Catrin M. Davies

Experimental Evaluation of the J or C * Parameter for a Range of Cracked Geometries

In the current ASTM Standard Test Method for Measurement of Creep Crack Growth Rates in Metals (E 1457) the experimental C* parameter is related to the load and creep load line displacement rate through the geometry related η factor. In this work η factors for a range of geometries are presented. The geometries examined are the compact tension specimen, C(T), single edge notch specimen in tension, SEN(T), and bending, SEN(B), double edge notch specimen in tension, DEN(T), middle crack specimen in tension, M(T) and the C-shaped specimen in tension CS(T). Calculations have been performed for a linear elastic-power law hardening material but the resulting η factors are applicable to either power law plastic or power law creeping materials. Values for ηLLD and ηCMOD, based on the load line displacement and crack mouth opening displacement, respectively, have been determined. A wide range of crack depths, 0.1⩽a/W⩽0.7, where a is crack length and W is specimen width, and hardening exponents, 3 ⩽N ⩽10, under plane stress and plane strain conditions have been examined using the finite element method. The influence of specimen length, crack length, material properties and out of plane stress state on the η factor has also been considered. It has been found that for shallow cracks the value of η depends quite strongly on the exponent, N in the material power law, regardless of whether η is defined based on the load line displacement or crack mouth opening displacement. The ηLLD factor has also been found to be strongly sensitive to plane stress/strain conditions imposed, a/W and specimen length, whereas ηCMOD depends more weakly on a/W and is almost independent of specimen length for the cases examined. There is, however, no clear trend in these variations over the range of specimen geometries and a/W examined. These results are found to be consistent with those in the literature. Recommendations are made regarding the most appropriate values for η, depending on the specimen type and geometry while taking into account the variability due to the material properties, out of plane stress state and variations between the numerical analyses.

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 550°C 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 550–750°C, 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 σ normalised by flow stress σ0·2 over the range 0·2<σ/σ0·2<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<σ/σ0·2<3 for temperatures between 550 and 750°C, 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 550°C. 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 550°C. 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 upper-bound NSW plane strain predictions for long term tests are overly conservative.

Effect of low transformation temperature weld filler metal on welding residual stress

Abstract The effect of weld filler metal austenite to acicular ferrite transformation temperature on the residual stresses that arise during the gas metal arc welding of a low carbon steel has been examined using a finite element model. It was found that the stress levels in the weld can be tailored by the appropriate selection of the filler metal and compressive, near zero or tensile residual stresses produced. Reasonable agreement was obtained between the model and the stresses measured using neutron diffraction both in welds using conventional and low transformation temperature filler metal.

Failure assessment diagram analysis of creep crack initiation in 316H stainless steel

In this work the time dependent failure assessment diagram (TDFAD) approach is applied to the study of crack initiation in Type 316H stainless steel, a material commonly used in high temperature applications. A TDFAD has been constructed for the steel at a temperature of 550 °C, and was found to be relatively insensitive to time. The TDFAD procedure is then applied to predict initiation times, at increments of creep crack growth Δa=0.2 and 0.5 mm, for tests on compact tension specimens and the results compared to experimentally determined values. It has been found that initiation time predictions are sensitive to the creep toughness values, and to the limit load (or reference stress) solution used. Conservative predictions of initiation times have been achieved through the use of the lower bound creep toughness values in conjunction with the plane strain limit load solution. The plane stress limit load solution has given conservative predictions for all bounds of creep toughness used.

Evaluation of the Testing and Analysis Methods in ASTM E2760-10 Creep-Fatigue Crack Growth Testing Standard for a Range of Steels

The development of the ASTM E2760-10, “Creep/Fatigue Crack Growth Testing” standard has initiated a phase of testing, analysis, and round robin initiatives associated with high temperature cyclic loading crack growth tests. Creep and fatigue are two complex independent mechanisms which may assist each other to drive the crack. A simplification of the macro creep/fatigue crack growth response of structures is therefore required. Using a linear cumulative damage method, as proposed in ASTM E2760, and other codes of practice, data for a range of steels are analysed. Limited crack growth data, using test methods similar to ASTM E2760, from previous collaborative projects for a range of steels are presented and analysed to identify the difficulties in the process. Creep-fatigue crack growth tests at frequencies ranging from 10 to 0.001 Hz are examined to consider the testing methodologies and in order to assess the effect of data scatter, dwell times, and the interaction region between creep crack growth (CCG) and fatigue crack growth mechanisms. Given the limited data available and the level of scatter, it can still be shown that the linear cumulative summation of static and high frequency data may be sufficient to predict creep/fatigue interaction both in terms of C* and ΔK, and the level of creep ductility will dictate appropriateness of the correlating parameter employed. Finally, in order to show that the level of data scatter can be dealt with in a predictive manner, the crack initiation and growth rates are also analysed using the NSW CCG model.

Comparison of methods for obtaining crack-tip stress distributions in an elastic-plastic material

Under linear elastic and elastic-plastic conditions the K field and the HRR (Hutchinson-Rice-Rosengren) field respectively are expected to provide an accurate representation of the stress field close to the crack tip in an elastic-plastic material. It has recently been proposed in French and UK defect assessment procedures that the Neuber method, originally developed for sharply curved notches, provides an alternative approach to estimate both notch and crack-tip stress fields, for use in conjunction with the sigma-d (σd) method to predict creep crack initiation times. In this work, the crack-tip stress fields under plane strain conditions, predicted from the Neuber approach, are compared with the HRR and K fields as well as those obtained from full-field finite element calculations. A compact tension and a single edge notched tension specimen have been examined; the material model used is the Ramberg-Osgood, power law plasticity model. As expected, the K field and HRR field have been found to provide a good representation of the near-tip fields at low and high loads respectively. Satisfactory solutions have also been obtained through the use of the reference stress to estimate the amplitude of the crack-tip stress in conjunction with the HRR field. The Neuber approach provides a good estimate of the equivalent (von Mises) stresses over the full range of load levels. However, but the use of the Neuber approach directly to predict the maximum principal stress in the plane of the crack provides a non-conservative prediction. A modified Neuber method, using an appropriate scaling function, has been proposed to determine the maximum principal stress in the plane of the crack from the equivalent (von Mises) stress predicted by the Neuber approach. Using the proposed method, a significantly improved estimate of the crack-tip stresses is obtained.

Creep Failure Simulations for 316H at 550°C

In this work a method to simulate failure due to creep is proposed using finite element damage analysis. The creep damage model is based on the creep ductility exhaustion concept. Incremental damage is defined by the ratio of incremental inelastic (plastic & creep) strain and multi-axial ductility. A simple linear damage summation rule is applied. When accumulated damage becomes unity, element stresses are reduced to almost zero to simulate progressive crack growth. The model is validated through comparison with experimental data on various sized compact tension, C(T), specimens of 316H stainless steel at 550 °C. The influence of the inelastic strain rate on the uniaxial ductility is considered. Good agreement is found between the simulated results and the experimental data.Copyright

Specimen Geometry Effects on Creep Crack Initiation and Growth in Parent Materials and Weldments

High temperature crack growth in weldments is of great practical concern in high temperature plant components. Cracking typically occurs in the heat affected zone (HAZ) and often propagates into adjacent parent material (PM). Recently, the importance of constraint effects on creep crack growth behaviour has been recognised and creep crack growth testing on a range of specimen geometries has been performed. Experimental crack growth testing has been performed at 550 °C on a range of fracture specimens using sections taken from a non-stress-relieved 316 steel weldment. These specimens include the compact tension, C(T), middle tension, M(T) and circumferentially cracked bar, CCB, geometries. Results are presented from two long-term creep crack growth (CCG) tests performed on M(T) weldment specimens and these are compared with available data on C(T) and CCB weldment specimens together with both long and short term tests on parent material for a range of specimen geometries. The creep crack initiation (CCI) and growth (CCG) behaviour from these tests has been analysed in terms of the C* parameter. As high levels of residual stress exist in non-stress-relieved weldments, the residual stresses remaining in the weldment specimens have therefore been quantified using the neutron diffraction technique. Long-term (low-load) tests are required on PM specimen to observe specimen constraint effects in 316 steel at 550 °C. When interpreted in terms of the C* parameter the CCG behavior of PM and Weldment materials follow the same trendline on low constraint geometries. However, significant difference is observed in the CCG behavior of PM and weldments on the high constraint C(T) geometry. Long term tests on C(T) specimen weldments are required to confirm the results found.Copyright

Prediction and assessment of springback in typical creep age forming tools

Creep age forming is often carried out under vacuum or autoclave loading conditions, where a sufficiently high, uniform pressure is required to force the workpiece into close contact with the tool surface. However, many creep age forming tests are performed to evaluate springback by clamping the workpiece to both ends of a cylindrical tool and forcing it to the tool surface, which is different from reality. In this study, a set of mechanistically based unified creep ageing constitutive equations have been incorporated into the commercial finite element code ABAQUS and used to analyse a common creep age forming tester, which employs a cylindrical tool shape. Two loading conditions are investigated: (1) end clamp and (2) uniform pressure. The amount of springback has been predicted, compared and analysed for both loading cases. A method has been introduced to assess the local curvature and springback variations. Good contact was achieved between the workpiece and tool surfaces for the uniform pressure condition (except at the plate end), providing that sufficient pressure was applied. However, for the end clamp condition, contact was limited to the vicinity of the clamps.

AN INVESTIGATION OF IRREGULAR CRACK PATH EFFECTS ON FRACTURE MECHANICS PARAMETERS USING A GRAIN MICROSTRUCTURE MESHING TECHNIQUE

A sub-grain size finite element modelling approach is presented in this paper to investigate variations in fracture mechanics parameters for irregular crack paths. The results can be used when modelling intergranular and transgranular crack growth where creep and fatigue are the dominant failure mechanisms and their crack paths are irregular. A novel method for sub-grain scale finite element mesh consisting of multiple elements encased in ~50–150 μm-sized grains has been developed and implemented in a compact tension, C(T), mesh structure. The replicated shapes and dimensions were derived from an isotropic metallic grain structure using representative random sized grain shapes repeated in sequence ahead of the crack tip. In this way the effects of crack tip angle ahead of the main crack path can be considered in a more realistic manner. A comprehensive sensitivity analysis has been performed for elastic and elastic-plastic materials using ABAQUS and the stress distributions, the stress intensity factor an...