Damage mechanism in weldment of 2.25Cr-1Mo steel under creep-fatigue loading

The purpose of this study is to clarify damage process of “Type IV cracking” in weldment of a 2.25Cr-1Mo steel and to propose a micro damage prediction method. From continuous observation under a creep-fatigue test, it was found that spherical shape voids initiated and grew on grain boundaries in fine grain region and these voids grow continuously by changing their shape to crack-like. Both spherical and crack-like void growth rate equations were derived from the proposed void growth model. It was indicated that measured void growth rate under the creep-fatigue loading was well predicted by the growth rate equations.     

Influence of dynamic strain aging pre-treatment on creep-fatigue behavior in 316L stainless steel

Stress controlled creep-fatigue tests were carried out for hot rolled and dynamic strain aging pre-treated 316L stainless steel different loading levels at 550 °C. For both pre-treatment state materials, dynamic strain aging was found to manifest itself macroscopically as displacement or mean strain abrupt jumps during cyclic deformation. Comparing with hot rolled material, by DSA pre-treatment, the creep-fatigue cyclic strain amplitude is decreased, the cycle at the 1st abrupt displacement jump is postponed and the material creep-fatigue life is effectively prolonged. Moreover, it is found that, the material creep-fatigue life monotonically increases with pre-deformation strain, no matter the maximum pre-deformation stress is larger than the maximum creep-fatigue applied stress or not.     

Remaining life assessment of welded pipes containing cracks

The presence of flaws, their size and location in the welded region affects the safe service life of pressure pipes operating at elevated temperature. A remaining life assessment was required to determine the probable remaining safe life of pressure pipes in a high-temperature steam distribution system in which crack-like defects had been identified in many welds. The crack-like defects indicated by NDE lacked the required information to fully identify their locations within the pipe wall. The effect of not knowing complete details of the location of the identified cracks was studied by considering a 360° circumferential crack with initial radial dimension a₀. The crack positions evaluated were: an internal surface crack, an external surface crack, and embedded cracks at various distances from the internal wall of the pipe. Crack growth was modeled using time-dependent fracture mechanics and a newly developed computer code.     

FATIGUE, CREEP AND CREEP/FATIGUE BEHAVIOUR OF A NICKEL BASE SUPERALLOY AT 700°C

Abstract— This paper presents the results of an experimental testing programme to examine the uniaxial creep, low cycle fatigue and creep/fatigue interaction behaviour of a Ni-base superalloy at 700°C. The material is used in the manufacture of aeroengine turbine discs. A creep continuum damage mechanics model is shown to be capable of accurately predicting the creep and creep rupture behaviour of the material. A healing term has been incorporated into the damage mechanics model to allow the behaviour under creep/fatigue conditions to be described.     

Multilevel approach to lifetime assessment of steam turbines

This paper presents a multilevel methodology for a steam turbine lifetime assessment based on the damage calculation, probabilistic analysis and fracture mechanics considerations. Creep-fatigue damage calculations serve as a basis for evaluating the current lifetime expenditure and for defining additional steps of analysis. The need for the use of probabilistic analysis results from the inherent uncertainty in estimating the lifetime expenditure primarily caused by scatter in material properties. Fracture mechanics considerations are helpful in determining additional safety margins for components containing cracks. This methodology has been illustrated using an example of the lifetime calculations of a high-temperature steam turbine rotor. The calculations were based on the results of 2D numerical simulations performed for steady state and transient operating conditions.     

High-temperature tensile-hold crack-growth behavior of HASTELLOY<Superscript>®</Superscript> X alloy compared to HAYNES<Superscript>®</Superscript> 188 and HAYNES<Superscript>®</Superscript> 230<Superscript>®</Superscript> alloys

The creep–fatigue crack-growth tests of HASTELLOY^® X alloy were carried out at the temperatures of 649°C, 816°C, and 927°C in laboratory air. The experiments were conducted under a constant stress-intensity-factor-range (ΔK) control mode with a R-ratio of 0.05. In the constant ΔK tests, a ΔK of 27.5 MPa\(\sqrt{\mathrm{m}}\) and a triangular waveform with a frequency of 0.333 Hz were used. Various tensile hold times at the maximum load were imposed to study fatigue and creep–fatigue interactions. Crack lengths were measured by a direct current potential drop method. In this paper, effects of hold time and temperature on the crack-growth rates are discussed. Furthermore, the crack-growth rates of the HASTELLOY^® X alloy are compared to those of the HAYNES^® 188 and HAYNES^® 230^® superalloys.     

蠕变—疲劳交互作用下结构缺陷评定方法的比较

电力装备以及石化设备的工作温度和压力呈现出日益增长的趋势,蠕变以及蠕变-疲劳交互作用成为高温设备失效的主要机制之一.文中介绍国际上几部主要缺陷评定标准,R5、BS7910、RCC-MR规程、日本JNC方法以及我国高温缺陷评定方法的最新进展,基于工程应用案例分析,比较不同规程缺陷评定方法的优劣,分析结果显示R5规程更符合实验结果,最后对进一步完善我国缺陷评定规范提出一些意见.     

Study on creep-fatigue evaluation procedures for high-chromium steels—Part I: Test results and life prediction based on measured stress relaxation

Strong demand for improving thermal efficiency of power generation plants promoted the use of high-chromium steels, which have high creep strength and corrosion resistance. Aiming at cost reduction for future nuclear power plants, these materials are also regarded as candidates for structural materials, being favoured for lower thermal expansion rate compared with austenitic stainless steels. In structural design and life management of these plants, failure due to the combination of fatigue and creep damages has been considered as an important phenomenon to be evaluated, in addition to simple creep failure under sustained loading such as inner pressure. The author has been conducting a series of creep-fatigue tests for three types of high-chromium steels used in fossil power plants and the applicability of life prediction methods has been studied. It was found that the time fraction rule gives a relatively small amount of creep damage and overpredicts the failure life, whereas a simple ductility exhaustion method provides very large creep damage which leads to too conservative prediction of failure lives. A modified ductility exhaustion method developed on the re-definition of creep damage as a ductility consumer gave a moderate amount of creep damage and provided reasonable life predictability. Moreover, an empirical formula was derived which can represent the life reduction in compressive hold tests as a function of pure fatigue life and hold time.     

A model for predicting the creep-fatigue life under stepped-isothermal fatigue loading

A model is developed herein for predicting the fatigue life of creep-fatigue damage interaction, which is induced by combined high frequency mechanical loading and low frequency temperature variation, i.e. stepped-isothermal fatigue loading. The model is derived from continuum damage mechanics. In the model, the interaction between creep and fatigue damage is considered to be nonlinear. To validate the proposed model, a cast aluminum alloy is fatigue tested at 200–350 °C and 350–400 °C. The results show that good agreement can be achieved between predicted life and experimental data.     

Influence of corrosion and creep on intergranular fatigue crack path in 2XXX aluminium alloys

In this paper, two examples of the influence of time-dependent processes on crack path in two 2XXX aluminium alloys are presented. The first example is concerned with corrosion-fatigue crack growth resistance of a 2024 T351 alloy cracked in the S-L direction in 3.5% NaCl solution at free corrosion potential. The second example deals with the elevated temperature crack growth resistance of a 2650 T6 alloy that might be used in future supersonic aircraft fuselage panels. The common idea is to correlate quantitative measurements of relevant fractographic features of crack path to the effects of time-dependent processes on crack growth rates.