Determination of mode I & II strain energy release rates in composite foam core sandwiches. An experimental study of the composite foam core interfacial fracture resistance
Introduction
Composite materials are at the forefront of innovative materials for the manufacture of all sorts of mechanical components. From aeronautics to automotive to the energy sector, their usage is ever increasing. Wind turbine blades used for the production of wind energy are no exception. Currently almost a 100% of wind turbine blade manufacturers (medium to large size) manufacture them with composite materials. The Blade structures also use sandwich panels extensively to promote stiffness and to save weight [1], Fig. 1.
The delamination and de-bonding of composite structures is one of the main sources of failure [3] [4] [5] [6]. During service the composite components undergo solicitations which can be normal and parallel to the interface plane. These solicitations cause transverse and in plane stresses. These stresses therefore, when attain a certain value can cause the initiation of cracks. These cracks usually originate at internal defects present within the structure or singularities due to stress concentrations as a result of design. Furthermore the internal structure such as ply stacking and fiber configurations such as weaved mats can be a source of initiation as well [7], in addition to complex loading patterns [8] [9].
The most common type of failure in composites is that of delamination and failure at interfaces of materials with different stiffness in sandwich structures [10] [11]. This is usually due to the fact that the matrix and foam core is a lot weaker in strength and stiffness than the fibers hence under the applied loads they tend to fail first. The same in tension; matrix failures apply to in-plane compression; where the load is applied in the plane of the plies. Kink band formations cause tensile stresses in the matrix thus causing it to fail, eventually giving rise to damage nuclei [12] [13].
This paper is an attempt to compare the different approaches to calculate SERR's for sandwich structures. Therefore in this paper a number of analytical approaches are used to calculate the SERR's, although cohesive models and their different variants are also used for calculation of SERR's [14]. The various studies show an effect of the shear stiffness of adhesive layers on the measured SERR's thus rendering the effects of thickness an important design parameter [15].
Section snippets
Materials tested
The materials used in this study are the ones taken directly used in a real wind turbine blade. They constitute of a 45 ° Bi-axial fiberglass mat of 0.286 mm thickness in a Polyester resin matrix. The core material is a 80 kg/m3 density PVC foam. The specimen geometries are shown in the corresponding sections.
The composite has been prepared using hand layup and cured under vacuum bagging under atmospheric pressure at room temperature. The composite mechanical properties are given in Table 1 and
Experimental setup: mode I opening mode
The specimens used for the characterization of the sandwich structure core – skin interfaces are shown schematically in Fig. 2. The sandwich specimens are made up of a 45 ° Biax type composite face plate joined to a PVC foam core of 80 kg/m3 density in 10, 20 and 30 mm thicknesses. The face plates for all of the specimens are 2 mm thick.
Fig. 2 shows the different DCB specimens used for the characterization of interfaces. The interfaces in question are between the foam core and face plates for
Conclusion
The SERR as seen from the results varies with the thickness of the core in the sandwich structures. However there is no linear relation between the SERR and the core thickness. The variation in the SERR can be attributed to the resin penetration in the core-face interface. This penetration of resin modifies the stiffness of the core material and hence the behavior of the interface. This effect is more evident in Mode II SERR where the shearing rate depends not only on the interfacial stiffness
Acknowledgments
This research work has been funded as a part of the WINFLO Project, by the Region of Brittany, France.
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