Composite materials in wind turbines are mainly joined with adhesives. Adhesive joining is preferable since it distributes the stresses over a larger area. This study shows how a defect can influence the fracture behaviour of adhesively joined composite. Repeated experiments are performed using double cantilever beam specimens loaded with bending moments. The specimens consist of two 8 mm thick GFRP-laminates which are joined by a 3 mm thick epoxy adhesive. A thin foil close to one of the laminates is used to start the crack. For some of the specimens a defect is created by an initial load-unload operation. During this operation, a clamp is used in order to prevent crack propagation in the main direction. For the specimens without defect, the crack propagates in the middle of the adhesive layer. For the specimens with defect, the crack directly deviates into the laminate. After about 25 mm propagation in the laminate, the crack returns to the adhesive. Compared to the adhesive the fracture energy for the laminate is significantly higher.

The paper proposes a methodology for reliable design and maintenance of wind turbine rotor blades using a condition monitoring approach and a damage tolerance index coupling the material and structure. By improving the understanding of material properties that control damage propagation it will be possible to combine damage tolerant structural design, monitoring systems, inspection techniques and modelling to manage the life cycle of the structures. This will allow an efficient operation of the wind turbine in terms of load alleviation, limited maintenance and repair leading to a more effective exploitation of offshore wind.

Different methods are commonly used for non-destructive testing in structures; among others, acoustic emission and ultrasonic inspections are widely used to assess structures. The research presented in this paper is motivated by the need to improve the inspection capabilities and reliability of structural health monitoring (SHM) systems based on ultrasonic guided waves with focus on the acoustic emission and acousto-ultrasonics techniques. The use of a guided wave based approach is driven by the fact that these waves are able to propagate over relatively long distances, and interact sensitively and uniquely with different types of defect. Special attention is paid here to the development of efficient SHM methodologies. This requires robust signal processing techniques for the correct interpretation of the complex ultrasonic waves. Therefore, a variety of existing algorithms for signal processing and pattern recognition are evaluated and integrated into the different proposed methodologies. As a contribution to solve the problem, this paper presents results in damage detection and classification using a methodology based on hierarchical nonlinear principal component analysis, square prediction measurements and self-organizing maps, which are applied to data from acoustic emission tests and acousto-ultrasonic inspections. At the end, the efficiency of these methodologies is experimentally evaluated in diverse anisotropic composite structures.

Three aluminium materials Al 1050, Al 5754, Al 7075 and pure Ti have been characterized by microscopy and mechanical testing for potential use under ballistic testing. Al 1050 and Al 5754 have been examined in the annealed state and after deformation to an ultrahigh strain by accumulative roll bonding and cold rolling, respectively, reducing the structural scale to 200-300 nm. The ballistic performance of 1 mm thick sheets has been estimated as a decrease in projectile kinetic energy showing a significant improvement in Al 1050 with the structural refinement, whereas an opposite effect has been found in Al 5754. An evaluation of the ballistic performances based on area weight showed a comparable behaviour of Al 1050 in the deformed state, Al 5754 and Ti, whereas superior performance characterized Al 7075.

As part of the Network of Excellence on Marine Structures (MARSTRUCT), a series of studies has been carried out into the buckling of glass-fibre-reinforced polymer plates with in-plane compression loading. The studies have included fabrication and testing of square, laminated panels with various thicknesses and initial geometrical imperfections, material testing, advanced finite element modelling studies and finally parametric studies covering a range of slendernesses and imperfection amplitudes. The paper provides an overview of the studies, which involved several participants in the Network.

Sandwich structures with thin, strong skin layers on both sides of a thick, compliant and light core combine low weight, high bending stiffness, high strength and high buckling resistance. These structures find use in many applications such as aircraft, marine applications and wind turbine blades. When used for wind turbine blades the sandwich structures are mainly made from FRP laminate skins and foam or balsa core. The shear properties of such sandwich shells are important design parameters. The shear stiffness of the core influences the deflections of the shell and the shear strength of the core may be limiting for the bending strength of the sandwich.