Analysis of the cohesive zone and crack length correction in the adhesively bonded double cantilever beam specimen

The double cantilever beam specimen has been increasingly employed to enable the development of cohesive zone models for adhesive joints. Evaluation of the traction–separation law (TSL) requires elaborate experimental techniques and usually relies on data measured until the crack initiation point. Nonetheless, current standards stipulate fracture energy measurements under steady-state crack propagation. This paper investigated the influence of the cohesive zone on the commonly used corrected beam theory data reduction scheme. Analytical solutions for the elastic–perfectly plastic, bilinear, and trapezoidal laws were developed using a beam model. The role of the elastic traction decay zone was found to be significant for high strength moderately tough adhesives. Nevertheless, the results showed that the sensitivity of the crack length correction to the cohesive zone can be exploited to obtain approximate TSLs.     

A Technique for the Measurement of Adhesive Fracture Energy by the Blister Method

The adhesive fracture energy, G_1c, of a model adhesive/adherend system, consisting of poly(methyl-methacrylate) plates bonded with a cyanoacrylate adhesive, has been evaluated using the Tapered Double Cantilever Beam and Blister test geometries. A refined Blister testing technique is described which, using relatively large diameter test plates [200mm], is capable of arresting the initial propagation of the-invariably less than naturally sharp-starter crack. This allows us to three subsequent G_c determinations for the same specimen from starter cracks of natural sharpness.

Adhesive fracture energy values determined for the model system using TDCB test pieces, 0.110±0.017 kJm^-2, were in good agreement with those obtained for Blister specimens in which arrested cracks had been repropagated, 0.119±0.013 kJm^-2. As is generally observed, values calculated from the initial propagation of starter cracks were somewhat higher for the TDCB specimens, 0.140±0.045 kJm^-2. Corresponding values for the Blister tests were significantly higher with more scatter, 0.194±0.074 kJm^-2.

It is concluded that meaningful G_1C data are only obtained from the Blister test if values obtained from the propagation of the initial starter crack are discarded. No matter how carefully prepared, these flaws will be less than naturally sharp. The precracking technique described here detects crack growth and, by releasing the hydraulic pressure driving it, arrests a propagating crack before catastrophic failure can take place.     

Predicting the Fatigue Life of Adhesively-Bonded Joints

The fatigue behaviour of adhesively-bonded joints, which consisted of an epoxy-film adhesive bonding fibre-composite substrates, has been studied. Using a double-cantilever beam specimen, the rate of crack growth per cycle has been measured as a function of the maximum strain-energy release rate, G_max. These data have then been modelled, and used to predict the fatigue lifetime of bonded single-lap joints. The agreement between the theoretical predictions and experimental results for the fatigue behaviour of the single-lap joints was found to be excellent.     

Influence of Residual Thermal Stresses on the Fracture Behavior of Hybrid Bonded Joints

The objective of this work is to study the influence of the residual thermal stresses on the fracture behavior of hybrid composite-aluminium bonded joints. A modified DCLS specimen is designed and the strain energy release rate is determined using both an analytical fracture mechanics based method and a finite element method. The residual thermal stresses, which appear in the adhesive because of the difference between the curing and the service temperatures, are evaluated with a two-ply laminate specimen. Some rupture tests are performed on the DCLS specimens at different temperatures and for different geometries and materials. The results show a good agreement between analytical calculations, numerical analysis and experiments. The residual thermal stresses are never negligible in the considered hybrid bonded joints and do influence their fracture behaviors. Although the designed specimen is not adapted to study fatigue crack propagation, it provides a simple way to study the crack initiation and thus to characterize the rupture properties of the joint.