N Rajic

Principal component thermography for flaw contrast enhancement and flaw depth characterisation in composite structures

This study reports on the application of principal component thermography to the nondestructive inspection of composite structures. The technique is based on a singular value decomposition of the measured response to pulsed thermal excitation as applied in the course of a normal flash thermographic inspection. Trials on synthetically generated data show evidence of excellent noise-rejection qualities which manifest high levels of flaw contrast relative to that present in the unprocessed data. A simple analytical expression is then derived that relates a characteristic time furnished by the decomposition to the flaw depth, providing a basis for flaw depth estimation. This framework is shown to provide flaw-depth estimates with good levels of precision and robustness. Finally, application to experimental data is considered. An AS4/3501 composite laminate sample containing an assortment of teflon inserts is subject to active thermographic inspection and the response data analysed using the proposed approach. The results conclusively demonstrate the practical efficacy of the approach, and confirm the attractive properties outlined in relation to the numerical tests.

Effects of local stiffness changes and delamination on Lamb wave transmission using surface-mounted piezoelectric transducers

The paper presents a set of numerical results on the use of surface mounted piezoelectric transducers to analyse the effects of impact damage and delamination of plate-like structures on the Lamb wave mode. The effects of the size, properties and orientation of the damage upon a propagating Lamb wave is qualitatively determined. In this paper, impact damage was simulated by a local change in the stiffness of the material in the structure and a delamination. The effects on the transmission of the incident Lamb wave when it propagates through a region of change in density are analysed. This paper will also demonstrate how the properties of a propagating Lamb wave can be affected by the existence of a delamination in a plate.

Smart structure application in bonded repairs

Abstract In the aerospace industries bonded composite patches are being increasingly used to extend the operational life of aging aircraft. The application of bonded composite patches to repair or reinforce defective metallic structures is widely acknowledged as an effective and versatile procedure. Such patches have been successfully applied to the repair of cracked structures, to the reinforcement of components subject to material loss due to corrosion damage and as a general means of stress reduction through the provision of a supplementary load path. However, certification requirements mandate the need for a methodology for monitoring the damage state of both the defective underlying structure and of the repair. In this case, the concept of smart structures can be used to detect damage in the repair itself as well as monitor damage growth in the parent structure. This paper will report on the development of a ‘perceptive repair’ or ‘smart’ system which will provide information on the in-service performance of the repair and the associated structure. In this respect, this paper will focus on the detection of disbond in the adhesive layer between adherend and the metallic parent structure. Since this is a relatively new area, a series of numerical studies were initially performed to reveal the salient features of the signals expected. These numerical findings were subsequently confirmed experimentally.

A comparison of image processing algorithms for thermal nondestructive evaluation

Thermography involves the application of heat to a structure and observation of surface temperature anomalies to reveal subsurface defects. Detection of subsurface defects can be greatly enhanced by the real time capture of a series of thermal images in time and the subsequent analysis of these images using various image processing algorithms. By applying image-processing algorithms, defects not readily observable can be detected and quantitatively characterized. The focus of this work is to investigate several of the numerous data reduction algorithms for thermal nondestructive evaluation by comparing results on a set of test samples. Some new types of data reduction algorithms have been recently developed with advantages such as noise reduction, file size compression, and material property measurements. By comparing various algorithms on factors such as computational speed, simplicity of use, robustness to noise, quantitative information, and optimum defect detection the most efficient algorithm may be chosen based on the user’s needs.

Integrity assessment of composite repair patch using propagating Lamb waves

In this paper, results from a numerical study on the use of Lamb wave propagation for the detection of disbond growth beneath a composite repair patch are presented. Comparison carried out against experimental results that were done previously shows good agreement. The Lamb wave simulated in the numerical study was generated by surface bonded ceramic piezoelectric transducer. The power transmission of Lamb wave can be used to quantify the relative growth of the disbond.

Smart structure for composite repair

This paper reports on an experimental study aimed at assessing the feasibility of detecting damage in a bonded composite structure by means of surface mounted PZT sensor/actuator elements. A boron/epoxy composite patch repair is artificially degraded through the introduction of teflon inserts at the bond line. PZT elements attached to the patch and host structure are then excited and sensed in both transfer function and self-impedance configurations and the measurements examined to assess whether bond degradation is detectable. These measurements indicate that both methods offer good prospect of detecting debonds however, some limitations have been identified.

Acoustic-wave-mode separation using a distributed Bragg grating sensor

This paper reports on the measurement and modal decomposition of structural plate waves using a single optical fibre sensor comprising an array of uniformly distributed Bragg gratings. Following a brief description of the design and fabrication of the sensor, numerical and experimental work is shown to demonstrate the fidelity of dynamic strain measurements furnished by the sensor at frequencies in excess of 200 kHz. The capacity of the sensor to provide a spatially resolved acoustic measurement represents an important advancement over conventional piezoelectric sensors as it allows for the decomposition of an elastic wave field into its constituent modes. This provides a potentially powerful diagnostic framework for structural health monitoring using guided waves. Experimental work on a metallic plate is presented to demonstrate the use of wave-mode conversion as a basis for the detection of structural damage, and its insensitivity to environmental effects.

Autonomous detection of crack initiation using surface-mounted piezotranducers

In this paper we report on the application of an in situ health monitoring system, comprising an array of piezoceramic wafer elements, to the detection of fatigue degradation in metallic specimens exposed to cyclic loading. Lamb waves, transmitted through a beam test coupon, are sensed using small surface-mounted piezotransducer elements, and the signals are then autonomously analysed for indications relating to the onset of structural degradation. The experimental results confirm the efficacy of the approach and provide a demonstration of good robustness under realistic loading conditions, emphasizing the great potential for developing an automated in situ structural health monitoring system for application to fatigue-prone operational structures, such as aircraft.

Detection of disbonding in a repair patch by means of an array of lead zirconate titanate and polyvinylidene fluoride sensors and actuators

This paper reports on a numerical study in which an array of surface mounted lead zirconate titanate and polyvinylidene fluoride sensors was used for the detection of disbond under a repair patch. The two techniques used for detecting these disbonds were an impedance method and the transfer function method. It was found that an array of smart materials could locate and determine the extent of damage. The results also showed that the location and size of the sensor used are dependent on the location and size of the disbond to be detected or monitored.

A numerical model for the piezoelectric transduction of stress waves

One of the most intensely studied forms of in situ structural health monitoring involves the use of piezoceramic disc elements to induce and transduce elastic stress waves. This paper reports on a numerical modelling facility for axisymmetric acousto-ultrasonic problems where a transversely-isotropic piezoelectric disc element is attached to or embedded in a structural host. The model accounts for both elasto-electromagnetic coupling and viscoelastic material behaviour. Experimental examples are shown to demonstrate good correlation between observed and predicted behaviour. Comparisons show that the predictive accuracy of the model depends profoundly on the quality of the material property data furnished. Where biased inputs are suspected, a remedy is proposed comprising stochastic optimization guided by observations of the system response.