Effect of heating rate on steam pressure induced in crack-like cavities in moisture saturated polymer matrix composites

When moisture saturated composites are rapidly heated, the steam pressure inside cavities can cause the composite to delaminate. We study the effect of heating rate on the steam pressure inside an isolated long thin “crack-like” cavity of thickness h assuming that the chemical potential of water is continuous across the cavity/polymer interface. For such a cavity in an infinite plate, we show there is sufficient moisture for the steam pressure to reach the saturated steam pressure, irrespective of the heating rate. However, for a plate of thickness L exposed to dry air, the cavity pressure reaches a maximum value, which depends only on the normalized plate thickness, α = L/h and normalized heating rate, $\beta =\dot{T}{h}^2/T_{0}D(T_0)$ where $\dot{T}$ is the heating rate, D(T₀) is the moisture diffusivity at the initial temperature T₀, before it decays to zero because of the dry air outside. For this case, the maximum steam pressure can be significantly less than the saturation pressure. The results in this work can also be used to study ‘popcorning’ observed in electronic packages.     

Gas-dynamic heating in cavities under the influence of pressure pulsations at the entrance

An investigation has been made of the gas-dynamic heating of gas in nearly closed cavities (tubes, channels, etc.) under the influence of given pressure pulsations (with and without a discrete component) at the entrance. The results are given of measurements of the wall temperature of the cavities and also the power of the gas-dynamic heating as a function of the relative cavity length 10< ln/d_n < 300, the relative level of the pressure pulsations ₀/p < 0.5 at the entrance, and the magnitude of the static pressure in the range p = 2–10 kg/cm². It was established that with increasing p and especially ₀/p the power of the gas-dynamic heating increases strongly.Translated fron Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 177–179, November–December, 1983.     

Strain measurements in tubes during rapid transient heating

A measuring apparatus using reluctance-type displacement transducers was successfully used for measuring radial thermal strains in 1-in.-diam × 8-in.-long thin-walled tubes of molybdenum, tantalum and 304 stainless steel. Wall-temperature rates of approximately 300° F/sec were accomplished by rapid heating with a plasma jet and strains at temperatures up to 2500° F were recorded. Excellent agreement between experimental results and a theoretical solution based on temperature profiles was found for temperatures to 2000° F.     

Modeling of diffusion in the presence of damage in polymer matrix composites

It is now well known that Fick’s Law is frequently inadequate for describing moisture diffusion in polymers and polymer composites. Non-Fickian or anomalous diffusion is likely to occur when a polymer composite laminate is subjected to external stresses that could give rise to internal damage in the form of matrix cracks. As a result, it is necessary to take into account the combined effects of temperature, stress, and damage in the construction of such a model. In this article, a modeling methodology based on irreversible thermodynamics applied within the framework of composite macro-mechanics is presented, that would allow characterization of non-Fickian diffusion coefficients from moisture-weight-gain data for laminated composites. A symmetric damage tensor based on continuum damage mechanics is incorporated in this model by invoking the principle of invariance with respect to coordinate transformations. For tractability, the diffusion-governing equations are simplified for the special case of a laminate, with uniformly distributed matrix cracks, that is subjected to a uniaxial tensile stress. The final form for effective diffusivity obtained from this derivation indicates that the effective diffusivity for this case is a quadratic function of crack density. A finite element procedure that extends this methodology to more complex shapes and boundary conditions is also presented. Comparisons with test data for a 5-harness satin textile composite are provided for model verifications.     

Experimental characterization of nonlinear,rate-dependent behavior in advanced polymer matrix composites

In order to support materials selection for the next-generation supersonic civilian-passenger transport aircraft, a study has been undertaken to evaluate the material stress/strain relationships needed to describe advanced polymer matrix composites under conditions of high load and elevated temperature. As part of this effort, this paper describes the materials testing which was performed to investigate the viscoplastic behavior of graphite/thermoplastic and graphite/bismaleimide composites. Test procedures, results and data-reduction schemes which were developed for generating material constants for tension and compression loading, over a range of useful temperatures, are explained. Paper was presented at the 1991 SEM Spring Conference on Experimental Mechanics held in Milwaukee, WI on June 9–13.     

Convective cooling/heating induced thermal stresses in a fluid saturated porous medium undergoing local thermal non-equilibrium

Local thermal non-equilibrium (LTNE) may have profound effects on the pore pressure and thermal stresses in fluid saturated porous media under transient thermal loads. This work investigates the temperature, pore pressure, and thermal stress distributions in a porous medium subjected to convective cooling/heating on its boundary. The LTNE thermo-poroelasticity equations are solved by means of Laplace transform for two fundamental problems in petroleum engineering and nuclear waste storage applications, i.e., an infinite porous medium containing a cylindrical hole or a spherical cavity subjected to symmetrical thermo-mechanical loads on the cavity boundary. Numerical examples are presented to examine the effects of LTNE under convective cooling/heating conditions on the temperature, pore pressure and thermal stresses around the cavities. The results show that the LTNE effects become more pronounced when the convective heat transfer boundary conditions are employed. For the cylindrical hole problem of a sandstone formation, the thermally induced pore pressure and the magnitude of thermal stresses are significantly higher than the corresponding values in the classical poroelasticity, which is particularly true under convective cooling with moderate Biot numbers. For the spherical cavity problem of a clay medium, the LTNE effect may become significant depending on the boundary conditions employed in the classical theory.     

A micromechanical study of residual stress and its effect on transverse failure in polymer–matrix composites

Cure residual stress and its effect on damage in unidirectional fibre-reinforced polymer–matrix composites under transverse loading were studied using a micromechanical unit cell model and the finite element method. The overall residual stress introduced from curing was determined by considering two contributions: volume shrinkage of matrix resin from the crosslink polymerization during isothermal curing and thermal contraction of both resin and fibre as a result of cooling from the curing temperature to room temperature. To examine the effect of residual stress on failure, a model based on the Maximum Principal Stress criterion and stiffness degradation technique was used for damage analysis of the unit cell subjected to mechanical loading after curing. Predicted damage initiation and evolution are clearly influenced by the inclusion of residual stress. Residual stress is always detrimental for transverse compressive loading and pure shear loading. For transverse tensile loading, residual stress is detrimental for relatively low resin strength and beneficial for relatively high resin strength. Failure envelopes were obtained for both biaxial normal loading and combined shear and normal loading and the results show that residual stress results in a shifting and contraction of the failure envelopes.     

Influence of Hall current and Joule heating on entropy generation during electrokinetically induced thermoradiative transport of nanofluids in a porous microchannel

A comprehensive theoretical study of entropy generation during electrokinetically driven transport of a nanofluid is of prime concern in the paper. The flow is considered to take place on a wavy channel under the action of an external transverse magnetic field and an external pressure gradient. Navier slips at the walls of the channel and thermal radiation have been taken into account in the study. The theoretical study has been carried out by developing a mathematical model by taking into account the effects of Joule heating, viscous dissipation, and the transverse magnetic field on heat transfer during the electrokinetic transport of the fluid. The derived analytical expressions have been computed numerically by considering the nanofluid as a mixture of blood and ferromagnetic nanoparticles. Variations in velocity, streaming potential, temperature distribution, Nusselt number, and Bejan number associated with the electrokinetic flow in capillaries have been investigated by the parametric variation method. The results have been presented graphically. The present investigation reveals that streaming potential decreases due to the Hall effect, while for the cooling capacity of the microsystem,we find an opposite behavior due to the Hall effect. The study further reveals that the fluidic temperature is reduced due to increase in the Hall current, and thereby thermal irreversibility of the system is reduced significantly. The results presented here can be considered as the approximate estimates of blood flow dynamics in capillaries during chemotherapy in cancer treatment.     

Effect of the recoil pressure induced by evaporation on motion of powder particles in the light field during laser cladding

A model is proposed, which takes into account acceleration of powder particles by a force induced by recoil of material vapors from the irradiated region of the particle surface. Results of a numerical analysis of heat and mass transfer in the case of motion of individual stainless steel powder particles in a gas flow and in a light field of laser radiation under conditions of laser cladding are presented. Acceleration of particles is found to depend on their diameter, carrier gas velocity, powder material properties, laser radiation power, and degree of attenuation of the power density in the laser beam in the direction of its action on the substrate. The calculated results are compared with experimental data on light-propulsion acceleration of individual particles (of aluminum, aluminum oxide, and graphite) under the action of pulsed laser radiation.