Micromechanical investigation of the influence of defects in high cycle fatigue

This study aims to analyse the influence of geometrical defects (notches and holes) on the high cycle fatigue behaviour of an electrolytic copper based on finite element simulations of 2D polycrystalline aggregates. In order to investigate the role of each source of anisotropy on the mechanical response at the grain scale, three different material constitutive models are assigned successively to the grains: isotropic elasticity, cubic elasticity and crystal plasticity in addition to the cubic elasticity. The significant influence of the elastic anisotropy on the mechanical response of the grains is highlighted. When considering smooth microstructures, the crystal plasticity have has a slight effect in comparison with the cubic elasticity influence. However, in the case of notched microstructures, it has been shown that the influence of the plasticity is no more negligible. Finally, the predictions of three fatigue criteria are analysed. Their ability to predict the defect size effect on the fatigue strength is evaluated thanks to a comparison with experimental data from the literature

Guardrails for Use on Historic Bridges: Volume 2—Bridge Deck Overhang Design

Bridges that are designated historic present a special challenge to bridge engineers whenever rehabilitation work or improvements are made to the bridges. Federal and state laws protect historically significant bridges, and railings on these bridges can be subject to protection because of the role they play in aesthetics. Unfortunately, original railings on historic bridges do not typically meet current crash-test requirements and typically do not meet current standards for railing height and size of permitted openings. The primary objective of this study is to develop strategies that can be used to address existing railings on historic bridges and to develop solutions that meet current design requirements. In addition to the modification, selection, and design of the bridge railing, the bridge deck is also impacted by changes made to the railing. Due to increased force levels recently required by AASHTO, deck overhangs require significantly more reinforcement than for past practice. These increases are being realized on all bridge decks and may pose particular challenges for the attachment of railing to historic bridges. Therefore, a secondary objective of this project is to investigate the design of the deck overhang and determine whether reduced amounts of reinforcement are possible. For Volume 1 (Replacement Strategies), three phases of research were conducted. First, an overview of current practice for addressing historic bridge railings was performed. Second, an investigation was conducted to document historic bridge railings in Indiana. Finally, rehabilitation solutions were developed to address the specific bridge railings found in Indiana. Based on this research, three retrofit strategies were developed which include an inboard railing, curb railing, and a simulated historic railing. These rehabilitation solutions can be used to address historic bridge railings not only in Indiana, but across the country. For Volume 2 (Bridge Deck Overhang Design), experimental testing of half-scale and full-scale overhang specimens was conducted, and the results were analyzed. Failures of in-service bridge railings were also evaluated. Based on this research, recommendations are provided for the more efficient and economic design of bridge deck overhangs. These recommendations are applicable not only for historic bridges, but for all concrete bridge decks

Is there an Aoki phase in quenched QCD?

We argue that quenched QCD has non-trivial phase structure for negative quark mass, including the possibility of a parity-flavor breaking Aoki phase. This has implications for simulations with domain-wall or overlap fermions.Comment: Parallel talk presented at Lattice2004(spectrum), Fermilab, June 21-26, 200

Neutral particle Mass Spectrometry with Nanomechanical Systems

Current approaches to Mass Spectrometry (MS) require ionization of the analytes of interest. For high-mass species, the resulting charge state distribution can be complex and difficult to interpret correctly. In this article, using a setup comprising both conventional time-of-flight MS (TOF-MS) and Nano-Electro-Mechanical-Systems-based MS (NEMS-MS) in situ, we show directly that NEMS-MS analysis is insensitive to charge state: the spectrum consists of a single peak whatever the species charge state, making it significantly clearer than existing MS analysis. In subsequent tests, all charged particles are electrostatically removed from the beam, and unlike TOF-MS, NEMS-MS can still measure masses. This demonstrates the possibility to measure mass spectra for neutral particles. Thus, it is possible to envisage MS-based studies of analytes that are incompatible with current ionization techniques and the way is now open for the development of cutting edge system architectures with unique analytical capability

Analysis of the mesoscopic high cycle fatigue strength of FCC metals with polycrystalline plasticity and extreme value probability methods

This study analyzes the influence of microstructure properties on the extreme value distributions of the fatigue indicator parameter (FIP) corresponding to the multiaxial HCF Dang Van criterion. The following loading cases are considered: uniaxial loading, proportional and non-proportional multiaxial loadings. The mesoscopic FIP determined from FE calculations on 2D polycrystalline synthetic aggregates using the ZeBuLoN code. The approach adopted in this work is to replace the RVE by random microstructure elements that can be considered as “statistical volume element” (SVE). A set of extreme values is constructed by determining the maximum value of the FIP for each SVE. The type of extreme value distribution is analyzed with a generalized extreme value function and is shown to follow a Gumbel type distribution. The shape factors of this distribution are compared for the different loading conditions. This comparison shows the limitations of the used criterion, especially in the case of multiaxial loadings. The effect of anisotropy on these distributions is finally investigated by comparing the results of two types of texture (isotropic and rolling). The introduction of a preferential texture reduces the shape factor and the criterion applied with the classic stress field becomes conservative, and also decreases the scatter parameter of the extreme value distributions of the FIP

Analysis of the multiaxial fatigue strength at the mesoscopic scale using 3D microstructure modeling and extreme value statistics

Fatigue life computing methods are generally based on putting into equation the mechanical quantities calculated at the micro or meso scale, the relevance of these selected quantities being validated by the capacity of the models to reproduce experimental results at the macroscopic scale. Although the scaling of the damage mechanisms involved in fatigue crack initiation processes are relatively well identified (grain scale, slip bands), their explicit consideration in fatigue criteria is still not well-developed. Furthermore, the existing methods do not consider the microstructure-sensitivity. The aim of this paper is to present the computational strategies developed to account for the microstructure-sensitivity in the calculation of fatigue strength. This work is based on three parts: (1) the development of 3D microstructure modeling tools (2) the analysis of the dispersion induced by the microstructure heterogeneities on the critical fatigue damage indicators and (3) the development of a statistical approach which provides a framework for analyzing calculation results in the HCF (High Cycle Fatigue) regime. In this background, a method of analysis based on the construction of statistical extreme value distributions from FEA calculation results was developed. The evolution of the scaling parameters of these distributions for different loading conditions informed us about the effect of non-proportional loading and microstructure. A design method based on these extreme value statistics is presented to obtain a new mesoscopic criterion sensitive to microstructure parameters. Finally, surface effects are discussed too

Analysis of the mesoscopic high cycle multiaxial fatigue strength of fcc metals with crystal plasticity and generalized extreme values probability

Multiaxial high cycle fatigue modeling of materials is an issue that concerns many industrial domains (automotive, aerospace, nuclear, etc) and in wich many progress still remains to be achieved. Several approaches exist in the litterature: invariants, energy, integral and critical plane approaches all of there having their advantages and their drawbacks. These different formulations are usually based on mechanical quantities at the micro or meso scales using localization schemes and strong assumptions to propose simple analytical forms. This study aims to revisit these formulations using a numerical approach based on crystal plasticity modelling coupled with explicit description of microstructure (morphology and texture). This work has three steps: First, 2D periodic digital microstructures based on a random grain sizes distribution are generated. Multiaxial cyclic load conditions corresponding to the fatigue strength at 107 cycles are applied to these microstructures. Then, the mesoscopic Fatigue Indicator Parameters (FIPs), formulated from the different criteria existing in the literature, are identified using the FE calculations of the mechanical fields. These mesoscopic FIP show the limits of the original criteria when it comes to applying them at the grain scale. Finally, a statistical method based on extreme value probability is used to redefine the parameters of these criteria. These new criteria contain the sensitivity of the microstructure variability

Lattice QCD with mixed actions

We discuss some of the implications of simulating QCD when the action used for the sea quarks is different from that used for the valence quarks. We present exploratory results for the hadron mass spectrum and pseudoscalar meson decay constants using improved staggered sea quarks and HYP-smeared overlap valence quarks. We propose a method for matching the valence quark mass to the sea quark mass and demonstrate it on UKQCD clover data in the simpler case where the sea and valence actions are the same.Comment: 15 pages, 10 figures some minor modification to text and figures. Accepted for publicatio