Wedge crack

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. 

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... 

McDevitt, N. T., et al., Accelerated Corrosion of Adhesively Bonded 7075 Aluminum Using Wedge Crack Specimens, Air Force Materials Laboratory Technical Report 77-184, October... 

Before writing down the solution we must first broach the subject of the geometric transformation relating the wedge crack problem to the allied problem of a free surface. If we consider a wedge crack with opening angle a, all points on the line z = must be mapped onto the line z = re. The transformation that... 

Ohr S. M., Chang S.-J. and Thomson R., Elastic Interaction of a Wedge Crack with a Screw Dislocation, J. Appl. Phys. 57, 1839 (1985). 

Fig. 6.82 Nucleation of a wedge crack due to pile-up dislocations on a slip plane [75]. With kind permission of Elsevier...

According to the theoretical presentation of Wu et al. [96], a wedge crack may be formed by the insertion extra material to create the head of a crack. An extra plane, present above a positive-edge dislocation, may serve as the source of a wedge crack. The idea is Stroh s [80], based on Zener s [23] original concept. 

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

Wedge crack length data to show the influence of varying the AC anodizing parameters and electrolyte temperature for the ACDC process note a 10 m, 20 V DC process has been deposited beneath each AC film (Cartwright 2005)... 

All of this should have come to an end in the middle 1970s when phosphoric-acid anodizing and optimized etch processes were established, particularly since Bethune at Boeing developed the simple wedge-crack test, ASTM D-3762, Marceau et al. (1977) that made it easy to distinguish between properly and improperly prepared bonding surfaces. The properly prepared surfaces had a stable oxide coating, with many pores that the primer could penetrate, as shown in O Fig. 44.7. 

Once reliable process specifications have been established, their application for metal bonding is customarily validated by two tests on coupons referred to as traveller coupons that are processed with the part. One of these tests is the lap-shear coupon (ASTM D-1002), tested at room temperature, and the other some form of peel test in a hot/wet (hostile) environment. Common peel tests are the wedge-crack test (ASTM D-3762 with a far more... 

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