High strain rate characterisation of composites using split-hopkinson bar method

Transcription

1 High strain rate characterisation of composites using split-hopkinson bar method Peter Kuhn / Dr. Hannes Körber A Comprehensive Approach to Carbon Composites Technology Symposium on the occasion of the 5 th anniversary of the Institute for Carbon Composites Research Campus Garching, September 11 th - 12 th 2014 Institute for Carbon Composites donated by

2 Agenda Motivation Introduction of Split-Hopkinson Bar test method Test Example with Compression Bar Setup Test Example with Tension Bar Setup Conclusion 2

3 Agenda Motivation Introduction of Split-Hopkinson Bar test method Test Example with Compression Bar Setup Test Example with Tension Bar Setup Conclusion 3

4 Motivation Side impact pole test [1] Foreign object damage [2] Increased applications in which fiber reinforced polymer matrix composites are loaded dynamically For FE-simulations, models capturing high-rate material response are required High-rate-loading experiments provide data to validate and further develop composite constitutive models and failure criteria 4

5 Agenda Motivation Introduction of Split-Hopkinson Bar test method Test Example with Compression Bar Setup Test Example with Tension Bar Setup Conclusion 5

6 Strain rate regimes and associated testing methods 10-6 Creep Strain rate [1/s] Quasi-static Intermediate High rate Impact Inertia forces neglected Isothermal Inertia forces important Adiabatic Conventional load frames (hydraulic, electro mechanical) Special servohydraulic frames Hopkinson Bars and Drop Tower Taylor Impact Test, Expanding Ring, Strain rate regimes and associated testing methods (adapted from [3]) 6

7 Classical Split-Hopkinson Bar Setup Compression (SHPB) SHPB Setup [4] Propagation of strain pulse The striker-bar impacts the free end of the incident-bar A longitudinal elastic compressive strain pulse is created, which propagates along the incident-bar The pulse is partly reflected at the incident-bar/specimen interface due to change of mechanical impedance The ratio of reflected to transmitted pulse defines the relative motion of the bar endfaces LCC-Setup: Ø 16, 18, 25 mm steel bars & Ø 16 mm aluminium bars 7

8 Split-Hopkinson Bar Setup Tension (SHTB) SHTB Setup [5] In principle, very similar to Split-Hopkinson Bar for compression Differences in loading mechanism Differences in specimen gripping methods LCC-Setup: Ø 16, 20, 25 mm titanium bars 8

9 Classical Analysis (SHPBA) Procedure SHPBA Raw data incident-bar strain gauge transmission-bar strain gauge Shifted strain waves 9

10 Classical Analysis (SHPBA) Limitation Ideal Non-planar interface deformation Inhomogeneous specimen deformation [4] [6] Correct calculation of specimen strain and strain rate not always possible using SHPBA Direct stain measurement on specimen is more accurate for composites Strain gauges on specimen Optical methods 10

11 Strain Measurement Digital Image Correlation (DIC) Digital Image Correlation Software (GOM ARAMIS) Setup for Optical Strain Measurement [4] Contactless measuring technique Full 2d strain field,, Verification of uniform specimen deformation and strain distribution High speed photography reveals deformation and failure mechanisms Principle of Digital Image Correlation (adapted from [7]) 11

12 Combined Analysis Procedure SHPBA Raw data F, σ S Synchronization ε S, S, Raw data DIC 12

13 Agenda Motivation Introduction of Split-Hopkinson Bar test method Test Example with Compression Bar Setup Test Example with Tension Bar Setup Conclusion 13

14 Setup for compression tests Specimen geometry and fixation HR1 10 HR2 Specimens are clamped between incident- and transmission-bar Bar-end surfaces are covered with MoS2 Same specimen types are used for quasi-static reference tests and dynamic SHPB tests to ensure comparability of results Specimen geometry Specimen fixation at SHPB Specimen fixation at electromechanical machine Already tested at LCC: UD-CFRP, 5HS CFRP, Plain-Weave GFRP, neat resin 14

15 Dynamic compression test Videos Material: 5-harness-satin carbon-epoxy Tested in 15 -, 30 -, 45 -off-axis and weft direction (video: 45 ) SHTB test setup: Steel bars, Ø 16 mm Two strain rates investigated Video sequence captured with high speed camera Photron SA5 high speed camera QS reference test setup: Electro-mechanical testing machine Velocity: 0,5 mm/min 3D ARAMIS system Axial strain field determined with ARAMIS DIC system 15

16 Compression test Comparison of quasi-static and dynamic material behaviour Axial stress-strain curves at different strain rates for 45 -specimens (failure points marked) Failure envelope in stress space Specimens were tested in 15 -, 30 -, 45 -off-axis and weft direction Strength components are transformed from loading coordinate system in material coordinate system A maximum stress failure criterion is well suited to approximate the failure envelop 16

17 Agenda Motivation Introduction of Split-Hopkinson Bar test method Test Example with Compression Bar Setup Test Example with Tension Bar Setup Conclusion 17

18 Setup for tension tests Specimen geometry and fixation Specimens are glued into slotted endcaps Threaded endcaps are screwed into bars Same specimen types are used for quasi-static reference tests and dynamic SHTB tests to ensure comparability of results Specimen geometry Specimen fixation at SHTB Specimen fixation at electromechanical machine Already tested at LCC: UD-CFRP, Plain-Weave GFRP, FML, neat resin 18

19 Dynamic tension test Videos Material: Plain-Weave E-glass-epoxy Tested in 0 -direction SHTB test setup: Titanium bars, Ø 16 mm Impact velocity: about 9 m/s Video sequence captured with high speed camera Photron SA5 high speed camera ( fps, 384x168 pixel²) QS reference test setup: Electro-mechanical testing machine Velocity: 0,5 mm/min 3D ARAMIS system Axial strain field determined with ARAMIS DIC system 19

20 Tension test Comparison of quasi-static and dynamic material behaviour plain-weave E-glass-epoxy Tested specimens (quasi-static) Tested specimens (high rate) All specimen failed at free length at the transition from gauge section to radius At strain rate 170 1/s, axial strength is 50,4% higher than under quasi-static conditions 20

21 Agenda Motivation Introduction of Split-Hopkinson Bar test method Test Example with Compression Bar Setup Test Example with Tension Bar Setup Conclusion 21

22 Conclusion The Split-Hopkinson bar method is ideally suited for dynamic material characterisation of composites in the strain rate range of 10² - 10³ 1/s Optical strain measurement techniques, such as Digital Image Correlation (DIC), are ideally suited to obtain all strain components of orthotropic materials and are further useful to evaluate the uniformity of the specimen deformation Reliable results can be achieved by using a combined analysis procedure, consisting of classical Split- Hopkinson Bar Analysis (SHPBA) and Digital Image Correlation (DIC) High speed photography reveals specimen deformation and failure mechanisms 22

23 Contact Peter Kuhn Room Tel Fax / / kuhn@lcc.mw.tu.de Address Technische Universität München Institute for Carbon Composites Boltzmannstraße Garching Institute for Carbon Composites donated by 23

24 Literature [1] [2] [3] Nemat-Nasser S., ASM Handbook Vol 8 Mechanical Testing and Evaluation, ch. Introduction to High Strain Rate Testing, ASM Int, 2000 [4] Koerber H., Mechanical Response of Advanced Composites under High Strain Rates (PhD) [5] Koerber H., Vogler M., Kuhn P., Camanho P.P., Experimental Characterisation and Modelling of non-linear stressstrain behaviour and strain rate effects for unidirectional carbon epoxy, ECCM16, 2014 [6] Gama B.A., Lopatnikov S.L., Gillespie J.W., Hopkinson bar experimental technique: A critical review, Applied Mechanics Reviews, vol. 57, no. 4, pp , 2004 [7] Pan B., Qian K., Xie H., Asundi A., Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review, Measurement Science and Technology, vol. 20, no. 6, p (17pp),