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51313-02270-An Approach to Measurement of Environmentally Assisted Small Crack Growth

Picture for An Approach to Measurement of Environmentally Assisted Small Crack Growth
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Product Number: 51313-02270-SG
ISBN: 02270 2013 CP
Author: Alan Turnbull
Publication Date: 2013
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A primary focus of much of current research on environmentally assisted cracking is to characterise the early stages of crack development including where appropriate crack precursors. There are intrinsic problems in generating data since a significant fraction of life is spent in the early stages of development and the growth rate of precursors and/or cracks is inherently slow. Nevertheless guidance to industry is essential to make engineering judgement on first inspection and to understand the potential impact of transients in service operation conditions. For steam turbine blade steels the damage process predominantly involves formation of corrosion pits and the transition to a crack usually under high frequency fatigue loading associated with steady on-load operation or low frequency loading associated with start-up and shut-down.

At Corrosion 2012 we presented short crack growth measurements using fracture mechanics specimens for a steam turbine blade steel under low frequency loading in simulated condensate for normal water chemistry and under static load condition (stress corrosion cracking) under upset conditions. This enabled direct comparison with long crack growth measurements. However a short crack in a fracture mechanics specimens is a through-thickness crack sampling lots of grains. It is not obvious that the growth rate date can be simply transferred to growth of a small crack from a pit or defect since the small crack samples only a small number of grains and the geometry is evolving with time.

To address this challenge a methodology for measuring small crack growth is being developed in which cracks are initiated either from corrosion pits or from FIB notches. Since removal of solution for detailed measurement of crack size at intervals is not possible because of perturbation of the environment oxide films and corrosion potential a combination of optical and direct current potential drop technique have been developed for in-situ measurement. Similar to the work of Marrow high resolution digital image acquisition techniques were utilised and combined with digital image correlation to characterise both surface crack length and crack opening. Intermittent staining of the crack walls by controlled variation in water chemistry enable assessment of the changing in crack shape with time.

The results of these measurements will be reported for fatigue loading conditions in simulated condensate water chemistry.
 

A primary focus of much of current research on environmentally assisted cracking is to characterise the early stages of crack development including where appropriate crack precursors. There are intrinsic problems in generating data since a significant fraction of life is spent in the early stages of development and the growth rate of precursors and/or cracks is inherently slow. Nevertheless guidance to industry is essential to make engineering judgement on first inspection and to understand the potential impact of transients in service operation conditions. For steam turbine blade steels the damage process predominantly involves formation of corrosion pits and the transition to a crack usually under high frequency fatigue loading associated with steady on-load operation or low frequency loading associated with start-up and shut-down.

At Corrosion 2012 we presented short crack growth measurements using fracture mechanics specimens for a steam turbine blade steel under low frequency loading in simulated condensate for normal water chemistry and under static load condition (stress corrosion cracking) under upset conditions. This enabled direct comparison with long crack growth measurements. However a short crack in a fracture mechanics specimens is a through-thickness crack sampling lots of grains. It is not obvious that the growth rate date can be simply transferred to growth of a small crack from a pit or defect since the small crack samples only a small number of grains and the geometry is evolving with time.

To address this challenge a methodology for measuring small crack growth is being developed in which cracks are initiated either from corrosion pits or from FIB notches. Since removal of solution for detailed measurement of crack size at intervals is not possible because of perturbation of the environment oxide films and corrosion potential a combination of optical and direct current potential drop technique have been developed for in-situ measurement. Similar to the work of Marrow high resolution digital image acquisition techniques were utilised and combined with digital image correlation to characterise both surface crack length and crack opening. Intermittent staining of the crack walls by controlled variation in water chemistry enable assessment of the changing in crack shape with time.

The results of these measurements will be reported for fatigue loading conditions in simulated condensate water chemistry.
 

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