Fatigue crack detection using smart sensor technologies
Wieslaw Jerzy Staszewski · Fatigue & Fracture of Engineering Materials & Structures · 2008
Maintenance of structures is important in many areas of aerospace, civil and mechanical engineering to ensure structural integrity and safety. It is well known that metals, such as aluminium, can develop cracks propagating under variable loading conditions. This is particularly relevant to air transportation where inspection of structures involves monitoring for fatigue cracks in airframes. There exist a variety of different Non-Destructive Testing (NDT) methods used for fatigue crack detection. The majority of these methods were developed in the 1960s. Ultrasonic inspection and eddy current technique are good examples of mature, well-established technologies which are widely used for crack detection. NDT techniques are often limited to single-point measurements and require scanning when large areas need to be monitored. Recent years have shown a range of new damage detection techniques and sensing technologies. These methods allow for global, on-line monitoring of large structures and fall into the Structural Health Monitoring (SHM) area. They are capable of achieving continuous monitoring for damage thanks to the application of new sensors. Damage monitoring systems which use smart sensor technologies are concerned with a design philosophy directed to the integration of actuators, sensors, and signal processing. The last twenty years have shown an enormous research effort in this area. Research developments include the application of new damage detection techniques (e.g. Lamb waves) new materials (e.g. piezoceramics, piezopolymers), new actuating and sensing devices (e.g. micro-electro-mechanical systems, optical fibres) and intelligent data processing (e.g. machine learning procedures, data fusion and novelty detection). This Special Issue selects and reports a few application examples of fatigue crack detection methods with smart sensors. The issue includes four different crack detection methods and four different sensor technologies. The first four papers use Lamb waves for crack detection. Lamb waves are the most widely used ultrasonic guided waves in engineering applications. The focus of the selected papers is on sensor technologies. The applications of three different types of sensors are presented. These are low-profile piezoceramic sensors, piezoelectric paint sensors and optical fibre Bragg gratings. The first paper of this group, by Yu et al, describes the application of piezoelectric wave active sensors (PWAS) for damage detection in thin-wall structures. The work includes a brief introduction to Lamb waves followed by analytical and experimental examples. The low-profile PWAS are used for Lamb wave generation and detection. The pulse-echo, pitch-catch, phased array and time-reversal approaches are demonstrated for damage fatigue crack and corrosion detection in metallic structures. Finally, the paper addresses the problem of wave propagation in composites. The analysis of Lamb wave responses is not a trivial task due to the complexity of wave propagation and wave interaction with damage. Various signal parameters resulting from wave attenuation/reflection and mode conversion are used in practice. The second paper of the group, by Chetwynd et al, demonstrates that the statistical technique of outlier analysis can be used to detect damage in metallic structures. Lamb waves were generated by large custom-made piezoceramic patches and were sensed by smaller piezoceramic sensors surrounding each patch. Outlier analysis was successfully able to distinguish between the damaged and undamaged cases with a very high classification rate. The work presented in this paper was part of a collaborative research project performed by academic and industrial aerospace partners under EU sponsorship. The majority of Lamb wave based damage detection approaches require baseline measurements representing undamaged condition. These measurements are then compared with current responses in order detect possible damage. Unfortunately changes in signal responses are often produced by operational and environmental variations making damage detection difficult. This problem is addressed by Kim and Sohn in the third paper. The method presented utilizes the polarity of piezoelectric materials and employs two pairs of collocated low-profile piezoceramic transducers placed on both sides of the aluminium plate to generate and measure Lamb waves. The existence of mode conversion due to the crack is identified from Lamb wave responses. The final article on the theme of Lamb waves, by Thursby et al, demonstrates the application of the multifunctional optical fibre sensor. The work presented demonstrates that the fibre Bragg grating sensor can be used to measure strain and Lamb waves. This important development offers real-time load monitoring and damage detection providing valuable data from which the lifetime of a structure can be predicted. The Special Issue proceeds by presenting four papers which demonstrate different methods utilizing smart sensors for crack detection. The first paper, by Staszewski et al, demonstrates the application of nonlinear acoustics for fatigue crack detection. The method utilizes modulation of the high-frequency acoustical wave by the low-frequency modal excitation in the presence of the crack. Low-profile piezoceramic transducers are used for both types of excitation. The study demonstrates that – for the case investigated – the method offers similar sensitivity of crack detection to the Lamb wave based approach. The second, paper by Li and Zhang, presents analytical investigations of piezoelectric paint. The focus is on material behaviour relevant to ultrasonic sensing. The paint is a composite material that is comprised of piezoelectric particles randomly dispersed in a polymer matrix. The paper demonstrates the feasibility of using the paint for acoustic emission sensing in crack detection. The next paper, by Zoughi and Kharkovsky, reviews recent advances in the area of near-field crack detection approaches. This includes methods based on microwaves and millimetre waves used for detecting fatigue cracks and determining crack tip locations. The frequency range of these waves can go up to 300 GHz. The paper discusses various waveguide probes and different approaches used for crack detection. The final paper of this special issue, by Park et al, demonstrates how piezoelectric sensors can be used for fatigue crack detection. The method proposes to use simultaneously high-frequency structural excitations and responses employing piezoelectric sensors to monitor the local area of a structure for changes in structural impedance. A new impedance model is proposed in the investigations. The study demonstrates also the application of wireless sensor technology that embeds data compression algorithms.