As the aircraft industry matures, and competition increases, the need for simple, low cost methods of effecting minor repairs to aircraft damaged in service has developed. The use of adhesives allows neater, lighter repairs to be carried out than are possible using conventional techniques.
Although adhesives are commonly used in the construction of the aircraft in the first instance, these adhesives generally require the application of heat and pressure during cure.
Fabrication with the factory environment means that cleanliness of components can be closely monitored, and surface preparation techniques such as acid etching can be carried out.
This, however is not an easy matter in the field, on a completed airframe. Many areas are relatively difficult to access to apply pressure or heat and total cleanliness is difficult to achieve. This has meant that repairs are generally carried out using cold cure adhesives, and surface preparation techniques are generally limited to abrasion, and/or solvent wiping.
As part of a programme of work initiated by British Airways, a number of Boeing wedge test samples (ASTM D3762) were prepared using 2024 clad aluminium alloy and carbon composite adherends. These were selected as being typical of the materials used in the construction of modern aircraft. These were bonded using a variety of commercial adhesives and surface preparation techniques, typical both of the initial construction, and those more suited to repair situations. These test specimens were immersed in water and tested at various intervals to determine the long term durability of the systems. Fortunately these specimens were retained, immersed in water, after the end of the initial programme of work, and became available to the forensic study programme for further investigation.
Aluminium alloy
Carbon fibre epoxy composite
Filled cold curing epoxy
Surface treatment by grit blast
Wedge test samples. Filled cold-cure epoxy bonded according to commercial practice.
Room temperature cure
Laboratory. Simulated initial construction and field conditions.
The final results obtained from the original wedge insertion continued to show a considerable degree of scatter in results from nominally identical specimens. This feature of quantitative results obtained from the Boeing wedge test can in part be explained by the dependence of the results on features such as the initial wedge during speed, bondline thickness, and the crack front profile, to name but a few. In addition, the test method is highly sensitive to the accuracy of measurement of the crack tip location, which can vary significantly along the crack front, and can not easily be measured.
The method does, however, provide a useful means of ranking the various adhesive and surface preparation combinations.
In general the results showed a significant increase in the rate of joint strength degradation, compared to that previously measured, although in the main the original ranking remained the same.
Where the original crack had penetrated deep into the adhesive, a single lap shear specimen was fabricated by cutting away part of one adherend. Tests carried out using this configuration were considerably less scattered than the other two mechanical test methods.
These tests confirmed the originally observed result, that the abraded composite specimens perform better than specimens prepared with a peel ply. It is possible that this was due to some residual release agent left on the surface of the composite after removal of the peel ply, or that some other form of contamination had affected the joint strength. The results also showed the importance of rigorous surface preparation as a pre-requisite to high strength, durable joints. Comparison of the shear strengths of joints with aluminium and composite substrates provided evidence for an interaction between the adhesive and the substrate at depths considerably deeper than the surface mono-layer.
Results obtained showed that cohesive failure of the bulk adhesive could occur when bonded to a composite adherend at a load less than that required to cause interfacial failure on an aluminium adherend. This suggests that there may be an inter-phase, where the adhesive properties are affected by the interaction with the adherend. This feature requires further investigation, before the complexities of adhesive durability can be fully understood.
Electron microscopy also gave evidence of differing properties at the interface. Several of the adhesives showed evidence of porosity within their bulk structure. At the interfaces, however, the adhesive had formed a smooth surface skin, which did not exhibit the same porous structure. Typically this skin layer thickness was of the order of 10µm. The initial locus of failure for these systems was generally cohesive within the porous region of the adhesive layer, at the junction between the porous layer and the skin. Microscopy also revealed that following the long term saturation, corrosion of the aluminium adherends had begun to take place under the adhesive bond, and the formation of a more friable oxide layer had caused the reduced joint strength.
Although it is difficult to draw firm conclusions from the results due to the large scatter and the possibly of a change in the interpretation of the crack front, the increased rate of degradation might indicate that some critical parameter has been reached, such as the leaching of a component of the adhesive to a critical level.
Certainly, after this length of immersion, the adhesive throughout the joint would have become fully saturated with water, a feature that several adhesive failure criteria use as a parameter.
Where the original wedge had fallen out, the adherends were pulled to simulate a double cantilever beam test. This test also produced a wide scatter on results from nominally identical specimens. Once again this means that the interpretation of quantitative results is difficult. Insufficient tests of this type were performed in this experimental programme to identify whether the method was suitable for ranking different systems.
Chemical analysis of the composition of the fracture surfaces using XPS confirmed that in many cases where the failure had initially been attributed to adhesive bond failure, the failure was actually occurring within the oxide layer of the adherend. This mode of failure was apparent in both joints made with corrosion inhibited adhesives and un-inhibited adhesives, suggesting that after long periods of saturation, the corrosion inhibiting component of the adhesive may be leached out. Experience with similar adhesives in industry however, has shown that if the adherend is protected from corrosion with a suitable primer, the resulting adhesive joint becomes considerably more durable.
This suggests that, In order to design a durable joint, effort needs to be concentrated on ensuring that the surface of the adherends remains firmly attached to the bulk material.
The method provides a useful means of ranking the various adhesive and surface preparation combinations.
In general the results showed a significant increase in the rate of joint strength degradation, compared to that previously measured. In the main the original ranking remained the same.
Interaction of the cold-cure epoxy adhesive with the adherend may be important in determining longevity.
The results indicated that cohesive failure of the bulk adhesive could occur when bonded to a composite adherend at a load less than that required to cause interfacial failure on an aluminium adherend. This feature requires further investigation.
More tests are needed to evaluate the suitability of the double-beam cantilever test as an alternative where the original wedge has fallen out.
In order to design a durable joint, effort needs to be concentrated on ensuring that the surface of the adherends remains firmly attached to the bulk material.
DTI MTS programme adhesives – Dissemination Guides ADH5CS6.DOC
NPL / British Airways/ ESR Technology Limited
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