Published: 01 January 1990
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The results of an interlaboratory program conducted to evaluate variability in measurements of the threshold stress intensity factor (KISCC) and stress corrosion crack growth rates (da/dt) for aluminum alloys are summarized. Precracked specimens machined from 7075-T6 aluminum alloy plate are exposed to a 3.5% NaCl (sodium chloride) solution by eleven participating laboratories. The effects of specimen configuration, starting stress intensity level, precracking procedure, and surface grooving are statistically evaluated, and sources of both interlaboratory and intralaboratory variability are identified. Data analysis indicates that variations in crack growth rates were related to difficulties associated with visual crack length measurements, especially on face-grooved specimens, and that more effective measurement techniques would more accurately characterize stress corrosion crack growth. In addition, test duration should be determined systematically, accounting for crack length measurement resolution, time for crack arrest, and experimental interferences. Variations in KISCC were related to the procedure used to measure the final bolt load. Final load measured by specimen reloading did not correlate well with that determined by compliance and appeared sensitive to factors such as corrosion product wedging and metallurgical features on the stress corrosion crack faces. Miniature instrumented load cells were used in several tests in a parallel program to monitor load relaxation during specimen exposure and to provide an indication of final load prior to specimen reloading. The load cell output during specimen exposure indicated that perturbations observed in crack length versus time curves may be related to real changes in load and should not be considered due only to crack length measurement difficulties. Compliance solutions were experimentally determined for both modified compact and double-beam wedge loaded specimens with different face-groove geometries and the results compared with published solutions. Both crack-mouth and load-line crack opening displacements were measured, and the data were used to define a relationship to correlate deflections at these measurement locations. The resulting relationship is compared with a previous solution determined by boundary collocation. No significant statistical variation in KISCC was observed with regard to specimen type, surface grooves, or specimen orientation, indicating both specimen configurations are suitable for stress corrosion testing and differences in KISCC determination were attributed to test procedures. Results of both the interlaboratory program and the additional studies should provide important contributions toward the development of a standard test method for stress corrosion testing and analysis using precracked specimens.
fatigue (materials), cracking, environmental effects, stress corrosion cracking, precracked specimens, fracture mechanics, modified compact bolt-loaded specimen, double-beam bolt-loaded specimen, aluminum alloys
Materials engineer, NASA Langley Research Center, Hampton, VA