Effective Thermal Stress Research Articles

Thermal effective stress in shales

Calculating velocities in shales in thermal production settings is important to refine time-lapse reservoir characterization from seismic. The effective stress concept is attractive to potentially reduce the amount of expensive core calibration data required. We propose a formulation for thermal effective stress in shales based on the idea of balancing undrained pore pressure increments from thermal expansion with an increase in the matrix stress to minimize pore deformation. This formulation is motivated by a desire to simplify forward modeling, reduce the number of dimensions that must be experimentally calibrated through core testing, and to leverage existing velocity-stress relations for thermal applications. The concept was tested on data from a well-known set of experiments consisting of two North Sea Kimmeridge shale core samples, which displayed a linear dependence of velocity on pressure and temperature. These data were found to be consistent with the proposed thermal effective stress model with a constant effective stress coefficient when considering elastic changes but do not prove that the concept is universally valid. Thermal effective stress coefficients were calculated for P- and S-wave velocities from the data and were found to lie from 0.66 to 1.22, demonstrating reasonable scaling for the proposed model.

Cyclic strain rate effects in fatigued face-centred and body-centred cubic metals

The present work deals mainly with the effect and the use of strain rate and temperature changes during cyclic deformation as a means to obtain valuable information on the thermally activated dislocation glide processes, based on the assessment of reversible changes of the thermal effective stress and of transient changes of the athermal stress. The importance of closed-loop testing in true plastic strain control with constant cyclic plastic strain rate throughout the cycle is explained and emphasized, especially with respect to the case of strain rate sensitive materials. Stress responses of face-centred cubic and body-centred cubic (bcc) metals to cyclic strain rate changes are presented to illustrate that the deformation modes of these two classes of materials differ characteristically at temperatures below that the so-called knee temperature of bcc metals. When such tests are performed in cyclic saturation, the temperature and strain rate dependence of bcc metals can be measured very accurately on one and the same specimen, permitting a thorough analysis of thermal activation.

The High Temperature Deformation Behavior of TiC0.98 Grown by the Floating Zone Technique

In order to investigate the high temperature deformation behaviors of TiC0.98, crystals were grown by the radio frequency (r.f.) floating zone technique and were deformed by a compression test at temperatures from 1283 to 2133 K and a strain rate from 5.87 × 10-4 to 6.01 × 10-3s-1. TiC0.98 exhibited a marked work-softening phenomena which is characteristic of covalently bonded materials such as Ge and Si, and the phenomena became less clear with an increasing temperature and with a decreasing strain rate. After the softening, work hardening was observed. The rate hardening decreased as the temperature increased. The stress-strain curve can be explained in terms of the increase in the athermal internal stress and the decrease in the thermal effective stress with the increase in dislocation density by the deformation. The upper yield stress was mainly determined by the frictional resistance to dislocation motion, and the athermal internal stress contribution was very small.

Characterization of 63Sn37Pb and 80Au2OSn solder sealed optical fiber feedthroughs subjected to repetitive thermal cycling

A highly accurate prediction of hermeticity lifetime is made for eutectic 63Sn37Pb and 80Au20Sn alloy solder sealed optical fiber-Kovar/sup TM/ nosetube feedthroughs subjected to repetitive thermal cycling. Thermal fatigue fracture of the Sn-Pb solder/Kovar/sup TM/ interface develops when cracks, initially generated from creep deformation of the solder, propagate gradually through the junction in the axial direction. A nonlinear axisymmetric finite element analysis of the 63Sn37Pb fiber feedthrough seal is performed using a thermo-elastic creep constitutive equation, and solder joint fatigue based on accumulated strain energy associated with solder creep imposed by temperature cycling is analyzed. Additionally, thermal effective stress and plastic strain is studied for alternative 80Au20Sn solder by the finite element method with results indicating significant increase in useful life as compared to 63Sn37Pb. SEM/EDX metallurgical analysis of the solder/Ni-Au plated Kovar/sup TM/ nosetube interface indicates that AuSn/sub 4/ intermetallic formed during soldering with 63Sn37Pb also contributes to joint weakening, whereas no brittle intermetallic is observed for 80Au20Sn. Hermetic carbon coated optical fibers metallized with Ni,P-Ni underplate and electrolytic Au overplating exhibit correspondingly similar metallurgy at the solder/fiber interface. Combined hermeticity testing and metallurgical analysis carried out on 63Sn37Pb and 80Au20Sn alloy solder sealed optical fiber feedthroughs after repetitive temperature cycling between -65 and +150/spl deg/C, and -40 and +125/spl deg/C validated the analytical approach.

A constitutive model for thermomechanical response of decomposing composites under high heating rates

The thermomechanical response model of chemically decomposing composites is established. Mass loss due to thermochemical decomposition is modeled by the Arrhenius equation. Phenomenological shrinkage models are proposed and the pore formation due to mass loss is examined from a micro-mechanical point of view. The effect of gas pore pressure caused by decomposition on the deformation of the composite is considered based on the thermal effective stress laws for linearly poroelastic composites. The coupling between the thermochemical decomposition and thermomechanical deformation is made clear throughout the formulation. The results of laboratory tests known as free thermal expansion tests and restrained thermal growth tests are theoretically predicted by the obtained model.

Thermal effective stress concentration problems in metal composites

Four examples of the fracture of metal composites are presented. After analysis, it is revealed that the cause of fracture can be attributed to high tensile stress concentration at free boundaries of the metal composite. The tensile stress concentration arises as a result of vastly different thermal expansion (or contraction) properties between the component materials of the composite. As the composite cools from high temperature (where residual stress is negligible) to room temperature, the component materials in the composite tend to contract differently so that tensile stress concentration is created. The fracture examples also demonstrate that high tensile stress concentration can be devastating to high hardness, brittle materials.

The anisotropic thermomechanical constitutive theory for a fluid-filled porous material with solid/fluid outer boundary

The anisotropic theromechanical constitutive theory for a fluid-filled porous material with a solid/fluid outer boundary is constructed along the same lines as in the author’s previous work. The notion that a representative volume element may be considered as a superposition of the porous solid material and the porous fluid material is employed. The constitutive theory with a uniform temperature increase is constructed by use of superposition. The thermal effective stress laws for various kinematical quantities are obtained.