Effects of Processing Residual Stresses on Fatigue Crack Growth Behavior of Structural Materials: Experimental Approaches and Microstructural Mechanisms

Fatigue crack growth mechanisms of long cracks through fields with low and high residual stresses were investigated for a common structural aluminum alloy, 6061-T61. Bulk processing residual stresses were introduced in the material by quenching during heat treatment. Compact tension (CT) specimens were fatigue crack growth (FCG) tested at varying stress ratios to capture the closure and Kmax effects. The changes in fatigue crack growth mechanisms at the microstructural scale are correlated to closure, stress ratio, and plasticity, which are all dependent on residual stress. A dual-parameter ΔK-Kmax approach, which includes corrections for crack closure and residual stresses, is used uniquely to connect fatigue crack growth mechanisms at the microstructural scale with changes in crack growth rates at various stress ratios for low- and high-residual-stress conditions. The methods and tools proposed in this study can be used to optimize existing materials and processes as well as to develop new materials and processes for FCG limited structural applications

The SPARC Toroidal Field Model Coil Program

The SPARC Toroidal Field Model Coil (TFMC) Program was a three-year effort between 2018 and 2021 that developed novel Rare Earth Yttrium Barium Copper Oxide (REBCO) superconductor technologies and then successfully utilized these technologies to design, build, and test a first-in-class, high-field (~20 T), representative-scale (~3 m) superconducting toroidal field coil. With the principal objective of demonstrating mature, large-scale, REBCO magnets, the project was executed jointly by the MIT Plasma Science and Fusion Center (PSFC) and Commonwealth Fusion Systems (CFS). The TFMC achieved its programmatic goal of experimentally demonstrating a large-scale high-field REBCO magnet, achieving 20.1 T peak field-on-conductor with 40.5 kA of terminal current, 815 kN/m of Lorentz loading on the REBCO stacks, and almost 1 GPa of mechanical stress accommodated by the structural case. Fifteen internal demountable pancake-to-pancake joints operated in the 0.5 to 2.0 nOhm range at 20 K and in magnetic fields up to 12 T. The DC and AC electromagnetic performance of the magnet, predicted by new advances in high-fidelity computational models, was confirmed in two test campaigns while the massively parallel, single-pass, pressure-vessel style coolant scheme capable of large heat removal was validated. The REBCO current lead and feeder system was experimentally qualified up to 50 kA, and the crycooler based cryogenic system provided 600 W of cooling power at 20 K with mass flow rates up to 70 g/s at a maximum design pressure of 20 bar-a for the test campaigns. Finally, the feasibility of using passive, self-protection against a quench in a fusion-scale NI TF coil was experimentally assessed with an intentional open-circuit quench at 31.5 kA terminal current.Comment: 17 pages 9 figures, overview paper and the first of a six-part series of papers covering the TFMC Progra

The Effects of Processing Residual Stresses on the Fatigue Crack Growth Behavior of Structural Materials

Residual stresses are a common and often undesired result of material processing, introduced through non-linear deformation and/or phase transformation of material under mechanical or thermo-mechanical loading. These macro stresses alter mechanical properties and the intrinsic fatigue crack growth characteristics of the material. Residual stress artifacts can introduce inconsistencies and significant errors when the true material behavior is needed for material development and optimization and for structural component design. The effects of quenching residual stresses on fatigue crack propagation behavior of various materials were investigated. In parallel, residual stresses similar in magnitude and distribution with the quenching residual stresses were generated using mechanical processes to decouple the effects of residual stresses from microstructural effects. Mechanical residual stress distributions predicted by 3D elastic-plastic finite element analysis showed good agreement with the stresses measured on fatigue crack growth testing specimens using fracture mechanics approaches. Crack propagation characteristics in fields with low and high residual stresses were studied using optical and scanning electron microscopy, and the effects of residual stress on crack path behavior were assessed. An original residual stress analytical correction to fatigue crack growth data was developed, compared to existing corrective methodologies, and validated using residual stress free data. Overall, the work provides tools to understand, control, and correct the effects of processing residual stresses on fatigue crack growth for accurate fatigue critical design and life predictions

VIPER: an industrially scalable high-current high-temperature superconductor cable

High-temperature superconductors (HTS) promise to revolutionize high-power applications like wind generators, DC power cables, particle accelerators, and fusion energy devices. A practical HTS cable must not degrade under severe mechanical, electrical, and thermal conditions; have simple, low-resistance, and manufacturable electrical joints; high thermal stability; and rapid detection of thermal runaway quench events. We have designed and experimentally qualified a vacuum pressure impregnated, insulated, partially transposed, extruded, and roll-formed (VIPER) cable that simultaneously satisfies all of these requirements for the first time. VIPER cable critical currents are stable over thousands of mechanical cycles at extreme electromechanical force levels, multiple cryogenic thermal cycles, and dozens of quench-like transient events. Electrical joints between VIPER cables are simple, robust, and demountable. Two independent, integrated fiber-optic quench detectors outperform standard quench detection approaches. VIPER cable represents a key milestone in next-step energy generation and transmission technologies and in the maturity of HTS as a technology

"Families" in international context: Comparing institutional effects across western societies

We review comparative evidence of institutional effects on families in Western societies. We focus on 2 key aspects of family life: gendered divisions of labor and people's transitions into, within, and out of relationships. Many individual-level models assume the effects are robust across countries. The international evidence over the past decade suggests instead that the socioeconomic and policy contexts strongly influence the significance and even direction of individual effects. A growing body of evidence also highlights important differences across social groups and family forms within countries. The pattern of relative gender, class, and other group equality varies across countries, as do related family experiences and outcomes. We conclude with suggestions for future comparative family research