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A model for studying the mechanical internal stresses into dielectric materials is developed in the present work. The model takes into account the formalism of inclusions in continuous media. The dielectric material is assumed to be partitioned in different cubes which form a sizeable bulk material in such a way that a given cube can be of the dipolar phase and its neighbor can be of the same phase or the matrix. The behavior of the internal stresses promoted by the electrostriction phenomenon can be monitored by studying the behavior of both the misfit coefficient related to the strain misfit and the transfer of elastic energy process. The equations obtained through the here presented model are calculable using magnitudes from mechanical tests, in particular dynamic mechanical analysis, which is very sensitive to changes in the microstructure. Dynamic mechanical analysis as a function of the electric field is reported, perhaps for the first time in literature, for the study of dielectric materials, giving rise to a useful tool for studying the behavior of internal stresses in dielectric materials. [ABSTRACT FROM AUTHOR]

In the present work, a study on the dependence of the Dielectric Strength (DS) as a function of the empty space and the internal stresses into polymer samples has been performed. The empty space was measured by means of positron annihilation lifetime spectroscopy. The internal stresses into the polymer matrix, due to the appearance of the crystalline zones, were determined by applying the inclusion theory results to the dynamic mechanical analysis measurements. Attention to the effect of the chain movement on the electrical breakdown process has been paid. The study has been performed at a mesoscopic scale with the aim of demonstrating the influence of the thermal, mechanical or electrical stresses on the chains movement and their effect on the dielectric breakdown phenomenon. It has been verified that, DS is a function of the empty space into the sample and also of the internal stresses acting at the polymer chains, which result from the crystalline state promoted by chemicrystallization. The empty space decrease and the internal stresses act overlapped against the electrical force operating on the polymer chains. It is deduced from the work, that the precursor phenomenon for the electrical breakdown is the bend of polymer chains, prior to the onset of their collective movement promoted by the electrical forces. [ABSTRACT FROM AUTHOR]

Mechanical spectroscopy (damping and elastic modulus as a function of temperature) and transmission electron microscopy studies have been performed in high purity polycrystalline molybdenum plastically deformed to different values of tensile and torsion strain. Mechanical spectroscopy measurements were performed from room temperature up to 1285 K. A relaxation peak in polycrystalline molybdenum related to the movement of dislocations into lower energy configurations near grain boundaries has been discovered to appear around 1170 K. The activation energy of the peak is 4.2 eV ± 0.5 eV. This relaxation phenomenon involves the interaction between vacancies and mobile dislocations near the grain boundaries. It should be highlighted that this relaxation process is controlled by the arrangement of vacancies and dislocations which occur at temperature below 1070 K. [ABSTRACT FROM AUTHOR]