Bonded joint analysis

Abstract: The present invention relates to a method of testing and or monitoring structural adhesively bonded joints by the introduction of transducers into the stress or strain field effected by the movement of said adhesively bonded joints under load, such that the load transfer characteristics between the bonded materials and the adhesive can be recorded thus allowing the quantification of the integrity of the adhesive bond lines in such joints. The method assesses the integrity of the bond by comparing the difference in maximum and minimum ratios and curve parameters relating to stress and strain with those of the "at assembly" values.

Description: This invention relates to the field of testing and monitoring structural characteristics of bonded joints and more specifically to a method of assessing and quantifying the integrity of adhesive bond lines in such joints. The testing and/or monitoring of the structural integrity of load carrying joints is one of the most important areas of engineering research. The ability to understand how structural joints are performing in service gives engineers the ability to refine and tailor their designs to maximise the strength of such structures, whilst at the same time minimising their weight. The advent of structural adhesives and the use of composite materials has, in industries such as aerospace, revolutionised structural material utilisation, with the number of available materials and adhesives for use in structural applications increasing markedly. The use of such technology has brought with it problems related to the inspectability of bonded joints due to there being no outwardly visible sign of connection (i.e. bolt or rivet heads protruding), and so alternative and more costly methods have had to be utilised to ensure the integrity of such assemblies. Various methods of structural health monitoring have been proposed, but the principle by which such methods give indications of reducing mechanical joint properties lies in their identification of areas of increased local strain or displacement. Such techniques are able to give indications of possible degradation of joint integrity, but can only give indications of possible areas of increased strain or displacement due to the nature of the state of the art transducers. State of the art methods utilised in the inspection of bonded structural joints involve the use of non-destructive test (NDT) techniques which, by the very nature of the scanning equipment required, can take a considerable amount of time and effort to undertake effectively. Additional problems occur when bonded joints are required to be inspected which are not readily accessible for the introduction of an NDT scanning head and therefore costly disassembly of areas of surrounding structure may be required. EP-A-0573778 discloses a method of measuring transverse strain at discrete locations on a bondline to which an axial load has been applied. A plurality of fibre optic sensors of different lengths are used, one for each location, and the respective strain measurements are summed and analysed. Our invention seeks to provide a method of analysing the integrity of bonded structural joints which is simpler to carry out than the known methods including the method disclosed in EP-A-0573778. Accordingly there is provided a method of analysing the integrity of a bonded structural joint under load, including the steps of; locating a transducer means in proximity of said joint, said transducer means being so disposed in relation to said bonded joint that parameters indicative of the load transfer characteristics between the bonded material and the adhesive can be recorded in response to the application of load, recording reference parameters of maximum strain or load and minimum strain or load under the application of a reference load after assembly of said bonded joint, and calculating the ratio of these reference parameters, subsequently recording parameters of maximum strain or load and minimum strain or load under the application of loads in service, and calculating the ratio of these subsequently obtained parameters, comparing the ratio of the reference parameters with the ratio of said subsequently obtained parameters to determine the integrity of the bond. Our invention offers a method of analysing the integrity of bonded structural joints by utilising sensor means capable of detecting the characteristics of load path/transfer through a bonded joint, without the requirement for such joints to be accessible for visible inspection. Additionally our method of analysing the integrity of the bond line utilises a relatively simple comparison of the joint's strain or load parameters, measured under reference and service load conditions respectively. The invention will now be described by way of example only in which reference will be made to the following drawings: Figure 1 - is a diagrammatic representation of a typical structural bonded lap joint. Figure 2 - is a graph showing the typical strain and load distribution for the typical bonded joint shown in figure 1. Figure 3 - shows a diagrammatic representation of a joint as in figure 1 with the introduction of a sensor means to the joint assembly. In figure 1, a structural bonded lap joint 1 is shown in an assembled condition, having two members 2 ,4 bonded by a layer of adhesive 6. The thickness of the structural adhesive 6 is shown as (t), and would typically be in the order of 0.1-0.2 mm. A co-ordinate axis system 8 is shown for reference in figure 2 along with typical applied loads (p) 10 Figure 2 shows the typical strain and load (ε, p) distribution (y) with distance (x) for the bonded lap joint 1 for an applied load 10 in direction (p), with reference to the given co-ordinate axis system 8. At x=0, the graph shows the strain or load at zero 12. As more of the load is attracted into the relatively less stiff adhesive 6 the strain or load increases rapidly to a peak 14 and then gradually decreases to a minimum as x increases along the length of the bondline at the x centreline 16. Due to the mechanical symmetry of the example joint, the strain or load then increases with increasing x position to another peak 18, before falling again to zero, 20, at the corresponding edge of the bond line, 6. Figure 3 shows a bonded structural lap joint 1, with an optical fibre 26 embedded within the adhesive bondline 6. The fibre 26 is attached to, and/or comprises a transducer means 28, which is so disposed so as to be capable of responding in a measurable way, and to produce some form of recordable output, to indicate the load transfer and distribution profile across the width of the bondline 6, due to the application of a load (p) 10. A typical example of a such a profile is shown in figure 2. The transducer/fibre 28/26, is connected to a processing / recording / display means 30, which is capable of processing the output of the fibre/transducer means 28 in response to the applied load (p) 10. This method of analysis therefore facilitates the recording of the load and/or strain distribution across the fibre 6, so as to enable a record of the particular load transfer characteristics of the bondline adhesive under evaluation to be made. In use, a bonded structural joint which is required to undergo constant, regular or periodic inspection for evaluation of it's structural integrity would have a fibre/transducer means 28 embedded in or attached to the adhesive, and connected to a processing/recording/display means 30. Once assembled, the joint would be put under load and the characteristic load transfer profile would be recorded (e.g.. figure 2). The parameters of maximum strain or load (y b ),14 and 28, and minimum load (y a ) at the centre of the joint 6 would be recorded and the ratio of these two parameters calculated. Additionally the shape of the load distribution curve generated between the peaks could be analysed and a measure of it's correlation or standard deviation made to quantify a parameter relating to the 'smoothness' or rate of change of load transfer across the bond line. Additionally the positions of these maxima and minima in the x plane would also give and indication of changes in the mechanical properties of the joint and bond line due to phenomena such as delamination material/ adhesive delamination and adhesive voids or stress concentrations. Armed with these initial, 'at assembly' parameters, the monitoring of the integrity of the joint in service would consist of the constant or periodic recording of the load transfer profile characteristics of the joint, and the comparison of newly recorded ratios and curve data with the initial 'at assembly' values. The integrity of the bond line can be assessed using this innovative technique by comparing the differences in max/min ratios and curve parameters to those of the 'at assembly' values. The invention will enable the engineer to monitor the performance and integrity of a load carrying bonded joint by comparing the inherent load transfer characteristics of the adhesive material and joined structural members, against a model of the same joint in the original 'at assembly' condition. This method therefore is not reliant on known initial data and does not require predictive models of specific material load transfer characteristics, but utilises the initial properties of such a joint to monitor it's degradation with time. Limit allowable changes in the measured parameters for differing materials can be assessed by structural testing of a range of structural adhesives, with such allowable being developed to meet the requirements of the particular applications to which this novel method of analysis is applied. Examples of industrial applications to which this invention could be applied are numerous and include aerospace, mechanical, civil and automotive engineering, along with any structural adhesive application which requires some form of repeated monitoring of bonded joint integrity. The fibre/transducer means could take the form of a series of point strain measuring means, or alternatively fibre optic technology could be applied by utilising integrated or etched gratings. In such a fibre optic grating application, the displacement and or strain characteristics of the bond line material would manifest itself in distortions of the embedded grating with the assembly under load, which, in combination with a method such as optical time domain reflectometry, could therefore be used to measure and record the required load transfer parameters required for joint analysis in accordance with the invention. Yet another method of obtaining the required bond line data would be the use of an optical, electrical or pitzo electric technique whereby measurable changes in material, electrical or optical properties the adhesive itself under applied load, could result in a recordable output or signal which in turn could be processed to produce the required parameters required to apply the invention.

Claims: A method of analysing the integrity of a bonded structural joint under load, including the steps of: locating a transducer means (28) in proximity of said joint (1), said transducer means (28) being so disposed in relation to said bonded joint (1) that parameters indicative of the load transfer characteristics between the bonded material (2, 4) and the adhesive (6) can be recorded in response to the application of load, characterised in that the method further includes the steps of: recording reference parameters of maximum strain or load and minimum strain or load under the application of a reference load after assembly of said bonded joint (1), and calculating the ratio of these reference parameters, subsequently recording parameters of maximum strain or load and minimum strain or load under the application of loads in service, and calculating the ratio of these subsequently obtained parameters, comparing the ratio of the reference parameters with the ratio of said subsequently obtained parameters to determine the integrity of the bond.

Inventors: BALL ANDREW STEPHEN, BALL, ANDREW STEPHEN, Ball, Andrew Stephen, Britisch Aerospace Def. Ltd.
Applicants: BAE SYSTEMS PLC
Application year: 1996
Main classification: G01M11
IPC codes: G01M11, G01N19