Piezoelectric sensors have made their way into polymer processing and rheometry applications, in particular when small pressure changes with very fast dynamics are to be measured. However, no validation of their use for steady shear rheometry is available in the literature. Here, a rheological slit die was designed and constructed to allow for the direct comparison of pressure data measured with conventional and piezoelectric transducers. The calibration of piezoelectric sensors is presented together with a methodology to correct the data from the inherent signal drift, which is shown to be temperature and pressure independent. Flow curves are measured for polymers showing different levels of viscoelasticity. Piezoelectric slit rheometry is validated and its advantage for the rheology of thermodegradable materials with viscosity below 100 Pa·s is highlighted.
A stationary experiment was performed to study the degradation of carbon-based materials by immersion in a plasma jet. In the experiment, graphite and C/C composite were chosen as the target materials, and the reactive plasma jet was generated by an air plasma torch. For macroscopic study of the material degradation, the sample's mass losses were measured as function of the exposure time under various temperatures on the sample surface. A microscopic analysis was then carried out for the study of microscopic aspects of the erosion of material surface. These experiments showed that the mass loss per unit area is approximately proportional to the exposure time and strongly depends on the temperature of the material surface. The mass erosion rate of graphite was appreciably higher than the C/C composite. The ablation rate in the carbon matrix region in C/C composite was also noticeably higher than that in the fiber region. In addition, the latter varied according to the orientation of fibers relatively to the flow direction. These tests indicated an excellent ablation resistance of the C/C composite, thus being a reliable material for rocket nozzles and heat shielding elements of the protection systems of hypersonic apparatuses from aerodynamic heating.
Silica/phenolic composite materials are often used in thermal protection systems (TPS) for atmospheric re-entry. The present work aims to compare two different approaches to assess heat transfer properties of these materials: i) using standard and specific experimental methods, and ii) with the development of 3D thermal transfer multiscale model using 2D (microscopy) and 3D (tomography) images. The latter procedure, based on computations on images, is a two-step change of scale from microscopic scale to mesoscopic scale and then to the macroscopic one. Two silica/phenolic composites with different spatial organizations are studied and their thermal properties are compared. Several experimental methods have been used, including space-resolved diffusivity determinations. Numerical results are compared to experimental ones in terms of transverse and longitudinal thermal conductivities of the composites, and were found to be in good agreement. A discussion is made on the different possible sources of uncertainty for both methods.
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