Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Experimental results and data base on the mechanical performance and failure/damage mechanisms

From the design of an aluminium-framed superstructure presented by VT Shipyards as an application case for Bondship, a series of adhesively bonded joints were tested to assess their strength and failure mode in a marine environment. The test programme included static tests on standard single lap shear specimens and butt strap joints with different adherends corresponding the materials used in the application case (Al 5083, Al 6082 and steel) and structural joints made of extruded box sections: deck to superstructure and unit-to-unit joints. Also, fracture mechanics tests have been carried out to assess fracture energy of the adhesive system through Double Cantilever Beam specimens.

The aim of the lap shear test programme was to assess the system adhesive / primer / surface preparation in two different conditions, namely: non-aged in laboratory conditions and environmentally degraded conditions humid and hot. Strength reduction from 8 to 28 % was observed depending on the type of adherend after ageing process.

Due to the design of the VT application case, based on butt strap joints, and the low production tolerance outlined by the shipyard, influence of the adhesive thickness on the strength and mode of failure of such joints were investigated. Four different adhesive thicknesses were considered: 1-, 3-, 5- and 10mm. Also, influence of three different ageing conditions was investigated: ideal laboratory conditions, 3 and 6 weeks in a 100% rh at 40°C. Tests showed an almost linear strength reduction between a 1-mm adhesive joint and a 10-mm one. Strength reduction is more significant with respect to the adhesive variation than to the ageing process. It has been outlined the importance of surface preparation and adhesive application process specially in the case of aged specimen where a good bondline ensures degradation due to infiltration of water.

Experimental modelling was carried out on structural joints to assess their tensile behaviour and flexural behaviour in a 4-point bending configuration. Deck to superstructure joints were shown to be fairly strong in tension whereas when tested in flexion, they were shown to have a similar strength to aluminium joints. These tests, together with butt strap and lap shear tests, have highlighted the fact that adhesion to steel presented a weaker bond than adhesion to aluminium, suggesting a more careful surface preparation concerning this adherend. Fatigue tests carried out to assess flexural behaviour have shown discrepancy in the results concerning deck to superstructure joints whereas unit-to-unit joints showed failure in the aluminium weld.

A fracture mechanics approach has been proposed in order to evaluate damage tolerance of large bonded structures. To initiate this assessment, Double Cantilever Beam (DCB) tests have been carried out to estimate the fracture energy of the adhesive system. DCB with different adherends and ageing conditions have been studied and fracture toughness derived with different analytical methods. Results were shown to be fairly consistent although a slight difference exists depending on the type of adherend used for the DCB test. All failure occurred cohesively except in few cases after environmental degradation process.

Potential applications for these results are for the design and construction of large bonded structures in shipbuilding. End-users of these results would be shipbuilders, repair yards and owners who would be interrested in implementing this technology in the construction of new units. The main innovative features of this work is a validated new approach for assessing adhesively bonded structures. Success factors will be a gain in weight in the structures, the possibility to fasten different materials and cost savings.

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Reported by

University of Southampton
SO9 5NH Southampton
United Kingdom
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