MEASUREMENT OF MECHANICAL PROPERTIES OF PVC FOAM USING A MODIFIED ARCAN FIXTURE

1. 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 EcolePolytechniqueFédérale de Lausanne (EPFL), Switzerland February 14-16, 2011

2. 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

3. 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.

4. 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)

5. A new bidirectional fixture Classical Arcan fixture with circular distribution of griping holes New fixture with spiral distribution of griping holes

6. Bidirectional Material Test Fixture (Patent No: PA 201100050) Quasi-spiral passed griping holes Fixture arm Butterfly shape specimen Metallic base bounded to foam specimen

7. Specimens for MAF fixture BS Shear & SD tensile BL compressive bidirectional Note: Thickness of all specimens is 15 mm

8. DIC system and setup Load cell Light CCD camera CCD camera • 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

9. DIC setup for modified Arcan fixture (MAF)

10. Tensile SD test results using DIC (raw data) Smoothing technique: Robust local regression using polynomial model (MATLAB)

11. DIC (Aramis) problem with large shear strain measurement Facet size: 60 pixels Steps: 30 pixels “Solution” techniques: Using a new pattern Dividing the images to two groups for analysis

12. New technique for pattern generation Classical technique New technique • Making white background surface (here using zinc oxide powder) • Spraying black speckles on white background Facets and overlap 12 Smearing black ink onto the surface Spreading white powder (zinc oxide) onto the surface Cleaning top of the surface to visualize cell walls

13. Dividing the images in two analyses New pattern DIC results up to 70% of failure strain (Stage I) Image 1 Image 60 Image 80 Image 100 • Last 30% of analysis (Stage II) Image 120 Image 140 • Facet:60 pixels • Step: 30 pixels • No smoothing applied to results

14. BS shear stress-strain response • 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 Stage I (new pattern) Stage II Old Pattern Smoothing technique: Robust local regression using polynomial model (MATLAB)

15. Different shear specimens(Butterfly shaped - BS) Fracture initiates at gauge section • Radius 6.67 mm • Radius 4.5 mm • Radius 2.5 mm

16. Correction factors for measured surface strains DIC camera a a Gauge section Gauge section Y Z X a a

17. Bilinear material approximation for nonlinear FEA Gauge line Gauge line (Strain%) (True strain)

18. Nonlinear FE modelling for shear test Gauge section shear strain Gauge line shear strain • 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)

19. Possible convergence problem? Nonlinear correction factors after FEA iterations

20. Shear and tensile stress-strain behaviour of H100 PVC foam after “corrections” FEA “corrected” stress-strain curves Experimental shear stress-strain data and “corrected” curve for H100 foam based on nonlinear FE analysis

21. MAF measurement of orthotropic properties – Divinycell H100 (cross-linked PVC foam) • * 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

22. 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

23. Ongoing and future work • Compressive testing • Thermal degradation measurements in thermal chamber using DIC • Nonlinar material modeling using ABACUS • Bidirectional shear-axial testing

24. 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.

25. Thank youQ & A