Stress-modified Fracture Strain Research Articles

Prediction of the irradiation effect on the fracture toughness for stainless steel using a stress-modified fracture strain model

This paper proposes a numerical method for predicting the reduction in the fracture toughness with neutron dose using the concept of an embrittlement constant. Tensile and fracture toughness test data for type 304 stainless steel in the literature [41,42] are used to validate the proposed method. Fracture toughness tests are simulated using a stress-modified fracture strain damage model implemented in an ABAQUS user-subroutine. The true stress-strain curve and one point of the fracture strain locus are obtained from the tensile test simulation. The remaining parameters of the fracture strain locus are determined by simulating a fracture toughness test. The fracture strain locus is modified to reflect the irradiation effect on the fracture toughness using the proposed irradiation embrittlement constant CI. The function of the constant with the neutron dose is calculated from the tensile tests and simulations. Subsequently, the modified damage model is used to predict the reduction in the fracture toughness. The predicted simulation results are compared with experimental data, showing good agreement.

Numerical method to characterize probabilistic energy distribution of drop weight tear test at Ductile-Brittle transition temperatures

This paper proposes a numerical method to characterize probabilistic energy distributions of drop weight tear tests (DWTTs) at ductile–brittle transition temperatures. The method employs finite element ductile–brittle fracture simulation using the stress-modified fracture strain model for ductile tearing and the maximum principal stress criterion for cleavage fracture. The probability of energy scatters is expressed using the two-parameter Weibull distribution in terms of the maximum principal stress. The maximum principal stress is then obtained as a function of the absorbed energy using FE simulation. The characterized energy scatters are compared with API X80 DWTT data at transition temperatures, showing good agreement. The proposed method can be used to effectively predict DWTT data at transition temperatures.

Finite element simulation of drop-weight tear test of API X80 at ductile-brittle transition temperatures

This paper proposes a method to simultaneously simulate interacting ductile and cleavage fracture in a DWTT (drop-weight tear test). The stress-modified fracture strain (SMFS) damage model is used to simulate ductile fracture and the maximum principal stress criterion is applied to simulate cleavage fracture. By incorporating the element size effect in the ductile and cleavage damage models, the numerical method to simulate the interacting ductile and cleavage fracture behavior in a DWTT is constructed. To validate the proposed method, the simulation results are compared with six API X80 data measurements at temperatures ranging between -97 °C to -20 °C. The comparison of the experimental load-displacement curve and fracture surface with the FE simulation results shows good agreement.

Fracture simulation model for API X80 Charpy test in Ductile-Brittle transition temperatures

This paper presents a method to simulate fracture patterns of interacting ductile and cleavage fracture and experimental validation using API X80 Charpy test data. For ductile fracture, the stress modified fracture strain damage model is used, whereas for cleavage fracture, the maximum principal stress criterion is proposed. By combining ductile and cleavage fracture models, a numerical method to simulate interacting ductile and cleavage fracture is proposed. The proposed numerical method is validated by comparing with experimental Charpy test data at ductile-brittle transition temperatures. Comparison of fracture surfaces and impact energies shows good agreements.

Applicability of Net-Section Collapse Load Approach to Multiple-Cracked Pipe Assessment: Numerical Study

In this paper, applicability of net-section collapse load approach to circumferential multiple-cracked pipe assessment is investigated using finite element (FE) damage analysis. The FE damage analysis based on the stress-modified fracture strain model is validated against limited fracture test data of two circumferential surface-cracked pipes. Then, the systematic parametric study is performed using the FE damage analysis for symmetrical and asymmetrical surface-cracked pipes. It is found that predictions using the net-section collapse load approach tend to be more accurate with increasing the distance between two symmetrical cracks. For asymmetrical cracks, it is found that the deeper crack plays a more important role and that the existing net-section collapse load expression can be potentially nonconservative. Idealization to symmetrical cracks based on the deeper crack is proposed.

Comparison of Ductile Tearing Simulation With Complex Cracked Pipe Test Data

This paper presents numerical ductile tearing simulation results of four complex cracked pipes made of two materials, performed at Battelle, Columbus, OH. In ductile tearing simulation, stress-modified fracture strain model is used, which is determined from tensile test results and fracture toughness test data. Overall good agreement is found in comparing simulation results with experimental load, crack length, and crack mouth opening displacements versus load-line displacement (LLD) data. The effect of the complex crack on J–resistance curves is discussed.

Blunt defect assessment in the framework of the failure assessment diagram

In order to reduce over-conservatism in fitness-for-service assessment procedures, experimental evidence and recent analytical developments recognise the importance of considering the actual shape of non-sharp flaws and/or the real geometric constraint conditions at the crack tip. This paper addresses the effect of blunt defects on the structural integrity assessment of reactor pressure vessel (RPV) and pipeline steels. Parametric studies for compact tension specimens with various notch root radii are performed using finite element analysis. The notch fracture toughness, the resistance to the onset of ductile cracking and the J-integral, quantifying the notch driving force, are evaluated. A stress-modified fracture strain model is used as a virtual testing method. The results are analyzed in the framework of the failure assessment diagram (FAD), showing that the existing shape of the FAD is also suitable for assessments of blunt defects and how the concepts introduced can be used to reduce the conservatism in defect assessment, define margins on failure and indicate when plastic collapse is the dominant failure mechanism.

Numerical Prediction of Maximum Load-Carrying Capacity of Cracked Alloy 690TT Steam Generator Tubes

This paper presents a finite element (FE) simulation technique to predict maximum load-carrying capacity of cracked steam generator tubes and its application to Alloy 690TT tubes. The simulation method is based on a simplified version of the stress modified fracture strain model. The damage model is determined from tensile test and one cracked tube test data. Predicted maximum pressures are compared with 23 test data of axial through-wall and surface cracked Alloy 690TT steam generator tubes. Comparison with experimental data shows good agreement.

Extracting ductile fracture toughness from small punch test data using numerical modeling

This paper presents a numerical method to extract ductile fracture toughness from notched small punch test data using FE ductile damage analysis based on the stress-modified fracture strain model. To validate the proposed method, analysis results are compared with series of mechanical (notched tensile, fracture toughness and small punch) test data for three different materials; low alloy steel SA508 Gr.3, TP316L austenitic stainless steel and CF8M cast austenitic stainless steel. Comparison with experimental fracture toughness data shows that the proposed method can predict fracture initiation values relatively well and tearing resistance conservatively.

Finite element simulation of creep crack growth using combined plastic-creep damage model

This paper compares a combined plastic and creep damage model with only creep damage model by simulating creep crack growth of P91. The previous model considered only creep damage is extended to consider effect of plastic damage to creep crack growth. The damage model is based on the stress-modified fracture strain model using the ductility exhaustion concept. Also, creep fracture strain depending on the strain rate is considered. Total damage is defined simply through linear addition of plastic and creep damage. When the accumulated damage becomes unity, all stress components at the finite element gauss point are reduced to a small value to simulate failure. Finite element damage analysis results using previous and proposed model are compared with experimental data for P91. The proposed model gives a better agreement with the experimental data compared with the previous model.

Application of engineering ductile tearing simulation method to CRIEPI pipe test

A method to simulate ductile tearing in large-scale pipes using finite element analysis is proposed, based on the stress-modified fracture strain model. An element-size-dependent critical damage model is also introduced in damage simulations. The damage model and associate parameters are determined from tensile and fracture toughness test data. The method is applied to simulate five bending tests of circumferential cracked carbon steel pipes. Simulated results agree overall well with two through-wall cracked pipe test data, but consistently over-predict the maximum loads for three surface cracked pipe tests. Advantages of the proposed method in practical application is briefly discussed.

Ductile Fracture Simulation of Full-scale Circumferential Cracked Pipes: (II) Stainless Steel

This paper reports ductile fracture simulation of full-scale circumferentially cracked pipes using finite element (FE) damage analysis. In the structural integrity, without experimental investigations or with few ones, it is not an easy task to properly evaluate the crack initiation and crack propagation of large-scale components with a crack-like defect. Unfortunately, from an economic perspective, performing experiments of large-scale components would be consequently unfavorable. For these reasons, ductile fracture simulation using FE damage analysis to predict crack behavior is one efficient way to replace the test procedures. In order to simulate ductile tearing of large-scale cracked pipes, element-size-dependent critical damage model based on the stress-modified fracture strain model is proposed. To evaluate fracture behavior of full-scale cracked pipes, tensile and C(T) specimens are calibrated by FE analysis technique. Tensile properties and fracture toughness of stainless steel at 288oC are taken from Battelle Pipe Fracture Encyclopedia. After calibrations, simulated results of the full-scale pipes with a circumferential crack are compared with test data to validate the proposed method.

Ductile Fracture Simulation of Full-scale Circumferential Cracked Pipes: (I) Carbon Steel

This paper reports ductile fracture simulation and comparison with carbon steel test data of full-scale cracked pipes via 3-D finite element damage analysis based on stress-modified fracture strain model. Although there are few test results and discussion of full-scale cracked pipes, it is not an easy task to properly evaluate fracture behaviour of large-scale components for various pipe sizes with different shape of a crack-like defect. In such a situation, finite element (FE) damage analysis can be a competitive alternative to characterize the fracture behaviour of cracked components such as crack initiation and maximum loads. In recent years, a simple FE method [1] to simulate ductile fracture has been developed based on the stress-modified fracture strain model [2,3]. The technique appropriately simulated ductile failure for miscellaneous cracked components [4,5]. However, for some cases, large-scale components using small element size, FE analysis couldn’t give reliable values because of numerical instability. Element-size-dependent critical damage model enhanced by taking the effect of element-size on the ductile fracture damage analysis is introduced to overcome this problem and to be applicable to the large-scale structures. In order to validate proposed method, two types of carbon steel (A106 Gr. B and SA333 Gr. 6) pipes with a circumferential crack taken from [6] are considered, subjected to four-point bending only and combined loading. It is shown that predicted crack initiation and maximum loads are compared with experimentally measured values, showing overall good agreements.

Hydrogen-assisted stress corrosion cracking simulation using the stress-modified fracture strain model

This paper proposes a numerical method to simulate hydrogen-assisted stress corrosion cracking, via coupled diffusion elastic-plastic finite element damage analyses based on phenomenological stress-modified fracture strain model. For validation, simulated results using the proposed method are compared with published experimental data of FeE 690T compact tension tests under air and hydrogen condition with various constant load-line displacement rates. The simulated results agree well with experimental data.

Comparison of fracture strain based ductile failure simulation with experimental results

This paper provides experimental validation of the approach for simulating ductile failure using finite element methods, recently proposed by the authors. The proposed method is based on a phenomenological stress-modified fracture strain model. Incremental damage is defined by the ratio of the plastic strain increment to the fracture strain, and total damage is calculated using linear summation. When the accumulated damage becomes unity, all stress components at the finite element gauss point are reduced to a small value to simulate progressive failure. The proposed method is validated against four experimental data sets of cracked specimens made of three different materials. Despite the simplicity of the proposed method, the simulated results agree well with experimental data for all cases considered, providing sufficient confidence in the use of the proposed method to simulate ductile failure.

A finite element ductile failure simulation method using stress-modified fracture strain model

This paper proposes a new method to simulate ductile failure using finite element analysis based on the stress-modified fracture strain model. A procedure is given to determine the stress-modified fracture strain as a function of the stress triaxiality from smooth and notched bar tensile tests with FE analyses. For validation, simulated results using the proposed method are compared with experimental data for cracked bar (tensile and bend) tests, extracted from API X65 pipes, and for full-scale burst test of gouged pipes, showing overall good agreements. Advantages in the use of the proposed method for practical structural integrity assessment are discussed.

A numerical method to simulate ductile failure of tensile plates with interacting through-wall cracks

This paper proposes a simple numerical method to simulate ductile failure behaviours of tensile plates with interacting through-wall cracks. The method is based on the finite element damage analysis using the stress-modified fracture strain damage model. To validate the proposed method, simulated results are compared with a total of 23 published experimental data of flat tensile plates with interacting through-wall cracks. Despite its simplicity, the proposed method well predicted the experimental maximum loads of tensile plates with interacting cracks, including the loads for crack coalescence. Systematic analyses are also performed to investigate the effect of the element size used in the finite element damage analysis.

A method to predict failure pressures of steam generator tubes with multiple through-wall cracks

This paper proposes a simple numerical method to predict burst pressures of steam generator tubes with multiple through-wall cracks, based on the stress-modified fracture strain damage model with stress reduction technique. For validation, simulated results using the proposed method are compared with 31 published experimental data of Alloy 600 plates and tubes with single or two through-wall cracks, showing that predicted loads are within 10% of experimentally-measured ones for all cases considered. Furthermore, a parametric study is performed to investigate the interaction effect of two through-wall cracks in Alloy 600 steam generator tubes under internal pressure.

Ductile failure analysis of API X65 pipes with notch-type defects using a local fracture criterion

A local failure criterion for API X65 steel is applied to predict ductile failure of full-scale API X65 pipes with simulated corrosion and gouge defects under internal pressure. The local failure criterion is the stress-modified fracture strain as a function of the stress triaxiality (defined by the ratio of the hydrostatic stress to the effective stress). Based on detailed finite element (FE) analyses with the proposed local failure criterion, burst pressures of defective pipes are estimated and compared with experimental data. For pipes with simulated corrosion defects, FE analysis with the proposed local fracture criterion indicates that predicted failure takes place after the defective pipes attain maximum loads for all cases, possibly due to the fact that the material has sufficient ductility. For pipes with simulated gouge defects, on the other hand, it is found that predicted failure takes place before global instability, and the predicted burst pressures are in good agreement with experimental data, providing confidence in the present approach.

Development of stress-modified fracture strain for ductile failure of API X65 steel

The present paper proposes ductile failure criteria in terms of true fracture strain (the equivalent strain to fracture) as a function of the stress triaxiality (defined by the ratio of the hydrostatic stress to the equivalent stress) for the API X65 steel. To determine the stress-modified fracture strain, smooth and notched tensile bars with four different notch radii are tested, from which true fracture strains are determined as a function of the notch radius. Then detailed elastic–plastic, large strain finite element analyses are performed to estimate variations of stress triaxiality in the tensile bars, which leads to true fracture strains as a function of the stress triaxiality, by combining them with experimental results. Two different failure criteria are proposed, one based on local stress and strain information at the site where failure initiation is likely to take place, and the other based on averaged stress and strain information over the ligament where ductile fracture is expected. As a case study, ligament failures of API X65 pipes with a gouge are predicted and compared with experimental data.