Post on 19-Dec-2015
Department of Mechanical and Manufacturing Engineering
MEASUREMENT OF MECHANICAL PROPERTIES OF PVC FOAM USING A
MODIFIED ARCAN FIXTURE
S T Taher1, O T Thomsen1, J M Dulieu-Barton2, S Zhang2
1 Department of Mechanical and Manufacturing Engineering, Aalborg University, Denmark
2 School of Engineering Sciences, University of Southampton, UK
5th International Conference on Composites Testing and Model Identification
Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
February 14-16, 2011
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Outline• Background and key-methods• Modified Arcan fixture (MAF)• Digital image correlation (DIC) setup• Tensile and shear testing• Nonlinear finite element analysis (FEA)• Conclusions • Ongoing and future work• Acknowledgement
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Background
• Polymer foam cored sandwich structures are often subjected to aggressive service conditions which may include elevated temperatures.
• Previously, the Arcan test rig has been used to measure bidirectional properties of polymer foams used for sandwich core materials, especially in the bidirectional tensile-shear stress region.
• A modified Arcan fixture (MAF) has been developed to characterize polymer foam materials with respect to their tensile, compressive, shear and bidirectional mechanical properties. In presented work tensile and shear properties obtained using short Dogbone (SD) and Butterfly Shape (BS) specimens.
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Key methods
• Measurement of mechanical properties of selected polymer foam core materials (with focus on closed cell PVC) subjected to tension, compression and shear loading using a modified Arcan fixture (MAF).
• A full field technique is used for non-contact measurement of the specimen deformations - Digital Image Correlation (DIC)
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A new bidirectional fixture
Classical Arcan fixture with circular distribution of griping holes
New fixture with spiral distribution of griping holes
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Bidirectional Material Test Fixture (Patent No: PA 201100050)
Quasi-spiral passed griping holes
Fixture arm
Butterfly shape specimen
Metallic base bounded to foam specimen
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Specimens for MAF fixture
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BS Shear & SD tensile BL compressive
bidirectional
Note: Thickness of all specimens is 15 mm
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DIC system and setup
System: • ARAMIS 4 M (GOM
GmbH)• Lenses: 50 mm (Family
C)• Resolution: 2048x2048
pixels• Strain accuracy: up to
0.01 % (ARAMIS hardware manual)
Setup:• 2D measurement• Measurement on both
sides of specimen• Synchronized with two
CCD cameras and load cell data
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CCDcamera
CCD camera
Load cell
Light
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DIC setup for modified Arcan fixture (MAF)
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Tensile SD test results using DIC (raw data)
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Smoothing technique: Robust local regression using polynomial model (MATLAB)
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DIC (Aramis) problem with large shear strain measurement
Facet size: 60 pixels Steps: 30 pixels
“Solution” techniques:
1. Using a new pattern
2. Dividing the images to two groups for analysis
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New technique for pattern generation
1. Smearing black ink onto the surface
2. Spreading white powder (zinc oxide) onto the surface
3. Cleaning top of the surface to visualize cell walls
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Classical technique New technique
Facets and overlap
1. Making white background surface (here using zinc oxide powder)
2. Spraying black speckles on white background
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New pattern DIC results up to 70% of failure strain (Stage I)
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Last 30% of analysis (Stage II)
• Facet:60 pixels• Step: 30 pixels• No smoothing applied to
results
Image 1 Image 60 Image 80 Image 100
Image 120 Image 140
Dividing the images in two analyses
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BS shear stress-strain response
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Smoothing technique: Robust local regression using polynomial model (MATLAB)
Stage I (new pattern) Stage II
Old Pattern
• DIC correlation lost around 40% of failure strain when using the first image as a reference for the analysis of all images (ARAMIS software)
• DIC correlation improved up to 70% of failure strain using new pattern (stage I)
• Rest of curve (stage II) computed in a new analysis using 70% strain image as the new reference image for the image correlation
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1 2 3 4 5 6 7 80
1
2
3
4
5
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Radius (mm)
Failure
str
ength
(M
Pa)
Different shear specimens(Butterfly shaped - BS)• Radius 6.67 mm• Radius 4.5 mm• Radius 2.5 mm
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Fracture initiates at gauge section
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Correction factors for measured surface strains
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a
aa
a
Y
Z X
Gauge
section Gauge
section
DIC
camera
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Bilinear material approximation for nonlinear FEA
Gauge line
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Gauge line
(True strain) (Strain%)
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20
1
2
3
4
5
6Through-thickness tensile
In-plane tensile
Shear
Bilinear through-thickness
Bilinear in-plane
Bilinear shear
Strain (mm/mm)
Str
ess (
MP
a)
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Nonlinear FE modelling for shear test
Gauge line shear strain
Gauge section shear strain
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• ANSYS 12.1• Nonlinear material model – bilinear at present
(sequentially linear in the future)• Large deformations• Element type: solid186 (higher order solid element)• Number of nodes: 35k
(True strain)
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Nonlinear correction factors after FEA iterations
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.181.00
1.02
1.04
1.06
1.08
1.10
1.12
1.14
Corection Factor (shear test)
Strain
Corr
ecti
on F
acto
r
Possible convergence problem?
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FEA “corrected” stress-strain curves
Experimental shear stress-strain data and “corrected” curve for H100 foam based on nonlinear FE analysis
Shear and tensile stress-strain behaviour of H100 PVC foam after “corrections”
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0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Shear data
Corrected curve
Shear strain (mm/mm)
Str
ess (
MP
a)
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MAF measurement of orthotropic properties – Divinycell H100 (cross-linked PVC foam)
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E1* (MPa) E2(MPa) ν12 G12(MPa) σ1max(MPa) σ2max(MPa) τ12max(MPa) ε1max (%) ε2max (%) ε12max
MAF 130.51 ±1.38
59.20±0.61 0.40±0.02 32.53±0.58 4.0±0.15 2.27±0.03 1.44±0.02 0.073±0.01 0.177±0.02 0.15±0.01
UoS** 132.78 ±0.88
58.70±0.89 0.41±0.01 30.12±0.18 N.A. N.A. N.A. N.A. N.A. N.A.
DIAB*** 130 N.A. N.A. 35 3.5 N.A. 1.6 N.A. N.A. 0.20
* Indices 1 and 2 represent the through-thickness and in-plane directions, respectively.** linear elastic properties measured using DIC at the University of Southampton*** Standard test data by DIAB
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Conclusions
• H100 Divinycell cross linked PVC foam show significant orthotropic material behaviour.
• BS specimen with smallest radius (2.5 mm) in shear test failed at gauge section and was selected as a reference shape for shear test by MAF.
• Nonlinear FEA was used to correct measured surface strains to obtain “corrected” stress-strain data. As expected, the strain correction factor obtained displayed it highest values in the linear region of the foam material.
• There is a good agreement between the material properties by standard tests data and the MAF data
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Ongoing and future work
• Compressive testing
• Thermal degradation measurements in thermal chamber using DIC
• Nonlinar material modeling using ABACUS
• Bidirectional shear-axial testing
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ACKNOWLEDGEMENT
The work presented has been co-sponsored by the Danish Council for Independent Research Technology and Production Sciences (FTP), Grant Agreement 274-08-0488, “Thermal Degradation of Polymer Foam Cored Sandwich Structures”, and the US Navy, Office of Naval Research (ONR), Grant Award N000140710227, The ONR program manager was Dr. Yapa D. S. Rajapakse. The financial support received is gratefully acknowledged.
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Thank you
Q & A
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