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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




Motivation1

Agenda


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


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

5

Conclusion5




Motivation1

Agenda


6

Strain rate regimes and associated testing methods


10510-1 104102 10310110010-210-310-410-5Strain 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


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


Tension (SHTB)

SHTB Setup [5]

9

SH

PB

AR

aw d

ata

Classical Analysis (SHPBA)


Procedure

incident-bar strain gauge transmission-bar strain gauge

Shifted strain waves

10

IdealInhomogeneous

specimen deformationNon-planar

interface deformation

Classical Analysis (SHPBA)


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]

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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


Digital Image Correlation (DIC)

Setup for Optical Strain Measurement [4]

Digital Image CorrelationSoftware (GOM ARAMIS)

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

12

SH

PB

AR

aw d

ata

Combined Analysis


Procedure

Raw

dat

aD

IC

Synchronization

εS, ��S,F, σS

13

Conclusion5




Motivation1

Agenda


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


Specimen geometry and fixation

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

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HR110 HR21010

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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


Videos

Video sequence captured with high speed camera

Axial strain field determined with ARAMIS DIC system

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Compression test


Comparison of quasi-static and dynamic material behaviou r

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 fo r45°-specimens (failure points marked)

17

Conclusion5




Motivation1

Agenda


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


Specimen geometry and fixation

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

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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


Videos

Video sequence captured with high speed camera

Axial strain field determined with ARAMIS DIC system

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Tension test


Comparison of quasi-static and dynamic material behaviou r

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

plain-weave E-glass-epoxy

Tested specimens (quasi-static) Tested specimens (hi gh rate)

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Conclusion5




Motivation1

Agenda


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

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Technische Universität MünchenInstitute for Carbon CompositesBoltzmannstraße 1585748 Garchingwww.lcc.mw.tum.de

Contact

Address

FaxEmail

TelRoom

+49 89 /+49 89 /

Institute for Carbon Composites donated by

Peter Kuhn

289 - 150965504.01.438

289 - [email protected]

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Literature


[1] http://www.bmwblog.com/2013/11/29/bmw-i3-earns-4-stars-euro-ncap-crash-tests-led/

[2] http://aviatic-world.blogspot.de/2010/11/flug-qf32-die-beinahekatastrophe-das.html

[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) 2010.

[5] Koerber H., Vogler M., Kuhn P., Camanho P.P., Experimental Characterisation and Modelling of non-linear stress-strain 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. 223-250, 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. 062001 (17pp), 2009

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