Papers by Keyword: Creep Crack Growth

Abstract: Modelling of Creep Crack Growth (CCG) using analytical and numerical methods is relevant to life assessment procedures of components operating at elevated temperatures. This paper compares an analytical crack prediction and a numerical based virtual CCG technique used in fracture mechanics components with sample experimental results. Two approaches are presented. First the well developed strain exhaustion model called the NSW and the modified NSW-MOD models which predict plane stress/strain bound crack initiation and growth rates for engineering alloys and the second a damage-based approach used to numerically predict the crack propagation rate in Finite Element models of fracture mechanics specimens. The results from both methods are correlated against an independently determined C* parameter. As an example the NSW and the extended NSW-MOD strain exhaustion models are applied to compare to the experimental data and FE predictions for two steels at Carbon-Manganese steel tested at 360 oC and a weld 316H stainless steel at 550 oC. For values of C* within the limits of the present creep crack growth data presented the plane strain crack growth rate predicted from the numerical analysis is found to be less conservative than the plane strain NSW model but more conservative than plane strain NSW-MOD model.

Abstract: The initiation and growth of micro-defects such as micro cracks and voids usually causes the failure of long term operated structural components at high temperature. In this study, the creep characteristics and void nucleation and growth characteristics of P92 steel which is used as main steam pipe material in power plant were investigated at several temperatures and loading conditions. The area fraction of void increased with increase of test temperature, stress, and load holding time. In case of internal defect presence, micro-voids initiated in the early stage of loading period and resulted in the increased load line displacement and crack growth rate. The microvoids were found to form along the prior austenite grain boundaries and at the martensite packet boundaries.

Abstract: The interaction behavior of two non-aligned through-wall cracks in flat plates is investigated by the finite element method (FEM) under extensive creep condition. The time-dependent fracture parameter C*-integral along the crack tips are calculated and compared to the results of a single crack of the same size. For comparison purpose, the interaction of stress intensity factors (SIFs) is also examined in the study. The results indicated that interaction of multiple cracks is different between the time- dependent fracture characterized by C*-integral and linear elastic fracture noted by SIF. The magnifying factors of time-dependent fracture are obviously larger than that of the linear elastic fracture cases. Therefore, the current re-characterization rule for multiple cracks developed from linear elastic fracture analysis may lead to a non-conservative result and should be modified when it is used in the assessment of time dependent failure.

Abstract: The results of cavity inspection in uniaxial creep tests and creep crack growth tests were compared to determine the cavity effect. The quantity of the cavities increased with increasing strain and crack growth rate as stress and temperature were increased. This resulted in the change of da/dt vs Ct. The increased rate in the number and size of the cavities in P92 steel with temperature was relatively fast, resulting in the high rate of da/dt against Ct. In this study, to determine the characteristics of defects in structure, the sub surface defects of P92 and P122 steels were investigated by an intelligent phased array ultrasonic inspection system. The system was found to be capable of detecting the creep crack length and the formation of cavities at the crack tip.

Abstract: A nondestructive inspection is required to check for defects inside of the actually used components and structures and to confirm their generation and growth rate. Using the backward radiated ultrasonic inspection system, we performed nondestructive inspection of cracks and micro cavities resulting from creep. From the ultrasound test results, the generation and growth of cracks and micro-cavities were confirmed, and the fracture life of the components and structures could be predicted. We confirmed degraded region by crack size and cavity based on the amplitude of the backward radiated ultrasound. The size of degraded region at 600oC was about 10mm, while that at 650oC was about 15mm. The size of crack and the cavity area fraction confirmed by ultrasound were very close to the actual size and cavity area fraction, indicating the validity of the predicted creep crack growth rate and creep.

Abstract: As the operation time of a power plant increases, the degradation and the cracks inside of the structure exposed to high temperature will increase gradually. Therefore, degradation rate, crack growth rate and fracture life of the structure can be evaluated according to the level of degradation and the growth of crack length. We performed creep rupture test and crack growth test with stress and temperature changes to evaluate the degradation rate, crack growth rate and fracture life. Degradation rate was evaluated using micro-cavities. The area fraction of the cavities increased with the increasing temperature and life fraction (t/tf). da/dt, the crack growth rate against Ct estimated from the relationship between load line displacement rate and cavity increase rate, was in good agreement with the result of da/dt vs Ct acquired from the test. It shows that the creep crack growth rate can be evaluated by the increase of cavity area fraction. It was also found that the predicted life calculated with the cavity growth rate was in good agreement with experimental results.

Abstract: The structures connected by welding are fractured often in Heat Affected Zone (HAZ) when cracks grow under high-temperature and high-pressure conditions. In this study, the creep crack growth tests of P92 and P122 steels were performed and the creep fracture characteristics were evaluated. Degradation rate by cavities was investigated and the influence of degradation rate on crack growth rate was evaluated. Crack growth rate of new material was compared with that of the degraded material to confirm the influence of the degradation rate by cavities on the crack growth rate. From these results, the crack growth rates in welded joints were determined. The crack growth rate of the step test specimen was much faster than that of new material under the same test conditions of step temperature, while it was slower than that of new material under the same step loading conditions. It implies that crack growth rate of the welded specimen is faster when degradation rate by cavities is high even under the same temperature and stress conditions. HAZ and weld metal have faster cavity nucleation and growth rates than base metal. Therefore, the HAZ and weld metal specimens showed shorter crack initiation time and the initial crack growth rate (da/dt)c of the HAZ and weld metal was faster than the base metal. If (da/dt)c is fast, fracture life will be reduced.

Abstract: Most heat resisting materials in structural components are used under multi-axial stress conditions and under such conditions ductile materials often exhibit brittle manner and low creep ductility at elevated temperature. Creep crack initiation and growth properties are also affected by multi-axial stress and it is important to evaluate these effects when laboratory data are applied to structural components. Creep crack growth tests using circumferential notched round bar specimens are a simple method to investigate multi-axial stress effects without using complicated test facilities. Creep crack growth tests have been performed using a 12CrWCoB turbine rotor steel. In order to investigate the effects of multi-axial stress on creep crack growth properties, the tests were conducted for various notch depths at 650°C. The circumferential notched round bar specimen showed brittle crack growth behaviour under multi-axial stress conditions. Creep crack growth rate was characterized in terms of the C* parameter. A 12CrWCoB turbine rotor steel has been tested using circumferential notched round bar specimens with different multi-axiality. Circumferential notched round bar specimens show increased brittle creep crack growth behaviour due to the multi-axial stress condition. Creep crack growth properties could be predicted by allowing for the decrease of creep ductility under multi-axial conditions.

Abstract: The growth of cracks in base steel P91 of 9Cr-1Mo class and in intercritical layer of HAZ is measured under creep coditions. For long term tests, a material degradation was detected consisting in an increase of crack growth rate and in a decrease of crack initiation time. An attempt is made to connect these effects with drop in ductility during thermomechanicalexposition.

Abstract: At high temperatures typical for service conditions in fossil power plants, the creep fracture is dominated by the formation, growth and coalescence of cavities. Using high temperature pipe materials, P92 and P122, the characteristics of creep crack growth were analyzed in this study according to the cavities. The characteristics of cavities play a critical role in creep crack propagation and load line displacement. The effect of the load line displacement rate(dv/dt) and crack growth rate(da/dt) on the da/dt–Ct relation of creep crack growth was evaluated at different temperatures and Ki(initial stress intensity factor) values. The number of cavities increased with increasing temperature and Ki. The crack growth rate and load line displacement rate increased with the increase in the cavity numbers. The kind and distribution of these internal flaws were investigated by an intelligent phased array ultrasonic method and they were utilized in deriving the relationship with the creep crack growth rate, which will predict the creep characteristics of these materials.