R.L. Holtz

Analysis of overload effects and related phenomena

Overloads and underloads perturb steady state fatigue crack growth conditions and affect the growth rates by retarding or accelerating the growth. Clear understanding of these transient effects is important for the reliable life prediction of a component subjected to random loads. The overload effects have predominately been attributed to either plasticity induced crack closure behind the crack tip, residual stresses ahead of the crack tip, or a combination of both. These effects are critically examined in the context of the Unified Approach proposed by the authors. Recent experimental and analytical evaluation of crack closure has confirmed its negligible contribution to crack growth and has demonstrated that changes in the stresses ahead of the crack tip are more important than closure behind the crack tip. It is shown that the overload effects and other transient effects arise due to perturbation of the stresses ahead of the crack tip, and these can be accounted for by the two parametric approach emphasized in the unified theory. It is shown that related phenomena including the role of Kmax, the existence of propagation threshold Kpr, and effects of overloads on Kpr and Kmax etc, are all accounted for by the Unified Approach.

Recent Developments in 2G HTS Coil Technology

Recent developments in 2G HTS coil technology are presented highlighting the ability of 2G HTS wire to function under difficult operating conditions without degradation. The challenges of use in various coil constructions and applications are discussed. Several applications where the conductor is subjected to high stress levels include high field insert coils and rotating machinery. While these applications present different challenges, the ability of the conductor to operate under high stress levels has been demonstrated in both direct sample measurement and test coils. The high winding current density that is available with SuperPowers thin 2G HTS wire was utilized in a high field insert coil demonstration generating central fields in excess of 26.8 T . The ability of the wire to be tailored (stabilization, insulation, ac losses) to fit various operating parameters will also be discussed.

Fatigue thresholds of Ni-Ti alloy near the shape memory transition temperature

The fatigue thresholds of nickel-titanium alloy with martensite start temperature of 80°C were measured in air from room temperature to 150°C and load ratios of 0.1 to 0.9. The ΔK versus K max fundamental threshold curves are characteristic of ductile metals at all temperatures tested and are consistent with the existence of two threshold parameters, ΔK th * and K max,th * , for cyclic and maximum stress intensity, respectively. ΔK th * increases linearly with temperature, while K max,th * , exhibits a transition between 80 and 100°C, decreasing by half as a function of temperature. Compliance curves suggest the existence of crack closure in the stable martensite regime below 100°C, but little or no crack closure in the stress-induced martensite regime above 100°C up to 150°C. The results suggest that both crack closure and the existence of an intrinsic fundamental threshold curve with both ΔK and K max thresholds as a function of load ratio are necessary to fully account for R dependence of ΔK th .

Extension of the Unified Approach to fatigue crack growth to environmental interactions

Abstract Environmental effects on fatigue crack growth were examined using the Unified Approach developed by the authors. According to this approach, K max and Δ K are two intrinsic parameters simultaneously required for quantifying fatigue crack growth data. The two parameters led to two intrinsic thresholds that must be simultaneously exceeded for a fatigue crack to grow. Environmental interactions being time and stress-dependent processes affect fatigue crack growth through K max parameter. Based on an extensive analysis of literature data, we have classified environmental effects into four basic types. In addition, we have defined a true reference state for an inert fatigue behavior. Classification procedures and methods of estimating the magnitude of environmental effects using the Unified Approach for fatigue crack growth were discussed taking examples from the literature.

Strain field and scattered intensity profiling with energy dispersive x-ray scattering

Two powerful synchrotron x-ray scattering techniques for residual strain depth-profiling and tomography-like scatter-intensity profiling of materials are presented. The techniques utilize energy dispersive x-ray scattering, from a fixed microvolume, with microscanning of the specimen being used to profile its interior. The tomography-like profiles exploit scattering-cross-section variations, and can be contrast enhanced by separately monitoring scattering from different crystal structures. The strain profiling technique is shown to finely chronicle the internal strain variation over several mm of steel. Detailed strain profiling for a cantilever spring demonstrates the interplay of residual and external stresses in elastic/plastic deformation. Since surface compression, by shot peening, is a classic method to fortify against fatigue failure, the strain profile for a shot-peened, surface-toughened material is determined and discussed in terms of a simple elastic–plastic stress/strain model. Finally the lattice strains in a WC/Co coated steel composite material are discussed.

A survey of sensitization in 5xxx series aluminum alloys

The 5xxx series (Al-Mg-based) aluminum alloys suffer from intergranular corrosion and intergranular stress corrosion cracking when the alloy has become “sensitized.” Sensitization refers to insidious precipitation of β phase (Mg2Al3), which is problematic when present at grain boundaries. The β phase is electrochemically active and may preferentially dissolve. This paper reviews the relevant works that have documented the degree of sensitization for various 5xxx series alloys, providing a holistic overview of the issue, along with attention to the bulk composition, heat treatment, and microstructure.

Transverse mechanical properties of BSCCO/Ag multifilamentary tapes

In a motor or magnet coil, the conductor experiences stresses both longitudinally and transversely to its length. Longitudinal tension is the most familiar stress configuration because of the well-known Lorentz force or hoop stress. Tension perpendicular to the flat face of a tape, termed c-axis tension, can result from Lorentz stresses in a potted pancake coil structure. While much is known about the response of multifilamentary BSCCO tapes to longitudinal tension, relatively little work has been done on the response to transverse stress. We report here measurements of the strength under c-axis tension at room temperature and 77 K of multifilamentary BSCCO-2223 tape, and a 3-ply reinforced tape.

Stress Gradient Induced Strain Localization in Metals: High Resolution Strain Cross Sectioning via Synchrotron X-Ray Diffraction

Strain localization in the presence of a stress gradient is a phenomenon common to many systems described by continuum mechanics. Variations of this complex phenomenon lead to interesting nonlinear effects in materials/engineering science as well as in other fields. Here, the synchrotron based energy dispersive x-ray diffraction (EDXRD) technique is used for high spatial resolution profiling of both compression and tension induced strain localization in important materials/engineering problems. Specifically, compression induced strain localization in shot peened materials and tension induced strain localization in the plastic zones adjoining the faces of a fatigue crack are profiled. The utility of the EDXRD synchrotron technique for nondestructively cross-sectioning strain variations on small length scales (down to 10-20 μm) is described. While the strain field profiling relies on the shift of the Bragg lines, the data show that plastic deformation regions can also consistently be seen in the broadening of the Bragg peaks through the full width at half maximum parameter. Quantitative correlations between the synchrotron based x-ray determined deformations and surface deformations, as measured by optical surface height profiling, are also made.

Fatigue crack growth “overload effect”: mechanistic insights from in-situ synchrotron measurements:

Synchrotron-based, high-energy X-ray diffraction measurements are used to study the local strain fields underlying the transient fatigue crack growth rate retardation produced by a single overload cycle known as the overload effect. Specifically, 4140 steel compact tension specimens fatigued for varying levels of crack growth after an overload cycle have been studied with in-situ diffraction under varying external loads. The load responses of the strain at the overload-position, versus at the crack tip, are focused upon in detail. The large compressive residual strain at the overload-point is observed to remain essentially unchanged even after the overload-point is left in the wake of the propagating crack tip. The differential strain-load response at the crack-tip/overload position before and immediately after the overload is seen to be unchanged. Once the overload point is behind the crack tip, a highly nonlinear behavior is observed in which the load response of the strain field transfers from the overload -point to the crack tip when the load exceeds a critical value. The results are discussed in terms of plasticity-induced crack face contact at the overload point as an important local mechanism contributing to the “overload effect” in this specific system.

Phenomenology of the effective stress intensity related to fatigue crack growth thresholds

Abstract The fatigue crack growth threshold conditions for effective stress intensity amplitude are examined using simple phenomenological models for crack face interference and internal stresses. We show that behaviors correlating with all pure fatigue classifications can be generated from a single ‘ideal fatigue’ behavior by accounting for internal stress and crack face interference. The possible threshold or near-threshold manifestations of an intrinsic KMAX threshold, independent of effective ΔK effects, are discussed.