Subcritical adhesion testing or stress corrosion cracking explores adhesive debonding in a range of crack velocities and applied strain energy release rates (SERR), G, that are significantly less than are observed from catastrophic failure. As a consequence, subcritical adhesion testing can closely simulate the failure occurring in the real-life application or service life of the adhesive. Information about the mechanism of failure is gathered from a log-log plot of the average debond velocity, v, as a function of G. A change in the slope is indicative of a change in the rate limiting step for crack advancement and includes such behavior as threshold behavior, stress-dependent chemical reaction or reaction-limited, and transport limited mechanism for debond advancement [1−3].
Traditionally, subcritical fracture test specimens have been triple-layered laminated beams, where the adhesive is bonded between two rectangular rigid adherends. Examples of the laminated beam type experiments are the DCB wedge test [4, 5], the asymmetric double cantilever beam test [6], the double cleavage drilled compression specimen [7, 8] and the four-point flexure samples . The advantages of laminated beam type specimens are: 1) the adhesive is loaded elastically away from the crack tip, 2) high strain energy release rates are obtainable, 3) the fracture mechanics models for analysis are well understood, and 4) the specimens can be self-loading. A disadvantage of these type of tests is that sometimes it is difficult to measure the debond length. For transparent adherends or for a clearly visible crack propagating at the edge of the specimen, an optical microscope may be the only instrument needed to measure the debond length. However, if the debond is not clearly visible, such as for opaque adherends, the use of sophisticated experimental equipment (video camera, acoustic or electrical methods) may be necessary [9, 10]. Additionally, for an adhesive sandwiched between two impermeable substrates, diffusion occurs only from the edge and the equilibration time may be long [11, 12]. This can slow down the speed of research as well as lead to a heterogeneous distribution of diffusant in the adhesive joint.
With respect to the laminated beam type specimens, there are advantages to utilizing the shaft-loaded blister test. The time for environmental saturation is relatively short, due to the exposed face and short diffusion path [11, 12]. In addition, the specimen geometry is axisymmetric, which reduces any misleading edge effects caused by the degradation of the interface away from the crack tip. Some disadvantages of the shaft-loaded blister test are that its fracture mechanics models have not been studied as extensively as models from the laminated beam specimens; and, like most coatings tests, the maximum value of the strain energy release rate before film rupture occurs is limited by the film's mechanical strength and the intrinsic interfacial toughness. Plastic yielding of the coating may lead to an overestimated blister height (relative to the elastic case) [13], possibly lead to creep [14] and therefore complicate the analysis. Furthermore, the strain energy release rate decreases as a function of a −4/3 and therefore will approach a threshold value of G more slowly than beam type specimens. For the DCB wedge test, the strain energy release rate decreases as a function of a−4. Therefore, a wide range of G values can be obtained for a relatively small change in crack length and the threshold energy can be rapidly approached. Neither the SLBT nor wedge test are particularly less sensitive than the other to the effects of residual stresses; in both test methods during subcritical testing the residual stress driven release rate may be the same magnitude as the threshold value, G Th, and should therefore be considered. [15−18]
In this work, the subcritical adhesion behavior between an epoxy and glass as a function of relative humidity (r.h.) was measured using the DCB wedge test and the SLBT. The results from the two tests are compared and the disadvantages and advantages of each test method are highlighted based on the experimental data.