Determination of Constraint-Modified J-R Curves for Carbon Steel Storage Tanks

Mechanical testing of A285 carbon steel, a storage tank material, was performed to develop fracture properties based on the constraint theory of fracture mechanics. A series of single edge-notched bend (SENB) specimen designs with various levels of crack tip constraint were used. The variation of crack tip constraint was achieved by changing the ratio of the initial crack length to the specimen depth. The test data show that the J-R curves are specimen-design-dependent, which is known as the constraint effect. A two-parameter fracture methodology is adopted to construct a constraint-modified J-R curve, which is a function of the constraint parameter, A2, while J remains the loading parameter. This additional fracture parameter is derived from a closed form solution and can be extracted from the finite element analysis for a specific crack configuration. Using this set of SENB test data, a mathematical expression representing a family of the J-R curves for A285 carbon steel can be developed. It is shown that the predicted J-R curves match well with the SENB data over an extensive amount of crack growth. In addition, this expression is used to predict the J-R curve of a compact tension specimen (CT), and reasonable agreement to the actual test data is achieved. To demonstrate its application in a flaw stability evaluation, a generic A285 storage tank with a postulated axial flaw is used. For a flaw length of 10% of the tank height, the predicted J-R curve is found to be similar to that for a SENB specimen with a short notch, which is in a state of low constraint. This implies that the use of a J-R curve from the ASTM (American Society for Testing and Materials) standard designs, which typically are high constraint specimens, may be overly conservative for analysis of fracture resistance of large structures.

Predicting How Crack Tip Residual Stresses Influence Brittle Fracture

A probability distribution model, based on the local approach to fracture, has been developed and used for estimating cleavage fracture following prior loading (or warm pre-stressing) in two ferritic steels. Although there are many experimental studies it is not clear from these studies whether the generation of local residual stress and/or crack tip blunting as a result of prior loading contribute to the enhancement in toughness. We first identify the Weibull parameters required to match the experimental scatter in lower shelf toughness of the candidate steels. Second we use these parameters in finite element simulations of prior loading on the upper shelf followed by unloading and cooling to lower shelf temperatures to determine the probability of failure. The predictions are consistent with experimental scatter in toughness following WPS and provide a means of determining the relative importance of the crack tip residual stresses and crack tip blunting. We demonstrate that for our steels the crack tip residual stress is the pivotal feature in improving the fracture toughness following WPS. The paper finally discusses these results in the context of the non-uniqueness and the sensitivity of the Weibull parameters.

Temperature and Residual Stress Fields in an Austenitic Circumferential Pipe Weld

The main target of a research programme “experimental and numerical analyses on the residual stress field in the area of circumferential welds in austenitic pipe welds”, sponsored by Technische Vereinigung der Großkraftwerksbetreiber e. V. (VGB) and carried out at MPA Stuttgart, was the validation of the numerical calculation for the quantitative determination of residual stress fields in austenitic circumferential pipe welds. In addition, the influence of operational stresses as well as the impact of the pressure test on the residual stress state had to be examined. By using the TIG orbital welding technique, circumferential welds (Material X 10 CrNiNb 18 9 (1.4550, corresponding to TP 347) were produced (geometric dimensions 255.4 mm I.D. × 8.8 mm wall) with welding boundary conditions and weld parameters (number of weld layers and weld built-up, seam volume, heat input) which are representative for pipings in power plants. Deformation and temperature measurements accompanying the welding, as well as the experimentally determined (X-ray diffraction) welding residual stress distribution, served as the basis for the verification of numeric temperature and residual stress field calculations. The material model on which the calculations were founded was developed by experimental weld simulations in the thermo-mechanical test rig GLEEBLE 2000 for the determination of the material behaviour at different temperatures and elasto-plastic deformation. The numeric calculations were carried out with the Finite Element program ABAQUS. The comparison of the calculation results with the experimental findings confirms the proven validation of the developed numerical calculation models for the quantitative determination of residual stresses in austenitic circumferential pipings. The investigation gives a well-founded insight into the complex thermo-mechanical processes during welding, not known to this extent from literature previously.

The Effect of Residual Stresses on the Fracture Resistance of Ductile Steels

Tensile residual stresses, such as those generated by welding, act as crack opening stresses and can have a negative effect on the fatigue and fracture performance of a component. In this work the effect of representative residual stress distributions on the fracture behaviour of a ferritic steel has been examined using finite element analysis. A Gurson-type void growth model is used to model the effect of ductile tearing ahead of a crack. For the cases examined it is seen that a tensile residual stress field may lead to a reduction in the toughness of the material (as represented by the J-resistance curve). The observed difference in toughness can be linked to the different constraint levels in the specimens due to the introduction of the residual stress field and can be rationalised through the use of a two parameter, J–Q approach.

Analysis of Residual Stresses and Assessment of Postulated Cracks in a Core Shroud Weld

In order to assess postulated cracks in weldments of a BWR core shroud residual stresses are calculated by simulating the welding process. In the numerical analysis, weld metal deposition and the sequence of weld passes follow the manufacture protocol. The calculations are performed using the finite element program ABAQUS and a material model with kinematic nonlinear hardening. Calculations of the crack driving parameter, the stress intensity factor, are carried out for postulated circumferential cracks using a numerical procedure, as well as by applying a weight function solution specially developed for cracks in a thin-walled cylinder. The results give rise to a discussion on the validity of linear elastic fracture mechanics for assessing defects in weldments. Additionally, for a complete circumferential crack the trend in the stress intensity factor is studied when the crack depth approaches the full wall thickness.

Influence of the Cutting Parameters on the Cutting Forces of Carbon Steel

Plain carbon steels and alloy steels have a great application in the manufacturing process especially due to their characteristic of high machinability and low cost. The machining of these materials, the study of the cutting forces, and the power required for the cutting process is one of the most important parameters to be evaluated. The relationship between this parameter and the other cutting variables process is crucial for the optimization of the machining process. The results of this research are empirical expressions, obtained from the cutting parameters (tool nose radius, feed rate and depth of cut) and the influence of these parameters on the cutting forces for each carbon steel studied (AISI 1020, AISI 1045 and AISI 4340), as well as a general expression which includes the mechanical properties of these carbon steels. These results enable the user to predict cutting forces when using a turning process.

Generation Behavior of Thermal and Residual Stresses Due to Phase Transformation During Welding Heat Cycles

Weld residual stress is possible to be controlled by considering and changing mechanical properties of the materials. History of thermal stress due to phase transformation and residual stress during welding heat cycles is studied in order to investigate the generating mechanism of residual stress and the effects of material properties on stress generation. Two materials of high-tensile strength steels are used in the numerical simulation and experiment. Material property of each microstructural phase is used and the time- and temperature-dependant proportion of microstructure are considered by using CCT-diagram in the analysis. Thermal stress history obtained by the simulation agrees well with the experimental result during welding heat cycles.

Experimental Investigation of Constraint Effects on Creep Crack Growth

In this work the effects of specimen size and type on creep crack growth rates in stainless steel are examined. Experiments have been carried out on high constraint compact tension specimens (CT) and low constraint centre cracked panels (CCP) of ex-service 316H stainless steel. All testing was carried out at 550°C. Constraint effects have been observed in the data, with the large CT specimens having the fastest crack growth rate and the small CCP specimens the slowest. These trends are consistent with those that would be predicted from two parameter (C*–Q) theories. However, it is found that a constraint dependent creep crack growth model based on ductility exhaustion overpredicts the constraint dependence of the crack growth data.

Fracture Toughness Properties of Savannah River Site Storage Tank ASTM A285 Low Carbon Steel

A materials test program was developed to measure mechanical properties of ASTM A285 Grade B low carbon steel for application to structural and flaw stability analysis of storage tanks at the Department of Energy (DOE) Savannah River Site (SRS). Under this plan, fracture toughness and tensile testing are being performed at conditions that are representative of storage tank conditions on steels that span compositions within ASTM A285 specifications. The testing is being done within the framework of a statistical test matrix and the data collected will be used to develop a predictive model for materials properties. The results presented herein are limited to a subset of data comparing for comparison of a recent vintage steel versus an older steel for fracture resistance behavior. These preliminary results indicate that dynamic loading rates result in a greater increase in the fracture toughness response in the case of the recent vintage steels of lower carbon content when compared to the archival heat of high carbon content. In addition, ductile tearing in the archival, high carbon steel was more likely to be interrupted by cleavage fracture at lower fracture energies than the modern, low carbon steel.

Effect of Small Amount of Bismuth on Corrosion Resistibility of Austenitic Stainless Steel Weld Metals

Flux-cored are welding (FCAW) has increased recently because of high welding efficiency. However, a small amount of bismuth in the weld metals was a residue from the flux component that was added for improving slag detachability. The effect of small amount of bismuth in austenitic stainless steel weld metal on corrosion behavior in wet corrosion environment is not adequately clear because there is little reported to date. In the present research, the effect of bismuth which remained in the weld metal on the corrosion behavior of 308-type weld metal in wet corrosive solution was examined by using different bismuth containing weld metal. Measurement of the anodic polarization curve in a sulfuric acid solution, determination of pitting potential and conducting stress corrosion cracking (SCC) tests in a chloride solution, and implementation of boiling nitric add solution tests were conducted. In addition, the corrosion behavior of the FCAW weld metal was compared with that of Bi-free weld metals by shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW) to clarify how the corrosion behavior varies between the welding processes. In this research, no noticeable adverse effect of bismuth remained in the 308-type austenitic FCAW weld metals was observed in corrosion test.