Лако-красочные материалы — производство

Once the performance of the interface bond was established by the previous delamination (durability) tests, contoured double cantilever beam (CDCB) specimens were designed to conduct mode-I fracture tests. In this study, bilayer CDCB specimens (see Fig. 6) were designed by the Rayleigh-Ritz method [8] and used for fracture toughness tests on bonded FRP-wood interfaces under both dry and wet conditions. The critical strain energy release rate, G1c, which is a measure of the fracture toughness, is given as:

where, Pc is the critical load, b the width of the specimen, and dC/da the rate of change of compliance C with respect to crack length a. The testing for fracture toughness of bonded interfaces using conventional constant-height double cantilever beam (DCB) specimens requires simultaneous measurements of critical load and crack length for each load step. The value of dC/da in Eq. (1) depends on the accuracy of the crack length measure­ment, which is generally a difficult task. The measurement of crack length can be avoided

adherend

adhesive

adherend

Contoured portions

Figure 6 A schematic view of a CDCB.

by contouring the DCB specimen, such that dC/da is a constant, and in this case, the specimen is known as the CDCB.

To use the CDCB configuration for interfaces of dissimilar bonded materials, it is convenient to use constant-thickness adherends bonded to contoured portions made of a material that is easy to shape, such as wood-based materials (Fig. 6). Owing to the relative complexity of defining the shape of a CDCB specimen, a numerical method based on the Rayleigh-Ritz solution was developed [8] to design the shape of the test specimens. For a given crack length, the CDCB specimen is modeled as a cantilever beam to obtain its compliance; the expression for compliance of the specimen is derived using the Rayleigh-Ritz solution and defined as a function of the crack length and the slope of the contour. A first-order shear deformation theory [11] is used to account for shear deformation, which is important for materials such as FRP and wood. In the following sections, the design procedures for CDCB specimens based on the Rayleigh-Ritz method [8] are briefly summarized; then, the predictions and calibrations of compliance rate change (dC/da) for linear-slope CDCB specimens are discussed. The applications of CDCB specimens for fracture toughness evaluations [using Eq. (1)] of phenolic and epoxy FRP-wood interfaces under both dry and wet conditions are presented.