Creep crack growth rate predictions in 316H steel using stress dependent creep ductility

Abstract

Short and long term trends in creep crack growth (CCG) rate data over test times of 500-30 000 h are available for Austenitic Type 316H stainless steel at 550uC using compact tension, C(T), specimens. The relationship between CCG rate and its dependence on creep ductility, strain rate and plastic strain levels has been examined. Uniaxial creep data from a number of batches of 316H stainless steel, over the temperature range 550-750uC, have been collected and analysed. Power-law correlations have been determined between the creep ductility, creep rupture times and average creep strain rate data with stress s normalised by flow stress s 0?2 over the range 0?2,s/s 0?2 ,3 for uniaxial creep tests times between 100 and 100 000 h. Creep ductility exhibits upper shelf and lower shelf values which are joined by a stress dependent transition region. The creep strain rate and creep rupture exponents have been correlated with stress using a two-stage power-law fit over the stress range 0?2,s/s 0?2 ,3 for temperatures between 550 and 750uC, where it is known that power-law creep dominates. For temperature and stress ranges where no data are currently available, the data trend lines have been extrapolated to provide predictions over the full stress range. A stress dependent creep ductility and strain rate model has been implemented in a ductility exhaustion constraint based damage model using finite element (FE) analysis to predict CCG rates in 316H stainless steel at 550uC. The predicted CCG results are compared to analytical constant creep ductility CCG models (termed NSW models), assuming both plane stress and plane strain conditions, and validated against long and short term CCG test data at 550uC. Good agreement has been found between the FE predicted CCG trends and the available experimental data over a wide stress range although it has been shown that upperbound NSW plane strain predictions for long term tests are overly conservative.

Prediction of creep crack growth behavior in Cr–Mo–V steel specimens with different constraints for a wide range of C∗

“…The values of the (A 1 , n 1 ) and (A 2 , n 2 ) are listed in Table 2 [38]. This 2RN behavior of the Cr-Mo-V steel is similar to that of the 316H steel [35,36].…”

“…For accurate CCG predictions for a wide stress range, the stress dependent creep ductility and strain rate model need to be used. In the recent work by Mehmanparast et al [35], by using the stress dependent creep ductility and strain rate model, the CCG behavior of 316H steel was successfully predicted, and good agreement has been found between the predicted CCG rates and available experimental data over a wide stress range. In the further work by Mehmanparast [36], the CCG predictions of 316H steel were performed on a wide range of specimen geometries by using the stress dependent creep ductility and strain rate model in creep damage calculations, and the results were validated through comparison with available experimental data.…”

“…Currently, the availability of the uniaxial creep ductility data of high temperature materials at a wide range of stresses and temperatures in the literature is very limited. The variation of creep ductility against the normalized stress at different temperatures and the estimation of lower shelf creep ductility at 600°C and 550°C for 316H steel have been collated by Mehmanparast et al [35], as shown in Fig. 5 of Ref.…”

“…This constant creep ductility is usually determined by the accelerated uniaxial creep tests at higher stress levels. However, a lot of experimental results and analyses have shown that the creep fracture mechanism depends on stress levels (strain rates), which leads to the stress-regime dependence of the creep ductility of materials [29][30][31][32][33][34][35]. For accurate CCG predictions for a wide stress range, the stress dependent creep ductility and strain rate model need to be used.…”

The creep of alloy 617 at 700 °C: Material properties, measurement of strain and comparison between finite element analysis and digital image correlation

“…(4) , it no longer has the ability to take the load. Thus it is assumed that the stress components in all directions will be zero in the elements concerned, which is simulated in a similar manner to Yatomi et al (2004) , Oh et al (2011) , Kim et al (2013) and Mehmanparast et al (2014) whereby the material properties of the damaged element (i.e. its stiffness) are reduced to a near-zero value.…”