Issue 10

A. Pirondi, Frattura ed Integrità Strutturale, 10 (2009) 21-28; DOI: 10.3221/IGF-ESIS.10.03 22 out about the influence of the thickness of the adhesive layer and of the adherends [9-11] that can be seen also as the evaluation of different degrees of constraint. The constraint effect in homogenous materials is well known to arise from the out-of-plane width and from the in-plane interaction of the crack with the boundaries. In the case of a crack in an adhesive joint, the adhesive layer thickness defines the distance of the boundaries (i.e. the adherends) from the crack tip. The flexibility of the boundaries plays of course also an important role in the constraint imparted to the crack. In a previous work [12] , 2D CZM analyses were carried out to simulate failure of T-peel bonded joints) with 1.5 and 3mm thick adherends, respectively. A trapezoidal cohesive law was used and a CZ was introduced using nonlinear springs. The fracture toughness and load-opening behaviour recorded in experiments on bonded DCB specimens [13] were taken as reference to calibrate CZM parameters. This work is aimed at extending those results. The failing interface is modeled with the cohesive elements available in this software. The influence of: i) different cohesive law shapes, ii) modelling the presence of the adhesive layer explicitly, is studied. E XPERIMENTAL Materials he DCB aluminum adherends (E=70GPa,  =0.3) were bonded with Loctite 330, a modified methacrilate ester supplied as viscous paste plus activator. Loctite 330 is a structural adhesive suitable for bonding of metals, wood, ceramics and plastics. Its advantage with respect to other structural adhesives is that it does not require specific surface preparation (only degreasing) and the joint can be handled after five minutes, allowing for higher production rates. Shear and tensile strength declared by the supplier are about 20MPa. The tensile behavior of the adhesive was modeled as a power law:  E   0 (1)  0  0  n  0 (1bis) where E = 878MPa,   = 4.8MPa, n = 0.44 and   =   /E were determined by tensile testing of bulk adhesive [13]. The Poisson's ratio was 0.15. The T-peel joints were made of italian standard UNI Fe 360 unalloyed steel, whose properties are typically E=210GPa,  =0.3,  u =360MPa,   =240MPa. DCB tests The specimen dimensions are shown in Fig. 1 . A four-step procedure was used for surface preparation of the substrates before bonding: i) polishing with sandpaper, ii) degreasing with acetone, iii) rinsing in hot water, iv) drying. The procedure was applied twice, initially using a coarse sandpaper and then a fine sandpaper. The recommended bondline thickness for Multibond 330  ranges between 0.05 to 0.5 mm. In all the experiments reported here, a thickness of 0.25 mm was adopted. Bond thickness control during specimen preparation was achieved placing two controlled-thickness copper shims between the adherends (see Fig. 1a) . A 0.05-mm-thick Teflon tape was placed just before bonding at mid-thickness of the adhesive layer to obtain a cohesive crack-like defect. The test were performed after at least a 24-hrs curing at room temperature, that is the minimum time to fully develop the mechanical strength according to the datasheet of the adhesive manufacturer. h h t a W b a 0 = 40 mm W = 120 mm h = 15 mm t = 0.3 mm b = 30 mm calibrated shims teflon  Figure 1 : Outline of the DCB joint. T