Bernie Riemer

Empirical model of effects of pressure and temperature on electrical contact resistance of metals

Abstract An important input property in the development of process models for resistance spot welding is electrical contact resistance. A model for the pressure and temperature dependence of electrical contact resistance was developed from established concepts of contact resistance. The key to developing the desired relationship is determining surface roughness characteristics, which is experimentally problematic. To overcome this difficulty the electrical resistance of contacting interfaces was measured as a function of the pressure applied across the interfaces. Using known information about the temperature dependence of bulk resistivity and mechanical properties, a curve fitting procedure was used to establish the desired relationship of contact resistance to pressure and temperature. This empirical model agrees well with experimental measurements in the regime of low applied pressure. At high pressures, predictions underestimate contact resistance, and this was attributed to strain hardening of asperities at the contacting interface. The model also predicts that the competing effects of bulk resistance and contact resistance will produce a peak in the variation of contact resistance with temperature. The model provides a suitable means for incorporating the pressure and temperature dependence of contact resistance into process models of the resistance spot welding process.

Target studies with BNL E951 at the AGS

We report initial results of exposing low-Z solid and high-Z liquid targets to 150-ns, 4/spl times/10/sup 12/ proton pulses with spot sizes on the order of 1 to 2 mm. The energy deposition density approached 100 J/g. Diagnostics included fiberoptic strain sensors on the solid target and high-speed photography of the liquid targets. This work is part of the R&D program of the Neutrino Factory and Muon Collider Collaboration.

SNS target tests at the LANSCE-WNR in 2001 – Part I☆

Testing of mercury filled targets in an 800 MeV proton beam was conducted at the Los Alamos Neutron Science Center-Weapons Neutron Research (LANSCE-WNR) facility on two occasions in 2001. The objective for the first test campaign was to investigate if target vessel cavitation damage could occur under transient pressure conditions much like the Spallation Neutron Source (SNS) target. Such an investigation was inspired after mechanical tests conducted by a Japan Atomic Energy Research Institute (JAERI/KEK) team revealed cavitation pitting in a mercury container having comparable pressure wave intensity. The first WNR test confirmed cavitation damage with 200 proton pulses on each of two test targets. As a result, concerns arose that the lifetime of the SNS target could be seriously limited. A second test campaign was then prepared and conducted to investigate if alternate target materials or geometries could reduce or eliminate the damage. Tested materials included Stellite, Nitronic-60 as well as 316LN stainless steel (the baseline SNS target material) that was cold worked and surface hardened. Theories that the original test target geometry caused the damage were checked with tests using thick beam windows and a target with a non-axisymmetric shape. This paper describes the test program and covers target preparation, irradiation conditions, post-test decontamination and an overview of the examinations performed. J.D. Hunn covers the detailed description of the metallurgical examinations in another paper here at IWSMT-5.

SNS target tests at the LANSCE-WNR in 2001 – Part II

Abstract It has been observed that stopping of an 800 MeV proton pulse in liquid mercury, such as in the United States Spallation Neutron Source (SNS), leads to cavitation that can affect the mercury vessel. This paper discusses pitting that was observed on mercury container walls after 100–200 proton pulses obtained at the Los Alamos Neutron Science Center Weapons Neutron Research facility (LANSCE-WNR). It was found that the degree of cavitation-induced pitting was dependent on the geometry and composition of the container. As expected, very hard surfaces were particularly effective for resisting deformation from cavity collapse.

An R&D program for targetry and capture at a neutrino factory and muon collider source

The need for intense muon beams for muon colliders and for neutrino factories based on muon storage rings leads to a concept of 1-4 MW proton beams incident on a moving target that is inside a 20-T solenoid magnet, with a mercury jet as a preferred example. Novel technical issues for such a system include disruption of the mercury jet by the proton beam and distortion of the jet on entering the solenoid, as well as more conventional issues of materials lifetime and handling of activated materials in an intense radiation environment. As part of the R&D program of the Neutrino Factory and Muon Collider Collaboration, an R&D eort related to

Detecting cavitation in mercury exposed to a high-energy pulsed proton beam

The Oak Ridge National Laboratory Spallation Neutron Source employs a high-energy pulsed proton beam incident on a mercury target to generate short bursts of neutrons. Absorption of the proton beam produces rapid heating of the mercury, resulting in the formation of acoustic shock waves and the nucleation of cavitation bubbles. The subsequent collapse of these cavitation bubbles promote erosion of the steel target walls. Preliminary measurements using two passive cavitation detectors (megahertz-frequency focused and unfocused piezoelectric transducers) installed in a mercury test target to monitor cavitation generated by proton beams with charges ranging from 0.041 to 4.1 muC will be reported on. Cavitation was initially detected for a beam charge of 0.082 muC by the presence of an acoustic emission approximately 250 mus after arrival of the incident proton beam. This emission was consistent with an inertial cavitation collapse of a bubble with an estimated maximum bubble radius of 0.19 mm, based on collapse time. The peak pressure in the mercury for the initiation of cavitation was predicted to be 0.6 MPa. For a beam charge of 0.41 muC and higher, the lifetimes of the bubbles exceeded the reverberation time of the chamber ( approximately 300 mus), and distinct windows of cavitation activity were detected, a phenomenon that likely resulted from the interaction of the reverberation in the chamber and the cavitation bubbles.

Influence of microstructure on the properties of resistance spot welds

An integrated model approach was proposed for relating resistance welding parameters to weldment properties. A key element of the approach is microstructure modeling. It was demonstrated that existing process models and microstructure models can be used to determine the spatial distribution of microstructures and properties in resistance spot welds of a plain carbon steel. It was also shown by finite element analysis that the existence of microstructure gradients in the welds is expected to reduce their ability to support shear loads by about 50%.

Optimizing moderator dimensions for neutron scattering at the spallation neutron source

In this work, we investigate the effect of neutron moderator dimensions on the performance of neutron scattering instruments at the Spallation Neutron Source (SNS). In a recent study of the planned second target station at the SNS facility, we have found that the dimensions of a moderator play a significant role in determining its surface brightness. A smaller moderator may be significantly brighter over a smaller viewing area. One of the immediate implications of this finding is that for modern neutron scattering instrument designs, moderator dimensions and brightness have to be incorporated as an integrated optimization parameter. Here, we establish a strategy of matching neutron scattering instruments with moderators using analytical and Monte Carlo techniques. In order to simplify our treatment, we group the instruments into two broad categories: those with natural collimation and those that use neutron guide systems. For instruments using natural collimation, the optimal moderator selection depends on the size of the moderator, the sample, and the moderator brightness. The desired beam divergence only plays a role in determining the distance between sample and moderator. For instruments using neutron optical systems, the smallest moderator available that is larger than the entrance dimension of the closest optical element will perform the best (assuming, as is the case here that smaller moderators are brighter).

The R&D program for targetry at a neutrino factory

The need for intense muon beams for muon colliders and for neutrino factories based on muon storage rings leads to a concept of 1-4 MW proton beams incident a moving target that is inside a 20-T solenoid magnet, with a mercury jet as a preferred example. Novel technical issues for such a system include disruption of the mercury jet by the proton beam and distortion of the jet on entering the solenoid, as well as more conventional issues of materials lifetime and handling of activated materials in an intense radiation environment. As part of the R&D program of the Neutrino Factory and Muon Collider Collaboration, R&D effort related to targetry is being performed within the context of BNL E951, first results of which are discussed here and in other contributions to this conference.

Gas Bubble Size Measurements in Liquid Mercury Using an Acoustic Spectrometer

A properly dispersed population of small bubbles can mitigate cavitation damage to aspallation neutron source target. In order to measure such a bubble population, anacoustic device was developed and implemented in a mercury loop at ORNL. The instru-ment generated pulses of various frequencies and measured their acoustic propagation inthe bubbly medium. It then deduced sound speed and attenuation at the various frequen-cies and used an inverse problem solver to provide near real-time measurements of bub-ble size distribution and void fraction. The measurements were then favorably comparedwith an optical method. [DOI: 10.1115/1.4026440]