Development of GTN Model Parameters for Simulating Ductile Fracture Behavior of X 70 Carbon Steel SENT Specimens

Single-Edge-Notched-Bending (SENB) specimen is mainly used for fracture characteristics test of pipe material. However, there is also a disadvantage in that it does not sufficiently simulate the fracture characteristics of thin pipes due to the difference in boundary effect between the SENB specimen and the actual pipe. The Single-Edge-Notched-Tensile (SENT) specimen can be used as complementary test method due to its less boundary effect compared to the SENB specimen. In this study, the SENT specimen which are fabricated with API X 70 carbon steel was simulated ductile fracture behavior by using finite-element-analysis (FEA). To simulate ductile fracture behavior, Gurson-Tvergaard-Needleman (GTN) model was applied. GTN model is a kind of damage model that describes the behavior of ductile fracture through three steps of void changes : nucleation, growth, and coalescence. And GTN model is composed of three constitutive equations and nine parameters. In order to develop the GTN ductile fracture model for API X 70 carbon steel, five kinds of tensile specimen tests were simulated by FEA. In addition, we analyzed the influence of parameters of GTN model through analysis and developed optimal material parameters for API X 70 carbon steel. Finally, the ductile fracture behavior of the SENT specimen was simulated and the FEA results of SENT specimen was compared with actual SENT specimen test.

Investigation of Fracture Process of S355JR Steel in Transition Region Using Metallographic and Fractographic Tests and Numerical Analysis

In the paper are presented test results of fracture process in brittle-to-ductile transition range for two microstructural types of S355JR steel – ferrite-pearlite and ferrite-carbides. For both kinds of S355JR steel obtained in temperature range of transition region the strength and plastic properties are similar, but the fracture toughness characteristics showed significantly are various. To clarify the differences in the course of trends in the mechanical characteristics performed metallographic and fractographic observations using the scanning electronic microscope. The fractographic examination showed that changes in the fracture surface morphology were dependent on the test temperature. It was also found that during the subcritical crack growth the region of ductile fracture extension reduced with decreasing temperature. The results of finite element method (FEM) calculation the stress fields in front of the crack of single edge notch in bending (SENB) specimens in the range of brittle-to-ductile transition are presented also. The FEM calculations were performed on the numerical model of SENB specimen using the ABAQUS program.

Numerical Prediction of Fracture Behavior for Austenitic and Martensitic Stainless Steels

Two types of stainless steels are compared in this paper, austenitic X15CrNiSi25-20 and martensitic X20Cr13, based on their numerically predicted fracture behavior. There are engineering applications where both of the steels can be considered for use and where these materials can be exposed to crack occurrence and growth, so proper distinction between them is desirable. Comparison is made on the basis of [Formula: see text]-integral values that are numerically determined using finite element (FE) stress analysis results. FE analysis is performed on compact tensile (CT) and single-edge notched bend (SENB) type specimens that are usually used in standardized [Formula: see text]-integral experimental procedures. Calculated [Formula: see text]-integral values are plotted versus crack growth lengths for mentioned specimens. Results show somewhat higher values of [Formula: see text]-integral for steel X20Cr13 than X15CrNiSi25-20. Further, when comparing [Formula: see text]-integral values obtained through FE model of CT and SENB specimen, it is noticed that CT specimens give somewhat conservative results. Results obtained by this analysis can be used in predicting fracture toughness assessment during design process.

Critical Strain and Damage Evolution for Crack Growth From a Sharp Notch Tip of High-Strength Steel

This paper proposes details of an approach that uses expressions of fracture strain and damage evolution as functions of stress triaxiality for notch-free specimens to predict their values for crack growth from a sharp notch tip of a single-edge-notched bend (SENB) specimen. Experimental testing and finite element (FE) modelling are used to determine the basic mechanical properties and deformation behaviour of those specimens, which are needed to calibrate model constants in the proposed approach and to validate prediction from the approach. Three types of mechanical testing were conducted, using standard smooth tensile, short-gauged tensile and standard SENB specimens. The FE modeling is to establish constitutive relationship between stress and strain for notch-free specimens so that the FE modeling can be used to determine parameters such as stress triaxiality and unloading modulus for the prediction of fracture strain and damage evolution at the sharp notch tip of SENB specimen. The study will then examine whether the proposed approach can predict the trend of variation for fracture toughness among three high-strength steels, which is an on-going study and the results will be presented in the conference.

Constraint Effect on the Ductile Fracture Behavior of High-Strength Pipeline Steels

This work presents an investigation of the ductile tearing properties for a pipe with internal and external circumferential cracks using 2D plane strain and axisymmetric models. Crack growth resistance curves were computed using the complete Gurson model. The pipes with various crack depths and internal pressures were analyzed. The results were compared with those of corresponding SENT and SENB specimens. It clearly indicated a significant effect of constraint on the resistance curves for internal and external cracked pipes. A minor effect of hoop stress induced by internal pressure on the CTOD-resistance curves is expected for deep-cracked pipes. The SENT specimen is a better representation of circumferentially flawed pipes and an alternative to the conventional standard SENB specimen for the fracture mechanics testing in engineering critical assessment of high-strength pipeline steels.

Determination of Path-Independent J-Integral for Cracks in Residual Stress Fields Using Finite Element Method

“Hot” residual stresses exist in metal welds due to welding thermal stresses, and “cold” residual stresses occur in mechanical damaged metallic pipes due to large plastic deformation. For a crack in ether a hot or cold residual stress field, residual stresses might have a strong effect on the crack-tip field and the fracture parameter, J-integral. To consider the effect of residual stresses and to ensure the path-independence of J, different correction methods have been developed over the years. Recently, the finite element analysis (FEA) commercial software ABAQUS adopted one of correction methods for determining the residual stress corrected J-integral. This paper intends to evaluate this new function of ABAQUS and to see if the residual stress corrected J-integral is path-independent. A brief review is first given to the J-integral definition, the conditions of J-integral path-independence or dependence, and the modifications of J-integral to consider the residual stress effect. A modified single edge-notched bend (SENB) specimen is then adopted, and a FEA numerical procedure is developed and used in the numerical tests to evaluate the path-independence of the residual stress corrected J-integral using ABAQUS. Detailed elastic-plastic FEA calculations are carried out for the modified SENB specimen in three-point bending. The residual stress field, crack-tip field, and J-integral with and without consideration of the residual stress effect are determined and discussed.