Competition between surface defect and grain size under fatigue loading - ARMCO iron

Interaction between a surface defect and grain size under high cycle fatigue loading: Experimental approach for Armco iron

Determination of allowable axial flaw sizes in stainless steel pipes based on probabilistic fracture mechanics

As part of the U. S. Department of Transportation safety regulations, seamless steel cylinders that are used to transport high-pressure gases are required to be periodically retested during their lifetime [1]. The safety regulations have recently been revised to permit the use of ultrasonic methods for retesting steel cylinders. These ultrasonic test methods permit the quantitative determination of the size of any flaws that are detected in the cylinders. Therefore, to use these ultrasonic test methods it is required that quantitative, “allowable flaw sizes” be established to set acceptance/rejection limits for the cylinders at the time of retesting. Typical flaws that can occur in seamless steel cylinders during service are line corrosion, gouges, local thin areas of corrosion, notches, and cracks. To establish “allowable flaw sizes” for seamless steel cylinders, an assessment of typical flaws that occur in seamless cylinders was first carried out to establish the “critical flaw sizes” (e.g. depth and length or area) for selected types of flaws. The critical flaw size is the size of the flaw that will cause the cylinders to fail at either the designated test pressure or at the marked service pressure. The API Recommended Practice 579 “Fitness-for-Service” was used to calculate the critical flaw sizes for a range of cylinder sizes and strength levels [2]. Several hundred monotonic hydrostatic, flawed-cylinder burst tests were conducted as part of an International Standards Organization (ISO) test program to evaluate the fracture performance of a wide range of steel cylinders [3]. The results of these tests were used to verify the calculated “critical flaw sizes” that were calculated using the API 579 procedures. These results showed that the analysis conducted according to API 579 always underestimated the actual flaw sizes to cause failure at test pressure or at service pressure. Therefore, the “Fitness for Service” assessment procedures can be used reliably to establish the “critical flaw sizes” for cylinders of all sizes and strength levels. After the “critical flaw sizes” to cause failure of the cylinders at both the test pressure and the service were established, the “allowable flaw sizes” were calculated for a wide range of the cylinder types and strength levels. This was done modifying (reducing) the size of the “critical flaw sizes” for each cylinder by adjusting for fatigue crack growth that may occur during the use of the cylinder. This results in the final “allowable flaw size” criteria that are used for defining the acceptance or rejection of the cylinders during retesting. This paper presents the results of the analytical and experimental work that was performed to establish the “critical flaw sizes” and “allowable flaw sizes” for a wide range of high-pressure gas cylinders.

As part of the U.S. Department of Transportation safety regulations, seamless steel cylinders that are used to transport high-pressure gases are required to be periodically retested during their lifetime [1]. The safety regulations have recently been revised to permit the use of ultrasonic methods for retesting steel cylinders. These ultrasonic test methods permit the quantitative determination of the size of any flaws that are detected in the cylinders. Therefore, to use these ultrasonic test methods it is required that quantitative, “allowable flaw sizes” be established to set acceptance/rejection limits for the cylinders at the time of retesting. Typical flaws that can occur in seamless steel cylinders during service are line corrosion, gouges, local thin areas of corrosion, notches, and cracks. To establish “allowable flaw sizes” for seamless steel cylinders, an assessment of typical flaws that occur in seamless cylinders was first carried out to establish the “critical flaw sizes” (e.g., depth and length or area) for selected types of flaws. The critical flaw size is the size of the flaw that will cause the cylinders to fail at either the designated test pressure or at the marked service pressure. The API Recommended Practice 579 “Fitness-for-Service” was used to calculate the critical flaw sizes for a range of cylinder sizes and strength levels [2]. Several hundred monotonic hydrostatic, flawed-cylinder burst tests were conducted as part of an International Standards Organization (ISO) test program to evaluate the fracture performance of a wide range of steel cylinders [3]. The results of these tests were used to verify the calculated “critical flaw sizes” that were calculated using the API 579 procedures. These results showed that the analysis conducted according to API 579 always underestimated the actual flaw sizes to cause failure at test pressure or at service pressure. Therefore, the “Fitness for Service” assessment procedures can be used reliably to establish the “critical flaw sizes” for cylinders of all sizes and strength levels. After the “critical flaw sizes” to cause failure of the cylinders at both the test pressure and the service were established, the “allowable flaw sizes” were calculated for a wide range of the cylinder types and strength levels. This was done modifying (reducing) the size of the “critical flaw sizes” for each cylinder by adjusting for fatigue crack growth that may occur during the use of the cylinder. This results in the final “allowable flaw size” criteria that are used for defining the acceptance or rejection of the cylinders during retesting. This paper presents the results of the analytical and experimental work that was performed to establish the “critical flaw sizes” and “allowable flaw sizes” for a wide range of high-pressure gas cylinders.

This paper presents a failure assessment diagram (FAD) and crack size based comparison of the ASME BVPC Section XI Nonmandatory Appendix C and Nonmandatory Appendix H and the British Standard BS 7910:2013 Option 1 assessment methods. The Section XI appendix C evaluates the acceptability of a flaw by determining the expected failure mechanism and by comparing the flaw size with allowable flaw size limits or by comparing the applied stress to the allowable stress. The Section XI appendix H and BS7910 employ a FAD based approach that simultaneously considers brittle fracture, ductile crack extension prior to reaching the limit load and exceedance of the limit load due to the gross plasticity in the cross section. The assessment is performed by calculating the assessment point coordinates and evaluating whether the point is located on the safe side of the FAD line. The three methods are compared for simplified austenitic and ferritic pipes under internal pressure and bending loads with postulated axial and circumferential internal surface flaws. The methods are applied to generate limiting flaw size diagrams for each component under the specified loads. Additionally, the limiting flaw size results are presented in the FAD plots. To maintain comparability between the results, identical input data are used with each analysis approach but using the method-specific formulae. The performed comparison shows that most often the limiting state is governed by the 75 % flaw depth rule in Section XI article IWB-3640. The largest differences between the methods are observed for cracks with a high length to depth ratio. The difference to the tabulated allowable planar flaws in Article IWB-3514 is typically high. When increasing the applied load to values approaching the limit load, differences in the limiting flaw sizes between the methods are observed, mostly due to the different limit load models and different assumptions on the utilization of the post-yield capacity. Besides the presented flaw size comparison, the paper presents a quick tool suitable for ranking different piping segments based on failure potential and for quick scoping evaluations of indications found in inspections. The case specific scoping tool is a map of yearly flaw size lines providing the information on which flaw sizes would grow to the final limiting size in a specified timeframe.

The objective of this paper is to present the technical basis used for developing acceptance/rejection limits for seamless, high pressure gas cylinders that can be used at the time of retesting the cylinders. The development of acceptance/rejection limits for cylinders is done in three steps. First, the “critical flaw sizes” (e.g. depth and length or area) for selected types of flaws are established by an analysis procedure that has been verified by experimental tests. Next the “allowable flaw sizes” are calculated by modifying (reducing) the size of the “critical flaw sizes” for each cylinder by adjusting for fatigue crack growth that may occur during the use of the cylinder. Finally the “acceptance/rejection criteria” is established to take into account other factors such as all the expected operating conditions that the cylinders may see in service and the reliability and detectability of the specific inspection equipment to be and to adjust the “allowable flaw sizes” to provide an additional margin of safety. This acceptance/rejection limits have been incorporated in recently published ISO Technical Report TR 22694: 2008 [1]. In this work, the API 579 “Recommended Practice for Fitness-for-Service” [2] was used to calculate the “critical flaw sizes” for a range of cylinder sizes and strength levels. For this study the “critical flaw size” is defined as the size of the flaw that will cause the cylinders to fail at the test pressure of the cylinder. The results of flawed-cylinder burst tests were used to experimentally verify the calculated “critical flaw sizes”. The “allowable flaw sizes” were then calculated by using well established fatigue crack growth rate data for steel and aluminum alloys to allow for the expected amount of fatigue crack growth that may occur during the specified retesting intervals. A limited number of tests were conducted to verify the “allowable flaw size” calculations. Further adjustments are made to the “allowable flaw sizes” to define the “acceptance/rejection criteria” to be used during cylinder retesting.

The objective of this paper is to present the technical basis used for developing acceptance/rejection limits for seamless, high pressure gas cylinders that can be used at the time of retesting the cylinders. The development of acceptance/rejection limits for cylinders is done in three steps. First, the “critical flaw sizes” (e.g., depth and length or area) for selected types of flaws are established by an analysis procedure that has been verified by experimental tests. Next the “allowable flaw sizes” are calculated by modifying (reducing) the size of the critical flaw sizes for each cylinder by adjusting for fatigue crack growth that may occur during the use of the cylinder. Finally, the “acceptance/rejection criteria” is established to take into account other factors, such as all the expected operating conditions that the cylinders may see in service, and the reliability and detectability of the specific inspection equipment to be used and to adjust the allowable flaw sizes to provide an additional margin of safety. This acceptance/rejection limits have been incorporated in a recently published ISO Technical Report No. TR 22694:2008 (2007, “Gas Cylinders—Methods for Establishing Acceptance/Rejection Criteria for Flaws in Seamless Steel and Aluminum Alloy Cylinders at Time of Periodic Inspection and Requalification,” The International Standards Organization, Geneva, Switzerland, Technical Report No. 22694). In this work, the API 579 “Recommended Practice for Fitness-for-Service” (2000, API 579: Recommended Practice for Fitness-for-Service, 1st ed., American Petroleum Institute, Washington, DC) was used to calculate the critical flaw sizes for a range of cylinder sizes and strength levels. For this study, the critical flaw size is defined as the size of the flaw that will cause the cylinders to fail at the test pressure of the cylinder. The results of flawed-cylinder burst tests were used to experimentally verify the calculated critical flaw sizes. The allowable flaw sizes were then calculated by using well established fatigue crack growth rate data for steel and aluminum alloys to allow for the expected amount of fatigue crack growth that may occur during the specified retesting intervals. A limited number of tests was conducted to verify the allowable flaw size calculations. Further adjustments are made to the allowable flaw sizes to define the acceptance/rejection criteria to be used during cylinder retesting.

Fatigue Strength Evaluation of Defects Embedded in Large-Sized Stud Bolt of Marine Engine

Risers and flowlines are an integral part of deepwater oil and gas field developments around the world. Risers, which serve as the interface between floating platforms and subsea flowlines, are subjected to low-stress high-cycle fatigue loading due to platform motions and vortex induced vibration (VIV). Flowlines are increasingly required to withstand high-stress range fatigue due to high pressure and high temperature (HP/HT) conditions causing lateral buckling along the flowline. Risers and flowlines are generally made by steel tubulars which are joined by girth welds for most subsea applications. Therefore, the quality of the girth welds is critical to the fatigue performance of risers and flowlines. Fatigue design of risers and flowlines is based on the SN fatigue approach. However, that approach does not address the potential for weld flaws to affect performance. Fracture mechanics based engineering critical assessments (ECAs) provide the technical basis for Non-Destructive Evaluation (NDE) and critical flaw acceptance criteria (FAC). The FAC should address maximum allowable flaw sizes at the fabrication stage to ensure that initial girth weld flaws do not grow excessively and cause unstable fracture or through wall failure over the entire service life. Where there is variability and/or uncertainty, ECAs use conservative assumptions and safety factors. However, as HP/HT developments are becoming more common, FAC resulting from ECA tend to be smaller. The specification of FAC plays an important role in the success of the project in terms of quality, cost and schedule. A more stringent FAC will have more weld rejections, which results in slower fabrication and higher cost, or may even become too small to be detected using automatic ultrasonic testing (AUT). In addition, weld repair will adversely affect the quality and increase probability of failure as girth weld failures are often found initiated at weld repairs. The result is questions about assumptions and safety factors applied. As part of this reliability-based assessment, this paper considers two design examples to address reliability based ECA flaw acceptance criteria and safety factors of risers and flowlines. The first example is a deepwater steel catenary riser (SCR) subjected to fatigue loads due to vessel motion, wave fatigue and riser VIV. The second example is a subsea flowline subjected to thermal fatigue loads. This paper offers valuable insights into a balanced approach for inputs selection in ECA by deriving a reliability based FAC and comparing it with the approach outlined in DNV-OS-F101 (Reference 1). It demonstrates FAC can be significantly increased by using reliability based ECA, and as such it will result in faster fabrication and reducing the project cost and schedule. This is of particular interest when considering fatigue performance and life extension of risers and flowlines, asset integrity management, and their relationships with project cost and schedule. Instead of the fit-for-purpose ECA which calculates fatigue life with known girth weld flaws, this paper discusses how to determine allowable initial flaw sizes to satisfy riser and flowline fatigue requirements by deriving a probability density function of the critical flaw acceptance criteria using a reliability based Monte-Carlo approach. This paper provides an approach which is beneficial not only for detailed design but also for tendering purposes during the very early stages of projects, with less conservatism.

Piping containing flaws that exceed the Acceptance Standards of Section XI of the ASME Code is evaluated using analytical procedures described in Section XI to determine plant operability for the evaluated time period. Subarticle IWB-3640 of Section XI provides allowable axial and circumferential part-through-wall flaws determined from limit load criteria. ASME Section XI Code Case N-494-3 also provides evaluation procedures based on use of a failure assessment diagram to determine allowable flaw sizes. To understand the allowable flaw sizes determined by the limit load criteria and the failure assessment diagram procedure, anstenitic stainless steel pipes with axial part-through-wall flaws with a wide range of pipe diameters were analyzed. The allowable flaw depth based on limit load from Code Case N-494-3 was determined to be very close to that determined from IWB-3640 of Section XI, when the predicted failure mode is elastic-plastic fracture. It was found that the allowable flaw depths derived from the failure assessment diagram procedure of Code Case N-494-3, are lower, but are not significantly different, from those determined from the limit load criteria of IWB-3640. This is due to the relatively high fracture toughness that was used for the austenitic stainless steel.

When a flaw is detected in power plant piping and it is judged to be allowable by the code consensus, the plant operation can be continued without the need for repair or replacement. The benefit of instrumented indentation technique (IIT) for the analysis of allowable flaw sizes is critically assessed in this study based on experimental data and code calculations. Intrinsic standard deviations of yield and ultimate tensile strengths for Type 304 stainless steels are discussed in this paper. As a result, IIT method is capable to estimate the flow stress for Type 304 stainless steel within 5% deviation from the result of conventional tensile tests. Then, the allowable circumferential flaw sizes for pipes subjected to tensile loading are obtained based on strength properties assessed by IIT. In addition, the allowable flaw sizes are compared to the allowable flaw sizes derived from the flow stress tabulated in the ASME (American Society of Mechanical Engineers) code section II, Materials. The conclusion is drawn that IIT is a beneficial method for the Limit Load Criteria analysis.

Defect size map for nodular cast iron components with ellipsoidal surface defects based on the defect stress gradient approach

The ASME Boiler and Pressure Vessel Code Section XI provides flaw size acceptance standards for ferritic steel pressure vessels. Section XI Table IWB-3510-1 presents allowable flaw size limits in terms of flaw depth, length and vessel thickness. These flaw size limits are based on linear elastic fracture mechanics calculations that assume a brittle fracture failure mode. As yet, no allowable flaw size standards are provided in Section XI for stainless steel reactor or non-reactor pressure vessels. This paper presents allowable flaw size limits for a stainless steel pressure vessel. These limits were based on elastic plastic fracture mechanics analyses that considered limit load and ductile tearing failure modes. Although the flaw acceptance levels were developed for a specific stainless steel vessel, insights gained from this work may be useful in a general methodology for ASME Code purposes. Tabulated flaw size acceptance levels, for several aspect ratios and inspection intervals, are presented for the axial shell welds. Results show the axial seam welds were the most flaw sensitive of the various welds analyzed. The acceptable flaw sizes were limited by the ductile tearing failure mode.

This paper presents the strength and damage results based on elastic-plastic analysis to address the design feasibility of pulling in a steel catenary riser (SCR) through a pull tube with various bend configurations in a Spar. The example riser system contains an SCR of typical size, a tapered stress joint, a vertical pull tube with multiple bend sections, guide supports for the pull tube, and the associated pull head and pull chain connected to the top of the riser. The design methods discussed in the paper include: (1) Modeling of riser and pull tube in ABAQUS for strength analysis of the SCR; (2) Strain-based strain-life method to assess the associated fatigue damage; and (3) Strain-based Level 3B ECA design method to derive the critical surface flaw sizes for weld qualification of the SCR inside the pull tube. Comparisons are also presented between results derived from elastic and elastic-plastic analysis methods. The pull-in load on the example SCR increases with the water depth as well as the number and curvature of the bends on the pull tube. Calculated riser pull-in loads are about 11% to 51% higher than the submerged weight of the SCR. The elastic-plastic analysis shows small plastic zone and also small plastic strain on the example SCRs passing through pull tubes of a large bend radius of 125 ft. It also shows large plastic zone but small plastic strain on the SCR in a triple-bend pull tube with a small bend radius of 70 ft. The overall fatigue damage caused by cyclic plastic straining on the example SCRs due to pull in is lower than 3.3%. The allowable surface flaw sizes for the example SCRs are on the order of a × 2c = 8 × 10mm and 2.5 × 40mm for low aspect-ratio and high aspect-ratio surface flaws, respectively. Critical flaw sizes determined by Level 2A ECA are about 25% smaller than the flaw sizes based on Level 3B ECA for low aspect-ratio surface flaws. The specified maximum allowable flaw sizes are not very sensitive to the pull tube configuration and the water depth under the present study. The strength and damage analyses of SCR from other installation methods such as reeling are not included in this paper.

BackgroundThe size of atrial septal defects (ASDs) is typically measured using echocardiography. In contrast, computed tomography (CT) images offer superior spatial resolution, and four-dimensional (4D) CT enables visualization of the defect hole during various cardiac phases. In this study, we compared the results of 4D CT measurements of defect sizes during atrial systole (As) and atrial diastole (Ad) in 7 of 18 cases of robot-assisted ASD closure (Robotic ASD closure) with measurements obtained by other methods.Case DescriptionRobotic ASD closure was performed in 18 patients between December 2019 and April 2025. Surgery was performed under general anesthesia, with a venous line from the right internal jugular vein and right femoral vein, and an arterial line on the right femoral artery to establish extracorporeal circulation. The procedure was conducted using the DaVinci Xi® surgical system. Image analysis was performed using VINCENT®, and the size of the defect was measured during As and Ad, and calculated using the formula: long radius × short radius × π. The mean age was 60.9 years (range, 34–82 years), and 10 (55.6%) patients were male. No residual shunt flow was detected on intraoperative or postoperative echocardiography. The defect size was measurable using 4D CT in 7 cases. The area of the defect was significantly different between As [65.9–207.2 mm2, mean 134.1 mm2, interquartile range (IQR) 58.9] and Ad (113.0–490.6 mm2, mean 226.1 mm2, IQR 94.2) (P=0.04).ConclusionsUsing 4D CT, it was possible to determine the morphology and area of ASD defects in As/Ad before surgery. This is useful for selecting surgical procedures and determining patch size and may contribute to reducing cardiac arrest and surgical time. To prove that 4D CT can accurately predict the size and morphology of the defects in As/Ad, further accumulation of cases and analyses are required.