International Journal of Adhesion & Adhesives 23 (2003) 215–223
Performance of bi-adhesive bonded aluminium lapjoints
, L. Quintino
, J.F. Durodola
*, A. Beevers
Instituto Superior Tecnico, Seccao de Tecnologia Mecanica, Av. Rovisco Pias 1049 001 Lisboa, Portugal
School of Engineering, Oxford Brookes University, Gypsy Lane, Headington, Oxford OX3 0BP, UK
Accepted 19 December 2002
The stress concentration towards the ends of a bonded lapjoint depends to some extent on the relative stiffnesses of the adherend
and the adhesive used. For a given adherend, the lower the stiffness of the adhesive used in the bondline, the lower the stress
concentration, giving rise to potentially higher joint strength. The paper presents the results of a study of the application of two
adhesives with different stiffnesses along the overlap length in single lap joints. A stiff adhesive was applied in the middle portion of
the overlap, while a low modulus adhesive was applied towards the edges prone to stress concentrations. The results show
measurable increase in strength of the bi-adhesive bonded joints compared with those in which single adhesives were used over the
full length of the bondline.
r 2003 Elsevier Science Ltd. All rights reserved.
Keywords: C. Lap-shear; C. Stress analysis; C. Stress distribution; E. Joint design; Multi-modulus bondline
Single lapjoints are by far the most widely used
adhesive joints and have been the subject of considerable
research over the years [1,2]. The stress distribution
within a joint varies with the adhesive and adherend
properties, and with the geometric dimensions. The
stresses induced by the externally applied forces to the
joints are non-uniformly distributed over the bonded
area [1–3]. Even when relatively low modulus adhesives
are employed, the stress is never uniformly distributed
through the bondline.
The loads in a single lapjoint are not co-linear, a
bending moment therefore exists which causes the joint
to rotate. This consequently exposes the adhesive layer
to shear, and peeling stresses. The adherends are
similarly subjected to tension and bending. It is quite
possible that both the adhesive and adherend may
become plastic, particularly in the highly stressed
regions. The commonly used metallic adherends used
in single lapjoint tests are often found to have large
plastic deformation, due to yielding, before failure .
Attempts to achieve very high shear strengths to
sustain high loads, has led to the introduction of high
stiffness and brittle adhesives . However, such
adhesives are susceptible to brittle failure due to the
high stress concentrations they develop. According to
Volkersen’s  shear lag analysis, the stress concentra-
tion factor n is proportional to the square root of the
ratio of the shear modulus, G; of the adhesive to the
Young’s modulus, E; of the adherend, i.e.
t are peak shear stress and mean shear stress,
respectively; l is the lapjoint overlaplength, t and d are
adherend and adhesive layer thicknesses, respectively.
Eq. (1) suggests that for a given adherend, the lower the
stiffness of the adhesive used in the bondline, the lower
the stress concentration giving rise to potentially higher
joint strength. The use of relatively low stiffness
adhesives at the ends in a bi-adhesive bondline can
reduce the stress concentration and therefore potentially
lead to higher joint strength. Functional gradation or
compositional gradation of material properties has been
of interest in recent years . The aim in compositional
gradation is to match structural requirements at a point
in a component with appropriate material properties at
*Corresponding author. Tel.: +44-1865-483-503; fax: +44-1865-
E-mail address: firstname.lastname@example.org (J.F. Durodola).
0143-7496/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.