Leon Chernin

Reliability analysis of rotor blades of tidal stream turbines

Tidal stream turbines are used for converting kinetic energy of tidal currents into electricity. There are a number of uncertainties involved in the design of such devices and their components. To ensure safety of the turbines these uncertainties must be taken into account. The paper shows how this may be achieved for the design of rotor blades of horizontal-axis tidal stream turbines in the context of bending failure due to extreme loading. Initially, basic characteristics of such turbines in general and their blades in particular are briefly described. A probabilistic model of tidal current velocity fluctuations, which are the main source of load uncertainty, is then presented. This is followed by the description of reliability analysis of the blades, which takes into account uncertainties associated with tidal current speed, the blade resistance and the model used to calculate bending moments in the blades. Finally, the paper demonstrates how results of the reliability analysis can be applied to set values of the partial factors for the blade design.

Cover cracking in reinforced concrete elements due to corrosion

The article considers analytical modelling of crack initiation in the concrete cover caused by corrosion of reinforcing steel. Initially, existing analytical models describing this phenomenon are critically reviewed. A new analytical model proposed by the authors is then presented and calibrated against available experimental data. The model is based on a thick-walled cylinder approach. To account for partial cracking of the concrete cover the cylinder is divided into two parts – a cracked inner cylinder and an uncracked outer one. The model ensures a consistent stress–strain formulation within both the inner and outer cylinders and enables to achieve complete continuity of stresses and strains on the boundary between the cylinders that distinguish it from the previously published analytical models. The model is then used to estimate the amount of corrosion products, which have diffused into concrete pores and cracks before full cracking of the concrete cover. It is shown that this amount may be larger than has been previously assumed. It is also shown that the assumption that corrosion products diffuse into concrete only until they fully fill the so-called ‘porous’ zone around a reinforcing bar leads to results, which are difficult to explain from a physical point of view. An alternative approach to account for the diffusion of corrosion products into concrete is proposed. Finally, a possible decrease in the corrosion rate with time and its influence on the prediction of the time to crack initiation are considered.