Thermo-mechanical Aspects Research Articles

Thermomechanical aspects of dynamic crack initiation

Dynamic fracture is generally addressed as an isothermal phenomenon. When specific phenomena such as shear band formation and propagation are involved, the analyses include thermomechanical conversion of strain and/or fracture energy into heat (thermoplasticity). In this case, it has been shown that very significant temperature rises can develop which cause softening of the crack-tip material. In these works, thermoelastic temperature changes at the tip of the crack are implicitly neglected. In a recent work, we have questioned this issue and shown that for a stationary crack subjected to transient loading, adiabatic thermoelastic effects were noticeable, thus causing a large temperature drop in the elastic zone surrounding the crack-tip (Rittel, 1998). In the present work, we pursue this line of investigation by presenting additional experimental results about temperature changes ahead of a dynamically loaded crack in commercial polymethylmethacrylate. We investigate mode I and mode II loading configurations. We observe, as expected, that the temperature drops for mode I loading while it rises for the mode II case. In each case, the crack initiates during the phase where the temperature changes (drop or rise). While showing that thermoelastic aspects of fracture should certainly be taken into account, the present results indicate that thermomechanical aspects in general should not be overlooked when addressing dynamic crack initiation.

Theory of sintering: from discrete to continuum

Theoretical concepts of sintering were originally based upon ideas of the discrete nature of particulate media. However, the actual sintering kinetics of particulate bodies are determined not only by the properties of the particles themselves and the nature of their local interaction with each other, but also by macroscopic factors. Among them are externally applied forces, kinematic constraints (e.g. adhesion of the sample's end face and furnace surface), and inhomogeneity of properties in the volume under investigation (e.g. inhomogeneity of initial density distribution created during preliminary forming operations). Insufficient treatment of the questions enumerated above was one of the basic reasons hindering the use of sintering theory. A promising approach is connected with the use of continuum mechanics, which has been successfully applied to the analysis of compaction of porous bodies. This approach is based upon the theories of plastic and nonlinear-viscous deformation of porous bodies. Similar ideas have recently been embodied in a continuum theory of sintering. The main results of the application of this theory for the solution of certain technological problems of sintering are introduced including their thermo–mechanical aspects.

Simulation of solidification

The field of solidification modelling covers micro- and macrosegregation, microstructure and defect formation, and thermomechanical aspects which include the formation of hot tears. Simulations are becoming increasingly more sophisticated and accurate as a result of refined physical models, improved thermodynamic, thermophysical and mechanical data, development of powerful numerical techniques and the availability of affordable and powerful computers.

The Yucca Mountain Project Drift Scale Test

The Yucca Mountain Project is conducting a Single Heater Test (SHT) in the Exploratory Studies Facility at Yucca Mountain, NV to evaluate coupled thermal-mechanical-hydrologic-chemical processes in situ. This paper describes the thermomechanical aspects of the SHT and presents preliminary results from the test. The 5-m long heater deployed in the SHT was energized on 26 August 1996 with the heater power set at 4 kW. Thermomechanical instrumentation was installed within and on the rock mass surrounding the SHT. The thermomechanical instrumentation includes temperature measurements using thermocouples, RTDs, and thermistors; displacement measurements using multiple-point borehole extensometers (MPBXs), tape extensometers, and surface-mounted wire extensometers; load measurements from load cells on rock bolts installed both within thermally perturbed and ambient regions; and rock mass modulus measurements using the NX borehole jack. Pretest analyses of the thermomechanical response were conducted using the thermohydrologic code TOUGH2 and the thermomechanical structural code JAC3D. The test design, preliminary data, and comparisons between pretest predictions and early data from selected thermal and mechanical sensors are presented.

Erosion and rift dynamics: new thermomechanical aspects of post-rift evolution of extensional basins

In this study, we investigate thermo-mechanical consequences of erosion of rift shoulders. Conventional models imply post-rift cooling that results in subsidence and strengthening of the lithosphere. Existing models neglect geodynamic consequences of surface processes (erosion and sedimentation). According to sedimentologic and geomorphologic data, surface processes modify the topography and thickness of sedimentary infill at rates comparable with the rates of the tectonic uplift/subsidence (few 0.1 mm/y). Consequently, a coupling between the surface and tectonic processes can be expected. An increase of the sedimentary load leads to localised inelastic weakening of the lithosphere. At the same time, erosional unloading of rift shoulders leads to local strengthening and flexural rebound. Subsidence of the rift “neck” (strongest layer of the thinned lithosphere) and uplift of the rift shoulders create pressure gradients sufficient to drive ductile flow in the low-viscosity lower crust. This flow, directed outward from the centre of the basin might facilitate uplift of the rift shoulders. It may even drive some post-rift “extension”. In the limiting case of slow erosion and sedimentation rates, gravitational stresses can reverse the flow, resulting in a retardation of basin subsidence rate, homogenisation of the crustal thickness, accelerated collapse of the shoulders and in some post-rift “compression”. These effects significantly change predictions of basin evolution inferred from the conventional back-stripping models.

Preliminary thermomechanical results of a heater test in welded tuff

The Yucca Mountain Project is conducting a Single Heater Test (SHT) in the Exploratory Studies Facility at Yucca Mountain, NV to evaluate coupled thermal-mechanical-hydrologic-chemical processes in situ . This paper describes the thermomechanical aspects of the SHT and presents preliminary results from the test. The 5-m long heater deployed in the SHT was energized on 26 August 1996 with the heater power set at 4 kW. Thermomechanical instrumentation was installed within and on the rock mass surrounding the SHT. The thermomechanical instrumentation includes temperature measurements using thermocouples, RTDs, and thermistors; displacement measurements using multiple-point borehole extensometers (MPBXs), tape extensometers, and surface-mounted wire extensometers; load measurements from load cells on rock bolts installed both within thermally perturbed and ambient regions; and rock mass modulus measurements using the NX borehole jack. Pretest analyses of the thermomechanical response were conducted using the thermohydrologic code TOUGH2 and the thermomechanical structural code JAC3D. The test design, preliminary data, and comparisons between pretest predictions and early data from selected thermal and mechanical sensors are presented.

Coupled HTM modelling of cold water injection in fractured hydrocarbon reservoirs

The Yucca Mountain Project is conducting a Single Heater Test (SHT) in the Exploratory Studies Facility at Yucca Mountain, NV to evaluate coupled thermal-mechanical-hydrologic-chemical processes in situ . This paper describes the thermomechanical aspects of the SHT and presents preliminary results from the test. The 5-m long heater deployed in the SHT was energized on 26 August 1996 with the heater power set at 4 kW. Thermomechanical instrumentation was installed within and on the rock mass surrounding the SHT. The thermomechanical instrumentation includes temperature measurements using thermocouples, RTDs, and thermistors; displacement measurements using multiple-point borehole extensometers (MPBXs), tape extensometers, and surface-mounted wire extensometers; load measurements from load cells on rock bolts installed both within thermally perturbed and ambient regions; and rock mass modulus measurements using the NX borehole jack. Pretest analyses of the thermomechanical response were conducted using the thermohydrologic code TOUGH2 and the thermomechanical structural code JAC3D. The test design, preliminary data, and comparisons between pretest predictions and early data from selected thermal and mechanical sensors are presented.

Experimental and numerical investigations of thermo-mechanically stressed micro-components

Reliability and functionality of microsystems, i.e. of small scale integrated electronic, mechanical and optical components, largely depend on their mechanical and thermal constitution. Thermo-mechanical aspects of component and system reliability become more and more important with growing miniaturization as the local physical parameters and field quantities show an increase in sensitivity due to inhomogeneities in local stresses, strains and temperature fields. Since there is usually a lack of information about the local material parameters, a pure field simulation cannot, as a rule, solve the problem. The state of the art of microsystem design more and more requires direct “coupling” between simulation tools (including e.g. FE modelling) and advanced physical experiments. The authors have combined various laser technique, scanning microscopy, and X-ray diffraction with numerical field simulation. The investigations have been directed towards current problems of mechanical and thermal reliability in electronic packaging, crack and fracture behaviour within the interconnected regions and life time estimation. Special problems of mechanical behaviour in solder joints are discussed taking into account local plasticity as well as creep effects.

Thermomechanical Modeling of Direct Chip Interconnection Assembly

Efforts to model thermomechanical aspects of the Direct Chip Interconnection (DCI) assembly process are described. DCI is a method to simultaneously attach and electrically interconnect bare chips to a substrate using Anisotropic Conductive Adhesive Films (ACAF). Emphasis has been placed on describing the numerical procedure used in the analysis. The major components of the analysis include a calibration procedure to “numerically measure” anisotropic properties of the film, a curing model to capture “frozen-in” stresses, a global analysis that considers the overall assembly station but does not resolve details of the interconnection, and a local model, coupled to the global model, that resolves details about the interconnection. Typical results are shown to demonstrate the capabilities of the model.

The origin of sedimentary basins: State of the art and first results of the task force

The origin of sedimentary basins is a key element of the geological evolution of the continental lithosphere. During the last decade substantial progress has been made in understanding the thermo-mechanical aspects of sedimentary basin formation and the isostatic response of the lithosphere to surface loads such as basins. Most of this progress has been made not so much in terms of the development of new modelling methodologies and insights into the rheological makeup of the lithosphere but rather in the processing of substantial, new, high-quality data sets from previously unexplored areas of the globe. Virtually all modelling carried out so far has been in terms of lithospheric displacements, thus refraining from a full examination of dynamic controls exerted by lithospheric stresses. This is because stresses are very sensitive to adopted lithospheric rheologies and these rheologies have been, by convention, unrealistically simple. This is true of models for both extensional and compressional sedimentary basins. For example, most models for extensional basin formation (Roberts et al., 1991) are keyed to lithospheric strain due to an unknown and unspecified stress field rather than to the strain response of the lithosphere to a known and/or realistic stress state. Moreover, changes in plate tectonic regimes and associated stress fields have been shown to be quite important in controlling the subsidence record and stratigraphic architecture of extensional basins. Similarly, models of basins developed in compressional environments (e.g., McClay, 1991) have been conventionally related to flexure profiles (displacement patterns), again not invoking the dynamic control of the compressional stresses intrinsic to this particular tectonic setting. Another reason that the relationship between lithospheric stresses and displacements in tectonic modelling has not received full attention is because little has been known about the actual stress state in the lithosphere. This situation has recently changed drastically as the result of the successful World Stress Map Project carried out in the framework of the International Lithosphere Program @back, 1992). Further, the application of structural techniques to establish the temporal evolution of paleo-stress fields has begun in a number of sedimentary basins (Letouzey, 1990). Not only has the dynamic element of lithospheric deformation been inadequately addressed but present quantitative models of the origin of basins are incapable of solving problems related to subsequent structural developments that may be intrinsically coupled with the basin formation. For example, late-stage compression during the post-rift evolution of extensional basins can largely explain current discrepancies between estimates of crustal thinning derived from structural analyses and subsidence data of rifted basins (Kooi and Cloetingh, 1989). Further progress in understanding the role of extensional faults in offshore areas like the North Sea requires detailed structural studies and modelling of exposed successions such as those found in intramontane transpressional basins. Post-rift compression of extensional or transtensional sedimentary basins leads to complex near-surface deformation patterns. In such cases of structural inversion, inher-

Some thermomechanical aspects of the subduction of continental lithosphere

On the basis of thermal and mechanical modeling, it is concluded that the subduction of continental lithosphere can lead to its breakup and the formation of a new plate contact within the middle or lower crust. As a result, (part of) the subducting continental crust is transferred to the upper plate. Breakup is caused by the resistive forces acting upon subducting continental crust, due to the buoyancy of crustal material and to friction at the plate contact, as well as the decrease in strength of the subducting crust once it has been subducted to a depth of a few tens of kilometers. The crustal thickness and the thermal and compositional structure of the continental crust just before the onset of subduction have a large influence on the depth to which continental crust can be subducted (prior to its breakup). The depth to which continental crust can be subducted coherently decreases as the surface heat flow or the crustal thickness of the subducting continental plate increases. In many cases, breakup is found to occur at a time when the upper surface of the continental plate has been subducted to a depth of 25–50 km. The subduction of a cold continental shield or of continental lithosphere with a relatively small crustal thickness, on the other hand, may lead to the subduction of both upper and lower crust to mantle depths.

A note on the use of the thermal response to deformation as a damage assessment tool

Considering the thermomechanical aspect of deformation leads also to the prospect of monitoring thermal emission as an indirect measure of dislocation activity. In this respect it may be considered to be analogous to acoustic emission and so could be viewed as a possible tool for assessing the damage state of components. With this in mind, a damage assessment was carried out on three identically sized and shaped specimens cut from fractured alloy 800H pipe that had been held under a tensile creep stress of 42 MPa at 900°C, failing after 122 h

Thermal/mechanical analyses of G-tunnel field experiments at Rainier Mesa, Nevada : Proc NEA Workshop on Excavation Response in Geological Repositories for Radioactive Waste, Winnipeg, 26–28 April 1988 P187–206. Publ France: OECD, 1989

Analysis methods (models) are currently being developed to support thermal, mechanical, and thermomechanical aspects of repository design and performance assessment of the candidate Yucca Mountain high-level nuclear waste site. Credibility of these models, and therefore of design and performance analyses, will in part be determined through comparison of calculated and measured response (validation) for large-scale field experiments. This paper discusses the models being developed, the rationale behind the model development, and analyses of experiments performed at G-Tunnel and planned as part of site characterization at Yucca Mountain. 25 refs., 21 figs.

Thermomechanical Aspects of the Liquid Metal Cooled Limiter

Analysis has been performed to evaluate the possibility of using liquid lithium as a coolant for the limiter. A global analysis was carried out to determine limiter's shape and configuration, and t...

Thermal-Hydraulic Study of the ESPRESSO Blanket for a Tandem Mirror Reactor

This paper deals primarily with the thermal-hydraulic design and some critical thermomechanical aspects of the proposed ESPRESSO blanket for the tandem mirror fusion reactor. This conceptual design was based on the same physics used in the MARS study. A computer code was developed to perform self-consistent calculations of the geometry, first-wall stresses, and canister thermal-hydraulic parameters. The neutronic data used as input were obtained from three-dimensional Monte Carlo computations. The design code also includes a simple steam power plant model to calculate the steam cycle thermal efficiency and the electrical power produced, as well as a cost subroutine for estimating the central cell contribution to the cost of electricity. Two different breeder/neutron multiplier configurations were finally chosen for detailed study based on their attractive overall potential:natural lithium oxide with no multiplier (case I) and 30% enriched gamma-lithium aluminate with a beryllium multiplier (case IV).

Thermomechanical aspects of mixed friction in concentrated contacts

Important aspects of partial elastohydrodynamic lubrication are discussed. For heavily loaded counterformed machine parts such as cams and followers, gear teeth, and ball- and roller-bearing elements working in the mixed friction regime, thermomechanical effects on the micro scale have been investigated. The temperature distribution resulting from friction has been determined, assuming that an axially symmetrical source of heat occurs on the peak of the contacting asperity. The thermal deformations and stresses in interacting asperities are obtained with the finite element method. A statistical analysis of the thermomechanical quantities has been carried out. Conclusions about the temperatures and the thermal stresses in contacting asperities have been formulated. A thermoelastic coupling of the interacting rough surfaces has been proposed as a mechanism of the scuffing mode of failure in concentrated contacts.

Super conducting magnet technologies for fusion reactor.

Fundamentals and application of superconducting magnet of a fusion reactor are stated. Regarding to a design of the magnet, thermomechanical aspect is referred to in terms of quench phenomena. A relation between stored magnetic energy and mass required to support electromagnetic force is discussed for the superconducting magnets.