Emerging Technology in Fitness-for-Service Assessment of Crack-Like Flaws

This paper describes several recent advances in crack assessment technology that have been or will be incorporated into the API 579 fitness-for-service standard. Four technology areas are addressed herein: • Stress intensity factor solutions. The 2016 edition of API 579 contains an extensive library of stress intensity solutions inferred from 3D finite element analysis. • A new equation for fitting elastic-plastic J solutions. A parametric equation that captures the elastic, fully-plastic and contained-yielding regimes of deformation provides an alternative definition of reference stress in the failure assessment diagram (FAD) method. • An enhanced constraint adjustment. A future edition of API 579 will include an improved version of the Wallin methodology for shifting the Master Curve reference temperature to account for constraint effects. • A procedure to account for non-ideal crack profiles. Most crack assessment methods assume an idealized flaw shape such as semi-elliptical, but many real-world flaws have complex shapes.

Effects of Flaw Shape (Idealization) on the Interaction of Co-Planar Surface Flaws

Engineering Critical Assessment (ECA) guidelines contain amongst others, rules to assess flaw interaction. Major flaw dimensions (depth or height and length) are typically characterized assuming the flaws to be contained entirely within a bounding rectangle through a procedure known as flaw idealization. In (computational) fracture mechanics based calculations, flaws are often assumed to be (semi-)elliptical. This paper investigates the interaction between identical co-planar surface breaking flaws. Two flaw shapes are considered and compared: “canoe-shaped” (quarter-circular ends and constant depth elsewhere) and semi-elliptical. Especially for long shallow flaws, the canoe-shaped approximates the bounding rectangle, whereas the semi-elliptical shape only touches the bounding rectangle at three points (deepest point and two points at the surface). Several flaw dimensions and spacing distances are studied through an extensive parametric study comprising elastic and elastic-plastic finite element simulations. The results, based on Stress Intensity Factor (SIF) and J-integral analysis, show how the flaw shape can affect the degree of interaction. Notably, the inconsistency is less in linear-elastic analysis, but becomes more pronounced at higher (elastic-plastic) loading levels. This work highlights a challenge of comparing analytical and numerical based evaluations of interaction with ECA guidelines.

Numerical Validations of Flaw Shape Idealization Methods to Burst Pressure Estimations of Steam Generator Tube With Axial Surface Flaws

This work investigates the applicability of the flaw shape idealization methods to carry out the structural integrity assessment of steam generator (SG) tubes under internal pressure with complicated axial inner and outer surface flaws that were typically found during the in-service-inspection (ISI). In terms of flaw shape, three different shapes of flaws which can be detected during an actual ISI are considered, i.e., long symmetric flaw, asymmetric inclined flaw and narrow, symmetric deep flaw. As for flaw shape idealization methods for the predictions of burst pressures of these flaws, four different flaw shape idealization models, i.e., semi-elliptical, rectangular, maximum length with effective flaw depth and weakest subcrack model proposed by the Electric Power Research Institute (EPRI) are employed in this work. In order to validate the applicability of these idealization methods, the burst pressures of SG tubes with these flaws are investigated by using the finite element (FE) analyses. By comparing the predictions of the burst pressures based on the four different flaw shape idealization methods with those based on actual flaw shapes, it is found that the weakest subcrack model proposed by the EPRI and maximum length with effective flaw depth model provide the better agreement with actual complex flaw.

Analytical Assessment of the Remaining Strength of Corroded Pipelines and Comparison With Experimental Criteria

Recently published analytical solutions for the remaining strength of a pipeline with narrow axial and axisymmetric volumetric flaws are described in this paper, and their experimental and numerical validation are reviewed. Next, the domains of applicability of each solution are studied, some simplifications suitable to steel pipelines are introduced, and an analytical model for the remaining strength of corroded steel pipelines is presented. This analytical solution is compared with the standards most widely used in the industry for assessment of corroded pipelines: ASME B31G, modified ASME, and DNV RP-F101. The empirical and analytical solutions are compared with respect to their most relevant parameters: critical (or flow) stress, flaw geometry parameterization, and Folias or bulging factor formulation. Finally, two common pipeline steels, API 5L grades X42 and X100, are selected to compare the different corrosion assessment methodologies. Corrosion defects of 75%, 50%, and 25% thickness reduction are evaluated. None of the experimental equations take into account the strain-hardening behavior of the pipe material, and therefore, they cannot properly model materials with very dissimilar plastic behavior. The comparison indicates that the empirical methods underestimate the remaining strength of shallow defects, which might lead to unnecessary repair recommendations. Furthermore, it was found that the use of a parameter employed by some of the empirical equations to model the assumed flaw shape leads to excessively optimistic and nonconservative results of remaining strength for long and deep flaws. Finally, the flaw width is not considered in the experimental criteria, and the comparative results suggest that the empirical solutions are somewhat imprecise to model the burst of wide flaws.

On a Flaw Shape Idealization of Axial Surface Flaws for Structural Integrity Assessment of Steam Generator Tubes

Nowadays, nuclear power plant (NPP) has become one of the most important energy sources to generate electricity in the world. Steam generator (SG) is a heat exchanger included in primary system of NPP. Alloy 600 MA is widely used for SG tube material and this is well-known as weakness of stress corrosion cracking. In recent year, according to increase the number of long-term operation NPP, many axial surface flaws have been found on SG tube during an in-service inspection. Therefore, many researches have been carried out to maintain structural integrity of SG tube. Commonly, flaw shape needs to be idealized to calculate a burst pressure because detected flaw shape is complicated. In this paper, validation of EPRI’s weakest sub-crack model, one of the well-known flaw idealization rule, is conducted through finite element (FE) analysis. For this, three actual flaws are assumed and these are idealized by using four flaw shape idealization methods; semi-elliptical crack model, rectangular crack model, maximum length with effective depth crack model and weakest sub-crack model. Burst pressure of each model is calculated and compared with burst pressure of actual shape crack model. As a result, if actual flaw is idealized by weakest sub-crack model, it is expected that conservative and efficient structural integrity assessment will be possible.

Applying Ultrasonic Testing to Detect Hole Defect Near the Surface

During materials manufacturing process will cause defects occasionally, if the defects located at near surface of material where the testing is not easy to implement. When the defects was stress by outside loading, it will grow up even become fracture, if the material apply ultrasonic testing then can increase material security to ensure structural safety. Practice material defect shape is variable and need complex product procedure. In this study, using wire cutting method to make practice defects specimen instead of the real flaws material, the specimen was made of medium carbon steel and aluminum alloy, defects away from the surface is 2-4 mm, defect shape including round shape hole and square shape hole, bore diameter was 1-3 mm. Using ultrasonic straight beam probe and delay line probe, straight beam probe have 5 MHz and 10 MHz frequency, and delay line probe have 5 MHz, 10 MHz frequency too, finally, compared the relationship between the accuracy and depth of flaws, pore size, flaw shape, material of specimen. Research results demonstrate that accuracy didn’t relate to the flaw shape, flaw size, depth of flaws and material of specimen. The accurately of 10 MHz delayed probe shown the depth of flaw smaller, the measurement than other probes.

Experimental Study on Damage Diagnosis of Reinforced Concrete Beam with Acoustic Emission

The concretes non-destructive inspection technology take does not harm the concretes component's operational performance as the premise, the application many kinds of physics principles and the chemical phenomenon, carries on the effective examination and the test to the structural element, in order to appraises their continuity, the integrity, the security reliability and certain physical property, including is examined in the material and the component whether to have the flaw, and to the flaw shape, the size, the position, the orientation, the distribution and situations and so on content carries on the judgment. The acoustic emission technology is one kind of dynamic non-destructive inspection technology, moreover sound transmitting message from flaw itself, therefore, may judge the flaw with the sofar efflux the gravity. A similar size, uniform quality flaw, when they receive the stress condition and locates when the position is different, is also different to the structure extent of damage, therefore their acoustic emission characteristic also has the difference.