Browse Topic: Advanced high-strength steels

Items (250)
Bake Hardening Behavior of DP, TBF, and PHS Steels with Ultimate Tensile Strengths Exceeding 1 GPa2020-01-05364/14/2020
Third generation advanced high strength steels (AHSS) have been developed combining high strength and formability, allowing for lightweighting of vehicle structural components. These AHSS components are exposed to paint baking operations ranging in time and temperature to cure the applied paint. The paint baking treatment, combined with straining induced from part forming, may lead to increased in-service component performance due to a strengthening mechanism known as bake hardening. This study aims to quantify the bake hardening behavior of select AHSS grades. Materials investigated were press hardenable steels (PHS) 1500 and 2000; transformation induced plasticity (TRIP) aided bainitic ferrite (TBF) 1000 and 1200; and dual phase (DP) 1000. The number designations of these grades refer to minimum as-received ultimate tensile strengths in MPa. Paint baking was simulated using industrially relevant times and temperatures from 15 to 60 min and 120 to 200 °C, respectively. Samples were prestrained 0, 2, or 5 pct to replicate part forming. Bake hardening values ranging from 90 to 140 MPa were observed for DP and TBF grades that were prestrained 2 pct and baked at 170 °C for 20 min. However, ductility diminished for these steels when subjected to 5 pct prestrain with uniform elongations after baking decreasing to 1 pct in some instances. PHS steels, on the other hand, showed substantial increases in yield strength without prestrain. Increases of 122 and 175 MPa were recorded for PHS 1500 and PHS 2000, respectively, following baking at 160 °C for 60 min. However, ultimate tensile strengths decreased due to reduced strain hardening, while total elongations decreased slightly.
Blesi, Brandon W.Smith, CharlesMatlock, David K.De Moor, Emmanuel
Frame Structure Durability Development Methodology for Various Design Phases2020-01-01964/14/2020
It is a challenging task to find an optimal design concept for a truck frame structure given the complexity of loading conditions, vehicle configurations, packaging and other requirements. In addition, there is a great emphasis on light weight frame design to meet stringent emission standards. This paper provides a framework for fast and efficient development of a frame structure through various design phases, keeping durability in perspective while utilizing various weight reduction techniques. In this approach frame weight and stiffness are optimized to meet strength and durability performance requirements. Fast evaluation of different frame configurations during the concept phase (I) was made possible by using DFSS (Design for Six Sigma) based system synthesis techniques. This resulted in a very efficient frame ladder concept selection process. Frame gauge optimization during the subsequent development phase (II) utilizing a newly developed damage based approach greatly reduced the number of design iterations relative to a typical stress based approach. In the light weighting phase (III) that followed, a method was established to effectively locate and optimize lightening holes using fatigue damage contours. In the final optimization phase (IV) custom Python® scripts were developed to optimize weld lengths at joints. This whole framework provides a fast and efficient way to optimize a frame structure for durability.
Thandhayuthapani, ChandraLin, BarryMao, JianghuiByali, RaghavendraNaik, Venkatesh
Development of Simplified Finite Element Model for Ultra-High-Strength Steel Resistance Spot Weld Fractures2020-01-02204/14/2020
This paper describes the development of a simplified fracture finite element (FE) model for resistance spot welds (RSW) of ultra-high-strength steel (UHSS) that can be incorporated into large-scale vehicle FE model. It is known that the RSW of UHSS generates two types of fracture modes: heat-affected zone (HAZ) and nugget zone fractures. Lap shear and peeling coupon tests using UHSS sheets found that the different RSW fracture modes occurred at different nugget diameters. To analyze this phenomenon, detailed simulated coupon tests were carried out using solid hexahedral elements. The analytical results revealed that RSW fractures are defined by both the application of plastic strain on the elements and the stress triaxiality state of the elements. A detailed model incorporating a new fracture criteria model recreated the different UHSS RSW fracture modes and achieved a close correlation with the coupon test results. Based on these results, a new simplified fracture model that can be incorporated into large-scale vehicle FE model was developed. This new model uses solid hexahedral elements to simulate the nugget zone and shell elements for the HAZ. Simulated coupon tests using the new model were also capable of recreating the different UHSS RSW fracture modes and correlated closely with the coupon test results.
Kawahara, KoshoKoga, MasatakaArimoto, ShinichiNomura, NorihisaNishimura, Ritsu
Metallurgical and Wear Behaviour of Stellite 6 Reinforced Stainless Steel 316 Joints by Nd-YAG Laser Welding Process2019-28-014710/11/2019
Laser welding process is a most effective and predominant method for joining of steel alloys when compared with other welding processes in practice due to their precise control of laser source across the bonding zone where it is crucial to control in other joining processes. In common the austenitic steels differ from ferritic based on two factors, thermal conductivity and expansion. Here, the selected Nd-YAG laser setup for joining of similar base material stainless steel 316 which is reinforced with and without stellite 6 powders. The experimental investigations (metallurgical survey and wear characteristics) were performed on all the samples. The powders were reinforced in the material directly by performing a drill across the bond line instead of a normal coating process which is in practice, later the powders were stuffed through the holes. Totally four samples were processed by varying the process parameters such as laser power (W), laser frequency (Hz) and keeping the time (s), feeding rate (mm/s) as constant. During the joining process the powders will get solidified with the molten steel alloy. Metallurgical study has been carried away across the weld zones for clear understanding of material behaviour and the survival of stellite powder particles. The wear characteristics were performed on the samples by keeping time, load and speed as constant, but the sliding distance is varied as (40, 50, 60) mm. The results revealed that a good agreement of stellite particles across the bonding region. It is also predicted from the experiments that joints made with addition of stellite powders instead of coating were shown a significant improvement in the grain boundary refinement and also a good wear resistance was observed which makes the alloy much more reliable than the existing one.
Arulvizhi, Varun KumarAlandur Somasundaram, SelvakumarMurugan, BalasrinivasanNatarajan, RavikumarKalam, Abdur Rahman
Lightweight Wheel Bearing with Dissimilar Materials for Vehicle2019-01-21349/15/2019
Limited fossil fuel resources, air pollution, and global warming all drive strengthening of fuel economy and vehicle emission standards globally. Much R&D continues to be dedicated to improve fuel efficiency of automobiles and to reduce exhaust gasses. These include improvement of engine/driveline performance for higher efficiency, development of alternative energy, and minimization of air resistance through aerodynamic design optimization. OEM weight reduction-focused research has extended into chassis components (steering knuckle, brakes, control arms, etc.) in sequence from body-in-white(BIW). Wheel bearings, one of the core components of a driveline and part of a vehicle’s unsprung mass, are also being required to reduce weight. Conventionally, wheel bearings have achieved “lightweighting” primarily through design optimization methods. They have been highly optimized today using steel based materials. Opportunities for further mass optimization are increasingly limited and so the focus of this study is integration of lighter-materials into steel bearing components for weight savings. Both aluminum and CFRP were considered in the study for partial integration into the steel hub flange which interfaces directly with the wheel. The application of lightweight materials was targeted on the specific area of the hub which is relatively less affected by impact loads. Hot forging and compressive molding were applied to induce denser bonding of the dissimilar materials. The slope angle design having an inverse draft at the combined boundary was applied to reinforce physical bonding and to enhance joint integrity. In addition, a Zinc-based coating was partially applied on the surface of the combined boundary to protect against galvanic corrosion between dissimilar materials. Bearings containing the new composite hubs achieved a 20% weight reduction compared to conventional wheel bearings made of full steel.
Lee, InhaLee, SeonhoShim, HeechanLee, JaeheeHong, Sung-TaePark, Jungyang
A Novel Method to Nondestructively Measure the Shear Edge Properties for Edge Cracking Evaluation with Advanced High Strength Steels2019-01-10904/2/2019
Nondestructive Evaluation (NDE) techniques are widely used in the manufacturing industry to control the quality of materials or final products. In the automotive industry, eddy current (EC) testing is one of the most extensively used NDE techniques for automatic in-line inspection of ferrous materials such as advanced high strength steels (AHSS). In addition, shearing is a very common forming operation in the automotive industry. With the increase of shearing clearance, the sheared-edge experiences significant work-hardening that normally decreases the formability of the sheared edge. In this paper, a novel, real-time monitoring NDE method based on the EC sensor was developed to characterize variations in shear edge quality for a DP980 steel. The developed NDE method was applied to scan the edges sheared at various clearances between 5% and 25% of the material thickness. The signal received was correlated with pre-straining introduced during the shearing process at various clearances. Microhardness measurements were taken to compare the trends obtained from the NDE tool with the hardness values. To evaluate the edge formability, half-specimen dome testing (HSDT) was conducted for the edges sheared at various clearances. A digital image correlation (DIC) system was used to record deformation during the HSDT. The failure strain of sheared edges was correlated with the NDE measurements for each clearance to assess the application of an NDE measurement in determining edge quality. The developed NDE method has great potential for significant financial and technical impact on blanking and piercing operations for blank processors or stampers, particularly for AHSS.
Pathak, NikkyGu, JiahuiKim, Hyunok
Research on the different Behavior of Edge Cracking Limit by Adopting the Laser Cutting Method2019-01-12644/2/2019
The edge fracture occurs more frequently during the forming procedure by using the material with higher strength. To avoid the edge fracture that happens during the manufacturing, the edge cracking limit at different pre-strain level needs to be determined. The edge of the part under forming is conventionally manufactured by mechanical cutting, and the edge cracking limit under this circumstance is already heavily studied. In recent years, laser cutting is more applied in the automotive industry to cutting the edge due to the following advantages over mechanical cutting: easier work holding, higher precision, no wearing, smaller heat-affected zone, etc. The change cutting method could lead to a different behavior to the edge cracking limit at different pre-strain level. In this paper, the edge cracking limits of sets of pre-strained coupons with different pre-strain levels are tested. Half of them is cut by the conventional punch method, and the other half uses laser cutting. These cut pre-strained half dog-bone coupons are loaded under a uniaxial tension to acquire the edge cracking limit. The thinning strain is measured to calculate the edge cracking limit using a 3D digital image correlation (3D-DIC) system. Also, the pre-strain level of each coupon is precisely determined using the 3D-DIC system. The detailed experimental setup, procedure, and results are described in this article. The different behavior of edge cracking limit at different pre-strain level by adopting the laser cutting method is discussed and concluded.
Li, JunruiXu, WanZhang, BoyangYang, Lianxiang
Effects of Nitrided and Chrome Plated Die Surface Roughness on Friction in Bending Under Tension2019-01-10934/2/2019
Different die surface polish conditions result in a noticeable effect on material flow in stamping, which can lead to splitting, wrinkling, or other surface stretching issues associated with different friction conditions. These occurrences are not only limited to the non-coated dies, but also nitrided and chrome plated dies. To ensure quality control of the stamped parts, the die conditions corresponding to different polishing procedures need to be developed based on measurable parameters such as surface roughness (Ra). The intent of this study is to investigate the effects of nitrided and chrome plated die surface roughness on friction. The Bending-Under-Tension (BUT) test was conducted to simulate the stamping process due to the test’s versatility and flexibility in changing test parameters. The test involves moving sheet metal across a 3/8-inch diameter pin, which substitutes for a die surface. The pin can be modified by material, heat treatment, coating, and surface roughness. Pins were made of D6510 steel and heat treated to 54-58 HRC. Two different die polishing procedures were developed. The first is the progressive polishing procedure, which is applied to the pins after heat treatment but before coating. The second polishing procedure added a second stage single polishing procedure after final die surface treatments were completed. Half of the pins were nitrided and the other half were chrome plated. The BUT test used hot-dipped galvanized (HDGI) bake-hardenable 210 MPa tensile strength steel (BH210). Results indicate the 600 grit# polish condition generates the lowest average friction coefficient among all polish conditions regardless of the die surface treatments. Additionally, 3.4 μin and 5 μin is the optimal die surface roughness (Ra) for the chrome plated dies and nitrided dies, respectively, in forming HDGI BH210 steel.
Shih, Hua-Chu
Strain Rate Effect on Martensitic Transformation in a TRIP Steel Containing Carbide-Free Bainite2019-01-05214/2/2019
Adiabatic heating during plastic straining can slow the diffusionless shear transformation of austenite to martensite in steels that exhibit transformation induced plasticity (TRIP). However, the extent to which the transformation is affected over a strain rate range of relevance to automotive stamping and vehicle impact events is unclear for most third-generation advanced high strength TRIP steels. In this study, an 1180MPa minimum tensile strength TRIP steel with carbide-free bainite is evaluated by measuring the variation of retained austenite volume fraction (RAVF) in fractured tensile specimens with position and strain. This requires a combination of servo-hydraulic load frame instrumented with high speed stereo digital image correlation for measurement of strains and ex-situ synchrotron x-ray diffraction for determination of RAVF in fractured tensile specimens. Specifically, the potentially competing effects of strain rate on austenite transformation to martensite were investigated to determine which predominate at nominal strain rates of 0.5 s-1, 5 s-1, 50 s-1 and 500 s-1. A corresponding decrease in austenite volume fraction at a fixed true strain with strain rate suggests that austenite transformation to martensite with strain is accelerated with increased strain rate despite potential energetic inhibition of the transformation due to adiabatic heating. Increased transformation rate to martensite results in increased work hardening rates, strengths, and elongations with strain rate increases to 500 s-1. Observations are discussed in the context of contributing mechanisms both favoring and inhibiting the strain-assisted transformation of austenite to martensite.
Enloe, CharlesSavic, VesnaPoling, WhitneyHector, LouisAlturk, Rakan
Estimation of Cutting Parameters in Two-Stage Piercing to Reduce Edge Strain Hardening2019-01-10924/2/2019
Edge fracture is a common problem when forming advanced high strength steels (AHSS). A particular case of edge fracture occurs during a collar forming/hole extrusion process, which is widely used in the sheet metal forming industry. This study attempts to relate the edge stretchability in collar forming to the strain hardening along the pierced edge; thus, Finite Element (FE) simulations can be used to reduce the number of experiments required to improve cutting settings for a given material and thickness. Using a complex-phase steel, CP-W 800 with thickness of 4.0 mm, a single-stage piercing operation is compared with a two-stage piercing operation, so called shaving, in terms of strains along the pierced edge, calculated by FE simulation. Results indicated that strains were reduced along the pierced edge by shaving. These results are correlated with experimental observations made at the Technical University of Munich (TUM) where better hole expansion ratios (HERs) were obtained using shaving as opposed to single-stage piercing. Moreover, in this study, the combination of cutting parameters that would generate the least edge strain hardening is evaluated based on simulation results. This approach could reduce the need for hardness measurements along the pierced edge as well as the number of experiments required to improve the shaving process.
Diaz-Infante, DavidNarayanan, AdvaithAltan, Taylan
Prediction of Ductile Fracture Propagation of High Strength Steels in Automotive Structures2019-01-10974/2/2019
Initiation and propagation of ductile fractures in crashed automotive components made from high strength steels are investigated in order to understand the mechanism of fracture propagation. Fracture of these components is often prone to occur at the sheet edge in a strain concentration zone under crash deformation. The fracture then extends intricately to the inside of the structure under the influence of the local stress and strain field. In this study, a simple tensile test and a 3-point bending test of high strength steels with tensile strengths of 590 MPa and 1180 MPa are carried out. In the tensile test, a coupon having a hole and a notch is deformed in a uniaxial condition. The effect of the notch type on the strain concentration and fracture behavior are investigated by using a digital imaging strain measurement system. For the 3-point bending test, hat-shaped parts having various types of notches and a hole located near the notch are examined to simulate ductile fracture in a crashed automotive part. The experimental results indicate that the shape of the notch and material properties influence the speed of fracture propagation. The location of the hole also influences the direction of propagation of the ductile fracture. Finite Element Method (FEM) is applied to predict the fracture propagation in 3-point bending deformation. In the numerical calculation, general FEM method and extended finite element method (nonlocal XFEM) are used to investigate the possibilities of the nonlocal XFEM in the crash simulation. The numerical results for both the 590 MPa and 1180 MPa steels, are validated in the force-stroke curves and ductile fracture propagation behavior by comparison with the experimental results. The difference of the general FEM and nonlocal XFEM are discussed for the prediction of ductile fracture propagation of high strength steels.
Sato, KentaroFutatsuka, TakayukiOkada, HideyukiEgawa, YasuhisaFukagawa, KenichiroAmaishi, Toshiro
Adhesive Failure Prediction in Crash Simulations2019-26-02971/9/2019
Structural adhesive is a good alternative to provide required strength at joinery of similar and dissimilar materials. Adhesive joinery plays a critical role to maintain structural integrity during vehicle crash scenario. Robust adhesive failure definitions are critical for accurate predictions of structural performance in crash Computer Aided Engineering (CAE) simulations. In this paper, structural adhesive material characterization challenges like comprehensive In-house testing and CAE correlation aspects are discussed. Considering the crash loading complexity, test plan is devised for identification of strength and failure characteristics at 0°, 45°, 75°, 90°, and Peel loading conditions. Coupon level test samples were prepared with high temperature curing of structural adhesive along with metal panels. Test fixtures were prepared to carryout testing using Instron VHS machine under quasi-static and dynamic loading. Various material models available in LSDYNA are studied and MAT169 (*MAT_ARUP_ADHESIVE) material model is selected for adhesive material characterization. Finite Element (FE) models inline to the test conditions were prepared. Sensitivity studies carried out to understand the significance of critical parameters in material model and modelling practices. Test and CAE correlation for all the test configurations are established considering quasi-static and dynamic scenarios. Methodology developed for adhesive material modelling and simulations are carried out at BIW and vehicle level using characterized adhesive material model.
Rao, Hrishikesh S.Tiwari, SourabhKoralla, SivaprasadGhosh, DebabrataDey, Susanta
Improving Hole Expansion Ratio by Parameter Adjustment in Abrasive Water Jet Operations for DP80005-11-03-00239/17/2018
The use of Abrasive Water Jet (AWJ) cutting technology can improve the edge stretchability in sheet metal forming. The advances in technology have allowed significant increases in working speeds and pressures, reducing the AWJ operation cost. The main objective of this work was to determine the effect of selected AWJ cutting parameters on the Hole Expansion Ratio (HER) for a DP800 (Dual-Phase) Advanced High-Strength Steel (AHSS) with s0 = 1.2 mm by using a fractional factorial design of experiments for the Hole Expansion Tests (HET). Additionally, the surface roughness and residual stresses were measured on the holes looking for a possible relation between them and the measured HER. A deep drawing quality steel DC06 with s0 = 1.0 mm was used for reference. The fracture occurrence was captured by high-speed cameras and by Acoustic Emissions (AE) in order to compare both methods. Results indicated that using, regardless of the material, a small standoff distance, high water pressure, and slow traverse speed and cutting the sample underwater will delay the fracture in a hole expansion operation. Furthermore, the AE have proven to be adequate to measure cracks when optical methods are not feasible. In conclusion, based on the impact of the aforementioned parameters, it is possible to select, appropriately, the AWJ operation parameters to achieve the edge stretchability required for each forming process.
Yilkiran, DenizWölki, KaiHübner, SvenDiaz-Infante, DavidAltan, Taylan
Lightweight Automobiles ALLIANCE Project: First Results of Environmental and Economic Assessment from a Life-Cycle Perspective2018-37-00275/30/2018
In the last years the research activities in the field of lightweighting have been advancing rapidly. The introduction of innovative materials and manufacturing technologies has allowed significant weight reduction. Despite this, novel technologies and materials have not reached a wide distribution. The reasons for this are mainly high production costs and environmental impacts of manufacturing that do not compensate benefits during operation. The paper deals with the AffordabLe LIghtweight Automobiles AlliaNCE (ALLIANCE) project which has the goal of developing novel advanced automotive materials and production technologies, aiming at an average 25% weight reduction over 100 k units/year, at costs of <3 €/kg. The article is focussed on Work Package 1 (WP1) of the project, aimed at estimating the full attributes of innovative design solutions by assessing costs, energy demand and GWP over the entire vehicle Life Cycle (LC). After an overview on project consortium and structure, the paper reviews the design strategies considered as well as the expected targets. Finally, the preliminary results of WP1 regarding the environmental and economic assessment are presented. Outcomes suggest that to reach the target in terms of GWP and cost reduction by means of innovative lightweight materials it is fundamental to bear in mind the potential trade-off between production and uses stages impacts/costs especially in the electric vehicle configuration. Moreover, the vehicle lifetime and the potential secondary effects would play a relevant role in the evaluation of lightweight benefits.
Delogu, MassimoDel Pero, FrancescoZanchi, LauraIerides, MarcosFernandez, VioletaSeidel, KristianThirunavukkarasu, DineshBein, Thilo
Test of Inclined Double Beads on Aluminum Sheets2018-01-12214/3/2018
Draw beads are widely used in the binder of a draw die for regulating the restraining force and control the draw-in of a metal blank. Different sheet materials and local panel geometry request different local draw bead configurations. Even the majority of draw bead is single draw bead, the alternative double draw bead does have its advantages, such as less bending damage may be brought to the sheet material and more bead geometry features available to work on. In this paper, to measure the pulling force when a piece of sheet metal passing through a draw bead on an inclined binder, the AA5XXX and AA6XXX materials were tested and its strain were measured with a digital image correlation (DIC) system. Five different types of double bead configurations were tested. The beads are installed in a Stretch-Bend-Draw-System (SBDS) test device. The clearance between a male and a female bead is 10% thicker than the sheet material. A tensile machine was used to record the pulling force. The comparison of pulling force and strain are reported in this paper. These experiment results can be used to validate the results of FEA simulation. It was found that the pulling force of a double bead is more sensitive than the single bead. Details of the sensitivity were illustrated and discussed in the paper.
Zhang, BoyangXu, WanYang, GuobiaoLi, JunruiYang, LianxiangZhou, DajunDu, ChangqingLi, KaipingRawya, Bazzi
Replacing Press Hardenable Steel with 980 MPa Generation 3 Steel for Automotive Pillars2018-01-01174/3/2018
Press hardenable ultra high strength steel (UHSS) is commonly used for automotive components to meet crash requirements with minimal mass addition to the vehicle. Press hardenable steel (PHS) is capable of forming complex geometries with deep sections since the forming takes place at elevated temperatures up to 900 degrees Celsius (in the Austenitic phase). This forming process is known as hot-stamping. The most commonly used PHS grade is often referred to as PHS1500. After hot-stamping, it is typically required to have a yield strength greater than 950 MPa and a tensile strength greater than 1300 MPa. Most automotive design and material engineers are familiar with PHS, the hot-stamping process, and their capabilities. What is less known is the capability of 3rd Generation advanced high strength steels (AHSS) which are cold stamped, also capable of forming complex geometry, and are now in the process of, or have recently completed, qualification at most automotive manufacturers. This paper will show that U. S. Steel’s Generation 3 steel with 980 MPa minimum tensile strength (980GEN3) has comparable crash performance to PHS1500 when used for automotive body-in-white (BIW) Pillars in a sedan. This paper will also show that these 980GEN3 BIW Pillars can achieve the same complex geometry as PHS1500, that the springback can be predicted and controlled, and that prototype parts can be assembled into a vehicle build and physically crashed without issues. Furthermore, correlation between cold stamped 980GEN3 BIW Pillars and forming simulation results will also be discussed.
McKune, PaulKhutorsky, AlexButala, Kapil
A Tailor Welded Blanks Design of Automotive Front Rails by ESL Optimization for Crash Safety and Lightweighting2018-01-01204/3/2018
Utilizing the tailor welded blanks (TWBs) design along with the latest AHSS grades for the front rails on a sedan was studied to reduce the weight of the vehicle and improve the crash safety performance. To find the most efficient material usage, the front rail parts were tailored into multiple blanks with varying thickness. A structural thickness optimization study of the tailored front rails was conducted for IIHS moderate overlap frontal crash, and the tailored blank thickness was set as design variable. The equivalent static loads (ESL) method was adopted for the thickness optimization, which allows many design variables to be optimized simultaneously. The torsion and bending stiffness of the sedan body in prime were set as design constraints, and would not be compromised. The optimal thickness configurations of the TWB designs by ESL optimization suggest that the weight of the frontal rails can be reduced by more than 30% while still maintaining the crash safety performance. These TWB designs were validated by US-NCAP full frontal impact and show similar performance with baseline. A 3rd gen AHSS, NEXMET™1000, was selected on four parts of the front rails to replace the baseline HSLA350. The optimal tailored frontal rail design using NEXMET™1000 grade was obtained through ESL thickness optimization and validated by US-NCAP full frontal impact. Compared with HSLA350, the NEXMET™1000 grade offers better crash safety performance with more weight reduction potential. An optimal thickness coefficient is proposed in this study to evaluate the material efficiency of the tailored blanks and the amount of thickness changes required for each blank to reach the most efficient material usage. The optimal TWB thickness configurations for HSLA350 and NEXMET™1000 grades through ESL were evaluated using this optimal thickness coefficient. The critical locations on front rails for crash safety were identified and the amount of thickness changes needed characterized. The tailor welded blanks technology can be implemented in the front rail design to reduce weight and improve crash safety. This optimal thickness coefficient can guide the automotive design for lightweighting.
Liang, JianyongPowers, JonathanStevens, Scott
Effects of AHSS Sheared Edge Conditions on Crash Energy Absorption in Component Bend Test2018-01-01134/3/2018
Edge fracture of advanced high strength steels (AHSS) can occur in both the stamping process and the crash event. Fracture due to poor sheared edge conditions in the stamping process was reduced with a recently developed optimal shearing process for AHSS. Currently, the improvement in the energy absorption due to the improved edge condition during crashes performed under different loading conditions had not been closely verified. The purpose of this study is to design and build a miniature component of AHSS and a three-point bending test for investigating the influence of various conditions of the sheared edge on the energy absorption in crashes. AHSS including DP600, TRIP780, DP980 and DP1180 were selected in the study. A small channel component was developed and fabricated using DP980 to simulate key features of the B-pillar. The exposed non-constrained, as-sheared edge was subject to stretch bending forces in three-dimensional space during the three-point bending test. Two bottom rollers equipped in the three-point bending test allowed the test specimen to bend freely without generating extra friction force between the test material and tooling. Previously developed shearing parameters were fine-tuned for generating the test specimen on the flexible shearing machine and then compared to the specimens made by water jet cutting and laser cutting. A life cycle analysis was also conducted by FEA to evaluate the new design of the shear blade. The experiment results indicate the improvement of the sheared edge conditions can also increase the material energy absorption in crashes. The laser cut sample has the best energy absorption capability, while the conventional cutting edge shows the worst. In addition to the three-point bending test, a wedge bend test was conducted on the flat sample and reached a similar energy absorption trend as in the component bend test. The wedge bend test on flat sample is recommended for baseline comparison in material bendability and crash energy absorption capability among different advanced high strength steels.
Shih, Hua-ChuChen, Guofei
A Material Efficiency Ratio to Evaluate the Methods for Improving the Torsional Rigidity of a Pickup Chassis Frame2018-01-10244/3/2018
While offering improved crash worthiness and significant lightweighting opportunities, the increased use of advanced high strength steels (AHSS) may compromise the stiffness and NVH performance of vehicles due to reduced part thickness. Different methods to improve the torsional rigidity were studied on a pickup chassis frame. These methods include adding bulkhead pairs as reinforcement, increasing the thicknes of frame parts, and enlarging the closed sections on the rails. Structural optimization was conducted for each stiffness improvement method and the minimal mass increase required to reach the improvement targets was obtained. A material efficiency ratio μ is proposed in this research and used as a criterion to evaluate the efficiency of a mass increase to improve the structural stiffness and NVH characteristics of vehicles. Based on this parameter, the methods to improve the torsional rigidity of the pickup frame in all design spaces were evaluated. The adding bulkhead pair option offers the highest material efficiency ratio, but the potential for improving the torsional rigidity is limited. Conversely, increasing the part thickness and enlarging the closed sections on rails give higher torsion improvement potential, while the material efficiency ratio is much lower. Structural optimization combining adding bulkhead pairs and enlarging the rail sections was conducted to fully utilize the advantages of both rigidity improvement methods. And the results show higher material efficiency and more potential for rigidity improvement than each individual method. This material efficiency ratio proposed is valuable in vehicle development to evaluate the efficiency of a design change for lightweighting.
Liang, JianyongPowers, JonathanStevens, Scott
The Influence of the Through-Thickness Strain Gradients on the Fracture Characterization of Advanced High-Strength Steels2018-01-06274/3/2018
The development and calibration of stress state-dependent failure criteria for advanced high-strength steel (AHSS) and aluminum alloys requires characterization under proportional loading conditions. Traditional tests to construct a forming limit diagram (FLD), such as Marciniak or Nakazima tests, are based upon identifying the onset of strain localization or a tensile instability (neck). However, the onset of localization is strongly dependent on the through-thickness strain gradient that can delay or suppress the formation of a tensile instability so that cracking may occur before localization. As a result, the material fracture limit becomes the effective forming limit in deformation modes with severe through-thickness strain gradients, and this is not considered in the traditional FLD. In this study, a novel bending test apparatus was developed based upon the VDA 238-100 specification to characterize fracture in plane strain bending using digital image correlation (DIC). Three punches with tip radii of 0.2, 0.4, and 1.0 mm were used to demonstrate the influence of the bend severity on the fracture limit in plane strain tension. Moreover, the influence of the through-thickness strain gradient on equi-biaxial stretching conditions was also investigated using hemispherical punches with radii of 5, 10, 25, and 50 mm. It was observed that using smaller radius, Nakazima punches can help to mitigate necking and provide a near-ideal biaxial strain path until fracture.
Cheong, KennethButcher, CliffordDykeman, James
Forming Limit Curves of Advanced High Strength Steels: Experimental Determination and Empirical Prediction2018-01-08044/3/2018
For the past decades, the adoption of empirical equations in the forming limit curve (FLC) calculation for conventional steels has greatly simplified the forming severity assessment in both forming simulations and on the stamping shop floor. Keeler’s equation based on the n-value and sheet thickness is the most popular one used in North America. However, challenges have been encountered on the validity of the equation for advanced high strength steels (AHSS) since Keeler’s equation was developed based on the FLC data mostly from mild steels and conventional high strength steels. In this study, forming limits of various AHSS grades under different strain conditions are experimentally determined using digital image correlation technique. Both Marciniak cup and Nakazima dome tests are exercised to demonstrate the differences in the resultant forming limits determined with different test methods. The effects of a few material-related aspects on the FLC of AHSS are elaborated, including (1) microstructures (C-Mn, dual phase and retained austenite containing steels), (2) material thickness (0.8 to 2.3 mm), and (3) coating process. Pertaining to these effects, the effectiveness of Keeler’s equation in the FLC estimation for various AHSS grades is evaluated with experimental FLC data. It has been illustrated that with proper modifications and compensations, the conventional empirical equations remain as useful references to approximate the FLC of AHSS. General recommendations and guidelines are provided regarding the practical applications of FLCs of AHSS grades. In the meantime, further development of FLC models is initiated and discussed based on statistical analysis to achieve improved reliability.
Huang, LuShi, Ming
Advanced high-strength steels (AHSS), due to their significantly higher strength than the conventional high-strength steels, are increasingly used in the automotive industry to meet future safety and fuel economy requirements. Unlike conventional steels, the properties of AHSS can vary significantly due to the different steelmaking processes and their fracture behaviors should be characterized. In crash analysis, a fracture model is often integrated in the simulations to predict fracture during crash events. In this article, crash simulations including a fracture criterion are conducted for a third-generation AHSS, that is, 980GEN3. A generalized incremental stress state dependent damage model (GISSMO) in LS-DYNA is employed to evaluate the fracture predictability in the crash simulations. The fracture strains of the 980GEN3 steel are experimentally characterized under various deformation modes encompassing shear, uniaxial tension, bending, plane strain, and balanced biaxial stretch conditions. The GISSMO parameters are determined and calibrated using fracture tests at these deformation modes for the 980GEN3 steel. Validation simulations are performed on three-point bending component crash tests and good correlations are achieved. The validated GISSMO card for the 980GEN3 steel can be used in crash simulations of automotive structures.
Chen, XiaomingChen, GuofeiHuang, Lu
Application of Nano-Indentation Test in Estimating Constituent Phase Properties for Microstructure-Based Modeling of Multiphase Steels2017-01-03723/28/2017
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. The results show that the estimated phase properties from the inverse method are similar to those from HEXRD, and that the Ultimate Tensile Strength (UTS) and Uniform Elongation (UE) predicted from the FE model based on the estimated phase properties are also similar to those of tensile experiment of the Q&P steel. Based on the results in this study, the nanoindentation-based inverse method appears to be a viable way in determining the phase properties of complex multiphase steels with submicrometer grain sizes.
Cheng, GuangChoi, Kyoo SilHu, XiaohuaSun, Xin
Ductile Fracture from Spot Weld and Flange Edge in Advanced High Strength Steels2017-01-03653/28/2017
A simple testing method is proposed in order to investigate ductile fracture in crashed automotive components made from advanced high strength steels. This type of fracture is prone to occur at spot-welded joints and flange edges. It is well known that the heat affected zone (HAZ) is a weak point in high strength steel due to the formation of annealed material around the spot-welded nugget, and the flange edge also has low ductility due to the damage caused by shearing. The proposed method is designed to simulate a ductile fracture which initiates from a spot-welded portion or a sheared edge in automotive components which are deformed in a crash event. Automotive steel sheets with a wide range of tensile strengths from 590MPa to 1470MPa are examined in order to investigate the effect of material strength on fracture behavior. The effects of material cutting methods, namely, machining and shearing, are also investigated. A digital imaging strain analysis system is applied into the fracture test to analyze the strain distribution at the spot weld and the flange edge. The experimental results indicate the criterion for fracture in spot-welded specimens. Based on the experimental results, a FE simulation is carried out considering the change in material properties of the heat-affected material around spot weld. The FE simulation showed good agreement with the experimental results in the force-stroke curve and the strain history around the spot weld. Further study is expected to improve the accuracy of the fracture prediction based on the experimental results obtained by the proposed simple experimental method.
Sato, KentaroFutatsuka, TakayukiHiramoto, JiroNagasaka, KeiAkita, AkiraKashiyama, Takeshi
Simplified Approach for Optimizing Lightening Holes in Truck Frames for Durability Performance2017-01-13453/28/2017
During development of new vehicles, CAE driven optimizations are helpful in achieving the optimal designs. In the early phase of vehicle development there is an opportunity to explore shape changes, gage reduction or alternative materials as enablers to reduce weight. However, in later phases of vehicle development the window of opportunity closes on most of the enablers discussed above. The paper discusses a simplified methodology for reducing the weight in design cycle for truck frames using parametric Design of Experiments (DOE). In body-on-frame vehicles, reducing the weight of the frame in the design cycle without down gaging involves introducing lightening holes or cutouts while still maintaining the fatigue life. It is also known that the lightening holes might cause stress risers and be detrimental to the fatigue life of the component. Thus the ability to identify cutout locations while maintaining the durability performance becomes very critical. This paper describes a method of effectively locating these lightening holes on the truck frame, thereby reducing the weight of the vehicle while preserving the durability performance. The process to incorporate these lightening holes is a multi-step approach beginning with a stress envelope creation. The load paths for each component are identified based on the stress envelops generated in the fatigue code using a complete set of proving ground loading events. A subsequent step includes tuning those lightening holes to meet the durability, strength and stiffness requirements via the automated process of resizing the lightening holes to their optimal sizes. The final verification is carried out with the regular analysis procedure to verify the lightening holes effect on the durability performance of the structure.
bhat, RamachandraSharma, NitinRivard, CliffordThomson, Kevin
A Method of Evaluating the Joint Effectiveness on Contribution to Global Stiffness and NVH Performance of Vehicles2017-01-03763/28/2017
While Advanced High Strength Steels (AHSS) and the next generation AHSS grades offer improved crash safety and reduced weight for vehicles, the global stiffness and NVH performance are often compromised due to reduced material thickness. This paper discusses an advanced method of evaluating the joint effectiveness on contribution to global stiffness and NVH performance of vehicles. A stiffness contribution ratio is proposed initiatively in this research, which evaluates the current contribution of the joints to the global stiffness and NVH performance of vehicles. Another parameter, joint effectiveness factor, has been used to study the potential of each joint on enhancing the global stiffness. The critical joints to enhance the vehicle stiffness and NVH performance can be identified based on above two parameters, and design changes be made to those critical joints to improve the vehicle performance. A Ford sedan BIW model was adopted in this study to demonstrate this method and torsion stiffness was chosen as the enhancement target. Based on this method, the global torsion stiffness, as well as bending stiffness and vibration modes, can be improved by enhancing the critical joints identified in this joint effectiveness study. Mass reduction target can also be achieved by degrading the joints that are found not efficient to global vehicle performance.
Liang, JianyongPowers, JonathanStevens, ScottShahidi, Behrooz
Impact of Powertrain Type on Potential Life Cycle Greenhouse Gas Emission Reductions from a Real World Lightweight Glider2017-01-12743/28/2017
This study investigates the life cycle greenhouse gas (GHG) emissions of a set of vehicles using two real-world gliders (vehicles without powertrains or batteries); a steel-intensive 2013 Ford Fusion glider and a multi material lightweight vehicle (MMLV) glider that utilizes significantly more aluminum and carbon fiber. These gliders are used to develop lightweight and conventional models of internal combustion engine vehicles (ICV), hybrid electric vehicles (HEV), and battery electric vehicles (BEV). Our results show that the MMLV glider can reduce life cycle GHG emissions despite its use of lightweight materials, which can be carbon intensive to produce, because the glider enables a decrease in fuel (production and use) cycle emissions. However, the fuel savings, and thus life cycle GHG emission reductions, differ substantially depending on powertrain type. Compared to ICVs, the high efficiency of HEVs decreases the potential fuel savings. BEVs are more efficient than HEVs but require heavy batteries to provide an acceptable driving range. A lightweight glider can allow a smaller battery to be used without sacrificing driving range. Battery downsizing is a secondary source of mass reduction that further decreases fuel use. A comparison of our results with those of other studies reveals inconsistencies and lack of powertrain-specific assumptions in the literature, which can mischaracterize the GHG emissions associated with producing lightweight vehicles, and those from potential fuel savings.
Luk, Jason M.Kim, Hyung ChulDe Kleine, RobertWallington, Timothy J.MacLean, Heather L.
Influence of Hardness Variation and Defects on Fatigue Behavior of Automotive Steels2017-01-03453/28/2017
Fatigue behavior of two types of automotive steel, quenched and tempered SUJ2 and carburized SCM820PRH, which are applied as powertrain parts are studied. These two types of steel are different in their hardness distribution from surface to core. The hardness of quenched and tempered SUJ2 is homogenous, in contrast to that of carburized SCM820PRH (SCM) which decreases from surface to core. These steels are investigated in terms of their monotonic tensile properties and fatigue behavior. A number of predictive methods were used to describe the fatigue behavior of these steels. A simple predictive method is based on approximation of S-N curve from ultimate tensile strength. The well-known Murakami’s defect area method was also applied for the prediction of the high cycle fatigue strength. It was found that the classic estimation of fatigue behavior which considers 700 MPa as fatigue limit at 106 cycles for materials with ultimate strength of more than 1400 MPa is reasonably close to fatigue behavior of SCM steel. However, considering half of ultimate strength as fatigue limit at 106 cycles resulted in a close prediction of fatigue behavior of SUJ2 steel. Also, Murakami’s defect area method resulted in reasonable predictions of fatigue limit for both steels while being closer to experimental data for SUJ2 which has a uniform distribution of hardness.
Cha, SungChulHong, Seung-HyunSharifimehr, Shahriar
Effect of Pre-Strain on Edge Cracking Limit for Advanced High-Strength Steel Using Digital Image Correlation2017-01-03943/28/2017
Advanced high-strength steel (AHSS) is gaining popularity in the automotive industry due to its higher final part strength with the better formability compares to the conventional steel. However, the edge fracture occurs during the forming procedure for the pre-strained part. To avoid the edge fracture that happens during the manufacturing, the effect of pre-strain on edge cracking limit needs to be studied. In this paper, digital image correlation (DIC), as an accurate optical method, is adopted for the strain measurement to determining the edge cracking limit. Sets of the wide coupons are pre-strained to obtain the samples at different pre-strain level. The pre-strain of each sample is precisely measured during this procedure using DIC. After pre-straining, the half dog bone samples are cut from these wide coupons. The edge of the notch in the half dog bone samples is created by the punch with 10% clearance for the distinct edge condition. The pre-strained half dog-bone samples are tested under a uniaxial tension to acquire the edge cracking limit. The thinning strain is measured to calculate the edge cracking limit using the DIC system. The detailed experimental setup, procedure, and results are described in this article. The effect of pre-strain on edge cracking limit for different material direction at different pre-strain level is discussed and concluded.
Li, JunruiYang, RuiyanLi, ZhenDu, ChangqingZhou, DajunYang, Lianxiang
Integrated Computational Materials Engineering (ICME) Multi-Scale Model Development for Advanced High Strength Steels2017-01-02263/28/2017
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. The ICME model calibration is implemented in the LS-OPT analysis tool as a component of an optimization process. The final result of the ICME Model calibration is a user-defined material subroutine, implemented in LS-DYNA finite element analysis software, which can be subsequently used in vehicle crashworthiness performance simulations.
Savic, VesnaHector, LouisBasu, UshnishBasudhar, AnirbanGandikota, ImtiazStander, NielenPark, TaejoonPourboghrat, FarhangChoi, Kyoo SilSun, XinHu, JunAbu-Farha, FadiKumar, Sharvan
New Mechanisms Governing Local Formability In 3 rd Generation AHSS2017-01-17043/28/2017
Automotive OEMs are compelled by increasingly stringent global emissions standards to find economic solutions for building higher efficiency vehicles without compromising safety and ride quality. This challenge requires new advanced high strength steels (AHSS) that will significantly reduce vehicle weight and improve fuel economy. In addition to providing higher strength, these automotive sheet steels must have exceptional formability to produce reduced gauge parts with increasingly complex geometries. Formability is comprised of two components, global and local. Global formability represents the ability of a sheet material to be deformed under various stress conditions and to be formed into a part without failure. It can be estimated using forming-limit diagrams or ductility measurements from conventional uniaxial tensile tests. However, these tests cannot reliably assess the local formability at the edges or at the internal holes of the blanks during stamping. Numerous correlations have been developed in an attempt to predict local formability such as yield strength to tensile ratio, true strain at fracture, and post uniform elongation but they are often inaccurate, especially for AHSS grades. In this paper, the local formability of a new class of 3rd Generation AHSS developed by NanoSteel will be shown utilizing hole expansion ratio (HER) testing. Analysis of the resulting HER values indicates new factors that play an important role in local formability related to the underlying deformation mechanisms and structural transformation.
Branagan, D.J.Frerichs, A.E.Meacham, B.E.Cheng, S.Sergueeva, A.V.
Determination of Fracture Strain of Advanced High Strength Steels Using Digital Image Correlation in Combination with Thinning Measurement2017-01-03143/28/2017
Fracture strain data provide essential information for material selection and serve as an important failure criterion in computer simulations of crash events. Traditionally, the fracture strain was measured by evaluating the thinning at fracture using tools such as a microscope or a point micrometer. In the recent decades, digital image correlation (DIC) has evolved as an advanced optical technique to record full-field strain history of materials during deformation. Using this technique, a complete set of the fracture strains (including major, minor, and thickness strains) can be approximated for the material. However, results directly obtained from the DIC can be dependent on the experiment setup and evaluation parameters, which potentially introduce errors to the reported values. To evaluate the capability of the DIC for fracture strain measurements, a validation study was performed to compare the fracture strains of a 980GEN3 steel measured with a DIC and a microscope, respectively. Briefly, Marciniak cup tool was employed to deform the specimens under different strain conditions (i.e., uniaxial tension, plane strain, and equi-biaxial stretch). Deviations in the DIC measurements from the thinning measurements were identified and understood via comparing the thinning at fracture measured with the DIC and the microscope, which demonstrated the limitation of the current DIC setup used in this work. In an effort to improve the accuracy of fracture strain measurements with current DIC configurations, a combined approach integrating the DIC with the thinning measurement was practiced. Using this combined method, fracture strain data for two advanced high strength steel grades were successfully generated.
Huang, LuShi, MingRussell, Patrick
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