Abstract: Plasticity and asperity-induced crack closure under mixed mode non-proportional loading are investigated, using numerical models. It is shown that the applied mode II can either increase or decrease the effective ΔKI, depending on the R-ratio and loading path. In some cases, mode II is found to have opposite effects on each of the two closure mechanisms. These results are coherent with some crack-length or roughness-dependent variations in mode I fatigue crack growth rate due to the presence of a static mode II reported in the literature. For 90° out-of-phase mixed-mode loading, mode II is predicted to reduce drastically or even suppress plasticity-induced closure.
Abstract: Bulk ultrafine-grained nickel specimens having grain sizes in the range of 0.25–5 μm were processed by a Spark Plasma Sintering method. The resulting microstructures were characterized by electron backscattering diffraction, transmission electron microscopy and X-ray diffraction analysis. Compression tests were carried out at room temperature and at a strain rate of 1.6 × 10−4 s−1. It was found that the fine-grained microstructure and the presence of NiO phase were the main strengthening factors in the as-processed bulk materials. The contribution of the oxide phase to strengthening was even more pronounced for lower grain sizes. This contribution was calculated as the difference between the measured strength and the value obtained from a Hall-Petch plot of oxide-free samples, and this yielded a flow stress increment of about 635 MPa for the lowest grain size studied here. In addition, a transition from work hardening to softening occurred for materials having a mean grain size smaller than about 300 nm and having boundaries that could have been weakened by the presence of a high amount of NiO phase.
Abstract: Bulk Nanocrystalline and ultrafine-grained metals are materials having grain size in the submicron range and have motivate considerable attention due to their interesting physical and mechanical properties. An important issue in the field of submicron grain-sized materials is how to achieve both high strength and high ductility? It has been suggested that, one strategy for enhancing the ductility of high-strength nanocrystalline materials is to develop a bimodal grain-size distribution, in which the fine grains provide strength, and the coarser grains enable strain hardening. In this paper, we report on the micromechanical behaviour of bulk nickel samples having bimodal microstructures. The samples were processed by hot isostatic pressing of blends of nano and micrometer-sized powder particles. The resulting microstructure is a bimodal randomly distributed grains considered here as a mixture of two unimodal log-normal distributions. An efficient modelling approach (i.e. a generalized self consistent approach) previously developed by Jiang and Weng (2004) is then applied to such experimental data to investigate, among others, local plastic strains and internal stresses fields as well as local magnitudes deviations.
Abstract: This paper deals with the bearing capacity of masonry walls under lateral loads. Four different series of experimental measures have been collected, representing a total number of 20 walls tested at the Scientific and Technical Center for Buildings (CSTB, France). The constitutive materials of the walls and the geometrical features of the walls are:
– Orthotropic blocks (masonry or concrete units), with either horizontal or vertical cells. Their geometrical dimensions are such that the thickness is either equal to 0.2 m or 0.38 m while the ratios (height/length) range from 0.4 up to 1. The compressive strength of the blocks are in relative ratios (horizontal/vertical strengths) ranging from 0.11 up to 3.11.
– Joints made of mortar or thin layer mortar. The vertical joints might be either empty or full while the horizontal joints are full for the whole experiments reported herein.
– Walls with lengths ranging from 1 m up to 3.75 m while the height range from 2.5 m up to 2.8 m.
An existing model, relying on the principle of wall failure by its diagonal in compression, has herein been applied and its results have been compared with the experimental values for the 20 available walls. The model for compressive diagonal provides results that range within the interval (0.52 up to 2.67) times the experimental bearing capacity of the masonry walls.
The authors have therefore developed a simplified model that assumes that the wall fail by induced tension in the perpendicular direction of the diagonal of either the blocks or the walls. Compared to the experimental values collected in this paper, this simplified mechanical model provides theoretical bearing capacity values that are in good accordance with the observed values.
