concrete, and across the delamination boundary. These characteristics can be used to accurately locate the delamination and its boundaries without increasing testing time. The applicability of the multisensor data fusion for impact-echo testing is validated using both numerical simulation and experimental testings.