1. Mechanistic Aspects double-notch bend testing shows that without H, fracture is strain-controlled, i.e., initiation occurs at the notch (left)double-notch bend testing shows that without H, fracture is strain-controlled, i.e., initiation occurs at the notch (left) with H, fracture is inter- granular and stress- controlled; initiation occurs ahead of the notch (right)with H, fracture is inter- granular and stress- controlled; initiation occurs ahead of the notch (right) Studies on the mechanical properties of high-strength steel (AISI 4340) characterize the marked deterioration in fracture strength with increase in hydrogen concentration.Studies on the mechanical properties of high-strength steel (AISI 4340) characterize the marked deterioration in fracture strength with increase in hydrogen concentration. Mechanical properties are used in the statistical model of hydrogen-assisted fractureMechanical properties are used in the statistical model of hydrogen-assisted fracture Hydrogen free 5 μm Hydrogen charged5 μm Hydrogen-induced Material Degradation: Brittle Decohesion Versus Plastic Flow Localization Petros Sofronis 1, Robert O Ritchie 2 1 University of Illinois at Urbana-Champaign, 2 University of California, Berkeley, DMR-0302470 Results for 4-point Bend Single Notched Specimens: Experiment vs Modeling

2. Hydrogen charged Hydrogen free In the presence of hydrogen, the fracture mode changes from microvoid coalescence to intergranular as seen in the micrographs at top. The finite element model indicates extensive plasticity at the notch root in the absence of hydrogen, and limited plasticity in the presence of hydrogen at the moment of failure as seen in the contour plots for the effective plastic strain at bottom.In the presence of hydrogen, the fracture mode changes from microvoid coalescence to intergranular as seen in the micrographs at top. The finite element model indicates extensive plasticity at the notch root in the absence of hydrogen, and limited plasticity in the presence of hydrogen at the moment of failure as seen in the contour plots for the effective plastic strain at bottom. Hydrogen-induced Material Degradation: Brittle Decohesion Versus Plastic Flow Localization Petros Sofronis 1, Robert O Ritchie 2 1 University of Illinois at Urbana-Champaign, 2 University of California, Berkeley, DMR-0302470 The statistical model predicts the location of the highest probability of failure (shown in the contour plot above) at 300 microns from the notch root. This compares favorably with the experimental evidence shown in the previous slide, which indicates that fracture initiates at a site at distance less than 400 microns from the notchThe statistical model predicts the location of the highest probability of failure (shown in the contour plot above) at 300 microns from the notch root. This compares favorably with the experimental evidence shown in the previous slide, which indicates that fracture initiates at a site at distance less than 400 microns from the notch Calculated Failure Probability

3. Hydrogen-induced Material Degradation: Brittle Decohesion Versus Plastic Flow Localization Petros Sofronis 1, Robert O Ritchie 2 1 University of Illinois at Urbana-Champaign, 2 University of California, Berkeley, DMR-0302470 Long term energy independence for US requires that petroleum be substituted as an energy source for industry and transportation. An alternative energy source is hydrogen, but the technology of large scale hydrogen transmission from central production facilities to refueling stations and stationary power sites is at present undeveloped. Among the problems which confront the implementation of this technology is the deleterious effect of hydrogen on structural material properties. The present project aims at developing and verifying a lifetime prediction methodology for failure of materials used in hydrogen gaseous environments. Development and validation of such predictive capability and strategies to avoid material degradation is of paramount importance to the rapid assessment of the suitability of using the current natural gas pipeline distribution system for hydrogen transport in the new hydrogen economy and of the susceptibility of new alloys tailored for use in hydrogen related applications. Our approach integrates mechanical property testing at the microscale, statistical microstructural analyses and transmission electron microscopy observations of the deformation processes of materials at the micro- and nano-scale, and finite element modeling and simulation at the micro- and macro-level. Broader impact