Key Engineering Materials Vol. 558

Abstract: This paper investigates the potential of single transducer pair guided waves time reversal to detect damage in composite laminates. According to dynamic reciprocity of Lamb waves propagation in linear media, the time reversal process should reconstruct the original signal. The similarity of original and reconstructed time signals is calculated for different damage types using numerical and experimental studies with the aim to investigate, if the interaction of the wave pulse with inhomogeneities introduces any nonlinearity that time reversibility breaks down and single transducer pair time reversal could be used as damage diagnostics tool. 3D explicit finite element analysis is used for the numerical simulation and laser Doppler vibrometry is used to capture out-of-plane displacement time histories excited by an adhesively bonded piezoceramic transducer disc in the experimental time reversal process. In the case of an undamaged composite laminate the similarity index used to quantify the similarity of the original and reconstructed wave pulses is better than 95%. The similarity index is smaller for laminates with artificial damages including embedded fluoro polymer films to simulate delamination damage, through holes and bonded mass inhomogeneities. Although numerical and experimental similarity indices are smaller at higher frequencies, there is no clear evidence that single transducer pair time reversibility breaks down and represents a reliable damage diagnostics tool.

Abstract: The Brillouin optical correlation domain analysis (BOCDA) technology is one of the distributed optical sensing technologies utilizing the Brillouin scattering phenomena. The authors are developing, verifying and validating this technology. They are also developing the installation technique of optical fiber sensors, evaluating the durability of the BOCDA system and manufacturing the new device compatible to avionics bays for size. Furthermore, the authors are also improving the monitoring ability for composite damages. This paper reports the developmental status of the BOCDA monitoring ability for composite damages such as debonding at adhesive joints and micro-damages at bolted joints. First, debonding detection tests were conducted using carbon fiber reinforced plastics (CFRP) stiffened or repaired panels. In this test, test plates with a sheet stringer or a repair patch were applied tension load, and expanding of debonding areas was monitored by measuring strain distribution changes near these areas. Second, bearing damage detection tests were conducted using CFRP bolted joint specimens. In this test, multi-fastener single-lap CFRP specimens were applied tension load, and occurrences of micro-damages near bolt holes was detected. Micro-damages were detected by monitoring shapes changes of the Brillouin Gain Spectrum, which is one of the BOCDA measurement results.

Abstract: A displacement measurement provides useful information for structural health monitoring (SHM) as it is directly related to stiffness of the structure. Most existing methods of direct measurement such as the Laser Doppler Vibrometer (LDV) and the Liner Variable Differential Transformer (LVDT) are known to have accurate performance but have difficulties particularly in the use of large-scale civil structures as the methods rely on fixed reference points. Alternatively, indirect methods have been developed and widely used methods are Global Positioning System (GPS), vision-based displacement measurement system and displacement estimation from acceleration record. Among the indirect method, the use of accelerometer provides simple and economical in term of both hardware installation and operation. The major problem using acceleration based displacement estimation is low frequency drift caused by double integration. Recently, dynamic displacement estimation algorithm that addresses low-frequency drift problem has been developed. This study utilizes Wireless Smart Sensor (WSN) for estimating dynamic displacement from acceleration measurement in combination with the recently developed displacement estimation algorithm. Integrated into WSN that are low-cost, wireless, compatible with accelerometers, and capable of onboard computation, the displacement can be measured without limit of location on large-scale civil structures. Thus, this approach has the significant potential to impact many applications that require displacement measurements. With the displacement estimation algorithm embedded, the WSN performs in-network data processing to estimate displacements at each distributed sensor location wirelessly using only measured acceleration data. To experimentally validate the performance of displacement estimation using WSN for the use in structures with multiple-degree of freedom, the random vibration test is conducted on the three-story shear building model. The estimated displacement is compared with the reference displacements measured from the laser displacement sensor and the result shows good agreement.

Abstract: Power spectral measurements are very ubiquitous for their utility in generating structural health monitoring (SHM) features, because of their clear physical interpretation and easy computation through Fourier transform. In most SHM applications, optimal features are always desired to perform whatever level of assessment is required. Optimal in this sense refers to a measure of performance capability to enhance decision-making, because structural health monitoring inevitably involves, at some level, a hypothesis test: in the binary case, the question becomes are the features extracted from data derived from a baseline condition (baseline can also mean linear, or any reference condition designated the null hypothesis) or ...from data derived from a different (test) condition. Inevitably, this decision involves stochastic data, as any such candidate feature is compromised by noise, which we may categorize as (i) operational and environmental, (ii) measurement, and (iii) computational/estimation. Regardless of source, this noise leads to the propagation of uncertainty from inception to final estimation of the feature; in all cases, the subsequent distribution of the features can lead to significant false positive (Type I) or false negative (Type II) errors in the classification of the features via the hypothesis test. Frequency domain approaches for SHM typically involve estimation of some form of transfer function, typically the usual frequency response function (FRF). Based upon the statistical modeling of the uncertainty of feature estimations, this paper evaluates the performance of two FRF-derived features, namely the dot-product difference (DPD) and Euclidian distance (ED), and statistical significance detection qualities are quantitatively compared. In each of the feature evaluations, the performance comparison is executed under the condition of best trade-off between sensitivity and specificity, adopting receiver operating characteristics as the performance indicator. Monte Carlo simulation and lab-scaled tests on plate-like structures are both implemented to validate the optimal feature selection process and demonstrate performance enhancement. The comparisons are facilitated through computation of receiver operating characteristics (ROCs), which are data-driven methods for comparing detection rates to error rates as a function of decision boundaries established between data distributions, independent of the actual underlying distribution.

Abstract: Existing damage imaging techniques rely on the use of active sensors, such as piezoelectric actuators, that can both transmit and receive guided waves. This paper presents a new time-reversal imaging approach to enable the use of passive sensors, such as optical fibre sensors and strain gauges, to augment active sensors for imaging structural damage. Computational simulations have revealed that damage size and severity can be accurately determined from the scattered wave using as few as six sensors: one active sensor and five passive sensors.

Abstract: The authors proposed fiber-optic-based damage monitoring of carbon fiber reinforced plastic (CFRP) bolted joints. Optical fibers were embedded along bolt holes and strain change along the optical fiber induced by internal damage was measured by a Brillouin Optical Correlation Domain Analysis (BOCDA), which is a high spatial resolution distributed strain sensing system. This study began by investigating damage modes of CFRP bolted joints after bearing failure. Effective embedding positions of optical fibers were then proposed and their feasibility was evaluated by finite element analysis simulating the damage propagation in the bolted joint and consequent strain change. Finally, verification tests were conducted using specimens with embedded optical fibers at various positions. It was clearly shown that damage could be detected using residual strain due to fiber-microbuckling (kinking) damage or permanent deformation of neighboring plies. Furthermore, damage size and direction could be estimated from the change in the strain distribution. The system developed is quite useful for a first inspection of large-scale composite structures in aerospace applications.

Abstract: Fatigue crack growth in metallic plates was monitored using Lamb waves which were generated and captured by surface-mounted piezoelectric wafers in a pitch-catch configuration. Instead of directly pinpointing signal segments to quantify wave scattering caused by the existence of crack damage and related severity, principal component analysis (PCA), as an efficient approach for information compression and classification, was undertaken to distinguish different structural conditions due to fatigue crack growth. For this purpose, a variety of statistical parameters in the time domain as damage indices were extracted from the wave signals. A series of contaminated counterparts with different signal-to-noise ratios were also simulated to increase the statistical size of the data set. It was concluded that PCA is capable of reducing the dimensions of a complex set of original data, whose information can be represented and highlighted by the first few principal components. With the assistance of PCA, the different structural conditions attributable to crack growth can be classified.

Abstract: The thermal protection systems of spacecraft are vulnerable to damage from impacts by foreign objects moving at high velocities. This paper describes a proposed novel structural health monitoring system that will detect, locate and evaluate the damage resulting from such impacts. This system consists of a network of intelligent local agents, each of which controls a network of piezoelectric acoustic emission sensors to detect and locate an impact, and a network of optical fibre Bragg grating sensors to evaluate the effect of the impact damage by means of a thermographic technique. The paper concentrates on two issues that are critical to the successful implementation of the proposed SHM system: measurement of the elastic properties of the thermal protection material, knowledge of which is essential to the design and operation of the acoustic emission sensor network; and investigation of the practical feasibility of a switched network of optical fibre sensors.

Abstract: Active evacuation guidance using a sensor agent is explored. The sensor agent robot is equipped with a high precision servo-accelerometer with a uplifting mechanism for axles to ensure accurate measurement of floor response. The story drift angle is estimated from the acceleration data obtained by the sensor agent robot to decide whether evacuation is needed or not. This sensor agent robot is used for services in the living space in a normal situation. Thus, the evacuation system can be economical and feasible as the system for this evacuation added to the robot is small.

Abstract: SHM systems are becoming feasible with the growth of computer and sensor technologies during the last decade. However, high implantation cost prevents SHM from becoming common in general buildings. The reason of this high cost is partially due to many accelerometers. In this research, we propose a mobile sensor agent robot with accelerometers and a laser range finder (LRF). If this robot can properly measure accurate acceleration data, the cost of SHM would be cut down and the SHM systems would become common. Our goal is to develop a platform for SHM using the sensor agent robot. We designed the prototype robot to detect the floor vibrations and acquire the micro tremor information correctly. When the sensor agent robot is set in the mode of acquiring the data, the dynamics of the robot should be tuned not to be affected by its flexibility. To achieve this purpose the robot frame was modified to move down to the floor and to provide enough rigidity to obtain good data. In addition to this mechanism, we tested an algorithm to know the location of the robot and the map of the floor correctly to be used in the SHM system using the LRF and Simultaneously Localization and Mapping (SLAM).