Papers by Keyword: Creep Damage

Abstract: Uncertainty of parameters like material mechanical properties are traditionally described by probabilistic model, which requires detailed statistical information of parameters limited to obtain in engineering application. In this paper, the problem is solved by using the non-probabilistic convex model that only needs the upper and lower bounds of variables instead of abundant statistical information, which greatly reduces the demand of data and avoids unsafe results caused by the scatter of data. Based on the non-probabilistic convex model, the non-probabilistic interval analysis method is introduced to calculate the creep damage of HK40 high temperature furnace tube and to discuss the influence of operation conditions on creep damage with operation pressure and operation temperature regarded as interval variables. Consequently, the effect of outer wall temperature on creep damage interval is greater at the beginning of furnace tube operation stage. With the increase of running time, the effect of inner wall temperature on creep damage interval is greater and greater, and at the same time greater than that of creep damage interval when the outer wall temperature considered as interval variables. When the operating pressure is treated as interval variable, the upper and lower bounds of creep damage are nearly in a straight line that indicates the effect of pressure change on damage is little. Meanwhile, the life of high temperature furnace tube is predicted according to the definition of non-probabilistic interval index, and the analysis results are compared with the results of probabilistic model. The analysis results sufficiently demonstrate that the non-probabilistic approach presented in this paper is viable.

Abstract: Routine non-destructive examination of the steam pipe elbow after more than 240 000 hours of operation at elevated temperature revealed the extensive creep damage on the outer surface of the pipe elbow. Complex metallographic analysis made in this area confirmed creep damage as well as the non-uniform nature of the cavitation. The density of cavities continuously decreased from the outer pipe surface towards the inner surface, but also its density rapidly waned beyond the damaged area in both directions, along the circumference as well as the length of the elbow. The actual extent of the material degradation was then evaluated by testing of mechanical properties, Charpy-V and fracture toughness testing and the results were used in calculation of the residual life of the pipe elbow. It was shown that although the creep damage was perhaps one of the worst detected in Czech Republic, the cracks in the pipe elbow would spread by the stable growth until the half of the pipe wall thickness. This result thus confirms the possibility of creep life extension far beyond the limit criteria used so far.

Abstract: The creep behavior of 2.25Cr-1Mo-0.25V ferritic steel was investigated using a set of physically-based creep damage constitutive equations. The material constants were determined according to the creep experimental data, using an efficient genetic algorithm. The user-defined subroutine for creep damage evolution was developed based on the commercial finite element software ANSYS and its user programmable features (UPFs), and the numerical simulation of the stress distribution and the damage evolution of the semi V-type notched specimen during creep were studied. The results showed that the genetic algorithm is a very efficient optimization approach for the parameter identification of the creep damage constitutive equations, and finite element simulation based on continuum damage mechanics can be used to analyze and predict the creep damage evolution under multi-axial stress states.

Abstract: The oxidation, environmental corrosion and creep damage are limiting factors in the life assessment of engineering components operating at elevated temperatures. Experimental observations have shown that environmental effects (i.e. oxidation) during components operation time can harden the material [1] and lead to evolution of damage in the form of surface cracks (see Figure 1). The regions subjected to environmental damage which show higher hardness are structurally weaker and more brittle. When components operate in the creep regime, the initiated micro cracks may lead to accelerated creep crack growth in the material. The aim of this paper is to highlight the importance of oxidation effects and microcracks on creep in component life assessments.

Abstract: Size effect for tension specimen in creep condition is not very well recognised phenomenon. The Weibull’s works on influence of nonhomogeneity of material properties on its strength suggest that such effect should take place also for creep. The most of authors regard that the stress redistribution occurring during creep considerably reduces size effect. To examine it the author performed a series of experiments for thin tin alloy wires creeping in room temperature as well as some numerical simulations. The results confirmed the existence of size effect for time to failure. The numerical analysis was made for time of first macroscopic damage occurrence t₁ and for time of cross-section failure t₂. Both these parameters show the influence of specimen size according to size effect law, but the dependence of parameter t₁ was more pronounced.

Abstract: Load processes and their interaction over long-term service exposition influence on degradation of construction materials in the equipment of conventional power plants. A wide range of destructive testing methods and number of test for each of them were used to clarify of the extent and mechanism of the damage steel components of the power plants pressure circuit. Changes of material properties were by those methods evaluated in state after operational exposition - till 190 000 hours and after heat treatment reprocessing, for all observed components in the entire profile of the wall thickness. Great attention was paid to metallographic evaluation of the creep damage also. The results of test were compared with the supposed properties of the undegraded material in origin state.

Abstract: An important issue to be considered in the life assessment of power plant components is the effects of prior creep damage on subsequent fatigue crack growth and fracture behavior. To examine these effects, creep damage has been introduced into 316H stainless steel material by interrupting creep crack growth (CCG) tests on compact tension, C(T), specimens at 550 °C. During the CCG tests, the specimen is loaded in tension, crept and unloaded after a small amount of crack extension. This process introduces compressive residual stress fields at the crack tip, which may subsequently affect the fatigue crack growth test results. In this work, neutron diffraction (ND) measurements have been conducted on interrupted CCG test specimens, which contain creep damage local to the crack tip, and the results are compared to predictions obtained from finite element (FE) simulations. Reasonable agreement has been found between the FE predictions and ND measurements.

Abstract: Life assessment of Type 316H stainless steel (SS) is of great concern to industry due to the widespread use of this material in power plant components that often operate at a temperature of around 550 °C. An important issue to be understood and considered in the life assessment of 316H components is the creep deformation and crack growth behaviour of this material in short term and long term operation times. Due to the limited long term test data available in the literature (i.e. [1, ), a new technique have been recently developed to estimate uniaxial creep ductility trends and subsequently creep crack growth (CCG) behaviour of the material at low load levels [. In this new method the creep ductility trends have been estimated as a function of the applied stress normalised by the temperature dependent 0.2% proof stress of the material, σ/σ_0.2, to include the plasticity effects on the creep deformation behaviour of the material. These trends have been implemented in finite (FE) simulations to predict the CCG behaviour of 316H in intermediate and long term tests at 550 °C and provisional results are presented in [.

Abstract: This paper reported a validation of computational software system for creep damage analysis using circular notched bar. The complete computational software system for creep damage analysis consists of pre-and post-processing and the numerical solver and here the validation is primarily for the pre-and post-processing section developed by the authors. The creep damage evolution with time was post-processed interactively via FEMSYS.

Abstract: Information revealed from 3D thermal-mechanical simulations of the innercasing over 0.2 million hours by finite element method(FEM) provides detailed local distributions of stress, temperature, and creep damage through the whole configuration. It found that the innercasing of a Ultra-supercritical(USC) turbine is subjected to the complex stress states. Due to this, the multiaxial effect is considered in the creep damage analysis.