Crack wedging

Fig. 3. Wedge test crack length as a function of maximum Cu buildup at the oxide-metal interface. The adhesive was Cytec FM-123. The surfaces were prepared with the Forest Products Laboratory etch. The oxide morphology was kept constant. Data are from Ref. 115].

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.

The slow rate of hydration for buried surfaces is desirable from a service point of view, but makes the study and evaluation of the durability of surface treatments difficult unless wedge tests (ASTM D3762) or similar tests are used to accelerate the degradation. As for the wedge test, the stress at the crack tip, together with the presence of moisture at the tip, make this a more severe test than soaked lap shear specimens or similar types and therefore a better measure of relative durability. 

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. 

Fig. 20. Wedge test results showing PAA, CAA, and BSAA treated aluminum bonds. (Note that this test involved thicker adherends than is typical and therefore crack lengths cannot be compared to those of other tests.)...

Fig. 21. Wedge test results for grit blast/silanc surface treatment compared to PAA. Both surfaces were primed with BR127. Note the expanded crack length scale compared to other figures. Data are from Ref [89].

Occasional in-service delaminations occurred at the aft tip of the wedge. Because there was no metal-to-metal closeout bond at the aft tip, any slight incidental damage could easily crack the bond slightly and allow moisture to enter the core. Eventually, freeze-thaw damage and delamination would occur. 

In order to supplement micro-mechanical investigations and advance knowledge of the fracture process, micro-mechanical measurements in the deformation zone are required to determine local stresses and strains. In TPs, craze zones can develop that are important microscopic features around a crack tip governing strength behavior. For certain plastics fracture is preceded by the formation of a craze zone that is a wedge shaped region spanned by oriented micro-fibrils. Methods of craze zone measurements include optical emission spectroscopy, diffraction... 

If the wedge of Fig. 8 is a rigid plate of constant thickness Y and if its front is x cm to the left of the crack orifice (x being a variable), then a relation between Y and x can be used instead of that between y and x in Eq. (52). In this manner, the relation... 

Wedge Test. The adhesive bond durabilities of the Inhibitor-treated 7075-T6 surfaces were evaluated by wedge tests (ASTM D-3762) on bonded specimens using the FM 123-2 epoxy adhesive to simulate the epoxy primer. The specimens were placed In a humidity chamber at 65°C and 95% relative humidity and removed at specified time Intervals to record the crack tip locations after each examination, they were returned to the humidity chamber. 

Fig. 16a and b. Wedge-test crack length (in.) of aluminum/thermoset (American Cyanamid FM123-2) joints as a function of exposure time to a 100% r. h.,60 °C environment, a) FPL, PAA, FPL + 10 ppm nitrilotris (methylene phosphonic acid (NTMP)831 and b) PAA and PAA + 300 ppm NTMP pretreatments were employed 1391 (Reprinted with permission from Chapman and Hall, LTD.)... 

Abstract—The effects of metal alkoxide type and relative humidity on the durability of alkoxide-primed, adhesively bonded steel wedge crack specimens have been determined. Aluminum tri-sec-butoxide, aluminum tri-tert-butoxide, tetrabutyl orthosilicate, and titanium(IV) butoxide were used as alkoxide primers. Grit-blasted, acetone-rinsed mild steel adherends were the substrates bonded with epoxy and polyethersulfone. The two aluminum alkoxides significantly enhanced the durability of the adhesively bonded steel, while the titanium alkoxide showed no improvement in durability over a nonprimed control. The silicon alkoxide-primed samples gave an intermediate response. The failure plane in the adhesively bonded samples varied with the relative humidity during the priming process. 

The XPS analysis of the samples primed at 34% RH indicated that there was a consistent failure of the wedge samples which occurred mainly within the alkoxide layer in all systems. Partial hydrolysis may have resulted in the formaton of a weak hydrated oxide layer and was the zone through which the crack propagated to debond the samples. Based on the relative humidity in the chamber during the priming process and the failure surface analysis results, it was concluded that this level of 34% RH was not sufficient to complete the hydrolysis of the alkoxides and produce a stabilized oxide structure. As noted above, however, the wedge crack results did not indicate any instability. 

There is an apparent optimum relative humidity level required to achieve good adhesion and durability. Priming the steel adherends at 18% RH caused failure in the wedge samples within the steel (oxide) layer. Adherends primed at 34% RH failed within the alkoxide primer layer, whereas at 51% RH failure occurred primarily within the adhesive layer. This change in locus of failure with humidity was not evident using the wedge crack test when the adherends were primed with aluminum alkoxides. A peel-type test would probably be more sensitive in detecting these shifts in failure mode. 

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