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Numerical modelling of the ratchetting effect under uniaxial and multiaxial loading conditions

100%

Kerkour-El M. A. , Abbadi M. , Radi M.

tom Vol. 57 nr 4

897--907

The main objective of the present work is to numerically test the performance of a micromechanical model under stress-controlled cyclic loading conditions. This simplified nonincremental model has a peculiarity of taking into account the grain shape effect and introduces an isotropic hardening variable for each slip system. The model shows an ability to predict accommodation, uni- and multiaxial ratchetting phenomena for complex loading paths. The uniaxial ratchetting is more pronounced for relatively higher mean stresses. Moreover, the evolution of intragranular isotropic hardening, mainly in path 1, is found to be dependent on both the sliding nature and the increase of the ACSS number in the case of multiaxial ratchetting. Finally, the main advantage of the explored multiscale approach is in its capability to describe local heterogeneities.

The Effect of Heavy Metal Speciation on Arbuscular Mycorrhizal Fungi Associated with Phoenix dactylifera L. Growing in Moroccan Urban and Peri-Urban Environments

75%

Radi M. , Anzid K. , Meddich A. , Boularbah A. , Hafidi M. , Lahcen O.

tom Vol. 25, iss. 5

154--166

Impacts of metal pollution on arbuscular mycorrhizal fungi (AMF) in rhizopheric soils of date palms in urban and peri-urban areas were studied. The objective was to assess the impact of rhizospheric soil contamination. Various chemical species of heavy metals on the AMF spore density associated with date palms were evaluated. A collection of date palm rhizospheric soil samples from eight sites including three boulevards, three gardens and two distinct areas of the Marrakesh palm grove was under study. These samples were used for counting endomycorrhizal fungal spores, for estimating mycorrhization state of root system and for physico-chemical analyses. A five-stage sequential extraction scheme was used to evaluate the fractionation of some heavy metals like lead (Pb), cadmium (Cd), copper (Cu), zinc (Zn) and iron (Fe). Pearson’s correlation coefficients between AMF spore’s density and metal species were determined and a multiple linear regression was tested to predict AMF spore density from the chemical species content of soil. A mycorrhizal dependency of date palm was indicated, since a high frequency from 61 to 98% and a middle intensity from 10 to 47% of root colonization by AMF were recorded. The spore density from all sites was found in increasing order of boulevards, garden then palm groves. A significant correlation between AMF spore density and some metal species rhizospheric soil content was recorded; negative for sulfide-bound lead (r = -0.81) and zinc (r = -0.70) and for exchangeable fraction copper (Cu), (r = -0.79) whereas it is positive for exchangeable fraction zinc (r = 0.70). AMF spore density predictions from sulfide-bound Zinc and exchangeable fraction copper exhibited a good fit, with higher R2 value (0.91, p = 0.002). Since Date palm has a mycotrophic nature, the sustainability of the microbial populations associated with their roots might be ensured by modifying some chemical forms of heavy metals like sulfide-bound zinc and exchangeable fraction copper.