Department of Mechanical and Manufacturing Engineering MEASUREMENT OF MECHANICAL PROPERTIES OF PVC FOAM USING A MODIFIED ARCAN FIXTURE S T Taher 1, O T Thomsen 1, J M Dulieu-Barton 2, S Zhang 2 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

Department of Mechanical and Manufacturing Engineering 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 2

Department of Mechanical and Manufacturing Engineering 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. 3

Department of Mechanical and Manufacturing Engineering 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) 4

Department of Mechanical and Manufacturing Engineering A new bidirectional fixture Classical Arcan fixture with circular distribution of griping holes New fixture with spiral distribution of griping holes 5

Department of Mechanical and Manufacturing Engineering Bidirectional Material Test Fixture (Patent No: PA ) Quasi-spiral passed griping holes Fixture arm Butterfly shape specimen Metallic base bounded to foam specimen 6

Department of Mechanical and Manufacturing Engineering Specimens for MAF fixture 7 BS Shear & SD tensile BL compressive bidirectional Note: Thickness of all specimens is 15 mm

Department of Mechanical and Manufacturing Engineering 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 8 CCD camera CCD camera Load cell Light

Department of Mechanical and Manufacturing Engineering DIC setup for modified Arcan fixture (MAF) 9

Department of Mechanical and Manufacturing Engineering Tensile SD test results using DIC (raw data) 10 Smoothing technique: Robust local regression using polynomial model (MATLAB)

Department of Mechanical and Manufacturing Engineering DIC (Aramis) problem with large shear strain measurement 11 Facet size: 60 pixels Steps: 30 pixels “Solution” techniques: 1.Using a new pattern 2.Dividing the images to two groups for analysis

Department of Mechanical and Manufacturing Engineering 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 12 Classical technique New technique Facets and overlap 1.Making white background surface (here using zinc oxide powder) 2.Spraying black speckles on white background

Department of Mechanical and Manufacturing Engineering New pattern DIC results up to 70% of failure strain (Stage I) 13 Last 30% of analysis (Stage II) Facet:60 pixels Step: 3 0 pixels No smoothing applied to results Image 1 Image 60 Image 80 Image 100 Image 120 Image 140 Dividing the images in two analyses

Department of Mechanical and Manufacturing Engineering BS shear stress-strain response 14 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

Department of Mechanical and Manufacturing Engineering Different shear specimens (Butterfly shaped - BS) Radius 6.67 mm Radius 4.5 mm Radius 2.5 mm 15 Fracture initiates at gauge section

Department of Mechanical and Manufacturing Engineering Correction factors for measured surface strains 16 a a a a Y Z X Gauge section DIC camera

Department of Mechanical and Manufacturing Engineering Bilinear material approximation for nonlinear FEA Gauge line 17 Gauge line (True strain) (Strain%)

Department of Mechanical and Manufacturing Engineering Nonlinear FE modelling for shear test Gauge line shear strain Gauge section shear strain 18 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)

Department of Mechanical and Manufacturing Engineering 19 Nonlinear correction factors after FEA iterations Possible convergence problem?

Department of Mechanical and Manufacturing Engineering 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” 20

Department of Mechanical and Manufacturing Engineering MAF measurement of orthotropic properties – Divinycell H100 (cross-linked PVC foam) 21 E 1 * (MPa)E 2 (MPa)ν 12 G 12 (MPa)σ 1max (MPa)σ 2max (MPa)τ 12max (MPa)ε 1max (%)ε 2max (%)ε 12max MAF ± ± ± ± ± ± ± ± ± ±0.01 UoS** ± ± ± ±0.18N.A. DIAB***130N.A N.A.1.6N.A * 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

Department of Mechanical and Manufacturing Engineering 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 22

Department of Mechanical and Manufacturing Engineering Ongoing and future work Compressive testing Thermal degradation measurements in thermal chamber using DIC Nonlinar material modeling using ABACUS Bidirectional shear-axial testing 23

Department of Mechanical and Manufacturing Engineering ACKNOWLEDGEMENT The work presented has been co-sponsored by the Danish Council for Independent Research Technology and Production Sciences (FTP), Grant Agreement , “Thermal Degradation of Polymer Foam Cored Sandwich Structures”, and the US Navy, Office of Naval Research (ONR), Grant Award N , The ONR program manager was Dr. Yapa D. S. Rajapakse. The financial support received is gratefully acknowledged. 24

Department of Mechanical and Manufacturing Engineering Thank you Q & A 25