Magnetomechanical damping in ferromagnetic materials has its source in stress-driven irreversible movement of magnetic domain boundaries. The maximum damping is proportional to λsE/σi, where λs is the saturation magnetostriction, E is Young's modulus, and σi is the average internal stress opposing domain boundary motion. In order to obtain the stress dependence of the damping, we include a distribution of internal stresses over a range ±Δσi about the average value. Inclusion of this distribution yields a theory in good quantitative agreement with experimental results for samples of 4% Si–Fe and cast iron.

The internal stress distribution theory of magnetomechanical damping is modified and extended to include effects of applied magnetic fields and applied static stresses. The present internal stress distribution function yields Rayleigh behavior at low vibration stress and has a more realistic shape than that of the previous model. The theory, besides giving a qualitative description of the damping behavior as a function of vibration stress, agrees quite well with measurements of the dependence of the damping maximum on applied field and static stress.

This work investigates experimentally a recent model of magnetomechanical damping based on a distribution of internal stresses, verifies some predictions and extends the model. Damping results are reported for cast irons and iron alloyed with silicon or germanium. As predicted, the maximum magnetomechanical damping ψmax is inversely proportional to the strain amplitude which produces ψmax. To explain the relatively slow decrease of ψmax with superimposed static shear stress or external magnetic field, the model must be modified to account for stress components or demagnetizing fields. By considering stress components along magnetization directions on opposite sides of a domain wall, we introduce a technique for describing the effect of single-crystal and grain orientations. This description qualitatively explains why bars with random grain orientations have larger damping in bending than in torsion. Although models relating damping to magnetic properties are in-exact, damping correlates experimentally with permeability and magnetic energy loss. These correlations are qualitatively explained by arguing that the 90° walls governing damping are less mobile than the 180° walls governing magnetic properties.

The internal-stress-distribution model of magnetomechanical hysteresis is extended to a calculation of the AE effect, the irreversible low-field permeability, and temperature~dependent magnetomechanical damping. The calculation of the strain dependence of the AE effect agrees well with experimental results for Fe-7.65% Ge. The model for permeability yields results for the product lI.O'i(lI.=magnetostriction, O',=average internal stress) in reasonable agreement with values from the Becker-Kersten model for reversible initial permeability /J.o and coercive force H •. The temperature-dependent damping observed in Ni by Roberts and Barrand is ascribed to an increase in domain-wall thickness as anistropy decreases. The model gives reasonable agreement with the measured temperature dependence.[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

A new formation-evaluation LWD tool that employs three ultrasonic transducers to determine tool standoff and borehole diameter is presented. The ultrasonic data are used to improve density- and neutron-porosity measurements obtained from the same tool. The paper describes the ultrasonic transducer hardware and operating characteristics, as well as a unique technique for setting the ultrasonic threshold. Data processing procedures and caliper equations are also presented. Downhole phenomena that adversely affect the measurements are discussed, along with solutions used to handle them. Log examples illustrate the effectiveness of the ultrasonic measurements and include evidence of sticking, lateral bouncing, and whirling of the tool string while drilling. Introduction Measurements of borehole caliper and tool standoff are useful in drilling, cementing, and formation evaluation. Caliper information helps drillers detect borehole instabilities and estimate the amount of cement required to fill between the casing and formation. Also, information from a three-transducer tool may be used to inform the driller when the drillpipe is sticking or undergoing unusual motions in the borehole. Furthermore, caliper and standoff data are needed to correct density, neutron, and other formation measurements. Despite the success of mechanical caliper devices in wireline logging, traditional designs will not survive the harsh conditions of drilling. In an effort to circumvent this problem, indirect methods are sometimes used to infer the borehole diameter from other logging-while-drilling (LWD) measurements. Recently, a more direct approach employing two ultrasonic transducers oriented 180 degrees apart was designed. Likewise, the LWD tool presented here uses a direct approach. The DNSC tool is a single module that makes density, neutron, standoff, and caliper measurements. Since it has three standoff transducers, more accurate caliper measurements are made than with a tool containing fewer transducers, particularly when the tool is sliding in the borehole. (We use the term sliding to refer to drilling with a mud motor, tripping into a well, or tripping out of a well.) Although tool standoff information is used to improve the density and neutron computations, this paper concentrates on standoff and caliper measurements. The paper describes the tool configuration and operation, the response of the ultrasonic transducers, the standoff and caliper calculations, the accuracy of the measurements, and the data processing techniques employed. Ultrasonic data obtained while drilling during development and testing are presented to illustrate the ability to flag several conditions that are adverse to the drilling process, as well as to illustrate the accuracy of the measurements under these conditions.