MDavid AJohnson ICCS 16

16th International Conference on Composite Structures (ICCS16)

Analysis of Crushing Response of Composite Crashworthy Structures

Matthew David and Alastair JohnsonDLR Institute of Structures and Design, Stuttgart

16th International Conference on Composite Structures , ICCS 16Analysis of Crushing Response of Composite Crashworthy Structures

M. David & A. JohnsonSlide 2

Overview

Introduction

Experimental Investigation (Strain rate)Testing and Analysis MethodologyIdentification of varied crush response modes

Experimental Investigation (Stacking sequence)

Conclusion

Outlook



IntroductionCrashworthy structures

During a crash scenario, the role of a crashworthy structure is to utilize energy absorbing mechanisms to absorb crash loads in a controlled progressive manner.

Goals of a crashworthy structure are: 1. Limit deceleration on occupants

2. Allow the safe evacuation of occupants by maintaining sufficient structural integrity

Full scale drop test conducted at CIRA (HeliSafe TA)

Bell UH-1D fitted with 3 dummies

Impact conditions (Vx = 11.1 m/s2, Vz = 9.3 m/s2)



IntroductionMotivation

Cooperation between CRC-ACS & DLR – Crashworthy Design (2007-2010)

Concepts for helicopter subfloor structures with improved energy absorption

Numerical design tools for predicting crash response of composite aircraft structures

Stiff Survivalspace

Energy absorbing structure

Drop tower test of a crashworthy structure (C. Kindervater et al., 2011)



IntroductionEnergy absorbing coupon specimen

Developed to understand the energy absorption and failure mechanisms of energy absorbing structures at an coupon level

Easy to fabricate and gives reproducible axial crush failures under quasi-static and dynamic loading conditions

Sub-floor box structure(CRASURV)

Coupon specimen



Experimental Investigation – Strain rateMethodology

Test Methodology

Crush load curves extracted from test machines

Video and still images of crushing process and specimens

High Resolution Computed Tomography (HRCT) scan

Analysis Methodology

Quasi-static crush test at a cross head velocity of 1.0 mm/s

Dynamic crush test at an impact velocity of 10.0 m/s

Dynamic crush test setup

Loading platen



Experimental Investigation – Strain rateTest specimen specifications

Material : Hexply M18/1/43%/G939/1230 five harness fabric pre-peg material system

Ply Layup : [(0/90)2/0/(90/0)2] configuration

Trigger Mechanism : 45º outside chamfer

Sketch of specimen Specimen affixed in clamp



Experimental Investigation – Strain rateCrushing process

Quasi-static

Dynamic



Experimental Investigation – Strain rate High-Resolution Computed Tomography (HRCT)

Non destructive analysis (NDT)

Nanotom® CT system by phoenix ׀ xray

Acquisition phase (2-D) to the reconstruction of the volumetric data (3-D) took ca. 5 hours

Voxel resolution of 7 microns

HRCT-scan image slices

Detailed analysis of cross section



Experimental Investigation – Strain rateHRCT-scan image of crushed coupon specimen

Quasi-static loading condition Dynamic loading condition



Experimental Investigation – Strain rateEnergy absorption performance parameters

Typical force-displacement curve(progressive failure mode)

Crush force efficiency (CE) : Peak Crush Force / SSCF

Energy Absorbed (EA) :Area under Crush Force-Displacement curve

Specific Energy Absorbed (SEA) :EA / Mass of crushed portion



Experimental Investigation – Strain rateEnergy absorption comparison



Experimental Investigation – Strain rateVaried crush response

Inter and intra molecular interaction during the load bearing phase

Quasi-static loading condition

Dynamic loading condition

Inter molecular interactions between polymer chains only due to time scale of the test

Difference attributed to the strain rate dependency of the epoxy resin :where the time scale of the load bearing phase dictates the extent of changes in the polymer chains

Brittle Behavior

Ductile Behavior



Experimental Investigation – Strain rateDynamic material properties (carbon-epoxy)

Tensile stress-strain curve Shear stress-strain curve

(K.S Raju et al. 2011)



Experimental Investigation – Strain rateSummary of varied crush responseGlobal splaying failure crush mode Local fragmentation failure crush mode



Experimental Investigation – Stacking sequenceTest specimen specifications and methodology

Material : ACG MTM44-1FR/468 four harness fabric pre-preg material system

Ply Layup : [(0/90) 2] s configuration[(+45) 2] s configuration

Test methodology : Dynamic crush test at an impact velocity of 10.0 m/s

Analysis methodology : As previously described



Experimental Investigation – Stacking sequenceCrushing process

Quasi-static

Dynamic



Experimental Investigation – Stacking sequenceEnergy absorption comparison



Conclusion

Successful experimental methodology implemented in testing and analysis of crush coupon specimens

Identification and detailed study of varied crushing response and failure modes (Global and Local) from increasing compression strain rates

Importance of understanding the role of strain rate dependency in the design of crashworthy structures highlighted from the work presented

Future outlook includes the adoption of the experimental methodology to study the crush response of crush coupons consisting of varied:

Dynamic loading ratesGeometriesMaterial systems



Acknowledgements

The authors would like to thank CRC-ACS for the supply of specimens, valuablediscussions and exchanges throughout the collaboration.

16th International Conference on Composite Structures (ICCS16)

Analysis of Crushing Response of Composite Crashworthy Structures

Matthew David and Alastair JohnsonDLR Institute of Structures and Design, Stuttgart

MDavid AJohnson ICCS 16

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