Comparison of Energy Absorption Characteristics of Thermoplastic Composites, Steel and Aluminum in High-Speed Crush Testing of U-Beams

Transcription

1 Comparison of Energy Absorption Characteristics of Thermoplastic Composites, Steel and Aluminum in High-Speed Crush Testing of U-Beams CELANESE ENGINEERED MATERIALS Michael Ruby October,

2 Overview Low carbon vehicle technology programme (UK) Comparing the energy management behavior of materials Materials selection: Defining performance criteria Materials selection: Ascertaining compliance with criteria Part development and materials of construction Composite process selection Producing beams for testing Results and summary 2

3 Low Carbon Vehicle Technology Programme West Midlands, UK Study Develop and deploy technologies and skills across local auto supply base Accelerate introduction of low-carbon technologies in auto industry Parent program broken into 15 work streams including lightweight structures Parent program participants: 3

4 Low Carbon Vehicle Technology Programme Goal of lightweight structures work stream Develop materials and process technologies capable of 20% mass reduction vs. conventional technology To reduce CO2 emissions during usage phase of complete vehicle lifecycle Must be achieved without incurring any detrimental environmental impacts in manufacturing or end-of-life phases Any material and process developments require proof to a concept level 4

5 Comparing Energy Management Behavior of Materials Criteria for materials-selection phase Performance Must be suitable for structural applications Must provide mechanical stability to 180 C for e-coat/ktl Manufacturing process Must provide rapid cycle times (60-90 sec) Must support medium-volume (30,000-50,000/yr) production without large capital investment Cost (ready and scalable supply) No adverse environmental performance 5

6 Materials Selection: Defining Performance Criteria 3 materials chosen: Conventional (baseline): Cold-rolled/dual-phase, high-strength structural steel (DP-600) Lightweight metallic: Cold-rolled/high-performance structural aluminum (AA5754) Composites: 60 wt.-% continuous / UD-fiber reinforced polyamide (PA) 6 (PA6-GF60) Selected for combination of cost, performance, processability and recyclability Available with carbon or glass reinforcement (CF 5x more costly) 6

7 Materials Selection: Defining Performance Criteria Verify mechanical stability of PA6-GF60 tape at elevated temperature via DMA testing Material retained >50% of dynamic stiffness to 200 C (only need to 180 C) Tan (δ) value measured matrix melting temperature at C 7

8 Stress (MPa) Materials Selection: Defining Performance Criteria Testing and CAE modeling verified performance of PA6-GF60 laminates Tensile properties vs. compression properties UD laminate 0 Single ply avg : Young s modules = 35 GPa Tensile strength : 730 MPa Compressive strength : 430 Mpa UD0 - tension 0/90 - tension tension UD0 - compression Tensile properties vs. compression properties UD laminate 90 Ply s average Young s modulus = 19 GPa Tensile strength = 340 MPa Axial strain () 8

9 Part Development and Materials of Construction Demonstrator part selected was scaled front longitudinal C- section with closure plate Part design similar to stamped steel structures from OEM partner s vehicle-development program Design modifications Downscaled in size (to fit test equipment) Simplified by removing slightly longitudinal taper (to facilitate manufacture and testing) Final beam section: 40-mm tall, 70-mm wide, 450-mm long 9

10 Part Development and Materials of Construction 3 variants were used in testing Beam A: (baseline metallic): Structural steel (DP600) Beam B: (lightweight metallic): Structural aluminum (AA5754) Beam C: (thermoplastic composite): Laminates from PA6-GF60 tapes Beam Designation Beam Material Beam Weight (g) Top-Hat Section Material and Thickness (mm) Closure Plate Material and Thickness (mm) A DP600 Steel (Baseline metallic) 1,880 DP600 steel (1.5) DP600 steel (1.5) B C AA5754 Aluminum (Lightweight Metallic) PA6-GF60 (Thermoplastic Composite) 1, AA5754 aluminum (2.5) PA6-GF60 (11 Plies) (3.0) AA5754 aluminum (2.5) PA6-GF60 (11 Plies) (3.0) 10

11 Composite Process Selection Steel and aluminum beams would be stamp-formed Composite beams manufactured via hot stamp-forming process Criteria for composite process selection Original target cycle time of sec was tightened to 60 sec to ensure compliance with highest target production volumes in study (50,000/yr.) Team sought production approach that could be implemented within existing auto supply chain Process had to be capable of producing parts >1 m2 using UD-fiber laminates Process and tooling had to be economical at both lower and higher ends of target production volumes (30,000-50,000/yr.) 11

12 Hot Stamp-Forming Tool Mounted in Compression Press 1,000 T Compression Molding Press Stamp-Form 1 m2 Part at 10 Mpa Matched-Metal Die set Preheating of Laminate in Heated Lamination Machine 12

13 Producing Beams for Testing Steel and aluminum beams stamp-formed from supplier Composite beams made at WMG 11-plies per Laminate Cross-ply stack & sequence not optimized or balanced Ply construction : [0 /90 /90 /0 /0 /90 /0 /0 /90 /90 /0 ] 0 orientation along length of beam; 90 orientation along width Preforms heated to 235 C and manually transferred to tool Cycle time (60 sec) included loading blanks & de-molding parts All metal and composite beams trimmed, cleaned and joined to closure plate via structural epoxy adhesive & rivets Uncured adhesive applied along beam's 2 flanges Closure plate added, then mechanically joined via self-piercing rivets in assembly fixture All parts cured in hot-air oven to replicate BIW e-coat process temps 13

14 Results and Summary 3-point Flexure Test Comparative to side-impact event where stiffness and energy absorption must be balanced to minimize intrusion and occupant accelerations Allowed researchers to assess comparative static structural performance for each type of beam using same section geometry while varying material thickness inwards from fixed outer surface Test conducted using cylindrical roller at speeds of 20mm/min and maximum deflection of 50mm Picture of the 3-point bend test set up note that the span was 400mm 14

15 Results and Summary Flexure test results Local crushing began at moderately low levels 3 Specific failure energy DP600 AA5754 PA6-GF60 All 3 types provided similar performance Composite beam showed toughness (matrix) and stiffness/strength (reinforcements) Composite also had better specific energy absorption (SEA) at peak load 15 Specific failure energy (J/g) Specific beam stiffness (N/mm per g) 2.5 Beam stiffness = resistance both to bending and local crushing Peak (failure) load = point when local crushing begins to dominate system response (beyond this point, beam starts to fold around load-introduction point) Specific beam stiffness

16 Central Sections of Beams Tested 3-Point Flexure Steel Aluminum PA6-GF60 DP600 Steel (Baseline metallic) 5754 Aluminum (Lightweight metallic) Celstran CFR-TP (Thermoplastic Composite) Ductile Steel and Aluminum Beams Show Characteristic V Shape from Local Crushing Composite Beam Shows Longitudinal and Transverse Cracking from Compressive Side- Wall Loading and Tensile Loading on Upper Beam Surface where it Conformed under Roller 16

17 Specific Load (N/g) Specific (Mass-Normalized) 3-Point Load vs. Deflection Curves DP600 AA5754 PA6-GF Crosshead displacement (mm) 17

18 Results and Summary Axial Crush Test (simulates to frontal crash) CAE model developed to ensure correlation to test data Testing done on instrumented spring-assist/drop-weight tower at 2 different energy levels 4 kj energy equivalent to impactor mass of 133 kg at speeds of 8 m/sec 8 kj energy equivalent to impactor mass of 74 kg at speeds of 14.6 m/sec Beams reduced from 450 to 375mm to stabilize crush column and prevent global buckling Failure initiators added to end of beams to ensure each test had stable and progressive crush mode 18

19 Comparison of Dynamic Axial Crush Exhibiting Progressive Failure Modes Aluminum PA6-GF60 19

20 Results and Summary Axial crush test Steel and Aluminum beams exhibited... Similar axial crush failure modes Characteristic uniform ductile folds Composite beams exhibited... Progressive failure along joint between rivet-bonded closure plate and stamped top-hat section Tearing failure of upper top-hat radii Formation of large-scale fronds with some fragmentation Specific Energy Absorption (SEA) was calculated and damage measured to determine mass of material destroyed / deformed from test 20

21 Specific energy absorption (SEA, J/g) Specific Energy Absorption (SEA) of Impact Rate for Beams (at 8 and 15 m/s) kj, 8 m/s 8 kj, 15 m/s DP600 steel 5754 aluminium PA6-GF60 TPC At 4 kj (8 m/sec), Composite Beam had SEA Value 3x Higher than Steel and 2x Higher than Aluminum. At 8 kj (14.6 m/sec), Strain-Rate Stiffening in Metal Beams and Slight Loss of Global Crush Stability in Composite Beams Reduced SEA of Composite Beams to >2x Steel and <2x Aluminum 21

22 Results and Summary Study demonstrated High-performance UD-fiber-reinforced thermoplastic composites can provide sufficient structural performance Validated that new hot stamp-forming process can produce parts at auto-industry-relevant production volumes Additional work (not covered here) confirmed materials substitution could provide environmental and economic benefits as well 22

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