The design of structural adhesive joints
This thesis details the work carried out under two research projects at the University
of Surrey. The first project titled The Design of Structural Adhesive Joints', was of
three years duration from September 1985 to August 1988 and was sponsored by
the Science and Engineering Research Council. The second project, sponsored by
Ford UK Ltd, and tided 'A General Joint Analysis Facility extended certain aspects
of the analysis work initiated In the first period of research.
The objective of the work was to address the problem of integrating structural
adhesives Into the design process and to provide procedures that would facilitate
this integration in a quantitative, rather than the more usual qualitative way. To be
effective, such an approach needed to consider not only a means of analyzing a
proposed joint but also a way of predicting the actual failure of that joint.
An extensive literature survey of analyses available to the design engineer has been
completed. The analyses investigated were found to be lacking in several critical
respects, and as part of this research, methods of analysis overcoming some of
these limitations have been developed. The analyses produced are based on earlier
approaches but extended and modified as appropriate. The work on all the analyses
produced has been carried out by considering a simple adherend-adhesive sandwich
Five different analyses, considering the sandwich to be modelled with differing
degrees of complexity, have been produced. In all of the analyses the adherends
are assumed to behave as cylindrically bent plates capable of sustaining both tensile
and shear forces and bending moments, with the adhesive transmitting both tensile
and/or shear loads.
Initially an elastic solution was obtained, adopting a relatively simple approach. This
enabled the subsequent enhancement of including non-linear material behaviour to
utilize the same governing equations, thus maintaining consistentcy. The General
Elastic Analysis (GEA) has been extensively simplified to produce a number of two
parameter design formulae suitable for use by an engineer at an early stage in the
design process. The two analyses produced by this simplification are called the
Simplified Peel Analysis (SPA) and the Simplified Shear Analysis (SSA), so called
because they consider the named component of stress in the adhesive layer only.
The GEA was then extended to include non-linear material properties in the adhesive
layer, and an analysis called the Non-linear Adhesive Analysis (NLAA) was produced.
A programme of validation using the NLAA and a non-linear finite element analysis
of similar joint configurations was carried out. Additional comparisons with existing
analyses have also been undertaken where possible. The NLAA has been shown to
produce extremely accurate results for the stresses in the adhesive layer when
compared with the component stresses predicted by the finite element method
The NLAA has been used successfully to determine the spread of yield in a single-lap
joint, giving dose agreement with results from analysis using the FEM, but with much
reduced computer and operator time.
The final stage of the work was concerned with the Inclusion of non-linear adherend
material properties, and an analysis called the Full Non-linear Analysis (FNLA) has
been produced which Incorporates this refinement to the general model. Again the
finite element method has been used to assess the accuracy of this new analysis,
and the results from this work are presented here. Derivations of both forms of the
elastic analysis and of the non-linear and full non-linear analyses are reported in
Chapters 4 and 5 and the software appropriate to each Is described fully.
The Initial survey of available literature has shown that there Is considerable lack of
knowledge about possible causes of joint failure. Specifically, It Is noted that a
criterion by which joint failure can be measured has not been uniquely defined. In
an attempt to provide a criterion or criteria to enable the prediction of joint failure a
'Failure Criteria' test and analysis programme has been completed. Joint configurations
were manufactured using a range of adhesives with different levels of
ductility, and adherends of different stiffnesses. Batches of these test coupons were
tested to failure under both predominantly mode I and mode 11ty pes of loading.
Both FEM and FNLA analyses of each test configuration have been carried out, and
the stress and strain distributions at the levels of failure load were established for
each batch and studied to establish any correlation between various proposed
failure criteria. Close agreement between certain factors and the equivalent bulk
material properties was noted for test batches.
The applicability of various failure criteria for both the mode I and mode II test
configurations and possible general criteria are discussed. The failure of the mode I
test configurations has been shown to be governed by the local level of maximum
principal stress at the end of the overlap. The mode II test configurations also show
dose agreement in terms of the maximum principal stress, but agreement with bulk
data Is poor. Therefore, a further failure criterion is proposed for the mode II joints
in terms of the 'global yielding' of the adhesive layer. The bulk property testing of
the adherend and adhesive materials to establish their physical properties for use
in the finite element analysis of the test programme Is also fully documented.