Wedge-crack propagation test

FIGURE 16.3 Typical bond durability data for Ti-6 Al-4 V adherends bonded with an epoxy adhesive and aged at 60°C and 100 percent RH. (a) Crack propagation versus time for the wedge crack propagation test. (b) Applied stress versus time to failure for the lap shear geometry. PF—phosphate fluoride MPF—modified phosphate fluoride DP—PasaJell 109 dry hone LP—PasaJell 107 liquid hone CAA-5—-5% solution CAA-10—10% solution TU—Turco 5578 etch DA—Dapcotreat.50... 

FIGURE 8. Schematic diagram of a wedge-crack propagation test for bond durability. 

Fig. 7. Schematic representation of hydration causing crack propagation in a wedge test specimen. The increase in volume upon hydration induces stresses at the crack tip that promote crack growth 19,391.

Most of the experimental results presented above were obtained for the case of a steady-state crack propagating around 1-5 pm/s. The crack velocity in the DCB experiment performed with a wedge is controlled by the velocity at which the wedge is pushed to separate the sample. It is therefore possible in principle to do tests over a range of velocities. However, a few studies have been reported where the velocity of crack propagation has been investigated in a systematic way. The trend in these studies, illustrated by Fig. 30 in the case of a PS/PVP interface reinforced with a dPS-PVP 800-870 diblock copolymer, is however always one of increasing Qc with crack velocity [60]. 

Because SHA is a relatively new preparation, SHA adherends were not included in the early studies. In wedge tests conducted in a humidity chamber over a wide range of 55-80°C and 95% relative humidity, epoxy-SHA bonded systems have shown durability comparable to epoxy-CAA systems.<60.62> jn one of these studies,< 0) the authors concluded that the SHA adherend was more durable than the CAA adherend. However, the crack propagation results showed only 2.5 mm difference between the two after 150 hours exposure. This difference is within the range of error in typical wedge tests< 3) and can be considered negligible. 

Figure 37 Cleavage test also called crack propagation (ISO 10354) or BOEING wedge test (a) test specimen (all dimensions in mm) (b) wedge (c) Test specimen after the test wedge has been pushed inside the test specimen (at a speed of 1 mm/min, for instance) original opening was o, which increased by Aa so that crack propagation was Aa (Aa is measured very accurately with a microscope).

Because of their greater thickness, CAA oxides serve to protect the metal surface from corrosion better than thinner oxides but the important factor for bond durability is the stability of the outer oxide structure when water diffuses to the oxide-polymer interphase. Accordingly, it would be expected that the performance of CAA treated adherends would be similar, although no better, than that of PAA, or BSAA. The wedge test data shown in Fig. 20 and other work [29,77,97,98] support this and demonstrate that when these processes are done correctly the wedge test crack will be forced to propagate entirely within the adhesive. Similar arguments are likely with BSAA adherends, also. 

---