Life Estimation Model Research Articles

Impact of Operating and Health Conditions on a Helicopter Turbo-Shaft Hot Section Component Using Creep Factor

The paper investigates the effects of various gas turbine operating and health conditions on its hot section component’s creep life via a simple relative creep life parameter known as Creep Factor. Using the Creep Factor, the correlation between individual gas turbine operating and health parameter and component’s creep life was established and the weight of the impact was measured. Analytical-parametric-based creep life estimation model combined with the Creep Factor approach was developed and integrated with an existing engine performance model to allow the estimation of various hot section component creep lives and the computation of the Creep Factors. The impact analysis was carried out on the high pressure turbine blade of a model turbo-shaft helicopter engine. The results indicate that for a clean engine, the change in engine rotational speed was seen to provide the highest impact on changing the blade’s creep life consumption while for a degraded engine, the presence of compressor fouling has the highest threat in changing the blade’s creep life. The analysis also shows that the Creep Factor is a good indicator of creep life consumption and provides a good technique to rank the influencing factor according to the threat they imposed.

Life estimation model of a cantilevered beam subjected to complex random vibration

ABSTRACTAn analytic methodology was developed to predict the fatigue life of a structure experiencing stationary, Gaussian random vibration excitation. This method allows the estimation of fatigue life using a frequency domain method, where only the input power spectral density and damping factor are required. The methodology uses linear elastic fracture mechanics for fatigue crack propagation and accounts for the frequency shifting that occurs due to fatigue crack evolution. Good results have been obtained comparing the analytic model to both finite element analysis (FEA) and experimental results, for mild‐steel cantilever beams.

Study on Probability Prediction Approach to Holistic Life Based on Fracture Theory

Fatigue life can be divided into crack initiation life and crack propagation life in fatigue damage process. Fatigue crack comes of the initial discontinuity states （IDS） of material, and goes through short crack , long crack and rupture failure. A estimation model based on fracture theory is built, in which includes of IDS, short crack, long crack and rupture failure. A fatigue holistic life (including fatigue life, the residual life and the calendar life ) estimation model was built on the basis of IDS in which the aircraft structures had not been corrosion damage. A lot of fatigue tests with different stress level and different stress ratio are carried out to validate the model. The results demonstrate that the approach is reasonable and feasible.

A cumulative damage model for fatigue life estimation of high‐strength steels in high‐cycle and very‐high‐cycle fatigue regimes

ABSTRACTA cumulative fatigue damage model is presented to estimate fatigue life for high‐strength steels in high‐cycle and very‐high‐cycle fatigue regimes with fish‐eye mode failure, and a simple formula is obtained. The model takes into account the inclusion size, fine granular area (FGA) size, and tensile strength of materials. Then, the ‘equivalent crack growth rate’ of FGA is proposed. The model is used to estimate the fatigue life and equivalent crack growth rate for a bearing steel (GCr15) of present investigation and four high‐strength steels in the literature. The equivalent crack growth rate of FGA is calculated to be of the order of magnitude of 10−14–10−11 m/cycle. The estimated results accord well with the present experimental results and prior predictions and experimental results in the literature. Moreover, the effect of inclusion size on fatigue life is discussed. It is indicated that the inclusion size has an important influence on the fatigue life, and the effect is related to the relative size of inclusion for FGA. For the inclusion size close to the FGA size, the former has a substantial effect on the fatigue life. While for the relatively large value of FGA size to inclusion size, it has little effect on the fatigue life.

Mechanism‐based life model for out‐of‐phase thermomechanical fatigue in single crystal Ni‐base superalloys

ABSTRACTThis paper describes an enhanced physics‐inspired model to predict the life of the second‐generation single crystal superalloy PWA 1484 experiencing out‐of‐phase (OP) thermomechanical fatigue (TMF). Degradation due to either pure fatigue or a coupling between fatigue and environmental attack are the primary concerns under this loading. The life model incorporates the effects of material anisotropy by utilizing the inelastic shear strain on the slip system having the highest Schmid factor while accounting for the effects of temperature‐dependent slip spacing and stress‐assisted γ′ depletion. Both conventional TMF and special bithermal fatigue (BiF) experiments were conducted to isolate and therefore better understand the interactions between these degradation mechanisms. The influences of crystallographic orientation, applied mechanical strain range, cycle maximum temperature and high temperature hold times were assessed. The resulting physics‐inspired life estimation model for OP TMF and OP BiF predicts the number of cycles to crack initiation as a function of crystal orientation, applied strain amplitude and stresses, temperature, cycle time (including dwells), and surface roughness within a factor of 2.

Stress intensity factor solutions for estimation of fatigue lives of laser welds in lap-shear specimens

Fatigue behavior of laser welds in lap-shear specimens of high strength low alloy (HSLA) steel is investigated based on experimental observations and two fatigue life estimation models. Fatigue experiments of laser welded lap-shear specimens are first reviewed. Analytical stress intensity factor solutions for laser welded lap-shear specimens based on the beam bending theory are derived and compared with the analytical solutions for two semi-infinite solids with connection. Finite element analyses of laser welded lap-shear specimens with different weld widths were also conducted to obtain the stress intensity factor solutions. Approximate closed-form stress intensity factor solutions based on the results of the finite element analyses in combination with the analytical solutions based on the beam bending theory and Westergaard stress function for a full range of the normalized weld widths are developed for future engineering applications. Next, finite element analyses for laser welded lap-shear specimens with three weld widths were conducted to obtain the local stress intensity factor solutions for kinked cracks as functions of the kink length. The computational results indicate that the kinked cracks are under dominant mode I loading conditions and the normalized local stress intensity factor solutions can be used in combination with the global stress intensity factor solutions to estimate fatigue lives of laser welds with the weld width as small as the sheet thickness. The global stress intensity factor solutions and the local stress intensity factor solutions for vanishing and finite kinked cracks are then adopted in a fatigue crack growth model to estimate the fatigue lives of the laser welds. Also, a structural stress model based on the beam bending theory is adopted to estimate the fatigue lives of the welds. The fatigue life estimations based on the kinked fatigue crack growth model agree well with the experimental results whereas the fatigue life estimations based on the structural stress model agree with the experimental results under larger load ranges but are higher than the experimental results under smaller load ranges.

Multiaxial fatigue: An overview and some approximation models for life estimation

A brief overview of some important issues in multiaxial fatigue and life estimation is presented. These include damage mechanisms and damage quantification parameters, material constitutive response and non-proportional hardening, cycle counting and damage accumulation in variable amplitude loading, and mixed-mode crack growth. It is shown that capturing the correct damage mechanism is essential to develop a proper damage quantification parameter for robust multiaxial fatigue life estimation. Additional cyclic hardening of some materials under non-proportional multiaxial loading and its dependence on the load path as well as material microstructure is also discussed. It is argued that critical plane damage models with both stress and strain terms are most appropriate since they can reflect the material constitutive response under non-proportional loading. Importance of a proper cycle counting method to identify cycles in a variable amplitude load history, and capability of the linear cumulative damage rule to sum damage from the counted cycles are also discussed. As mixed-mode crack growth can constitute a significant portion of the total fatigue life, analysis of crack growth rates and correlations under combined stresses is presented. Several models as well as some simple approximations in capturing the aforementioned effects in multiaxial fatigue life estimations are also described. The approximation models include an estimation model for obtaining material non-proportional cyclic hardening coefficient, and a simple multiaxial fatigue life estimation model for steels based on hardness as the only required material property.

Multiaxial fatigue life estimation based on a piecewise ruled S– N surface

A multiaxial model for fatigue life estimation, formulated in terms of a piecewise ruled S– N surface, is proposed. The first ruled surface considers the sum of a deviatoric stress amplitude and the maximum hydrostatic stress as an exponential function of the number of cycles to failure. For small magnitudes of the hydrostatic stress, another surface is defined by considering only the effect of the deviatoric stress amplitude upon the expected fatigue life. The deviatoric stress amplitude was computed by the maximum prismatic hull concept. The model proved successful when compared with other multiaxial criteria for available data.

A new model for life estimation of creep

This paper deals with a new model for life estimation of creep derived from the law of energy conservation and the momentum conservation principle. By mathematical technique, the expressions of these two rules are transformed into a simple function, which can describe the relationship between the internal energy and creep life. This model supposes that the internal energy change of a specimen is a constant from the initial state to the failure one under certain conditions. Moreover, owing to its simple expression, it is very easy to use this model to predict the creep life. To check the validity of the new model proposed in this paper, the published data of 2.25Cr–1Mo, 1Cr–0.5Mo, 18Cr–12Ni–Mo and 1Cr–1Mo–0.25 V steels have been studied and analyzed. A well agreement is noted between the model and the results.

ANOVA parameters influence in LCF experimental data and simulation results

1 Abstract The virtual design of components undergoing thermo mechanical fatigue (TMF) and plastic strains is usually run in many phases. The numerical finite element method gives a useful instrument which becomes increasingly effective as the geometrical and numerical modelling gets more accurate. The constitutive model definition plays an important role in the effectiveness of the numerical simulation [1, 2] as, for example, shown in Figure 1. In this picture it is shown how a good cyclic plasticity constitutive model can simulate a cyclic load experiment. The component life estimation is the subsequent phase and it needs complex damage and life estimation models [3-5] which take into account of several parameters and phenomena contributing to damage and life duration. The calibration of these constitutive and damage models requires an accurate testing activity. In the present paper the main topic of the research activity is to investigate whether the parameters, which result to be influent in the experimental activity, influence the numerical simulations, thus defining the effectiveness of the models in taking into account of all the phenomena actually influencing the life of the component. To obtain this aim a procedure to tune the parameters needed to estimate the life of mechanical components undergoing TMF and plastic strains is presented for commercial steel. This procedure aims to be easy and to allow calibrating both material constitutive model (for the numerical structural simulation) and the damage and life model (for life assessment). The procedure has been applied to specimens. The experimental activity has been developed on three sets of tests run at several temperatures: static tests, high cycle fatigue (HCF) tests, low cycle fatigue (LCF) tests. The numerical structural FEM simulations have been run on a commercial non linear solver, ABAQUS®6.8. The simulations replied the experimental tests. The stress, strain, thermal results from the thermo structural FEM simulations have been used to assess specimens life by means of several damage models implemented in two dedicated softwares. One of them is commercial life estimation software and the second one has been developed by the present research group. The life estimations results obtained from the two softwares and from the damage models have been compared with experimental duration in different test conditions. A statistical analysis of life estimation results allowed evaluating the effectiveness of the procedure.

Verification of Life Estimation Model for Space Lithium-Ion Cells

Large capacity Li-ion cells with 100 Ah for satellite application had been developed in 1999, and calendar and cycle life characteristics of the cells had been evaluated under various test conditions with wide range of temperature (0°C–60°C), depth of discharge (3%–80% DOD), and state of charge (0%–100% SOC). These tests were started in 1999, and the data have been accumulated until 2008 for around ten years. From these results, we have confirmed that GYT Space Li-ion cells have sufficient capability to achieve the mission life requirements for several kinds of artificial satellites. Furthermore, we have discovered that our simple life estimation model needs to be modified to consider SEI growth blocking mechanism. It means that the SEI growth is blocked by the adjacent SEI layers, therefore calendar capacity loss is affected by not only its test term, temperature, and state of charge but also its calendar capacity loss values. Our modified estimation formula is that a rate of calendar capacity loss is decreased in proportion to the 2.4th power of the calendar capacity retention. By using the modified formula, the estimation results show very good fitting with the long term cell test data for ten years.

Fatigue behavior of spot friction welds in lap-shear and cross-tension specimens of dissimilar aluminum sheets

Fatigue behavior of spot friction welds or friction stir spot welds in lap-shear and cross-tension specimens of dissimilar aluminum 5754-O and 7075-T6 sheets is investigated based on experimental observations and three fatigue life estimation models. Optical micrographs of the 5754/7075 and 7075/5754 welds before and after failure under quasi-static and cyclic loading conditions are examined to understand the fracture and failure mechanisms of the welds. Under cyclic loading conditions, the micrographs show that the 5754/7075 welds in lap-shear specimens mainly fail from the kinked fatigue crack through the lower sheet thickness. Also, the micrographs show that the 7075/5754 welds in lap-shear specimens mainly fail from the kinked fatigue crack through the lower sheet thickness and from the fracture surface through the upper sheet thickness. Under cyclic loading conditions, the micrographs show that the 5754/7075 and 7075/5754 welds in cross-tension specimens mainly fail from the fatigue crack along the interfacial surface and from the fracture surface through the upper sheet material. A kinked fatigue crack growth model based on the stress intensity factor solutions for finite kinked cracks, a structural stress model based on the closed-form structural stress solutions, and a through-nugget fatigue crack growth model based on the analytical stress intensity factor solutions for cracks growing through the weld nugget are adopted to estimate the fatigue lives of the 5754/7075 and 7075/5754 welds in lap-shear and cross-tension specimens. The fatigue life estimations based on the kinked fatigue crack growth model, the structural stress model and the through-nugget fatigue crack growth model agree well with the experimental results. Finally, the effective stress intensity factor and J integral solutions at the critical locations of the welds obtained from three-dimensional finite element analyses appear to be good fracture mechanics parameters to correlate the experimental fatigue data for the 5754/7075 and 7075/5754 welds in lap-shear and cross-tension specimens.

Fatigue behavior of aluminum 5754-O and 6111-T4 spot friction welds in lap-shear specimens

Fatigue behavior of aluminum 5754-O and 6111-T4 spot friction welds in lap-shear specimens is investigated based on experimental observations and two fatigue life estimation models. Optical micrographs of the 5754 and 6111 welds made by a concave tool and a flat tool, respectively, before and after failure under quasi-static and cyclic loading conditions are examined. The micrographs show that the failure modes of the 5754 and 6111 welds under quasi-static and cyclic loading conditions are quite different. Under quasi-static loading conditions, both types of welds mainly fail from the nearly flat fracture surface through the nugget. Under low-cycle loading conditions, both types of welds mainly fail from the kinked crack through the upper sheet thickness and the fracture surface through the nugget. Under high-cycle loading conditions, both types of welds mainly fail from the kinked cracks through the upper and lower sheet thicknesses. A kinked fatigue crack growth model based on the stress intensity factor solutions for finite kinked cracks and a structural stress model based on the closed-form structural stress solutions at the critical locations of the welds are adopted to estimate the fatigue lives of both types of welds. The fatigue life estimations based on the kinked fatigue crack growth model and the structural stress model appear to agree well with the experimental results for both types of welds.

Continuum damage mechanics-based creep–fatigue-interacted life prediction of nickel-based superalloy at high temperature

This paper presents a creep–fatigue-interacted lifetime prediction methodology based on continuum damage mechanics for the Ni-based superalloy under cyclic loading at high temperature. A unified life estimation model has been developed enabling stress–strain curves and cumulative damage to be predicted. Four types of low cycle fatigue tests were carried out at 650 °C. The effects of the levels of strain range and strain hold time together with frequency characteristics of fatigue behavior on the lifetime were investigated.

Estimating Service Lives of Organic Vapor Cartridges III: Multiple Vapors at All Humidities

A published model for estimating service lives of organic vapor (OV) air-purifying respirator cartridges has been extended to include multiple organic vapors at all humidities. Equilibria among the OVs are calculated using Ideal Adsorbed Solution Theory, whereas the effects of adsorbed water are considered as due to micropore volume exclusion. Solubilities of OVs in water must also be taken into account. Adsorption kinetics of components of mixtures are based on published correlations of the effects of covapors and water vapor. The dynamics of adsorption and competition are incorporated using expanding zones within the carbon bed, taking into account vapor and water displacements. Measurements of breakthrough curves for two ternary OV mixtures at high humidities have been done for a single cartridge type. The service life estimation model, implemented as a spreadsheet and a computer program, has been tested against these data as well as data for OV mixtures from literature sources. Good agreements were obtained between model predictions and experimental breakthrough times at dry conditions and humid conditions.

Robustness improvement of tool life estimation assisted by a virtual manufacturing cell

As the cutting condition changes dramatically in flexible manufacturing systems, the developed tool condition monitoring (TCM) system cannot easily acquire cutting parameters in cutting process and thus it weakens its robustness. This paper presents a new approach to improve the robustness of on-line tool life estimation in NC milling with the assistance of a virtual manufacturing cell from which most of the cutting variables and motor powers can be calculated to improve the feature extraction and decision-making of a TCM system. A robust model for tool life estimation is established and its effectivity is validated by experimental results.

Fatigue crack growth in stiffened steel tubular joints in seawater environment

Welded steel tubular structures commonly used for offshore platforms are susceptible to fatigue failure due to stress concentration at welded joints and environmental loading which is cyclic in nature. Offshore structures contain flaws ranging from microscopic material defects to macroscopic geometric imperfections, due to fabrication and construction process. Such flaws can develop into cracks due to influence of cyclic wave loading in an aggressive marine environment. Due to this and many other causes, maintaining structural integrity of these structures has been a major concern for the oil industry. Hence a model for crack initiation life and crack growth life estimation has been developed by conducting constant amplitude corrosion fatigue tests on internally ring-stiffened steel tubular T and Y joints under freely corroding conditions. Biologically inert synthetic seawater was used as the corrosive environment. Tests in seawater were carried out with a loading frequency of 0.2 Hz. The studies were made with respect to tropical environment similar to Indian offshore. The corrosion fatigue crack growth data obtained during the tests and the Fracture Mechanics (FM) model developed are discussed in this paper.

601 CFRP接着補強構造物の疲労強度評価(OS 接着・界面)

The structure made of aluminum alloy, which was stiffness-reinforced with CFRP bonded adhesively to decrease vibration, was put to practical use. To put CFRP adhesive structure to practical use, it was necessary to obtain the fatigue life. In current standard test methods, there wasn't an suitable test method for the design of this application. Therefor, we had to use the full scale model equivalent to actual structure each time. In this study, we suggested a compact model for the fatigue life estimation of reinforced structure combined with CFRP bonded adhesively. And fatigue tests using full scale models and compact models were carried out. As the result, we obtained the good interrelation between the fatigue life of the full scale model and the life of the compact model.

Application of a two-surface plasticity model for thermal ratcheting and failure life estimation in structural model tests

Abstract In the structural design of elevated temperature components of liquid metal fast breeder reactor plants, detailed inelastic analysis needs to be applied when design code criteria for preventing excessive deformation or creep-fatigue failure cannot be satisfied by elastic design routes. Because the results of inelastic analysis are fully dependent on the material deformation models used, development of a reliable inelastic constitutive model is essential for application of detailed inelastic analysis to design assessment. In this study, one of the recent plastic constitutive models developed by the author, which belongs to a category of two-surface models, was applied to inelastic analysis of two types of type 304 stainless steel structural models subjected to repeated thermal cycles. The conventional life estimation method employed in the present design rules was used for the prediction of failure life due to thermal fatigue or creep-fatigue interaction. Reasonable agreement was obtained between the test results and the predictions in regard to thermal ratcheting behaviour and crack initiation life.

Effect of heat treatment on low cycle fatigue properties of zircaloy-4: Xiao, L. and Gu, H.C. Xiyou Jinshu Cailiao yu Gongcheng (Rare Metal Materials and Engineering) (Oct. 1993) 22(5), 61–66 (in Chinese)

Effect of cycling frequency on fatigue behavior of neat, talc filled, and short glass fiber reinforced injection molded polymer composites was investigated by conducting load-controlled fatigue tests at several stress ratios (R = −1, 0.1, and 0.3) and at several temperatures (T = 23, 85 and 120 °C). A beneficial or strengthening effect of increasing frequency was observed for some of the studied materials, before self-heating became dominant at higher frequencies. A reduction in loss tangent (viscoelastic damping factor), width of hysteresis loop, and displacement amplitude, measured in load-controlled fatigue tests, was observed by increasing frequency for frequency sensitive materials. Reduction in loss tangent was also observed for frequency sensitive materials in DMA tests. It was concluded that the fatigue behavior is also time-dependent for frequency sensitive materials. A Larson–Miller type parameter was used to correlate experimental fatigue data and relate stress amplitude, frequency, cycles to failure, and temperature together. An analytical fatigue life estimation model was also used to consider the strengthening effect of frequency in addition to mean stress, fiber orientation, and temperature effects on fatigue life.