Residual Life Assessment Research Articles

Degradation and residual life assessment of thermocouples with damaged sheaths in corrosive environments

Mineral-Insulated Metal-Sheathed thermocouples exhibit extended lifespan and resilience in harsh environments, owing to their protective sheath. However, these thermocouples are prone to failure in a highly acidic medium such as nuclear fuel reprocessing plants. This paper investigates the degradation phenomena and its effect on measurement accuracy when a thermocouple with damage to its protective sheath is immersed in a corrosive medium. The objective is to assess the thermocouple's reliability by micro machining a small hole to simulate sheath damage, facilitating direct contact between the corrosive medium and the sensing materials of a K-type thermocouple. The thermocouples with sheath defects were subjected to aging in different concentrations of acidic medium at a constant temperature, leading to eventual failure. The time-to-failure data of thermocouples at each concentration is analyzed using the Weibull distribution. This study establishes a thermocouple accelerated life-stress relationship using the Inverse Power Law under a ruptured sheath condition. The study also explores the correlation between the residual life and the position of simulated ruptures. ANOVA is employed to test the hypotheses regarding the influence of rupture position on thermocouple performance. It is shown that the location of damage is statistically significant in determining the thermocouple's residual life under damaged sheath condition.

Reliability Index for Steel Structure’s Technical State and Residual Life Assessment

The article is dedicated to problem of technical state assessment of real steel truss under operation for residual life prediction after technical investigation. The reliability index is calculated based on design, real and prospective loads. Method of statistical data analysis adopted for stress-strain determination also for all kinds of applied loads. The residual life with the reliability index determined as a transition from satisfactory to emergency technical state. The method provides the possibility for estimation of future safety steel structures operation with technical investigation results only that exclude any subjective opinion.

Advancing the Reliability of Residual Life Assessment for Turbines through Strain Gauge Measurements: Key Aspects and Latest Developments

Abstract With the need for flexible operation and an aging hydropower fleet, the importance of residual life assessment for turbines has been on the rise. While prediction methods are continuously improving, the most reliable data about the health status of a machine is obtained through measurement campaigns. Unlike fatigue evaluations during the design stage that include safety factors, residual life assessments require higher precision to enable owners to evaluate their assets and investments. The increased precision of a measurement campaign justifies the additional costs incurred. This article emphasizes the significance of a careful review of all aspects, including load history, measurement data, transposition method, and material data. It also presents the latest developments in accordance with IEC 63230 and provides an example prototype measurement to underscore the importance of each aspect. Special attention is given to the transposition of strain data from the measurement location to the hot spot position and its impact on load sequences and stochastic loads. Through a sensitivity study, the article aims to provide a better understanding of the importance of each aspect in the residual life evaluation process.

Residual Fatigue Life Estimation of Damaged Welded Structures

This paper presents a finite element modeling procedure to determine of crack growth behaviour of butt welded joints under the subject of load for mode I. This paper presents a computation procedure to determine the ratio of fatigue crack growth in butt welded plates for mode I fracture mechanics loading conditions. The presence of residual stresses in welded structures can significantly affect the material’s resistance to fatigue under cyclic loading. The presence of tensile residual stresses adversely affects the fatigue crack growth rate increased it. Change of microstructure and hardening material as a result of the welding process also has a negative impact on the growth of the crack. Accurate prediction and reliable assessment of the residual stress are important for the structural integrity and residual life assessment of welded parts design. Although there are several techniques for the determination of residual stresses, the finite element method (FEM) is one of the most convenient and useful. This paper presents a finite element modeling procedure to determine of crack growth behaviour of butt welded joints under tensile load for mode I. Keywords: Welding, Residual stress, Residual life, Crack growth, FEM, Butt welded joint

Material volume reduction for creep testing using composite cantilevers and its application for residual life assessment

Digital image correlation (DIC) and finite element analysis (FEA) demonstrate that creep deformation in bending occurs primarily in ≈30 % of the cantilever volume near the fixed end, especially when the creep stress exponent ranges from 5 to 7. As an alternative approach to minimize the material volume required for testing, the concept of fabricating composite cantilevers is proposed and validated in this study. A composite cantilever sample consists of an “active” creeping portion (e.g., T22 boiler steel) and an additively extended “passive” non-creeping portion (e.g., IN718). The volume reduction process involved varying the length of the “active” section, a, while keeping the total length of the cantilever, L, constant. The DIC measurements conducted at 600 °C to assess the creep behavior of T22 steel revealed that analytical expressions for monolithic cantilevers could aptly predict the constitutive steady-state creep laws from the composite cantilevers if measurements are made in a region at a critical distance away from the interface. FEA indicates that accurate stress estimation enables predicting monolithic creep behavior using composite samples with “a/L” ratios of as small as 5 %. Using the developed approach, the loss of creep resistance of T11 boiler steel that was in service for ∼ 240,000 h was ascertained in high throughput fashion using a composite cantilever having only 30 vol % of the “active” material. Guidelines to minimize the volume fraction of the “active” portion in the composite cantilever and the implications of the observations for estimating the residual life of in-service high-temperature components are discussed.

Fatigue research of metro car body based on operational loads

PurposeThe principle of infinite life design currently directs fatigue resistance strategies for metro car bodies. However, this principle might not fully account for the dynamic influence of operational loads and the inevitable presence of defects. This study aims to integrate methods of service life estimation and residual life assessment, which are based on operational loads, into the existing infinite life verification framework to further ensure the operational safety of subway trains.Design/methodology/approachOperational loads and fatigue loading spectra were determined through the field test. The material test was conducted to investigate characteristics of the fracture toughness and the crack growth rate. The fatigue strength of the metro car body was first verified using the finite element method and Moore–Kommers–Japer diagrams. The service life was then estimated by applying the Miner rule and high-cycle fatigue curves in a modified form of the Basquin equation. Finally, the residual life was assessed utilizing a fracture assessment diagram and a fitted curve of crack growth rate adhered to the Paris formula.FindingsNeither the maximum utilization factor nor the cumulative damage exceeds the threshold value of 1.0, the metro car body could meet the design life requirement of 30 years or 6.6 million km. However, three out of five fatigue key points were significantly influenced by the operational loads, which indicates that a single fatigue strength verification cannot achieve the infinite life design objective of the metro car body. For a projected design life of 30 years, the tolerance depth is 12.2 mm, which can underscore a relatively robust damage tolerance capability.Originality/valueThe influence of operational loads on fatigue life was presented by the discrepancy analysis between fatigue strength verification results and service life estimation results. The fracture properties of butt-welded joints were tested and used for the damage tolerance assessment. The damage tolerance life can be effectively related by a newly developed equation in this study. It can be a valuable tool to provide the theoretical guidance and technical support for the structural improvements and maintenance decisions of the metro car body.

Residual Life Assessment of Steel Truss Bridge due to Coupled Action of Corrosion and Fatigue

Residual Life Assessment of Steel Truss Bridge due to Coupled Action of Corrosion and Fatigue

Eddy current non-destructive testing for inspection of reformer tubes applying machine learning

HP austenitic stainless steel undergoes microstructural aging due to prolonged exposure to oxidizing and corrosive atmospheres in steam reforming furnaces. The derived aging states are classified by its service temperature and microstructural markers and monitoring it is important to residual life assessment. In this regard it was used a portable Eddy Current inspection system with the aid of machine learning classification tools, characterizing aging states in HP steel in real-time. The classification profile of a 12-meter tube was acquired, validated through Field Metallurgical Replication. The developed Eddy Current inspection system successfully differentiates three regions, revealing a progression of aging states.

Corrosion fatigue behaviour of Q690D high-strength steel considering the effect of coupling

During the service period of steel structures, the condition of suffering from environmental corrosions and fatigue loadings simultaneously widely exists. In this article, focusing on the corrosion fatigue behaviour of Q690D high-strength steel with consideration of the coupling effect, experimental research was conducted and the coupling effect was analysed. Corrosion fatigue coupling tests were performed with the method of electrolytic accelerated corrosion. The corrosion characteristics were studied and compared with the corrosion tests without loadings. Then, the corrosion fatigue S-N curve was established and compared with non-corroded and pre-corroded conditions. Morphology observations were carried out to further analyse the mechanisms of corrosion fatigue coupling. The results show that the fatigue resistance of Q690D high-strength steel deteriorated greatly compared with non-corroded and pre-corroded conditions, and the adverse effect of corrosion fatigue coupling on fatigue resistance cannot be neglected. The coupling effect under real engineering service conditions could be well represented with the adopted test method, taking into account the time dimension correlation between fatigue and corrosion. The established Q690D S-N curve under the corrosive environment provides important references for the residual fatigue life assessment of high-strength steel fatigue considering the effect of environmental deterioration.

Expression of concern “An investigation on residual stress and fatigue life assessment of T-shape welded joints” [Eng. Fail. Anal. 141 (2022) 106685

Expression of concern “An investigation on residual stress and fatigue life assessment of T-shape welded joints” [Eng. Fail. Anal. 141 (2022) 106685

Residual life assessment of deep rolled railway axles considering the effect of process parameters

Deep rolling is an efficient technique to increase the fatigue life of railway axles. The application introduces compressive residual stresses into the area near the surface, which are the most effective positive influence on fatigue behaviour and remaining service life in presence of a crack. This paper investigates the remaining service life of deep rolled railway axles. Previous research studies are used as basis for this work whereas, firstly, an elaborated crack growth model for common railway axle steels was developed. This model is based on experimental crack propagation results and can consider crack closure as well as load sequence effects and local residual stress conditions. Secondly, a numerical deep rolling simulation model validated with experimental residual stress measurements to determine the introduced residual stress state and to investigate the influence of various process parameters. In this study, the comprehensive outcome of both studies is connected in order to evaluate and compare the residual life of railway axles with the presence of different deep rolling-induced residual stress distributions in-depth. The crack growth study involves combinations of the deep rolling force as one of the most influencing process parameters, load scenarios as well as initial crack sizes and finally discusses the comprehensive results.

Creep damaged microstructure and mechanical properties of Cr–Mo–V steel subjected to long-term service exposures

Evaluating the degree of creep damage is of high importance for the residual life assessment of high temperature and pressure steam piping systems of power plants. This study aims at assessing creep damaged microstructure and mechanical properties of low alloy Cr–Mo–V steel after long-term service exposures. Variations in mechanical properties have been assessed by means of tensile tests, hardness measurement and instrumented indentation testing (IIT). Optical and scanning electron microscopies were employed for the identification of creep damaged microstructure. The effect of heat treatment on the recovery of microstructure and mechanical properties was also studied. Correlations of IIT and hardness results between surface and bulk values have been obtained. The results of IIT agree well to the results of tensile tests. Moreover, relationships between hardness and tensile properties have been assessed. Eventually, the evaluation of the degree of creep damage by employing IIT and hardness measurement was attempted.

Effect of compressive residual stress and press fitting on residual life assessment of deep-rolled EA4T steel railway axles

Deep rolling surface strengthening is an effective way to introduce compressive residual stress (CRS) along the railway axle, which can improve the fatigue resistance of the material. The CRS field was rebuilt adopting the proportional-integral (PI) iteration method and the unit pressure method (UPM) to study the fatigue cracking behavior of deep-rolled EA4T railway axles. The remaining life prediction model and the stress intensity factor (SIF) solution method were then validated using simulations and testing with small-sized specimens. The remaining life of the EA4T railway axle was then calculated by applying a realistic vertical load spectrum. The results showed that CRS prevented crack propagation, enhanced the critical propagation size of railway axle cracks, and prolonged the remaining life of the axle. Additionally, the remaining life of the axle may not support the following maintenance period if a crack with an extendable size emerges at the unloading groove of the railway axle. This work provides a theoretical basis for the optimization of the non-destructive testing (NDT) interval of deep-rolled EA4T railway axles.

ON THE ISSUE OF PREDICTING THE REMAINING SERVICE LIFE OF NUCLEAR POWER PLANT ELECTRICAL EQUIPMENT

This article addresses the issues related to the assessment and prediction of residual life of power plant equipment, taking into account the wear rate of its main components. Theoretical aspects arising during probabilistic calculations for predicting the residual life of three-phase induction motors are investigated. The study assumes that the prediction of residual life is based on determining its minimum estimate through calculation. A comparison is made between the minimum estimate result and the normative permissible level of wear of the investigated objects. The authors propose using a probabilistic-physical approach to objectively assess the technical condition and predict the residual life of this technical equipment. Within this approach, a probabilistic model based on the application of the diffusion-monotonic distribution for failures is considered for assessing longevity. The parameters of the model used have a physical interpretation. The key parameters taken into account are the average rate of change of the determining parameter, namely the insulation absorption coefficient and its resistance, as well as the coefficients of variation of selected degradation processes, assuming the possibility of their measurement or the use of prior estimation for the variation coefficient of processes. The article presents criteria for assessing the residual life of the investigated objects, which allows reducing operational costs by increasing the maintenance intervals and establishing the actual service life of the motors.

Endurance Strength and Residual Life Assessment of T23 Steel after 96000 Hours of High Temperature and High Pressure Service

The macroscopic morphology, oxide layer characteristics, rupture strength and residual life of T23 steel after 96000 hours of service at high temperature and high pressure were analyzed. The results show that T23 steel pipe has good macroscopic appearance, without obvious abnormal features such as cracks, folds, rolling breaks, double skin, scabs, delaminations and sharp scratches. The chemical composition of different pipe sections conforms to the standard, and the structure is mainly bainite, with uniform distribution and no obvious coarse phase. There is an obvious interface between the metal substrate and the internal oxide layer, and there is an obvious wavy line at the interface. The oxide layer is relatively dense and continuous. The tensile properties at room temperature and high temperature are good. At room temperature, the yield strength exceeds 420MPa, the tensile strength exceeds 540MPa, and the elongation exceeds 19%. After 96000 hours of use, the remaining creep life of T23 split platen superheater submitted for inspection is 84000 hours, and the remaining creep life of T23 rear platen superheater submitted for inspection is 40000 hours.

Modern approaches to strength design substantiation for critical elements of WWER NPP reactor internals

This study presents the theoretical and applied results on the development and application of refined models and methods for solving actual problems of mechanics related to the substantiation of strength and forecasting the service life of elements of the primary circuit equipment of WWER nuclear power plants (NPPs). The scientific and applied developments take into account current trends in global practice and contain new conceptual approaches to solving nonlinear boundary value problems in the mechanics of deformed structures. Refined mathematical models have been proposed and theoretically substantiated to describe non-isothermal inelastic deformation processes, taking into account the deformation history of thermal and power loading, radiation effects of hardening, swelling and creep of irradiated metal, and accumulated damage of brittle-ductile fracture. The obtained scientific results are the basis for the methodology of refined design strength analysis and residual life assessment of critical elements of the equipment of the first circuit of WWER reactor units to solve urgent problems of ensuring the conditions for safe operation of Ukrainian NPPs and justifying the terms of their service life extension

Fretting fatigue crack growth simulation and residual life assessment of railway press-fitted axle

To evaluate the inspection interval and safety margin of a railway press-fitted axle with fretting cracks, we conducted a fretting fatigue crack growth simulation and residual life assessment of a railway press-fitted axle by employing an actual load spectrum. The predicted results were compared with the measured crack to examine the effectiveness of various crack orientation criteria for assessing crack growth. Based on the validated crack orientation criteria, a finite element model was built to perform incremental fretting fatigue crack growth analysis. Then, the fretting fatigue crack growth life of the railway axle was predicted using two conventional crack growth rate models and compared with the experimental results. The results indicate that the max(ΔkI*) criterion provides a better approximation of the fretting fatigue crack growth path. The Paris equation can yield excessively conservative prediction life due to the strong load dependence of the effective stress ratio of fretting cracks, which can vary with the crack length and is influenced by press fitting. In contrast, the NASGRO equation yielded prediction results that were closer to the test results and can be utilised to determine the inspection interval. This study can serve as a vital reference for the operation and maintenance strategies for damaged axles with fretting cracks.

Residual Creep Life Assessment of High-Temperature Components in Power Industry.

A large percentage of power, petroleum, and chemical plants over the world were in operation for a long duration with the corresponding critical components being used beyond the design life of 30 to 40 years. It is generally more cost-effective to refurbish or modernize the degraded equipment or components, rather than to construct a new plant. Therefore, a reliable plant life extension assessment that can evaluate the critical components is needed. The key element in plant life extension is the residual life assessment technology. However, at present, there is still no general consensus among the industry players on the approach to adopt when performing residual life assessment for such a critical damage mechanism as creep. In this article, a three-level residual life assessment methodology is proposed as a general approach for high-temperature components prone to creep. A detailed validation of the selected guidelines and calculation models is also described. Eventually, an application of the three-level methodology to a real industrial case study is presented.

Assessment of Residual Useful Life of Sun Gear in a Planetary Gearbox Based on Dynamic Wear Behaviors Analyses

For proactively arranged downtime and maintenance of the gearbox, the accurate assessment of residual useful life (RUL) for worn-surface gear is a critical problem that needs to be solved. In light of this, a novel dynamic assessment methodology for RUL of sun gear was proposed by coupling Archard’s wear model and a nonlinear dynamics model. By modifying the internal excitation model, the effect of tooth surface wear is included in the dynamics model. Then, the effect of dynamic behavior on the wear process of the tooth surface is further fed back by the dynamic meshing force. On this basis, the influence of input speed, torque load and initial wear depth on the RUL was analyzed using a planetary gear NGW11-6 as an example. The results indicate that the torque load has the greatest influence on the RUL, followed by the initial wear depth, and that the input speed has the least influence. The established methodology will provide a theoretical basis for the RUL assessment and scheduled maintenance of transmission systems with worn-surface gears.

Asphalt Pavement Residual Life Assessment Based on SmartRock Sensors

Real-time structural health monitoring (SHM) is important for the performance evaluation and maintenance management of asphalt pavement. This study applies SmartRock, a wireless embedded sensor widely used in track health monitoring, to the long-term SHM of asphalt pavement to achieve its residual life assessment. According to the structural characteristics of asphalt pavement, a novel processing method of SmartRock monitoring data is proposed. Firstly, An improved genetic algorithm for back-calculation of pavement dynamic modulus using SmartRock monitoring data is proposed. Then, the evaluated dynamic moduli are used to predict the residual rutting life and residual fatigue life of the pavement structure. Finally, the two types of residual life are combined to achieve the pavement residual life. This research uses the real-time monitoring data of a test road in Shandong Province, China, to carry out a case study to verify the proposed approach.