The automotive industry is currently increasing its use of structural adhesives to reduce vehicle weight and improve crash resistance. The development
of design criteria for structural adhesives are essential to increase designers’ confidence and consequently its utilization. A recent approach
consists in the use of Cohesive Zone Model (CZM) through Finite Element Method (FEM) programs. A CZM requires a traction-separation (σ-δ) law to
represent the adhesive being modeled between two surfaces. To define a σ-δ law essentially three parameters are necessary: the adhesive’s stiffness,
K, the cohesive traction, σi0,
and the critical fracture toughness, Gic ,
in the respective mode of loading i. This paper deals with an experimental/numerical approach to investigate the bond thickness effect into the σ-δ
law parameters in mode I. The adhesive tested, Betamate 1496s, is a one component, heat curing, high toughness structural epoxy adhesive, commonly
employed on the bonding of vehicles body structure. Double Cantilever Beam (DCB) tests were conducted on four bond line thickness: 0.30, 0.56, 0.81
and 1.57 mm. The Compliance Based Beam Method (CBBM) was used to obtain the critical fracture toughness in mode I,
GIc,
and the others σ-δ law parameters in mode I were obtained by fitting the numerical force-displacement (P-δ) curves to the experimental P-δ curves. The
sensitivity analysis performed revealed that the P-δ curves can be fitted with either a triangular or a trapezoidal σ-δ law. Furthermore, although
parameters could be calibrated to fit the P-δ loading phase with good accuracy, it wasn’t possible to simultaneously calibrate the delamination phase.
Further work is needed to address this particular problem in the employed model. The results revealed a linearly increasing dependency of
GIc
with the adhesive thickness from 3.43 N/mm for t = 0.30 mm to 7.89 N/mm for
t = 1.57 mm. The parameters σI0 and
K are almost unaffected by variations on bond thickness staying close to the values of 24 MPa and 120
N/mm respectively.