International Journal of Adhesion & Adhesives 22 (2002) 119–127
Determination of dynamic shear modulus of structural adhesives
in thick adherend shear test specimens
M.R. Maheri, R.D. Adams*
Department of Mechanical Engineering, University of Bristol, Queen’s Building, University Walk, Bristol BS8 1TR, UK
Accepted 20 August 2001
Abstract
The viability of using the thick-adherend shear test (TAST) specimen to find the dynamic shear modulus of structural adhesives
has been studied. Two different modes of vibratory deformation have been considered. In both cases, it has been shown that, in
practical situations, simple vibratory models are inadequate for representing the modal vibration of TAST specimens. Expressions
based on empirical results have been presented, and it has been shown that such expressions can adequately yield the adhesive shear
modulus from dynamic tests. Experimental verification has been carried out using a number of different adhesives. The tests are
nondestructive and the specimens may then be subjected to the normal quasi-static loading to fracture. r 2002 Elsevier Science Ltd.
All rights reserved.
Keywords: C. Dynamic mechanical analysis; C. Lap-shear; D. Mechanical properties of adhesives; Thick adherend shear test
1. Introduction
In order to determine the stresses and strains in
adhesive joints in a variety of configurations [1–4], it is
necessary to know the mechanical properties, particu-
larly the stress-strain curve and the moduli [1–5]. While
tensile lap-shear specimens are commonly used for
quality control, it is almost impossible to determine
the adhesive properties from such tests owing to the
complex stress and strain distributions inside the joints.
Sometimes, it is possible to produce bulk (solid)
specimens of cured adhesive which can be shaped and
tested in tension or shear like metals. However, owing to
the difficulty of making defect-free specimens, and to the
problems of exothermic temperature rise in all but very
thin sections, bulk specimens are usually restricted to
room temperature curing materials. Also, it is difficult to
make bulk specimens from film adhesives.
The alternative is to use thin layers of adhesive
between two metallic sheets or plates. If the plates are
thick enough, and made from steel or aluminium, the
adhesive forms a compliant sandwich between two
relatively stiff members.
In an attempt to simplify the stress distribution, the
thick-adherend shear test (TAST) specimen has been
proposed by Althof [6] and Krieger [7]. In these
specimens, by using a thick adherend and a short
overlap, the specimen rotation and the adherend
compliance under load are largely eliminated, rendering
a state of stress in the adhesive almost entirely
dominated by uniform shear. However, since the
compliance of the adherend under load is not totally
eliminated [5], this makes measuring the shear modulus
difficult, because it necessitates accurate measurement of
small displacements and correcting for adherend defor-
mations over a thin adhesive layer. Even so, the method
is adequate for determining maximum strains and
stresses.
On the other hand, although structural adhesive test
specimens such as the lap-shear and TAST specimens
are almost always used for testing under static condi-
tions, the adhesive material can show a degree of
viscoelastic behaviour such that its mechanical proper-
ties under dynamic loading can be significantly different
from those under static loading. Since many structural
adhesives are used in transport applications where the
bonded joints are subject to dynamic as well as static
loading, the adhesive dynamic properties can be of
significance. An early work [8] on determining the
dynamic moduli of thin, constrained adhesive films
*Corresponding author. Tel.: +44-117-928-7743; fax: +44-117-929-
4423.
E-mail address: r.d.adams@bristol.ac.uk (R.D. Adams).
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