1 Simulation von Kerbeffekten durch Löcher und Schweißpunkte in Karosseriebauteilen aus Ultra Hochfesten Stählen April 18th, 2012, Bad Nauheim R&A Europe: H. Lanzerath, Aleksandar Bach, Hueseyin Cakir, Udo Brüx Overview 2 Introduction Need for Failure Modelling in Crash Simulation Summary
Introduction Crash Performance Evaluation 3 legal requirement public domain ROOF FMVSS new rule 216 roof strength IIHS roof crush assessment (4,2 veh. weight) FMVSS 301 rear impact fuel system integrity Rear deformable EU barrier (70% 80kph 1250kg) REAR FRONT SIDE ECE 94.01 front offset FMVSS 208 front impact Occupant protection FMVSS 301 front impactfuel system integrity Euro NCAP front offset 64kph USNCAP front impact IIHS front offset impact Increasing Safety Performance Acceptance Criteria ECE 95.01 side impact FMVSS214 side impact IIHS side impact Euro NCAP side impact 50kph USNCAP side crabbed barrier USNCAP side oblique pole Increasing Demand for Affordable Light Weight Designs Increasing number of derivates on the same, global platform Introduction UHSS Applications in Ford Focus 4 47 % Mild Steel 26 % Conventional HSS 18 % Advanced HSS 9 % Ultra HSS
Introduction UHSS Applications in Ford Focus 5 Door Beams Rocker Reinforcement MSW1200 DOCOL1400 Rear Bumper Beam System MSW1200 cross member 1.8mm, MSW1200 2.6kg Introduction UHSS Applications in Ford Focus 6 Implementation 2006 (CD-Car) Standard Boron technologies Monolitic hot-formed parts Implementation 2011 (Focus) Advanced Boron technologies Tailor Rolled Blanks / hot-formed Weight Save Boron vs. DP600 m = 5.2 kg/vehicle Weight Reduction = 28 % Additional Weight Reduction = 1.4 kg
Overview 7 Introduction Need for Failure Modelling in Crash Simulation Summary Crash Properties of Steels 8 CONVENTIONAL steels AHSS/UHSS Low Strength High Strength High Ductility Low Ductility Ductility IF - Steel Mild Steel BH and HS IF Steel TRIP Steel Sensitive to notches Martensitic steels cold formed (e.g. DOCOL1400 M) hot formed (e.g. BORON1500) Dual Phase Steel Micro Alloyed High Strength Steel Complex Steel Dual Phase Steel Martensitic Steel Boron Steel Strength Light weight design use of more AHSS/UHSS (thickness reduction) Light weight potential of UHSS cannot be utilised if material failure limits are not known
Observations in Prototype Parts 9 t = 2mm Crack starts at rectangular hole. Prototype Parts shown Cracks start at welded areas. Failure mode and failure strain depend on: the local load situation (multiaxiality) the local material history the local strain rate Failure Modelling - Simulation Approach 10 Materialcharacterisation -E, ρ, ν -Flowcurve -Yield Locus -Fracture Limit Curves Material Models (User Subroutines) MF GenYld Elasto-plastizität CrachFEM Versagensmodell Commercial Solver Radioss LS-Dyna (explizit)
Flow Curve 11 Swift - equation σ n m ( ε + ε ) & eq = a 0 eq εv 1.6 Strain rate sensitivity m = ln ln ( σ (1) σ (2) ) (& ε & ε ) (1) (2) Anisotropic behaviour True Stress / σ y [-] 1.4 1.2 1.0 0.8 Tensile Test Layer Compression Test Swift_Approximation 0.0 0.1 0.2 0.3 0.4 0.5 0.6 True Strain [-] Uniform Elongation Failure Modelling Material Model 12 FRACTURE PROPERTIES (failure limit curves) Shear Fracture plastic strain at fracture DNF DSF INST Normal Fracture DNF DSF INST stress state ductile normal fracture ductile shear fracture instability Instability More testing & material characterisation required!
Failure Modelling Systematic Validation 13 MATERIAL MODEL Systematic Validation Procedure & Version Checker CAE vs. Test One Element Tests no Component Tests yes no no no Substructure Tests 0 DGOF CORRELATION TEST-SIMULATION? yes F V=const= 2.5mm/s s m ax = 500mm 0 DGOF yes Full Vehicle Tests Robustness Failure Modelling: Tool for Material Selection 14 Example: Component tests on bumper beams 2 different material grades investigated: MAT_1 MAT_2 MAT_1 MAT_2 CAE without failure modeling MAT_1 MAT_2 CAE with failure modeling Without failure modeling: MAT_1 would be the initially selected material
Different Failure Mechanisms 15 1. Pure Material Failure 2. Geometrical Notches (Holes, Cut-edges, Rivets, ) 3. Metallurgical Notches (RSW, MIG-Welds, Laser-Welds, ) Different Failure Mechanisms - Pure Material Failure 16 F/2 F 3Point Bending of a Boron Beam TEST F/2 Boron 19001500 / N-mode Force F TEST_1 TEST_2 CAE Tests on Prototype Parts CAE 0 40 80 120 Displacement [mm]
Different Failure Mechanisms - Notches 17 METALLURGICAL NOTCHES heat affected zones (HAZ) of spot welds in UHSS GEOMETRICAL NOTCHES (holes, cut-outs, ) Ø Ø 10 10 section A-A 14 A A 14 FAILURE MODELLING of NOTCHES CAE methodology to predict material failure due to notches cut-outs HAZ (heat in affected B-pillar reinforce zone) ment Different Failure Mechanisms - Notches 18 3PB of Boron beam (with & without notches) Force F NO NOTCH HOLE SPOT WELD 0 10 20 30 40 50 60 Displacement [ mm ] NO NOTCH HOLE SPOT WELD Significantly reduced energy absorption and sudden failure!
Geometrical Notches Holes 19 Impactor Force CAE_STANDARD CAE_Standard CAE_REFINED TEST CAE_Refined Discplacement [mm] eps PL high low Refined mesh more accurate prediction of deflection behaviour including fracture increase of CPU time Metallurgical Notches 20 Hardness HV Section A-A Base HAZ Nugget HAZ Base A A Direction 0 Direction 45 Direction 90 HAZ = Heat Affect Zone
Metallurgical Notches Conventional Modelling 21 Typical Crash-Modell: - ~ 2 Mio. Finite Elements - Edge Length 5-10 mm - ~ 3500 Spot Welds Spring-Element Metallurgical Notches Validation on Component Level 22 Conventional Modelling Test vs. Simulation Impactor: Force over Displace ment Conventional model for RSW not applicable to predict HAZ effects! Tests on Prototype Parts
Metallurgical Notches Micro-Modelling 23 HAZ = Heat Affect Zone Section A-A Base HAZ Weld-Nugget A A Properties: - detailled geometrical representation - local material properties - Mesh dependency Metallurgical Notches Validation on Component Level 24 Micro-Modelling Test vs. Simulation Impactor: Force over Displace ment Tests on Prototype Parts
Comparison of CPU Time Full Car Simulation 25 Simulation Time in h New CAE Methods are required, which are 1. Accurate, 2. Efficient. Conventional Micro What is Multi-Domain for Crash Simulation? 26 Arbitrary number of domains can be specified within a Full Car Crash Model E.g. non-crash relevant parts can be modelled with coarse meshes, while safety critical components are modelled in detail Each domain can have own time step (element size) Optimized CPU usage Improved Failure Prediciton Model 1 Model 2 RAD2RAD Model 1 Model 2 Increased computation accuracy at low CPU time increase DOMAIN A DOMAIN B DOMAIN C source: Maciek Wronski (Altair), HTC Conference, 2010
Multi-Domain CAE: Validation on Full Vehicle Level 27 Side Pole Impact Standard CAE Multi-Domain CAE Computational Time Refined Mesh CAE STANDARD CAE no prediction of failure Multi-Domain CAE accurate failure prediction Tests on Prototype Parts 28 Design Solutions
Reduce Notch Effects Tailored Tempering 29 Source: TKSE Reduce Notch Effects Tailored Properties 30 Sourc e: GESTAMP
Reduce Notch Effects Redesign 31 Cut-edge moved in less critical area Further Challenges for CAE 32 New Boron Technologies (1900 MPa, Tailored Properties, Softzones, ) New Manufacturing Processes (e.g. Form Blow Technology, ) Mechanical joining technologies (e.g. RIVTAC, SPR, FDS, laser-welding,friction-element welding, ) RIVTAC EJOT-Weld FDS Sourc e: Böllhoff Sourc e: EJOT Sourc e: EJOT
Summary 33 The trend in body structure design is to use more and more materials that offer lightweight potential at affordable costs. UHSS offer significant weight saving potential In order to utilize UHSS in the best way Design guidelines need to be followed CAE optimization including the capability to predict failure modes is required The consideration of these aspects Supports proper material selection Enables robust designs and efficient d pment processes Avoids bad surprises at first prototypes