Cross Section Optimization for Axial and Bending Crushing Using AHSS

Transcription

1 Cross Section Optimization for Axial and Bending Crushing Using AHSS Guofei Chen United States Steel Corporation

2 Outline Introduction Dimension Effect Axial Crush Mode Stability Optimization Study Test Verification Weight Reduction Potentials Summary

3 Introduction Frontal crash: axial crash mode Crash components: Front/rear rails Side impact and roof crush: bending mode Crash components: A-pillar, B-pillar, roof rail, rocker, X-member Validated FEA models were utilized for the axial crush mode and crosssection optimization study The critical thickness-to-width ratios were proposed for both DP59 and DP78 steels to achieve the regular folding mode Optimal cross-sections were automatically searched for improved axial and bending crush performance Potential weight savings with optimized cross-section using DP59, DP78 and DP98 steels

4 Dimension Effect Width 3 mph axial crash on 2.5 mm thick DP59 and DP78 samples Average Force (KN) F ~ b.37 R 2 =.9783 DP59 FEA Data Power (FEA Data) Width b (mm) Average Force (KN) b F ~ b.39 R 2 =.927 DP Width b (mm) b FEA Data Power (FEA Data)

5 Dimension Effect Thickness 3 mph axial crash on DP59 and DP78 samples 2 DP59 b=84mm Average Force (KN) F ~ t 1.75 R 2 =.9975 FEA Data Power (FEA Data) Thickness t (mm) Average Force (KN) b DP78 F ~ t 1.77 R 2 =.9872 FEA Data Power (FEA Data) Thickness t (mm)

6 Axial Crush Mode Stability Static Elastic Buckling Stress for A Square: σ cr 2 E = π 3(1 v 2 ( t ) / b) For sections with small t/b and σ cr < material s yield strength: Elastic buckling occurs before material yields Mode of Collapse is influenced mainly by geometry instead of materials 2 b

7 Axial Crush Mode Stability Experiments have shown that when σ cr <.6*σ y, the irregular crumpling mode occurs The threshold t/b ratio for plastic buckling: 2 ( t / b) cr >.48[ σ y (1 v ) / E ] 1/ 2 Crumping Regular folding

8 Axial Crush Mode Stability DP59 Square b=84mm Stress (MPa) Max. Average Stress Elastic Buckling Stress Yeild Strength Stress (MPa) Max. Average Stress Elastic Buckling Stress Threshold t/b Ratio Gauge (mm) t/b For gauges less than 1.5 mm or t/b ratios less than.18: σ cr << σ y and σ cr < σ max elastic buckling occurred before material yielding an irregular crumpling mode

9 Axial Crush Mode Stability DP59 Square t=1.25 mm or t/b=.15 t=2. mm or t/b=.24 Quasi-static b b=84mm 3 mph

10 Axial Crush Mode Stability DP78 Square b=84mm Stress (MPa) Max. Average Stress Elastic Buckling Stress Yield Strength Gauge (mm) Stress (MPa) Max. Average Stress Elastic Buckling Stress threshold t/b Ratio t/b For gauges less than 1.75 mm or t/b ratios less than.21: σ cr << σ y and σ cr < σ max elastic buckling occurred before material yielding an irregular crumpling mode

11 Axial Crush Mode Stability DP78 Square t=1.5 mm b=84mm t/b=.18 t=2. mm b=84mm t/b=.24 t=1.5 mm b=63mm t/b=.24 Quasi-static 3 mph

12 Cross-Section Optimization Study Shape Variables For the 84mm Square Shape1 Shape4 Shape6 Shape1 HyperMorph Shape3 Shape8 Shape3 Shape2 Shape7 Shape2 Shape1 Shape5 Shape1 Shape6 1.5mm DP78 1.5mm DP78

13 Shape Variables #1 - #3 Shape #1 Shape #2 Shape #3

14 Crush Models Optimization Software: HyperStudy Crash Simulation Solver: LS-Dyna Quasi_static bending Objective: Maximize energy absorption Constraint: Mass and Peak Load 3 mph Axial Objective: Minimize crush distance Constraint: Mass

15 Optimization for Axial Crush Crush distance (mm) Crush distance Mass Iteration Mass (kg) Shape variable Shape1 Shape2 Shape3 Shape4 Shape5 Shape6 Shape7 Shape8 Iteration

16 Optimization for Axial Crush Crush Force (KN) Square Baseline Optimized Displacement (mm) Energy Absorption (KJ) Square Baseline Optimized Displacement (mm)

17 Optimization for Bending Crush Energy Absorption (KJ) Best Iteration Peak Force (KN) Iteration Mass (Kg) Iteration Shape Variables Shape1 Shape2 Shape3 Shape4 Shape5 Shape6 Shape7 Shape8 Iteration

18 Optimized Section for Bending Crush Force (KN) Square Baseline Optimized Energy Absorption (KJ) Square Baseline Optimized Displacement (mm) Displacement (mm)

19 Mass Reduction Bending Crush Design Variable Description Range Initial Max. Dist. (mm) Shape1 Diagonal move, 1, 4 Shape2 Side bead depth -1, 1-15, 15 Objective: Min. Mass Constraint: Energy Shape3 Side bead width -1, 1-15, 15 Shape4 Top bead depth, 1, 15 Shape5 Bot. bead depth, 1, 15 Shape6 T/B bead width -1, 1-15, 15 Shape7 Section width -3, 1-3, 1 Shape8 Section depth -3, 1-3, 1 DV9 Gauge.6, , 2 DV1 Steel Grade 1, 4 1 NA Baseline: 2mm HSLA35 DV1 for steel Grade: 1 HSLA35 2 DP59 3 DP78 4 DP98

20 Constraint on Cost Relative Material Cost Factors From A/SP Future Generation Passenger Compartment Project: Material Name Relative Cost IF 14/27 1. HSLA DP DP DP The mass is assigned twice as much importance as cost: A cost increase of 1% is justified if that results in a 2% mass saving

21 Mass Reduction Bending Crush Mass (kg) 9% Mass (kg) Energy (KJ) Iteration Energy absorption Design variable Iteration Shape1 Shape3 Shape5 Shape7 Grade Shape2 Shape4 Shape6 Shape8 Gauge 1.29mm DP98 7% 5% 3% 1% Mass Savings Cost Savings 34.5% 13.2% -1% % Iteration

22 Test Verification on 12-Sided Cross-Section 3 12-Side At 15 mm crush distance Crush force (KN) Octagon Hexagon Circle Displacement (mm) Steel: 1.55mm DP59 Crush Test: 3mph axial Energy absorption (KJ) Energy Absorption (KJ) % increase w.r.t. 12-Sided Octagon Hexagon Circle Side Octagon Hexagon Circle Displacement (mm)

23 Weight Reduction Strategy Material and Dimension Effects 3 mph Axial Crush Shape Square Steel Grade HSLA Width (mm) Gauge (mm) Weight (kg) Weight Saving (%) NA DP59 DP

24 Weight Reduction Strategy Cross-section Effect Shape Steel Grade 3 mph Axial Crush Width (mm) Gauge (mm) Weight (kg) Weight Saving (%) Square DP Hexagon DP Octagon DP Sided DP Shape Steel Grade Width (mm) Gauge (mm) Weight (kg) Weight Saving (%) Square DP Hexagon DP Octagon DP Sided DP

25 Weight Reduction Strategy Shape Square Hexagon Octagon 12-Sided Steel Grade 3 mph Axial Crush Width (mm) Gauge (mm) Weight (kg) Weight Saving (%) HSLA NA DP DP DP DP DP DP DP DP

26 Weight Reduction Strategy Comparisons with The Aluminum Extrusion Design Shape Aluminum Extruion Hexagon Octagon 12-Sided Steel Grade 3 mph Axial Crush Width (mm) Gauge (mm) Weight (kg) Weight Difference (%) Aluminum 661-T NA DP % DP % DP % DP % DP % DP %

27 Summary The axial crush performance of thin-walled columns is much more sensitive to gauge than width. More weight reduction can be achieved by downsizing the width along with downgauging using AHSS. The thickness-to-width ratio is recommended to be larger than.18 for thinwalled DP59 columns and.21 for DP78 columns to achieve regular folding mode in axial crush. Significant mass savings can be achieved by the applications of AHSS together with optimized cross-sections. The 12-sided cross-section was found to be superior in axial crush, while the square cross-section with shallow outward beads was optimal in bending crush performance. The 12-sided cross-section using DP59 or DP78 is superior in axial crush performance and is an excellent candidate to replace extruded aluminum frame design.

28 Acknowledgment Special thanks to Dr. Tau Tyan of Ford Motor Company for many valuable discussions and for coordinating the tests and providing the test data Thanks to Ford Safety Laboratory for conducting the crush test

29 Great Designs in Steel is Sponsored by: AK Steel Corporation, ArcelorMittal, Nucor Corporation, Severstal North America w w w. Inc. a u and t o s United t e e States l. o r gsteel Corporation