119 research outputs found
Evaluation of faba beans for resistance to sclerotinia stem rot caused by Sclerotinia trifoliorum
No full textSclerotinia stem rot, a fungal disease caused by Sclerotinia trifoliorum, is often a serious problem in faba beans (Vicia faba). The levels of resistance to sclerotinia stem rot were evaluated in 23 faba bean cultivars originating from various European countries, 18 cultivars from ICARDA, five cultivars from Canada, 58 populations originating from various provinces of Greece, and five V. narbonensis populations. Evaluation was done under controlled conditions following artificial inoculation with carrot root pieces colonized by the fungus. Faba bean plants were scored for resistance on a 0 to 3 severity scale. There was significant variability for resistance to S. trifoliorum among faba bean cultivars and among populations. Seven cultivars and 15 Greek faba bean populations showed a satisfactory level of resistance (≤ 1.50), whereas 22 cultivars and 25 populations were susceptible (> 2.00). However, all V. narbonensis populations were resistant to S. trifoliorum (disease severity from 1.11 to 1.44) without significant variability within populations. Variability in the resistance of faba beans to S. trifoliorum would allow selection within appropriate genotypes for breeding in an effort to provide an effective alternative for sclerotinia stem rot management in this crop.La pourriture à sclérotes, une maladie fongique causée par le Sclerotinia trifoliorum est souvent un grave problème pour la production de féverole (Vicia faba). À la suite d’une inoculation artificielle avec des morceaux de carottes colonisés par le champignon, l’intensité de la résistance à la pourriture à sclérotes a été évaluée en conditions contrôlées chez 23 cultivars de féverole provenant de divers pays européens, 18 cultivars de l’ICARDA, cinq cultivars canadiens, 58 populations issues de diverses provinces de Grèce et cinq populations de V. narbonensis. Une cote de résistance a été attribuée à chaque plante sur une échelle d’intensité de maladie allant de 0 à 3. Il y avait des différences significatives pour la résistance au S. trifoliorum parmi les cultivars et parmi les populations de féverole. Sept cultivars et 15 populations grecques de féverole ont démontré un degré satisfaisant de résistance (≤ 1,50), alors que 22 cultivars et 25 populations étaient sensibles (> 2,00). Par contre, toutes les populations de V. narbonensis ont été résistantes au S. trifoliorum (intensité de maladie entre 1,11 et 1,44) sans qu’il n’y ait de différence significative entre les populations. Les différences de résistance au S. trifoliorum dans la féverole signifient qu’il serait possible de procéder à de la sélection au sein de génotypes choisis afin d’offrir l’amélioration génétique comme moyen efficace de lutte contre la pourriture à sclérotes de cette culture
Progressive Damage Accumulation Process of CFRP Cross-Ply Laminates during the Early Fatigue Life
No full textThe present work aims at investigating the progressive damage accumulation process of CFRP laminates in an interactive scheme, with a special focus on the early fatigue life where mainly matrix-dominant damage accumulates and stiffness degrades significantly. An in-situ damage monitoring system, containing edge observation, digital image correlation and acoustic emission techniques, was established to characterize and quantify the accumulation of transverse cracks and delamination. Two cross-ply configurations ([0/902]s and [02/904]s) and different stress levels were involved in the experimental campaign. Dependent crack ratio was proposed to reflect the interaction among transverse cracks, and saturated crack density was used to represent the interactive level between transverse cracks and delamination. Results showed that generation of transverse cracks and their interaction govern the early fatigue damage accumulation of the [0/902]s laminates, while not only the interaction among cracks but also the interaction between both damage mechanisms were observed for the [02/904]s laminates.Structural Integrity & Composite
Experimental and numerical validation of an inter-ply friction model for thermoset based fibre metal laminate under hot-pressing conditions
No full textHot-pressing can be an attractive fabrication method that enables the forming of hybrid materials like thermoset based fibre metal laminates. However, the process simulation on press forming requires accurate material characterization and boundary conditions to facilitate part design for a defect-free component. In order to improve the overall predictive simulation quality, the inter-ply sliding at metal-prepreg interfaces which is one of the critical deformation mechanisms is considered. An inter-ply friction model has been established using an experimental friction test apparatus and the effect of slip rate, normal force and temperature is taken into consideration. To validate the proposed friction model, a comparative study between results obtained from the numerical model and the experimental ones is carried out. The research demonstrates that the inter-ply friction model can be a valuable building block for the finite element simulation of the hot-pressing process for thermoset based fibre metal laminates.Aerospace Manufacturing Technologie
Development and characterization of hybrid thin-ply composite materials
No full textThin-ply composites are recognized as a key solution for the manufacturing of high-performance composite structures due to the unique mechanical properties and the increased design versatility that they offer. They are obtained with state-of-the-art fiber spreading methods where high-count (6-24K filaments) tows of technical fibers (carbon, glass) are thinned by spreading into flat unidirectional tapes which are then combined with a polymer matrix to create pre-impregnated (prepregs) tapes of reduced thickness. In recent years, the industrialization of fiber spreading and impregnation processes enabled the large-scale production of homogenous thin-ply prepregs with thicknesses down to about 15μm per ply, which attracted the interest of the research community. However, the high production cost due to the complexity of the manufacturing methods and the inherent brittleness of thin-ply composites limit their wider adoption by the composites industry[1]. Fiber hybridization (i.e combining at least two types of fibers in a common matrix) is emerging as a promising approach for alleviating these drawbacks towards laminates with balanced characteristics in terms of mechanical properties and cost-efficiency. Currently, most studies on thin-ply hybrids employ simple interlayer (ply-by-ply) configurations mainly due to difficulties in manufacturing of more complex hybrid architectures[2]. However, simulation tools predict that notable improvements can be obtained from more complex intralayer (tow-by-tow) and intrayarn (fiber-by-fiber) hybrid architectures[3]. This work focuses on the study of existing fiber spreading methodologies, the development of equipment, and the optimization of composite processing at North Thin Ply Technology (NTPT) Renens, Switzerland, that allowed the manufacturing of hybrid composites with a high degree of fiber dispersion and controlled microstructure. Hybrid prepregs were produced by combining various ratios of dissimilar fibers following different processing routes. Composite laminates were manufactured and a versatile microstructural analysis tool was developed that enabled correlations between the manufacturing route, the resulting microstructural features describing the degree of co-dispersion, and the mechanical performance of the final part. Acknowledgments The research leading to these results has been performed within the framework of the HyFiSyn project and has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 765881. Delamination growth in fibre reinforced polymer composites is generally evaluated with experiments that have been standardized for quasi-static load conditions. These tests characterize unidirectional delamination growth in mode I (DCB), mode II (ELS or ENF) of mixed mode conditions (MMB). However, little attention is paid in literature to the applicability of these tests to in-service delamination problems that are generally characterized by planar delamination growth. In this study, the relation between planar delamination growth, induced by transverse quasi-static indentation loading, and these unidirectional delamination tests was investigated. To that aim, prior planar delamination growth tests reported in literature, performed at EPFL, were analysed to identify up to what extent this planar growth could be correlated to the concepts of strain energy release and strain energy density. Once this appeared to successful, an experimental setup was designed to measure the delamination boundary during the transverse indentation loading of planar delamination specimens made of nontransparent carbon fibre reinforced polymer composites. With that set-up, quasi-static and fatigue planar delamination growth experiments were performed, and delamination contours could be successfully captured. While the quasi-static tests revealed limited growth, evaluation with numerical simulations revealed that the indentation force required to extend the delamination quasi-statically would cause damage to the specimen. This is attributed to the increasing length of the delamination contour when delaminations expand, which is not the case with standard unidirectional specimen. With the fatigue tests, however, delamination growth was achieved, but interestingly enough two phases were observed; first the delamination propagated in a planar fashion, while at some point in time work did not exceed an apparent threshold. Instead of no growth, however, the delamination still increased but then in a transverse manner. What makes this study of particular interest, is that the strain energy density as criterion could capture the strain energy offered (work) along the entire delamination contour, while the strain energy release rate described the resistance to delamination growth. This latter observation is in agreement with the original concept employed by Griffith when he formulated the basis of linear elastic fracture mechanics. This presentation present the experiments performed, the analysis of results, and will conclude with a proposal how to relate standard unidirectional tests to planar growth, considering that these standard tests contain little to no information on transverse phenomena with respect to strain energy density (work) and strain energy release (dissipation).Aerospace Manufacturing Technologie
The role of matrix boundary in the microstructure of unidirectional composites
No full textFinding new ways to evaluate the variability of microstructures, and its effect on macroscopic properties such as permeability and mechanical performance [1,2] is of increasing interest in the composite field. The variability of microstructural features at a three-dimensional level is not fully understood and its effect on macroscale properties is not well established, and mostly analyzed at a phenomenological level [3]. We introduced in recent work a method based on X-ray Computed Tomography for the threedimensional reconstruction of the fibrous microstructure of unidirectional tapes at a single fibre resolution [4]. A schematic of the workflow is represented in Figure 1. Three descriptors are introduced in the work to describe increasing level of complexity in the microstructural organization, from a single fiber path level with differential tortuosity, to group behavior with collective motion, to fibre network connectivity with length of contact. These descriptors and their interdependence highlight local effects like edge-core segregation in microstructural characteristics. However, in order to achieve a more complete definition of the unidirectional tape domain, understanding of matrix-based features and its interrelation with fiber architecture descriptors is needed. In this work, we expand the methodology of Gomarasca et al. [4], to account for matrix-based phenomena such as tape boundary variability, and void formation and morphology. This will be showcased on a unidirectional composite tape including both fiber-based and matrix-based analysis. These methods enable advanced characterization and modelling of microstructural formation and evolution during composite manufacturing.Aerospace Manufacturing TechnologiesAerospace Structures & Computational Mechanic
Tailoring mechanical properties of randomly oriented tape (ROT) composites: An experimental study
No full textDiscontinuous fiber composites (DFC) have properties such as notch insensitivity, short processing times, and large shaping freedom [1-3]. However, the mechanical behavior of ROT composites is less predictable compared to continuous fiber composites due to the mesoscopic heterogeneity of the material [4]. Controlling the tape alignment is a compelling approach for tailoring the mechanical properties of the ROT composites, enabling better control and prediction of the material properties such as tensile strength and stiffness [5]. However, for a successful implementation of the alignment method in industry, a fast tape alignment method is needed. In this study, a quantitative assessment has been made between three alignment methods. The methods have been evaluated in respect of the level of alignment and the manufacturing process by means of a decision-making matrix. The level of alignment of each method has been determined using computer vision on the orientation of individual tapes. The final alignment method has been selected which uses vertical mesoscopic sieves with a high aspect ratio to rotate tapes in a preferred direction during deposition. With this alignment method, ROT have been aligned at -45˚, 0˚, and +45˚ with respect to the loading direction. With these alignment tools, CF/PEEK ROT have been deposited inside a cavity followed by a consolidation cycle at 45 bar and 385˚C. The effect of alignment was examined by comparing the mechanical response of samples with: ROT, being longitudinally aligned, and a laminate of [+45˚,-45˚]s aligned pseudo-layers. The results of tensile tests showed that by aligning tapes in the longitudinal direction, the average tensile stiffness and strength increased by 145% (from 32.5 to 79.7 GPa) and 96% (from 202 to 396 MPa) respectively compared to randomly orientated tapes. These results show the potential gain in material properties and the ease of implementation of the method.Aerospace Manufacturing Technologie
Enhancement of mode I fracture toughness of adhesively bonded secondary joints using different layup patterning of CFRP
No full textDelamination growth in fibre reinforced polymer composites is generally evaluated with experiments that have been standardized for quasi-static load conditions. These tests characterize unidirectional delamination growth in mode I (DCB), mode II (ELS or ENF) of mixed mode conditions (MMB). However, little attention is paid in literature to the applicability of these tests to in-service delamination problems that are generally characterized by planar delamination growth. In this study, the relation between planar delamination growth, induced by transverse quasi-static indentation loading, and these unidirectional delamination tests was investigated. To that aim, prior planar delamination growth tests reported in literature, performed at EPFL, were analysed to identify up to what extent this planar growth could be correlated to the concepts of strain energy release and strain energy density. Once this appeared to successful, an experimental setup was designed to measure the delamination boundary during the transverse indentation loading of planar delamination specimens made of nontransparent carbon fibre reinforced polymer composites. With that set-up, quasi-static and fatigue planar delamination growth experiments were performed, and delamination contours could be successfully captured. While the quasi-static tests revealed limited growth, evaluation with numerical simulations revealed that the indentation force required to extend the delamination quasi-statically would cause damage to the specimen. This is attributed to the increasing length of the delamination contour when delaminations expand, which is not the case with standard unidirectional specimen. With the fatigue tests, however, delamination growth was achieved, but interestingly enough two phases were observed; first the delamination propagated in a planar fashion, while at some point in time work did not exceed an apparent threshold. Instead of no growth, however, the delamination still increased but then in a transverse manner. What makes this study of particular interest, is that the strain energy density as criterion could capture the strain energy offered (work) along the entire delamination contour, while the strain energy release rate described the resistance to delamination growth. This latter observation is in agreement with the original concept employed by Griffith when he formulated the basis of linear elastic fracture mechanics. This presentation present the experiments performed, the analysis of results, and will conclude with a proposal how to relate standard unidirectional tests to planar growth, considering that these standard tests contain little to no information on transverse phenomena with respect to strain energy density (work) and strain energy release (dissipation).Structural Integrity & Composite
Promoting extrinsic bridging of adhesively-bonded CFRP joints through the adhesive layer architecture
No full textCarbon fiber-reinforced polymers (CFRPs) have widely attracted the aerospace and automotive industries due to high stiffness and lightweight. Secondary adhesive bonding of CFRPs is a promising research field to fully explore their potential. However, multiple challenges have limited the further application of adhesively-bonded composite joints since it is difficult to inspect the premature debonding, which leads to catastrophic failure once initiated. Thus, it is crucial to introduce crack arrest features, to slow down (or even stop) the crack growth and achieve progressive failure. Various methods have been reported to introduce crack arrest features, including z-pins and corrugated substrates. Our previous work directly utilized the adhesive layer to bridge the separating CFRP parts, through the extrinsic bridging of adhesive ligaments. The bridging adhesive ligaments are triggered by the patterning of distinct surface treatments. These extrinsic bridging ligaments largely enhance the energy release rate (ERR) and successfully arrest the crack propagation. However, a large portion of the required energy for the further crack propagation is stored elastically in the stretching ligaments, which would cause catastrophic fast joint debonding after the failure of ligaments. In this work, the adhesive layer was architected in order to improve its plasticity. By promoting the plastic energy dissipation, the bridging, stretching, and failure of generated adhesive ligaments could result in tougher and safer joints. CFRP substrates were alternatively patterned by two distinct surface treatments to achieve different interfacial strength and toughness values. Then, double-cantilever beams (DCB) were manufactured by bonding treated substrates with the architected adhesive material, such as integrating 3D-printed nylon wires or newly synthesized adhesive material. Results showed that the proposed joint toughening strategy could improve ERR compared to conventional uniform treatments and increasd adhesive plasticity could also stabilize the crack propagation, leading to a safer joint.Structural Integrity & Composite
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