Mechanical Characterization of PVC Foam Using Digital Image Correlation and Nonlinear FE Analysis

olymer foam cored sandwich structures are often subjected to aggressive service conditions which may include elevated temperatures. The mechanical properties of polymer foam cores degrade significantly with elevated temperatures, and significant changes in the properties may occur well within the operating range of temperatures. The material properties of foam cored sandwich structures depend on the temperature field imposed, and this is usually ignored in engineering analysis and design. As an example, the thermal degradation problem for wind turbine blades is especially associated with the use of polymer foam cores in the wing shells when these are exposed to high temperatures. This occurs most severely under hot climate conditions, but can also occur in temperate climates. An example would be very high gusting winds increasing on a warm/hot summer day, for instance due to the development of a thunder storm.

Furthermore sandwich core materials may experience multidirectional mechanical stress states. In a conventional sandwich panel the in-plane and bending loads are carried by the face sheets, while the core resists the transverse shear loads. A well known failure mode of such sandwich panels is ‘core shear failure’ in which the core fails due shear stress overloading. However, although the shear stress is often the main core stress, there are conditions in which the normal stresses in the core are of comparable size or even higher than the shear stresses. Such conditions may occur in the vicinity of concentrated loads or supports and also in the vicinity of geometrical and material discontinuities. Under such condition a material element in the core is subjected to a multidirectional state of stress. Therefore, proper design of sandwich structures requires the characterization of the core material response under multi-directional stress states.
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 [1]. A modified Arcan fixture (MAF) has been developed to characterize polymer foam materials with respect to their tensile, compressive, shear and bidirectional mechanical properties at room and at elevated temperatures, and including the elastic coefficients and the stress-strain response to failure. The MAF enables the realization of pure compression or high compression to shear bidirectional loading conditions that are not possible with conventional Arcan fixtures. The MAF is attached to a standard universal test machine equiped with an environmental chamber using specially designed grips that do not constrain the specimen rotation, and hence reduces paristic effects due to misalignment.