R. A. Winholtz

Use of position-dependent stress-free standards for diffraction stress measurements

Abstract Residual stress measurements using neutron diffraction are inherently triaxial in nature due to the low adsorption, and thus high penetration, of neutrons in samples of interest. This means that stress-free reference standards are required to convert measured changes in peak position to strain and stress tensors. Althogh a number of empirical and analytical approaches have been utilized to obtain accurate stress-free reference cell parameters, they all presume the presence of a single reference value for the material being measured. However, important cases can arise for which the local stress-free cell parameter varies from point to point. One such case, a circumferentially welded cylinder, is presented here. A method of point-to-point correction is employed, involving the sectioning of a companion piece into small cubes corresponding to the positions of stress measurement. The variations in peak position and peak breadth are presented. It is shown that accurate stress tensors can be obtained and that use of a constant value of stress-free reference cell parameter from the unaffected base metal leads to errors of up to 700 MPa in some stress tensor components. Effects on the principal stresses are also presented.

Visual Observation of Oscillating Heat Pipes Using Neutron Radiography

Qualitative observation of flow patterns in water and nanofluid oscillating heat pipes was conducted at various heat inputs and condenser temperatures. Images of the liquid flow within the copper tubing were first captured at 30 frames per second using neutron radiography. Neutron radiography allows direct observation of a fluid position because liquid water is hydrogen rich and opaque while water vapor (because it is much less dense) and the other materials in the oscillating heat pipes are transparent. Flow visualization was conducted on an 8-turn water oscillating heat pipe, an 8-turn nanofluid oscillating heat pipe, and a 12-turn nanofluid oscillating heat pipe. The water oscillating heat pipe was filled with high performance liquid chromatography grade water. The 12-turn nanofluid oscillating heat pipe was filled with 1% by volume (35.0 g-ml -1 ) diamond nanoparticles in high performance liquid chromatography water and the 8-turn oscillating heat pipe contained 0.016% by volume (0.5 mg ml -1 ) diamond nanoparticles high performance liquid chromatography water. The diamond nanoparticles were 5 to 50 nm in diameter. All oscillating heat pipes were charged at a filling ratio of 50%. Visual observation shows for all heat pipes that at low heat inputs, fluid oscillation is very random and intermittent. Increasing the heat input causes a steady flow pattern to appear. For all tested oscillating heat pipes, increased heat load or operating temperature resulted in an increased fluid velocity. Also, nucleation was never observed in the tested oscillating heat pipes.

Experimental Investigation of Miniature Three-Dimensional Flat-Plate Oscillating Heat Pipe

An experimental investigation on the effects of condenser temperatures, heating modes, and heat inputs on a miniature three-dimensional (3D) flat-plate oscillating heat pipe (FP-OHP) was conducted visually and thermally. The 3D FP-OHP was charged with acetone at a filling ratio of 0.80, had dimensions of 101.60×63.50×2.54 mm 3 , possessed 30 total turns, and had square channels on both sides of the device with a hydraulic diameter of 0.762 mm. Unlike traditional flat-plate designs, this new three-dimensional compact design allows for multiple heating arrangements and higher heat fluxes. Transient and steady-state temperature measurements were collected at various heat inputs, and the activation/start-up of the OHP was clearly observed for both bottom and side heating modes during reception of its excitation power for this miniature 3D FP-OHP. The neutron imaging technology was simultaneously employed to observe the internal working fluid flow for all tests directly through the copper wall. The activation was accompanied with a pronounced temperature field relaxation and the onset of chaotic thermal oscillations occurring with the same general oscillatory pattern at locations all around the 3D FP-OHP. Qualitative and quantitative analysis of these thermal oscillations, along with the presentation of the average temperature difference and thermal resistance, for all experimental conditions are provided. The novelty of the three-dimensional OHP design is its ability to still produce the oscillating motions of liquid plugs and vapor bubbles and, more importantly, its ability to remove higher heat fluxes.

Thermal and Visual Observation of Water and Acetone Oscillating Heat Pipes

A visual and thermal experimental investigation of four oscillating heat pipes (OHPs) was conducted to observe fluid flow of liquid plugs and vapor bubbles in the OHP and its effect on the temperature distribution and heat transfer performance in an OHP. These four OHPs consist of an open loop water OHP, an open loop acetone OHP, a closed loop water OHP, and a closed loop acetone OHP. These copper OHPs were constructed identically with all six turns in the same plane. They were constructed out of 1.65 mm inner diameter copper tubing and copper heat spreading plates in the evaporator and condenser regions. The heat pipes were charged at a filling ratio of about 50%. The results show that the acetone OHP at low power performs better than the water OHP, while at high power the water OHP exceeds the acetone OHP. The experimental results show that both the acetone and water closed loop OHPs had reduced movement in the connecting turn between the two sides. However, in the water closed loop OHP, this prevented circulation altogether. Comparing the water closed loop OHP to the water open loop OHP, their flow patterns were similar. Therefore, improving the fiow in this turn should increase the closed loop OHPs performance.

Residual stress and stress gradients in polycrystalline diamond compacts

Abstract Thermal residual macrostresses and their gradients were studied in a series of polycrystalline diamond compacts (PDC) using neutron diffraction. The specimens comprised WC–Co cemented carbides with high temperature/high pressure (HTHP) sintered polycrystalline diamond (PCD) layers. Residual stresses were investigated in two as-sintered variants and after several post-sinter thermal treatments and bonding processes. Measurements were made of (1) the average in-plane stress in the diamond layer for each sample and (2) the average in-plane stress gradient in both the WC–Co substrate and the diamond layer in a subset of the samples. Average in-plane stresses in the diamond layer ranged from −250 to −582 MPa. Sintering process parameters, thermal treatments, and bonding were all found to affect residual stress levels and stress gradient characteristics. Measured average in-plane stress gradients are shown to differ substantially in some cases from linear elastic predictions.

Load sharing of the phases in 1080 steel during low-cycle fatigue

By means of X-ray diffraction, the stress response of the individual phases in a 1080 steel were measured. Specimens with pearlitic, spheroidal, and tempered martensitic microstructures were subjected to low-cycle fatigue and the stress-strain hysteresis loops were separated into components for the carbide and matrix phases. Calculations of the microstresses formed by differential plastic deformation of the matrix and inclusions accurately model the spheroidite. Measured microstresses in the pearlite are smaller than the predicted values, probably due to yielding of the cementite and limitations on modeling the morphology. Work-hardening rates associated with the microstresses also qualitatively agree with the measurements. The tempered martensite cyclically softens with fatigue loading. The increased plastic strain range in the tempered martensite with cyclic softening is accompanied by an increase in the microstresses. These microstresses are significantly larger than predicted.

Heat Transport Capability and Fluid Flow Neutron Radiography of Three-Dimensional Oscillating Heat Pipes

An experimental investigation into the parameters affecting heat transport in two three-dimensional oscillating heat pipes (OHPs) was implemented. A three-dimensional OHP is one in which the center axis of the circular channels containing the internal working fluid do not lie in the same plane. This novel design allows for more turns in a more compact size. The OHPs in the current investigation is made of copper tubings (3.175 mm outside diameter, 1.65 mm inside diameter) wrapped in a three-dimensional fashion around two copper spreaders that act as the evaporator and condenser. The two OHPs have 10 and 20 turns in both the evaporator and condenser. The 20-turn OHP was filled to 50% of the total volume with a high performance liquid chromatography grade water. Transient and steady state temperature data were recorded at different locations for various parameters. Parameters such as heat input, operating temperature, and filling ratio were varied to determine its effect on the overall heat transport. Neutron radiography was simultaneously implemented to create images of the internal working fluid flow at a rate of 30 frames per second. Results show the average temperature drop from the evaporator to condenser decreases at higher heat inputs due to an increase in temperature throughout the condenser region due to greater oscillations. These large oscillations were visually observed using neutron radiography. As the operating temperature is increased, the thermal resistance is reduced. A decrease in filling ratio tends to create more steady fluid motion; however, the heat transfer performance is reduced.

I Know This Is Supposed to Be More Like the Real World, but . . .: Student Perceptions of a PBL Implementation in an Undergraduate Materials Science Course.

This qualitative case study examines the initial implementation of a problem-based version of an undergraduate course in materials science for the purpose of identifying areas of improvement to the curriculum prior to a planned second implementation. The course was designed around problems that students work in small teams to solve under the guidance of facilitators, with early sequence problems designed to foster the problem-solving skills required to succeed in the course. This report describes students’ impressions of and experiences in the course as they worked to solve the final problem at the end of the semester and compares those impressions, where applicable, to impressions gathered after they had completed the first problem near the beginning of the semester. Using grounded theory techniques to analyze the data, six central themes emerged from the implementation: course structure, facilitation roles, student roles, group processes, coconstruction, and resources. Implications for practice and potential instructional design solutions that may aid in future implementations are discussed.

Relation of elastic strain distributions determined by diffraction to corresponding stress distributions

The problem of converting elastic strain distributions determined by diffraction measurements to stress distributions is discussed. The relation of mean strains measured by powder diffraction to the corresponding mean stresses is well known and regularly employed in residual stress measurements. However, the relation between strain variance and stress variance is not so straightforward. The elastic strain distribution depends on both the variation of strain from point to point in a given direction in a volume, and in all directions at each point in the same volume. In this paper, relations between the strain variance and corresponding stress variance are developed in both two and three dimensions for the case of microstress variations in particulate systems. The two components cannot be separated from a diffraction measurement of the strain distribution but the relations developed provide upper and lower limits on the variance in the stress distribution and are useful for relating diffraction results to models of the material. The results are applied to elastic strain distributions obtained in composite systems from measurements of diffraction peak breadths and modeled using two-dimensional finite element methods.

Changes in the macrostresses and microstresses in steel with fatigue

Abstract The residual stresses in both the ferrite and cementite phases of fatigued 1080 steel specimens with pearlite, spheroidite and tempered martensite microstructures were measured using X-ray diffraction giving both the macrostresses and microstresses. Specimens with no initial stresses showed little changes with fatigue. Specimens with initial macrostresses and microstresses showed fading of the stresses, the fading being slowest for the strongest microstructure. Hydrostatic microstresses are present after heat treatment owing to the differential thermal properties of the cementite and ferrite.