Residual Life Assessment Research Articles

Prediction of fatigue crack propagation lives of turbine discs with forging-induced initial cracks

This paper presents a study on residual life assessment of turbine discs in a military aircraft engine that were found to be susceptible to fatigue cracking failure due to forging flaws formed in the original manufacturing process. As these flaws were not considered in the original life assessment, it is important to predict the residual lives of affected turbine discs and to determine the safe inspection intervals in order to prevent possible failures during service. The examination of the cracked disc revealed that the flaw was formed during the hammer forging. A systematic analysis approach was developed to analyse all four turbine discs and to predict the fatigue crack growth (FCG) rates by using advanced finite element (FE) and numerical FCG predictions. The critical locations for these discs were found to be on the aft neck face of disc web. The predicted FCG for the cracked disc correlated reasonably well with the striation counting from the cracked disc. The residual lives for representative discs at the critical location and associated inspection intervals are determined for life management of the affected turbine discs.

Studies on Broad-Sense Sample Method in Data Mining

With the development of science and technology, people pay more and more attention to the reliability of the products, especially in some special field, such as aerospace, military products, and some products of high reliability and long life. As a part that runs through the whole life cycle of products, reliability test provides an important source of data for the design, batch production and residual life assessment of the product development. For some expensive, new products put into use, they are not quite little in amount, having the characteristics of small sample. In this case, how to use the existing data to predict product life, reliability of calculating the reliability of a product more accurately and other related parameters is particularly important.

Residual Fatigue Life Assessment of Damaged RC Bridge Slabs Based on Site-Inspection for Cracking: A Quantitative Discussion

There exist in-service RC bridge slabs suffering greatly from the fatigue loads of moving vehicles and serious cracking may often be observed. For quantitative assessment of residual fatigue life of bridge slabs under expected future loads, the past traffic loads' record is not available in most cases. Then, this paper aims to present pseudo-cracking approach to remaining fatigue life assessment of existing RC bridge slabs based on site crack inspection data alone. First, inspected crack width and its spacing are converted to the space-average strain field, on which finite element analysis can be carried out to predict the residual fatigue life of the cracked slabs. To prove the applicability of this simulation system proposed, parametric analysis is conducted to compute the S-N diagram of the damaged slabs and comparison is made with simplified deterioration grades specified in the manual of maintenance based upon experiences of bridge management. Furthermore, two bridge slabs in service with real crack inspection data were examined as a case study of application.

Noise Models in Numerical Analysis of Stochastic Creep Damage Growth and Residual Life Assessment

Experimental observations of creep life in structural systems exhibit significant scatter. As a result, probabilistic methods that incorporate the associated uncertainties in residual life assessment methodologies have been developed. Most studies in the literature adopt Gaussian models for the noise. However, Monte Carlo simulations with such models lead to possibilities of physically unrealistic negative damage growth increments in certain sample damage growth realizations. This study investigates the use of alternative models for noise in damage growth analyses and compares these predictions with those obtained when Gaussian models are used. A continuum damage mechanics based approach is used for obtaining the thermal creep damage growth in a nuclear power plant component. Numerical results are presented to highlight the salient features arising from this study.

Remaining life assessment of a high pressure turbine casing in creep and low cycle service regime

Thick walled components such as high pressure (HP) steam turbine casings operating under high parameter conditions are subjected to a complex stress state. As a result of that stress state, some parts of HP turbine casing undergo to the creep fatigue caused by the combination of thermal fatigue resulted from repeated start/stop operation and the creep which occurs during long-term operation at high temperature and high-pressure. It is well known that domestic thermal power plants have been in use over 100000 h which means that significant cost is required not only for maintenance, but often for renewal of equipment. Based on comprehensive investigation, the results of residual life assessment of one high pressure steam turbine casing, which belongs to the older turbine generation, taking into account simultaneous action of thermal fatigue and creep, are presented in this paper. Also, the critical flaw crack size of HP turbine casing is determined because this parameter has a strong influence on casing integrity and residual life. The results of residual life assessment provide not only a basis for further maintenance, but also estimated time for reparation or renewal.

Development of Creep Residual Life Assessment Method for Steam Pipes with Bainite Structures

Development of Creep Residual Life Assessment Method for Steam Pipes with Bainite Structures

Change of precipitate free zone during long-term creep in 2.25Cr–1Mo steel

This work presents a residual life assessment method based on the quantitative analysis of changes in the fraction and width of a precipitate free zone (PFZ) around grain boundaries during long-term creep. Such changes resulted from precipitation and coarsening of carbides on grain boundaries and within the ferrite matrix during long-term creep exposure. Testing was conducted at creep temperatures ranging from 450 to 650°C, for a maximum time of 142415.1h. A relationship between the fraction of grain boundary with PFZ, PFZ width, and the Larson-Miller Parameter (LMP) was established, and the mechanism of PFZ formation was discussed. The increase in width, governed by the diffusion of alloy elements to grain boundaries, was mathematically modeled. The relationship between the width and creep exposure time was fitted to a square root law, and the PFZ broadening rate constant and activation energy for width growth were calculated. The proposed model will be applicable to determine the operating parameters of 2.25Cr–1Mo steel components in high-temperature equipment.

Measurement of Local Initiation, Early Growth and Retardation of Physically Short Fatigue Cracks Using Amended DCPD Method

Different experimental methods are being used in laboratories for automatic measurement of fatigue crack growth rates and threshold values. Such data belong to mechanical properties essential for an assessment of residual life of components and structures containing cracks of length more than several millimetres. However, if the material contains small crack-like defects like inclusions or pores, knowledge about local resistance against fatigue growth of physically short cracks becomes very important. Damage mechanisms are even more complicated in case of a presence of subsurface residual stress field, e.g. due to technological effects. Experimental difficulties connected with investigation of short fatigue crack growth (FCG) can be reduced in case of use of automatic indirect methods. The aim of the work described in this paper was to explore possibilities and limits of the use of DCPD (direct current potential drop) method for physically short crack measurement and to use the optimised method for an evaluation of local damage process of initiation and early growth of short fatigue crack in residual stress field induced by shot peening.

SMC structural health monitoring benchmark problem using monitored data from an actual cable-stayed bridge

A structural health monitoring (SHM) system provides an efficient way to diagnose the condition of critical and large-scale structures such as long-span bridges. With the development of SHM techniques, numerous condition assessment and damage diagnosis methods have been developed to monitor the evolution of deterioration and long-term structural performance of such structures, as well as to conduct rapid damage and post-disaster assessments. However, the condition assessment and the damage detection methods described in the literature are usually validated by numerical simulation and/or laboratory testing of small-scale structures with assumed deterioration models and artificial damage, which makes the comparison of different methods invalid and unconvincing to a certain extent. This paper presents a full-scale bridge benchmark problem organized by the Center of Structural Monitoring and Control at the Harbin Institute of Technology. The benchmark bridge structure, the SHM system, the finite element model of the bridge, and the monitored data are presented in detail. Focusing on two critical and vulnerable components of cable-stayed bridges, two benchmark problems are proposed on the basis of the field monitoring data from the full-scale bridge, that is, condition assessment of stay cables (Benchmark Problem 1) and damage detection of bridge girders (Benchmark Problem 2). For Benchmark Problem 1, the monitored cable stresses and the fatigue properties of the deteriorated steel wires and cables are presented. The fatigue life prediction model and the residual fatigue life assessment of the cables are the foci of this problem. For Benchmark Problem 2, several damage patterns were observed for the cable-stayed bridge. The acceleration time histories, together with the environmental conditions during the damage development process of the bridge, are provided. Researchers are encouraged to detect and to localize the damage and the damage development process. All the datasets and detailed descriptions, including the cable stresses, the acceleration datasets, and the finite element model, are available on the Structural Monitoring and Control website (http://smc.hit.edu.cn). Copyright © 2013 John Wiley & Sons, Ltd.

Structural health monitoring of wind towers: residual fatigue life estimation

In a recent paper (Benedetti et al 2011 Smart Mater. Struct. 20 055009), the authors investigated the possibility of detecting cracks in critical sites of onshore wind towers using a radial arrangement of strain sensors around the tower periphery in the vicinity of the base welded joint. Specifically, the strain difference between adjacent strain sensors is used as a damage indicator. The number of sensors to be installed is determined by the minimum crack size to be detected, which in turn depends on the expected extreme wind conditions and programmed inspection/repair schedule. In this companion paper, we address these issues by investigating possible strategies for residual fatigue life assessment and management of onshore wind towers once the crack has been detected. For this purpose, fracture mechanics tests are carried out using welded samples to quantify the resistance to fatigue crack growth as well as the elastic–plastic fracture toughness of the welded joint at the tower base. These material strength characteristics are used to estimate (i) the critical crack size for structural integrity on the basis of fracture toughness tests, elastoplastic finite element analyses and loading spectra under extreme wind conditions, (ii) the residual life before structural collapse, applying a frequency-domain method to typical in-service wind actions and wind directionality.

J042013 ベイナイト組織を有する配管のクリープ余寿命評価法の開発([J042-01]工業材料の力学応答)

J042013 ベイナイト組織を有する配管のクリープ余寿命評価法の開発([J042-01]工業材料の力学応答)

Investigation of Fracture Behavior of Steam Generator Tubes of Indian PHWR using PLT Specimens

Abstract Steam generator (SG) tubes made of Ni-Cr-Fe based alloys such as Alloy-800 are used in nuclear power plants like Indian Pressurized Heavy Water Reactors (PHWRs) in the form of thin-walled tubular structural members as heat exchangers. These tubes are exposed to aggressive environments such as high temperature, internal pressure and flow-induced vibration etc. Corrosion aspects like intergranular-stress-corrosion-cracking (IGSCC) of such alloys received much attention in literature, whereas work on fracture behavior of these materials is limited, particularly for thin-walled tubes. For structural integrity assessment of SG tubes, a valid fracture mechanics parameter must be obtained. As their thickness is only about 1 mm, it is not feasible to machine standard fracture mechanics specimens from them. Moreover, the plane-strain condition of state of stress cannot be achieved at the crack-tip of these tubes which is a requisite of ASTM standard tests for fracture toughness. Thus, non-standard tests have been carried out on axially-cracked tubular specimens, directly machined from SG tubes in a test-setup known as Pin-Loading-Tension (PLT) setup for derivation of shape function from compliance for determining stress intensity factor and fracture toughness. Finite element analysis (FEM) has also been done to determine the shape function and the other required geometric functions for obtaining J from experimental data. A load-normalization technique was used for estimation of crack growth. The results obtained will be highly useful for residual life assessment of SG tubes of Indian PHWRs.

Piezo-impedance transducers for residual fatigue life assessment of bolted steel joints

This article presents a new approach for fatigue life assessment of bolted steel joints using the equivalent stiffness determined by surface-bonded piezo-impedance transducers. The piezo transducers acquire electromechanical admittance signatures comprising the conductance (real part) and the susceptance (imaginary part), from which the equivalent system parameters of the joint are identified. The equivalent stiffness, one of the identified parameters, is experimentally found to be related to the residual stiffness of the joint and is correlated with the remaining life of the component in terms of the cycles of loading that can be further sustained by the joint. Through tests on three prototype steel joints, empirical equations are derived to relate the residual fatigue life to the loss of equivalent identified stiffness. The most promising feature of the proposed approach is that it employs the admittance signature of the surface-bonded piezo transducers directly, thereby circumventing the determination of the in situ stiffness of the joint.

Finite Element based Bayesian Particle Filtering for the estimation of crack damage evolution on metallic panels

A lot of studies are nowadays devoted to structural health monitoring, especially inside the aeronautical environment. In particular, focusing the attention on metallic structures, fatigue cracks represent both a design and maintenance issue. The disposal of real time diagnostic technique for the assessment of structural health has led the attention also toward the prognostic assessment of the residual useful life, trying to develop robust prognostic health management systems to assist the operators in scheduling maintenance actions. The work reported inside this paper is about the development of a Bayesian particle filter to be used to refine the posterior probability density functions of both the damage condition and the residual useful life, given a prior knowledge on damage evolution is available from NASGRO material characterization. The prognostic algorithm has been applied to two cases. The former consists in an off-line application, receiving diagnostic inputs retrieved with manual structure scanning for fault identification. The latter is used on-line to filter the input coming from a real-time automatic diagnostic system. A massive usage of FEM simulations is used in order to enhance the algorithm performances.

Evaluation of Creep Strength Reduction Factors for Welded Joints of Modified 9Cr-1Mo Steel

Creep strength of welded joint for high Cr ferritic heat resisting steels decreases due to Type-IV failure in heat-affected zone (HAZ) during long-term use at high temperatures. In order to review the allowable creep strength of these steels, creep rupture data of base metals and welded joints have been collected, and long-term creep strength has been evaluated in the SHC (strength of high-chromium steel) committee in Japan. In the present paper, the creep rupture data of 370 points for welded joint specimens of modified 9Cr-1Mo steel (ASME Grade 91 steel) offered from seven Japanese companies and institutes were analyzed. These data clearly indicated that the creep strength of welded joints was lower than that of base metal due to Type-IV failure in HAZ at high temperatures. From the activities of this committee, it was concluded that the weld strength reduction factor (WSRF) should be taken into consideration for the design and residual life assessment of boiler components in fossil power plants. The committee recommended the WSRF for 100,000 h creep of Gr.91 steel as 0.85 at 575 °C, 0.75 at 600 °C, 0.74 at 625 °C, and 0.70 at 650 °C. The master curve for residual life assessment of Gr.91 steel welds using Larson-Miller parameter was also proposed.

Assessment of Accrued Damage and Remaining Useful Life in Leadfree Electronics Subjected to Multiple Thermal Environments of Thermal Aging and Thermal Cycling

Electronic systems are often stored for long periods prior to deployment in the intended environment. Aging has been previously shown to effect the reliability and constitutive behavior of second-level leadfree interconnects. Deployed systems may be subjected to cyclic thermo-mechanical loads subsequent to deployment. Prognostication of accrued damage and assessment of residual life is extremely critical for ultrahigh reliability systems in which the cost of failure is too high. The presented methodology uses leading indicators of failure based on microstructural evolution of damage to identify impending failure in electronic systems subjected to sequential stresses of thermal aging and thermal cycling. The methodology has been demonstrated on area-array ball-grid array test assemblies with Sn3Ag0.5Cu interconnects subjected to thermal aging at 125 °C and thermal cycling from -55 to 125 °C for various lengths of time and cycles. Damage equivalency methodologies have been developed to map damage accrued in thermal aging to the reduction in thermo-mechanical cyclic life based on damage proxies. Assemblies have been prognosticated to assess the error with interrogation of system state and assessment of residual life. Prognostic metrics including α - λ metric, sample standard deviation, mean square error, mean absolute percentage error, average bias, relative accuracy (RA), and cumulative RA have been used to compare the performance of the damage proxies.

Intelligent Diagnostic Method for Ageing Analysis of Transformer

The fuzzy method is proposed for ageing analysis of transformer. The fuzzy controller is used for various input and one output, strictly depends on the number of membership function and there rule base and the type of the defuzzification method. The ageing of transformers is influenced by short term and long term over loads, number and intensity of short circuits, incidence of lightning, and internal faults. The recent development of various techniques for detecting the incipient fault conditions have been improved to some extent; the life expectancy of transformers by resorting to corrective actions in time. The ageing behavior is likely to be different for different types of transformers. The life span of the transformer, thus depends initially on the design and quality of manufacture and later on service conditions and maintenance standard, these factors vary considerably and affect the useful span of service life which therefore needs to be taken into account for residual life assessment. During the natural ageing of transformers, the insulation of winding deteriorates. Cellulose insulation degrades due to heating or electrical breakdown which is dissolved in oil. Hence, the chemical analysis of the Transformer oil gives evidence of changes that are taking place in the winding insulation during operation. Deterioration in transformer cellulose decreases both its electrical and mechanical strength. In this paper a novel fuzzy based algorithm has been implemented on three samples of power transformer oil.

Residual life assessment for the metallic structures of hoisting machines

The potentials of the three parameters: Brinell hardness, coefficient of variation of the material hardness, and oercitivity, to be used as the residual life indicators for metallic crane structures were examined. It was shown that the coefficient of variation and coercitivity may be employed in the expert evaluation of metallic structures of hoisting machines to determine their condition. The hardness parameter is not applicable for solving this problem.

Prognostication of Residual Life and Latent Damage Assessment in Lead-Free Electronics Under Thermomechanical Loads

Requirements for system availability for ultrahigh reliability electronic systems such as airborne and space electronic systems are driving the need for advanced health monitoring techniques for the early detection of the onset of damage. Aerospace electronic systems usually face a very harsh environment, requiring them to survive the high strain rates, e.g., during launch and reentry, and thermal environments, including extremely low and high temperatures. Traditional health monitoring methodologies have relied on reactive methods of failure detection often providing little or no insight into the remaining useful life of the system. In this paper, a mathematical approach for the interrogation of the system state under cyclic thermomechanical stresses has been developed for six different lead-free solder alloy systems. Data have been collected for leading indicators of failure for alloy systems, including Sn3Ag0.5Cu, Sn0.3Ag0.7Cu, Sn1Ag0.5Cu, Sn0.3Ag0.5Cu0.1Bi, Sn0.2Ag0.5Cu0.1Bi0.1Ni, and 96.5 Sn3.5Ag second-level interconnects under the application of cyclic thermomechanical loads. The methodology presented resides in the prefailure space of the system in which no macroindicators such as cracks or delamination exist. Systems subjected to thermomechanical damage have been interrogated for the system state and the computed damage state correlated with the known imposed damage. The approach involves the use of condition monitoring devices which can be interrogated for damage proxies at finite time intervals. The interrogation techniques are based on the derivation of damage proxies and system prior-damage-based nonlinear least square methods, including the Levenberg-Marquardt algorithm. The system's residual life is computed based on residual-life computation algorithms.

Residual Life Assessment for Ferromagnetic Components Based on Metal Magnetic Memory Technology

The fatigue crack growth experiment was carried out at room temperature for the standard SEB(3) specimens of 40Cr steel, the distribution of the surface magnetic field of the specimen under same load and different cycle numbers (N) was studied. By analyzing relations between the magnetic signals and the crack propagation life, a new damage parameter model has been established by magnetic signals. According to this model, we can evaluate the residual life of ferromagnetic components excellently. This belong to basic research on early damage inspecting and residual life assessment for ferromagnetic materials, it confirmed that residual life assessment and safety estimating for ferromagnetic components based on metal magnetic memory technology is feasible.