Spring 2007: Society for Adhesion and Adhesives, One Day Symposium on Structural Validation and Health Monitoring of Adhesive Joints

The Society for Adhesion & Adhesives
30 Apr 2007

One Day Symposium on Structural Validation and Health Monitoring of Adhesive Joints, held at the Society of the Chemical Industry, 15, Belgrave Square, London, 19th April 2007

This meeting was well attended and useful discussions took place. The topic is of increasing interest as bonded metal joints and composite structures are forming an ever larger proportion of modern aircraft structures. Monitoring methods are useful during flight testing and are being developed to monitor structures in service to reduce the amount of inspection required. 

The first paper, entitled ‘In-situ Strain and Displacement Measurement Using Digital Image Correlation’ was given by Jeff Sargent from the BAE Advanced Technology Centre Sowerby, at Filton, Bristol. He discussed the use of this method on adhesively bonded wing structures - stringer to skin bonding - on the BAE Jetstream Series and 146 aircraft. These were bonded with 3Ms AF-163 a tough and well-known film adhesive. The objective was to compare predicted deformations with those measured by digital imaging. 

Jeff has found that quantitative measurements of adhesive/adherend strains and displacements and in-situ imaging under load provide a useful way of understanding the micromechanical behaviour of an adhesively bonded joint. Comparisons were made of digital images taken at zero load with images taken at successive load levels. Small test pieces were used - lap shear and wedge test specimens - and mounted on the stage of an optical microscope. Results were compared with finite element predictions. It was concluded that the optical image correlation method provided a consistent, quick and relatively simple method for making measurements of adherend displacements and maps of strains at intermediate sensitivity and accuracy in comparison with, for example, holographic or interferometric methods. 

The full paper should be read for more detail but this is an interesting method that could prove very useful. 

The second paper, entitled ‘Experimental and Numerical Mapping of Strain in Bonded Joints Using Neutron Diffraction and High-Resolution Moiré Interferometry’ was given by Ian Ashcroft of Loughborough University. This paper compared experimentally measured strains generated from neutron diffraction and moiré interferometry experiments with those predicted from finite element analysis. In general good agreement was found between experimental and predicted values and it was considered that many of the common simplifying assumptions used in the finite element analysis of bonded joints appear acceptable. In this case Cytec’s FM 73 film adhesive was used with 7075-T6 aluminium alloy test pieces or IM7/8552 carbon fibre reinforced laminates. Two types of double lap joint were used, one with only aluminium adherends and another with aluminium and CFRP adherends. Results using neutron diffraction were compared with others using Moiré interferometry and also with FEA. 

Some differences were found when using CFRP. The full paper should be read for more detail. 

The third paper, entitled ‘Measuring Fatigue Damage in Bonded Joints Using the Back-Face Strain Technique’ was given by Alejandro Graner-Solana from the University of Surrey.  

This method uses single lap joints with strain gauges attached to the back face a short distance from the joint ends. Changes in back-face strain are measured as the damage evolves; the location of the gauges is critical. It can be shown that maximum sensitivity can be obtained by locating the gauge just inside the overlap region of the joint. In these tests six gauges were used and placed 1mm inside the overlap region. The joints were made from either 2014-T6 or 7075-T6 aluminium alloy bonded with Cytec’s FM 73 adhesive. Fatigue tests were carried out at 30, 40 and 50% of the static shear strength. 

At 50% of shear strength, failure began after about 12,000 cycles. One of the main aims of this project was to use the experimental back-face strain data to validate fatigue damage models for the adhesive. Initially a one–phase damage model was used but it was found necessary later to use a two-phase model, which is now under development. 

The reason for this is that, although the one-phase model fitted the experimental data, at a given load, very well, if the same damage parameters were used, it was limited when applied at different loads; see the full paper.  

Paper four, entitled ‘Modified Acoustic Emission for Structural Damage Monitoring of Complex Structures’ was given by Dr Christophe Paget of Airbus UK. 

This paper described Acoustic Emission (AE) equipment called Vigilant developed by Ultra Electronics and Airbus. Its purpose is to locate structural damage in complex structures such as aircraft. It would be useful to have continuous monitoring in critical areas to save regular inspections and to find defects at an earlier stage. This equipment consists of sensing elements, a pre-amplifier and a recording system.   It must be able to triangulate so as to locate the defect in addition to detecting it. It has been used during flight tests of the Airbus A.380 and is recommended for use on the lead aircraft in any fleet to give early warning of any problems. Work is being done on stringer debonding, bonded repairs and hybrid structures. For more detail see this paper or contact Dr Paget. 

The fifth paper was given by Steve Ogin from the University of Surrey with the title, ‘Disbond Monitoring in Bonded Composite Joints Using Embedded Chirped-Fibre Bragg Grating Sensors’. 

Once again disbond and disbond growth were of interest. This paper presents recent work on a novel optical technique using chirped fibre Bragg gratings [CFBGs],which can both detect disbond initiation and monitor disbond growth to within about 2mm [depending on sensor position and joint materials]. In this technique, the optical sensor is embedded within a composite adherend, and not within the bond line.   The method can be used for composite adherends bonded either to other composites or to other materials [e.g. composite/aluminium joints]. As the sensor is embedded within an adherend, it cannot act as the site for defect initiation within the joint. In work carried out to date it has been shown that both disbond initiation and disbond growth due to fatigue loading can be identified clearly in a single lap joint by simple changes to the reflected CFBG spectrum. 

It can be concluded form this work that disbond initiation and disbond propagation can be monitored using CFBG sensors embedded in composite adherends. If the CFBG sensor extends the full length of the overlap of a single lap joint, disbond development at either end of the overlap length can be measured. Please see the diagrams in this paper.  

The sixth paper, entitled ‘Dielectric Analysis of Ageing Structural Adhesives’ was given by Professor Dick Pethrick of The University of Strathclyde. 

Dielectric analysis can be used to monitor the cure process and ensure that an adhesive has been cured properly. It can also be used to monitor changes occurring in the bond line as a structure is used and it is this second use that is discussed in this paper.  

Prof. Pethrick shows that both water ingress into an adhesive and the changes this produces at the interface in aluminium alloys, by effects on the oxide layer, can be monitored. He says the changes in the oxide layer often parallel a decrease in mechanical strength of the joint and a change from cohesive to adhesive failure. [Note: The paper says changes from adhesive failure to cohesive failure but I think this must be a mistake]. This is a complex paper that needs careful reading. 

The seventh paper, entitled ‘Planes, Trains and Automobiles: Ultrasound Technologies for the Characterisation of Adhesives and the Inspection of Adhesively Bonded Engineering Structures’, was given by Dr Richard Fremantle of Wavelength NDT Limited. 

As the title suggests, this paper discussed problems from a wide range of industries.  Topics discussed were skin to stringer bonds in aircraft wings, masts for large yachts and inspection of adhesive bonds in cars. Fig. 1 showed that the cure process in adhesives can be monitored using ultrasound. New, high resolution equipment was mentioned, which shows great improvement in the sensitivity of this method of inspection. Ultrasonic NDT can now be related to FEA calculations. 

The eighth and final paper was given by Simon Pickering from the University of Bath and was entitled ‘Inspection of Adhesive Bonding Using Pulsed Thermography’.  

This technique of pulsed transient thermography involves using a short duration, high intensity flash of light to heat up the surface of a sample. The heat then diffuses into the body of the sample. The sample’s surface temperature is recorded by an infrared camera and computer system as it decays after this flash heating. The images showing the surface temperature are processed and the temperature decay profile at any point can be seen by looking at these data. Subsurface defects produce anomalies in the surface cooling characteristics and are captured by an infrared camera. The technique is gaining wide acceptance in aerospace, automotive and power generation industries. It adds another useful technique to the list of Non-Destructive Inspection methods.  Please contact Simon for further information.

This was another very interesting day in which a significant amount of new information was presented.