The elastic constants of particulate composites are evaluated employing a theoretical cube-within-cube formation. Two new models of four and five components, respectively, formed by geometrical combination of three-component models existing in the literature, are used as Representative Volume Elements. Using the governing stress and strain equations of the proposed models, two new equations providing the static elastic and shear moduli of particulate composites are formulated. In order to obtain the dynamic elastic and shear moduli, the correspondence principle was applied successively to components connected in series and/or in parallel. The results estimated by the proposed models were compared with values evaluated from existing formulae in the literature, as well as with values obtained by tensile, dynamic, and ultrasonic experiments in epoxy/iron particulate composites. They were found to be close to values obtained by static and dynamic measurements and enough lower compared with values obtained from ultrasonic experiments. The latter is attributed to the high frequency of ultrasonics. Since measurements from ultrasonic's and from dynamic experiments depend on the frequency, the modulus of elasticity estimated by ultrasonic's is compared with that (storage modulus) estimated by dynamic experiments.
Hydrogen produced after exposure of a low – carbon steel to corrosive NaCl – Water solution may affect various its tensile mechanical and magnetic microstructural behaviour in a complex manner. This was investigated by introducing a relevant micromagnetic specific emission (ME) - response of this ferromagnetic material, where related processes and parameters of micromagnetic activity and mechanical response were implemented. In this manner, it was demonstrated that an increase in the hydrogen accumulation with corrosion time leads to an associated increase in the embrittling effect expressed by a substantial loss in the ductility of material. The competive and opposing effects of cumulative hydrogen, applied stress and plastic strain – induced microstructural damage were related to the specific ME- response parameter by which an increased magnetic hardening tendency of material with corrosion time was possible to establish. In this fashion and by using a stress as well as strain mode of presentation- aided combined approach, the complex interplay between micromagnetic activity, hydrogen accumulation and applied stress-strain was better revieled and analysed. It was also shown that the embrittlement is a product of hydrogen accumulation introduced by two highly localized processes. As such, accumulation occurs in two characteristic parallel ways: one of a common lattice diffusion and one of hydrogen transport and redistribution by moving dislocation towards the affected sites. Concerning the highly localized effects the dominating role of hydrogen – induced damage in form void initiation and growth over the hydrogen – assisted stress relief was reasonably demonstrated by using a simple modelling approach. Based on a mechanism of moving dislocation – assisted interaction between commulative hydrogen and magnetic domain walls, a Portervin – Le Chatelier – type micromagnetic process of a cooperative-corelated domain wall transport was proposed to explain certain subtle, quasiperiodic behaviour of ME- response. In the frame of the above findings the superior sensivity of ME – response compared to the mechanical one in early detecting cumulative hydrogen – assisted microstructural damage changes can be d educed.
In the first part of this complex study the thermomechanical and fractural properties of particle reinforced polymer composites were experimentally obtained. The experimental values for modulus of elasticity, fracture stress, fracture strain and thermal expansion coefficient were compared with those derived from theoretical formulae existing in the literature and also from a theoretical model assuming the existence of an interphase between the two main phases the filler and the matrix. This model was used to obtain theoretical expressions for modulus of elasticity and thermal expansion coefficient. The mechanical properties of the material used in this investigation were determined from tensile experiments carried out with a composite material made of epoxy resin reinforced with iron particles the volume fraction of which varies from 0 to 25% and in some cases up to 40%. To obtain information concerning the thermal expansion coefficient and glass transition temperature of the same material thermomechanical analysis (TMA) measurements were performed. The effects of heating rate and filler content on the glass transition temperature were examined. In the second part of this study an attempt is made to explain on a more phenomenological basis the relative big discrepancies observed between theoretical models and experiments concerning certain strength parameters such as fracture stress and strain presented in the first part. This was possible by assuming a delayed kind of fracture behavior which was simulated by a subcritical crack growth based on the theory of elastic-small yielding fracture mechanics and by an arrest micromechanism. The above simulation in turn was achieved by a procedure of a semiquantitative gross estimation approach which has taken into consideration certain experimental fractographical and microstructural data such as interfacial decohesion features between grain and matrix, grain size and interinclusion spacing, parameters estimated by Scanning Electron Microscopic (SEM) measurements.
In this work, the influence of corrosion – induced hydrogen accumulation on a stressed low- carbon steel after exposure to NaCl - water solution was investigated by means of its combined tensile mechanical and Micromagnetic emission (ME) - response. The investigation was conducted by employing certain relevant parameters and processes of mechanical and magnetic microstructural changes. The mechanical and Micromagnetic response data were reduced to the ultimate tensile strength as well as to maximum (ME) - response respectively where certain critical- characteristic microstructural- transitional changes take place. Under these conditions and by an appropriate procedure of “consecutive - selective discrimination steps” of the related affecting factors their differential influence on the mechanical and ME – response was better revealed, compared and analyzed. In this manner it was demonstrated that the detrimental influence of cumulative hydrogen arises in from of mechanical embrittlement which can be related to a parallel magnetic hardening trend of the material. The explanations are given on the basis of highly localized and competitive or opposing processes of void initiation- growth and stress relive, resulting by a common lattice diffusion, as well as moving dislocation- aided transport of hydrogen to the affected sites. Within the frame of the above findings it was shown that the ME-response presents, compared with mechanical response, an increased sensitivity making the first a superior technique in early detecting hydrogen- assisted microstructural damage in loaded steel components
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