Abstracts:Using ultrasonic arrays to detect and characterise surface breaking cracks is important in the non-destructive evaluation (NDE) field. It can provide early warning of failure and useful information for component integrity assessment. Typically, cracks are approximated by machined slots and used to examine and assess defect detection and characterisation methods. In this paper, real surface breaking cracks are fabricated in 3-point bending specimens following ASTM standard E1820 and used to examine the performance of two array defect characterisation methods: image-based and scattering matrix sizing. In both cases, an array is used to record the full matrix capture (FMC). In image-based sizing, the total focusing method (TFM) is used to form an image from which the defect size is measured directly. This approach is shown to work well for cracks greater than two wavelengths in size. The FMC is also used to extract the defect scattering matrix which is then compared to a pre-computed smooth-crack scattering matrix database. The best match between experiment and this database is found by cross-correlation and used to characterise the defect. This approach is shown to work well for defects in the range of 0.78–1.84 wavelengths. Within these ranges of applicability, both methods show excellent agreement between the known crack length and that measured ultrasonically, with errors less than 19% in all cases.

Abstracts:The in-situ strength of concrete is often evaluated by destructive compressive strength tests conducted on cylinders or cube specimens cast alongside the main structure. Various non-destructive testing (NDT) techniques are also available in the industry. In the past decade, the use of piezoelectric based wave propagation (WP) techniques for monitoring the curing process of concrete has attracted considerable interest from researchers in the civil engineering discipline and hence there is a need to summarize the state of knowledge in order for future research efforts to be focused and relevant. This paper therefore presents a review of recent research and development of the abovementioned research area. Existing NDT techniques and piezoelectric based techniques for the monitoring of concrete curing are initially reviewed. Developments associated with the WP technique employing both embedded and surface bonded piezoelectric transducers are then presented. Finally, successful applications of the WP technique are provided. A wide range of parameters adopted by different researchers as concrete strength indicators are also summarized. Theoretical and numerical models available are reviewed, followed by a summary of practical issues related to the application of this technique. Several studies to date have proven the capability of the WP technique in monitoring the curing process of concrete. More rigorous studies are, however, required before a mature technique is developed that can entice commercial interest. Thus, future areas of investigation are identified in the final section. This article is expected to serve as an introduction for researchers interested in venturing into this area and as a valuable summary to inspire existing researchers for further improving the technique.

Abstracts:The carbon fiber reinforced polymer (CFRP) has been widely used in aerospace, automobile and sports industries. In laminated composite materials, cyclic stresses and impact will cause internal defects such as delamination and debonding. In order to guarantee internal quality and safety, optical pulsed thermography (OPT) nondestructive testing has been used to detect the internal defects. However, current OPT methods cannot efficiently tackle the influence from uneven illumination, and the resolution enhancement of the defects detection remains as a critical challenge. In this paper, a hybrid of thermographic signal reconstruction (TSR) and automatic seeded region growing (ASRG) algorithm is proposed to deal with the thermography processing of CFRP. The proposed method has the capability to significantly minimize uneven illumination and enhance the detection rate. In addition, it has the capacity to automate segmentation of defects. It also overcomes the crux issues of seeded region growing (SRG) which can automatically select the growth of image, seed points and thresholds. The probability of detection (POD) has been derived to measure the detection results and this is coupled with comparison studies to verify the efficacy of the proposed method.

Abstracts:Post-tensioning systems provide safe and efficient construction solutions for long span bridges. Despite the improved grouting practices over the past decade, existing post-tensioning systems may have significant amount of grout defects, which could lead to corrosion of the strands. Condition assessment of post-tensioning systems is necessary to allow bridge owners to take timely, proactive actions to mitigate or prevent further deterioration and unanticipated tendon failures. A detailed experimental study conducted to assess the performance of nondestructive evaluation techniques in detecting grout defects within internal tendons is presented herein. Nondestructive evaluation techniques that include Ground Penetrating Radar, Impact Echo, Ultrasonic Tomography, and Ultrasonic Echo are evaluated in terms of detecting the location and severity of fabricated grout defects in a full-scale post-tensioned U-girder mock-up specimen. While Ground Penetrating Radar can identify the location and profile of the internal tendons, particularly metal ducts due to strong reflections, this method did not provide any information about the defect conditions within the tendon. Both Impact Echo and Ultrasonic Echo techniques are effective in identifying the location of grout defects, but could not differentiate between water, void, or compromised grout conditions. The study clearly demonstrates the need for NDE techniques that are applicable to anchorage regions, and that are capable of estimating the severity and nature of grout defects in internal tendons.

Abstracts:Flaw characterization in eddy current testing usually requires to solve a non-linear inverse problem. Due to high computational cost, Markov Chain Monte Carlo (MCMC) methods are hardly employed since often needing many forward evaluations. However, they have good potential in dealing with complicated forward models and they do not reduce to only providing the parameters sought. Here, we introduce a computationally-cheap surrogate forward model into a MCMC algorithm for eddy current flaw characterization. Due to the use of a database trained off-line, we benefit from the MCMC algorithm for getting more information and we do not suffer from the computational burden. Numerous experiments are carried out to validate the approach. The results include not only the estimated parameters, but also standard deviations, marginal densities and correlation coefficients between two parameters of interest.

Abstracts:In this work, we model ACU transmission in delaminations, with focus in the interference effects resulting from multiple ultrasonic reflections within the delamination layers and the resulting changes in the amplitude and time of flight. For this purpose, we propose a simplified analytical model, which we cross-validate with full-wave finite-difference time-domain (FDTD) simulations. Both models show a very high agreement on the predicted ultrasound waveforms, with amplitude deviations less than 0.15 dB and time deviations below 0.1 μs. The reduction of ultrasound signal amplitude at debonding was validated with experiments. A simple engineering formula in function of delamination gap and transducer frequency was sufficient to model experimental transmission values for a gap thickness range from 70 to 2000 μm with an uncertainty below 2 dB. Furthermore, consistent resonance frequencies were identified in both experiments and simulations. The use of pulsed ultrasound signals reduces undesired resonances and provides a consistent amplitude reduction across the full range of delamination gaps. Apart from interference effects, the effect of the finite size of the ultrasound transducers as well as diffraction effects were empirically investigated. As a result of these, the lateral resolution is reduced and the sound tends to propagate through the bonded region next to the delamination. Diffraction effects are strongly influenced by wood anisotropy, with lower lateral resolution in the grain direction.