Browsing by Author "Hadjov K."
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Item Assessment of modulated hot wire method for thermophysical characterization of fluid and solid matrices charged with (nano)particle inclusions(2010-01-01) Chirtoc M.; Henry J.; Turgut A.; Tavman I.; Hadjov K.; Schuchmann H.; Sauter C.; Antoniow J.; Fudym O.; Tavman S.Recently we reported on simultaneous thermal conductivity k and thermal diffusivity a measurement of liquids and in particular of nanofluids in a configuration using an ac excited hot wire combined with lock-in detection of the third harmonic (3ω method) [1]. The conductive wire is used as both heater and sensor. The requirements for the asymptotic validity of the line heat source model are fulfilled at low modulation frequencies below a few Hz. The study of the relative sensitivity of signal amplitude and phase to changes in k and a indicates that there is an optimum frequency range for accurate and stable results. We extend by up to two decades the feasible frequency range for 3ω measurements by considering various more elaborate models for the heat transfer between the wire and the fluid. Finally we show that the same ac hot wire method can be applied to soft solid, composite materials. We measured the k enhancement of a poly(ethylene vinyl acetate) EVA polymer matrix charged with various fractions of graphite. © 2010 IOP Publishing Ltd.Item Behavior of rubbers in the case of diffusion and variable temperature(2013-12-17) Hadjov K.; Hallil G.; Milenova M.In this work the authors using an approximate solution of Fick's differential equation and applying integral time transformation obtain analytical expression concerning the enhancement and space distribution of fluid concentration in rubber at variable temperature. Experimental comparisons for water in poly-isoprene rubber are made.Item Cyclic loading of rubbers - amplitude spectrum and payne effect(2017-01-01) Hadjov K.; Aleksandrova V.Introducing nonlinear integral constitutive equation with singular kernels the authors have obtained the stressstrain hysteresis curves in the case of imposed large strains (stresses), respectively. The respective solutions of the integral equations are represented in Fourier series, which coefficients being convergent are used to obtain the Fourier amplitude spectrum and the amplitude dependence (decreasing) of the storage modulus - the so called Payne effect. The respective enhancement of the compliance with the imposed stress amplitude is also discussed. The experimental data obtained for polyisoprene rubber well agree with the theoretical predictions.Item Nonlinear elastoviscousity of rubbers by cycling loading(2013-12-17) Hadjov K.In this work in analytical form are used instantaneous stress-strain relations incorporated in the hereditary integral equations of Volterra to predict the creep and relaxation of rubbers and rubberlike materials at large deformations. One takes into account the non-linearity of the viscous behavior in the presence of similarity in the isochronal stress-strain curves. These integral equations well describe the stress response by cycling in the case of imposed strains and vice-versa. Using this approach we have obtain the hysteresis loop for different imposed strains and the frequency and amplitude dependence of the loss factor by damping. Our theoretical results are compared with experimental data for polyisoprene rubber. The theoretical curves agree very god with the test ones.