Time-of-flight diffraction method for joint with linear misalignment

The time-of-flight diffraction (TOFD) method is known as one of the most accurate flaw sizing methods among the various ultrasonic testing techniques. However, the standard TOFD method cannot be applied to weld joints with linear misalignment because of its basic assumptions. In this study, a geometric model of the TOFD method for weld joints with linear misalignment is introduced and an exact solution for calculating the flaw tip depth is derived. Since the exact solution is extremely complex, a simple approximate solution is also derived assuming that the misalignment is sufficiently small relative to the probe spacing and the flaw tip depth. The error in the approximate solution is confirmed to be negligible if the assumptions are satisfied. Numerical simulations are conducted to assess the flaw sizing accuracy of both the exact and approximate solutions considering the constraint of the probe spacing and the influence of the excess metal shape. Finally, experiments are conducted to prove the applicability of the proposed method. As a result, the proposed method is proven to enable accurate flaw sizing of weld joints with linear misalignment.

Estimates of Probability of Detection and Sizing of Flaws in Ultrasonic Time of Flight Diffraction Inspections for Complex Geometry Components With Grooved Surfaces

In a reliability assessment of ultrasonic time-of-flight diffraction (TOFD) inspection, probability of detection (POD) and sizing (POS) curves are developed. Experiments are performed on a complex geometry specimen with the grooved inspection surface simulating the gland seal area of a steam turbine rotor. In the reliability experiment, it is assumed and confirmed that the distribution of signal responses is normal. The effects of probe center spacing on detection and sizing are observed. The PODs developed here have a decreasing trend with flaw size which is in contrary to the generally observed increasing trend in conventional ultrasonic amplitude-based flaw sizing techniques. The reason for this decreasing POD with crack height is explained in the present study. The curves developed in this work are specific to the geometry and dimensions of the specimen with the set of notches and the probes used in the experiment. Hence, these curves can only be used under similar conditions. In TOFD inspection of similar type of complex shaped structures, e.g., turbine, the POD and POS curves developed here can be used in taking an appropriate engineering decision with respect to run, repair, or replace.

Advanced NDE Techniques for Reliable Crack Inspection and Fitness-for-Service Assessments

Reliable and accurate NDE (Non-Destructive Evaluation) techniques are required to conduct FFS (Fitness-For-Service) assessments for crack type flaws in equipment. FFS assessments rely on certain flaw detection and flaw sizing assumptions which can only be provided through NDE. The detection and sizing provided by the NDE work is highly dependent on both inspection methodology (e.g., operator, equipment, and procedure) as well as the specific component and application (e.g., part thickness and geometry, flaw type, damage mechanism, etc.). Therefore, the accurate determination of these detection and sizing assumptions requires thorough consideration, modeling, assessments, and determination by NDE specialists. This paper will describe factors that affect flaw detection and sizing results by NDE, the use of synthetic flaws to assess and improve NDE performance, the verified performance of advanced NDE techniques on natural flaws, and the integration of NDE performance results with FFS assessments.

Ultrasonic Inspection of Dissimilar Metal Welds for Probabilistic Reliability Assessment of Pipe Welds

There are several sources of uncertainties which need to be considered in a probabilistic reliability and lifetime assessment of safety-relevant components. In addition to the probabilistic distribution of material properties, the size and properties of flaws present in a component contribute to uncertainties in the lifetime analysis. In a current reactor safety research project, a methodology for a probabilistic fracture mechanics assessment of reliability for components with austenitic and dissimilar metal welds will be developed using the combined results from statistical evaluation of material properties and ultrasonic inspection (UT). Dissimilar metal welds present a particular challenge for ultrasonic testing due to the effects of the material anisotropy on ultrasonic propagation and scattering. Evaluation of inspection results is typically done using image-based techniques. As there is no simple relationship between UT response and flaw size, criteria for image evaluation and flaw sizing have to be defined taking into account the influence of various factors such as material and flaw properties on the UT response. In this paper, we present results from a study of the influence of grain structure on the ultrasonic inspection results and discuss the challenges of extracting data for probability of detection (POD) analyses.

A Remediation of Flaws in Stainless Steel Cladding on Hydroprocessing Reactors

Hydroprocessing reactors are made of heavy wall Cr-Mo steels with stainless steel cladding and installed in the most of oil refineries in the world during past half century. After long time operation, they found several material degradation and/or damages due to high pressure and high temperature hydrogen services. This paper presents typical remediation method of flaws in stainless steel cladding of reactors including flaw sizing, FFS assessment and repair procedures.