scholarly journals Estimation of the Onset of Crack Growth in Ductile Materials

Materials ◽  
2018 ◽  
Vol 11 (10) ◽  
pp. 2026 ◽  
Author(s):  
Andrzej Neimitz ◽  
Jaroslaw Galkiewicz ◽  
Sebastian Lipiec ◽  
Ihor Dzioba
Keyword(s):  
Plastic Strain ◽  
Ductile Fracture ◽  
Crack Growth ◽  
Crack Front ◽  
Stress Triaxiality ◽  
Experimental Program ◽  
Effective Plastic Strain ◽  
Ductile Materials ◽  
Ductile Crack Growth ◽  
Different Temperatures

In this paper, the ductile fracture mechanism is discussed. The results of numerical and experimental analyses were used to estimate the onset of crack front growth. It was assumed that the ductile fracture in front of the crack starts at the location along the crack front where the accumulated effective plastic strain reaches a critical value. According to numerous research articles, the critical effective plastic strain depends on the stress triaxiality and the Lode angle. The experimental program was performed using five different specimen geometries, three different materials, and three different temperatures of +20 °C, −20 °C, and −50 °C. Using the experimental data and results of the finite element computations, the critical effective plastic strains were determined for each material and temperature. However, before the critical effective plastic strain was determined, a careful calibration of the stress–strain curves was performed after modification of the Bai–Wierzbicki procedure. It was found that critical effective plastic strain was a function of triaxiality factor and Lode parameter, as expected, and that the fracture locus was useful to estimate the onset of ductile crack growth.

scholarly journals Estimation of the Onset of Crack Growth in Ductile Materials

2018 ◽  
Author(s):  
Andrzej Neimitz ◽  
Jaroslaw Galkiewicz ◽  
Sebastian Lipiec ◽  
Ihor Dzioba
Keyword(s):  
Plastic Strain ◽  
Ductile Fracture ◽  
Crack Front ◽  
Stress Triaxiality ◽  
Critical Value ◽  
Effective Plastic Strain ◽  
Ductile Materials ◽  
Different Temperatures ◽  
Fracture Tests ◽  
Lode Angle

In this paper, the ductile fracture mechanism is discussed. The results of the numerical and experimental analyses are used to estimate of the onset of the crack front growth . It is assumed that the ductile fracture in front of the crack starts at the location along the crack front where the accumulated effective plastic strain reaches a critical value. It is also assumed that the critical effective plastic strain depends on the stress triaxiality and the Lode angle. The experimental programme was performed using five different specimen geometries, three different materials and three different temperatures of +20°C, -20°C and -50°C. Using the experimental data and the results of the finite element computations, the critical effective plastic strains are determined for each material and each temperature. However, before the critical effective plastic strain is determined, a careful calibration of the stress–strain curves was performed after modification of the Bai–Wierzbicki procedure. Finally, by analysing the experimental results recorded during the interrupted fracture tests and scanning microscopy observations, the research hypothesis is verified.

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.

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.

Verification of a Cohesive Zone Model for Ductile Fracture

1996 ◽  
Vol 118 (2) ◽  
pp. 192-200 ◽  
Author(s):  
Huang Yuan ◽  
Guoyu Lin ◽  
Alfred Cornec
Keyword(s):  
Ductile Fracture ◽  
Crack Growth ◽  
Cohesive Zone ◽  
Cohesive Zone Model ◽  
Normal Modes ◽  
Stable Crack Growth ◽  
Small Region ◽  
Zone Model ◽  
Cohesive Law ◽  
Ductile Crack Growth

In the present paper, ductile crack growth in an aluminium alloy is numerically simulated using a cohesive zone model under both plane stress and plane strain conditions for two different fracture types, shear and normal modes. The cohesive law for ductile fracture consists of two parts—a specific material’s separation traction and energy. Both are assumed to be constant during ductile fracture (stable crack growth). In order to verify the assumed cohesive law to be suitable for ductile fracture processes, experimental records are used as control curves for the numerical simulations. For a constant separation traction, determined experimentally from tension test data, the corresponding cohesive energy was determined by finite element calculations. It is confirmed that the cohesive zone model can be used to characterize a single ductile fracture mode and is roughly independent of stable crack extention. Both the cohesive traction and the cohesive fracture energy should be material specific parameters. The extension of the cohesive zone is restricted to a very small region near the crack tip and is in the order of the physical fracture process. Based on the present observations, the cohesive zone model is a promising criterion to characterize ductile fracture.

Constraint Corrected Cleavage Fracture Assessment Based on Tests of Standard and Small Scale Cruciform Specimens

2009 ◽  
Author(s):  
Jo¨rg Hohe ◽  
Volker Hardenacke ◽  
Dieter Siegele
Keyword(s):  
Numerical Simulation ◽  
Plastic Strain ◽  
Cleavage Fracture ◽  
Crack Front ◽  
Stress Triaxiality ◽  
Local Stress ◽  
Fracture Model ◽  
Additional Stress ◽  
Small Scale ◽  
Fracture Assessment

The objective of the present study is the determination and validation of a constraint corrected probabilistic cleavage fracture model considering both, the nucleation and the possible instability of potentially critical microdefects. For this purpose, the local mechanical conditions at the cleavage triggering point are analyzed by a numerical simulation of different fracture mechanics experiments using C(T), M(T) and SE(B) specimens with deep and shallow cracks. The data base is extended by experiments on novel small scale cruciform bending specimens allowing the application of an additional stress acting longitudinally to the crack front. Based on the results, an enhanced local cleavage fracture model is proposed, which accounts for the nucleation of microdefects in terms of a combined parameter of the local accumulated plastic strain and the local stress triaxiality ratio.

Assessment of Ductile Crack Growth Based on a Local Approach

2006 ◽  
Author(s):  
Tae-Rin Lee ◽  
Yoon-Suk Chang ◽  
Jae-Boong Choi ◽  
Young-Jin Kim
Keyword(s):  
Crack Growth ◽  
Fracture Resistance ◽  
Stress Triaxiality ◽  
Mechanical Damage ◽  
Ductile Crack ◽  
Local Approach ◽  
Crack Growth Behaviour ◽  
Damage Models ◽  
Ductile Crack Growth ◽  
Comparison Results

The influence of stress triaxiality was a matter of concern on ductile fracture to explain geometry dependent fracture resistance characteristics of specimens and real structures during past two decades. Regarding the issue, recently, interests for local approach and micro-mechanical damage models are increased again in accordance with progress of computational environments. In this paper, the applicability of a local approach is investigated through a series of finite element (FE) analyses incorporating both a modified GTN model and a Rousselier model as well as fracture toughness tests. The ductile crack growth behaviour of SA515 Gr.60 carbon steel is assessed to guarantee transferability of fracture resistance curve from typical specimens with different in-plane and out-of-plane sizes. The material parameters are determined by calibration of test results and corresponding numerical analyses results, and used to simulate the fracture behaviour of CT specimens. Then, a comparison is drawn between the numerically estimated crack resistance curves and experimentally determined ones. Finally, the Rousselier model is applied to estimate J-R curves of circumferential through-wall cracked pipe. The comparison results showed that the two damage models can be used as promising solutions for ductile crack growth simulation.

Effect of Constraint on the Initiation of Ductile Fracture in Shear Loading

2004 ◽  
Vol 261-263 ◽  
pp. 183-188 ◽  
Author(s):  
Majid R. Ayatollahi ◽  
David John Smith ◽  
M.J. Pavier
Keyword(s):  
Fracture Toughness ◽  
Ductile Fracture ◽  
Crack Growth ◽  
Void Growth ◽  
Crack Tip ◽  
Shear Loading ◽  
Mode Ii ◽  
Ductile Crack ◽  
Ductile Crack Growth ◽  
Void Growth And Coalescence

Research studies for mode I cracks have shown that fracture toughness or the critical value of J for fracture initiation, Jcrit is not merely a material property but depends also on the geometry and loading configurations. The geometry dependency of fracture toughness can be attributed to the effect of the crack tip constraint. In this paper, the constraint effect is studies for the initiation stage in mode II ductile crack growth. Two major mechanisms of ductile fracture: 'void growth and coalescence' and 'shear band localization and de-cohesion' are considered. A boundary layer model is simulated using the finite element method and the effect of far-filed T-stress on the relevant stress parameters near the crack tip is studied. It is shown that the initiation of the ductile crack growth in mode II is influenced significantly by T for the mechanism of void growth and coalescence and is insensitive to T for the mechanism of shear localisation and de-cohesion.

Three-Dimensional Crack Growth in Ductile Materials: Effect of Stress Constraint on Crack Tunneling

2008 ◽  
Vol 130 (3) ◽  
Author(s):  
Jianzheng Zuo ◽  
Xiaomin Deng ◽  
Michael A. Sutton ◽  
Chin-Shan Cheng
Keyword(s):  
Fracture Toughness ◽  
Crack Growth ◽  
Crack Front ◽  
Fracture Criterion ◽  
Three Dimensional ◽  
Stress Constraint ◽  
Ductile Materials ◽  
Fracture Simulation ◽  
Crack Tunneling ◽  
Stable Tearing

Crack tunneling is a crack growth feature often seen in stable tearing crack growth tests on specimens made of ductile materials and containing through-thickness cracks with initially straight crack fronts. As a specimen is loaded monotonically, the midsection of the crack front will advance first, which will be followed by crack growth along the rest of the crack front, leading to the formation of a thumbnail shaped crack-front profile. From the viewpoint of fracture mechanics, crack tunneling will occur if the operating fracture criterion is met first in the midsection of the crack front, which may be due to a higher fracture driving force and∕or a lower fracture toughness in the midsection. A proper understanding of this fracture behavior is important to the development of a three-dimensional fracture criterion for general stable tearing crack growth in ductile materials. In this paper, the phenomenon of crack tunneling during stable tearing crack growth in a single-edge crack specimen is investigated by considering the effect of stress constraint on the fracture toughness. Crack growth in the specimen under nominally Mode I loading conditions is considered. In this case, crack tunneling occurs while the initially flat crack surface (which is normal to the specimen’s lateral surfaces) evolves into a final slanted fracture surface. A mixed-mode crack tip opening displacement (CTOD) fracture criterion and a custom three-dimensional (3D) fracture simulation code, CRACK3D, are used to analyze the crack tunneling event (but not crack slanting) in the specimen. Results of this investigation suggest that the critical CTOD value (which is the fracture toughness) has a clear dependence on the crack-front stress constraint Am (the constraint measure in this work is the stress triaxiality, which is the ratio of the mean normal stress to the von Mises effective stress). For simplicity, this dependence can be approximated by a straight line within the range of stress constraint values found, with the toughness decreasing as the constraint increases. It is found that crack tunneling in this case is mainly the result of a higher stress constraint (hence a lower fracture toughness) in the midsection of the crack front. Details of the crack growth simulation and other findings of this study will also be presented.

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