Life Prediction Of Components Research Articles

Life Prediction for Turbopropulsion Systems Under Dwell Fatigue Conditions

The objective of this investigation was to develop an innovative methodology for life and reliability prediction of hot-section components in advanced turbopropulsion systems. A set of three generic time-dependent crack growth models was implemented and integrated into the Darwin® probabilistic life-prediction code. Using the enhanced risk analysis tool and material constants calibrated to IN 718 data, the effect of time-dependent crack growth on the risk of fracture in a turboengine component was demonstrated for a generic rotor design and a realistic mission profile. The results of this investigation confirmed that time-dependent crack growth and cycle-dependent crack growth in IN 718 can be treated by a simple summation of the crack increments over a mission. For the temperatures considered, time-dependent crack growth in IN 718 can be considered as a K-controlled environmentally-induced degradation process. Software implementation of the generic time-dependent crack growth models in Darwin provides a pathway for potential evaluation of the effects of multiple damage modes on the risk of component fracture at high service temperatures.

Cumulative fatigue damage and life prediction of elastomeric components

ABSTRACTElastomeric components are widely used in many applications due to their good damping and energy absorption characteristics. The type of loading normally encountered by these components in service is variable amplitude cyclic loading. Therefore, fatigue failure is a major consideration in their design. In this work capabilities of Rainflow cycle counting procedure, maximum principal strain as a damage criterion, and Miner's linear cumulative damage rule are evaluated with both specimen and component tests. An automotive cradle mount is used as an illustrative component. Comparison of predicted and experimental fatigue lives in both specimen and cradle mount variable amplitude load tests indicate satisfactory predictions in both cases.

Assessing Long-Term Performance of CANDU®Out-of-Core Materials

As so-called second-generation power reactors are approaching the end of their original design lives, assessments are being made to determine the feasibility and economics of extending plant life. Although components exposed to neutron and gamma irradiation are often those of most concern in terms of in-service ageing and continued fitness for service, ageing of out-of-core components can also limit the possibility of extended service life beyond design life. In CANDU®reactors, life extension decisions occur when the Zr-2.5Nb pressure tubes reach end of life, typically after about 25 years of service for the first CANDU-6 units. At the time of pressure tube replacements, the remaining life predictions for several other major components or systems provide the information required to determine life extension feasibility. Several CANDU reactors are currently being refurbished, with others planned, and experience to date shows that the steam generators, heat transport system piping and various balance of plant piping systems are typically those requiring careful assessment to ensure successful refurbishment. In this paper, we discuss AECL R&D that is oriented towards providing the chemistry and materials inputs required to assess current condition and predict future ageing of CANDU reactor out-of-core components and systems, and in particular steam generators (Alloy 800 tubing and carbon steel internals), feeder pipes and related heat transport system piping (carbon steel flow accelerated corrosion, feeder cracking). Systems and components that may impact future life will also be discussed, along with the related R&D, and this includes balance of plant system piping (feedwater piping and buried piping), cables and concrete structures.

Three-Dimensional Fracture Analysis of Circumferentially Cracked Boiler Tubes

A comprehensive methodology is developed to understand and characterize the fracture behavior of circumferentially cracked boiler tubes in this study. Weld overlay is applied on the coal-fired boiler tubes in order to prevent the degradation of corrosive and erosive environment that the boiler tubes are exposed to in the power plants. Finite element modeling and analysis are employed for all of the computations including steady-state and transient stress intensity factor (SIF) calculations in this study. Circumferential cracking has been one of the failure modes in waterwall boiler tubes, which results in high maintenance and replacement costs. Thermomechanical stresses and corrosive environment are basically the two remarkable contributors that bring about this failure mode. The former one is investigated and quantified in this study in order to explain the fracture behavior of weld overlay coatings during the power plant operation. Periodic soot blowing operations cause cyclic transient thermomechanical stresses on the weld overlay coating that results in crack propagation and fatigue failure. Three-dimensional fracture analysis of circumferentially cracked boiler tubes is examined using enriched finite element method in this study. Transient temperatures and thermomechanical stresses are computed using ANSYS for five different periodic crack spacing values (h), which are 2, 4, 6, 10, and 20 mm in the axial direction. 3D fracture analysis was performed, and stress intensity factors were computed using FRAC3D, which is Finite Element Analysis (FEA) software developed at Lehigh University. The maximum stress intensity factor is obtained at the deepest penetration of the crack in the model which has the largest periodic axial crack spacing, h = 20 mm. The stress intensity factors due to welding residuals decrease as the axial crack spacing, h, decreases. The FEA methodology developed in this research would provide the engineers with the ability to understand the fracture problem and predict component life and improve the reliability of the weld overlay coated boiler tubes utilized in the power plants.

Material deformation and fatigue behavior characterization for elastomeric component life predictions

AbstractFailure analysis and prediction of fatigue life of elastomers are important issues due to wide usage of elastomeric components in many applications. Material deformation and fatigue characterization including both crack nucleation and crack growth are typically required for such analysis and predictions. This article discusses relevant material deformation properties obtained from experiments conducted under stress states of simple tension and planar tension. Cyclic transient behavior including Mullin's effect is also discussed. Then, fatigue crack initiation approach and experimental procedure and obtained data are discussed including modeling of the R ratio effect. Finally, based on the fracture mechanics approach, fatigue crack growth test procedure and properties are presented, including a model for R‐ratio effect. It is also shown that using the crack growth approach as a total fatigue life predictor gives reasonable results, as compared with the crack nucleation approach. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Reliability Design for Allowable Stress of Creep Rupture Strength of 4Cr25Ni35 Steel

Creep rupture data plays vital role in life prediction and safety assessment of high temperature components. In order to describe the scattering of the data, a statistical analysis of creep rupture data for 4Cr25Ni35 steel was performed by Z-parameter method. With the application of Z-parameter, reliability design for allowable stress of creep rupture strength was carried out according to design life. It is found that Manson-Haferd method appears better correlation results with experimental data. Statistical analysis shows that the scattering of Z-parameter for 4Cr25Ni35 steel is supported by normal distribution. Compared with safety factor method, the method based on Z-parameter can perform reliability design for allowable stress of creep rupture strength by considering the dispersibility of the rupture data. Reliability design based on Z-parameter is more agree with experimental data.

Reliability Assessment of Solder Joints in Power Electronic Modules by Crack Damage Model for Wind Turbine Applications

Wind turbine reliability is an important issue for wind energy cost minimization, especially by reduction of operation and maintenance costs for critical components and by increasing wind turbine availability. To develop an optimal operation and maintenance plan for critical components, it is necessary to understand the physics of their failure and be able to develop reliability prediction models. Such a model is proposed in this paper for an IGBT power electronic module. IGBTs are critical components in wind turbine converter systems. These are multilayered devices where layers are soldered to each other and they operate at a thermal-power cycling environment. Temperature loadings affect the reliability of soldered joints by developing cracks and fatigue processes that eventually result in failure. Based on Miner’s rule a linear damage model that incorporates a crack development and propagation processes is discussed. A statistical analysis is performed for appropriate model parameter selection. Based on the proposed model, a layout for component life prediction with crack movement is described in details.

Critical Life Prediction of Ship-Borne Helicopter Metallic Dynamic Components Based on Corrosion Damage

In an effort to improve the reliability and reduce the operating costs of helicopter structure, an increasing emphasis is being placed on the damage tolerant approach for life management of helicopter structure. Corrosion damage of helicopter dynamic components and life reliability prediction approach has been systematically analyzed. Based on the conclusion of fatigue life under normal environment, the model for life reliability analysis of helicopter dynamic components has been established. A hybrid approach based on a mixture of the traditional safe-life and damage tolerance techniques can be used as an optimal strategy for ensuring helicopter structural integrity; the life of a helicopter main rotor blade under corrosion environment has been obtained according to flaw-tolerance method. According to probabilistic fracture mechanics theory, the analytical model of structures with initial corner cracks has been established. Thus the service life and inspection intervals according to the request of reliability can be determined; it is very valuable in engineering for life prediction and monitoring of helicopter dynamic components under corrosion environment.

A modelling framework for the calculation of structural loads for fatigue life prediction of helicopter airframe components

A modelling framework for determining structural dynamic loads in airframe components has been developed. This paper addresses the flight dynamics and structural modelling tools of the framework and presents the first validation results. Validation of the calculated component strains has been done by means of comparison with strain gauge measurements on the aft-pylon engine frame during scheduled operational flights. Results show a good agreement for the 3/rev vibrations of the component. Trend analyses provides insight in weight, flight speed and altitude dependencies. The proposed framework is capable of calculating structural dynamic loads of an airframe component in a relatively simple and cost-effective way.

Research on Aircraft Components Life Prediction Based on Grey Relational Analysis

Aiming at life prediction for aircraft components, the life distribution types and the parameter estimation methods were introduced, then a method using grey relational analysis to determine the type of the life distribution was proposed, from which can calculate the remaining life. In this paper, the typical aircraft components were taken as an example to demonstrate the feasibility of the method. It has some certain reference value in the maintenance decision-making.

Study on Fatigue Life Prediction of Mechanical Components with Stochastic Cyclic Load Application

Based on the nominal stress method for fatigue life prediction, the model for predicting the fatigue life of mechanical structures under random cyclic load is developed in this paper. The uncertainty of the cyclic loads applied on the mechanical structures is analyzed. With the number of load application as the life index, the fatigue life prediction models of mechanical structures under random cyclic load are developed with the probability weighted method and the Miner linear cumulated damage rule, when the relationships between fatigue life and stress can be expressed as the exponential function and the power function, respectively. Finally, the models proposed are used to predict the fatigue life of train axis.

Aerodynamics and Heat Transfer for a Cooled One and One-Half Stage High-Pressure Turbine—Part I: Vane Inlet Temperature Profile Generation and Migration

As controlled laboratory experiments using full-stage turbines are expanded to replicate more of the complicated flow features associated with real engines, it is important to understand the influence of the vane inlet temperature profile on the high-pressure vane and blade heat transfer as well as its interaction with film cooling. The temperature distribution of the incoming fluid governs not only the input conditions to the boundary layer but also the overall fluid migration. Both of these mechanisms have a strong influence on surface heat flux and therefore component life predictions. To better understand the role of the inlet temperature profile, an electrically heated combustor emulator capable of generating uniform, radial, or hot streak temperature profiles at the high-pressure turbine vane inlet has been designed, constructed, and operated over a wide range of conditions. The device is shown to introduce a negligible pressure distortion while generating the inlet temperature conditions for a stage-and-a-half turbine operating at design-corrected conditions. For the measurements described here, the vane is fully cooled and the rotor purge flow is active, but the blades are uncooled. Detailed temperature measurements are obtained at rake locations upstream and downstream of the turbine stage as well as at the leading edge and platform of the blade in order to characterize the inlet temperature profile and its migration. The use of miniature butt-welded thermocouples at the leading edge and on the platform (protruding into the flow) on a rotating blade is a novel method of mapping a temperature profile. These measurements show that the reduction in fluid temperature due to cooling is similar in magnitude for both uniform and radial vane inlet temperature profiles.

Bi-variable damage model for fatigue life prediction of metal components

Based on the theory of continuum damage mechanics, a bi-variable damage mechanics model is developed, which, according to thermodynamics, is accessible to derivation of damage driving force, damage evolution equation and damage evolution criteria. Furthermore, damage evolution equations of time rate are established by the generalized Drucker’s postulate. The damage evolution equation of cycle rate is obtained by integrating the time damage evolution equations, and the fatigue life prediction method for smooth specimens under repeated loading with constant strain amplitude is constructed. Likewise, for notched specimens under the repeated loading with constant strain amplitude, the fatigue life prediction method is obtained on the ground of the theory of conservative integral in damage mechanics. Thus, the material parameters in the damage evolution equation can be obtained by reference to the fatigue test results of standard specimens with stress concentration factor equal to 1, 2 and 3.

Fatigue life prediction of notched components: a comparison between the theory of critical distance and the classical stress-gradient approach

Fatigue life prediction for machine components is a key factor in the industrial world and several methods can be traced in technical literature to estimate life of notched components. The paper correlates the classical stress-gradient approach, here after called support factor (SF) method, proposed by Siebel, Neuber and Petersen with the modern theory of critical distance (TCD) approach by Tanaka and Taylor. On the one hand, the main asset of the SF method is that it relies only on the knowledge of the maximum stress and stress gradient in the hot spot. By contrast, the TCD needs the calculation of the stress distribution for a finite depth inside the material. On the other hand, the main drawback of the SF method is that the material parameter ρ* is available only for a limited collection of materials and moreover the experimental procedure to retrieve this parameter is not clearly defined in the technical literature. In order to overcome this limitation, the paper investigates the correlation between the material parameter ρ* and the critical distance L of the TCD by relying on a specific stress function. A comparison between the SF method and the TCD is then performed by considering three different benchmark geometries: a general V-notch in a plate, a pressure vessel and an industrial oleo-hydraulic distributor. Effective stresses are analytically retrieved and compared using both methods for the first two benchmarks and with the help of an elastic finite element analysis for the last one. The resus appear good in terms of fatigue life prediction, especially for the industrial case study.

Evolução microestrutural e alteração de dureza na bainita e na perlita em aços 2,25Cr1Mo após tratamento de envelhecimento

Os aços 2,25Cr-1Mo são largamente utilizados no parque de geração termelétrica no Brasil, estando em boa parte das caldeiras atualmente em operação, e podem apresentar microestrutura composta de ferrita-perlita ou ferrita-bainita. Submetido à fluência, esse material tem suas propriedades degradadas em serviço através de modificações microestruturais típicas, como o coalescimento dos carbonetos originalmente presentes. A extensão das mudanças microestruturais representa, nesse caso, perda de resistência mecânica no material. Como existe uma aceleração no grau de deformação, que depende da tensão, da temperatura e do tempo, com a degradação, a extensão da deterioração microestrutural pode ser usada como uma medida de dano. Dessa forma, é importante conhecer essas modificações para fornecer suporte às técnicas de previsão de vida residual de componentes fabricados com esse tipo de material. A degradação do material ferrítico-perlítico é bastante conhecida, mas a literatura ainda não apresenta resultados consistentes quanto à do ferrítico-bainítico. Trata-se, aqui, de um estudo comparativo da evolução microestrutural do aço 2,25Cr-1Mo com as duas microestruturas, perlítica e bainítica, em temperaturas entre 550 e 600°C até 2.000 h. Os resultados mostraram que a microestrutura do aço ferrítico-bainítico é mais estável que a do ferrítico-perlítico. Entretanto, a estrutura bainítica não obedece aos mesmos estágios de degradação, identificados por Toft e Marsden utilizados para classificar a perlita.

Neural networks for critical high temperature component life prediction

The aim of the present research was to subject a well known parametric model and a neural network model to an acid test of extrapolation in order to determine which can produce improved long term creep rupture life predictions for 2·25Cr–1Mo steels. Linear, squared and cubic parametric models were used and the accuracy of the predictions assessed by calculating the mean percentage absolute error. Many different neural network geometries were developed and the accuracy of the predictions was assessed again by calculating the mean percentage absolute error. As the predictions are concerned with the long term rupture life of components, the accuracy below 60 MPa is of the greatest importance. A neural network model with a 2–5–5–1 architecture provided the lowest error for predictions below 60 MPa, compared to other neural networks and the parametric models, and is therefore the optimum model from this study for predictions of long term rupture creep life for a 2·25Cr–1Mo steel.

Effects of Thermal Aging on Ductile-to-Brittle Transition Temperature Behavior of Welded A516 Steel

Prolonged high temperature exposure of welded C-Mn steels is likely to cause microstructural changes leading to an inrease in the ductile-to-brittle transition temperature (DBTT) of the welded joint. Consequently, such degrading material properties should be quantified in view of establishing accurate component life prediction model. This study examined effects of isothermal aging on DBTT behavior of the heat affected zone (HAZ) in welded Type A516 Gr 70 steels. Microstructures of the as-received weld region revealed the presence of pearlite and ferrite in the base metal while upper and lower bainite are found in the HAZ and weld metal, respectively. Hardness measures for the weld metal region, HAZ and base steel are 172, 209 and 150, respectively. Aging at 420 oC, 500 hours lowers hardness value of the HAZ by 20 %. A series of Charpy impact tests on V-notched specimens are performed for as-received and thermally aged samples at 420 oC for 500, 800 and 1200 hours. Results showed that the absorbed impact energy displays a sigmoidal variation with test temperatures. DBTT ranges from -60 to 5 oC for HAZ while narrow range from -25 to 12 oC for weld metal region. Absorbed impact energy variations in samples aged for durations up to 800 hours display another saturation level over test temperatures between -30 to 10 oC. Fractographic analysis on HAZ fracture surface indicated brittle fracture at -60 oC while ductile failure dominated at 27.7 oC. A mix-mode fracture mechanism is displayed for test conducted at -38 oC.

Geometrical uncertainty in turbomachinery: Tip gap and fillet radius

In turbomachinery, the prediction of component life is crucial. It has been found that the variability of gaps, fillets and small geometries, due to manufacturing process and in service operation, can be higher than expected. This has direct consequences on both component life and efficiency. The question arising is how to take into account the aleatory distribution due to manufacturing uncertainty during the design phase. The use of state of the art Computational Fluid Dynamics allows considering the uncertainties in the estimation of performance parameters. Uncertainty is treated including the statistical distribution of the variability in the geometrical models. In this work the methodology for the analysis of the impact of geometrical variations on components life and machine reliability is presented. A representative high pressure stage is used as a reference and fillet sizes and gaps are modified in order to generate the response surface to geometrical uncertainty. For that machine, the nominal rotor tip gap G 0 is 1.5% of the span. A preliminary analysis has been performed considering a value for the fillet radius r/ G 0 = 50%, to underline the high impact of geometrical variations on the flow field. Then, five fillet radii at the rotor tip and three tip gaps have been studied ( r/ G 0 = 0.0, 0.03, 0.075, 0.15, 0.3 and G/ G 0 = 1, 1.5, 2). A two dimensional Gaussian distribution for the fillet radius and tip gap is arbitrarily assumed. Starting from this distribution the probabilistic functions of corresponding mass-flow through the rotor gap and the heat loading of rotor tip are obtained. The overall variation of mass-flow can be more than 60% and the heat loading is modified by 15% if compared to nominal conditions. Moreover this work shows that by including the mean value without the probability distribution the discrepancy is about 6% for mass-flow and 7% of heat loading.

Fracture Analysis and Fatigue Remaining Life Prediction of Concrete Structural Components Accounting for Tension Softening and Size Effects

This paper presents methodologies for fracture analysis and fatigue remaining life prediction of concrete structural components with and without accounting for tension softening and size effects. Stress intensity factor (SIF) is computed by conducting finite element analysis (FEA) and compared with that obtained by using analytical approach. The domain integral method has been used to calculate the strain energy release rate (SERR) and SIF by post-processing the FEA results. Nonlinear fracture mechanics principles (NLFM) have been used for crack growth analysis and remaining life prediction. Details of size effect in the computation of SIF and remaining life prediction have been presented. Size effect has been accounted for by modifying the Paris law, leading to size adjusted Paris law. Numerical studies have been conducted for fracture analysis, crack growth studies and remaining life prediction. The predicted remaining life values with the combination of tension softening & size effects are in close agreement with the corresponding experimental values available in the literature.

Prediction of creep crack growth properties of P91 parent and welded steel using remaining failure strain criteria

Old grades of creep resistant materials such as P11 and P22 have been studied in depth and data and prediction models are available for design and fitness for service assessment of creep rupture, creep crack growth, thermo-mechanical fatigue, etc. However, as the 9%Cr material is relatively new, there is relatively limited data available and understanding with respect to quantifying the effect of variables on life prediction of components fabricated from P91 is more difficult. Since grade P91 steel was introduced in the 1980s as enhanced ferritic steel, it has been used extensively in high temperature headers and steam piping systems in power generating plant. However, evidence from pre-mature weld failures in P91 steel suggests that design standards and guidelines may be non-conservative for P91 welded pressure vessels and piping. Incidences of cracking in P91 welds have been reported in times significantly less than 100,000 h leading to safety and reliability concerns worldwide. This paper provides a review and reanalysis of published information using properties quoted in codes of practice and from recent research data regarding the creep crack growth of P91 steel, and uses existing models to predict its behaviour. Particular areas where existing data are limited in the literature are highlighted. Creep crack growth life is predicted based on short-term uniaxial creep crack growth (CCG) data. Design and assessment challenges that remain in treating P91 weld failures are then addressed in light of the analysis.