Christian Berggreen

Application and Analysis of Sandwich Elements in the Primary Structure of Large Wind Turbine Blades

The present work studies the advantages of applying a sandwich construction as opposed to traditional single skin composites in the flanges of a load carrying spar in a future 180 m wind turbine rotor. A parametric finite element model is used to analyze two basic designs with single skin and sandwich flanges, respectively. Buckling is by far the governing criterion for the single skin design. Introducing a sandwich construction results in a globally more flexible structure making tower clearance the critical criterion. Significant weight reduction up to 22.3% and increased buckling capacity is obtained. Moreover, the study showed that proper choice of core material is important to prevent face wrinkling. Geometric nonlinear analysis showed sensitivity to imperfections. A consistent submodeling technique is presented for verifying the response from the global model in any section of interest.

Design Analysis of the Mixed Mode Bending Sandwich Specimen

A design analysis of the mixed mode bending (MMB) sandwich specimen for face—core interface fracture characterization is presented. An analysis of the competing failure modes in the foam cored sandwich specimens is performed in order to achieve face—core debond fracture prior to other failure modes. The analysis facilitates selection of the appropriate geometry for the MMB sandwich specimen to promote debond failure. An experimental study is performed using MMB sandwich specimens with a H100 PVC foam core and E-glass—polyester faces. The results reveal that debond propagation is successfully achieved for the chosen geometries and mixed mode loading conditions.

The Effect of Face Sheet Wrinkle Defects on the Strength of FRP Sandwich Structures

Wrinkle defects may reduce the compressive strength of a face laminate for in-plane loading applied perpendicularly to the line of the wrinkle. To be able to decide whether a repair is needed, it is necessary to know the magnitude of the strength reduction for a given wrinkle geometry. In the studies reported here, the influence of wrinkle defects on the in-plane compressive strength of quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates used in PVC foam-cored sandwich panels has been investigated by three approaches: testing of sandwich beam specimens in four-point bending, testing of sandwich panels with in-plane compression, and finite element simulation. Wrinkles involving different numbers of plies have been considered. Two different sandwich lay-ups typical of deck and hull bottom panels in naval ships have been included.

Fabrication, Testing and Analysis of Steel/Composite DLS Adhesive Joints

This paper describes experimental and numerical techniques to study the structural design of double lap shear joints that are based on thick-adherend steel/steel and steel/composite, with epoxy adhesive. A standard practical fabrication method was used to produce specimens of various dimensions and materials. These specimens consist of 10 mm steel inner adherend and various outer adherend materials including composite and steel of various thicknesses and overlaps. The composite is largely based on carbon fibre-reinforced plastic. The specimens were tested under monotonic tensile loading and the results showed that joint strength depends largely on materials combination and overlap length. The testing also included the use of an advanced imaging system to determine failure initiation and propagation. Two-dimensional finite element analysis (FEA) stress models were applied and showed the importance of modelling the composite layers adjacent to the adhesive bondline in order to account for the critical local stresses. The FEA results also showed that overall shear stress distributions can be used to characterise joint failure. The paper presents the experimental and numerical details with key conclusions.

Ultimate Failure of Debond Damaged Sandwich Panels Loaded with Lateral Pressure: An Experimental and Fracture Mechanics Study

In this study a tool for assessing residual strength of debond damaged laterally loaded sandwich panels is developed. The analysis tool consists of a parametric finite element model and a fracture mechanics calculation procedure to determine the residual strength. The parametric approach allows variation of all geometric and material entities. The fracture mechanics calculation uses crack flank displacements obtained from the finite element analysis solution and experimentally measured mixed-mode fracture toughness values to determine the ultimate failure load. The analysis tool is validated with a number of different ship type panels. Debond criticality is evaluated by using the developed tool and by comparing the test results from panel experiments. The comparison shows that the analysis tool predicts both failure load and failure mode well. The tool can be used to determine the residual strength of different damage cases and has a considerable potential for further development.

Reliability Analysis of a Composite Wind Turbine Blade Section Using the Model Correction Factor Method: Numerical Study and Validation

Reliability analysis of fiber-reinforced composite structures is a relatively unexplored field, and it is therefore expected that engineers and researchers trying to apply such an approach will meet certain challenges until more knowledge is accumulated. While doing the analyses included in the present paper, the authors have experienced some of the possible pitfalls on the way to complete a precise and robust reliability analysis for layered composites. Results showed that in order to obtain accurate reliability estimates it is necessary to account for the various failure modes described by the composite failure criteria. Each failure mode has been considered in a separate component reliability analysis, followed by a system analysis which gives the total probability of failure of the structure. The Model Correction Factor method used in connection with FORM (First-Order Reliability Method) proved to be a fast and efficient way to calculate the reliability index of a complex composite structure.

A Modified TSD Specimen for Fracture Toughness Characterization - Fracture Mechanics Analysis and Design

The tilted sandwich debond (TSD) specimen has been recognized as a viable candidate for characterization of the face/core fracture resistance. Analysis, however, shows that the range of phase angles that can be realized by altering the tilt angle and other parameters of the test is quite limited. A method to extend the range of mode-mixities of the TSD specimen is to introduce a larger amount of transverse shear by reinforcing the loaded upper face with a stiff metal plate. Analysis shows that this method extends the range of phase angles to a practically useful range. Guidelines on selection of thicknesses of the reinforcement, and design considerations for further modifications are provided.

Energy-Release Rate and Mode Mixity of Face/Core Debonds in Sandwich Beams

Closed-form algebraic expressions for the energy-release rate and the mode mixity are obtained for a debonded sandwich (trimaterial). The most general case of an “asymmetric” sandwich is considered (i.e., the bottom face sheet not necessarily of the same material or thickness as the top face sheet). The energy-release rate is obtained by use of the J-integral, and the expression is derived in terms of the forces and moments at the debond section. Regarding the mode mixity, a closed-form expression is derived in terms of the geometry, material, and applied loading, and it is proven that, in the trimaterial case, just as in the bimaterial case, the mode mixity can be obtained in terms of a single scalar quantity ω, which is independent of loading; the ω value for a particular geometry and material can be extracted from a numerical solution for one loading combination. Thus, this analysis extends the existing formulas in the literature, which are for either a delamination in a homogeneous composite or an int...

Bend-twist coupling potential of wind turbine blades

In the present study an evaluation of the potential for bend-twist coupling effects in wind turbine blades is addressed. A method for evaluation of the coupling magnitude based on the results of finite element modeling and full-field displacement measurements obtained by experiments is developed and tested on small-scale coupled composite beams. In the proposed method the coupling coefficient for a generic beam is introduced based on the Euler-Bernoulli beam formulation. By applying the developed method for analysis of a commercial wind turbine blade structure it is demonstrated that a bend-twist coupling magnitude of up to 0.2 is feasible to achieve in the baseline blade structure made of glass-fiber reinforced plastics. Further, by substituting the glass-fibers with carbon-fibers the coupling effect can be increased to 0.4. Additionally, the effect of introduction of bend-twist coupling into a blade on such important blade structural properties as bending and torsional stiffness is demonstrated.

Residual Strength of In-Plane Loaded Debonded Sandwich Panels: Fracture Mechanical Modelling

This paper presents a FEM based numerical model for prediction of residual strength of damaged sandwich panels. As demonstrated, the model can predict the maximum load carrying capacity of real-life panels with debond damages, where the failure is governed by face-sheet buckling followed by debond growth. Comparison of the theoretical predictions is carried out against a series of large-scale experiments described in Lundsgaard-Larsen et al. [1].