Crystal Plasticity Modeling of Hydrogen and Hydrogen-Related Defects in Initial Yield and Plastic Flow of Single-Crystal Stainless Steel 316L

Understanding of and accounting for various mechanisms that affect inelastic deformation of crystalline metals in the presence of hydrogen remains an unsettled issue. Macroscopic experimental observations contradict limited atomistic simulations, complicating the situation. In this work, we extend a recent physically based crystal viscoplasticity framework to include constitutive equations with a direct dependence on relevant hydrogen and hydrogen-related defect concentrations. Focusing on initial yield and post-yield strain hardening, we consider hydrogen solute drag on mobile dislocations as well as the role of dilute concentrations of hydrogen-vacancy complexes as obstacles to dislocation motion. Furthermore, the evolution of hydrogen and hydrogen-affected defect concentrations is explicitly considered via evolving hydrogen trap concentrations. The resulting framework is used to investigate hydrogen effects on the quasistatic, monotonic, strain-controlled uniaxial loading of single-crystal stainless steel 316L smooth specimens at room temperature in an attempt to connect atomistic insight and the resulting mesoscale model framework with experimental interpretations. Attributing the primary role of hydrogen in this manner is shown to produce good agreement with experiments in the initial yield and post-yield regime. The dominance of various hydrogen effects mechanisms is discussed.

     

A Finite Element Formulation of Baraff-Witkin Cloth

The Baraff-Witkin [BW98] model has been a popular formulation for cloth for 20 years. However, its relationship to the finite element method (FEM) has always been unclear, because the model resists being written as an isotropic, hyperelastic strain energy. In this paper, we show that this is because the Baraff-Witkin model is actually a coupled anisotropic strain energy. We show that its stretching term approximates the isotropic As-Rigid-As-Possible (ARAP) energy, and its shearing term is a cross-fiber coupling energy common in biomechanics. While it has been known empirically for some time that the model can produce indefinite force Jacobians, the conditions under which they occur has never been clear. Our formulation enables a complete eigenanalysis that precisely characterizes exactly when indefiniteness occurs, and leads to fast, analytic, semi-positive-definite projection methods. Finally, our analysis suggests a generalized Baraff-Witkin energy with non-orthogonal warp and weft directions.     

Fast and Robust Stochastic Structural Optimization

Stochastic structural analysis can assess whether a fabricated object will break under real-world conditions. While this approach is powerful, it is also quite slow, which has previously limited its use to coarse resolutions (e.g., 26 × 34 × 28). We show that this approach can be made asymptotically faster, which in practice reduces computation time by two orders of magnitude, and allows the use of previously-infeasible resolutions. We achieve this by showing that the probability gradient can be computed in linear time instead of quadratic, and by using a robust new scheme that stabilizes the inertia gradients used by the optimization. Additionally, we propose a constrained restart method that deals with local minima, and a sheathing approach that further reduces the weight of the shape. Together, these components enable the discovery of previously-inaccessible designs.     

Static fatigue of Hi-Nicalon™-S fiber at elevated temperature in air,steam, and silicic acid-saturated steam

A facility for testing SiC fiber tows in static fatigue and creep at elevated temperatures in air and steam was developed. Static fatigue of Hi-Nicalon™-S fibers was investigated at 800°C-1100°C at applied stresses between 115 and 1250 MPa in air, in Si(OH)₄(g)-saturated steam, and in unsaturated steam. Fibers tested in Si(OH)₄(g)-saturated steam and in air had silica scales throughout the test sections, but those tested in unsaturated steam did not develop scales near the steam injection point. Fiber lifetimes in static fatigue were shortest in unsaturated steam, intermediate in Si(OH)₄(g)-saturated steam, and longest in air. Failure strains did not exceed 0.3%. Steady-state strain rates and static fatigue lifetimes are modelled empirically by the Monkman-Grant relationship. Failure mechanisms are discussed.     

Experimental evaluation of the seismic performance of steel MRFs with compressed elastomer dampers using large-scale real-time hybrid simulation

Real-time hybrid simulation combines experimental testing and numerical simulation, and thus is a viable experimental technique for evaluating the effectiveness of supplemental damping devices for seismic hazard mitigation. This paper presents an experimental program based on the use of the real-time hybrid simulation method to verify the performance-based seismic design of a two story, four-bay steel moment resisting frame (MRF) equipped with compressed elastomer dampers. The laboratory specimens, referred to as experimental substructures, are two individual compressed elastomer dampers with the remainder of the building modeled as an analytical substructure. The proposed experimental technique enables an ensemble of ground motions to be applied to the building, resulting in various levels of damage, without the need to repair the experimental substructures, since the damage will be within the analytical substructure. Statistical experimental response results incorporating the ground motion variability show that a steel MRF with compressed elastomer dampers can be designed to perform better than conventional steel special moment resisting frames (SMRFs), even when the MRF with dampers is significantly lighter in weight than the conventional MRF.     

The Effects of Omega‐3 Polyunsaturated Fatty Acid Consumption on Mammary Carcinogenesis

The consumption of omega‐3 polyunsaturated fatty acids (n‐3 PUFA) is associated with a reduced risk of breast cancer. Studies in animals and in vitro have demonstrated mechanisms that could explain this apparent effect, but clinical and epidemiological studies have returned conflicting results on the practical benefits of dietary n‐3 PUFA for prevention of breast cancer. Effects are often only significant within a population when comparing the highest n‐3 PUFA consumption group to the lowest n‐3 group or highest n‐6 group. The beneficial effects of n‐3 PUFA eicosapentaenoic and docosahexaenoic on the risk of breast cancer are dose dependent and are negatively affected by total n‐6 consumption. The majority of the world population, including the most highly developed regions, consumes insufficient n‐3 PUFA to significantly reduce breast cancer risk. This review discusses the physiological and dietary context in which reduction of breast cancer risk may occur, some proposed mechanisms of action and meaningful recommendations for consumption of n‐3 PUFA in the diet of developed regions.     

Electrically Tunable Nanoporous Carbon Hybrid Actuators

A novel nanoporous carbon/electrolyte hybrid material is reported for use in actuation. The nanoporous carbon matrix provides a 3D network that combines mechanical strength, light weight, and low cost with an extremely high surface area. In contrast to lower dimensional nanomaterials, the nanoporous carbon matrix can be prepared in the form of macroscopic monolithic samples that can be loaded in compression. The hybrid material is formed by infiltrating the free internal pore volume of the carbon with an electrolyte. Actuation is prompted by polarizing the internal interfaces via an applied electric bias. It is found that the strain amplitude is proportional to the Brunauer‐Emmett‐Teller (BET) mass specific surface area, with reversible volume strain amplitudes up to the exceptionally high value of 6.6%. The mass‐specific strain energy density compares favorably to reported values for piezoceramics and for nanoporous metal actuators.