Duncan, B C; Crocker, L; Urquhart, J M; Arranz, E; Mera, R D; Broughton, W R (2001) Failure of flexible adhesive joints. NPL Report. MATC(A)36

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Abstract

Flexible adhesives, characterised by low modulus and large extensions to failure, have a long history of use in non-structural applications. However, the advantageous properties of flexible adhesives in sustaining large strains and distributing peel forces more evenly on the bonded substrates is leading to their use for structural joining applications. This is driving the need to improve the understanding of their mechanical properties that have received little attention in comparison to structural adhesives. The study of means of characterising the deformation and failure of flexible adhesives has formed the core of the Flexible Adhesives project of the Performance of Adhesive Joints programme sponsored by the DTI.
The performances of two flexible adhesive types – an elastomer and a polyurethane – have been studied through bulk specimen and adhesive joint tests. Experimental measurements and Finite Element (FE) model predictions, for lap shear, scarf and T-peel joints, have been compared, at different strain rates and temperatures, in an attempt to identify possible failure criteria. No single failure criterion has been identified although a large volume of observations have been made on the behaviour of flexible adhesives. The strength of the adhesive joints correlates well with the tensile strength of the bulk adhesive material measured under corresponding test conditions. Specimens tested at high temperatures and low strain rates have the lowest strength.
Maximum stress values correlate with the tensile strength within a joint configuration but there is no obvious correlation between different joint types. Interpretation is complicated by the dependence of the results on the FE element size. The results suggest that the elastomeric adhesive, where the joints always fail cohesively, can sustain joint stresses that are a higher proportion of the tensile strength than the polyurethane adhesive, where failure modes are more variable. Cracks have been observed to form and grow within the adhesive layer before the maximum load has been reached. Any design or failure criterion will need to relate to the conditions of crack formation rather than ultimate joint strength. Having the predicted maximum principal stress lower than the tensile strength scaled by an adhesive dependant factor of less than one may be a reasonable design criterion.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	MATC(A)36
Subjects:	Advanced Materials Advanced Materials > Adhesives
Last Modified:	02 Feb 2018 13:17
URI:	http://eprintspublications.npl.co.uk/id/eprint/2286

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