Automated counting of off-axis tunnelling cracks using digital image processing

An automated method for counting propagating matrix tunnelling cracks for use in mechanical testing of Glass Fibre Reinforced Plastic (GFRP) laminates under quasi-static and fatigue loading is presented. White light images are captured from specimens during the loading. The transmitted light is used to detect the cracks in the images, which are then processed to count the cracks as they develop and grow through the duration of the test. The reproducibility and accuracy of the image processing is demonstrated using simulated transverse crack densities and patterns. The methodology is demonstrated and validated experimentally using two different laminate stacking sequences of the type [0/??/0/ ?]s. The results related to the crack density evolution are shown to be consistent with results from the literature

Structural collapse of a wind turbine blade. Part A: static test and equivalent single layered models

The overall objective is a top-down approach to structural instability phenomena in wind turbine blades, which is used to identify the physics governing the ultimate strength of a generic wind turbine blade under a flap-wise static test. The work is concerned with the actual testing and the adoption of a phenomenological approach, and a discussion is conducted to assess and evaluate the wind turbine blade response during loading and after collapse by correlating experimental findings with numerical model predictions. The ultimate strength of the blade studied is governed by instability phenomena in the form of delamination and buckling. Interaction between both instability phenomena occurs causing a progressive collapse of the blade structure

Interlaminar/interfiber failure of unidirectional glass fiber reinforced composites used for wind turbine blades

A unidirectional glass fiber/epoxy composite was characterized under multi-axial loading by testing off-axis specimens under uniaxial tension and compression at various angles relative to the fiber direction. Iosipescu shear tests were performed with both symmetric and asymmetric specimens. Tests were performed on both 1-2 and 1-3 material coordinate planes. Strain gauges and Digital Image Correlation were used to record the stress–strain responses. A new approach was used to define a ‘failure initiation strength’ by analyzing the recorded stress–strain curves. The experimentally determined failure stresses were compared with the predictions of the maximum stress, Tsai-Wu and Northwestern University failure criteria. It was found that using the approach of analyzing the stress–strain curve to define a point of material failure initiation, it was possible to obtain good correlation between the experimental data and predictions by both the Tsai-Wu and the NU failure criteria

A stochastic multiaxial fatigue model for off-axis cracking in FRP laminates

A model to predict off-axis crack evolution in multidirectional laminates subjected to multiaxial loading is proposed. The model applies multi-scale stress and local fracture mechanics analyses to distinguish between two microscopic damage mechanisms, which govern the damage evolution, as well as determining the magnitude of the damage evolution rate associated with each microscopic damage mechanism. The multi-scale analysis is based on the GLOB-LOC off-axis crack damage model, which is extended to include substantial new capabilities such as the influence of crack face sliding interaction and prediction of variations in the stress field due to the local crack density. The extension of the GLOB-LOC model introduces physically based multiaxial fatigue criteria for off-axis crack initiation and mixed-mode off-axis crack propagation. The extended GLOB-LOC model is implemented and it is demonstrated that good predictions are obtained for the damage evolution under various multiaxial stress conditions. Furthermore it is shown that the model only requires input from two different multiaxial stress states to obtain the material parameters for a given FRP ply

Investigation of failure mechanisms in GFRP sandwich structures with face sheet wrinkle defects used for wind turbine blades

Wrinkle defects can be formed during the production of wind turbine blades consisting of composite monolithic and sandwich laminates. Earlier studies have shown that the in-plane compressive strength of a sandwich panel with wrinkle defects may decrease dramatically. This study focuses on the failure modes of sandwich specimens consisting of thick GFRP face sheets with a wrinkle defect and a balsa wood core subjected to in-plane compression loading. Three distinct modes of failure were found, and the strain distributions leading up to these failures were established by use of digital image correlation (DIC). Finite element analyses were subsequently conducted to model the response of the test specimens prior to failure, and generally a very good agreement was found with the DIC measurements, although slight differences between the predicted and measured strain fields were observed in the local strain values around the wrinkle defect. The Northwestern University (NU) failure criterion was applied to predict failure initiation, and a good correlation with the experimental observations was achieved

A damage-based model for mixed-mode crack propagation in composite laminates

A model for the off-axis crack propagation in laminated fibre reinforced polymer composites subjected to multiaxial fatigue loadings is presented. On the basis of several observations reported in the literature, the crack propagation phenomenon can be seen as the result of a series of micro-scale events occurring ahead of the crack tip within a process zone. The mixed mode loading condition defines the type of the micro-scale events which occur in the process zone and lead to fatigue crack propagation. Based on this evidence and by using a multiscale approach to determine the micro-scale stress fields in the matrix, two simple parameters are defined for predicting the crack growth rate through a Paris-like law. By extracting the proposed parameters from experimental data obtained from the literature, it is demonstrated that the crack propagation data are all included into two Paris-like scatter bands covering the whole mode-mixity range