Materials Science and Engineering A xxx (2006) xxx--xxx

The mechanical behavior and microstructure of pure iron subjected to dominant shear loading has been characterized over a wide range of strain rates. Pure iron is found to be highly strain-rate sensitive. Iron exhibits marked strain softening at # 850 MPa that is unexpected for the annealed material, as characterized by TEM, but is identical to that of iron preshocked at 40 GPa [G.M. Weston, J., Mater. Sc. Lett. 11 (1992) 1361]. The microstructure is found to undergo significant refinement with increasing strain rate, from large initial grains (50 #m), through dislocation cells and large twinning, and finally micro-twins and dynamically recrystallized 200 nm grains at the higher strain rates. In situ temperature measurements indicate the release of an external heat source, other that the thermomechanical conversion of plastic work, which is identified as dynamic recrystallization. The present results suggest the operation of the # (BCC) # (HCP) phase transition that is known to occur during hydrostatic or shock loading at 13 GPa. The combination of the high strain-rate sensitivity and dominant shear loading conditions seem to trigger this phase transition, thus supporting recent work [K.J. Caspersen, A. Lew, M. Ortiz, M., E.A. Carter, Phys. Rev. Lett. 10 (2004) 115501] emphasizing the role of shear

Relationship between processing parameters and mechanical properties of thick glass fibre reinforced pmma composites

micro- and macroscopic scales. More precisely, composite plates are infused at different temperatures, and the evolution of the microstructure along the thickness of each sample is studied. Emphasis is placed on the characterization of porosity distribution and morphology by optical microscopy and X-ray microtomography. Additionally, chromatographic chemical analysis is carried out for the assessment of molecular weight distribution and residual monomer content. The local mechanical response is evaluated by carrying out nano-indentation tests inside the matrix pockets, while in situ transverse compression experiments are performed within a SEM. Thermal analysis as well as a simple thermomechanical method are used to quantify the amplitude of the residual stresses. In parallel, the viscoelastic-viscoplastic response of the bare matrix is determined by combining various macroscopic tests in order to allow micromechanical modelling of representative volume elements. Current results suggest that, while the in situ micromechanical properties of the methacrylic matrix barely vary with the infusion temperature, it strongly affects the amount and distribution of porosity in the composite part – which, in turn, governs the macroscopic mechanical properties of the final composite part and their variability

Effect of viscoplasticity of the epoxy matrix on long-term stress redistribution around fibre breaks in a composite subjected to high static tensile load

important for the time-dependent failure of composites. The creep strain development was therefore characterized for a commercial epoxy resin system (North Thin Ply Technology 736LT). These measurements were then plugged into a micro-mechanical finite element model. This model represented a standard composite microstructure consisting of T700 carbon fibres at a 50% volume fraction and was used to assess the time-dependent effect of stress redistribution around a fibre break. As the shear stresses in the matrix went down over time, the stress transfer region spreads out in fibre direction, which effectively lowers the stress concentration in the break plane but increases the effective overload in the longitudinal direction. The results of the modelled representative volume element can be translated into a timedependent local load sharing approximation, which is used as input data for the fibre break model from Swolfs et al. [2]. The advanced fibre break model is able to determine the strength degradation over time of a unidirectional composite bundle exposed to high static tensile load. By studying the development of the degradation paired with the respective probability for a certain path, better lifetime predictions for continuously loaded components can be made. Modelling predictions are intended to be verified with the help of Synchrotron Computed Tomography at the submicron scale by holding a specimen at a constant displacement. The analysis of the scan data is still ongoing

Studies on plasma-sprayed thermal barrier coatings

The emphasis of this work has been the fundamental characterization of the material properties of thermal barrier coatings. Tests have been carried out on plasma sprayed ZrO//2-8wt%Y//2O//3 and ZrO//2-12wt%Y//2//3 (YSZ) coatings. The thermal expansion coefficient ( alpha ) is anisotropic in the longitudinal direction (planar to the substrate) compared to the transverse orientation (perpendicular to the substrate). The magnitude of alpha is dependent on prior heat treatment of the coating. The coating, deposited onto a superalloy substrate, was thermally cycled to 1200 degree C and failure followed via acoustic emission (AE) techniques. AE count rate distribution varies with temperature. Relationships of this nature lead to an understanding of cracking mechanisms within the coating