Allowable Cracks Related to Penetration for Part-Through Cracks in Pipes Subjected to Bending Stresses

Fully plastic collapse stresses for circumferentially part-through cracked pipes subjected to bending stresses are estimated by Limit Load Criteria provided by the ASME Code Section XI. Allowable crack depths were determined by using the Limit Load Criteria and that are tabulated in the ASME Code Section XI for different plant service level conditions. On the other hand, crack penetration bending stresses for part-through cracked pipes were estimated by using the Local Approach of Limit Load Criteria. By using these Criteria, the study presented in this paper obtained allowable crack depths at penetration for circumferentially part-through cracked pipes. Comparing the allowable crack depths obtained by both methods for each service level, it is evident that the allowable crack depths at penetration calculated by the Local Approach of Limit Load Criteria are almost always smaller than those at fully plastic collapse stresses calculated by the Limit Load Criteria. It was found that the allowable crack depths provided by the ASME Code Section XI are less conservative for crack penetrations.

CC(T) Specimen Load-Bearing Capacity Related to Yield Strength and Upper-Shelf Charpy-V Energy

The load-bearing capacity of a CC(T) specimen (Center-Cracked Tension) in the ductile fracture regime is usually controlled by plastic collapse. If the material’s tearing resistance is sufficiently low, the load-bearing capacity can drop below the plastic collapse value. Here, a recently developed simple fracture mechanics-based Charpy-V impact energy criterion for plastic collapse was used to provide a best estimate assessment of the CC(T) specimen load-bearing capacity.

A Bayesian updating of crack distributions in steam generator tubes

The structural failure of steam generator tubes is a common problem that can a ect the availability and safety of nuclear power plants. To minimize the probability of occurrence of failure, it is needed to implement maintenance strategies such as periodic nondestructive inspections of tubes. Thus, a tube is repaired or plugged whenever it has detected a crack which a threshold size is overtaken. In general, uncertainties and errors in crack sizes are associated with the nondestructive inspections. These uncertainties and errors should be appropriately characterized to estimate the actual crack distribution. This work proposes a Bayesian approach for updating crack distributions, which in turn allows computing the failure probability of steam generator tubes at current and future times. The failure criterion is based on plastic collapse phenomenon, and the failure probability is computed by using the Monte-Carlo simulation. The failure probability at current and future times is in good agreement with the ones presented in the literature.

Improvement and Assessment of the Plastic Collapse Bending Moment Equations in Circumferentially Cracked Pipe

The plastic collapse bending moment in a pipe cross-section with a circumferential crack is defined in ASME B&PV Code Section XI, Appendix C using simplified equilibrium equations by approximating the pipe mean radius Rm and the neutral axis angle β. In previous papers it was demonstrated by the authors that, for externally cracked pipes, those simplified equilibrium equations are not conservative and hence improved equations were developed and proposed which account for the cracked pipe ligament mean radius Rmc. In this paper, it is demonstrated that the accuracy of the collapse bending moment equation can be refined by taking into account the neutral axis position Yna of the cracked pipe section. This leads to exact collapse bending moment equations without any approximation on the pipe mean radius Rm nor on the neutral axis angle β. In this framework, it is shown that, for externally cracked pipes, the Appendix C equations could lead to more than 20% less conservative collapse bending moment than with the exact equations. An extended finite element method analysis completes this study to assess the relevance of the model used to determine the plastic collapse bending moment.

Hydrogen Induced Cracking Damage Estimation and Evaluation

New non-destructive testing (NDT) inspection technology, quantification of damage, and tensile testing, has enabled the assessment of conservatism associated with the hydrogen induced cracking (HIC) damage parameter (DH) currently used in API 579 – 1/ASME Fitness-For-Service (FFS) Part 7. To address HIC damage from an FFS perspective, the general requirements include addressing protection against plastic collapse and protection against cracking. The focus of this body of work is only to address conservatism regarding the protections against plastic collapse. The detrimental effects of HIC damage on plastic collapse is modeled through the DH parameter, currently set at 0.8 or an 80% strength loss in HIC damage material for the Level 2 HIC Assessment Procedure. Original development of the DH parameter was based on tensile testing of HIC damaged material performed in air, where the HIC damage was not sized or quantified, and a 30% margin was added to the maximum measured reduction in tensile strength to get to the 80% strength loss. Modification of the DH parameter is allowed in the Level 3 HIC Assessment Procedure, provided supporting testing data justifying a reduction is also provided with the assessment. For the tensile testing in air, the quantified HIC damage and tensile testing results are consistent with an 80% strength loss, without an added margin. A rigorous ultrasonic testing inspection using conventional phased array ultrasonic testing (PAUT) and PAUT with full matrix capture using the total focusing method (FMC/TFM) was performed on ex-service SA-212 Grade B material. Locations with service generated HIC damage were extracted and tensile tested in air and in gaseous hydrogen. Examination of the tensile specimen fracture surfaces allowed for quantification of HIC damage associated with final tensile failure. HIC damage measured with NDT was similar to the HIC damage on the fracture surface when characterized using the crack sensitivity ratio (CSR). The hydrogen tensile testing results suggested that for material still charged with hydrogen (not currently explicitly addressed in API 579 – 1/ASME FFS Part 7), the loss in strength may be larger than 80%.

Effect of structural deformations and bend angle on the collapse load of pipe bends subjected to in-plane closing bending moment

Purpose The purpose of this study is to investigate the effect of structural deformations and bend angle on plastic collapse load of pipe bends under an in-plane closing bending moment (IPCM). A large strain formulation of three-dimensional non-linear finite element analysis was performed using an elastic perfectly plastic material. A unified mathematical solution was proposed to estimate the collapse load of pipe bends subjected to IPCM for the considered range of bend characteristics. Design/methodology/approach ABAQUS was used to create one half of the pipe bend model due to its symmetry on the longitudinal axis. Structural deformations, i.e. ovality (Co) and thinning (Ct) varied from 0% to 20% in 5% steps while the bend angle (ø) varied from 30° to 180° in steps of 30°. Findings The plastic collapse load decreases as the bend angle increase for all pipe bend models. A remarkable effect on the collapse load was observed for bend angles between 30° and 120° beyond which a decline was noticed. Ovality had a significant effect on the collapse load with this effect decreasing as the bend angle increased. The combined effect of thinning and bend angle was minimal for the considered models and the maximum per cent variation in collapse load was 5.76% for small bend angles and bend radius pipe bends and less than 2% for other cases. Originality/value The effect of structural deformations and bend angle on collapse load of pipe bends exposed to IPCM has been not studied in the existing literature.

Effect of Plastic Anisotropy on the Collapse of a Hollow Disk under Thermal and Mechanical Loading

Plastic anisotropy significantly affects the behavior of structures and machine parts. Given the many parameters that classify a structure made of anisotropic material, analytic and semi-analytic solutions are very useful for parametric analysis and preliminary design of such structures. The present paper is devoted to describing the plastic collapse of a thin orthotropic hollow disk inserted into a rigid container. The disk is subject to a uniform temperature field and a uniform pressure is applied over its inner radius. The condition of axial symmetry in conjunction with the assumption of plane stress, permits an exact analytic solution. Two plastic collapse mechanisms exist. One of these mechanisms requires that the entire disk is plastic. According to the other mechanism, plastic deformation localizes at the inner radius of the disk. Additionally, two special solutions are possible. One of these solutions predicts that the entire disk becomes plastic at the initiation of plastic yielding (i.e., plastic yielding simultaneously initiates in the entire disk). The other special solution predicts that the plastic localization occurs at the inner radius of the disk with no plastic region of finite size. An essential difference between the orthotropic and isotropic disks is that plastic yielding might initiate at the outer radius of the orthotropic disk.