Anisotropic ductile fracture criterion based on linear transformation

Lou, Yanshan,Yoon, Jeong Whan Elsevier 2017 International journal of plasticity Vol.93 No.-

<P><B>Abstract</B></P> <P>An anisotropic ductile fracture criterion is proposed for ductile fracture of lightweight metals. The ductile fracture criterion couples effect of stress triaxiality on void growth, and assumes the shear linking-up of voids governed by the largest shear stress. The criterion is developed based on an isotropic strain rate potential computed from an isotropic damage equivalent strain rate vector, which is mapped from the plastic strain rate vector by a forth order linear transformation tensor. The proposed anisotropic ductile fracture criterion is applied to depict anisotropic ductile fracture of AA 6K21 in shear, uniaxial tension and plane strain tension along different loading directions, and the balanced biaxial tension. The predicted fracture strain and fracture locus are compared with experimental results for the verification of the proposed criterion. The comparison demonstrates that the ductile fracture criterion properly models the anisotropy in ductile fracture under shear, uniaxial tension, plane strain tension and balanced biaxial tension with high accuracy. It shows that the proposed anisotropic ductile fracture criterion can be utilized to predict the onset of ductile fracture in plastic deformation and metal forming of lightweight metals with approximately proportional loading.</P> <P><B>Highlights</B></P> <P> <UL> <LI> An anisotropic ductile fracture criterion is newly proposed based linear transformation. </LI> <LI> Strain-rate potential is introduced to calibrate the anisotropic fracture. </LI> <LI> Tests are conducted in shear, uniaxial tension and plane strain tension along RD, DD and TD, and balanced biaxial tension. </LI> <LI> The criterion can model fracture in shear, uniaxial and plane strain tension of RD, DD and TD and balanced biaxial tension. </LI> </UL> </P>

유한요소해석을 이용한 금속 판재용 전단 파단 시편 설계

김찬양,봉혁종,이명규 한국소성∙가공학회 2023 소성가공 : 한국소성가공학회지 Vol.32 No.2

In this study, shear fracture specimens are designed using finite element analyses for the characterization of ductile fracture criteria of metal sheets. Many recently suggested ductile fracture criteria require experimental fracture data at the shear stress states in the model parameter identification. However, it is challenging to maintain shear stress states in tension-based specimens from the initial yield to the final fracture, and the loading path can be different for the different materials even with the same shear specimen geometries. To account for this issue, two different shear fracture specimens for low ductility/high ductility metal sheets are designed using the sensitivity tests conducted by finite element simulations. Priorly mechanical properties including the Hosford-Coulomb fracture criterion of the aluminum alloy 7075-T6 and DP590 steel sheets are used in the simulations. The results show that shear stress states are well-maintained until the fracture at the fracture initiation points by optimizing the notch geometries of the shear fracture specimens.

Comparative Study on Various Ductile Fracture Models for Marine Structural Steel EH36

Sung-Ju Park,Kangsu Lee,Burak Can Cerik,Joonmo Choung 한국해양공학회 2019 韓國海洋工學會誌 Vol.33 No.3

It is important to obtain reasonable predictions of the extent of the damage during maritime accidents such as ship collisions and groundings. Many fracture models based on different mechanical backgrounds have been proposed and can be used to estimate the extent of damage involving ductile fracture. The goal of this study was to compare the damage extents provided by some selected fracture models. Instead of performing a new series of material constant calibration tests, the fracture test results for the ship building steel EH36 obtained by Park et al. (2019) were used which included specimens with different geometries such as central hole, pure shear, and notched tensile specimens. The test results were compared with seven ductile fracture surfaces: Johnson-Cook, Cockcroft-Latham-Oh, Bai-Wierzbicki, Modified Mohr-Coulomb, Lou-Huh, Maximum shear stress, and Hosford-Coulomb. The linear damage accumulation law was applied to consider the effect of the loading path on each fracture surface. The Swift-Voce combined constitutive model was used to accurately define the flow stress in a large strain region. The reliability of these simulations was verified by the good agreement between the axial tension force elongation relations captured from the tests and simulations without fracture assignment. The material constants corresponding to each fracture surface were calibrated using an optimization technique with the minimized object function of the residual sum of errors between the simulated and predicted stress triaxiality and load angle parameter values to fracture initiation. The reliabilities of the calibrated material constants of B-W, MMC, L-H, and HC were the best, whereas there was a high residual sum of errors in the case of the MMS, C-L-O, and J-C models. The most accurate fracture predictions for the fracture specimens were made by the B-W, MMC, L-H, and HC models.

Ductile Fracture Analysis of Welded Hollow Spherical Joints Subjecting Axial Forces with Micromechanical Fracture Models

Yue Yin,Xinyu Che,Zhenyu Li,Jinlong Li,Qinghua Han 한국강구조학회 2019 International Journal of Steel Structures Vol.19 No.6

Two micromechanical fracture models, void growth model (VGM) and stress modifi ed critical strain (SMCS) model, were adopted to distinguish the failure mechanism of welded hollow spherical (WHS) joints under axial load based on FE analysis. Ductile fracture was successfully predicted for WHS joints under axial tension. The predicted fracture location is at the weld toe between WHS joints and circular hollow section members, which is consistent with corresponding test results. The predicted fracture load is lower than the peak load on the load–displacement curve, which indicates that the failure mechanism of WHS joints under axial tension is fracture due to inadequate strength and the fracture load should be taken as the ultimate load bearing capacity of the joints. A simplifi ed SMCS model was proposed and verifi ed for ductile fracture prediction of WHS joints under axial tension. Micromechanical fracture analysis was also conducted on WHS joints under axial compression. It was obtained by both VGM and SMCS model that no fracture would occur before the load reached its peak value, the reason of which was discussed by tracing the variation of the equivalent plastic stain and stress triaxiality at the potential location of fracture. Therefore, the failure mechanism of WHS joints under axial compression is losing stability with the depression of the sphere cap and the peak load on the load–displacement curve should be taken as the ultimate load bearing capacity of WHS joints.

Comparative investigation of ductile fracture with 316L austenitic stainless steel in small punch tests: Experiments and simulations

Kubí,k, Petr,Š,ebek, Františ,ek,Petruš,ka, Jindř,ich,Hů,lka, Jiř,í,Park, Namsu,Huh, Hoon Elsevier 2018 Theoretical and applied fracture mechanics Vol.98 No.-

<P><B>Abstract</B></P> <P>This paper deals with a small punch test and ductile failure prediction of austenitic stainless steel 316L. Comprehensive investigation of fracture surfaces was conducted as well as examination of δ-ferrite and σ-phase present in the material in order to reveal the reason of occurrence of nonstandard type of fracture. Three phenomenological ductile fracture criteria were selected for comparative simulations regarding the prediction of crack initiation and propagation during the SPT. These criteria were calibrated by using the fracture tests of smooth cylindrical specimens, notched cylindrical specimens, notched tube specimens and cylinders with spherical recess. The whole material model including the fracture criteria with von Mises plasticity and element deletion technique for simulation of the crack initiation was implemented by using the user subroutine Vectorised User MATerial into the commercial finite element code of Abaqus. Finally, the simulation results with three ductile fracture criteria are compared and discussed based on the experimental results. Only one of the failure criteria captured the displacement at fracture correctly.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Wide range of stress states is experimentally described for AISI 316L. </LI> <LI> Detailed study of fracture surfaces produced by penetration tests. </LI> <LI> Non-standard cracking is observed due to ferrite phase. </LI> <LI> Uncoupled ductile fracture criteria are calibrated for austenitic stainless steel. </LI> </UL> </P>

A New Fracture Analysis Technique for Charpy Impact Test Using Image Processing

박태창,김범석,손지희,여영구 대한금속·재료학회 2021 대한금속·재료학회지 Vol.59 No.1

The Charpy impact test is used to identify the transition between ductility and brittleness. The percentages of ductile and brittle fractures in steel can be evaluated based on each fracture area, which is presently determined by an analyzer with the naked eye. This method may lead to subjective judgement, and difficulty accurately quantifying the percentage. To resolve this problem, a new analysis method based on image processing is proposed in this study. A program that can automatically calculate the percentage of the ductile and brittle fractures has been developed. The analysis is performed after converting an RGB fracture image into a binary image using image processing techniques. The final binary image consists of 0 and 1 pixels. The parts with the pixel values of 1 correspond to the brittle fracture areas, and the pixel values of 0 represent the ductile fracture areas. As a result, by counting the number of 0 pixels in the entire area, it is possible to automatically calculate the percentage of ductile fracture. Using the proposed automatic fracture analysis program, it is possible to selectively distinguish only the brittle fracture from the entire fracture area, and to accurately and quantitatively calculate the percentages of ductile and brittle fractures.

The Effect of Surface Defects on the Cyclic Fatigue Fgacture of HEROShaper Ni-Ti rotary in a Dynamic Model : A Fractographic Analysis

Lee, Jung-Kyn,Kim, Eui-Sung,Kang, Myoung-Whai,Kum, Kee-Yeon 대한치과보존학회 2007 Restorative Dentistry & Endodontics Vol.32 No.2

본 연구의 목적은 니켈티타늄 전동파일의 피로파절에 있어서 표면 결함의 역할을 규명하고자 fatigue tester에서 반복적인 fatigue force 를 부여한 후 파절된 단면을 주사전자현미경으로 관찰하여 파절 역학을 규명하는 것이다. 총 45개의 #30/.04 taper와 21 mm의 HEROShaper 니켈-티타늄 전동파일을 15개씩 3개의 군으로 분류하였다. 제 1군은 결함이 없는 새 HEROShaper파일, 제 2군은 제조과정에서 metal rollover나 machining marks와 같은 표면결함을 갖는 HEROShaper파일, 제 3군은 임상에서 4 - 6개의 구치부 근관의 확대에 사용한 HEROShaper 파일을 사용하였다. 모든 파일들은 회전속도(300 rpm)와 pecking distance (3 mm)가 일정하게 맞춘 fatigue tester에서 파절될 때까지 시간을 측정한 후 통계분석을 통해 각 군간의 유의성을 분석하였고, 파절 단면의 farctographic analysis 를 통해 파절역학을 규명하고자 하였다. 실험결과 평균 파절시간에 있어서 group 1과 2, group l과 3 사이에는 통계학적으로 유의할 만한 차이가 있었으나 (p < 0.05), group 2와 3 사이에는 통계학적인 차이가 없었다. Fractographic analysis 결과 대부분의 파절면에서 microvoid와 dimple 소견을 갖는 ductile fracture양상이 관찰되었다. 또한 brittle fracture가 일어난 파절면에서는 파절선 전방에 수 많은 striation 들이 관찰되었고 transgranular 및 intergranular cleavage 소견도 보였다. 표면결함이 있는 제 2, 3 군의 파절단면에서는 모든 시편에서 표면결함이 관찰되었다. 이와 같은 결과로 미루어 보아 표면결함이 반복 피로파절에서 미세균열의 기시점으로 중요한 역할을 하며 fractography 분석법은 Ni-Ti 파일의 파절역학을 규명하는데 유용함을 알 수 있었다. This in vitro study examined the effect of surface defects on cutting blades on the extent of the cyclic fatigue fracture of HEROShaper Ni-Ti rotary files using fractographic analysis of the fractured surfaces. A total of 45 HEROShaper (MicroMega) Ni-Ti rotary files with a #30/.04 taper were divided into three groups of 15 each. Group 1 contained new HEROShapers without any surface defects. Group 2 contained HEROShapers with manufacturing defects such as metal rollover and machining marks. Group 3 contained HEROShapers that had been clinically used for the canal preparation of 4-6 molars. A fatigue-testing device was designed to allow cyclic tension and compressive stress on the tip of the instrument whilst maintaining similar conditions to those experienced in a clinic. The level of fatigue fracture time was measured using a computer connected the system. Statistical analysis was performed using a Tukey's test. Scanning electron microscopy (SEM) was used for fractographic analysis of the fractured surfaces. The fatigue fracture time between groups 1 and 2, and between groups 1 and 3 was significantly different (p < 0.05) but there was no significant difference between groups 2 and 3 (p > 0.05). A low magnification SEM views show brittle fracture as the main initial failure mode. At higher magnification, the brittle fracture region showed clusters of fatigue striations and a large number of secondary cracks. These fractures typically led to a central region of catastrophic ductile failure. Qualitatively, the ductile fracture region was characterized by the formation of microvoids and dimpling. The fractured surfaces of the HEROShapers in groups 2 and 3 were always associated with pre-existing surface defects. Typically, the fractured surface in the brittle fracture region showed evidence of cleavage (transgranular) facets across the grains, as well as intergranular facets along the grain boundaries. These results show that surface defects on cutting blades of Ni-Ti rotary files might be the preferred sites for the origin of fatigue fracture under experimental conditions. Furthermore, this work demonstrates the utility of fractography in evaluating the failure of Ni-Ti rotary files.

Revisiting MARSTRUCT benchmark study on side-shell collision with a combined localized necking and stress-state dependent ductile fracture model

Cerik, Burak Can,Ringsberg, Jonas W.,Choung, Joonmo Elsevier 2019 Ocean engineering Vol.187 No.-

<P><B>Abstract</B></P> <P>The MARSTRUCT benchmark study on a small-scale double hull structure penetrated by a hemispherical punch was revisited by employing a combined localized necking and stress-state dependent ductile fracture model. By using the limited information provided to the participants of the benchmark study, the plasticity and fracture model parameters were identified. To model the material behavior beyond moderate plasticity, a combination of the Swift and Voce strain hardening laws was used. The damage indicator framework using the Hosford–Coulomb fracture model, combined with the Domain of Shell-to-Solid-Equivalence (DSSE) concept, was adopted to predict the initiation and propagation of ductile fracture. Using the adopted approach, the predicted instant and force levels corresponding to the fracture initiation in the upper and lower plates were found to be in good agreement with the test results. The deformation of the structural elements was also accurately captured. The benefits of adopting the damage indicator framework and distinguishing different failure modes were investigated.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Localized necking dominates the failure of punch-loaded double-hull testmodel. </LI> <LI> Constant fracture strain criterion does not predict well the failure process of dented web-girders. </LI> <LI> Non-proportional loading paths and bending deformation is observed in the failure of outer and inner shells. </LI> <LI> Employing advanced ductile fracture models has considerable benefits for maritime crash simulations. </LI> </UL> </P>

Prediction of Shear-induced Crack Initiation in AHSS Deep Drawing Operation with a Phenomenological Fracture Model

Meng Luo,Yaning Li,Joerg Gerlach,Tomasz Wierzbicki 한국소성가공학회 2010 기타자료 Vol.2010 No.6

Advanced High Strength Steels (AHSS) draws enormous attentions in automotive industry because it has great potential in reducing weight and improving fuel efficiency. Nonetheless, their relatively low formability also causes many problems in manufacturing processes, such as shear-induced fracture during deep drawing or stamping. This type of fracture could not be predicted using traditional necking-based Forming Limit Diagram (FLD), which is commonly used by the forming community. In the present paper, a recently developed Modified Mohr-Coulomb (MMC)[1] ductile fracture model is employed to make up the deficiency of FLD. In the limiting case of plane stress, the MMC fracture locus consists of four branches when represented on the plane of the equivalent strain to fracture and the stress triaxiality. A transformation of above 2D fracture locus to the space of principal strains was performed which revealed the existence of two new branches not known before. The existence of those branches explains the formation of shear-induced fracture. As an illustration of this new approach, initiation and propagation of cracks in a series of deep drawing tests is predicted and compared with the experimental observations. It was shown that the location of fracture as well as the magnitude of punch travel corresponding to first fracture was correctly predicted by MMC fracture model for both square and circular punch cases.

A Study on Fracture Locus of St12 Steel And Implementation Ductile Damage Criteria

Mohammad Khataei,Mehrdad Poursina,Mahmoud Kadkhodaei 한국소성가공학회 2010 기타자료 Vol.2010 No.6

In a metal forming process, the state of stress is one of the most important parameters on forming and behavior of the material. According to ductile damage criteria, the magnitudes of fracture strain for various stress triaxiality values should be determined for prediction of the place and time of ductile fracture. In this paper, the magnitudes of fracture strain of St12 steel is measured using several tensile tests on notched samples. Johnson-Cook equation for fracture strain as a function of stress triaxiality is calibrated for St12 steel, using the obtained experimental data. The accuracy of this function is achieved by comparison of the FEM results with experimental data which are achieved during simple tension and Erichsen tests. The simulation results have shown that the ductile damage model is a suitable criterion for prediction of fracture in St12 steel. In addition, notched samples tensile tests are suitable for calibration of Johnson-Cook equation for St12 steel.