W.K. Chiu

Composite repairs to cracks in thick metallic components

This paper presents a series of experimental and numerical studies into the repair of cracks in thick structural components. To this end, after first summarising the design guidelines for repairs to thin structural components, a variety of problems are considered; semi-elliptical surface flaws, interacting surface flaws, cracked fastener holes, and cracked lugs.

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.

Damage detection in bonded repairs using piezoceramics

A smart structure is one that senses its internal state and external environment, and based on the information gained responds in a manner that fulfils its functional requirements. The primary advantage of moving towards smart structures technology is the potential cost benefit of condition-based maintenance strategies and the prospective life extension that may be achieved through in situ health monitoring. The monitoring of operational health and performance, and diagnosis of any faults as they occur, is a relatively new concept that is being developed globally to provide advantages of safer , more reliable and affordable structures. Health monitoring can be achieved by positioning (embedded or surface mounted) sensor systems on a structure to measure those physical parameters that are informative with respect to the state of the structural health. Information relating to the severity and location of damage, as well as to the nature of the loading is of obvious importance to this endeavour. In the aerospace industry bonded composite patches are increasingly being used to extend the operational life of ageing 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 reports 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 focuses on the detection of disbond in the adhesive layer between adhered and the metallic parent structure. One of the criteria of this smart system is that it must be economical, reliable and, preferably, self-powered. To this end, it was proposed that piezoceramic of piezoelectric material be utilized because of their ease of application. These materials were chosen because they can be used both as a actuator and as a sensor. This paper presents a set of numerical investigations performed to highlight the viability of using this material system, and the associated signal analysis that can be employed to detect the presence or the development of disbonds in the adhesive in a bonded repair situation.

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.

Numerical study of detection of disbond growth under a composite repair patch

The application of the impedance method and the transfer function method to detect disbonds in bonded repairs has been investigated using finite-element analysis. The damages simulated are typical disbonds that can occur in bonded repairs. This paper shows that both the impedance method and the transfer function method can be used to detect disbond growth under the repair patch within the limitations set out. By strategically locating the sensor and actuator pair, the location, type and severity of a disbond in a repair patch can be determined.

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.

Weight functions for composite repairs to stiffened panels

Abstract This paper presents a weight function technique for calculating the stress intensity factors for composite repairs to cracks emanating from an internal notch, corrosion blend out, or a free edge under arbitrary loading in rib stiffened panels. The predictions are compared with both finite element and experimental values. This methodology represents a significant extension to existing assessment and design formulae that are currently limited to the case of uniform loading and flat unstiffened panels.