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  • Institute for Carbon Composites donated by

    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 11th - 12th 2014

  • 2

    Conclusion5

    Test Example with Tension Bar Setup4

    Test Example with Compression Bar Setup3

    Introduction of Split-Hopkinson Bar test method2

    Motivation1

    Agenda

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

  • 3

    Conclusion5

    Test Example with Tension Bar Setup4

    Test Example with Compression Bar Setup3

    Introduction of Split-Hopkinson Bar test method2

    Motivation1

    Agenda

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

  • 4

    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

    Motivation

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Side impact pole test [1] Foreign object damage [2]

  • 5

    Conclusion5

    Test Example with Tension Bar Setup4

    Test Example with Compression Bar Setup3

    Introduction of Split-Hopkinson Bar test method2

    Motivation1

    Agenda

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

  • 6

    Strain rate regimes and associated testing methods

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    10510-1 104102 10310110010-210-310-410-5 Strain rate [1/s]

    Creep

    10-6

    Quasi-static Intermediate High rate Impact

    Inertia forces neglected Inertia forces important

    Conventional load frames (hydraulic, electro mechanical)

    Special servo- hydraulic frames

    Hopkinson Bars and

    Drop Tower

    Taylor Impact Test, Expanding

    Ring,…

    Isothermal Adiabatic

    Strain rate regimes and associated testing methods (adapted from [3])

  • 7

    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

    Classical Split-Hopkinson Bar Setup

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Compression (SHPB)

    SHPB Setup [4] Propagation of strain pulse

  • 8

    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

    Split-Hopkinson Bar Setup

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Tension (SHTB)

    SHTB Setup [5]

  • 9

    S H

    P B

    A R

    aw d

    at a

    Classical Analysis (SHPBA)

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Procedure

    incident-bar strain gauge transmission-bar strain gauge

    Shifted strain waves

  • 10

    Ideal Inhomogeneous

    specimen deformation Non-planar

    interface deformation

    Classical Analysis (SHPBA)

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Limitation

    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

    [4] [6]

  • 11

    Contactless measuring technique

    Full 2d strain field ��� , ��� , ��� �

    Verification of uniform specimen deformation and strain distribution

    High speed photography reveals deformation and failure mechanisms

    Strain Measurement

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Digital Image Correlation (DIC)

    Setup for Optical Strain Measurement [4]

    Digital Image Correlation Software (GOM ARAMIS)

    Principle of Digital Image Correlation (adapted from [7])

  • 12

    S H

    P B

    A R

    aw d

    at a

    Combined Analysis

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Procedure

    R aw

    d at

    a D

    IC

    Synchronization

    εS, ��S,F, σS

  • 13

    Conclusion5

    Test Example with Tension Bar Setup4

    Test Example with Compression Bar Setup3

    Introduction of Split-Hopkinson Bar test method2

    Motivation1

    Agenda

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

  • 14

    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

    Already tested at LCC: UD-CFRP, 5HS CFRP, Plain-Weave GFRP, neat resin

    Setup for compression tests

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Specimen geometry and fixation

    Specimen geometry Specimen fixation at SHPB Specimen fixation at electro- mechanical machine

    20

    HR110 HR210 10

  • 15

    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

    Photron SA5 high speed camera

    QS reference test setup:

    Electro-mechanical testing machine

    Velocity: 0,5 mm/min

    3D ARAMIS system

    Dynamic compression test

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Videos

    Video sequence captured with high speed camera

    Axial strain field determined with ARAMIS DIC system

  • 16

    Compression test

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Comparison of quasi-static and dynamic material behaviour

    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

    Failure envelope in

    � � stress spaceAxial stress-strain curves at different strain rates for 45°-specimens (failure points marked)

  • 17

    Conclusion5

    Test Example with Tension Bar Setup4

    Test Example with Compression Bar Setup3

    Introduction of Split-Hopkinson Bar test method2

    Motivation1

    Agenda

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

  • 18

    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

    Already tested at LCC: UD-CFRP, Plain-Weave GFRP, FML, neat resin

    Setup for tension tests

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Specimen geometry and fixation

    Specimen geometry Specimen fixation at SHTB Specimen fixation at electro- mechanical machine

  • 19

    Material:

    Plain-Weave E-glass-epoxy

    Tested in 0°-direction

    SHTB test setup:

    Titanium bars, Ø 16 mm

    Impact velocity: about 9 m/s

    Photron SA5 high speed camera (100.000 fps, 384x168 pixel²)

    QS reference test setup:

    Electro-mechanical testing machine

    Velocity: 0,5 mm/min

    3D ARAMIS system

    Dynamic tension test

    09/11/2014 | Kuhn | LCC-Symposium | High strain rate characterisation of composites using split-Hopkinson bar method

    Videos

    Video sequence captured with high speed camera

    Axial strain fiel