Jonathan Duff

In situ observation of intergranular crack nucleation in a grain boundary controlled austenitic stainless steel

Grain boundary engineering has been proposed to increase the lifetime performance of sensitized austenitic stainless steel in aggressive environments. Increased microstructure resistance is typically associated with higher fractions of twin (Σ3) grain boundaries, but there is uncertainty about the properties and role of other boundaries. To develop predictive models for stress corrosion crack nucleation, more information is required about how grain boundary crystallography and the orientations of the grain boundary plane and its surrounding grains affect crack development. Digital image correlation combined with electron backscatter diffraction has been used to characterize the microstructure and to observe, in situ, the nucleation and propagation of short stress corrosion cracks in thermo‐mechanically processed type 304 stainless steel. The crack path and its growth rate have been determined and are found to be influenced by the microstructure.

Preliminary Evaluation of Digital Image Correlation for In-situ Observation of Low Temperature Atmospheric-Induced Chloride Stress Corrosion Cracking in Austenitic Stainless Steels

Digital image correlation has been used to observe the growth of atmospheric-induced chloride stress corrosion cracking in type 304L stainless steel under controlled conditions of temperature, relative humidity and chloride-deposition density in a nondestructive manner. The technique is capable of detecting changes in crack dimensions that are difficult to discern via conventional optical microscopy, i.e. crack growth beneath salt layers and adherent corrosion product deposits, and measurement of crack opening displacements. Our results also demonstrate that suitable specimen design, combined with digital image correlation, will provide the means of comparing the growth behaviour of short atmospheric-induced chloride stress corrosion cracks with data obtained from conventional pre-cracked compact tension specimens as a function of mechanical “driving force”.

IN-SITU OBSERVATION OF CRACK NUCLEATION IN NUCLEAR GRAPHITE BY DIGITAL IMAGE CORRELATION

To study the fracture behaviour of nuclear graphite, a full-field digital image correlation technique has been applied to large specimens of isotropic Gilsocarbon graphite. Optical images of the tensile surface in four-point bend tests were recorded throughout the loading history, with a 100 × 100 mm viewing area. Although crack nucleation was not observable in these raw images, the high sensitivity of digital image correlation to the small displacements allows cracks to be detected. Strain maps are derived from the displacements, and surface cracks with lengths from 1 mm can be seen due to the high effective strain that results from crack opening. Post-processing of the strain maps can track the development of every such defect. These unique observations show the distribution of cracks and their sub-critical development and interactions prior to unstable fracture. This information may be used to validate models for the effects of sample size and stress gradient on component fracture strength.Copyright

In-Situ Observations of Intergranular Stress Corrosion Cracking

The development and validation of predictive models for intergranular stress corrosion cracking requires knowledge of short crack growth kinetics in response to mechanical driving forces. A new experimental method for in-situ observation of the early stages of crack growth during stress corrosion cracking, via full field Digital Image Correlation, is described and data for crack growth development are presented. Intergranular stress corrosion cracks were nucleated in sensitised 304 stainless steel under static uniaxial flexural deflection, within a potassium tetrathionate environment. High resolution optical images of a 2mm by 2mm area are recorded through the test solution during the experiment. The raw images show no observable cracking. However, the high sensitivity of digital image correlation allows small crack opening displacements to be detected. The derived strain map of the sample surface thereby enables imaging of the cracks. Surface cracks with lengths exceeding approximately 30μm can be observed. Post processing of the strain maps is then used to track the development of the cracks.Copyright

Development of a Microfluidic Setup to Study the Corrosion Product Deposition in Accelerated Flow Regions

CRUD (Chalk River Unidentified Deposit) forms in the water circuits of nuclear reactors due to corrosion of structural materials and the consequent release of species into the coolant. The deposition of CRUD is known to occur preferentially in regions of the primary circuit of pressurised water reactors (PWRs) where the water flow accelerates. In order to investigate this phenomenon, a micro-fluidic system, recreating plant conditions while using a simplified experimental set-up, was realised. A flow cell, comprising a stainless steel disc with a central micro-orifice, was used to create accelerated flow under representative operating conditions. By monitoring the pressure drop across the cell, the build-up rate (BUR) of CRUD within the micro-orifice was monitored in real time. By this setup, the conditions inducing deposition of CRUD under PWR conditions were emulated and CRUD deposition was induced in the accelerated flow region. Further effects associated with the presence of lithium hydroxide were investigated in real-time.Corrosion: Reproducing deposition in nuclear power plantsA simplified micro-fluidic system can successfully emulate corrosion products deposition in nuclear reactor water circuits. A team led by Fabio Scenini at the University of Manchester in the U.K. used a stainless steel disc with a micro-orifice and a micro-fluidic cell to build a system recreating the accelerated flows of an operating power plant. They monitored the pressure drop and build-up rate of corrosion products in real time, showing that more efficient setup reproduced corrosion seen under plant conditions, and that spallation of built-up oxide was a consequence of competition between its complex hydrodynamic and electrokinetic preferential deposition and its removal at high velocities. When adding lithium to the water, corrosion oxide formation was limited. Applying this methodology may help us better understand corrosion in nuclear reactors.

SCC Initiation in the Machined Austenitic Stainless Steel 316L in Simulated PWR Primary Water

Annealed and cold-worked stainless steel 316L samples with machined and polished surfaces were tested in simulated pressurized water reactor (PWR) primary water under slow strain rate tensile (SSRT) test conditions to investigate stress corrosion cracking (SCC) initiation. Roughness, residual stress and cross-sectional microstructure of the as-machined samples were characterized before SSRT tests. Plan view and cross-sectional examinations were performed after the test. Pre-test characterization indicated that a deformation layer was present on the machined surfaces. This deformation layer consisted of an ultrafine-grained layer on the top and deformation bands underneath. The thickness of the deformation layer on the annealed material was greater than that on the cold-worked material. Post-test characterization revealed that the SCC initiation behaviors of the as-machined and polished surfaces were different for both annealed and cold-worked materials. Machining increased SCC initiation susceptibility of the annealed material as many shallow cracks initiated along the machining marks in the machined surface, and it decreased the SCC initiation susceptibility of the cold-worked material as a reduced number of cracks were identified in the machined surface compared to the polished surface. The factors influencing SCC initiation are also discussed.