Lars Pilgaard Mikkelsen

Technical University of Denmark | DTU · Department of Wind Energy

This dataset includes four large field-of-view scanning electron microscopy (SEM) images together with associated Matlab scripts aimed for the analysis used in the joint publication. Each of the four stitched images is generated from a large number (between 15500 and 24500) high-resolution (195nm/pixel) scans, which have been stitched into four ima...

This work presents a methodology to evaluate the effect of wrinkle defects in the stiffness response of laminate characteristic of wind turbine blades. The assessment is carried out through numerical models and experimental tests with coupon specimens embedded with artificial wrinkles. Specimens are manufactured with two types of defects, prone to...

Microstructural effects on the energy release rate and the mode mixity for tunneling cracks are investigated. A detailed microstructure is based on scanning electron microscopy of a real laminate. Based on a newly developed two-dimensional off-axis finite element formulation, the dependency of the Mode I, Mode II, and Mode III energy release rates...

A periodic 2D finite element model is proposed to identify the axial and transverse stiffness degradation for arbitrarily oriented parallel tunneling cracks. This is achieved with a recently developed off-axis framework taking the 3D deformation into account via a special kinematic formulation. The proposed model is successfully validated against a...

This work presents observations of fatigue damage in a quasi-unidirectional polymer reinforced composite made from basalt fibre non-crimp fabric and epoxy. Through observations over large areas, the study provides quantitative observations of the damage caused by the cyclic bending loads, with focus on the damage in the tension-tension loaded regio...

This paper presents an analytical model that quantifies the stress ratio between two test specimens for the same probability of failure based on the Weibull weakest link theory. The model takes into account the test specimen geometry, i.e., its shape and volume, and the related non-constant stress state along the specimen. The proposed model is a v...

This work presents a methodology to evaluate the effect of wrinkles defects in the stiffness response of laminates characteristic of wind turbine blades. The assessment is carried out through numerical models and experimental tests with coupon specimens embedded with artificial wrinkles. Specimens are manufactured with two types of defects, prone t...

A nondestructive probe for mapping elastic strains within polymers would be an asset for materials engineering: for validation of mechanical models and for understanding damage nucleation. As a step towards this aim we demonstrate an X-ray wide angle diffraction methodology probing strain tensor components on mesoscopic length scales, in the range...

Microscale fatigue damage evolution of off-axis tunnelling cracks was investigated for non-crimp fabric composites used in wind turbine blades. Test specimens fabricated from a multidirectional laminate of the layup sequence [0b/60b/0b/−60b]s (with subscript ’b’ representing backing) with glass fibres and epoxy resin, were subjected to cyclic tensi...

Among the manufacturing defects of wind turbine blades, wrinkles are critical to the blade performance, affecting the structural integrity. Therefore, this paper introduces three-dimensional numerical models of laminates representative of a wind turbine blade with artificial wrinkles. The models are verified against static tension-compression exper...

Off-axis oriented tunnel cracking in a laminated structure is modeled using a special 2D off-axis finite element formulation, thus replacing full 3D finite element simulations with much less demanding 2D simulations. The 2D off-axis element is formulated as a user element in the commercial finite element code ABAQUS, and the user element code is ma...

X-ray computed tomography scans can provide detailed information about the state of the material after manufacture and in service. X-ray computed tomography aided engineering (XAE) was recently introduced as an automated process to transfer 3D image data to finite element models. The implementation of a structure tensor code for material orientatio...

Accurate modelling of fibre-reinforced composites requires anisotropic material models. Structure tensor analysis of X-ray 3D images has been shown to provide fast and robust estimation of local structural orientations in fibre-reinforced composites. We present two mapping algorithms which can be used to map estimated local orientations onto finite...

This study presents a holistic segmentation procedure, which can be used to obtain individual fibre inclination angles from X-ray computed tomography. The segmentation approach is based on principal component analysis and was successfully applied for a unidirectional and an air-textured glass fibre–reinforced composite profile. The inclination resu...

This paper presents an experimental method for tensile testing of unidirectional carbon fibre composites. It uses a novel combination of a new specimen geometry, protective layer, and a robust data analysis method. The experiments were designed to test and analyze unprotected (with conventional end-tabs) and protected (with continuous end-tabs) car...

Important properties of fiber-reinforced composites, such stiffness, compression strength, and fatigue resistance, are sensitive to fiber alignment. In this paper, we use structure tensor analysis on CT images to characterize the fiber orientations in three samples of unidirectional fiber-reinforced composites: pultruded carbon, pre-preg carbon, an...

A novel approach is presented on how the growth of off-axis tunnelling cracks in a composite laminate subjected to biaxial loads can be studied on an equivalent laminate with an another off-axis angle under uniaxial load. The primary objective is to determine the off-axis angle of the equivalent laminate such that the mode mixity of tunnelling crac...

The fatigue damage evolution depends on the local fiber volume fraction as observed in the co-submitted publication [1]. Conventionally, fiber volume fractions are determined as an averaged overall fiber volume fraction determined from small cuts of the laminate. Alternatively, automatically stitching of scanning electron microscopy (SEM) images ca...

A micromechanical model is developed for the prediction of the growth rate of fatigue damage zones in unidirectional fibre composites subjected to cyclic tension-tension loading. The fatigue damage occurs by progressive fibre breaks caused by the debond crack tip stress fields of fibres that have broken in earlier load cycles. The debond crack tips...

The steady-state energy release rate for tunneling cracks under mixed-mode loading is determined using finite element analyses. The balanced and symmetric laminate layup [0/θ/0/ − θ ] s is investigated, where the tunneling crack is located parallel to the fiber direction of the central off-axis oriented layer. It is found that for the steady-state...

This data in brief article describes a dataset used for an X-ray computer tomography aided engineering process consisting of X-ray computer tomography data and finite element models of non-crimp fabric glass fibre reinforced composites. Additional scanning electron microscope images are provided for the validation of the fibre volume fraction. The...

The mechanical properties of composite fiber materials are highly dependent on the orientation of the fibers. Micro-CT enables acquisition of high-resolution 3D images, where individual fibers are visible. However, manually extracting orientation information from the samples is impractical due to the size of the 3D images. In this paper, we use a S...

In this study two automated segmentation methodologies of an X-ray computer tomography based numerical analysis are compared. These are then assessed based on their influence on the stress distribution results of finite element models of glass fibre reinforced composites made out of non-crimp fabrics. Non-crimp fabrics reinforced composites are com...

In this study a complete procedure is presented of how to generate finite element models based on X-ray computer tomography data on the fibre bundle scale for non-crimp fabric reinforced composites. Non-crimp fabric reinforced composites are nowadays extensively used in the load carrying parts of wind turbine blades. Finite element analysis based o...

This research is focused on studying the tension-tension fatigue behaviour of a unidirectional (UD) glass-fibre wind turbine composite. The damage features, their progression and the associated strain fields are tracked in a representative volume by employing a novel correlative approach bringing together x-ray computed tomography (XCT) and digital...

The primary objective of this work is to demonstrate how the condition for biaxially loaded off-axis tunnelling crack growth can be reproduced in a modified laminate under uniaxial loading. More specifically, laminates with the stacking sequences [0/θ/0/−θ]s and [0/α/0/−α]s have been considered for the biaxial and uniaxial loadings, respectively. A...

Accurate characterization of fibres is crucial for the understanding the properties and behaviour of fibre-reinforced composite materials. Fibre properties are key parameters for composite design, modelling and analysis. In this study, characterization of mechanical properties of glass and carbon fibres has been performed using a semi-automated sin...

Load carrying components of modern wind turbine blades are manufactured from composites, consisting of non-crimp fabrics infused with polymer resins. The effective stiffness of the resulting laminate is a combination of the properties of its building blocks i.e. fibers, and matrix as well as from the fabric texture imperfections e.g. fiber undulati...

Load carrying components of modern wind turbine blades are manufactured from composites, consisting of non-crimp fabrics infused with polymer resins. The effective stiffness of the resulting laminate is a combination of the properties of its building blocks i.e. fibers, and matrix as well as from the fabric texture imperfections e.g. fiber undulati...

Free-edge effects in fibre reinforced composites can produce premature damage at the composite edges due to mechanical, geometrical and hygro–thermal effects. Observation of damage on the free-edge of a composite plate is therefore different than what can be observed inside the material. The amount of individually broken fibres in a composite is co...

Micromechanisms of leading edge erosion of wind turbine blades are studied with the use of X‐ray tomography and computational micromechanics simulations. Computational unit cell micromechanical models of the coatings taking into account their microscale and nanoscale structures have been developed and compared with microscopy studies. It was observ...

Insegt Fibre is a tool for volumetric fibre segmentation, that is appropriate for densely packed fibres. The robustness to image quality of the underlying dictionary-based algorithm has opened up new opportunities for micro-structural characterisation of fibre-reinforced polymers. The recent user-friendly version of the robust fibre-detection algor...

This data article contains lab-based micro-computed tomography (μCT) data of unidirectional (UD) non-crimp fabric (NCF) carbon fibre reinforced composite specimens that have been deformed by compression. The specimens contain UD fibres with off-axis angles of 0°, 5°, 10°, 15° and 20° and the compression testing induces kink-band formation. This dat...

Structure-mechanical properties relationships of nanocellulose reinforced polymer composites are studied in computational experiments. A code for the generation of 3D unit cell finite element models of nanocellulose reinforced polymers with “snake”-shaped nanocellulose fibrils is developed. The code allows the generation of pre-defined nanocomposit...

Kink-band formation is the governing failure mechanism for compressive failure of fibre reinforced composites. Here, kink-plane orientation, describing the direction of kink-band formation, is studied using X-ray computed tomography (CT). Unnotched unidirectional specimens with off-axis angles ranging from 0° to 20° are tested in compression. The m...

Understanding the fatigue damage mechanisms in composite materials is of great importance in the wind turbine industry because of the very large number of loading cycles rotor blades undergo during their service life. In this paper, the fatigue damage mechanisms of a non-crimp unidirectional (UD) glass fibre reinforced polymer (GFRP) used in wind t...

The data published with this paper is obtained during fatigue testing of a unidirectional non-crimp fabric based glass fibre composite by means of ex-situ X-ray CT and in-situ transilluminated white light imaging experiments. The data experimentally shows the damage initiation and progression under fatigue loading both in terms of off-axis cracks i...

This work investigates hygrothermal aging degradation of unidirectional glass/epoxy composite specimens through a combination of experiments and numerical modeling. Aging is performed through immersion in demineralized water. Interlaminar shear testes are performed after multiple conditioning times and after single immersion/redrying cycles. Degrad...

The current paper proposes a low cost scheme for enhancement of the fatigue life properties of fibre reinforced composites through minimization of residual stresses induced from the composite cure cycle. The enhancement scheme works on the presumptions that a low processing temperature at the event of resin gelation causes reduced residual stresses...

In the presented study, it has been studied how tensile/tensile fatigue damage will develop in quasi-unidirectional non-crimp fabric based glass fibre epoxy matrix composites. A material system conventionally used as the load carrying laminates in wind turbine blades. It will be demonstrated by stopping a tensile/tensile fatigue test before the fin...

High performance polymer matrix composites (PMC) have a high specific stiffness and can be used to easily manufacture highly complex components. Many types of defects can occur during molding. Flaws and damage degrade the resulting mechanical properties of the composites material. It is difficult to assess the actual stiffness, strength and fatigue...

The current study presents a workflow to import a fibre bundle structure of a non-crimp fabric based fibre composite obtained by X-ray CT to a solvable 3D model in the finite element software ABAQUS. The considered fibre composite is similar to that used for the load carrying parts of wind turbine blades, and each layer of the non-crimp fabric cont...

A unidirectional (UD) glass fibre reinforced polymer (GFRP) composite was scanned at varying resolutions in the micro-scale with several imaging modalities. All six scans capture the same region of the sample, containing well-aligned fibres inside a UD load-carrying bundle. Two scans of the cross-sectional surface of the bundle were acquired at a h...

The current work studies the fatigue damage initiation and progression in a quasi-unidirectional non-crimp fabric based fibre composite used for wind turbine blades. This is done by combining in-situ transilluminated white light imagining (TWLI) with ex-situ X-ray computed tomography (CT) experiments along with tension clamp X-ray CT experiments. T...

Imaging with X-ray computed tomography (CT) enables non-destructive 3D characterisations of the micro-structure inside fibre composites. In this paper we validate the use of X-ray CT coupled with image analysis for characterising unidirectional (UD) fibre composites. We compare X-ray CT at different resolutions to optical microscopy (OM) and scanni...

Computational and analytical studies of degradation of wind turbine blade materials at the macro-, micro-, and nanoscale carried out by the modelling team of the Section Composites and Materials Mechanics, Department of Wind Energy, DTU, are reviewed. Examples of the analysis of the microstructural effects on the strength and fatigue life of compos...

The data published with this article are high resolution X-ray computed tomography (CT) data obtained during an ex-situ fatigue test of a coupon test specimen made from a non-crimp fabric based glass fibre composite similar to those used for wind turbine blades. The fatigue test was interrupted four times for X-ray CT examination during the fatigue...

Due to the high number of fatigue load cycles during the life of a wind turbine blade, fatigue is one of the main design concerns. However, it is still not possible to realistically predict the fatigue life of the non-crimp fabric based fibre composites commonly used for the main load carrying parts of wind turbine blades. Existing modelling attemp...

This work studies the tension fatigue damage progression of a uni-directional glass fibre composite made from a non-crimp fabric similar to those used for the main load carrying parts of a wind turbine blade. The spatial damage progression in a chosen region of a test specimen is monitored on a micro-structural scale by ex-situ X-ray computed tomog...

Unidirectional (UD) glass fibre composites are used for wind turbine blades due to their high stiffness to weight ratio. One of the main limiting factors of increasing the blade length is the lack of knowledge on fatigue damage progression, making it necessary to include high safety factors. In this study on the fatigue damage progression mechanism...

Perthes' disease is a destructive hip joint disorder characterized by malformation of the femoral head in young children. While the morphological changes have been widely studied, the biomechanical effects of these changes still need to be further elucidated. The objective of this study was to develop a method to investigate the biomechanical alter...

Poly(l-lactide) (PLLA) is a bioabsorbable polymer with high stiffness and strength compared to the other commercially available bioabsorbable polymers. The properties of PLLA can be improved by straining, causing deformation-mediated molecular orientation. PLLA tubes were biaxially strained above their Tg for improvement of their strength, in a two...

The aim of this paper is to characterise the fibre orientation in unidirectional fibre reinforced polymers, namely glass and carbon fibre composites. The compression strength of the composite is related to the orientation of the fibres. Thus the orientation is essential when designing materials for wind turbine blades. The calculation of the fibre...

In this study, the progression of tension-tension fatigue (R = 0.1) damage in a uni-directional (UD) composite made from a non-crimp glass fibre fabric used for wind turbine blades is investigated using multi-scale 3D X-ray computed tomography (CT). Initially, a representative volume is examined at one specific damage level. UD fibre fractures are...

FBG_SiMul V1.0 is a tool to study and design the implementation of fibre Bragg grating (FBG) sensors solutions in any arbitrary loaded structure or application. The software removes the need for a fibre optic expert user and makes the sensor response of a structural health monitoring solution using FBG sensors more simple and fast. The software use...

This study presents a method for monitoring the 3D fatigue damage progression on a micro-structural level in a glass fibre/polymer coupon test specimen by means of laboratory X-ray Computed Tomography (CT). A modified mount and holder made for the standard test samples to fit into the X-ray CT scanner along with a tension clamp solution is presente...

A uni-directional (UD) glass fibre composite made from a non-crimp fabric (NCF) was investigated by 3D X-ray computed tomography (CT) to study the fatigue damage at different stages of the fatigue life. The damage was found to appear as local UD fibre fracture regions close to the so-called supporting backing layer, which seemed to have an importan...

Based on a simplified beam model, the loads, stresses and deflections experienced by a wind turbine blade of a given length is estimated. Due to the simplicity of the model used, the model is well suited for work investigating scaling effects of wind turbine blades. Presently, the model is used to predict the weight of the load carrying beam when u...

In this study, the progress of damage under tension-tension fatigue of a uni-directional (UD) glass fibre composite made from a non-crimp fabric is studied using transilluminated white light imaging (TWLI) and X-ray computed tomography (CT). TWLI images are automatically captured throughout the fatigue test, and at two damage levels the test is sto...

An approach is developed for determining the stress-strain law and a failure stress appropriate for micro-mechanical models of polymer materials. Double cantilever beam test specimens, made of an epoxy polymer with notches having finite root radius, were subjected to pure bending moments in an environmental scanning electron microscope. The recorde...

A novel method to obtain independent strain and temperature measurements using embedded Fibre Bragg Grating (FBG) in polymeric tensile test specimens is presented in this paper. The FBG strain and temperature cross-sensitivity was decoupled using two single mode FBG sensors, which were embedded in the specimen material with a certain angle between...

A novel method to assess a crack growing/damage event in fibre reinforced plastic, using conventional single mode Fibre Bragg Grating sensors embedded in the host material is presented in this article. Three different damage mechanisms that can change the sensor output, longitudinal strain \( \varepsilon_{xx} \), transversal stress \( \sigma_{yy,zz...

In a fibre-reinforced polymer (FRP) structure designed using the emerging damage tolerance and structural health monitoring philosophy, sensors and models that describe crack propagation will enable a structure to operate despite the presence of damage by fully exploiting the material's mechanical properties. When applying this concept to different...

Poly(L-lactide) (PLLA) in its L-form has promising mechanical properties. Being a semi-crystalline polymer, it can be subjected to strain-induced crystallization at temperatures above Tg and can thereby become oriented. Following a simultaneous (SIM) biaxial strain process or a sequential (SEQ) biaxial strain process, the mechanical properties of b...