Electromechanical Impedance Technique Research Articles

Transformation temperatures of shape memory alloy based on electromechanical impedance technique

This paper presents the use of Electromechanical Impedance (EMI) technique as an alternative tool to measure phase transformation temperatures of Shape Memory Alloys (SMAs). Differential Scanning Calorimetry (DSC) is the most common tool used to identify the transformation temperatures of SMAs. Meanwhile this work proposes a method based on a different operating principle, named as EMIH (Electromechanical Impedance Heterostructure). Samples composed of Lead Zirconate Titanate (PZT) ceramics and Ni-Ti alloys were investigated. The proposed method is quantitatively evaluated by means of impedance magnitude and phase angle measurements with different fixed frequencies, besides sweep frequency analysis. These analyses are intended to observe the phase transformation process by means of an impedance-based technique under thermal cycling. PZT/Steel, PZT/Copper and PZT/Brass samples were also analyzed by EMI in order to compare results with PZT/SMA, and for validating purposes. Experimental phase transformation temperatures present errors between 4.9% and 35.2% when compared to DSC method.

Strength development monitoring and dynamic modulus assessment of cementitious materials using EMI-Miniature Prism based technique

Electromechanical impedance (EMI) technique provides an alternative means of characterizing strength development of early age concrete on a real-time basis. However, most existing studies employing the technique heavily rely on statistical tools for strength development characterization. This article proposes a new impedance-based approach to strength and dynamic modulus assessment of cementitious materials. In this approach, a lead zirconate titanate patch is surfaced-bonded on a customized cementitious material specimen, known as ‘Miniature Prism’, in which the conductance signatures throughout the curing process are acquired. A 3D coupled field finite element (FE) model is then developed to compute the conductance signatures and model updating is performed using the experimental results. The conductance signatures computed by the updated FE model are found to be in good agreement with experimental results. The key contribution of this approach is the use of ‘Miniature Prism’ which ensures consistency of the resonance peaks in the conductance spectrum between identical specimens. This has been very difficult, if not impossible, to achieve with the conventional EMI technique. This merit allows for modelling of the electromechanical system and hence parametrically predicting the dynamic modulus of elasticity of the cementitious material throughout the curing process. Comparative study is also conducted on various conventional and advanced techniques and results indicate that the proposed technique is effective in strength assessment of cementitious materials. In addition, the technique is suitable for autonomous online monitoring purpose, and thus exhibits promising potential to substitute the conventional non-destructive testing methods.

Numerical and experimental study on crack identification based on the piezoelectric ceramic lead zirconate titanate impedance technology

The structural health monitoring system with electromechanical impedance technique is studied numerically and experimentally in this article. A three-dimensional numerical model is developed to acc...

Quantitative assessment of the concrete gravity dam damage under earthquake excitation using electro-mechanical impedance measurements

A rare but remarkable accident is the dam failure due to earthquake that can trigger catastrophes on economic and life losses. It is essential to monitor the presence and development of seismic damage of dam structures in the early stage of emergence of damage and take steps to prevent the full structural failure. In this research, only seismic damage and dynamic performances of a roller compacted concrete (RCC) dam were investigated using an electromechanical impedance (EMI)–based structural health monitoring (SHM) technique enabled by lead-zirconate-titanate (PZT) ceramic sensors network. The finite element model of the PZT sensor and damage monitoring method were verified by comparing the scanning conductance signals obtained from the experimental tests. A quasi 3D finite element model of the RCC dam using concrete damaged plasticity (CDP) constitutive model was built and assumed to be subjected to different degrees of earthquake excitations to initiate the dam damage due to tension. The conductance signals were measured by the PZT sensors network which are excited by inputting unit harmonic voltage. In addition, the normalized root-mean-square deviation (RMSD) was used as a damage index to quantify and evaluate the variations in conductance signals measured at the PZT sensors. Furthermore, two damage indices, namely the RMSD based damage volume ratio (DVR) and RMSD based displacement, were proposed to predict the damage levels and maximum horizontal displacement at the crest of dam under seismic loading. It is clearly demonstrated that the EMI technique based PZT sensors network can keenly detect the seismic damage of dam and the proposed damage indices can effectively quantify the damage levels and predict the dynamic response performances.

Looseness Monitoring of Bolted Spherical Joint Connection Using Electro-Mechanical Impedance Technique and BP Neural Networks.

The bolted spherical joint (BSJ) has wide applications in various space grid structures. The bar and the bolted sphere are connected by the high-strength bolt inside the joint. High-strength bolt is invisible outside the joint, which causes the difficulty in monitoring the bolt looseness. Moreover, the bolt looseness leads to the reduction of the local stiffness and bearing capacity for the structure. In this regard, this study used the electro-mechanical impedance (EMI) technique and back propagation neural networks (BPNNs) to monitor the bolt looseness inside the BSJ. Therefore, a space grid specimen having bolted spherical joints and tubular bars was considered for experimental evaluation. Different torques levels were applied on the sleeve to represent different looseness degrees of joint connection. As the torque levels increased, the looseness degrees of joint connection increased correspondingly. The lead zirconate titanate (PZT) patch was used and integrated with the tubular bar due to its strong piezoelectric effect. The root-mean-square deviation (RMSD) of the conductance signatures for the PZT patch were used as the looseness-monitoring indexes. Taking RMSD values of sub-frequency bands and the looseness degrees as inputs and outputs respectively, the BPNNs were trained and tested in twenty repeated experiments. The experimental results show that the formation of the bolt looseness can be detected according to the changes of looseness-monitoring indexes, and the degree of bolt looseness by the trained BPNNs. Overall, this research demonstrates that the proposed structural health monitoring (SHM) technique is feasible for monitoring the looseness of bolted spherical connection in space grid structures.

Performance assessment method for crack repair in concrete using PZT-based electromechanical impedance technique

Concrete cracks are considered as an important indicator for potential damage and durability and are generally monitored in structural maintenance. A typical repair method in practice for concrete cracks involving with durability issues is to use alternative materials to fill the cracks. As the recovery quality of the repair is difficult to determine, an effective assessment approach is necessary. The electromechanical impedance (EMI) technique is a widely favored method; however, the determination of the frequency range and corresponding damage indicator is a governing factor for practical applications. Furthermore, its application to the evaluation of the crack repair performance has not been reported in the literature to date. This study presents an applicability of the PZT-based EMI technique for the quality assessment of crack repair in concrete. To determine an optimal frequency range that strongly reflects the structural condition, the trends of four damage indicators are investigated with multiple damage severities in each sub-frequency range. Subsequently, the performance of the crack repair material is evaluated using the predefined optimal frequency range with the damage indicators. The proposed approach is shown to effectively assess the recovery quality of the crack repair material over time.

Damage Detection Performance of the Electromechanical Impedance (EMI) Technique with Various Attachment Methods on Glass Fibre Composite Plates.

Composite materials such as glass and carbon fibre composites have become popular and the preferred choice in various applications due to their many advantages such as corrosion resistance, design flexibility, high strength and light weight. Combining materials with different mechanical properties make composites more difficult to evaluate where the damage mechanisms for composites are more complex than traditional materials such as steel. A relatively new non-destructive testing (NDT) method known as the electromechanical impedance (EMI) technique has been studied by various researchers, but the damage detection performance of the method on composite structures still requires more investigations before it can be accepted for field application, especially in aerospace industry due to the high standard of safety. In this paper, the detection capabilities and performance of the EMI technique subjected to different PZT attachment methods have been investigated. To this end, glass fibre composite plates with various attachment methods for the sensor have been prepared and detection of common defects such as delamination and crack with the EMI technique under study has been performed. The performance of each attachment method for identifying different damage types has been analysed and finite element analysis (FEA) was carried out for verification of the experimental results.

Comparative analysis of signal processing techniques for impedance-based SHM applications in noisy environments

Structural health monitoring (SHM) systems have been extensively studied in recent decades to determine the health statuses of mechanical, naval and aerospace engineering components. Currently, one of the most promising non-destructive tests for the detection of structural damage is the electromechanical impedance (EMI) technique, which uses low-cost, surface-bonded piezoelectric transducers operating both as sensors and actuators. Although many studies have reported the effectiveness of the EMI method, numerous practical issues, such as signal noise effects caused by environmental conditions, may severely affect the detection and quantification of damage. Therefore, this paper presents a comparative analysis of three signal processing techniques used in the context of the EMI method, which have the potential to enhance the detection of structural damage under environmental signal noise effects. These signal analysis methods include (i) damage indices, such as the correlation coefficient deviation metric, computed directly on impedance signatures; (ii) the wavelet transform computed on transducer response signals in the time domain; and (iii) a novel approach of damage feature extraction in the EMI method based on the Hinkley criterion. Experimental tests were carried out to analyse the three signal processing techniques on a damaged aerospace composite carbon fibre structure subjected to various signal noise levels. The experimental results revealed that conventional damage indices and wavelet transform were significantly affected by noise, whereas the proposed approach based on the Hinkley criterion was more effective for detecting damage in noisy environments.

New signal processing approach for structural health monitoring in noisy environments based on impedance measurements

The electro-mechanical impedance (EMI) technique is one of the most promising structural health monitoring (SHM) approaches for material damage detection, which is based on impedance measurements of low-cost piezoelectric transducers. However, numerous practical issues such as signal noise effects caused by environmental conditions can alter signal measurements and limit the capabilities of the EMI technique when the characterization of damage is performed using conventional basic indices. Therefore, this paper proposes a new index for structure feature extraction based on the cross-correlation signal processing technique that can be applied in real noisy environment. The proposed index was evaluated in the frequency domain, where the damage detection is performed directly on the electrical impedance measurements of the transducer, as well as on the time domain, which is based on the wavelet transform applied to the transducer response signal. Experimental tests were carried out on a damaged aluminium structure subject to various signal noise levels. Experimental results revealed that the proposed approach for material feature extraction under noisy environments proved to be effective for detecting damage, thus enhancing the reliability and expanding the applicability of the EMI technique.

Dressing Tool Condition Monitoring through Impedance-Based Sensors: Part 2-Neural Networks and K-Nearest Neighbor Classifier Approach.

This paper presents an approach for impedance-based sensor monitoring of dressing tool condition in grinding by using the electromechanical impedance (EMI) technique. This method was introduced in Part 1 of this work and the purpose of this paper (Part 2) is to achieve an optimal selection of the excitation frequency band based on multi-layer neural networks (MLNN) and k-nearest neighbor classifier (k-NN). The proposed approach was validated on the basis of dressing tool condition information obtained from the monitoring of experimental dressing tests with two industrial stationary single-point dressing tools. Moreover, representative damage indices for diverse damage cases, obtained from impedance signatures at different frequency bands, were taken into account for MLNN data processing. The intelligent system was able to select the most damage-sensitive features based on optimal frequency band. The best models showed a general overall error lower than 2%, thus robustly contributing to the efficient automation of grinding and dressing operations. The promising results of this study foster the EMI-based sensor monitoring approach to fault diagnosis in dressing operations and its effective implementation for industrial grinding process automation.

Piezoelectric-based monitoring of the curing of structural adhesives: a novel experimental study

Structural adhesives are employed as bonding agents in the construction industry, to externally bond fibre-reinforced polymer (FRP) composite materials onto existing structures for strengthening and repair purposes. The strength development of such structural adhesives throughout the curing process is critical to the success of the system. Monitoring such strength development is thus important, to ensure that the adhesive has been suitably installed. This paper reports a proof-of-concept experimental study that assesses the feasibility of adopting lead zirconate titanate (PZT)-based monitoring techniques for real-time monitoring of the curing process of structural adhesives. More specifically, the electromechanical impedance (EMI) technique is used. As verification of the EMI technique, the well-established wave propagation (WP) technique is employed. Tensile tests, which are conducted in conjunction with the PZT-based monitoring tests, serve as control specimens. For the EMI technique, both the structural and PZT resonance peaks increased rapidly in the first 24 h of curing. The rate of movement of the resonance peaks, however, decreased for curing durations ranging from 24 h to seven days. The peaks then showed negligible movement for curing durations of seven to 21 days. A similar observation is found in the velocity of the elastic wave obtained from the WP technique. Overall, both monitoring techniques correlate well with the physical changes of the structural adhesive throughout the curing process and the tensile tests. The piezoelectric-based technique is therefore confirmed as a viable technique for monitoring the curing process of structural adhesives.

Measurement of Unit Weight of Dry Sand Using Piezoelectric Sensor

Recently, an electro-mechanical impedance (EMI) technique has found wide application in the monitoring of concrete structures. This EMI technique was employed for measuring the unit weights of various grain sizes of sand and the relevant process is reported herein. A piezoelectric sensor (PZT, i.e., lead zirconate titanate sensor) was imbedded into small, medium, and large-sized grains of the Nakdong river sand and a surcharge was applied. The effect of an increase in unit weight owing to the surcharge was investigated in terms of the resonant frequency (peak frequency) and corresponding conductance at resonant frequency (peak conductance). A ceramic-coated PZT sensor was used to prevent the occurrence of a short circuit. The measured peak frequency and conductance from the PZT sensor were correlated with the unit weight of sand. With an increase in the unit weight of sand, the peak PZT frequency was found to have increased, however, the peak conductance was seen to have decreased. The peak PZT frequency and conductance were seen to be strongly correlated with the unit weight of Nakdong river sand except for the peak conductance of large grain sized sand.

Loosening Monitoring of a Threaded Pipe Connection Using the Electro-Mechanical Impedance Technique-Experimental and Numerical Studies.

Threaded connections are the most common pipe fittings used in oil and gas transportation systems. Due to external vibrations, cyclic loads, and pollution, the fitting parts may start getting loose, which could result in pipeline leaks and other environmental disasters. It is of great significance to develop a reliable technique that could provide real-time monitoring of the looseness of pipeline fittings. In this paper, a piezoceramic-based active sensing method combined with the electro-mechanical impedance (EMI) technique was developed to monitor the health condition of threaded pipe connections in real time. Two pipe segments coupled with a threaded coupling fitting were assembled in the laboratory, and a lead zirconate titanate (PZT) patch was surface bonded onto the coupling part. In the experiment, the PZT impedance signatures were measured at each simulated loosening condition. A root-mean-square deviation (RMSD) method was employed to build a looseness index from the measured impedance signatures. To verify the effectiveness of the developed EMI technique, the experimental results were compared with those computed from a numerical simulation. The good agreement from experimental and numerical results highlights that the developed piezoceramic-based EMI technique has great potential for determining early looseness, as well as for monitoring the health status of the pipeline fitting during its service life.

Numerical evaluation of nonbonded piezo sensor for biomedical diagnostics using electromechanical impedance technique.

Directly bonded piezo sensor, conventionally employed in the electromechanical impedance (EMI) technique, although a proven candidate for structural health monitoring, is severely constrained in its application in the biomedical field due to its bonding requirement. In contrast, nonbonded piezo sensor (NBPS) provides a viable platform to assess the condition of human bones, tissues, and other biomedical subjects using the EMI technique without inflicting pain or irritation to the skin. The name NBPS was coined to emphasize that there was no direct bonding between the PZT patch and the live subject; instead, the PZT patch was bonded to a supporting medium, which maintains the mechanical interaction between the PZT patch and the subject. However, there are several aspects in the analysis of NBPS configuration that cannot be addressed completely through experimental study due to measurement constraints, cost, and time. For example, experimentally changing the density of bone continuously to study the osteoporosis effect is a tedious task warranting large number of specimens. This paper presents a detailed parametric study based on finite element method covering condition monitoring of human bones using the NBPS configuration. It is for the first time that 3D analysis for specimen identification and damage detection in bones using NBPS has been carried out. In addition to the validation of the numerical model against the previously established experimental studies involving bones, quantification of the extent of damage and its localization has been investigated. The density changes due to osteoporosis in bones are comprehensively investigated by the NBPS including the quantification aspect of osteoporosis/damage. Definite acquisition of bone signature and detection of physiological changes in bones are achieved even with the presence of skin, muscle, and fat layers on the bone.

EMI based monitoring of early-age characteristics of concrete and comparison of serial/parallel multi-sensing technique

Changes in admittance signatures measured using Electro-Mechanical Impedance (EMI) technique with a lead zirconate titanate (PZT) transducer, embedded in concrete is an indirect estimate of the localised changes in dynamic stiffness of the concrete in the proximity of the sensor. Experiments and observations of the EMI signatures obtained from PZT sensors bonded onto identical plates and embedded in concrete cubes with two sets of curing have been presented. Also, experiments are conducted on a concrete beam embedded with six PZT sensors bonded on carrier plates of differing thickness by subjecting the beam to differential curing. Feature extraction on the admittance signatures is carried out to identify the early age strength gain characteristics in the concrete. EMI measurements have been continued for 28 days until the signatures asymptote to vanishing changes. The contribution of the present work is the application of serial/parallel connected sensors embedded in concrete (in a Multi Input Single Output, MISO mode), that can be typically deployed in a large scale structural health monitoring (SHM) scenario. A novel methodology for separating the electrical and mechanical coupling in a multiplexed sensing scenario is a notable contribution. A hitherto unreported invariant factor which is obtained as the ratio of the parallel and serial admittance signatures is reasoned out and the removal of this results in separating the constant electrical admittance from the mechanical admittance. Well separated resonant peaks in the admittance signature is a pre-requisite for tracking and observing the changes, which is achieved through plates of different thickness resulting in different ultrasonic resonant modes. In the composite EMI signature which is representative of many sensors, individual peaks could be associated with separate sensors and their influencing regions.

Electro-Mechanical Impedance (EMI) Based Interlayer Slide Detection Using Piezoceramic Smart Aggregates-A Feasibility Study.

Interlayer slide damage is one of the main causes of landslide hazard, inflicting huge economic losses and casualties. It is urgent to accurately detect the initiation and development of the interlayer slide damage in real time. In this paper, a study on the feasibility of using the electro-mechanical impedance (EMI) technique to detect the interlayer slide damage was presented. The main purpose of this paper is to investigate the application of the EMI technique for interlayer slide detection using piezoceramic smart aggregates (SAs). In the experimental study, three small landslide specimens with a weak interlayer in the middle were fabricated. For each specimen, three piezoceramic SAs were post-embedded at specific positions, which were located above the weak interlayer inside the structure. The specimens were subjected to a compressive test to initiate an interlayer slide along the weak layer. The whole loading process was monitored with a precision impedance analyzer by measuring the admittance (reciprocal of impedance) of the SAs over time. The statistic metrics, including root mean square deviation (RMSD) and mean absolute percentage deviation (MAPD), were introduced to quantify the variations in admittance signatures due to interlayer slide damage. It was found that the admittance signatures and statistic metrics were sensitive to the interlayer slide damage. The experimental results verify the feasibility and practicality of using EMI technique to detect the interlayer slide.

Damage and retrofitting monitoring in reinforced concrete structures along with long-term strength and fatigue monitoring using embedded Lead Zirconate Titanate patches

This article aims at developing a generic system for the damage and retrofitting monitoring along with long-term strength and first-stage fatigue monitoring of reinforced concrete structures using embedded Lead Zirconate Titanate sensors in the form of concrete vibration sensors. The concrete vibration sensor is a ready-to-use sensor, and its unique packaging renders it very compatible for embedment in reinforced concrete structures. In addition to cost-effectiveness, the concrete vibration sensors are also characterized by excellent structure-compatibility and durability. In this article, both finite element method and experimental investigations have been employed to establish the feasibility of using curvature (second-order derivative) and other higher order derivatives of displacement mode shapes for damage detection and retrofitting assessment. The experiments are conducted on a real-life-sized reinforced concrete beam. The concrete vibration sensors embedded on the outer faces of the reinforced concrete beam are coupled to obtain the curvature and higher order mode shapes of the beam in pristine, damaged and retrofitted conditions. It is found that the curvature mode shape–based response of concrete vibration sensors can successfully identify the location of damage both numerically and experimentally. However, the third-order mode shape is unable to correctly identify the location of damage. Before introducing damage in the beam, the effect of long-term dynamic loading from Day 6 to Day 108 after casting of the reinforced concrete beam is also monitored. Both the global monitoring technique (in which flexural rigidity of the beam is monitored) and the local electro-mechanical impedance technique (where the equivalent stiffness identified by concrete vibration sensors is monitored) successfully detected the decreasing fatigue strength of the reinforced concrete beam. Degradation of the strength of reinforced concrete beam results due to the development of micro-cracks in the concrete because of the continuous vibrations (9.3 million load cycles) experienced by it via shaker. This is the first-of-its-kind proof-of-concept application of equivalent stiffness concept for monitoring curing of a large-sized reinforced concrete structure. It is also the first study on first-stage fatigue monitoring carried out before the ‘retrofitting-stage’ of the structure. Complete experimental investigations after the ‘retrofitting-stage’ covering all three stages of fatigue have been covered by the authors in their related publication.

A New Structural Health Monitoring Strategy Based on PZT Sensors and Convolutional Neural Network.

Preliminaries convolutional neural network (CNN) applications have recently emerged in structural health monitoring (SHM) systems focusing mostly on vibration analysis. However, the SHM literature shows clearly that there is a lack of application regarding the combination of PZT-(lead zirconate titanate) based method and CNN. Likewise, applications using CNN along with the electromechanical impedance (EMI) technique applied to SHM systems are rare. To encourage this combination, an innovative SHM solution through the combination of the EMI-PZT and CNN is presented here. To accomplish this, the EMI signature is split into several parts followed by computing the Euclidean distances among them to form a RGB (red, green and blue) frame. As a result, we introduce a dataset formed from the EMI-PZT signals of 720 frames, encompassing a total of four types of structural conditions for each PZT. In a case study, the CNN-based method was experimentally evaluated using three PZTs glued onto an aluminum plate. The results reveal an effective pattern classification; yielding a 100% hit rate which outperforms other SHM approaches. Furthermore, the method needs only a small dataset for training the CNN, providing several advantages for industrial applications.

Electromechanical impedance (EMI) technique as alternative to monitor workpiece surface damages after the grinding operation

Electromechanical impedance (EMI) technique has been employed in detection of structural failure in civil and mechanical structures because of its non-destructive property and easy implementation of small and inexpensive piezoelectric transducers that are attached to the structures, which lead to cost reduction as well as lesser dependence of manual inspection methods. In this technique, the capsule is excited by applying a sinusoidal voltage to generate waves to propagate throughout the structure. From the impedance signature of the structure without any damage, any structural change can be detected by measuring the electrical impedance of the piezoelectric (PZT) patch. Based on its real potentiality and because of its non-destructive characteristics, this work aimed to employ the EMI technique as the first alternative to monitor workpiece surface damages after grinding operation with a conventional abrasive grinding wheel. EMI measurements were performed by using a low-cost PZT transducer and under controlled environmental conditions. Microhardness and surface roughness of the machined surfaces, as well as grinding power, were also measured to detect any damage in the machined surface and to stablish relationship with the EMI technique. From the damage indices root mean square deviation (RMSD) and correlation coefficient deviation metric (CCDM), surface alterations on the ground surfaces were inferred by the EMI method. Also, it was observed a good correlation between the EMI technique and the other output parameters that were investigated in this work, such as surface roughness and power grinding, thereby posing as a non-destructive, low-cost, and viable technique to monitor workpiece surface damages in the grinding operation.

Modified Dual Piezo Configuration for Improved Structural Health Monitoring Using Electro-Mechanical Impedance (EMI) Technique

This paper presents a new variant of the electro-mechanical impedance (EMI) technique encompassing sensor-actuator dual configuration for improved damage assessment. In this arrangement, called as dual piezo configuration (DPC), an outer piezoelectric ring acts as the actuator and an inner piezoelectric disc as the sensor. The configuration yields better results than the conventional EMI configuration employing the same piezo patch as the sensor as well as the actuator. This paper presents a more practical and modified version of DPC entailing a group of normal commercially available PZT patches. The piezo sensor configurations evaluated in the paper are: (a) single piezo configuration (SPC), which is conventionally employed in the EMI technique; (b) dual piezo configuration (DPC), which consists two different patches in ring type configuration; and (c) modified dual piezo configuration (MDPC), which uses four outer piezo patches for the actuation and a centrally located piezo patch for the sensing purpose. The paper first covers the comparison of the three configurations through finite element method (FEM), using an aluminum block of dimensions 48 × 48 × 10 mm as the host structure and lead zirconate titanate (PZT) patches of size 10 × 10 × 0.3 mm, analyzed through coupled field analysis. The simulation study is followed by an experiment on the aluminum block of the same size and finally on a prototype steel plate of dimensions 1200 × 970 × 8 mm. Simulation results and experimental data prove that the proposed MDPC is much more sensitive to the occurrence than the conventional SPC. Also, MDPC ensures much larger zone of influence as compared to the conventional SPC as well as the DPC and it commands higher sensitivity. Hence, it can be employed on the large structures to detect damage, which is not possible by using the SPC which may warrant very large number of sensors. Thus, the proposed MDPC approach can be practically employed to detect the damage in large civil, mechanical and aerospace structures.