Prediction on fatigue life of U‐notched PMMA plate

In this paper, fatigue life prediction of U‐notched polymethyl methacrylate (PMMA) plate is numerically investigated based on the combination of fatigue damage mechanism and fatigue crack propagation mechanism. First, strength and stiffness degeneration criterions during the fatigue process are established on the basis of nonlinear progressive damage evolution, and the fatigue crack initiation life is estimated. Second, fatigue crack propagation phase is analysed through virtual crack closure technique. The fatigue crack propagation life before totally fracture is also predicted. Finally, finite element models of PMMA plate weakened by lateral symmetric U‐notch are built up using ABAQUS, and the total fatigue life of notched plate is calculated by combining the crack initiation life with the crack propagation life. These results will play an important role for evaluating the fatigue life of U‐notched PMMA plate.     

An efficient fatigue and creep‐fatigue life prediction method by using the hysteresis energy density rate concept

Fatigue damage, time‐dependent creep damage and their interaction are considered as the main failure mechanisms for many high temperature structural components. A generalized methodology for predicting both the high temperature low cycle fatigue (HTLCF) and creep‐fatigue lives by using the hysteresis energy density rate (HEDR) and fatigue damage stress concepts was proposed. Experimental data for HTLCF and creep‐fatigue in Alloy 617, Haynes 230 and P92 steel were respectively collected to validate the method. A better prediction capacity and most of the data points that fall within a 1.5 scatter band were obtained compared with the traditional energy‐based method, time fraction rule and ductility exhaustion model. Moreover, a creep‐fatigue damage diagram was also constructed by using the proposed approach.     

High-temperature creep and long-term strength of structural elements under cyclic loading

We present a method for solving problems of high-temperature cyclic creep and damage accumulation in structural elements. The asymptotic expansion and averaging techniques both over the period of forced vibrations of a body and that of slowly varying loads are used for the set of equations describing the creep and damage processes in thin-walled structural elements. __________ Translated from Problemy Prochnosti, No. 5, pp. 45–53, September–October, 2008.     

航空MDYB-3定向有机玻璃蠕变行为温度效应试验研究

对航空MDYB-3定向有机玻璃不同温度下的蠕变行为进行了试验研究.得到了有机玻璃在20,50℃和75℃和不同应力下的蠕变曲线,结果表明:有机玻璃蠕变分为三个阶段,随着温度和应力的增大,有机玻璃的蠕变速率加快,蠕变断裂伸长增大,抗蠕变断裂时间变短.运用陈化理论,Norton公式和指数公式分别描述了蠕变三个阶段的特性.     

Improvement of life prediction in AISI H11 tool steel by integration of thermo-mechanical fatigue and creep damage models

In hot forging operations, the die surfaces and the nearest surface layers of the die undergo mechanical and thermal cycles which significantly influence their service life. The real thermal and mechanical cycles have been previously investigated in forging plants by measurements and numerical simulation, and a reasonable variation window of process parameters has been determined. A new simulative test applied to AISI H11 hot working die steel has been used to generate failure data in conditions similar to those of the forging dies, but under a more controlled and economical method. Fracture surfaces of specimens for different tests observed by scanning electron microscopy (SEM) indicate that both thermo-mechanical fatigue (TMF) and creep phenomena can be considered to be main damage mechanisms and their contribution to the failure differs as testing conditions vary. As a result of the experiments, the failure is affected by both thermo-mechanical cycle and resting time at high temperature. Therefore, the authors developed a new lifetime prediction model obtained by combining the damage evolution laws, at each cycle, for pure creep and pure TMF. This combination was based on the linear accumulation rule. The damage evolution law for pure creep is obtained by modifying Rabotnov's law in order to suit the case of actual hot forging cycles, where temperature and stress vary widely. The damage evolution law for pure TMF is based on a generalization of the Wöhler–Miner law. This law is modified in order to take into account the presence of thermal cycle and thermal gradient. Comparison between the experimental cycles to failure and the predicted ones was performed using tests excluded in the determination of the coefficients. The conclusion was that the accuracy of prediction appears to be quite good and that the linear accumulation and interaction of TMF and creep is confirmed.     

Micro mechanics based on vacancy diffusion coupled with damage mechanics related to creep deformation and prediction of creep fracture life

Abstract

In situ observations of crack growth and damage progression were conducted under creep conditions for P92 and titanium aluminides inter-metallic compound. A proposed analysis of stress induced particle diffusion was applied to stress induced vacancy diffusion. Results obtained from this analysis were successfully correlated with the experimental behaviour of macroscopic damage progression and a theoretical characteristic of creep deformation was derived. It was found to be in good agreement with experimental characteristics of creep deformation. Furthermore, the experimental characteristics of creep damage progression which concern voids and micro crack formations at grain boundary were found to be well correlated with those of deformation. From these results, correlation between vacancy diffusion in nano-scale, creep damage in mezzo-scale and creep deformation in macro-scale were successfully realized.     

A coupled creep damage evolution model and creep life evaluation

Due to the damage accumulation during creep deformation, creep failure after a certain service time is the most important failure mode for metal structures working at high temperatures. Considering the coupled damage evolution of geometric and material’s damage, a creep life evaluation method based on continuum damage mechanics has been proposed and examined. It is found that the geometric damage evolution model can be deduced theoretically from the creep constitutive equation, while the material’s damage evolution can be assumed in the same way as that for static fatigue problems. Through solving the coupled damage evolution models, creep lives under various stress levels and temperatures can be evaluated in a unified way, just by several material constants which can be determined by some creep tests only.     

Damage modelling: the current state and the latest progress on the development of creep damage constitutive equations for high Cr steels

This paper reviews the fundamentals of the development of creep damage constitutive equations for high Cr steels including (1) a concise summary of the characteristics of creep deformation and creep damage evolution and their dependence on the stress level and the importance of cavitation for the final fracture; (2) a critical review of the state of art of creep damage equation for high Cr steels; (3) some discussion and comments on the various approaches; (4) consideration and suggestion for future work. It emphasises the need for better understanding the nucleation, cavity growth and coalesces and the theory for coupling method between creep cavity damage and brittle fracture and generalisation.     

Damage mechanics based probabilistic high‐cycle fatigue life prediction for Al 2024‐T3 using non‐intrusive polynomial chaos

This paper proposes a low‐cost method for predicting probabilistic high‐cycle fatigue life for Al 2024‐T3 based on continuum damage mechanics and non‐intrusive polynomial chaos (NIPC). To randomize Lemaitre's two scale fatigue damage model, parameters S and s are regarded as random variables. Based on small sample of test life, inverse analysis is performed to obtain samples of the two parameters. Statistic characteristics of the two parameters are calculated analytically through coefficients of NIPC. Fatigue test of aluminum alloy 2024‐T3 standard coupon and plate with hole under different spectrum loading shows that the proposed method is effective.     

Linear matching method on the evaluation of plastic and creep behaviours for bodies subjected to cyclic thermal and mechanical loading

On the basis of the previously proposed Linear Matching Method (LMM), a new LMM model and its corresponding numerical procedure are developed in this paper to allow for the evaluation of plastic, creep and ratchet strains of structures subjected to a general load condition in the steady cyclic state. The constant and varying residual stress fields associated with differing mechanisms are obtained as well as the steady cyclic stress state of the whole component for further structural design and assessment. The total strain range for use in fatigue assessment, including the effect of creep and plastic strains, is obtained. A typical example of 3D holed plate subjected to cyclic thermal load and constant mechanical load is analysed here in detail to verify the applicability of the proposed numerical technique. The LMM results in the paper are compared with those obtained by ABAQUS step‐by‐step inelastic analyses. This comparison demonstrates that the LMM has both the advantage of programming method and the capacity to be implemented easily within a commercial finite element code, in this case ABAQUS. The LMM provides a general‐purpose technique for the evaluation of creep/fatigue interaction in the steady cyclic state. Copyright © 2006 John Wiley & Sons, Ltd.     

两级蠕变下损伤演变及寿命估算

在两级或多级加载下,材料的蠕变寿命分数之和与加载顺序密切相关,这表明加载历史对损伤演变过程具有显著的影响。本研究采用考虑损伤和硬化影响的蠕变律,得到了蠕变损伤演变过程受加载历史的影响,讨论了两组蠕变加载时损伤演变和寿命估算问题。总结表明,本方法与试验结果吻合较好,可以反映加载效应的影响。     

The effect of prior cyclic loading variables on the creep behaviour of ex-service Type 316H stainless steel

Abstract

Initial tests have been conducted for a systematic survey of the effect of prior cyclic loading on subsequent creep properties. Samples of 316H stainless steel were subjected to prior cyclic loading at 550°C at different combinations of strain range and cycles experienced. These samples were then remachined into uniaxial creep specimens and tested under a constant load at 250 MPa at 550°C.

The initial results from specimens subject to prior cyclic loading show significant decreases in the minimum true creep strain rate of between 30 and 94%. A consistent decrease in the minimum creep strain rate was found with increases in both the strain range of the prior cyclic loading and the number of cycles experienced by the sample. In addition, the prior-cyclic loading has significantly changed the shape of the creep curve to varying degrees depending upon the applied cyclic loading.     

Three‐dimensional analyses of unified characterization parameter of in‐plane and out‐of‐plane creep constraint

Based on extensive three‐dimensional finite element analyses, the unified characterization parameter A_c of in‐plane and out‐of‐plane creep constraint based on crack‐tip equivalent creep strain for three specimen geometries (C(T), SEN(T) and M(T)) were quantified for 316H steel at 550 °C and steady‐state creep. The distributions of the parameter A_c along crack fronts (specimen thickness) were calculated, and its capability and applicability for characterizing a wide range of in‐plane and out‐of‐plane creep constraints in different specimen geometries have been comparatively analysed with the constraint parameters based on crack‐tip stress fields (namely R*, h and T_Z). The results show that the parameter A_c in the centre region of all specimens appears uniform distribution and lower value (higher constraint), and in the region near free surface it shows protuberant distribution and higher value (lower constraint). The parameter A_c can simultaneously and effectively characterize a wide range of in‐plane and out‐of‐plane creep constraints, while the parameters R*, h and T_Z based on crack‐tip stress fields cannot achieve this. The different capabilities of these parameters for characterizing in‐plane and out‐of‐plane creep constraints originate from their underlying theories. The parameter A_c may be useful for accurately characterizing the overall constraint level composed of in‐plane and out‐of‐plane constraints in actual high‐temperature components, and it may be used in creep life assessments for improving accuracy.     

Numerical simulation and life prediction on T22 boiler tubes with steam-side and fire-side oxide scale

In this study based on Larsen-Miller empirical formula, two simulation models of T22 boiler tube of a super-heater are analyzed by Finite Element Method (FEM). Model one only considers the steam-side oxide scale, and model two considers both the steam-side and the fire-side oxide scale. The results show that the average temperature and Von Mises stresses of the tube increase gradually due to the existence of the steam-side and fire-side oxide scale, in which the effect of fire-side oxide scale is more remarkable. Based on the simulation results, Kachanov-Rabotnov creep damage model was employed to assess the creep life of the tube. It was found out that at the initial stage of the service time the fire-side oxide scale can protect the tube and delay the creep damage obviously, and the critical creep damage of the tube is only about 0.5 before failure occurs.     

Accelerated LCF‐creep experimental methodology for durability life evaluation of turbine blade

This paper proposes an accelerated low cycle fatigue (LCF)‐creep experimental methodology in laboratory to investigate the durability life of turbine blades. A typical mission profile of the turbine blade was obtained by means of rain flow counting method, considering both the actual flight condition and ground test data. Finite element analysis (FEA) was conducted to obtain the stress and temperature fields of turbine blade. A test system was constructed to conduct LCF‐creep experiments of turbine blades, simulating the stress and temperature distributions of critical section properly. LCF‐creep experiments of full‐scale turbine blades were performed under a trapezoidal loading spectrum. Experiment results showed that the durability life of turbine blade based on numerical method was longer than that based on this experimental methodology, even an order of magnitude. Furthermore, this experimental methodology helped to extend the service life of this blade safely, and its validity was verified in actual service condition.     

Analysis on the fatigue damage evolution of notched specimens with consideration of cyclic plasticity

This paper presents a damage mechanics method applied successfully to assess fatigue life of notched specimens with plastic deformation at the notch tip. A damage‐coupled elasto‐plastic constitutive model is employed in which nonlinear kinematic hardening is considered. The accumulated damage is described by a stress‐based damage model and a plastic strain‐based damage model, which depend on the cyclic stress and accumulated plastic strain, respectively. A three‐dimensional finite element implementation of these models is developed to predict the crack initiation life of notched specimens. Two cases, a notched plate under tension‐compression loadings and an SAE notched shaft under bending‐torsion loadings including non‐proportional loadings, are studied and the predicted results are compared with experimental data.     

Microstructure evolution and analysis of a single crystal nickel-based superalloy during compressive creep

During compressive creep, the cubical γ′ phase in [0 0 1] orientation single crystal nickel-based superalloy is transformed into the rafted structure along the direction parallel to the applied stress axis. By means of the elastic stress-strain finite element method (FEM), the von Mises stress distributions of the cubical γ′/γ phases are calculated for investigating the influence of the applied stress on the stress distribution and the directional coarsening regularity of γ′ phase. Results show that the stress distribution of the cubical γ/γ′ phases may be changed by the applied compressive stress, and the coarsening orientation of γ′ phase is related to the von Mises stress distribution of the γ matrix channel. Thereinto, under the action of applied compressive stress, the bigger von Mises stress produced on (0 0 1) plane of the cubical γ′ phase is thought to be a main reason of the microstructure evolution. The expression of the driving force for the elements diffusion and the directional growing of γ′ phase during compressive creep are also proposed.     

An energy‐based critical fatigue life prediction method for AL6061‐T6

An energy‐based critical fatigue life prediction method is developed and analysed. The original energy‐based fatigue life prediction theory states that the number of cycles to failure is estimated by dividing the total energy accumulated during a monotonic fracture by the strain energy per cycle. Because the accuracy of this concept is heavily dependent on the cyclic behaviour of the material, a precise understanding of the strain energy behaviour throughout each failure process is necessary. Examination of the stress and strain during fatigue tests shows that the cyclic strain energy behaviour is not perfectly stable as initially presumed. It was discovered that fatigue hysteresis energy always accumulates to the same amount of energy by the end of the stable energy region, which has led to a new ‘critical energy’ material property. Characterization of strain energy throughout the fatigue process has thus improved the understanding of an energy‐based fatigue life prediction method.     

Crack evolution law and failure mode of red sandstone under fatigue–creep interaction

To explore the fracture characteristics of rock under the interaction of fatigue load and creep load, fatigue–creep interactive loading experiments were performed on red sandstone with prefabricated cracks. The crack evolution process and failure mode were analyzed using acoustic emission technology and digital image correlation. The results showed that crack growth mainly occurred in the fatigue loading stage; the crack evolution of the sample could be divided into three stages: nucleation and initiation ( ), stable expansion ( ), and unstable fracture ( ). There were distinct differences in the crack propagation modes of the rock samples with different prefabricated crack angles. The relationship between the crack initiation angle and prefabricated crack angle was analyzed based on the maximum circumferential stress theory. Moreover, with an increase in the prefabricated crack angle, the rock sample gradually changed from compression–shear failure to tension–shear failure.