Thermal Degradation of Polymeric Foam Cored Sandwich Structures

1. Fluid Structure Interactions Research Group Thermal Degradation of Polymeric Foam Cored Sandwich Structures S.Zhang1, J.M.Dulieu-Barton1 , R.K.Fruehmann1, and O.T.Thomsen2 1 Faculty of Engineering and the Environment; University of Southampton 2 Department of Mechanical and Manufacturing Engineering; University of Aalborg, Denmark sz2g09@soton.ac.uk • Background • Polymeric foam cored sandwich structures are being widely used in wind turbine (see Figure 1), naval, transportation and civil industries. • At elevated temperatures, the linearity, stability and failure mode/strength of sandwich structures could be altered due to the degradation of material mechanical properties together with the mismatch of thermal expansion as shown in Figure 2 [1]. • The thermo-mechanical interaction on sandwich structures is not well understood at present, especially in the viewpoint of experimental validation. Aims • Develop a methodology to obtain the elastic properties of polymeric foam materials at elevated temperatures using DIC (digital image correlation). • Construct an experimental system to characterise the thermo-mechanical interaction on polymeric cored sandwich structures; validate the behaviour predicted by the high-order sandwich panel theory [1]. Experimental apparatus • The experimental apparatus to study the thermo-mechanical behaviour of sandwich structures is shown in Figure 4. • Sandwich beam specimens were manufactured with a 25 mm thick Divinycell H100 PVC foam core and 0.9 mm thick aluminium face-sheets/0.8 mm thick E_glass/Epoxy composite face-sheets. The specimen length and width were 450 mm and 50 mm, respectively. • A three-point bending load was applied to the specimen. The ends of the specimen was constrained in three different manners: only constrain the vertical displacement of the bottom surface (BC1); constrain both vertical and horizontal displacements of the bottom surface (BC2); in addition to BC2, constrain the horizontal displacements of the top face-sheet (BC3). The full-field deformation of specimen was characterised using DIC. • An infra-red lamp was used to heat the top face-sheet of the sandwich beam specimen. The temperature distribution was monitored by an infra-red camera. A linear temperature gradient was achieved through the specimen thickness. Composite Figure 2: Behaviour of a sandwich beam at elevated temperatures [1] Figure 1: Application of sandwich structures on wind turbine blades Figure 4: Experimental apparatus to study the mechanical behaviour of sandwich structures with a thermal gradient through the specimen thickness Foam core Temperature dependence of foam material Results and discussion • Initially, the influence of temperature on the load-deflection behaviour was studied, as shown in Figure 4. The specimen was constrained in BC1 manner and subjected by a load of 1 kN at the mid-span. • The experimental result agrees well with the corresponding FE study. It was concluded that the overall bending stiffness reduces greatly with increasing temperature due to the stiffness loss of the core material. • Future work will focus on validating predictions of the non-linear geometrical deformation triggered by elevated temperatures. The influence of temperature on the failure mode and failure strength will also be investigated. • This work will form the first experimental characterisation of the thermo-interaction effects on sandwich structures. • A novel methodology based on DIC was developed to characterise the temperature dependence of the elastic properties (tensile, compressive, shear) of polymeric foam materials [2]. • This methodology was verified with good repeatability and can be applied for most polymeric foam core materials. • Figure 3 shows the thermal degradation of elastic properties of Divinycell H100 PVC foam. The normal and shear moduli reduce linearly with increasing temperatures from room temperature to 70ºC, and then significant non-linear reduction occurs. At 90ºC, the stiffness has reduced by over 50%. • The thermal degradation behaviour was found to depend only on the properties of the base polymer material rather than the cell structure. A master curve to describe the thermal degradation path of foam Divinycell H100 – H200 using a combination of a linear and second order polynomial was proposed: Figure 4: Mid-span deflection of a sandwich beam at different temperatures; (top: top face-sheet, bot: bottom face-sheet) Normalised modulus References: 1: Frostig, Y. and O.T. Thomsen, Non-linear thermal response of sandwich panels with a flexible core and temperature dependent mechanical properties. Composites Part B: Engineering, 2008. 39(1): p. 165-184. 2: Zhang, S., Dulieu-Barton, J., Fruehmann, R.K. and Thomsen, O.T., A Methodology for Obtaining Material Properties of Polymeric Foam at Elevated Temperatures. Experimental Mechanics, 2011. Online first: 1-13. T(°C) Figure 3: Normalised thermal degradation path of tensile, compressive and shear modulus of Divinycell H100 FSI Away Day 2012