Application of Damage Mechanics Modeling to Strain Based Design With Respect to Ductile Crack Initiation

2010 ◽  
Author(s):  
Nobuyuki Ishikawa ◽  
Hitoshi Sueyoshi ◽  
Satoshi Igi
Keyword(s):  
Crack Initiation ◽  
Plastic Strain ◽  
Critical Condition ◽  
Equivalent Plastic Strain ◽  
Material Behavior ◽  
Specimen Geometry ◽  
Small Scale ◽  
Ductile Crack ◽  
Material Parameters ◽  
Linepipe Steels

Limit state condition in the tensile failure for the strain based-design (SBD) currently considering is the point of maximum load which is evaluated by curved wide plate (CWP) testing or full scale pipe tensile testing. Maximum loading point is understood as the onset of instability of the structure. However, the material behavior controlling structural instability is not well understood since it includes many aspects of material response such as local strain concentration, ductile crack initiation and stable crack growth. In order to clearly specify the material property suitable for SBD, it is important to understand the fundamental behavior of the linepipe steels that leads to ductile crack initiation and following ductile tearing. In this paper, critical condition for ductile crack initiation was investigated by both small scale and large scale testing, notched round bar and wide plate testing, by using X80 and X100 linepipe steels and welds. Two different analytical procedures, equivalent plastic strain criterion and damage mechanical analysis, were applied to evaluate the local material conditions for ductile crack initiation. As was already verified by many other researches, the critical equivalent plastic strain can be used as the local criterion for ductile crack initiation which is not affected by specimen geometry. However, equivalent plastic strain is still macroscopic parameter that is not reflected by microscopic feature of the steel. Therefore, the Gurson-Tvergaard damage mechanical model was applied to further understand microscopic material behavior to ductile crack initiation. Material parameters for G-T model were carefully evaluated depending on the microscopic characteristics of each steel. By selecting appropriate material parameters, the critical condition for ductile crack initiation was estimated by the critical void volume fraction, which is independent of specimen geometry. Effect of microstructural characteristics on crack initiation was also investigated in this study.

Application of Ductile Cracking Criterion to the Assessment of Girth Weld Integrity for High Strength Linepipe

2005 ◽  
Author(s):  
Nobuyuki Ishikawa ◽  
Shigeru Endo ◽  
Satoshi Igi ◽  
Teruki Sadasue
Keyword(s):  
Crack Initiation ◽  
Plastic Strain ◽  
Large Deformation ◽  
Material Properties ◽  
Equivalent Plastic Strain ◽  
High Strength ◽  
Small Scale ◽  
Pipeline System ◽  
Ductile Crack ◽  
Notch Tip

Fracture behavior of high strength linepipes with weld defects is of great interest for the integrity of pipeline system. Especially, in the seismic or permafrost area, where large ground displacement can be expected, linepipe materials need to have sufficient resistance against brittle and ductile fracture under large deformation. Wide plate tensile test with surface flaw in the girth weld metal of X100 linepipe demonstrated that tensile limit is dominated by ductile crack initiation and its propagation. Conditions for ductile crack initiation for the base materials and girth weld joints of Grade X80 and X100 linepipes were investigated in this study. It was shown that ductile cracking occurs in the notch tip region of the wide plate specimen when notch tip equivalent plastic strain reaches the same critical value as determined by the small-scale tests. Therefore, “the equivalent plastic strain” in the critical regions can be used as a transferable parameter to predict ductile crack initiation behavior. Assessment methodology for tensile limit of high strength linepipe girth weld with respect to preventing ductile cracking was proposed. The effect of strength matching of girth weld and base metal Y/T ratio on limit remote strain as well as allowable defect size was investigated analytically. Increasing strength matching and lowering Y/T ratio of base material can lead to higher limit strain to ductile cracking of girth weld. These effects of material properties were validated by weld wide plate tensile tests. Therefore, careful selection of material properties should be important to improve resistance against ductile cracking of linepipe girth welds under large deformation field.

Two-Parameter Criteria for Ductile Crack Initiation and Propagation in High-Grade Line Pipes

2007 ◽  
Vol 348-349 ◽  
pp. 493-496
Author(s):  
Yoichi Kayamori ◽  
P.S.J. Crofton ◽  
Roderick A. Smith
Keyword(s):  
Crack Initiation ◽  
Plastic Strain ◽  
Driving Force ◽  
Equivalent Plastic Strain ◽  
Stress Triaxiality ◽  
High Grade ◽  
Ductile Crack ◽  
Force Curve ◽  
Crack Driving Force ◽  
The Relationship

Full-scale burst test data of high-grade line pipes for high-pressure gas pipelines were referred to, and 3-D elastic-plastic finite element analysis was carried out using the test data for the calculation of fracture parameters. Ductile crack initiation was evaluated by the intersection of a toughness locus and a crack driving force curve, where the toughness locus was indicated by the relationship between the critical equivalent plastic strain and the stress triaxiality, and the crack driving force curve was shown by a history of the equivalent plastic strain and the stress triaxiality at characteristic distance. In addition, ductile crack rapid propagation was assessed by the relationship between the critical CTOA and the global constraint factor, where the critical CTOA remained almost constant because of high constraint.

Ductile Crack Initiation Behavior of Prestrained Steels

2011 ◽  
Author(s):  
Takehisa Yamada ◽  
Yoichi Yamashita
Keyword(s):  
Crack Initiation ◽  
Void Nucleation ◽  
Equivalent Plastic Strain ◽  
Stress Triaxiality ◽  
Ductile Crack ◽  
Round Bar ◽  
Point Bend ◽  
Materials Used ◽  
Cracked Specimens ◽  
Triaxiality Factor

Ductile crack initiation behaviors were experimentally and analytically investigated using compressively prestrained notched round bar specimen (lower stress triaxiality condition) and four point bend specimen (higher stress triaxiality condition). The materials used were SM400B and HT780. It was observed that ductile crack initiation of notched round bar specimen occurred at the center of specimen and was caused by coalescence of micro voids. Ductile limit curves, which are the relationships between equivalent plastic strain and stress triaxiality factor at ductile crack initiation, were obtained using FE-analyses and experimental results. Ductile crack initiation of four point bend specimens as cracked specimens could be evaluated using ductile crack initiation limit curves obtained from notched round bar specimens if the positions of void nucleation at crack tip are properly considered. It has been found that ductile crack initiation limits can be evaluated using ductile limit curves under both low and high constraint conditions (stress triaxiality conditions.

scholarly journals On the micromechanism of inclusion driven ductile fracture and its implications on fracture toughness

2020 ◽  
Author(s):  
Yu Liu ◽  
Xinzhu Zheng ◽  
shmuel osovski ◽  
Ankit Srivastava
Keyword(s):  
Fracture Toughness ◽  
Ductile Fracture ◽  
Crack Growth ◽  
Volume Fraction ◽  
Void Nucleation ◽  
Small Scale ◽  
Crack Advance ◽  
Crack Growth Behavior ◽  
Ductile Crack ◽  
Material Parameters

The objective is to identify the micromechanism(s) of ductile crack advance, and isolatethe key microstructural and material parameters that a?ect these micromechanisms andfracture toughness of ductile structural materials. Three dimensional, ?nite element, ?nitedeformation, small scale yielding calculations of mode I crack growth are carried out forductile material matrix containing two populations of void nucleating particles using anelasto-viscoplastic constitutive framework for progressively cavitating solid. The larger par-ticles or inclusions that result in void nucleation at an early stage are modeled discretelywhile smaller particles that require large strains to nucleate voids are homogeneously dis-tributed. The size, spacing and volume fraction of inclusions introduce microstructure-basedlength-scales. In the calculations, ductile crack growth is computed and fracture toughness ischaracterized. Several features of crack growth behavior and dependence of fracture tough-ness on microstructural and material parameters observed in experiments, naturally emergein our calculations. The extent to which the microstructural and material parameters a?ectthe micromechanisms of ductile crack advance and, hence, the macroscopic fracture tough-ness of the material is discussed. The results presented provide guidelines for microstructuralengineering to increase ductile fracture toughness, for example, the results show that for amaterial with small inclusions, increasing the mean inclusion spacing has a greater e?ect onfracture toughness than for a material with large inclusions.

Evaluation of Ductile Cracking Criterion for Grade X100 Linepipe and Relationship to Large Scale Fracture Behavior

2003 ◽  
Author(s):  
Nobuyuki Ishikawa ◽  
Shigeru Endo ◽  
Alan Glover ◽  
David Horsley ◽  
Masao Toyoda
Keyword(s):  
Plastic Strain ◽  
Ductile Fracture ◽  
Fracture Behavior ◽  
Large Scale ◽  
Notch Root ◽  
Equivalent Plastic Strain ◽  
High Strength ◽  
Fracture Initiation ◽  
Linepipe Steels ◽  
Ductile Fracture Initiation

Recent developments in the manufacturing process of steel plate for high strength linepipe have enabled superior toughness to prevent brittle fracture of the pipe body. Techniques for non-destructive inspection have also improved, and large flaws that could lead to brittle fracture are highly unlikely in recent high strength pipelines. However, large amounts of plastic deformation can be expected in seismic or permafrost regions. Prevention of ductile fracture of the pipe body or weldment therefore becomes a key issue in defining the tensile strain limit. Ductile fracture is considered to occur by growth and coalescence of voids, and is affected by stress triaxiality and plastic straining at the cracked region. Although many studies have been carried out to evaluate ductile cracking criteria, its transferability to large-scale fracture behavior has not been thoroughly investigated. In this study, ductile cracking of high strength linepipe steels, Grade X80 and X100, was investigated. Notched round bar specimens with different notch root radii were tested to determine the precise conditions for initiation of ductile fracture. Stress and strain conditions at the notch regions were evaluated by FE analysis, and the “critical equivalent plastic strain” was defined at conditions corresponding to ductile fracture initiation in the experimental small specimen tests. Ductile crack initiation behavior was also determined for wide plate test specimens by making close observations of the notch root area. 3-D FE analysis of the wide plate tensile test showed that the equivalent plastic strain at the point of ductile fracture initiation was in close agreement with that in the notched round bas specimen. Thus, the “critical equivalent plastic strain,” determined by small notched round bar specimens, can be considered as a transferable criterion to predict large-scale fracture behavior in wide plate tests. Concepts of strain based design in terms of preventing ductile failure from a surface flaw by applying critical strain to cracking were also discussed in this paper. Results were compared to conventional grade linepipe steels and structural steels, showing that recent high strength linepipe steels have higher resistance to ductile cracking than conventional structural steels. In addition, 3-D FE analyses were used in a parametric study to determine the effects of Y/T and uniform strain on the onset of ductile cracking behaviour. The results of these analyses show the relative importance of materials properties on the resistance to ductile cracking.

DUCTILE FRACTURE CHARACTERIZATION OF ALUMINUM ALLOY 2024-T351 USING DAMAGE PLASTICITY THEORY

2009 ◽  
Vol 01 (02) ◽  
pp. 267-304 ◽  
Author(s):  
LIANG XUE ◽  
TOMASZ WIERZBICKI
Keyword(s):  
Aluminum Alloy ◽  
Plastic Strain ◽  
Equivalent Plastic Strain ◽  
Calibration Procedure ◽  
Plasticity Theory ◽  
Material Parameters ◽  
Wide Range ◽  
Lode Angle ◽  
Aluminum Alloy 2024 ◽  
Fracture Envelope

This paper presents the calibration procedure for aluminum alloy 2024-T351 using a recently developed damage plasticity theory. The damage plasticity theory consists of a full three dimensional damage evolution law where the pressure sensitivity and the Lode angle dependence are included in a fracture envelope and the equivalent plastic strain is used as a time-like variable to determine the damage rate. Because of the coupled nature of the plastic strain and the damage, material parameters are calibrated from a parallel study of numerical simulations and experimental measurements. A set of 10 tests that cover a wide range stress states for both the hydrostatic pressure and the Lode angle are conducted in order to capture the fracture envelope in the interested stress range. The experimental setups include un-notched and notched round bars with three different notch radii, a doubly grooved flat plate and compressed cylinders of three different heights at two friction conditions. The detailed numerical and experimental procedure of calibration is demonstrated by using four of these tests. The accuracy of the calibrated material parameters is further assessed by the remainder of tests. Notch sensitivity in tensile round bars and the friction conditions in upsetting tests are discussed in detail. Good agreement in the tested load conditions is achieved for both the fracture patterns and the load-displacement curves.

Application of Equivalent Damage Concept to Evaluation of Ductile Cracking for Linepipe Under Large Scale Seismic Loading

2003 ◽  
Author(s):  
Mitsuru Ohata ◽  
Masao Toyoda
Keyword(s):  
Cyclic Loading ◽  
Crack Initiation ◽  
Safety Assessment ◽  
Large Scale ◽  
Seismic Loading ◽  
Steel Pipe ◽  
Small Scale ◽  
Ductile Crack ◽  
Hardening Material ◽  
Cyclic Straining

A large scale seismic loading sometimes produces local buckling in onshore or offshore linepipe and subsequent loading can lead to ductile cracking followed by ductile failure. It is important to assess the ductile crack initiation of linepipe subjected to a large scale cyclic straining induced by seismic loading for safety assessment of linepipe. This paper is mainly paid attention to the applicability of the damage concept proposed by authors for evaluation of ductile cracking of steel pipe under large scale cyclic loading. The damage concept is based on the “two-parameter criterion”, using the effective plastic strain, which is taken into account mechanical and microstructural aspects of Bauschinger effect of steel. The transferability of small scale tensile test results to the assessment of ductile crack initiation of steel pipe under seismic loading by using the effective damage concept is verified by conducting cyclic bending tests for straight pipe with initial deflection. The effective damage strain under cyclic loading, which is derived from the evolution of back stress, was calculated by FE-analysis employing a combined (isotropic/kinematic) hardening material model. It is found that the critical safety assessment of ductile crack initiation can be conducted based on the strain-based criterion in accordance with the proposed damage concept.

Effect of Strength Mismatch on Ductile Crack Initiation Behavior from Notch Root under Static Loading

2005 ◽  
Vol 297-300 ◽  
pp. 756-761 ◽  
Author(s):  
Gyu Baek An ◽  
Mitsuru Ohata ◽  
Masahito Mochizuki ◽  
Han Sur Bang ◽  
Masao Toyoda
Keyword(s):  
Crack Initiation ◽  
Static Loading ◽  
Notch Root ◽  
Equivalent Plastic Strain ◽  
Stress Triaxiality ◽  
Critical Conditions ◽  
Ductile Crack ◽  
Surface Cracking ◽  
Triaxial Stresses ◽  
Ductile Fractures

It has been well known that ductile fractures of steels are accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using two-parameter criterion based on equivalent plastic strain and stress triaxiality. It has been demonstrated by authors using round-bar specimens with circumferential notch in single tension that the critical strain to initiate ductile crack from specimen center depends considerably on stress triaxiality, but surface cracking of notch root is in accordance with constant strain condition. This study fundamentally clarifies the effect of strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining under static loading, on critical conditions for ductile cracking from the pre-notch root. In order to evaluate the stress/strain state in the pre-notch root of specimens, a thermal elastic-plastic finite element (FE) analysis has been carried out.

Sign in / Sign up

Export Citation Format

Share Document