Ply Waviness Research Articles

The Effect of Ply Waviness for the Fatigue Life of Composite Wind Turbine Blades

Wind turbine blade failure is one of the most prominent types of damage occurring in operating wind turbine systems. Glass fibre materials are mainly used for producing wind turbine blades and they consist of large tows, stitched fibres, which are prone to generate imperfections such as ply waviness. The primary objective of this research was to evaluate the effect of ply waviness on fatigue life of typical composites used in wind turbine blades. This study involves two main areas: Finite elements analysis (FEA) and experimental testing. The Finite element analysis was carried out using the commercial code from ESI group. The experimental work was carried out by manufacturing specimens from glass fibre with regular waviness and testing for static and fatigue failure. The tensile static test data was used to identify the effect of ply waviness for failure strength. Cycles to failure (S-N) curves were developed for two different wave profiles based on fatigue testing data. The long term aim of this ongoing research is to improve current fatigue life estimation methods applicable to composite wind turbine blades.

Interaction of compression failure mechanisms in composites with ply waviness defects

This article presents results from experimental and computational investigations of composite laminates with embedded ply waviness defects of different geometries under compression. The effect of the ply waviness defect on the initiation, interaction, and evolution of failure mechanisms of surrounding plies are studied. Using high-resolution imaging and digital correlation analysis, the strain localization phenomenon is monitored, and the sequence of failure mechanisms is established. The experimental observations are supported with computational finite element progressive failure analyses of the same composites with ply waviness defects using an energy based continuum damage mechanics approach. For the case of laminates with ply waviness defects of high angles, a delamination crack between the wavy plies and adjacent transverse plies is the dominant mode of failure, which interacts strongly with the fiber compression and matrix tension modes. For the case of laminates with ply waviness defects of low angles, failure does not initiate at the waviness defect but at locations where the laminate strength was locally reduced due to other microstructural defects. The combined experimental and computational analyses presented here advanced the understanding of the interaction of compression failure mechanisms in composites with ply waviness defects, which is highly challenging to experimentally measure.

Effect of Hybridization and Ply Waviness on the Flexural Strength of Polymer Composites: An Experimental and Numerical Study.

The study aims to ascertain the influence of hybridisation and ply waviness on the flexural behaviour of polymer composites. Two different resin systems, namely epoxy and Poly(methyl methacrylate)-PMMA, were chosen for the study, wherein two batches of carbon/glass hybrid composites (CGHC) were fabricated with the two resin systems. In addition to CGHC samples, four other neat batches with waviness (glass/epoxy and glass/PMMA) were prepared to study the effect of out-of-plane ply waviness. Two sets were additionally made with in-plane waviness (angles ranging from 15–35°) with epoxy to further understand the effect of waviness on flexural behaviour. Thereafter, two more batches of samples with neither waviness nor hybrid architectures were tested to achieve a better understanding of hybridization and the presence of waviness. It was seen that the hybridization of polymer composites introduces a pseudo-ductile behaviour in brittle composites, which makes the failure more predictable. An energy-based model was implemented to quantify the ductility introduced by hybridization. The presence of in-plane waviness increased the flexural load but reduced the modulus considerably. The presence of out-of-plane waviness decreased the flexural properties of composites drastically, though the displacement rate was seen to increase considerably. From the comparison between epoxy and PMMA, it was seen that PMMA exhibited similar flexural properties vis-à-vis epoxy. PMMA is easy to re-cycle and thus could serve as an ideal replacement for epoxy resin. Finally, a numerical model was built based on an LS-DYNA commercial solver; the model predicted the flexural behaviour close to what was seen in the experiments. The model could be calibrated correctly by ascertaining the influence of failure strain in the longitudinal direction, which is fibre dependent, and the failure strain in the transverse direction, which is matrix dependent.

Effect of in-plane fiber waviness defects on the compressive properties of quasi-isotropic thermoplastic composites

The influence of in-plane fiber waviness defects on the compressive properties of quasi-isotropic (QI) carbon polyether-ether-ketone (C/PEEK) composites was investigated experimentally. Specimens with localized waviness were manufactured using a stamp forming process resulting into laminates featuring multiple wavy plies, i.e. from one to three wavy 0° plies in a 24-ply QI composite and a range of maximum waviness angle between 23° to 60°. No significant influence of the waviness was found on the global laminate stiffness. Compression tests coupled with high-speed camera monitoring were performed to study the failure process. It was confirmed that the waviness defects act as a trigger for the initiation of damage, predominantly by the kinking mechanism, resulting into an early failure and significantly lower ultimate strength than the baseline when loading in 0° direction. Furthermore, it was found that all specimens with waviness and with the same layup have a similar strength, indicating that the maximum waviness angle within the range studied in this work did not significantly influence the ultimate compressive strength. However, the presence of waviness in multiple plies clearly affected the strength. It was found that the ultimate compressive strength decreased proportionally to the percentage of plies oriented in the loading direction that is wavy.

Influences of ply waviness and discontinuity on automated fibre placement manufactured grid stiffeners

Advanced grid structures can realise significant weight savings, compared to conventional stringer stiffened structures. Overlaps of the transverse tows create fibre drop-off around the intersection region. As a result, this creates a potential weak point of whole advanced grid stiffened structures. Localised buckling at the intersection region is imminent under compressive loading. In order to overcome this shortfall, an automated fibre placement based method to improve the microstructure of grid stiffener is proposed in this paper. In this method, discontinuous plies are introduced into rib to remove excessive material at the intersection. The influences of fibre waviness and discontinuous plies on the microstructure, mechanical performance of grid stiffener are investigated using experimental and finite element methods. Results show that structural efficiency of grid stiffener can be improved significantly with appropriate ratio of discontinuous plies in the intersection of grid stiffener. Corresponding finite element models are developed for verification and established good correlation with the experimental response. The finite element analyses also provide an insight on the failure mechanisms. The results of this study can be further used towards the design and manufacture of practical grid stiffeners for the mechanical performance improvement.

Delamination behavior of L-shaped composite beam with manufacturing defects

Composite structures are more susceptible to manufacturing defects than conventional materials. Fiber misalignment, cracks, and voids are typical types of manufacturing defects in laminated composites. Defects can greatly reduce structural integrity and load carrying capacity, so their effects on material and structural strengths must be understood. In this paper, the effect of manufacturing defects on the progressive delamination behavior was studied for carbon/epoxy composite laminated L-beam under four-point bending test conditions. The defects of wavy plies and pure resin that had flowed out were characterized from surface photography and then transformed into finite element modeling using semi-automatic approach to which pre-delamination and void were included. Next, progressive failure analyses were performed with the interface delamination and matrix direction failure accounted for by cohesive zone modeling. Numerical results were examined focusing on the effects of defects on the peak load reduction and the variation of delamination pattern. The stacking sequence effect was also investigated.

Understanding the impact of fibre wrinkle architectures on composite laminates through tailored gaps and overlaps

Among a variety of defects associated with composite technology, fibre wrinkles are known to be a major source of failure for fibre reinforced plastic and closely associated with the ever popular automated fibre placement technique. In the present work, with the aim to investigate their influence on the failure of carbon fibre reinforced plastics (CFRP), composite laminates containing fibre wrinkles of varying architectures have been designed and prepared utilizing gaps and overlaps based on the understanding of wrinkle formation mechanisms. The effects of these engineered wrinkles with different combination of gaps and overlaps on the mechanical performance of CFRP were systematically investigated. A maximum of 21% and 37% drop in the tensile and compressive strength were recorded for the most severe combination of gaps and overlaps. A simple analytical model was proposed to relate the wrinkle angle to the defect characteristic parameters and a good agreement with experimental results was obtained. Furthermore, numerical simulations were conducted to inform the failure mechanisms where different wrinkle models were generated using previously developed pre-processing tools. Good agreements with experimental observations were obtained in terms of ply waviness, strength and failure modes. These findings provide certain practical insights for controlling defects in composite manufacturing processes.

A multi-parameter model for stiffness prediction of composite laminates with out-of-plane ply waviness

In this paper, an analytical multi-parameter model based on the classical lamination theory (CLT) is developed and the study of the stiffness response for unidirectional composite laminates with out-of-plane ply waviness is followed. A range of parameters associated with the waviness defect, including the waviness geometry, the fiber off-axis angle, the extent of wavy region and the waviness pattern, are incorporated into the analytical model. The fiber volume fraction and the type of reinforcement are also taken into account in the parametric analysis. The three-dimensional stiffness parameters of thick wavy laminates are investigated systematically for different cases. An observation of the negative in-plane Poisson’s ratios for wavy laminated composites reinforced with carbon fibers is presented under the given assumptions of CLT. This phenomenon provides a possible, new technique to design the auxetic materials. The modeling approach provides an effective procedure to quantify the effects of ply waviness on the stiffness of composite laminates.

Micromechanics model for three-dimensional effective elastic properties of composite laminates with ply wrinkles

This paper presents an analytical micromechanics model for predicting the three-dimensional effective elastic properties of multidirectional composite laminates with ply wrinkle defects. A representative volume element (RVE) was chosen for analysis and the geometry of wavy plies in the wrinkled laminates was described by sinusoidal functions. The derivation of the effective elastic moduli was based on the mixed boundary conditions, and a two-step homogenization technique was proposed. The analytical predictions were compared with the published numerical and experimental results for unidirectional and cross-ply carbon/epoxy laminates. The model was also applied to quasi-isotropic laminates, and the effect of wrinkles on their effective properties was examined. The developed analytical micromechanics model was found to accurately predict the in-plane and out-of-plane effective properties of the wrinkled laminates, making it a useful tool for providing information on the relationships between wrinkle defects and the macroscale response of composite laminates and for designing composite structures.

Effect of Gap Induced Waviness on Compressive Strength of Laminated Composites

During manual/automated prepreg lamination, intraply gaps and overlaps are likely to occur due to contoured mould surface over which layup is done. Unnoticed gaps and improperly filled gaps result in localized resin rich regions followed by fiber waviness in components. It becomes indispensable to assess the safety of the laminated composite structure in the presence of such defects. In the present study, four different cases of specimens with embedded gaps in plies at multiple locations are under consideration. These specimens are fabricated and tested, under compression loading and analyzed through finite element analysis. The optical images of cut specimens both longitudinal and transverse of all four cases are used for creating 3-D finite element models with ply waviness to compare their behavior with baseline reference specimens. The strain behavior seen during test is compared to validate the finite element modeling approach. The wave factor, which is a measure of the ply waviness, calculated using optical images is used to study the effect of fiber waviness on the compressive behavior of the specimens experimentally.

Through-thickness fatigue behavior of non-crimp fabrics featuring manufacturing defects

The use of cost efficient CFRP materials is growing especially in the automotive applications. Here, a challenge is delamination occurring during service. Out-of-plane loading should be avoided in composite structures, since it is the weakest loading direction. However, this is not always possible in more complex geometries or at joints. Hence, there is a demand for through-thickness fatigue properties, for design and analysis of CFRP structures.Fatigue damage is often initiated by manufacturing imperfections or defects, such as voids, in-plane or out-of-plane ply waviness or folds of plies. In this research work, the influence of a fold defect on the through-thickness mode I fatigue fracture behavior of an automotive non-crimp fabric laminate is assessed, through the means of both experimental testing and numerical simulation. The selected test method is the direct through-thickness tensile load application. This method allows microscopic analysis of the fracture surfaces of the delaminated specimens, which contributes to a better understanding of the influence the manufacturing defect has on the damage behavior and material performances.Fractography shows, that interlaminar fracture is predominant due to stitching yarn concentration and resin rich areas. A knockdown of 10% due to the presence of the fold defect is observed and can be further fed back into the fatigue material properties for simulation and analysis of structural composite parts. In this way, the presence of defects can be quantified and accounted for during the design stage.

A continuum damage model to predict the influence of ply waviness on stiffness and strength in ultra-thick unidirectional Fiber-reinforced Plastics

Ply waviness is a commonly observed manufacturing defect of ultra-thick composite materials essentially affecting stiffness, strength, and fatigue behavior of the composite. A specimen's geometry is designed to represent the failure mechanisms in thick wavy laminates, typically observed in spar caps of today's wind turbine blades. A material model is developed to simulate the phenomenological processes in wavy laminates to obtain a strength knock-down. Particular attention is taken on the nonlinear shear behavior and kink band formation. The presented model correlates well with results achieved by experiments. This paper shows a significantly higher influence of compressive compared with tensile loading on the mechanical material behavior of wavy laminates.

Influence of out-of-plane ply waviness on elastic properties of composite laminates under uniaxial loading

In this paper the influence of out-of-plane ply waviness on elastic properties of composite laminates subjected to uniaxial loading is investigated. A three-dimensional analytical method based on the classical lamination theory is developed to quantitatively determine the effective elastic constants of thick composite laminates with ply waviness. Various influential factors associated with ply waviness, such as the waviness geometry, the off-axis angle, the extent of wavy region, and the waviness pattern, are incorporated in the multi-parameter model. Analytical results for the Young’s moduli, the shear moduli and the Poisson’s ratios of wavy fiber composite laminates are obtained and discussed for a specific range of fiber volume fractions and two different lay-ups. Negative Poisson’s ratios are found for unidirectional laminates nested wavy ply/plies. The present method provides a useful and effective tool to evaluate the effects of out-of-plane ply waviness on three-dimensional elastic properties of composite laminates.

Strength prediction of ply waviness in composite materials considering matrix dominated effects

Ply waviness (PW) is a commonly observed manufacturing defect in composite materials leading to degradation of the mechanical properties. The effects of matrix systems on the strength behavior of unidirectional laminates containing ply wavinesses are analyzed for compressive loading. An analytical approach based on Hsiao and Daniel (1996) allows an estimation of stiffness and strength in dependence on waveform and material. While the geometry of the waviness can be characterized by its amplitude to wavelength ratio and its wave shape (uniform or graded), composite constituents can be considered individually. The approach according to Hsiao and Daniel is enhanced by the Puck failure criterion Schürmann (2005) and is implemented as a graphical user interface (gui) using the software matlab. The analytical approach is validated against literature data. Strength properties are individually varied, indicating that failure behavior of wavy laminates is mostly affected by matrix dominated shear strength R12.

Modelling the effect of gaps and overlaps in automated fibre placement (AFP)-manufactured laminates

AbstractIn automated fibre placement (AFP) process, gaps and overlaps parallel to the fibre direction can be introduced between the adjoining tapes. These gaps and overlaps can cause a reduction in strength compared with pristine conditions. Finite element modelling is an effective way to understand how the size and distribution of such gaps and overlaps influences the strength and failure development. Many modelling work showed that out-of-plane waviness and ply thickness variations caused by gaps and overlaps play an important role in inducing the strength knock-down; however, there has been a lack of effective way to explicitly model the ply waviness, which constrained the relevant research. In this work, 3D meshing tools were developed to automatically generate ply-by-ply models with gaps and overlaps. Intra-ply and inter-ply cohesive elements are also automatically inserted in the model to capture the influence of splitting and delamination. Out-of-plane waviness and ply thickness variations caused by gaps and overlaps are automatically modelled. Models with various sizes and distribution of gaps and overlaps were built to predict the reduction of strength as a function of the magnitude and type of the defects. Results of gap and overlap models will be used to guide future experimental characterization of simulated AFP process defects, manufactured by hand layup from pre-preg tape.

Manufacturing of CFRP specimens with controlled out-of-plane waviness

In this paper, the manufacturing of carbon fibre reinforced plastic laminates with intentionally, reproducibly and mathematically defined high quality out-of-plane waviness made from unidirectional prepreg material is described. This methodology is employed to manufacture tension test specimens with included controlled out-of-plane ply waviness. The waviness ratio is A/L = 0.09, where A and L denote the wave amplitude and the wavelength, respectively. A number of quality assurance verifications is carried out on these specimens before the tension test. These verifications are to corroborate that the realised graded waviness is consistent with the theoretically defined waviness function. The manufacturing process of these specimens is described in detail. The tension test of the specimens is performed by means of a hydraulic testing device. The force–displacement curves of the tension test are plotted to analyse the material behaviours. Experimental investigations are carried out in conjunction with non-destructive 3D optical measurement methodology to identify the failure mechanisms. The results of quality assurance verifications on the specimens exhibit a perfect matching between the geometrically graded shapes and the theoretical function. Furthermore, the results of tension qualification tests show that all specimens have very similar damage behaviour and the failure events have occurred in the waviness area.

Failure mechanisms under compression loading in composites with designed out-of-plane fibre waviness

Compressive strength reduction due to fibre waviness is a key concern in composite structures. Specimens with fibre misalignments representative of those that can occur at ply drops and resin rich areas in components were manufactured by incorporating either two or 10 discontinuous plies oriented either in 0° (‘longitudinal’) or 90° (‘transverse’). Longitudinal specimens did not fail in compression but failed by delamination initiating at the ply drops. Transverse specimens did not fail directly due to the waviness: specimens with 8° waviness failed at similar stresses and strains as fully aligned specimens, while 30° waviness specimens failed in the aligned plies at a slightly reduced overall stress due to redistribution of load from the less stiff wavy plies. None of the specimens failed directly in compression due to misalignment in the wavy plies, indicating that this is not necessarily the critical failure mechanism, and highlighting the importance of delamination and load redistribution at ply drops.

Study of induced ultrasonic deviation for the detection and identification of ply waviness in carbon fibre reinforced polymer

Ply waviness is a major defect, which can appear in certain composite materials such as CFRP. Attenuation and ultrasound velocity measurements allow the detection, but not the identification, of ply waviness. In the present study it is shown that in order to identify this ply waviness, it is important to take the deviation of the ultrasonic beam into account. Two different methods allowing such deviations to be detected are proposed. On the one hand, the deviation produces an asymmetrical behaviour in the responses obtained at oblique incidence angles. This phenomenon is revealed through the study of incidence angle ranges, which can normally be superimposed. On the other hand, the double scanning technique allows the deviation of the energy maxima of the transmitted acoustic field to be determined. In both cases, the study of induced deviation reveals that it is sensitive to the presence of ply waviness. These methods have been experimentally validated and their potential use, depending on the thickness of the component, is discussed.

Finite element mesh generation for composites with ply waviness based on X-ray computed tomography

A method for automated generation of finite element meshes for unidirectional composites with waviness defects is proposed. Images used as input for mesh generation are recorded with X-ray computed micro-tomography. Quality and contrast of the scanned images is such that fiber directions cannot be detected everywhere. To generate finite elements mesh that follow fiber directions it is suggested to interpolate available fiber slope data using radial basis functions and to create mesh nodes by integrating ordinary differential equations for fiber slopes. Examples demonstrate practical steps of detecting waviness from volume slice images and generation of meshes that model waviness defects with acceptable accuracy.

Experimental investigation and characterisation techniques of compressive fatigue failure of composites with fibre waviness at ply drops

Internal features, such as fibre waviness and ply drops, have an impact on fatigue performance of composite structures. In this investigation, specimens were manufactured with wavy plies sandwiched between straight plies by placing blocks of discontinuous plies anti-symmetrically on either side of the continuous wavy plies. The specimens were representative of the kind of fibre misalignment that can occur in components at ply drops and resin rich areas. Experimental characterisation techniques using strain gauge software were also developed for detecting the initiation of delamination and its propagation, by measuring interlaminar transverse displacements. Specimens with longitudinal discontinuous plies did not fail in compression but rather failed by delamination initiating at the ply drops. None of specimens with transverse discontinuous plies failed directly in compression due to misalignment in the wavy plies. The fatigue failure mechanisms are the same as those observed under static loading in previous work, which facilitates development of a fatigue model.