Improved Failure Assessment Diagrams to Describe Delayed Hydride Cracking Initiation at a Blunt Flaw

2003 ◽  
Author(s):  
Douglas A. Scarth ◽  
Ted Smith
Keyword(s):  
Process Zone ◽  
Peak Stress ◽  
Mode I ◽  
Mode Iii ◽  
Exact Relation ◽  
Practical Applications ◽  
Failure Assessment ◽  
Mode I Loading ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

Delayed Hydride Cracking (DHC) in Zr-2.5 Nb alloy material is of interest to the CANDU (CANada Deuterium Uranium) industry in the context of the potential to initiate DHC at a blunt flaw in a CANDU nuclear reactor pressure tube. The material is susceptible to DHC when a hybrided region forms at the flaw tip. The hydrided region could then fracture to the extent that a crack forms, and is able to grow by the DHC crack growth mechanism. A process-zone based methodology for evaluation of DHC initiation at a blunt flaw that takes into account flaw geometry has been developed. In a paper presented at the 2000 ASME PVP Conference, the process-zone methodology was used to develop failure assessment diagrams in such a way that the geometry dependence of the failure assessment curves was minimized. This was achieved by defining the ordinate of the failure assessment diagrams in terms of the ratio of the applied elastic peak stress divided by the threshold peak stress for DHC initiation at the tip of a deep flaw. However, the resultant failure assessment curves for Mode I loading did not have the simple form as the curves for Mode III loading, where the Mode III case was modelled in order to clearly see the interplay between material and geometry parameters. The present paper demonstrates that the irregular shapes of the Mode I curves were due to the relation for the threshold peak stress for the deep flaw that was used in the Mode I failure assessment curves. In the 2000 ASME PVP Conference paper an exact relation for the threshold peak stress was used for Mode III loading, while an approximate relation was used for Mode I. In the present paper a more accurate relation for the threshold peak stress for a deep flaw was used for Mode I loading, and the resultant Mode I failure assessment curves have a simpler form, which leads to more practical applications of the approach. Agreement between the improved Mode I failure assessment diagram predictions and experimental results is reasonable.

The Behaviour of a Process Zone at the Root of a Deep Sharp Flaw for Mode I Loading

2003 ◽  
Author(s):  
E. Smith
Keyword(s):  
Stress Distribution ◽  
Process Zone ◽  
Fracture Initiation ◽  
Radius Of Curvature ◽  
Mode I ◽  
Zone Size ◽  
Mode Iii ◽  
Root Radius ◽  
Mode I Loading ◽  
Linear Behaviour

The process zone representation of non-linear behaviour at the root of a flaw or crack is frequently used to quantify fracture initiation scenarios. In a paper presented at the 2002 PVP Conference, the author showed that with regard to a process zone at the root of a deep sharp flaw for Mode III loading conditions, its behaviour can be described, to a good degree of accuracy, by inputting the stress distribution ahead of the actual flaw root into an analysis of a process zone at the root of a semi-infinite crack in an infinite solid. The present paper describes an analysis of the analogous Mode I loading situation, with the results being compared with those obtained by a procedure which blends the small and large s/ρ behaviours where s is the process zone size and ρ is the flaw root radius of curvature.

Delayed Hydride Cracking Initiation at Notches in Zr-2.5Nb Alloys

2009 ◽  
Vol 131 (4) ◽  
Author(s):  
Jun Cui ◽  
Gordon K. Shek ◽  
D. A. Scarth ◽  
Zhirui Wang
Keyword(s):  
Hydrogen Diffusion ◽  
Process Zone ◽  
Notch Root ◽  
Threshold Value ◽  
Operating Conditions ◽  
Test Results ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Notch Tip ◽  
Hydride Cracking

Delayed hydride cracking (DHC) is an important crack initiation and growth mechanism in Zr-2.5Nb alloy pressure tubes of CANDU nuclear reactors. DHC is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth, and fracture of a hydrided region at a flaw tip. In-service flaw evaluation requires analyses to demonstrate that DHC will not initiate from the flaw. The work presented in this paper examines DHC initiation behavior from V-notches with root radii of 15 μm, 30 μm, and 100 μm, which simulate service-induced debris fretting flaws. Groups of notched cantilever beam specimens were prepared from two unirradiated pressure tubes hydrided to a nominal hydrogen concentration of 57 wt. ppm. The specimens were loaded to different stress levels that straddled the threshold value predicted by an engineering process-zone (EPZ) model, and subjected to multiple thermal cycles representative of reactor operating conditions to form hydrides at the notch tip. Threshold conditions for DHC initiation were established for the notch geometries and thermal cycling conditions used in this program. Test results indicate that the resistance to DHC initiation is dependent on notch root radius, which is shown by optical metallography and scanning electron microscopy to have a significant effect on the distribution and morphology of the notch-tip reoriented hydrides. In addition, it is observed that one tube is less resistant to DHC initiation than the other tube, which may be attributed to the differences in their microstructure and texture. There is a reasonable agreement between the test results and the predictions from the EPZ model.

Cycle-Wise Process-Zone Model for Prediction of Delayed Hydride Cracking Initiation Under Flaw-Tip Hydride Ratcheting Conditions

2018 ◽  
Author(s):  
Steven X. Xu ◽  
Jun Cui ◽  
Douglas A. Scarth ◽  
David Cho
Keyword(s):  
Structural Integrity ◽  
Process Zone ◽  
Operating Conditions ◽  
Zone Model ◽  
Evaluation Procedures ◽  
Fuel Bundle ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Technical Basis ◽  
Hydride Cracking

Flaws found during in-service inspection of Zr-2.5Nb pressure tubes in CANDU(1) reactors include fuel bundle scratches, debris fretting flaws, fuel bundle bearing pad fretting flaws and crevice corrosion flaws. These flaws are volumetric and blunt in nature. A key structural integrity concern with in-service blunt flaws is their susceptibility to delayed hydride cracking (DHC) initiation, particularly for debris fretting flaws under flaw-tip hydride ratcheting conditions. Hydride ratcheting conditions refer to situations when flaw-tip hydrides do not completely dissolve at normal operating temperature, and accumulation of flaw-tip hydrides occurs with each reactor heat-up/cool-down cycle. A significant number of in-service flaws are expected to be under hydride ratcheting conditions at late life of pressure tubes. DHC initiation evaluation procedures based on process-zone methodology for flaws under hydride ratcheting conditions are provided in CSA (Canadian Standards Association) N285.8-15. The process-zone model in CSA N285.8-15 predicts whether DHC initiation occurs or not for given flaw geometry and operating conditions, regardless of the number of reactor heat-up and cool-down cycles. There has been recent new development. Specifically, a cycle-wise process-zone model has been developed as an extension to the process-zone model in CSA N285.8-15. The cycle-wise process-zone model is able to predict whether DHC initiation occurs or not during a specific reactor heat-up and cool-down cycle under applied load. The development of the cycle-wise process-zone model was driven by the need to include flaw-tip stress relaxation due to creep in evaluation of DHC initiation. The technical basis for the development of the cycle-wise process-zone model for prediction of DHC initiation under flaw-tip hydride ratcheting conditions is described in this paper.

Delayed Hydride Cracking Initiation at Simulated Secondary Flaws in Zr-2.5 Nb Pressure Tube Material

2004 ◽  
Author(s):  
Jun Cui ◽  
Gordon K. Shek ◽  
Douglas A. Scarth ◽  
William K. Lee
Keyword(s):  
Hydrogen Diffusion ◽  
Process Zone ◽  
Pressure Tube ◽  
Experimental Program ◽  
Small Surface ◽  
Integrity Assessment ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
The Impact ◽  
Hydride Cracking

Flaws in Zr-2.5 Nb alloy pressure tubes of CANDU nuclear reactors are susceptible to a crack initiation and growth mechanism called Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth of the hydrided region and fracture of the hydrided region at the flaw-tip. The presence of small surface irregularities, or secondary flaws, at the bottom of service-induced fretting flaws in pressure tubes requires an integrity assessment in terms of DHC initiation. Experimental data and analytical modeling are required to predict whether DHC initiation can occur from the secondary flaws. In the present work, an experimental program was carried out to examine the impact of small secondary flaws with sharp radii on DHC initiation from simulated fretting flaws. Groups of cantilever beam specimens containing blunt notches with and without secondary flaws were prepared from unirradiated pressure tube materials hydrided to a nominal concentration of 50 wt ppm hydrogen. The specimens were subjected to multiple thermal cycles to form hydrides at the flaw-tip at different applied stress levels, which straddled the threshold value for DHC initiation. The threshold conditions for DHC initiation were established for different simulated fretting and secondary flaws. The experimental results are compared with predictions from the engineering process-zone DHC initiation model.

A Comparison of Finite Element Cohesive-Zone Modelling With the Process-Zone Approach for the Prediction of Delayed Hydride Cracking

2013 ◽  
Author(s):  
Michael Martin
Keyword(s):  
Finite Element ◽  
Stress Level ◽  
Cohesive Zone ◽  
Threshold Stress ◽  
Process Zone ◽  
Zirconium Alloys ◽  
Cohesive Elements ◽  
Cohesive Zone Modelling ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

Zirconium alloys, as used in water-cooled nuclear reactors, are susceptible to a time-dependent damage mechanism known as Delayed Hydride Cracking, or DHC. Corrosion of the zirconium alloy in the presence of water generates hydrogen that subsequently diffuses through the metallic structure towards stress concentrating features such as notches or cracks. Canadian standard CSA N285.8–10 uses a process-zone modelling approach to define a threshold stress level beyond which DHC is predicted to occur. The process-zone analysis to calculate the threshold stress level generally proceeds by representing the process-zone as a crack, the length of which is determined by the superposition of stress intensity factors obtained from handbook solutions or cracked-body finite element models. Process-zone models are a subset of the more general class of cohesive-zone models and cohesive elements are available in a number of standard finite element codes. Cohesive elements can be used to simulate the process-zone response, or indeed more complex cohesive behaviour. In this paper, the stress and displacement results from finite element based cohesive-zone modelling of a sharp crack and blunt notches of various root radii are compared with analytical process-zone solutions. The cohesive-zone results are also compared with the process-zone formulation used in CSA N285.8–10. The results show that finite element based cohesive-zone analysis can be used to replicate the process-zone results. The key benefit of finite element based cohesive-zone modelling is that it provides a framework for investigating the DHC characteristics of arbitrary hydride distributions, using readily available techniques.

Validation of Weight Function Based Process-Zone Model for Evaluation of Delayed Hydride Cracking Initiation in Zr-2.5Nb Pressure Tubes

2016 ◽  
Author(s):  
Steven X. Xu ◽  
Dennis Kawa ◽  
Jun Cui ◽  
Heather Chaput
Keyword(s):  
Weight Function ◽  
Process Zone ◽  
Matrix Material ◽  
Pressure Tube ◽  
Zone Model ◽  
Hydrogen Atoms ◽  
Fuel Bundle ◽  
Pressure Tubes ◽  
Delayed Hydride Cracking ◽  
Hydride Cracking

In-service flaws in cold-worked Zr-2.5 Nb pressure tubes in CANDU(1) reactors are susceptible to a phenomenon known as delayed hydride cracking (DHC). The material is susceptible to DHC when there is diffusion of hydrogen atoms to a service-induced flaw, precipitation of hydrides on appropriately oriented crystallographic planes in the zirconium alloy matrix material, and development of a hydrided region at the flaw tip. The hydrided region could then fracture to the extent that a crack forms and DHC is said to have initiated. Examples of in-service flaws are fuel bundle scratches, crevice corrosion marks, fuel bundle bearing pad fretting flaws, and debris fretting flaws. These flaws are volumetric in nature. Evaluation of DHC initiation from the flaw is a requirement of Canadian Standards Association (CSA) Standard N285.8. This paper describes the validation of the weight function based process-zone model for evaluation of pressure tube flaws for DHC initiation. Validation was performed by comparing the predicted threshold load levels for DHC initiation with the results from DHC initiation experiments on small notched specimens. The notches in the specimens simulate axial in-service flaws in the pressure tube. The validation was performed for both un-irradiated and pre-irradiated pressure tube material.

The Mode I Elastic Stress Distribution Near the Root of a Deep Notch With a Parabolic Root Profile

2007 ◽  
Author(s):  
E. Smith
Keyword(s):  
Stress Distribution ◽  
Elastic Stress ◽  
Notch Root ◽  
Peak Stress ◽  
Fracture Initiation ◽  
Radius Of Curvature ◽  
Mode I ◽  
Mode Iii ◽  
Root Radius ◽  
Notch Root Radius

The basis of a fracture mechanics type methodology for fracture initiation at a blunt notch is the elastic stress distribution immediately ahead of a notch root. Earlier work, presented at the 2006 ASME PVP Conference [7], for the Mode III deformation of a deep notch with a parabolic root profile, has shown that the shear stress σ(x) at a distance x ahead of the root is uniquely defined by the peak stress σp, x and ρ (the notch root radius of curvature), when x and ρ are both small compared with the notch depth, irrespective of the loading characteristics and the notch shape away from the root. This paper is concerned with the corresponding Mode I deformation situation, with the objective of providing support for the viability of a similar conclusion for Mode I deformation.

The Peak Stress Method applied to fatigue assessments of steel tubular welded joints subject to mode-I loading

2010 ◽  
Vol 77 (11) ◽  
pp. 2100-2114 ◽  
Author(s):  
Giovanni Meneghetti ◽  
Bruno Atzori ◽  
Giampiero Manara
Keyword(s):  
Welded Joints ◽  
Peak Stress ◽  
Mode I ◽  
Mode I Loading
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