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Item type:Publication, Access status: without files , Guidelines for test conditions and documentation of buckling experiments with cylindrical CFRP shells(Queen's University Belfast, 2023-08); ; ; Different types of loads, such as axial compression, torsion or bending, cause thin-walled cylindrical shells to collapse due to buckling. Because of numerous kinds of possible imperfections, this usually occurs significantly below the buckling load of the ideal shells. When using fibre composites, there are even more influencing factors. Therefore, test data are indispensable for the design of new shells. This data is needed to validate models and new design approaches. However, the available database remains a major challenge. Many experimental data are not freely accessible or were generated under different conditions. In this contribution, the available database is gathered and evaluated. Differences and irregularities are identified. Subsequently, it is shown which data the most widespread or common design approaches require. Based on the analysis of the available database as well as the required data for the individual design approaches, recommendations for testing thin-walled cylindrical shells and for publishing new test data are formulated. Finally, a test series of 12 thin-walled CFRP cylinder shells is used to show how these recommendations are to be applied. It is illustrated that the scatter varies depending on the quality of the test, thus underlining the importance of profound documentation of test procedures, setup and results.Publicationtype: Conference Paper93 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Simulation of the progressive failure of CFRP at elevated temperature(International Committee on Composite Materials, 2023-01-01); ; ; Publicationtype: Conference Paper25 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Component testing of sandwich structures with more realistic boundary conditions(International Committee on Composite Materials, 2023); ; As part of the design process for aircraft cabin monuments, the loads on the fasteners, like inserts are determined using global FEM and are compared with allowables of the fasteners. The allowables are determined in component tests, which currently have highly idealized boundary conditions, resulting in high safety factors being added. In this paper, application-oriented component tests with realistic boundary conditions for insert systems are proposed, experimental investigations are performed on them, and virtual models are implemented. The studies show that the boundary conditions have a great influence on the structural behavior and the failure mechanisms of the sandwich structure and the insert system. The results further prove that tests with application-oriented loading situations should be used to validate detailed virtual models, e.g. to ensure their use as submodels in the global FEM analysis of large sandwich structures. In addition, an example is given to show how such a component test with realistic boundary conditions can be used in the design optimization of the load introduction.Publicationtype: Conference Paper39 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Vibro-acoustic modulation measurements for a lifetime evaluation of composite materials(International Committee on Composite Materials, 2023); ; ; ; This paper investigates the influence of different damage types and their interaction on the vibroacoustic behaviour of glass fibre and carbon fibre reinforced polymers (GFRP and CFRP) under stepwise tensile and fatigue loading. For this purpose, different GFRP layups were used to correlate visible matrix cracks with VAM signal. Additionally, for CFRP the layer thicknesses are varied for cross-ply laminates with constant areal fibre weight (268 g/m2) and quasi-isotropic laminates with different areal fibre weights (30, 60 and 120 g/m2). After damage introduction under tension-tension loading (R=0.1), high-frequency Lamb waves are introduced via bonded piezo ceramic actuator and modulated by a low-frequency longitudinal wave using a servo-hydraulic testing machine. In this study, the analysis and interpretation of the vibro-acoustic response signal can be used to demonstrably identify different damage types and conditions of the CFRP specimens. The sideband amplitudes and the modulation index are used as VAM characteristics in the frequency domain and the shape of the modulation envelope in time domain as well as X-ray scans to prove the damage progress after various cycles. It is shown that regardless of the lay-up, both inter-fibre failures, as well as extensive delaminations, can be reliably and sensitively detected using a simple vibration-based test setup, providing a promising nondestructive testing methodology. Finally, it is demonstrated that for certain GFRP specimens the modulation index and lifetime can be predicted after only a few loading cycles.Publicationtype: Conference Paper54 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Effects of wet aging on the damage progress in multidirectional GFRP composites under fatigue loading(International Committee on Composite Materials, 2023); ; The impact of harsh environments with variable temperatures, high humidity, or direct water contact poses a challenge for lightweight construction materials, such as polymer composites, because of their hydrophilic and temperature-dependent properties. Since the most widely used glass fiber reinforced polymers are known to suffer from the above-mentioned conditions, precise knowledge of their long-term durability is of high interest. In addition to the pure change of mechanical properties due to water absorption or temperature, damage evolution and lifetime performance are important. This is what the present study addresses, bringing together the initiation and development with the effects on fatigue life. In contrast to what is regularly reported in the literature, a lifetime increase was found for a specific multidirectional (quasi-isotropic), epoxy-based laminate after short- and long-term wet-aging. The reasons for this outcome were investigated using interrupted fatigue tests, microscopy, and residual strength analysis. As a result, significant differences in damage initiation and progress could be identified and related to the quasi-static and fatigue result. The main reason for the increase in lifetime is expected to be the distributed energy dissipation in the wet-aged condition. Here, damage occurs simultaneously in several regions, which leads to the fact that the introduced energy is divided among numerous defects at the same time. In the dry case, on the contrary, the growth of the few, individual damages terminates in an early catastrophic failure.Publicationtype: Conference Paper37 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Bearing strength high performance fibre metal thin-ply laminates(Queen's University Belfast, 2023); ; ; ; Publicationtype: Conference Paper51 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Numerical simulation of the bearing failure mechanism of thin-ply metal laminates(International Committee on Composite Materials, 2023); ; ; ; Within this study, the influence of a local hybridisation of Thin- and Thick-Ply laminates under bearing conditions was investigated by numerical simulations. The hybridisation was carried out by substituting 90° layers with stainless steel patches locally in the area of the load introduction. The stainless steel patches or foils had the same thickness as the prepreg layers. This prevents any unintentional change or disturbance of the laminate geometries. The CompDam - Deformation Gradient Decomposition (DGD) damage model was chosen for the numerical investigations. The study results showed that a reduction in layer thickness is associated with increased bearing strength. The bearing strength improvement of hybrid specimens is significantly higher for Thin- and Thick-Ply specimens. An improvement of almost 50 % could be achieved. Fibre kinking led to final failure for all configurations. The initiation of fibre kinking occurred in the 0° layers, which has the highest compressive stress. However, it could be observed that using stainless steel patches reduces the maximum compressive stress in the middle 0° layer. Thus, the initiation of fibre kinking shifted to higher strains or stresses, resulting in a higher bearing strength.Publicationtype: Conference Paper34 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Novel manufacturing process for natural fibre composites out of biobased and biodegradable materials(International Committee on Composite Materials, 2023); ; ; The use of natural fibres is an approach to address the issue of sustainability in the field of composites. With regard to potential recycling strategies for composites, natural fibres are particularly relevant in conjunction with biodegradable matrix materials. In addition to polysaccharides and lipids, proteins offer potential. Compared to conventional matrix materials such as epoxy resins, these kinds of natural materials cause some challenges in terms of processing. Combining two existing processes made it possible to produce flax fibre composites with a matrix based on the protein zein sourced from agricultural industry residues. A pre-impregnation of the flax fibres with the matrix ensures adequate infiltration of the flax fibres. By alternating stacking with films of the same matrix material, a good fibre-matrix bonding could be achieved, as shown with transverse tensile tests. By further modifying the matrix material, the mechanical properties were significantly improved. The developed manufacturing technique allows the reproducible fabrication of laminates with high quality. Therefore, unidirectional and cross-ply [0/90]2s flax fibre biocomposites were manufactured and characterised in terms of mechanical performance and optical damage analysis. While the unidirectional specific tensile strength of the novel biocomposites with protein-based matrix already reaches approx. 80 % of the best epoxy-based flax fibre composites available on the market, the achieved stiffness is already on the same level. Consequently, a comparison with other “green composites” highlights the competitiveness of fully biodegradable materials. The major advantage of the presented biocomposites, which makes them superior to all others in terms of sustainability, is the degradation by biological activity demonstrated in home composting. Recycling is thus possible even in the simplest conceivable process. Industrial composting at elevated temperatures and shredding of the composites can additionally lead to faster decomposition rates.Publicationtype: Conference Paper102