Review of Environmentally Assisted Cracking
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THE purpose of our analysis is to extract some generic principles governing the embrittlement phenomena that encompass a wide variety of materials, material microstructures, and environments. The materials include brittle materials such as glasses and ceramics, more ductile metals, alloys, single crystals, and complex engineering materials with relatively high toughness in an inert environment. Environments include external gaseous, aqueous, liquid metal environments as well as internal hydrogen or internal embrittling elements such as Pb and Cd. There is an extensive literature on each of these materials and environments, and many review articles exist summarizing, classifying, or unifying the principles underlying the embrittlement phenomenon.[1–20] Gangloff[11] provided an exhaustive list of literature references in his review on hydrogen embrittlement. In the present article, we only highlight general behavior and cover only those references and analyses that are pertinent to the generic principles involved in embrittlement K. SADANANDA, Consultant, is with Technical Data Analysis, Falls Church, VA 22042. Contact e-mail: kuntimaddisada@yahoo. com A.K. VASUDEVAN, Scientific Officer, is with the Office of Naval Research, Arlington, VA 22203. Manuscript submitted December 7, 2009. Article published online December 9, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
phenomenon. Our analysis pertains specifically to stress corrosion and not to general corrosion, exfoliation, or time-dependent creep.
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GENERAL BEHAVIOR
A. Crack Nucleation Three distinct types of specimens have been used to experimentally characterize and quantify the embrittlement process: (1) smooth specimens under uniaxial loading; (2) fracture mechanics specimens with initially sharp fatigue precracks; and (3) notch tensile specimens with finite depth, which can be considered as an intermediate between the first two. In the limit of infinite notch-tip radius, they behave like smooth tensile specimens, and in the limit of zero radius, they behave like cracked specimens. We may note here that atomically sharp crack cannot be sustained in a material due to singularity in stresses at the crack tip. Hence, a realistic case involves crack tip core relaxations in brittle materials such as glasses[21] or localized crack tip plasticity resulting in a finite radius in ductile materials. The assumption of some minimum critical crack tip radius is not far from reality,[22,23] to characterize the behavior. Figure 1 highlights the behavior of smooth, notch, and cracked specimens. Under sustained load or stress VOLUME 42A, FEBRUARY 2011—279
in an aggressive environment, a smooth specimen fails in time. Experimental results indicate that the stress to fail reaches an asymptotic limit, indicated by a threshold stress, rth. In the case of a fracture mechanics specimen, the asymptotic limit corresponds to a threshold stress intensity factor, Kth. There are predominately two factors that govern these thresholds: one is the chemical concentration and the other is materia
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