Wedge tests

ASTM D3762-03 Standard test method for adhesive bonded surface durability of aluminum (wedge test). 

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A meehanical analysis of the near surface stresses in a wedge test as a function of surface geometry shows that tan a is equal to the ratio of the shear stress to the peel stresses (Fig. 4). 

Wedge test fracture energy (from Eqs. 1 and 2) vs. adherend surface treatment (from ref. [3 )... 

Eqs. 1-5 hold whether failure is interfacial or cohesive within the adhesive. Furthermore, Eq. 5 shows that the reversible work of adhesion directly controls the fracture energy of an adhesive joint, even if failure occurs far from the interface. This is demonstrated in Table 5, which shows the static toughness of a series of wedge test specimens with a range of adherend surface treatments. All of these samples failed cohesively within the resin, yet show a range of static toughness values of over 600%. 

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. 5. Wedge test results for Ti adherends with several different surface treatments having differing degrees and. scales of roughness. Specimens were exposed to 100% relative humidity at 60 C. Data from Ref. 132. ...

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.

Fig. 9. Wedge test results of aluminum adherends with the following surface preparations FPL, PAA, and FPL followed by an NTMP treatment. Adapted from Ref. [42].

Another means of providing a hydration-resistant surfaee is its treatment with a hydration inhibitor [41]. Fig. 9 shows wedge tests results for a Forest Produet Laboratory (FPL) bond [43], an FPL bond pretreated with nitrilotrismethylenephos-phonie (NTMP) aeid [42,44,45], and a PAA bond. The monolayer eoverage of NTMP stabilizes the FPL surfaee against hydration and provides wedge test bond performanee similar to that of PAA-treated adherends. 

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. 

Optimized grit blast/silane treatments can provide wedge test durability as good as PAA with failure entirely cohesive within the adhesive (Fig. 21) [89]. Maintaining the process parameters within acceptable tolerances is critical with the heat drying of the silane on the treated surface being the most sensitive process parameter. 

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].

Sol-gel film.s deposited on a grit-blasted aluminum surface give performance close to PAA bonds (Fig. 22) with generally cohesive failures observed in wedge tests. Given that one application of this treatment is repair, the performance is... 

Wedge test results suggest that the curing process (e.g., percent crosslinking) of the epoxy-polyamide primer system is not affected by the addition of organosilanes, but may be affected by NTMP. The results of substrate surface characterization, adsorption behavior of applied films, and evaluation of candidate inhibitors by chemical, mechanical, and electrochemical test methods are presented. Mechanisms to explain the observed behavior of the various phosphonate and silane polymer systems are discussed. 

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. 

FPL Surface. A second wedge test was performed to evaluate... 

Primer Epoxy vs. Nitrile-Modified Epoxy. The compatibility of the epoxy-polyamide primer with the nitrile-modified epoxy adhesive facsimile and the aluminum oxide surface was also evaluated by the wedge test, since earlier tests using the primer as the adhesive had failed immediately. As shown in Fig. 8, the addition of the primer directly to the prepared... 

Figure 5. Wedge test results for inhibitor-treated SAA 7075-T6 aluminum specimens.

Figure 8. Wedge test results for primed FPL 7075-T6 specimens.

The hydration resistance of the organosilane compounds was reflected by the wedge test performances of our silane-... 

The ionic phosphonates like NTMP are effective hydration inhibitors because they can form an insoluble complex with the oxide surface. They are useful as epoxy adhesive couplers in cases where the adhesive and its curing cycle are compatible with the adsorbed phosphonate molecule. (14) Wedge test results indicate that in two epoxy-aluminum systems studied, certain organosilanes tend to both increase the epoxy-metal bond durability and maintain hydration resistance. The results of anodic polarization experiments further suggest that these silane films are effective against localized pitting. 

Fig. 9. Schematic diagram of the failure mechanism proposed by Venables et al.85), in aluminum/poly-mer joint systems during wedge testing in humid environments. (Reprinted with permission from Chapman and Hall LTD.)...

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.)... 

Majowicz Jacobs (Ref 1) and Campbell et al (Ref 2) first used the wedge test to study shock initiation of solid expls. The test is named for the wedge-shaped expl sample that is shocked by a booster-and-attenuator system as shown in Fig 1... 

Fig 1 Experimental Arrangement for Most Wedge Test Shots (from Ref 3)... 

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