Olivier Lavigne

Water irradiation by high-frequency ultrasonic wave: effects on properties of passive film formed on stainless steel.

In this paper an aqueous solution was irradiated with a 1.63MHz ultrasonic wave. It is shown that if stainless steel can passivate under dynamic polarization in this medium, under static polarization, the latter does not show any repassivation behaviour with time. This is attributed to a diminution of the diffusion layer thickness that is developed at the electrode/electrolyte interface, which is associated with a production of H(2) species by sonolysis and which maintains reductive conditions at the interface. The oxide film formed under ultrasonic irradiation for 1h at a passive potential of+0.2V(SCE) shows an early stage of passivation and an increased disordered state, which implies a considerable decrease in the corrosion resistance behaviour of the sample. The polarization resistance of the stainless steel R(p) is divided by a value of 4.5 under ultrasonic conditions.

Bond Slip Models for Uncorroded and Corroded Steel Reinforcement in Class-F Fly Ash Geopolymer Concrete

AbstractGeopolymer concrete is an innovative construction material that utilizes industrial by-product waste materials to form a cement replacement for concrete manufacture. In order to simulate the behavior of reinforced concrete at all load levels, an understanding of the bond between the reinforcement and the concrete is required. That is, at the serviceability limit state, the bond between the reinforcement and the concrete controls the formation of cracks, crack widening, and tension stiffening. Similarly, adequate bond between the reinforcement and the concrete is required at the ultimate limit state to ensure the full capacity of the reinforcement is obtained. Over time, the bond between the reinforcement and concrete can deteriorate due to corrosion, thus impacting the overall performance of a structure. This paper presents a wide-ranging study of the bond between reinforcement and geopolymer concrete including an investigation of the influence of corrosion. This study involved 102 pull-out test s...

High-pH inclined stress corrosion cracking in Australian and Canadian gas pipeline X65 steels

High-pH stress corrosion cracking is a form of environmental degradation of gas pipeline steels. The crack path is intergranular by nature and typically perpendicular to the maximum applied (hoop) stress (i.e. perpendicular to the pipe outer surface). Some unusual instances of cracks have been observed in Canadian and Australian X65 pipes, where cracks grow away from the perpendicular for considerable distances. This paper presents a comparative study in terms of crack morphology, mechanical properties and crystallographic texture for these Australian and Canadian pipe steels. It is shown that the crack morphologies are quite similar, the main difference being the angle at which the cracks propagate into the material. This difference could be explained by the different through-wall texture and grain aspect ratio measured in the two materials. The interdependency of crack tip plasticity, crack tip electrochemistry and anisotropy in microstructural texture seems to heavily affect the resulting inclined crack path.

Influence of Strain on Current Densities and Stress Corrosion Cracking Growth Rates in X65 Pipeline Steel

High pH stress corrosion cracking (SCC) in gas pipelines has been seen to grow along an incline angle rather than perpendicular to the outer surface. Crack tip strain enhanced electrochemistry has been previously postulated as a reason for the inclination, and recent computer simulations that take this effect into account produce realistic crack paths. This study attempts to determine the effect of strain on the electrochemical response of X65 steel, and the impact that the strain has on growth rates for inclined SCC. Potentiodynamic tests were conducted on X65 tensile specimens with residual plastic strain or in situ elastic strain. An increase of the current density up to 300% was observed within the SCC potential range. Computer simulations were also conducted to show the qualitative effect of this increase in current density on the growth rate, and results indicated that inclined SCC could grow to 50% wall thickness faster than straight SCC if the current density along the inclined angle is 20% higher...

Effect of demineralization time on the mineral composition and mechanical properties of remineralized dentin

The aim of this study was to determine whether recovery of mineral levels restored the mechanical properties of dentin subjected to different durations of demineralization. Dentin at the floor of class 1 cavities (n = 12) was demineralized for 1, 2, and 3 weeks. Half the demineralized cavity floor was coated (control side), and a Fuji IX restoration was placed. The remaining half was therefore in contact with the Fuji IX restoration (test side). Simulated dentin tubular fluid was then supplied to each pulp chamber for 6 weeks. After remineralization, the teeth were detached from the system and sectioned. Concentrations of calcium, phosphorus, fluorine, and strontium and mechanical properties (hardness and Youngs modulus) of the test and control sides were determined by electron probe microanalysis and nano-indentation, respectively. For remineralized dentin demineralized for 1 week, the substantial uptake of mineral elements restored hardness and Youngs modulus at depths of 50 to 200 μm from the lesion front. For longer periods of demineralization (2 and 3 weeks), structural damage to the demineralized dentin was severe and impeded recovery of mechanical properties, despite mineral uptake.

In situ neutron diffraction measurement of strain relaxation in welds during heat treatment

ABSTRACT Neutron diffraction (ND) is commonly used to investigate the stress redistribution before and after post-weld heat treatment (PWHT) in welded structures. However, there is a lack of information on the evaluations of strains during PWHT. The present work employed in situ ND to measure the relaxation of residual strains during conventional PWHT in multi-pass high-strength low-alloy steel welds. It was found that strain relaxation occurs principally during the heating stage of the heat treatment. The findings have important economic bearings and can be used to characterise comparable material combinations and optimise the PWHT process for high-strength low-alloy weld joints. This unique information also provides a valuable benchmark for the finite element modelling of this complex process.

Susceptibility of Acicular Ferrite and Upper Bainite Microstructures to Hydrogen Assisted Cold Cracking Propagation

Acicular ferrite (AF) and upper bainite (UB) are microstructural constituents commonly found in ferritic weld metals. Both microstructures are formed within a similar temperature range and by the same type of transformation mechanisms. They have however, substantially different morphologies and microstructural features that govern both their mechanical properties and hydrogen embrittlement susceptibility. This work shows that despite substantial microstructural differences, the mechanical properties of both microstructural constituents were quite similar. However, the microstructural differences were found to significantly affect the hydrogen crack propagation resistance. Hydrogen assisted cold cracking (HACC) propagates along a path of least resistance through the surrounding microstructure. The unit crack path was significantly shorter for AF than for UB, which implied more frequent changes in direction and thus increased dissipation of energy from the crack driving force. These results suggest that AF, possessing fine interlocking grains and high angle grain boundaries (HAGB), increases the localised resistance to HACC propagation more than UB due to the impediment of brittle, cleavage-like crack propagation at HAGB’s.

Characterisation of Fracture and HAC Resistance of an Individual Microstructural Constituent with Micro-Cantilever Testing

This paper focuses on the application of miniaturized fracture tests to evaluate the fracture and hydrogen assisted cracking (HAC) resistance of a selected microstructural constituent (acicular ferrite, AF) which only occurs in microscopic material volumes. Site-specific Focused Ion Beam (FIB) micro-machining was used to fabricate sharply notched micro-cantilevers into a region fully constituting of AF. The micro-cantilevers were subsequently tested under uncharged and hydrogen charged conditions with a nanoindenter. The load displacement curves were recorded and analysed with a simplified plastic hinge model for the uncharged specimen, as AF demonstrated an essentially ductile behaviour. The simplified model assisted with FE simulations provided values of the critical plastic crack tip opening displacement (CTOD). A value of the conditional fracture toughness was thereby determined as 12.1 MPa m1/2. With LEFM, a threshold stress intensity factor, Kth, to initiate hydrogen crack propagation in AF was found to range between 1.56 MPa m1/2 and 4.36 MPa m1/2. All these values were significantly below the corresponding values reported for various ferrous alloys in standard macro-tests. This finding indicates that the fracture and HAC resistance at the micro-scale could be very different than at the macro-scale as not all fracture toughening mechanisms may be activated at this scale level.

Simulation of Water Radiolysis by Sonochemistry: Effects on the Electrochemical Behavior of a Stainless Steel

In this study, water radiolysis occurring in nuclear power plants was simulated by sonochemistry. Generated hydroxyl radicals can recombine in others species such as H2O2 and H2. It is shown that solution conductivity is an important parameter on the evolution of open circuit potential due to the thickness variation of the diffusion layer which may contain sonolysed species (OH, H2, H2O2) in different concentrations. Current densities increase under ultrasonic irradiation due to an increase of mass transport and charge transfer and to the presence of sonolysed species. The oxide film formed under ultrasonic for 1 h at a passive potential of +0.2 VSCE shows early stage of passivation and higher disordered state which imply a great decrease of the corrosion resistance behavior of the sample. The polarisation resistance Rp, of the stainless steel is divided by a value of 4.5 under ultrasonic irradiation.