Javier Gil Sevillano

A comparison of FEM and upper-bound type analysis of equal-channel angular pressing (ECAP)

Abstract In this paper the pressures needed for non-friction equal-channel angular pressing (ECAP) of perfectly plastic or strain-hardening materials are analysed using analytical approximations (upper-bound type) and numerical (finite-elements) methods. The approximate solutions agree very well with the FEM results for different ECAP die angles or materials. The convenience of using back-pressure for improving the strain pattern homogeneity of deformed strain-hardening materials is highlighted.

Towards a reliable procedure for the measurement of elastic modulus in instrumented indentation

Unloading stiffness is a critical magnitude when extracting elastic modulus in instrumented indentation. Any phenomenon which interacts with its measurement may affect the final calculation of the modulus. Analytical and numerical calculations have been carried out to determine the influence of thermal drift and creep response on its measurement, and the predictions were in good agreement with experimental results. Since the influence of thermal drift is depth-dependent, it determines the effective resolution of an indentation device for a given material. In contrast, indentation creep significantly alters unloading stiffness even for weakly rate-sensitive materials (sensitivity exponent, m < 0.05) but its effect could be smoothed down due to measurement artefacts (unloading curve fitting strategy). For instance, for an ultra-fine grained (UFG) pure niobium at room temperature (m ∼ 0.015 and H/Er ∼ 0.02), the error in the measurement of elastic modulus with a typical nanoindentation procedure (5 s of holding time and 65 s of unloading time) can be as high as 15%. This paper proposes simple rules for a reliable experimental procedure to avoid both thermal drift and creep effects on the measurement of elastic modulus, which are especially relevant for the new generation of high temperature instrumented indentation facilities.

Mechanical and thermal stability of heavily drawn pearlitic steel wire

Having interlamellar spacings on the nanometer scale, there is no doubt about considering heavily drawn pearlitic steel wire as a nano-layered material. This extremely fine structure is of great technical importance: indeed, as the interlamellar distance determines the onset of plastic flow, the wire can be brought to a tensile strength beyond 4,000 MPa and is therefore one of the strongest materials on the market nowadays. At extremely large strains (well beyond {var_epsilon} = 4) and/or at moderate temperatures, the pearlitic steel loses its strength. Several possible failure mechanisms, like fragmentation of the cementite or thermal and strain-induced cementite dissolution, are put forward, but until now, there is no definite understanding of the really active mechanism. In the present work, the calorimetric differential scanning technique, in combination with thermopower measurements and the high-resolution atomic force microscopy, have turned out to be most promising tools to reveal some of the mechanisms that are responsible for the degradation of the lamellar aggregate.