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Journal Article

Applicability of Micromechanics Model Based on Actual Microstructure for Failure Prediction of DP Steels

2009-04-20
2009-01-0469
In this paper, various micromechanics models based on actual microstructures of DP steels are examined in order to determine the reasonable range of martensite volume fraction where the methodology described in this study can be applied. For this purpose, various micromechanics-based finite element models are first created based on the actual microstructures of DP steels with different martensite volume fractions. These models are, then, used to investigate the influence of ductility of the constituent ferrite and martensite phases and also the influence of voids in the ferrite phase on the overall ductility of DP steels.
Journal Article

Application of Nano-Indentation Test in Estimating Constituent Phase Properties for Microstructure-Based Modeling of Multiphase Steels

2017-03-28
2017-01-0372
For multiphase advanced high strength steels (AHSS), the constituent phase properties play a crucial role in determining the overall mechanical behaviors. Therefore, it is important to accurately measure/estimate the constituent phase properties in the research of AHSS. In this study, a new nanoindentation-based inverse method that we developed was adopted in estimating the phase properties of a low alloy Quenching and Partitioning (Q&P) steel. A microstructure-based Finite Element (FE) model was also generated based on the Electron BackScatter Diffraction (EBSD) and Scanning Electron Microscopy (SEM) images of the Q&P steel. The phase properties estimated from nanoindentation were first compared with those estimated from in-situ High Energy X-Ray Diffraction (HEXRD) test and, then, employed in the generated FE model to examine whether they can be appropriately used as the input properties for the model.
Technical Paper

Characterization of the Fracture Toughness of TRIP 800 Sheet Steels Using Microstructure-Based Finite Element Analysis

2009-04-20
2009-01-0800
Recently, several studies conducted by automotive industry revealed the tremendous advantages of Advanced High Strength Steels (AHSS). TRansformation Induced Plasticity (TRIP) steel is one of the typical representative of AHSS. This kind of materials exhibits high strength as well as high formability. Analyzing the crack behaviour in TRIP steels is a challenging task due to the microstructure level inhomogeneities between the different phases (ferrite, bainite, austenite, martensite) that constitute these materials. This paper aims at investigating the fracture resistance of TRIP steels. For this purpose, a micromechanical finite element model is developed based on the actual microstructure of a TRIP 800 steel. Uniaxial tensile tests on TRIP 800 sheet notched specimens were also conducted and tensile properties and R-curves (Resistance curves) were determined.
Technical Paper

Combined Synchrotron X-Ray Diffraction and Digital Image Correlation Technique for Measurement of Austenite Transformation with Strain in TRIP-Assisted Steels

2016-04-05
2016-01-0419
The strain-induced diffusionless shear transformation of retained austenite to martensite during straining of transformation induced plasticity (TRIP) assisted steels increases strain hardening and delays necking and fracture leading to exceptional ductility and strength, which are attractive for automotive applications. A novel technique that provides the retained austenite volume fraction variation with strain with improved precision is presented. Digital images of the gauge section of tensile specimens were first recorded up to selected plastic strains with a stereo digital image correlation (DIC) system. The austenite volume fraction was measured by synchrotron X-ray diffraction from small squares cut from the gage section. Strain fields in the squares were then computed by localizing the strain measurement to the corresponding region of a given square during DIC post-processing of the images recorded during tensile testing.
Technical Paper

Describing the Formability of Tailor Welded Blanks

2002-07-09
2002-01-2085
This paper presents two methods of characterizing and describing the formability of tailor welded blanks (TWB). The first method involves using miniature tensile specimens, extracted from TWB weld material, to quantify mechanical properties and material imperfection within TWB welds. This technique combines statistical methods of describing material imperfection together with conventional M-K method modeling techniques to determine safe forming limit diagrams for weld material. The second method involves the use of an extended M-K method modeling technique, which places multiple material thickness and material imperfections inside one overall model of TWB performance. These methods of describing TWB formability and their application to specific aluminum TWB populations are described.
Technical Paper

Effects of Constituent Properties on Performance Improvement of a Quenching and Partitioning Steel

2014-04-01
2014-01-0812
In this paper, a two-dimensional microstructure-based finite element modeling method is adopted to investigate the effects of material parameters of the constituent phases on the macroscopic tensile behavior of Q&P steel and to perform a computational material design approach for performance improvement. For this purpose, a model Q&P steel is first produced and various experiments are then performed to characterize the model steel. Actual microstructure-based model is generated based on the information from EBSD, SEM and nano-indentation test, and the material properties for the constituent phases in the model are determined based on the initial constituent properties from HEXRD test and the subsequent calibration of model predictions to tensile test results. The influence of various material parameters of the constituents on the macroscopic behavior is then investigated.
Technical Paper

Effects of Forming Induced Phase Transformation on Crushing Behavior of TRIP Steel

2010-04-12
2010-01-0216
In this paper, results of finite element crash simulation are presented for a TRIP steel side rail with and without considering the phase transformation during forming operations. A homogeneous phase transformation model is adapted to model the mechanical behavior of the austenite-to-martensite phase. The forming process of TRIP steels is simulated with the implementation of the material model. The distribution and volume fraction of the martensite in TRIP steels may be greatly influenced by various factors during forming process and subsequently contribute to the behavior of the formed TRIP steels during the crushing process. The results indicate that, with the forming induced phase transformation, higher energy absorption of the side rail can be achieved. The phase transformation enhances the strength of the side rail.
Technical Paper

Effects of Fusion Zone Size on Failure Modes and Performance of Advanced High Strength Steel Spot Welds

2006-04-03
2006-01-0531
This paper examines the effects of fusion zone size on failure modes, static strength and energy absorption of resistance spot welds (RSW) of advanced high strength steels (AHSS). DP800 and TRIP800 spot welds are considered. The main failure modes for spot welds are nugget pull-out and interfacial fracture. Partial interfacial fracture is also observed. The critical fusion zone sizes to ensure nugget pull-out failure mode are developed for both DP800 and TRIP800 using the limit load based analytical model and the microhardness measurements of the weld cross sections. Static weld strength tests using cross-tension samples were performed on the joint populations with controlled fusion zone sizes. The resultant peak load and energy absorption levels associated with each failure mode were studied using statistical data analysis tools. The results of this study show that the conventional weld size of can not produce nugget pull-out mode for both the DP800 and TRIP800 materials.
Technical Paper

Effects of Manufacturing Processes and In-Service mperature Variations on the Properties of TRIP Steels

2007-04-16
2007-01-0793
This paper examines some key aspects of the manufacturing process that “ Transformation Induced Plasticity” (TRIP) steels would be exposed to, and systematically evaluate how the forming and thermal histories affect final strength and ductility of the material. We evaluate the effects of in-service temperature variations, such as under hood and hot/cold cyclic conditions, to determine whether these conditions influence final strength, ductility and energy absorption characteristics of several available TRIP steel grades. As part of the manufacturing thermal environment evaluations, stamping process thermal histories are included in the studies. As part of the in-service conditions, different pre-straining levels are included. Materials from four steel suppliers are examined. The thermal/straining history versus material property relationship is established over a full range of expected thermal histories and selected loading modes.
Technical Paper

Effects of Pore Distributions on Ductility of Thin-Walled High Pressure Die-Cast Magnesium

2013-04-08
2013-01-0644
In this paper, a microstructure-based three-dimensional (3D) finite element modeling method is adopted to investigate the effects of porosity in thin-walled high pressure die-cast (HPDC) magnesium alloys on their ductility. For this purpose, the cross-sections of AM60 casting samples are first examined using optical microscope and X-ray tomography to obtain the general information on the pore distribution features. The experimentally observed pore distribution features are then used to generate a series of synthetic microstructure-based 3D finite element models with different pore volume fractions and pore distribution features. Shear and ductile damage models are adopted in the finite element analyses to induce the fracture by element removal, leading to the prediction of ductility.
Technical Paper

Failure Predictions for Aluminum Tube Hydroforming Processes

2006-04-03
2006-01-0543
Two analytical tools for failure predictions in free-expansion tube hydroforming, namely “Process Window Diagram” (PWD) and forming limit curve (FLC), are discussed in this paper. The PWD represents the incipient failure conditions of buckling, wrinkling and bursting of free-expansion tube hydroforming processes in the plane of process parameters, e.g. internal pressure versus axial compression. The PWD is a useful tool for design engineers to quickly assess part producibility and process design for tube hydroforming. An attempt is also made to draw the differences between FLCs for sheet and tube so that the appropriate FLC could be used to estimate the bursting or fracture limits in free-expansion tube hydroforming processes.
Technical Paper

Formability Investigation of Aluminum Extrusions under Hydroforming Conditions

2000-10-03
2000-01-2675
The transportation industry is finding an ever-increasing number of applications for products manufactured using the tubular hydroforming process. Most of the current hydroforming applications use steel tubes. However, with the mounting regulatory pressure to reduce vehicle emissions, aluminum alloys appear attractive as an alternative material to reduce vehicle weight. The introduction of aluminum alloys to tubular hydroforming requires knowledge of their forming limits. The current work investigates the forming limits of AA6061 in both the T4 and T6 tempers under laboratory conditions. These experimental results are compared to theoretical forming limit diagrams calculated via the M-K method. Free hydroforming results and forming limit diagrams are also compared to components produced under commercial hydroforming conditions.
Technical Paper

Formability and Fatigue of Aluminum Tailor Welded Blanks

2000-10-03
2000-01-2664
Tailor welded blanks are finding increasing application in automotive structures as a powerful method to reduce weight through material minimization. As consumer demand and regulatory pressure direct the automotive industry toward improved fuel efficiency and reduced emissions, aluminum alloys are also becoming an attractive automotive structural material with their potential ability to reduce vehicle weight. The combination of aluminum and tailor welded blanks thus appears attractive as a method to further minimize vehicle weight. Two major concerns regarding the application of aluminum tailor welded blanks are the formability and durability of the weld materials. The current work experimentally and numerically investigates aluminum tailor welded blanks ductility, and experimentally investigates their fatigue resistance.
Technical Paper

Integrated Computational Materials Engineering (ICME) Multi-Scale Model Development for Advanced High Strength Steels

2017-03-28
2017-01-0226
This paper presents development of a multi-scale material model for a 980 MPa grade transformation induced plasticity (TRIP) steel, subject to a two-step quenching and partitioning heat treatment (QP980), based on integrated computational materials engineering principles (ICME Model). The model combines micro-scale material properties defined by the crystal plasticity theory with the macro-scale mechanical properties, such as flow curves under different loading paths. For an initial microstructure the flow curves of each of the constituent phases (ferrite, austenite, martensite) are computed based on the crystal plasticity theory and the crystal orientation distribution function. Phase properties are then used as an input to a state variable model that computes macro-scale flow curves while accounting for hardening caused by austenite transformation into martensite under different straining paths.
Journal Article

Loading Path Dependence of Forming Limit Diagram of a TRIP800 Steel

2011-04-12
2011-01-0019
In this paper, the microstructure-based finite element modeling method is used in investigating the loading path dependence of formability of transformation induced plasticity (TRIP) steels. For this purpose, the effects of different loading path on the forming limit diagrams (FLD) of TRIP steels are qualitatively examined using the representative volume element (RVE) of a commercial TRIP800 steel. First, the modeling method was introduced, where a combined isotropic/kinematic hardening rule is adopted for the constituent phases in order to correctly describe the cyclic deformation behaviors of TRIP steels during the forming process with combined loading paths which may include the unloading between the two consecutive loadings. Material parameters for the constituent phases remained the same as those in the authors' previous study [ 1 ] except for some adjustments for the martensite phase due to the introduction of the new combined hardening rule.
Technical Paper

Modeling of Failure Modes Induced by Plastic Strain Localization in Dual Phase Steels

2008-04-14
2008-01-1114
Microstructure level inhomogeneities between the harder martensite phase and the softer ferrite phase render the dual phase (DP) steels more complicated failure mechanisms and associated failure modes compared to the conventionally used low alloy homogenous steels. This paper examines the failure mode DP780 steel under different loading conditions using finite element analyses on the microstructure levels. Micro-mechanics analyses based on the actual microstructures of DP steel are performed. The two-dimensional microstructure of DP steel was recorded by scanning electron microscopy (SEM). The plastic work hardening properties of the ferrite phase was determined by the synchrotron-based high-energy X-ray diffraction technique. The work hardening properties of the martensite phase were calibrated and determined based on the uniaxial tensile test results. Under different loading conditions, different failure modes are predicted in the form of plastic strain localization.
Journal Article

Predicting Stress vs. Strain Behaviors of Thin-Walled High Pressure Die Cast Magnesium Alloy with Actual Pore Distribution

2016-04-05
2016-01-0290
In this paper, a three-dimensional (3D) microstructure-based finite element modeling method (i.e., extrinsic modeling method) is developed, which can be used in examining the effects of porosity on the ductility/fracture of Mg castings. For this purpose, AM60 Mg tensile samples were generated under high-pressure die-casting in a specially-designed mold. Before the tensile test, the samples were CT-scanned to obtain the pore distributions within the samples. 3D microstructure-based finite element models were then developed based on the obtained actual pore distributions of the gauge area. The input properties for the matrix material were determined by fitting the simulation result to the experimental result of a selected sample, and then used for all the other samples’ simulation. The results show that the ductility and fracture locations predicted from simulations agree well with the experimental results.
Technical Paper

Relationship between Material Properties and Local Formability of DP980 Steels

2012-04-16
2012-01-0042
A noticeable degree of inconsistent forming behaviors has been observed for the 1st generation advanced high strength steels (AHSS) in production, and they appear to be associated with the inherent microstructural-level inhomogeneities for various AHSS. This indicates that the basic material property requirements and screening methods currently used for the mild steels and high strength low alloys (HSLA) are no longer sufficient for qualifying today's AHSS. In order to establish more relevant material acceptance criteria for AHSS, the fundamental understandings on key mechanical properties and microstructural features influencing the local formability of AHSS need to be developed. For this purpose, in this study, DP980 was selected as model steels and eight different types of DP980 sheet steels were acquired from various steel suppliers.
Technical Paper

The Formability of Friction Stir Welds in Automotive Stamping Environments

2005-04-11
2005-01-1258
Automobile body and truck cab structures are composed primarily of stampings formed from monolithic and constant gage blanks. Cost and weight penalties can arise when strength or other requirements in one small area of the part leads to the use of a material or gage that is overmatched to the needs of the rest of the stamping. Tailor Welded Blanks (TWBs) are hybrid sheet products composed of either different materials or different thickness sheets that are joined together, then subjected to a stamping operation to create a formed assembly. The strategy is employed generally to save weight and material costs in the formed assembly by placing higher strength or thicker sections only where needed. The forming or stamping process requires the joint to be severely deformed along with the parent sheets. Aluminum TWBs for automotive applications are particularly problematic because of the low formability of aluminum weld metal.
Technical Paper

The Role of Second Phase Hard Particles on Hole Stretchability of Two AA6xxx Alloys

2017-03-28
2017-01-0307
The hole stretchability of two Aluminum Alloys (AA6111 and AA6022) are studied by using a two stages integrated finite element framework where the edge geometry and edge damages from the hole piercing processes were considered in the subsequent hole expansion processes. Experimentally it has been found that AA6022 has higher hole expansion ratios than those of AA6111. This observation has been nicely captured by finite element simulations. The main cause of differences have been identified to the volume fractions of the random distributed second phase hard particles which play a critical role in determining the fracture strains of the materials.
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