The Importance of Nodule Size in the Management of Ruptured Thyroid Nodule After Radiofrequency Ablation: A Retrospective Study and Literature Review

BackgroundNodule rupture is a relatively uncommon yet severe complication of radiofrequency ablation (RFA). When nodule rupture occurs, determining suitable therapeutic management is a critical issue. A study herein aimed to identify the predictive factors affecting the management of post-RFA nodule rupture.MethodsPost-RFA nodule rupture data of 9 patients were enrolled from 2 medical centers. A literature investigation was performed, uncovering nodule rupture data of 17 patients. A total of 26 patients were analyzed and divided into two groups, categorized as patients requiring either invasive or conservative therapeutic management. Data including initial symptoms, imaging, therapeutic management, and prognosis were reviewed and compared between the two groups.ResultsSignificant differences in nodule diameter, and the ablation time of the course prior to rupture (RUP time) were noted between the two groups (p = 0.045 and 0.008, respectively). Logistic regression analysis indicated the initial nodule diameter and RUP time significantly affected the requirement of invasive treatment (OR 1.99 and 1.11, respectively). Considering practicality, when a nodule with an initial maximum diameter of >4.5cm ruptured, invasive management was suggested (sensitivity 69% and specificity 79%).ConclusionThough nodule ruptures can be managed conservatively, a ruptured nodule with an initial maximum diameter of >4.5cm may require invasive management. Understanding the significant clinical and imaging features will help physicians make an appropriate risk assessment to determine the correct treatment in a timely manner.

Basic Creep-Fatigue Models Considering Cavitation

Cavitation plays a central role during creep-fatigue. During recent years, fundamental models for initiation and growth of creep cavities that do not involve any adjustable parameters have been developed. These models have successfully been used to predict creep rupture data for austenitic stainless steels again without using adjustable parameters. However, it appears that basic models have not yet been applied to creep-fatigue assessments. A summary of the fundamental cavitation models is given. A model for monotonous deformation is transferred to cyclic loading. The parameter values are kept except that the dynamic recovery constant is raised due to increased interactions between dislocations during cycling. This model is successfully compared with observed LCF and TMF hysteresis loops. A new model for cavity growth due to plastic deformation is presented. The model is formulated in such a way that the condition for constrained growth is automatically satisfied. In this way, it is avoided to overestimate the growth rate.

Extrapolation of Creep Rupture Data Using Parametric Numerical Isothermal Datum (P-NID) Method for Inconel 617

The development of advanced power plants requires alloys to operate at elevated temperature and pressure for an extended period of time. It is critical to consider creep during the design process to avoid catastrophic failure. Creep rupture data are often not available for desired operating conditions. Accurate extrapolation of creep life is necessary. One of the earliest and most widely used life prediction model is the classic Larson-Miller Parametric (LM) model. Over time numerous time-temperature parametric (TTP) models have been proposed such as Manson-Haferd, Orr-Sherby-Dorn, Manson-Succop, Graham-Walles, Goldhoff-Sherby parametric models. Non-TTP models such as the Wilshire equation is available. The prediction models vary in mathematical form, and number of material constants but shares a common calibration approach. Each model is calibrated against data for every available isotherm. A recently proposed model calibration approach is the parametric numerical isothermal datum (P-NID) method that can be applied to an existing model for improved long-term extrapolation. The P-NID approach is different than the traditional approach as the data are transferred to a datum temperature followed by model calibration against the transferred data at the datum temperature. The calibrated model is then transferred back to the original temperatures. In this study, the P-NID method is applied to the LM model to perform extrapolation for Inconel 617 alloy. Creep rupture data for five isotherms ranging from 800 to 1000°C and stress levels from 9MPa to 170 MPa are used. A detail step by step procedure is provided for the application of the P-NID method to calibrate the LM model (LM-NID). The extrapolation performance of the classic LM and LM-NID models are compared. Normalized Mean Squared Error (NMSE) is used to analyze prediction accuracy. It is observed that the LM-NID model provides a realistic inflection free prediction compared to the LM model. A 10% data-cull from the lowest stress data is performed to assess the reliability of extrapolation. Based on the comparison a recommendation is provided.

Evaluation of Stress Rupture Factors for Grade 91 Weldments

Stress rupture factors and weld strength reduction factors for Grade 91 steel weldments in the codes and literatures have been reviewed. Stress rupture factors for weld metals proposed for code case N-47 in the mid 1980's was defined as a ratio of average rupture strength of the deposited filler metal to the average rupture strength of the base metal. Remarkable drop in creep rupture strength of weldments is significant issue of Grade 91, especially in the low-stress and long-term regime. A premature failure of Grade 91 steel weldments in the long-term, however, is caused by type IV failure which takes place in the fine grain heat affected zone (FG-HAZ), rather than fracture in the deposited weld metal. The stress rupture factor of the Grade 91 steel, therefore, was based on the creep rupture strength of cross weld test specimens. Creep rupture data of Grade 91 steel weldments reported in the publication of ASME STP-PT-077 were integrated with the creep rupture data collected in Japan and used for this study. Time- and temperature-dependent stress rupture factors for Grade 91 steel have been evaluated based on the consolidated database as a ratio of average creep rupture strength of cross weld test specimen to the average creep rupture strength of base metal.

Metamodeling Time-Temperature Creep Parameters

There exist many time-temperature parameter (TTP) models for creep rupture prediction of components including the Larson–Miller (LM), Manson–Haferd (MH), Manson–Brown (MB), Orr–Sherby–Dorn (OSD), Manson–Succop (MS), Graham–Walles (GW), Chitty–Duval (CD), Goldhoff–Sherby (GS) models. It remains a challenge to determine which model is “best”, capable of accurate interpolation and physically realistic extrapolation of creep rupture data for a given material. In this study, metamodeling is applied to create a unified TTP metamodel that combines and regresses into twelve TTP models (eight existing and four newly derived). An analysis of the mathematical problems that exist in TTP models is provided. A matlab code is written that can: (1) calibrate the material constants of any of the twelve TTP models (using the metamodel); (2) determine the most suitable stress-parameter function; (3) and report the normalized mean square error (NMSE) of rupture predictions for a given material database. Using the metamodel, and code, a design engineer can make an intelligent selection of the “best” TTP model for creep resistant design. This process is demonstrated using four isotherms of alloy P91 creep rupture data. To assess the influence of material, further validation is performed on alloys Hastelloy X, 304SS, and 316SS. It is determined that the “best” model is dependent on material type and the quality and quantity of available data.

Influence of Oxidation on Estimation of Long-Term Creep Rupture Strength of 2.25Cr–1Mo Steel by Larson–Miller Method

The influence of oxidation on the estimation of long-term creep rupture strength is investigated for 2.25% chromium (Cr)–1% molybdenum (Mo) steel specified as JIS STBA 24, JIS SCMV 4 NT, and ASTM A542/A542M by the Larson–Miller method using creep rupture data in the National Institute for Materials Science (NIMS) Creep Data Sheets at 450–650 °C for up to 313,000 h. The creep rupture data exhibit a change in slope of the stress versus time to rupture curves due to oxidation in air during 600 °C creep tests at 15,000–40,000 h and 650 °C tests at 2000–3500 h for different size specimens, which indicates degradation in creep life by the oxidation. The estimated 100,000 h creep rupture strength using regression analysis is increased by the elimination of long-term data degraded by the oxidation. Several metallurgical factors, such as the initial strength represented by the 0.2% proof stress at the creep test temperature and the concentration of aluminum (Al) impurity, also affect the creep life of the tested steel.