In fracture mechanics, a flaw behavior in pressure vessels is assessed with respect to the material fracture toughness. Fracture toughness which most Fitness-for-Service (FFS) codes relies on, only considers mode-I crack opening. However, in presence of tilted flaws, like quasi-laminar hydrogen flakes, this mode-I toughness may be too severe, and a mixed mode I+II fracture toughness seems to be more appropriate. In order to address the assessment of the fracture toughness curve, mixed mode I+II tests were performed by the authors on ferritic steel samples by adjusting the standard mode I CT specimen geometry to a geometry subjected to mixed mode I+II. Then, XFEM simulations of the mixed mode tests were performed in order to calculate the J-integral along the crack front. Based on tests and calculations results, the paper explains how the authors work towards proposing a method to measure the material fracture toughness in case of flaws subjected to mixed mode (I+II) loading.
{"title":"Towards a Process for the Assessment of Mixed Mode I+II Fracture Toughness","authors":"Afaf Bouydo, V. Lacroix, R. Chaouadi, V. Mareš","doi":"10.1115/PVP2018-84575","DOIUrl":"https://doi.org/10.1115/PVP2018-84575","url":null,"abstract":"In fracture mechanics, a flaw behavior in pressure vessels is assessed with respect to the material fracture toughness.\u0000 Fracture toughness which most Fitness-for-Service (FFS) codes relies on, only considers mode-I crack opening. However, in presence of tilted flaws, like quasi-laminar hydrogen flakes, this mode-I toughness may be too severe, and a mixed mode I+II fracture toughness seems to be more appropriate.\u0000 In order to address the assessment of the fracture toughness curve, mixed mode I+II tests were performed by the authors on ferritic steel samples by adjusting the standard mode I CT specimen geometry to a geometry subjected to mixed mode I+II. Then, XFEM simulations of the mixed mode tests were performed in order to calculate the J-integral along the crack front.\u0000 Based on tests and calculations results, the paper explains how the authors work towards proposing a method to measure the material fracture toughness in case of flaws subjected to mixed mode (I+II) loading.","PeriodicalId":128383,"journal":{"name":"Volume 1A: Codes and Standards","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-07-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"129240286","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Cr-Mo and Ni-Cr-Mo high-strength low-alloy steels are candidate materials for the storage of high-pressure hydrogen gas. Forging materials of these steels have been used for such an environment, while there has been a strong demand for a higher performance material with high resistance to hydrogen embrittlement at lower cost. Thus, mechanical properties of Cr-Mo and Ni-Cr-Mo steels made of quenched and tempered seamless pipes in high-pressure hydrogen gas up to 105 MPa were examined in this study. The mechanical properties were deteriorated in the presence of hydrogen that appeared in reduction in local elongation, decrease in fracture toughness and accelerated fatigue-crack growth rate, although the presence of hydrogen did not affect yield and ultimate tensile strengths and made little difference to the fatigue endurance limit. It is proposed that pressure vessels for the storage of gaseous hydrogen made of these seamless line pipe steels can be designed.
{"title":"Hydrogen Compatibility and Suitability of (Ni)-Cr-Mo High-Strength Low-Alloy Seamless Line Pipe Steels for Pressure Vessels for Hydrogen Storage","authors":"A. Nagao, N. Ishikawa, T. Takano","doi":"10.1115/PVP2018-84726","DOIUrl":"https://doi.org/10.1115/PVP2018-84726","url":null,"abstract":"Cr-Mo and Ni-Cr-Mo high-strength low-alloy steels are candidate materials for the storage of high-pressure hydrogen gas. Forging materials of these steels have been used for such an environment, while there has been a strong demand for a higher performance material with high resistance to hydrogen embrittlement at lower cost. Thus, mechanical properties of Cr-Mo and Ni-Cr-Mo steels made of quenched and tempered seamless pipes in high-pressure hydrogen gas up to 105 MPa were examined in this study. The mechanical properties were deteriorated in the presence of hydrogen that appeared in reduction in local elongation, decrease in fracture toughness and accelerated fatigue-crack growth rate, although the presence of hydrogen did not affect yield and ultimate tensile strengths and made little difference to the fatigue endurance limit. It is proposed that pressure vessels for the storage of gaseous hydrogen made of these seamless line pipe steels can be designed.","PeriodicalId":128383,"journal":{"name":"Volume 1A: Codes and Standards","volume":"5 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2018-07-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"116819328","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Morley, Marius Twite, N. Platts, A. Mclennan, C. Currie
High temperature water environments typical of LWR operation are known to significantly reduce the fatigue life of reactor plant materials relative to air environments in laboratory studies. This environmental impact on fatigue life has led to the issue of US-NRC Regulatory Guide 1.207 [1] and supporting document NUREG/CR-6909 [2] which predicts significant environmental reduction in fatigue life (characterised by an environmental correction factor, Fen) for a range of actual and design basis transients. In the same report, a revision of the fatigue design curve for austenitic stainless steels and Ni-Cr-Fe alloys was proposed [2]. This was based on a revised mean curve fit to laboratory air data and revised design factors to account for effects not present in the test database, including the effect of rough surface finish. This revised fatigue design curve was endorsed by the NRC for new plant through Regulatory Guide 1.207 [1] and subsequently adopted by the ASME Boiler and Pressure Vessel (BPV) Code [3]. Additional rules for accounting for the effect of environment, such as the Fen approach, have been included in the ASME BPV Code as code cases such as Code Case N-792-1 [4]. However, there is a growing body of evidence [5] [6] [7] and [8] that a rough surface condition does not have the same impact in a high temperature water environment as in air. Therefore, application of Fen factors with this design curve may be unduly conservative as it implies a simple combination of the effects of rough surface and environment rather than an interaction. Explicit quantification of the interaction between surface finish and environment is the aim of a number of recent proposals for improvement to fatigue assessment methods, including a Rule in Probationary Phase in the RCC-M Code and a draft Code Case submitted to the ASME BPV Code as described in References [9] and [10]. These approaches aim to quantify the excessive conservatism in current methods due to this unrecognised interaction, describing this as an allowance for Fen effectively built into the design curve. A number of approaches in various stages of development and application are discussed further in a separate paper at this conference [11]. This paper reports the results of an extensive programme of strain-controlled fatigue testing, conducted on two heats of well-characterised 304-type material in a high-temperature simulated PWR environment by Wood plc. The baseline behaviour in environment of standard polished specimens is compared to that of specimens with a rough surface finish bounding normal plant component applications. The results reported here substantially add to the pool of data supporting the conclusion that surface finish effects in a high-temperature water environment are significantly lower than the factor of 2.0 to 3.5 assumed in construction of the current ASME III fatigue design curve. This supports the claim made in the methods discussed in [9] [10] and [11] that the fati
{"title":"Effect of Surface Condition on the Fatigue Life of Austenitic Stainless Steels in High Temperature Water Environments","authors":"A. Morley, Marius Twite, N. Platts, A. Mclennan, C. Currie","doi":"10.1115/PVP2018-84251","DOIUrl":"https://doi.org/10.1115/PVP2018-84251","url":null,"abstract":"High temperature water environments typical of LWR operation are known to significantly reduce the fatigue life of reactor plant materials relative to air environments in laboratory studies. This environmental impact on fatigue life has led to the issue of US-NRC Regulatory Guide 1.207 [1] and supporting document NUREG/CR-6909 [2] which predicts significant environmental reduction in fatigue life (characterised by an environmental correction factor, Fen) for a range of actual and design basis transients. In the same report, a revision of the fatigue design curve for austenitic stainless steels and Ni-Cr-Fe alloys was proposed [2]. This was based on a revised mean curve fit to laboratory air data and revised design factors to account for effects not present in the test database, including the effect of rough surface finish. This revised fatigue design curve was endorsed by the NRC for new plant through Regulatory Guide 1.207 [1] and subsequently adopted by the ASME Boiler and Pressure Vessel (BPV) Code [3]. Additional rules for accounting for the effect of environment, such as the Fen approach, have been included in the ASME BPV Code as code cases such as Code Case N-792-1 [4].\u0000 However, there is a growing body of evidence [5] [6] [7] and [8] that a rough surface condition does not have the same impact in a high temperature water environment as in air.\u0000 Therefore, application of Fen factors with this design curve may be unduly conservative as it implies a simple combination of the effects of rough surface and environment rather than an interaction. Explicit quantification of the interaction between surface finish and environment is the aim of a number of recent proposals for improvement to fatigue assessment methods, including a Rule in Probationary Phase in the RCC-M Code and a draft Code Case submitted to the ASME BPV Code as described in References [9] and [10]. These approaches aim to quantify the excessive conservatism in current methods due to this unrecognised interaction, describing this as an allowance for Fen effectively built into the design curve. A number of approaches in various stages of development and application are discussed further in a separate paper at this conference [11].\u0000 This paper reports the results of an extensive programme of strain-controlled fatigue testing, conducted on two heats of well-characterised 304-type material in a high-temperature simulated PWR environment by Wood plc. The baseline behaviour in environment of standard polished specimens is compared to that of specimens with a rough surface finish bounding normal plant component applications. The results reported here substantially add to the pool of data supporting the conclusion that surface finish effects in a high-temperature water environment are significantly lower than the factor of 2.0 to 3.5 assumed in construction of the current ASME III fatigue design curve. This supports the claim made in the methods discussed in [9] [10] and [11] that the fati","PeriodicalId":128383,"journal":{"name":"Volume 1A: Codes and Standards","volume":"128 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2015-07-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133868425","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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