R5 Volume Research Articles

Probabilistic structural integrity: methodology and case-study in the creep regime

ABSTRACTThis paper presents a methodology for conducting probabilistic assessments for structural integrity applications based on the Monte Carlo method. This starts with the definition of the underlying procedure for assessing the failure mechanisms of interest, followed by the statistical modelling of the key input parameters, leading to the estimation of desired probabilities. A case-study assessing a plant component (the tubeplate) for creep-fatigue crack initiation using the R5 Volume 2/3 procedure was conducted to provide context and demonstrate the utilities of implementing a probabilistic framework. Expanding on previous work, this paper highlights the main areas of focus for the proposed probabilistic methodology: probabilistic representation of input parameters, correlations, treatment of loading uncertainties, conducting post-assessment sensitivity analyses, the extrapolation of assessment location probabilities to component-level and, thereafter, population-level estimates. Through presenting a case-study implementing the full probabilistic methodology, the aim is to promote wider application and acceptance within the international structural integrity community, and further development of the methodology and constituent methods.

Glottal Aperture and Buccal Airflow Leaks Critically Affect Forced Oscillometry Measurements

The forced oscillation technique (FOT) measures respiratory resistance and reactance; however, the upper airways may affect the results. We quantified the impact of glottal aperture and buccal air leaks. In the glottal aperture study (1) 10 healthy subjects (aged 34 ± 2 years) performed a total lung capacity maneuver followed by 10-s breath-hold with and without total glottal closure and (2) the effects of humming (incomplete glottal narrowing) on FOT measurements were studied in six healthy subjects. Glottal narrowing was confirmed by direct rhinolaryngoscopy. In the air leak study, holes of increasing diameter (3.5, 6.0, and 8.5 mm) were made to the breathing filters. Eleven healthy subjects (aged 33 ± 2 years) and five patients with COPD (aged 69 ± 3 years) performed baseline FOT measurements with the three modified filters. Narrow glottal apertures and humming generated whole-breath resistance at 5 Hz (R5) peaks, increased R5 (1.49 ± 0.37 kPa/L/s vs 0.34 ± 0.01 kPa/L/s, P < .001), and decreased whole-breath reactance at 5 Hz (X5) values (-2.10 ± 0.51 kPa/L/s vs -0.09 ± 0.01 kPa/L/s, P < .001). The frequency dependency of resistance was increased. Holes in the breathing filters produced indentations on the breathing trace. Even the smaller holes reduced R5 in healthy subjects (0.33 ± 0.02 to 0.24 ± 0.02 kPa/L/s, P < .01) and patients with COPD (0.50 ± 0.04 to 0.41 ± 0.04 kPa/L/s, P < .05), whereas X5 became less negative (from -0.09 ± 0.01 to -0.05 ± 0.01 in healthy subjects, P < .01; from -0.22 ± 0.06 to -0.11 ± 0.03 kPa/L/s in patients with COPD, P < .05). Visual inspection of the data is required to exclude glottal narrowing and buccal air leaks identified as R5 peaks and volume indentations, respectively, because these significantly affect FOT measurements.

Creep Fatigue Crack Initiation Assessment for Weldment Joint Using the Latest R5 Assessment Procedure

Boiler tube to tubeplate welds are often seen in the power generation industry. In the nuclear power industry, especially in Advanced Gas-Cooled Reactors, the welds are exposed to long term creep temperature and are subjected to fatigue cycles due to start-up and shutdown. The creep-fatigue crack initiation life assessment for the weld is required for the purposes of ensuring of safe operation and life extension. This paper presents the detailed assessment methodologies to perform a creep-fatigue crack initiation assessment using the latest R5 volume 2/3 (Issue 3 Revision 002, 2014) for the boiler superheater header tubeplate to tube Type 316 welds. The key changes of the revised R5 procedure are also outlined in the paper.

An investigation of effects of welding residual stresses on creep crack growth for a low alloy butt weld

R5 is an assessment procedure for the high temperature response of structures and R5 Volume 7 specifies the creep crack growth assessment for low alloy welds. In power plants, almost all welds operating in creeping regime have been post weld heat treated (PWHT). However, low levels of welding residual stresses may still exist. It is known that the welding residual stresses in weldments can be difficult to estimate, as their magnitude and distribution depend on materials, geometry, weld procedure and heat treatment.For high temperature low alloy steel, CrMoV, butt welds, R5 Volume 7 suggests that a bending welding residual stress of 60 MPa should be assumed for a circumferential defect following a post weld heat treatment. For conservatism, the relaxation of this low level of the welding residual stress in a service component operating at high temperatures for a long period time has been discounted in R5. As the actual welding residual stresses can be difficult to estimate, it is necessary to carry out a detailed investigation for the effects of the welding residual stresses on the creep crack growth and a number of associated crack growth parameters.In this paper, for a typical low alloy steel butt weld in ½Cr½Mo¼V (‘½CMV’) pipework with 2¼Cr1Mo weld metal under a given set of internal pressure, temperature and system moment, a series of creep crack growth analyses have been conducted using the R5 Volume 7 assessment procedure, assuming the welding residual stresses of 10 MPa–100 MPa and three different crack initiation times. A postulated service-initiated internal circumferential defect (semi-elliptical surface-breaking surface defect) of 2.3 mm deep by 15 mm long has been used.The investigation results are presented by a series of graphs which show the effects of the welding residual stresses and different crack initiation times on the creep crack growth for the weld studied. After a detailed discussion, conclusions can be drawn.

Derivation of shakedown factors (Ks) for Type 316 and Type 321 stainless steel at high temperature

The shakedown factor, Ks, is defined in R5 Volume 2/3 as the factor applied to the minimum 0.2% proof stress, Sy, in order to obtain the material ratchet limit, in terms of the maximum stress level at which the materials exhibits cyclically stable behaviour. This experimentally derived factor is used in the R5 assessment procedure to perform simple checks for shakedown and also when a more detailed shakedown analysis is required. In addition, it is also used to determine the start-of-dwell stress as part of a creep-fatigue damage assessment.This paper presents the derivation of this shakedown factor, Ks, from a number of cyclic load-controlled tests, which were carried out on several casts of Types 316H and 321 steel between room temperature and 650°C. These tests involved subjecting uniaxial specimens to blocks of continuous load controlled cycles, the stress range remaining constant during each block, but then being increased until material cyclic instability was observed. Since the type of cycle was expected to be related to reactor trip or fault conditions, and only a few hundred were expected in the lifetime of the plant, the tests were based on blocks of 500 repeated cycles. Tensile tests were also carried out on the same materials in order to provide values of the 0.2% and 1% proof stress which are used to derive the Ks parameter. The Ks factor is sensitive to the strain rate used in the tensile tests.Finally, the technique used to evaluate the shakedown factor is discussed, along with possible modifications to the R5 methodology to improve the representation of stress-strain cycles at elevated temperatures.

Recent developments in the R5 procedures for assessing the high temperature response of structures

The R5 procedures have been developed within the UK power generation industry to assess the integrity of nuclear and conventional plant operating at high temperatures. Within R5, there are specific procedures for assessing creep-fatigue crack initiation in initially defect-free components (Volume 2/3) and for assessing components containing defects (Volume 4/5).The current R5 Volume 2/3 procedure for assessing creep-fatigue initiation in weldments involves analysis assuming homogeneous parent material properties with a single Fatigue Strength Reduction Factor (FSRF) used to account for both the reduction in fatigue endurance and enhancements in strain due to material mismatch and local geometry effects. A new development of this approach involves splitting the FSRF into a Weldment Endurance Reduction (WER), which accounts for reduced fatigue endurance due to weld imperfections, and a Weldment Strain Enhancement Factor (WSEF), which accounts for material mismatch and local geometry effects. The new approach results in less conservative predictions of creep damage than are obtained using the existing approach as it is only the WSEF that affects the stress at the start of the creep dwell rather than the larger FSRF in the existing approach. However, the new approach has little effect on the calculated fatigue damage as the combined effects of the WER and the WSEF are essentially equivalent to the previous FSRF.In addition to methods for predicting creep and creep-fatigue crack growth for load-controlled situations, R5 Volume 4/5 gives advice on the prediction of crack growth for situations involving combined primary and secondary loading. This advice has recently been extended to cover cases involving significant amounts of crack growth, C(t) estimation methods for elastic-plastic-creep behaviour and explicit advice on the treatment of combined loading for situations involving significant welding residual stresses. This extended advice can be used to predict the growth of defects in non-stress relieved austenitic weldments operating at elevated temperatures.This paper briefly outlines the current R5 Volume 2/3 and Volume 4/5 procedures and then focuses on these two significant recent developments in R5, both of which are highly relevant to real instances of creep and creep-fatigue cracking that have been observed in high temperature austenitic plant components.

Derivation of shakedown factors (Ks) for Type 316 and Type 321 stainless steel at high temperature

The shakedown factor, Ks, is defined in R5 Volume 2/3 as the factor applied to the minimum 0.2% proof stress, Sy, in order to obtain the material ratchet limit, in terms of the maximum stress level at which the materials exhibits cyclically stable behaviour. This experimentally derived factor is used in the R5 assessment procedure to perform simple checks for shakedown and also when a more detailed shakedown analysis is required. In addition, it is also used to determine the start-of-dwell stress as part of a creep-fatigue damage assessment.This paper presents the derivation of this shakedown factor, Ks, from a number of cyclic load-controlled tests, which were carried out on several casts of Types 316H and 321 steel between room temperature and 650°C. These tests involved subjecting uniaxial specimens to blocks of continuous load controlled cycles, the stress range remaining constant during each block, but then being increased until material cyclic instability was observed. Since the type of cycle was expected to be related to reactor trip or fault conditions, and only a few hundred were expected in the lifetime of the plant, the tests were based on blocks of 500 repeated cycles. Tensile tests were also carried out on the same materials in order to provide values of the 0.2% and 1% proof stress which are used to derive the Ks parameter. The Ks factor is sensitive to the strain rate used in the tensile tests.Finally, the technique used to evaluate the shakedown factor is discussed, along with possible modifications to the R5 methodology to improve the representation of stress-strain cycles at elevated temperatures.

Analysis of cross-weld creep rupture data on the ½Cr½Mo¼V type IV zone

For the ferritic steel ½Cr½Mo¼V, the operating experience of medium and long term creep failures in power station welds has been of cavitation and cracking in the heat affected zone (HAZ) adjacent to the parent material, in what is referred to as the Type IV zone. This experience has led to the generation of uniaxial cross-weld creep rupture data on ½CrMoV weldments and to the development of life assessment procedures such as R5 Volume 7, which uses the rupture strength of the Type IV zone to calculate the life of power station components. Recently, the ECCC Working Group 3A has conducted a collation of the UK and German cross-weld creep rupture data on ½CrMoV weldments. Creep failure in ½CrMoV cross-weld specimens occurs in a variety of weldment zones; typically in the parent, the Type IV zone or the weld metal. Post test examination of the specimens has enabled those tests that failed in the Type IV zone to be identified and a creep rupture data assessment has been performed to derive a new model for the rupture strength of the Type IV zone.

Use of the R5 Volume 4/5 procedures to assess creep–fatigue crack growth in a 316L(N) cracked plate at 650 °C

The R5 procedures have been developed within the UK power generation industry to assess the integrity of nuclear and conventional plant operating at high temperatures. Within R5, Volume 4/5 describes the procedures for assessing components containing defects under creep and creep–fatigue loading. This paper describes the use of these procedures to assess creep–fatigue crack growth in a 316L(N) cracked plate tested at 650 °C. It is shown that the R5 Volume 4/5 predictions of crack growth are in close agreement with the experimental observations.

Behaviour of single angle beam-column element subjected to fatigue loading: Knight, G.M.S. and Santhakumar, A.R. Conf. Fatigue and Fracture in Steel and Concrete Structures, ISFF '91, Madras, India, Dec 1991 Vol 2 (Oxford & IBH Publishing Co. Pvt. Ltd, New Delhi, India, 1991) 867–881

R5 is an assessment procedure for the high temperature response of structures and R5 Volume 7 specifies the creep crack growth assessment for low alloy welds. In power plants, almost all welds operating in creeping regime have been post weld heat treated (PWHT). However, low levels of welding residual stresses may still exist. It is known that the welding residual stresses in weldments can be difficult to estimate, as their magnitude and distribution depend on materials, geometry, weld procedure and heat treatment.For high temperature low alloy steel, CrMoV, butt welds, R5 Volume 7 suggests that a bending welding residual stress of 60 MPa should be assumed for a circumferential defect following a post weld heat treatment. For conservatism, the relaxation of this low level of the welding residual stress in a service component operating at high temperatures for a long period time has been discounted in R5. As the actual welding residual stresses can be difficult to estimate, it is necessary to carry out a detailed investigation for the effects of the welding residual stresses on the creep crack growth and a number of associated crack growth parameters.In this paper, for a typical low alloy steel butt weld in ½Cr½Mo¼V (‘½CMV’) pipework with 2¼Cr1Mo weld metal under a given set of internal pressure, temperature and system moment, a series of creep crack growth analyses have been conducted using the R5 Volume 7 assessment procedure, assuming the welding residual stresses of 10 MPa–100 MPa and three different crack initiation times. A postulated service-initiated internal circumferential defect (semi-elliptical surface-breaking surface defect) of 2.3 mm deep by 15 mm long has been used.The investigation results are presented by a series of graphs which show the effects of the welding residual stresses and different crack initiation times on the creep crack growth for the weld studied. After a detailed discussion, conclusions can be drawn.