Creep-fatigue crack growth behaviour of P91 steels

The importance of predicting failure due to combined creep-fatigue crack growth in high temperature power-plant components has become of great importance importance due to the need for plant to ‘load follow’ in response to fluctuations in demands and the availability of renewables. P91 steel has been widely utilized in conventional plant components. Creep fatigue crack growth (CFCG) tests have been performed on compact specimens at temperatures ranging between 600° C to 625° C. The experimental results have been compared to static creep, high cycle fatigue and CFCG test data available in literature on P91 steel. The CFCG data has been characterised using stress intensity factor range parameter, ΔK and C* parameter. The crack growth per cycle and ∆K relationship shows that at high frequency, the CFCG behaviour tends to that of high cycle fatigue crack growth and at low frequency, the contribution of creep becomes increasingly more significant. The correlation between crack growth rate and C* parameter, shows that most CFCG data fall within the creep crack growth (CCG) P91 data band which may indicate that the crack growth behaviour is dominated by creep processes. Fractography has also shown an intergranular, ductile fracture surface indicating creep dominance for the conditions considered. A linear cummulative rule has ben used to predict the CFCG experimental result

Experimental and numerical investigation of the weld geometry effects on Type IV cracking behaviour in P91 steel

The focus of the present study is on creep crack growth behaviour in Type IV region of P91 steel weldments at 650 °C. In the experimental studies on small- and large-scale single-edge notched specimens in tension, SENT, the effects of weld dimensions and specimen size on the creep crack growth behaviour of the material are investigated. The experimental results demonstrate that the crack starts to propagate at an angle normal to the loading direction, subsequently deviates towards the Type IV region and the specimen eventually ruptures when the crack growth angle becomes parallel to the loading direction. The creep rupture data for SENT specimens compared well with those of the round bar specimens for P91 welded joints. In addition, the data for crack growth rates from the deviating crack path were correlated with the C* fracture mechanics parameter and showed good agreement with standard compact tension test data. To predict the creep crack growth behaviour in the Type IV region, finite element simulations were performed in conjunction with a multiaxial ductility damage criterion at the weld/base metal interface. Given that a lower failure strain along the Type IV region is prominent, it is shown that the cracking, in line with the experiments, followed the HAZ region and led to the final creep rupture in the net sectio

The Fracture Mechanics Concept of Creep and Creep/Fatigue Crack Growth in Life Assessment

There is an increasing need to assess the service life of components containing defect which operate at high temperature. This paper describes the current fracture mechanics concepts that are employed to predict cracking of engineering materials at high temperatures under static and cyclic loading. The relationship between these concepts and those of high temperature life assessment methods is also discussed. A model for predicting creep crack growth initiation and growth in terms of C* and the creep uniaxial ductility is presented and it is shown that this model gives good agreement with the experimental results. The effects of cyclic loading on crack growth behaviour are considered and fractography evidence is shown to back a simple cumulative damage concept when dealing with creep/fatigue interaction. Finally a discussion is presented which highlights the important aspect of life assessment methodology for high temperature plant