Robust topology optimization accounting for misplacement of material

The use of topology optimization for structural design often leads to slender structures. Slender structures are sensitive to geometric imperfections such as the misplacement or misalignment of material. The present paper therefore proposes a robust approach to topology optimization taking into account this type of geometric imperfections. A density filter based approach is followed, and translations of material are obtained by adding a small perturbation to the center of the filter kernel. The spatial variation of the geometric imperfections is modeled by means of a vector valued random field. The random field is conditioned in order to incorporate supports in the design where no misplacement of material occurs. In the robust optimization problem, the objective function is defined as a weighted sum of the mean value and the standard deviation of the performance of the structure under uncertainty. A sampling method is used to estimate these statistics during the optimization process. The proposed method is successfully applied to three example problems: the minimum compliance design of a slender column-like structure and a cantilever beam and a compliant mechanism design. An extensive Monte Carlo simulation is used to show that the obtained topologies are more robust with respect to geometric imperfections.     

Understanding spatial concentrations of road accidents using frequent item sets

This paper aims at understanding why road accidents tend to cluster in specific road segments. More particularly, it aims at analyzing which are the characteristics of the accidents occurring in "black" zones compared to those scattered all over the road. A technique of frequent item sets (data mining) is applied for automatically identifying accident circumstances that frequently occur together, for accidents located in and outside "black" zones. A Belgian periurban region is used as case study. Results show that accidents occurring in "black" zones are characterized by left-turns at signalized intersections, collisions with pedestrians, loss control of the vehicle (run-off-roadway) and rainy weather conditions. Accidents occurring outside "black" zones (scattered in space) are characterized by left turns on intersections with traffic signs, head-on collisions and drunken road user(s). Furthermore, parallel collisions and accidents on highways or roads with separated lanes, occurring at night or during the weekend are frequently occurring accident patterns for all accident locations. These exploratory results show the potentiality of the frequent item set method in addition to more classical statistical techniques, but also suggest that there is no unique countermeasure for reducing the number of accidents.     

Antimony leaching from uncarbonated and carbonated MSWI bottom ash

The recycling potential of municipal solid waste incinerator (MSWI) bottom ash may be limited by the leaching of antimony (Sb). Therefore, treatment methodologies need to be developed. The pH-dependent leaching behaviour of this oxyanion-forming element in fresh and weathered bottom ash is, however, not understood. Sb leaching was investigated in a wide range of both pH and extent of carbonation. Sb came close to equilibrium with calcium antimonate (Ca[Sb(OH)(6)](2)) at acid and neutral pH. Therefore, adsorption experiments with synthetic calcite (CaCO(3)), ettringite (Ca(6)Al(2)(SO(4))(3)(OH)(12) x 26H(2)O), gypsum (CaSO(4) x 2H(2)O), and portlandite (Ca(OH)(2)) and adsorption modelling to hydrous ferric oxides (HFO) and amorphous aluminium minerals (AAM) were conducted to investigate which minerals decrease Sb leaching below equilibrium with calcium antimonate. At pH>12, calcium antimonate comes into solution due to portlandite formation, but the subsequent increase in Sb leaching is reduced due to strong interaction of Sb with portlandite and ettringite. Ettringite appears to be an important host mineral for Sb at the natural pH of mildly weathered bottom ash (11.8) because a minimum in leaching is observed. When pH is decreased below 10.5, ettringite dissolves and Sb comes into solution, approaching equilibrium with calcium antimonate near pH 9. Gypsum showed no affinity for Sb. The interaction of calcite with Sb was not clear. Adsorption modelling suggested that HFO, rather than AAM, control Sb leaching when pH<9. During carbonation, Sb leaching first increased, most likely due to dissolution of ettringite. Then, Sb leaching decreased, since the pH became low enough to allow sorption by HFO.