Ultrasonic Quantification Of Corroded Surfaces
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macroscopic measurables that quantify the system state, and the relationships between these measurables and parameters such as residual strength, residual lifetime and the impacts on fatigue damage and initiation of multi-site damage. An early study on the effect of corrosion on fatigue life [1] demonstrated that the surface root-mean-square (rms) roughness could be used as a single quantitative measure for damage. This parameter changes in a predictable manner with corrosion exposure time and can be used to predict fatigue lifetimes in a repeatable way. These results suggested that the damage could be quantified by a single parameter that is not strongly influenced by subjective interpretations Oe. not like the ASTM exfoliation corrosion rating scale). Unfortunately, the results were completely empirical and cannot be quantitatively used for any differing material/environment systems. Fractal geometry has been found useful for quantitatively describing the irregular shapes associated with fracture surfaces [2-5] and corrosion [6,7]. The corrosion studies seek to relate the measured "fractal" dimension of the surface to the type and/or severity of exposure. The experimental evidence is convincing and it is universally recognized that the surface morphology is a direct result of either random [8,9] or high order "chaotic" behavior [10-12]. The experimental results, however, do not provide an a prioriprediction of the measured fractal dimension for a corroding system, nor the evolution of this parameter. In addition a methodology for relating this measured parameter to the residual service life of the system is not yet presented in the literature. 37 Mat. Res. Soc. Symp. Proc. Vol. 503 0 1998 Materials Research Society Downloaded from https://www.cambridge.org/core. Gothenburg University Library, on 30 Jan 2020 at 21:32:01, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-503-37
The purpose of this note is to present experimental results which demonstrate the relationships observed in the literature, and a very simple model for the corrosion process capable of predicting this behavior. General conclusions about measurement of the system are drawn from the model results. ULTRASONIC CHARACTERIZATION The samples examined in this study were tensile coupons cut from 2024-T3 sheet. These were then subjected to an ASTM G44 stress-corrosion cracking test, alternate immersion in 3.5% NaCl solution, unstressed for 3 and 6 weeks. Following the exposure, the samples were cleaned according to the ASTM G1 procedure, cleaning in a hot H3P0 4 , Cr0 3 and HNO 3 solution followed by a rinse in room temperature HNO3 . The samples were then tested to failure in fatigue. The front and back surface of a representative sample is shown in Figure 1. Both sides of each sample were examined using a custom-designed, high-precision scanning acoustic microscope (HIPSAM) from the Non-Destructive Evaluation Branch of the Air Force Wright Laboratory [13]. The analysis metho
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