Elsevier

Composite Structures

Volume 87, Issue 2, January 2009, Pages 175-181
Composite Structures

Damage detection in repairs using frequency response techniques

https://doi.org/10.1016/j.compstruct.2008.05.010Get rights and content

Abstract

Structural health monitoring (SHM) technology may be applied to composite bonded repairs to enable the continuous through-life assessment of the repair’s efficacy. Adhesively bonded joints are an ideal starting point for real-time, in situ monitoring due to known mechanisms and locations of failure. Similarly, the ability to accurately monitor the health of a joint has potential to aid acceptance of adhesive bonding. This paper describes the development of an SHM technique for the detection of debonding in composite bonded patches based on frequency response. Two commonly used repair schemes, the external doubler repair and the scarf repair, are examined. The paper outlines an experimental investigation on the frequency response of the repairs with and without defects under different boundary conditions. It was found that damage could be readily detected through changes in frequency response for both types of repair. The results are discussed with implications for the development of a technology to monitor the integrity of composite bonded repairs.

Introduction

Adhesive bonding is ideally suited to joining advanced composite structures. Bonded joints show greater fatigue resistance, can sustain higher loads and are lighter than mechanical interfaces [1]. Unfortunately, these advantages are often offset by inspection difficulties [2]. Consequently, quality assurance is commonly achieved through careful management of the bonding process [3]. Nevertheless, sound process management alone is unable to predict or prevent bond failure due to aging or poor environmental durability.

The most common aerospace application of adhesive bonding is repair. Traditionally, an inability to monitor bond condition has forced conservative repair design. For a component to qualify for repair it must be able to carry limit load without the patch. A repair can only be used to increase the residual strength of a component from above limit load to the design load [4]. The development of a structural health monitoring (SHM) technology that is able to provide accurate and reliable information on the integrity of a bonded repair in real-time could reduce maintenance effort and allow the repair of more significant damage. SHM technologies are a system consisting of sensors and supporting infrastructure including connections, power supplies and processing equipment coupled with diagnostic and prognostic methodologies [5]. The majority of SHM techniques currently under development seek to monitor subtle changes in strain or vibration as a result of damage.

Damage detection by measuring natural frequency reductions has been shown to be an effective technique for the assessment of structural health. One of the most widely used and cost effective vibration based non-destructive testing techniques is the coin tap test. Unlike the ‘wheel tap’ technique applied in the rail industry where the health of a large steel wheel is ascertained by tapping anywhere on the wheel and listening to its ‘ring’, the coin tap test is highly localized [6]. Unless a defect is immediately below the region struck it will not be detected by the coin tap test. Fortunately, in the case of a bonded repair, the region requiring inspection is small and can be easily identified. The coin tap test combined with accurate frequency response measurement could potentially form a simple SHM system for bonded repairs and joints. As aerospace structures are typically thin any debonding would significantly degrade stiffness lowering natural frequencies by a detectable amount. It is feasible that a SHM system could be developed using piezoelectric devices to produce and monitor precise ‘taps’.

This paper details a study conducted to assess the frequency response technique for the detection of debonds in typical aerospace structural repairs. Two different types of repair schemes commonly used for aerospace structures are examined. They are the external doubler repair and the scarf repair. The latter is used where surface flushness is required for aerodynamic and stealth considerations. Piezoelectric transducers were used to excite the structures and measure their vibration response for the purpose of damage identification.

Section snippets

Specimen manufacture

As careful control of boundary conditions was required, the test specimens were designed to be mounted with bolts to a rigid alloy supporting frame with dimensions of 290 mm × 290 mm. Bolts were selected rather than rivets to make it simple to mount and remove the specimens from the frame as well as allowing different boundary conditions by altering bolt torque.

Testing

Two different types of boundary conditions were examined in the study, which were free–free and fixed–fixed. To achieve the free–free boundary conditions, specimens were suspended by a string from a retort stand. Five tests of both the control and damaged specimen were made; each test consisted of ten taps which were averaged to develop a transfer function. Testing alternated between the two specimens (damaged and undamaged) in each repair configuration to test repeatability. For the

External doubler repair

Transfer functions were generated based on the arbitrary square wave input signal as no significant difference was identified between input impulses. Transfer functions for free–free and fixed–fixed boundary conditions are presented in Fig. 4, Fig. 5, respectively, for each of the five tests. Each transfer function was averaged from ten recorded taps.

For both boundary conditions investigated the presence of debonding appears to cause changes in frequency response over the frequency range

Conclusions

An experimental investigation was conducted into the use of frequency response techniques for the detection of damage in adhesively bonded composite to composite repairs. Both the external doubler repair and the scarf repair were examined. Piezoelectric transducers were used to actuate and then measure the frequency response of the test specimens, which consisted of a control specimen and one with significant debonding damage for each repair configuration. Two types of boundary conditions were

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