Browse Publications Technical Papers 2018-36-0071
2018-09-03

Effects of adhesive thickness on the mode I cohesive zone model parameters of a high toughness structural adhesive 2018-36-0071

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.

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