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Technical Paper

Springback Prediction Improvement Using New Simulation Technologies

2009-04-20
2009-01-0981
Springback is a major concern in stamping of advanced high strength steels (AHSS). The existing computer simulation technology has difficulty predicting this phenomenon accurately even though it is well developed for formability simulations. Great efforts made in recent years to improve springback predictions have achieved noticeable progress in the computational capability and accuracy. In this work, springback simulation studies are conducted using FEA software LS-DYNA®. Various parametric sensitivity studies are carried out and key variables affecting the springback prediction accuracy are identified. Recently developed simulation technologies in LS-DYNA® are implemented including dynamic effect minimization, smooth tool contact and newly developed nonlinear isotropic/kinematic hardening material models. Case studies on lab-scale and full-scale industrial parts are provided and the predicted springback results are compared to the experimental data.
Technical Paper

Development of Shear Fracture Criterion for Dual-Phase Steel Stamping

2009-04-20
2009-01-1172
Forming Limit Diagrams (FLD) have been widely and successfully used in sheet metal stamping as a failure criterion to detect localized necking, which is the most common failure mechanism for conventional steels during forming. However, recent experience from stamping Dual-Phase steels found that, under certain circumstances such as stretching-bend over a small die radius, the sheet metal fails earlier than that predicted by the FLD based on the initiation of a localized neck. It appears that a different failure mechanism and mode are in effect, commonly referred to as “shear fracture” in the sheet metal stamping community. In this paper, experimental and numerical analysis is used to investigate the shear fracture mechanism. Numerical models are established for a stretch-bend test on DP780 steel with a wide range of bend radii for various failure modes. The occurrences of shear fracture are identified by correlating numerical simulation results with test data.
Journal Article

Fracture Modeling of AHSS in Component Crush Tests

2011-04-12
2011-01-0001
Advanced High Strength Steels (AHSS) have been implemented in the automotive industry to balance the requirements for vehicle crash safety, emissions, and fuel economy. With lower ductility compared to conventional steels, the fracture behavior of AHSS components has to be considered in vehicle crash simulations to achieve a reliable crashworthiness prediction. Without considering the fracture behavior, component fracture cannot be predicted and subsequently the crash energy absorbed by the fractured component can be over-estimated. In full vehicle simulations, failure to predict component fracture sometimes leads to less predicted intrusion. In this paper, the feasibility of using computer simulations in predicting fracture during crash deformation is studied.
Journal Article

AHSS Shear Fracture Predictions Based on a Recently Developed Fracture Criterion

2010-04-12
2010-01-0988
One of the issues in stamping of advanced high strength steels (AHSS) is the stretch bending fracture on a sharp radius (commonly referred to as shear fracture). Shear fracture typically occurs at a strain level below the conventional forming limit curve (FLC). Therefore it is difficult to predict in computer simulations using the FLC as the failure criterion. A modified Mohr-Coulomb (M-C) fracture criterion has been developed to predict shear fracture. The model parameters for several AHSS have been calibrated using various tests including the butter-fly shaped shear test. In this paper, validation simulations are conducted using the modified (M-C) fracture criterion for a dual phase (DP) 780 steel to predict fracture in the stretch forming simulator (SFS) test and the bending under tension (BUT) test. Various deformation fracture modes are analyzed, and the range of usability of the criterion is identified.
Journal Article

Development of Empirical Shear Fracture Criterion for AHSS

2010-04-12
2010-01-0977
The conventional forming limit curve (FLC) has been widely and successfully used as a failure criterion to detect localized necking in stamping. However, in stamping advanced high strength steels (AHSS), under certain circumstances such as stretching-bending over a small die radius, the sheet metal fails much earlier than predicted by the FLC. This type of failure on the die radius is commonly called “shear fracture.” In this paper, the laboratory Stretch-Forming Simulator (SFS) and the Bending under Tension (BUT) tester are used to study shear fracture occurring during both early and later stages of stamping. Results demonstrate that the occurrence of shear fracture depends on the combination of the radius-to-thickness (R/T) ratio and the tension/stretch level applied to the sheet during stretching or drawing. Based on numerous experimental results, an empirical shear fracture limit curve or criterion is obtained.
Technical Paper

An Evaluation of the Dynamic Dent Resistance of Automotive Steels

1991-02-01
910287
The effects of sheet thickness, yield strength, strain aging and prestrain on the dynamic dent resistance of sheet steel are investigated using an instrumented drop-weight test. It is found that the dynamic dent resistance is less dependent on the sheet thickness and the yield strength of the material than the static dent resistance. The dent resistance of automotive steels under dynamic loading conditions increases with prestrain in a manner similar to static denting. The relative ranking of different strength steels in the performance of dynamic denting is provided at various strain levels. An empirical relation of dynamic denting force and energy with the sheet thickness and the yield strength of the material is derived for a flat panel. Data confirm that dynamic dent resistance is improved using high strength steels including bake hardenable steels and rephosphorized steels.
Technical Paper

Prestrain Effects on Static Dent Resistance of Automotive Steels

1991-02-01
910288
In previous investigations, it has been shown that the dent resistance of an auto body panel depends upon the yield strength of the material. However, it is known that the yield strength of steel increases with prestrain due to strain hardening. Panel design and material selection based on the material properties obtained from unstrained sheet steels may lead to inaccurate prediction of the dent resistance of the formed panel. In this study, the effect of prestrain on the static dent resistance of auto body panels was investigated. Using existing empirical relationships between dent resistance and panel properties, it was found that the static dent resistance of an auto panel depends not only on the part geometry and material properties but also on the strain level in the panel. The improvement in dent resistance resulting from a material change from an AKDQ steel to a bake hardenable steel or a high strength steel was determined at different strain levels.
Technical Paper

An Evaluation of Interface Friction in Different Forming Models for Coated Steel Sheets

1992-02-01
920633
Interface friction between sheet metal and tooling in sheet metal forming is examined in different forming modes using laboratory simulative tests. Stretchability is studied by the limiting dome height test; drawability is investigated by a four inch Swift cup draw test and the coefficient of friction is measured by the draw bead simulator under bending and unbending deformation. The responses of the interface friction in six different coated and uncoated steel sheets are studied using seven different lubricants. It is found that the interface friction between sheet metal and tooling is very sensitive to the forming mode and the type of coating. For the same lubricant and coated material, two different forming modes may produce very different results in interface friction. However, overall good and bad lubricants for all forming modes can be determined for a given coated material using these three tests.
Technical Paper

Strain Rate Sensitivity of Automotive Steels

1992-02-01
920245
Strain rate sensitivity is an important material property in the formability of sheet metal. In this study, strain rate sensitivity is evaluated for several different grades of steel. Strain rate sensitivity varies from 0.01 to 0.022 for the steels tested. It was found that formable steels such as IF and AKDQ steels have both high n-value (strain hardening) and m-value (strain rate sensitivity). Positive strain rate sensitivity results in a significant increase in the yield strength and tensile strength at higher strain rates. The n-value decreases with strain rate for all of the steels. The total elongation decreases slightly with strain rate for the lower strength steels but is constant or even increases slightly with strain rate for high strength steels. For a typical AKDQ steel, the increase in yield strength can be as high as 43% for an increase in strain rate from 0.002 /s to 2.0 /s.
Technical Paper

Studies on Edge Strain Hardening Produced by Trimming Operations

2013-04-08
2013-01-1774
Advanced high strength steels (AHSS) are widely used in the automotive industry for various applications especially for structural and safety parts. One of the concerns for AHSS in stamping operations is edge fracture originating from sheared blanked edges. This type of failure cannot be predicted by computer simulations using the conventional forming limit as the failure criterion. The reason for this is that edge damage produced by the blanking operation is not incorporated into the computer models to properly simulate the material edge formability. This study presents a method to evaluate edge damage in terms of the residual stress at the sheared edge produced by the blanking operation. The method uses the level and distribution of edge strain hardening (ESH) through the material thickness as an index to characterize the edge damage caused by the shearing operation.
Technical Paper

Development of Dynamic Dent Resistance Testing Procedures

2003-03-03
2003-01-0607
The dent resistance of an automotive body panel has been used as one of key design parameters for automotive body panels. Quasi-static dent testing procedures have been well documented in North America using A/SP Standard Dent Resistance Test Procedures and numerous publications in static denting are also available. However, test procedures under dynamic denting are not very well documented and limited data exist on dynamic denting performance of automotive body panels. In this paper, dynamic dent tests are carried out using different impact velocities and different test procedures. The advantages and disadvantages of test procedures are discussed. Different ways to characterize the dynamic dent test results are investigated and discussed. Due to higher impact velocity during the dynamic dent testing, the acceleration effect must be considered in the data analysis. Experiments were carried out on a hydraulic controlled dynamic dent tester.
Technical Paper

Numerical Investigation of Effects of Frame Trigger Hole Location on Crash Behavior

2005-04-11
2005-01-0702
The front rail plays a very important role in vehicle crash. Trigger holes are commonly used to control frame crush mode due to their simple manufacturing process and flexibility for late changes in the product development phase. Therefore, a study, including CAE and testing, was conducted on a production front rail to understand the effects of trigger hole shape, size and orientation. The trigger hole location in the front rail also affects crash performance. Therefore, the effect of trigger hole location on front rail crash behavior was studied, and understanding these effects is the main objective of this study. A tapered front rail produced from 1.7 mm thick DP600 steel was used for the trigger hole location investigation. Front rails with different trigger spacing and sizes were tested using VIA sled test facility and the crash progress was simulated using a commercial code RADIOSS. The strain rate, welding and forming effects were incorporated in the front rail modeling.
Technical Paper

Experimental and Numerical Studies of Crash Trigger Sensitivity in Frontal Impact

2005-04-11
2005-01-0355
Advanced High Strength Steels (AHSS) along with innovative design and manufacturing processes are effective ways to improve crash energy management. Crash trigger hole is another technology which can been used on front rails for controlling crash buckling mode, avoiding crash mode instability and minimizing variations in crash mode due to imperfections in materials, part geometry, manufacturing, and assembly processes etc. In this study, prototyped crash columns with different trigger hole shapes, sizes and locations were physically tested in frontal crash impact tests. A corresponding crash computer simulation model was then created to perform the correlation study. The testing data, such as crash force-displacement curves and dynamic crash modes, were used to verify the FEA crash model and to study the trigger sensitivity and effects on front rail crash performance.
Technical Paper

Mass Efficient Cross-Sections Using Dual Phase Steels For Axial and Bending Crushes

2007-04-16
2007-01-0978
Because of their excellent crash energy absorption capacity, dual phase (DP) steels are gradually replacing conventional High Strength Low Alloy (HSLA) steels for critical crash components in order to meet the more stringent vehicle crash safety regulations. To achieve optimal axial and bending crush performance using DP steels for crash components designed for crash energy absorption and/or intrusion resistance applications, the cross sections need to be optimized. Correlated crush simulation models were employed for the cross-section study. The models were developed using non-linear finite element code LS-DYNA and correlated to dynamic and quasi-static axial and bending crush tests on hexagonal and octagonal cross-sections made of DP590 steel. Several design concepts were proposed, the axial and bending crush performance in DP780 and DP980 were compared, and the potential mass savings were discussed.
Technical Paper

A Failure Criterion for Stretch Bendability of Advanced High Strength Steels

2006-04-03
2006-01-0349
Studies in an Angular Stretch Bend Test (ASBT) have demonstrated that the failure location moves from the side wall to punch nose area. This occurs as the R/T ratio decreases below a certain limit and applies to most low carbon steels with the exception of Dual Phase (DP) steels. Such behavior in DP steels indicates that bending effects have a severe impact on the formability of DP materials. Therefore, the traditional criterion using the forming limit curve (FLC) is not suitable to assess the formability at punch radius areas for DP steels due in part to its uniqueness of unconventional microstructures. In this paper, a new failure criterion, ‘Bending-modified’ FLC (BFLC), is proposed by extending the traditional FLC using the “Stretch Bendability Index” (SBI) concept for the stretch bendability assessment.
Technical Paper

Prediction of Stretch Flangeability Limits of Advanced High Strength Steels using the Hole Expansion Test

2007-04-16
2007-01-1693
More and more advanced high strength steels (AHSS) such as dual phase steels and TRIP steels are implemented in automotive components due to their superior crash performance and vehicle weight reduction capabilities. Recent trends show increased applications of higher strength grades such as 780/800 MPa and 980/1000 MPa tensile strength for crash sensitive components to meet more stringent safety regulations in front crash, side impact and roll-over situations. Several issues related to AHSS stamping have been raised during implementation such as springback, stretch bending fracture with a small radius to thickness ratio, edge cracking, etc. It has been shown that the failure strains in the stretch bending fracture and edge cracking can be significantly lower than the predicted forming limits, and no failure criteria are currently available to predict these failures.
Technical Paper

Strain Hardening and Forming Limits of Automotive Steels

1995-02-01
950700
The formability window of a material depends upon the forming limit and its strain distribution ability. For two materials with the same forming limit, the formability performance is governed by their strain distribution ability. In this study, strain hardening behavior of different strength steels was investigated using a uniaxial tension test and the forming limit was studied using both Marciniak cup and dome tests. The n-value of steels varies with strain. Different strain hardening behaviors are found between mild steels and high strength steels. Strain distribution ability of steels increases with the increase in the overall n-value. The peak n-value at a low strain level enhances the strain distribution ability of the steel. It has been shown from both theoretical and experimental studies that a constant thickness strain line exists on the left side of the forming limit curve.
Technical Paper

Dynamic Dent Resistance Performance of Steels and Aluminum

1993-03-01
930786
Body panel performance properties such as denting force, oil canning/critical buckling load, initial and secondary stiffnesses under dynamic loading (drop weight test) were measured for different strength steels and two aluminum alloys using both flat and curved sheets. It was found that all these properties varied with the drop velocity. For the steels, the denting force steadily increased with the increase in drop velocity. For the aluminum alloys, the denting force increased with the drop velocity at lower velocities and decreased or remained unchanged at higher velocities. The oil canning/critical buckling load increased with the increase in drop velocity and initial and secondary stiffnesses decreased with the increase in drop velocity for both steel and aluminum. The dent resistance performance for some aluminum alloys with thicker gauge is comparable to steels dent tested at lower velocities.
Technical Paper

A Comparison of the Response of HSLA and Dual Phase Sheet Steel in Dynamic Crush

2001-10-16
2001-01-3101
Continuing pressure to reduce mass and cost of vehicles is driving the development of new high strength steel products with improved combinations of strength and formability. Galvanized, cold rolled dual phase steel products are new alternatives to conventional high strength low alloy (HSLA) steel for strength limited applications in vehicles. These steels have higher tensile strengths than HSLA products with nearly equivalent formability. This paper compares the performance of HSLA and dual phase sheet steel products in a series of drop tower tests. Samples were prepared by stamping the steel sheets into typical rail-type parts using a production-intent die process. The parts were sectioned, and subsequently fabricated into hat-shaped assemblies that were then dynamically crushed by a drop weight. The experiments were designed such that the entire energy input by the drop weight was absorbed by the samples.
Technical Paper

An Experimental Study of Springback for Dual Phase Steel and Conventional High Strength Steel

2001-10-16
2001-01-3106
An experimental study of springback was conducted for a hat channel section with varying cross sections and controlled gap between punch and die. The channel section was formed in a single step forming process with upper pressure pad. DP590 steel was compared to a group of high strength steels (HSS), e.g. HSLA270, 340 and 420. In addition, sidewall curl phenomenon was studied utilizing bending under tension test. This paper describes methodology of experiment and discusses springback related results. It also offers recommendations that can be applied to die-punch gap control or material substitution situations. The results of this investigation can be used to verify accuracy of springback predictions in finite element analysis (FEA).
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