A CDM-based study of fatigue–creep interaction behavior

Under stress control mode, the damage evolution during fatigue, creep and their interaction behavior actually is a ductility exhaustion process in response to cyclic and static creep. Based on the ductility dissipation theory and effective stress concept of continuum damage mechanism (CDM), a new fatigue–creep interaction damage model has been developed in this paper, where the change of the inelastic strain energy density is used to define the damage variable. To assess this damage model, high temperature fatigue–creep interaction experiments have been carried out for 1.25Cr0.5Mo steel under stress control mode employing a trapezium waveform. The damage evolution laws of 1.25Cr0.5Mo steel under various combinatorial conditions with different maximum stresses and stress amplitudes are derived. Results indicate that the damage parameter and damage model presented in this paper are applicable to describe the damage evolution for fatigue–creep interaction.     

Overview of the HIDA Project

HIDA is a recently completed four-year-duration large research, development and validation project aimed at harmonising and expanding current high temperature crack assessment procedures and practices with a view to developing a unified European/International procedure. The project was started in 1996 and completed in early 2000. The work involved analysis of currently available published and unpublished data and generation of data from various standard fracture mechanics and validation type feature specimen tests on a range of steels used in high temperature plants. The project covered creep, high temperature fatigue and creep-fatigue interaction.This paper outlines the background issues and methodology applied to achieve the project objectives. Details are covered in other HIDA papers presented at the first HIDA Conference (held at CEA Saclay, France, in April 1998),¹ and also in the HIDA papers being presented at this Conference.The project Consortium now intends to expand this work through a new European Commission funded project ‘HIDA Applicability’ to incorporate sensitivity and probabilistic assessment.     

Creep crack growth in welded components — a numerical study and comparison with the R5 procedures

In the present study, creep crack growth (CCG) in a circumferentially welded low alloyed pipe is numerically investigated for a number of different combinations of weldment constituent material properties. A creep ductility based damage model describes the accumulation of creep damage ahead of the crack tip where a constraint parameter and the creep strain rate perpendicular to the crack plane are used as characterising parameters. It is assumed that a fully circumferential creep crack, located in the heat affected zone with a depth of one quarter of the pipe thickness, is growing at a constant rate from the outer surface towards the inside. The numerical results reveal that not only the properties of the zone containing the crack, but also the deformation properties of the surrounding material influence the CCG behaviour. This influence can be noted on the characterising parameters used for the CCG rate predictions as well as on the CCG rate itself. The mismatch influence on corresponding C^* values is, however, marginal. This indicates that determination of the CCG rate in weldments, based on the C^* value only, may result in uncertain estimates.The numerically investigated cases are also assessed by use of the R5 procedures for the sake of comparison. Considering the stress re-distribution, due to the mismatch effect, the CCG rate is determined for the different weldment configurations. The comparison shows that the assumption of plane strain or plane stress conditions in the R5 analysis is essential for the agreement of the results between R5 and the two-parameter approach. Assuming plane stress conditions at the crack tip results in a relatively good agreement for the axial stress dominated cases investigated. However, for the hoop stress dominated cases, the R5 procedures predict higher CCG rates by an order of magnitude.     

Creep crack growth in welds: a damage mechanics approach to predicting initiation and growth of circumferential cracks

The results of damage mechanics finite element analyses have been used to estimate the initiation and growth of type IV cracks in a series of internally pressurised circumferential pipe welds, in main steam pipelines made of 1/2CrMoV steel. The material properties used, for the various zones of new, service-aged and repaired welds, were produced from creep test data at 640°C. Damage distributions and accumulation with time within the HAZ are presented, from which the crack initiation times and positions for these welds, under a closed-end condition, and with additional axial (system) loading, were identified. By investigating the propagation of damage through the wall thickness, the remaining lives of the various weld types were estimated. The method provides a means for predicting the initiation and growth of type IV cracks in these CrMoV weldments, and for estimating the length of time a weld can safely be left in service, after damage, or type IV cracking, is identified during inspection.     

Creep crack growth of high temperature weldments

The experimental programme of the EC supported project (SMT 2070) SOTA aimed at addressing a technical and industrial need to provide guidelines for creep crack growth (CCG) testing and data analysis of weldments. Mechanical and creep properties were determined on two pressure vessel steels of P22 (2.25Cr1Mo) and P91 (9Cr1MoVNb). The specimens were taken from pipe welds for weld metal (WM) tests, and simulated heat affected zone (HAZ) material for the HAZ tests.The CCG tests were carried out on cross-weld compact tension (CT) specimens machined out from weldment of pipes. The tests were done at 550 and 600°C on P22 and P91 materials, respectively. The CT specimens were notched using electrical discharge method, for a sharp starter crack. This method of initiating sharp starter crack was chosen to make sure that all partners will test specimens with starter crack location as specified in the work programme to study crack initiation and growth in WM and HAZ (both in the centre and type IV region). The CCG tests were carried out following the ASTM E1457-92 [ASTM E1457-92, Standard test method for measurement of creep crack growth rates in metals, ASTM, Philadelphia, PA 19103, USA]. The partners assessed their data and sent both assessed and their raw data to be further assessed centrally. All the data from partners were analysed and compared with those of partners' own assessed data.The present paper reports on the analyses of CCG data obtained in the programme including six laboratories from six European countries. The programme addresses the differences and difficulties in testing and the assessment of weldments, and provides guidelines for harmonisation of testing procedures for reliable data production for remanent life assessment of plants with welded components.     

Residual life assessment of steam turbine casing containing crack defect

In steam turbine casing made of cast steel of type 0.6Cr0.5Mo0.25V ( SN 42 2744), which run about 25% over designed life, several crack defects were found. The most serious case was represented by a crack in valve chest, which reached nearly up to one half of wall thickness. In spite of this it was necessary to operate the turbine for some time. The aim of this contribution is to describe procedures used for assessment of safe residual life.Material data necessary for this life assessment were evaluated during research of material properties of another steam turbine casing made of the same type steel whose operating time was finished after 1.23×10⁵ h. The mentioned material data are also presented.     

锅炉锅筒鼓包原因分析及预防

采用化学分析及金相检验等方法 ,对锅炉锅筒产生的鼓包原因进行了分析。结果表明 ,锅炉严重结垢使得锅筒底部钢板长期过热产生蠕变 ,从而导致锅炉锅筒产生鼓包变形     

ZnAl27合金基复合材料高温蠕变性能研究

利用HBE-750型高低温硬度仪和H-800型TEM研究了SiC     

输送带蠕动变形的定量分析

输送带的变形和蠕动,是在输送机设计中应考虑的因素。本文对带式输送机胶带的蠕动量和蠕动速度进行了定量分析和计算,有利于提高滚筒表面的牵引力或制动力。     

快堆包壳用ODS铁素体合金的蠕变性能

对比正在快堆中服役的奥氏体钢和主要候选材料之一的铁素体－马氏体钢的蠕变性能,对作为先进快堆包壳用最有前途的三种ODS（Oxide dispersion strengthened)铁素体合金的蠕变行为进行了研究。