A Generalized Damage Model Accounting for Instability and Ductile Fracture for Sheet Metals

2014 ◽  
Vol 611-612 ◽  
pp. 106-110 ◽  
Author(s):  
Jun He Lian ◽  
Xiao Xu Jia ◽  
Sebastian Münstermann ◽  
Wolfgang Bleck
Keyword(s):  
Stress State ◽  
Ductile Fracture ◽  
Automotive Industry ◽  
Void Nucleation ◽  
Damage Model ◽  
High Stress ◽  
Forming Limit ◽  
Dual Phase ◽  
Sheet Metals ◽  
Hard Phase

With the requirement of vehicle performance and fuel economy, dual-phase (DP) steels as one of the advanced high stress steels (AHSS) are increasingly used in the automotive industry due to the excellent combination of the tensile strength and ductility. On a microscale the ductile fracture is governed by the void nucleation, growth and coalescence mechanism. In the dual-phase steels this damage mechanism exhibits a rather complex situation: voids are generated by the debonding of the hard phase from the matrix and the inner cracking of the hard phase besides by inclusions. On a macroscale fracture of these materials is observed in the automotive industry with the absence of strain localization or minimal post-necking deformation. Consequently the failure during the forming process is caused by a competitive or combined mechanism of internal damage evolution and metal instability. In this study, the target is to develop a simple and generalized model for metal forming processes accounting for instability, damage and ductile fracture. Theoretical predictions of metal instability by the Hill–Swift necking criterion and the modified maximum force criterion are considered. The damage model is developed by the combination of the prediction of metal instability and ductile fracture of sheet metals. The model is developed in 3D triaxial stress state and the accumulation of damage is stress state dependent. Furthermore, the influence of the hardening curve effected by damage on the forming limit curve is investigated.

Development of Grade X80 High Charpy Energy Linepipe by MA Formation Control

2016 ◽  
Author(s):  
Hideyuki Kimura ◽  
Tomoyuki Yokota ◽  
Nobuyuki Ishikawa ◽  
Shinichi Kakihara ◽  
Joe Kondo
Keyword(s):  
Ductile Fracture ◽  
Formation Control ◽  
Single Phase ◽  
Void Nucleation ◽  
Controlled Rolling ◽  
Accelerated Cooling ◽  
Dual Phase ◽  
Long Distance ◽  
Dual Phase Steels ◽  
Homogeneous Microstructure

Higher grade linepipes such as grade X80 have been developed and applied to long distance pipelines in order to reduce the cost of pipeline construction by using thinner pipes than is possible with conventional grades. Service pressures have also been increased in recent years for efficient gas transportation. In addition to the requirement of higher strength, running ductile fracture should be prevented in long distance and high pressure pipelines. Resistance to ductile fracture, as evaluated by Charpy energy, is an important material property for higher grade linepipes. It has been reported that bainite single-phase steel tends to show higher Charpy energy than ferrite-bainite or bainite-MA (martensite-austenite constituent) dual-phase steels, since void nucleation is suppressed in single-phase steels compared with dual-phase steels. However, in higher grade steels with a bainite single phase, a small amount of MA grains generally remains due to the chemical stability of MA. Therefore, further reduction of MA is key to improving Charpy energy for higher grade linepipe steels. In order to achieve high Charpy energy by MA formation control, the optimum conditions of the plate manufacturing process were investigated. As a result, a high Charpy energy was achieved by the combination of controlled rolling and precise control of the accelerated cooling conditions, by which the MA phase was minimized. Based on the above investigation, grade X80 high Charpy energy linepipes were trial-produced by applying JFE Steel’s optimized accelerated cooling (ACC) system with a high cooling rate and homogeneous temperature profile. Stable higher Charpy energy was achieved by minimizing MA formation and achieving a homogeneous microstructure by advanced cooling control.

Ductile Fracture Behavior of Bainite-MA Dual Phase Steels

2014 ◽  
Author(s):  
Junji Shimamura ◽  
Kyono Yasuda ◽  
Nobuyuki Ishikawa ◽  
Shigeru Endo
Keyword(s):  
Ductile Fracture ◽  
Fracture Behavior ◽  
Volume Fraction ◽  
Void Nucleation ◽  
Ground Movement ◽  
Dual Phase ◽  
Dual Phase Steels ◽  
Boundary Area ◽  
Strain Capacity ◽  
Shelf Region

In order to achieve safety and reliability of the pipeline installed in seismic region, it is quite important to apply the high-strength linepipes with sufficient strain capacity against buckling and weld fracture by the seismic ground movement. Dual-phase microstructure control is an essential measure for improving strain capacity of linepipe steels. Ferrite-bainite or bainite-MA microstructures are practically applied to the linepipes for the strain-based design to achieve higher deformability which has low Y/T (Yield/Tensile strength) ratio and high uniform elongation even after pipe coating. On the other hand, dual-phase steels tend to show lower Charpy energy in the upper shelf region than single-phase steel. It is considered that void nucleation and growth is enhanced in the dual-phase steels due to the strain concentration at the boundary between two different phases, resulting in early cracking in the specimen that leads to lower Charpy energy. The Charpy energy of the bainite-MA dual-phase steels was strongly affected by the volume fraction and size of MA. In the case of Bainite-MA steels with fewer volume fraction of MA and smaller size of MA, the sample showed higher Charpy energy. Ductile fracture behavior was investigated through several kinds of Charpy impact tests in order to clarify the effect of these microstructure differences on the Charpy energy in the upper shelf region. From the SEM observation, it was found that void nucleation was enhanced in the sample with higher volume fraction of MA and larger size of MA. It is considered that the increase of boundary area that works as void nucleation site affected these results. Experimental results were mainly discussed in this paper.

Application of Barlat Yld-96 Yield Criterion for Predicting Formability of Pre-Strained Dual Phase Steel Sheets

2016 ◽  
Author(s):  
Shamik Basak ◽  
Sushanta Kumar Panda
Keyword(s):  
Dual Phase Steel ◽  
Yield Criterion ◽  
Thin Sheet ◽  
Damage Model ◽  
Forming Limit Diagram ◽  
Plasticity Theory ◽  
Forming Limit ◽  
Dual Phase ◽  
Strain Paths ◽  
Anisotropy Coefficients

The selection of advanced material model considering the anisotropy mechanical properties of the thin sheet is vital in order to estimate stress based forming limit diagram (σ-FLD). In present study associative plasticity theory was applied indulging Barlat Yld-96 anisotropy yield function and the Swift hardening law was implemented for estimating the limiting stresses from the conventional strain FLD (ε-FLD) of an automotive grade dual phase steel DP600. Three different approaches were made to evaluate Yld-96 anisotropy coefficients using experimental results of stack compression and tensile tests. To impose complex strain path, two stage stretch forming processes were simulated in finite element solver LS-DYNA. After biaxial pre-straining, the sample geometries were varied to achieve different strain paths during the second stage of deformation. The results indicated that there was negligible difference in limiting stress estimated by Yld-96 plasticity theory when the anisotropy coefficients were calculated based on plastic strain at ultimate tensile strength compare to that by minimum plastic work method. It was concluded that the dynamic shift of ε-FLD could be restricted by σ-FLD estimated using Yld 96 plasticity theory, and hence it was proposed to be a suitable damage model to evaluate formability of pre-strained DP600 steels.

Prediction of Forming Limit of Dual-Phase 500 Steel Sheets Using the GTN Ductile Damage Model in an Innovative Hydraulic Bulging Test

JOM ◽  
2018 ◽  
Vol 70 (8) ◽  
pp. 1542-1547 ◽  
Author(s):  
Xiao-Lei Cui ◽  
W. W. Zhang ◽  
Zhi-Chao Zhang ◽  
Yi-Zhe Chen ◽  
Peng Lin ◽  
...  
Keyword(s):  
Damage Model ◽  
Ductile Damage ◽  
Forming Limit ◽  
Dual Phase ◽  
Steel Sheets ◽  
Bulging Test ◽  
Hydraulic Bulging

Numerical Simulation of Void Nucleation in S355 Steel

2016 ◽  
Vol 250 ◽  
pp. 244-249
Author(s):  
Wiktor Wcislik
Keyword(s):  
Numerical Simulation ◽  
Stress State ◽  
Civil Engineering ◽  
Void Nucleation ◽  
Iron Carbide ◽  
High Stress ◽  
Nucleation Process ◽  
Engineering Structures ◽  
The Matrix

The study presents the numerical simulation of the void nucleation process due to decohesion of the interface between the matrix and inclusion of iron carbide Fe3C, under high stress state triaxiality ratio, which is equal to 1.345. The analysis was conducted for S355J2G3 steel, commonly used in civil engineering structures. Special attention was paid to the determination of the value of void nucleation strain.

scholarly journals Study of Fatigue Fractography of Mild Steel Used in Automotive Industry

2019 ◽  
Vol 15 (1) ◽  
pp. 82-88
Author(s):  
Ahmed Ameed Zainulabdeen
Keyword(s):  
Ductile Fracture ◽  
Fatigue Test ◽  
Automotive Industry ◽  
High Stress ◽  
Low Carbon Steel ◽  
Xrd Analysis ◽  
Tensile Tests ◽  
Void Coalescence ◽  
Low Carbon ◽  
High Stress Level

Fatigue failure is almost considered as the predominant problem affecting automotive parts under dynamic loading condition. Thus, more understanding of crack behavior during fatigue can strongly help in finding the proper mechanism to avoid the final fracture and extent the service life of components. The main goal of this paper is to study the fracture behavior of low carbon steel which is used mostly in automotive industry. For this purpose, the fractography of samples subjected to high and low stress levels in fatigue test then was evaluated and analyzed. Hardness and tensile tests were carried out to determine the properties of used steel. Also, the samples were characterized by microstructure test and XRD analysis to examine the constitute phases. The fatigue test (S-N curve) was done at stress ratio (R= -1), and the fracture examination was perform using Scanning Electron Microscope (SEM). The results of microstructure and XRD analysis were indicating that the Ferrite and a little amount of pearlite are the dominant phases of this steel. Whereas, the fractography observations reveal that the void coalescence ductile fracture is the main failure mode in samples with high stress level, while the ductile fracture (void coalescence) with Transgranular Cleavage fracture was noticed in low stress fatigue mode for this alloy.

Effect of Martensite Fraction on Void Nucleation Behavior and Ductility in DP Steels

2014 ◽  
Vol 783-786 ◽  
pp. 875-879 ◽  
Author(s):  
Daisuke Maeda ◽  
Osamu Kawano
Keyword(s):  
Ductile Fracture ◽  
Void Fraction ◽  
Void Nucleation ◽  
Small Strain ◽  
The Other ◽  
Dual Phase ◽  
Nucleation Behavior ◽  
Martensite Fraction ◽  
Effective Factor ◽  
Dp Steels

Void nucleation behavior of ferrite/martensite dual phase steels varying martensite fraction was investigated. As easily recognized, void fraction was increased with strain induced, and more voids were nucleated in the sample with higher martensite fraction. On the other hands, void fraction at ductile fracture was decreased with increasing martensite fraction. Void nucleation was observed to occur due to the fracture of martensite in DP steels. In the sample with high martensite fraction, many micro voids were nucleated at initial deformation with small strain and lead to ductile fracture. Inter-voids distance at the fracture was almost same among the DP steels with various martensite fractions. It is considered that the most effective factor on ductile fracture of DP steels was not 2nd phase fraction but the distance between 2nd phases which caused micro voids.

Sign in / Sign up

Export Citation Format

Share Document