Failure Diagram for Chemically Assisted Crack Growth
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INTRODUCTION
CHEMICALLY assisted crack growth involves chemical potential gradients contributing to a reduction in the mechanical stresses required to cause crack nucleation and growth, leading to a component failure. The chemically assisted crack growth occurs during stress corrosion crack growth in aqueous solutions (SCC), sustained load crack growth, liquid metal embrittlement (LME), solid metal–induced embrittlement, and hydrogen-assisted cracking. An aggressive environment provides the chemical driving force by adsorption, absorption, or crack tip reaction, and diffusion of damaging chemical species to the region ahead of the crack tip, causing embrittlement by reduction in cohesive forces or reduction in surface energy. In the absence of a pre-existing crack, heterogeneous deformations of favorably oriented grains set up the localized stress concentrations that form loci for the diffusing chemical reactants contributing to crack initiation and growth. Chemically assisted crack growth is very common and sometimes contributes to catastrophic failures, as in the case of the recent failure of the Mississippi Bridge in 2007. Quantification and life prediction under the combined chemical and mechanical forces are important in the selection, diagnostics, and prognostics of load-bearing materials in service. K. SADANANDA Consultant, is with Technical Data Analysis, Falls Church, VA 22143. Contact e-mail: kuntimaddisada@yahoo. com A.K. VASUDEVAN, Scientific Officer, is with the Office of Naval Research, Arlington, VA 22203. Manuscript submitted December 8, 2009. Article published online December 15, 2010 296—VOLUME 42A, FEBRUARY 2011
Extensive literature exists[1–8] in analyzing the crack nucleation and its growth behavior in a variety of materials including metals, alloys, ceramics, and composites in divergent environments including aqueous, gaseous, and liquid metals with environments external and internal to the specimens. While the detailed evaluation of the chemically assisted crack growth has been presented elsewhere,[9] we focus here on the representation of the material response in the form of a failure diagram, to facilitate its subsequent usage in the life prediction models. We approach this problem in the same way as we did in earlier articles where we combined the stages of nucleation and propagation of cracks under cyclic loads, using a modified Kitagawa failure diagram for fatigue.[10]
II.
MODIFIED KITAGAWA DIAGRAM FOR FATIGUE
In the original proposed Kitagawa diagram,[11] the endurance limit of a smooth tensile specimen that characterizes the minimum remote stress amplitude (Dre) to failure (taking 107 cycles as failure limit) was combined with the minimum threshold stress intensity range (DKth) for a crack growth from a fracture mechanics specimen. We recently modified this diagram[10] for fatigue incorporating several key features. (a) Fatigue involves two loading parameters, the stress amplitude DK and the peak stress Kmax. Correspondingly, there are two limiting endurance values for a smooth specimen, Dr
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