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.

Prediction of Ductile Crack Growth from Ductility of Steel

2007 ◽  
Vol 539-543 ◽  
pp. 2186-2191 ◽  
Author(s):  
Mitsuru Ohata ◽  
Takuya Fukahori ◽  
Fumiyoshi Minami
Keyword(s):  
Mechanical Properties ◽  
Crack Initiation ◽  
Crack Growth ◽  
Damage Model ◽  
Stress Triaxiality ◽  
Local Strain ◽  
Strength Properties ◽  
Ductile Crack ◽  
Ductile Crack Growth ◽  
Point Bend

This study pays attention to reveal the material properties that control resistance curve for ductile crack growth (CTOD-R curve) on the basis of the mechanism for ductile crack growth, so that the R-curve could be numerically predicted only from those properties. The crack growth tests using 3-point bend specimens with fatigue pre-crack were conducted for two steels that have different ductile crack growth resistance with almost the same CTOD level for crack initiation, whereas both steels have the same “Mechanical properties” in terms of strength and work hardenability. The observation of crack growth behaviors provided that different mechanisms between ductile crack initiations from fatigue pre-crack and subsequent growth process could be applied. It was found that two “Mechanical properties” associated with ductile damage of steel could mainly influence CTOD-R curve; one is a resistance of ductile crack initiation estimated with critical local strain for ductile cracking from the surface of notched specimen, and the other one is a dependence of stress triaxiality on ductility obtained with circumferentially notched round-bar specimens. The damage model for numerically simulating the R-curve was proposed taking the two “ductile properties” into account, where ductile crack initiation from crack-tip was in accordance with critical local strain based criterion, and subsequent crack growth GTN (Gurson-Tvergaard-Needleman) based triaxiality dependent damage criterion. The proposed model accurately predicted the measured R-curve for the two steels used with the same “strength properties” through ductile crack initiation to growth.

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.

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.

scholarly journals Investigation of Notch-Induced Precise Splitting of Different Bar Materials under High-Speed Load

Materials ◽  
2020 ◽  
Vol 13 (11) ◽  
pp. 2461 ◽  
Author(s):  
Yuanzhe Dong ◽  
Yujian Ren ◽  
Shuqin Fan ◽  
Yongfei Wang ◽  
Shengdun Zhao
Keyword(s):  
Crack Initiation ◽  
High Speed ◽  
Splitting Method ◽  
Equivalent Plastic Strain ◽  
Stress Triaxiality ◽  
Initiation Zone ◽  
New Type ◽  
Notch Tip ◽  
Maximum Impact Force ◽  
1045 Steel

A notch-induced high-speed splitting method was developed for high-quality cropping of metal bars using a new type of electric-pneumatic counter hammer. Theoretical equations and FE models were established to reveal the crack initiation and fracture mode. Comparative tests were conducted for notched and unnotched bars of four types of steels, i.e., AISI 1020, 1045, 52100, and 304, and the section quality and microfracture mechanism were further investigated. The results show that damage initiates at the bilateral notch tips with peak equivalent plastic strain, and propagates through the plane induced by the notch tip; the stress triaxiality varies as a quasi-sine curve, revealing that the material is subjected to pure shearing at the notch tip, and under compression at the adjacent region. High precision chamfered billets were obtained with roundness errors of 1.1–2.8%, bending deflections of 0.5–1.5mm, and angles of inclination of 0.7°–3.4°. Additionally, the notch effectively reduced the maximum impact force by 21.6–23.9%, splitting displacement by 7.6–18.6%, and impact energy by 27.8–39.1%. The crack initiation zone displayed quasi-parabolic shallow dimples due to shear stress, and the pinning effect was larger in AISI 52100 and 1045 steel; the final rupture zone was characterized by less elongated and quasi-equiaxial deeper dimples due to the combination of shear and normal stress.

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.

Ductile Crack Extension Analysis for X80 Bending Deformation Using Damage-Based Model

2014 ◽  
Author(s):  
Satoshi Igi ◽  
Mitsuru Ohata ◽  
Takahiro Sakimoto ◽  
Kenji Oi ◽  
Joe Kondo
Keyword(s):  
Plastic Strain ◽  
Simulation Model ◽  
Crack Growth ◽  
Crack Extension ◽  
Damage Model ◽  
Stress Triaxiality ◽  
Crack Growth Behavior ◽  
Ductile Crack ◽  
Ductile Crack Growth ◽  
Notch Tip

This paper presents experimental and analytical results focusing on the strain limit of X80 linepipe. Ductile crack growth behavior from a girth weld notch is simulated by FE analysis based on a proposed damage model and is compared with the experimental results. The simulation model for ductile crack growth accompanied by penetration through the wall thickness consists of two criteria. One is a criterion for ductile crack initiation from the notch-tip, which is described by the plastic strain at the notch tip, because the onset of ductile cracking can be expressed by constant plastic strain independent of the shape and size of the components and the loading mode. The other is a damage-based criterion for simulating ductile crack extension associated with damage evolution influenced by plastic strain in accordance with the stress triaxiality ahead of the extending crack tip. The proposed simulation model is applicable to prediction of ductile crack growth behaviors from a circumferentially-notched girth welded pipe with high internal pressure, which is subjected to tensile loading or bending (post-buckling) deformation.

scholarly journals Comparison of Modified Mohr–Coulomb Model and Bai–Wierzbicki Model for Constructing 3D Ductile Fracture Envelope of AA6063

2021 ◽  
Vol 34 (1) ◽  
Author(s):  
Jianye Gao ◽  
Tao He ◽  
Yuanming Huo ◽  
Miao Song ◽  
Tingting Yao ◽  
...  
Keyword(s):  
Ductile Fracture ◽  
Fracture Criterion ◽  
Fracture Strain ◽  
Stress Triaxiality ◽  
Ductile Fracture Criterion ◽  
Forming Process ◽  
Tension Tests ◽  
Wide Range ◽  
Round Bar ◽  
Fracture Envelope

AbstractDuctile fracture of metal often occurs in the plastic forming process of parts. The establishment of ductile fracture criterion can effectively guide the selection of process parameters and avoid ductile fracture of parts during machining. The 3D ductile fracture envelope of AA6063-T6 was developed to predict and prevent its fracture. Smooth round bar tension tests were performed to characterize the flow stress, and a series of experiments were conducted to characterize the ductile fracture firstly, such as notched round bar tension tests, compression tests and torsion tests. These tests cover a wide range of stress triaxiality (ST) and Lode parameter (LP) to calibrate the ductile fracture criterion. Plasticity modeling was performed, and the predicted results were compared with corresponding experimental data to verify the plasticity model after these experiments. Then the relationship between ductile fracture strain and ST with LP was constructed using the modified Mohr–Coulomb (MMC) model and Bai-Wierzbicki (BW) model to develop the 3D ductile fracture envelope. Finally, two ductile damage models were proposed based on the 3D fracture envelope of AA6063. Through the comparison of the two models, it was found that BW model had better fitting effect, and the sum of squares of residual error of BW model was 0.9901. The two models had relatively large errors in predicting the fracture strain of SRB tensile test and torsion test, but both of the predicting error of both two models were within the acceptable range of 15%. In the process of finite element simulation, the evolution process of ductile fracture can be well simulated by the two models. However, BW model can predict the location of fracture more accurately than MMC model.

Sign in / Sign up

Export Citation Format

Share Document