Elsevier

Composites Part B: Engineering

Volume 111, 15 February 2017, Pages 134-142
Composites Part B: Engineering

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

https://doi.org/10.1016/j.compositesb.2016.11.044Get rights and content

Abstract

The use of composite materials is on the rise in different engineering fields. Following this trend the wind turbine industry has adopted composites as their primary material of choice. For wind turbine blades having large unsupported functional aerodynamic surfaces; the structural stiffness is very important. Stiffness is required to keep the deformations to a minimum under aerodynamic forces. The blade is thus stiffened using sandwich structures at high strain locations within the structure. The lightweight foam cored sandwiches though add stiffness, at the same time pose a challenge for design as the difference in stiffness of both the face-plate and the foam core is very high. The resistance to fracture in any part of the structure is an important design parameter to be determined. The determination of fracture resistance quantified here as the Strain Energy Release Rate (SERR) poses some unique challenges when dealing with highly heterogeneous materials in terms of stiffness. In this study some approaches have been analyzed while others are developed to tackle this problem and to measure the Mode I & II SERR of the face-plate foam-core interface. The sandwich core varies in both thickness and density depending on the loading and thus the location along the blade length. However for this study we have used a single density of foam core for the most part of the turbine blade. Different thicknesses of the foam cores are used to determine the effect of scale on the calculated SERR.

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.

References (29)

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