Fatigue
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Global-local thermomechanical analysis of fracture in polycrystalline silicon shells using a phase-field approach.The photovoltaic (PV) modules containing multiple polycrystalline silicon solar cells (PSSCs) are one of the most common and widely used devices for the production of solar energy. However, the energy production efficiency degrades during their lifespan, which can be primarily associated with the cracking in a polycrystalline silicon wafer (PSW). The aim of this joint research project (ISD - Leibniz Universität Hannover, IAM - TU Braunschweig) is to evaluate the overall stiffness degradation of polycrystalline silicon solar cells (PSSCs) due to microcracking. PSSCs are intricate component consisting of multiple materials and modeling becomes computationally expensive. Therefore, modeling reduction techniques such as numerical homogenization were employed for evaluating the effective material properties of PSSCs including cracks. The cracks are to be modeled using a phase-field approach. An improved Voronoi-tessellation scheme was used to generate polycrystalline patterns of the PSW and a mean-field homogenization scheme was employed to determine the homogenized response of PSSCs. The accuracy of the homogenization scheme was verified and the material response of the heterogeneous and homogeneous PSSCs was compared.Led by: Prof. Dr-Ing. habil. Raimund RolfesTeam:Year: 2018Funding: DFG, German Research FoundationDuration: 01.08.2018 - 31.07.2021
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New methods for failure and fatigue analysis of suction panels for laminar flow controlAlthough the suction panel concept holds a high potential to increase the sustainability of future aircrafts, it comes with some structural mechanical challenges that need to be carefully examined. With the panel’s underlying backbone structure adopting the load-carrying function of the outer airfoil in the suction area (see Fig. 1), the stress flux in the airfoil is considerably disturbed, resulting in multiple, potentially critical stress concentrations. To ensure a sufficient robustness of the suction panel concept in terms of static strength and fatigue resistance, the backbone structure is to be analyzed numerically by means of finite element simulations. With deep knowledge in the field of continuum damage mechanics and progressive fatigue analysis, ISD will perform high fidelity strength and fatigue analyses of the backbone structure to identify sufficiently robust designs of the backbone structure. To calibrate the numerical methods, experimental coupon tests of the backbone structure’s base material are scheduled to identify respective static and fatigue-related material properties. Beside the identification of mechanically robust designs of the suction panel, the numerical simulations are also to address topics like scalability of the suction concept and the benefits of thin ply laminates, which are well known to feature a superior fatigue resistance.Led by: Prof. Dr-Ing habil Raimund RolfesTeam:Year: 2019Funding: DFG, German Research FoundationDuration: 01.04.2019-31.12.2022
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SE2A-Excellence Cluster sustainable and energy efficient aviationAlthough the suction panel concept holds a high potential to increase the sustainability of future aircraft, it comes with some structural mechanical challenges that need to be carefully examined. With the panel’s underlying backbone structure adopting the load-carrying function of the outer airfoil in the suction area, the stress flux in the airfoil is considerably disturbed, resulting in multiple, potentially critical stress concentrations. To ensure sufficient robustness of the suction panel concept in terms of static strength and fatigue resistance, the backbone structure is to be analyzed numerically employing finite element simulations. With deep knowledge in the field of continuum damage mechanics and progressive fatigue analysis, ISD will perform high-fidelity strength and fatigue analyses of the backbone structure to identify sufficiently robust designs of the backbone structure. From the mechanical standpoint, thin-ply (TP) laminates are known to have better static strength and fatigue resistance in contrast to conventional laminates. A well-established fatigue damage model (FDM) was calibrated and modified in order to consider the influence of ply thickness under static and cyclic loading.Led by: Prof. Dr-Ing. habil. Raimund RolfesTeam:Year: 2019Funding: DFG, German Research FoundationDuration: 01.04.2019-31.12.2022
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Experimental analysis and numerical modelling of microcrack induced delaminations under cyclic loading with load reversalsThe aim of this joint research project (ISD - Leibniz Universität Hannover, ILK - TU Dresden) is to develop a profound understanding of the damage process during delamination growth in fibre-reinforced polymer laminates (FRP) based on existing inter fibre failures during cyclic loading with load reversals. By analysing and quantifying the relevant damage processes during loading, the influence of the level and direction of the applied load on delamination growth in FRP laminates is clarified. Based on the experimental work, detailed numerical simulations on a macro- and mesoscopic level are developed which allow a purposeful analysis of the delamination process in DCB- and ENF- as well as in laminate experiments. Thus, the delamination length-dependent analysis of the fracture modes, which cannot be implemented experimentally, is made possible. Consequently, it is analysed whether characteristic values determined by standardised crack propagation investigations (DCB, ENF etc.) can be transferred to embedded layers. In addition, the investigations provide extensive experimental results on the delamination process in FRP laminates under in-plane loading and thus create a basis for the development of suitable analytical and numerical models. It is investigated to what extent existing numerical damage models (e.g. cohesive zone approaches) allow reliable and efficient modelling of cyclic delamination growth and how the mesoscopic simulation results can be used for macroscopic simulations in terms of continuum mechanics.Led by: Prof. Dr.-Ing. habil. Raimund RolfesTeam:Year: 2021Funding: German Research Foundation (DFG) - Project number 457043708Duration: 01.09.2021-31.08.2023
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Fatigue behavior and fatigue damage prediction of short fiber - reinforced adhesives in Blades of wind turbine (Add2ReliaBlade)The reliability of rotor blades has a special importance for a safe and economic operation of wind turbines (WT). Turbine manufacturers have gained a lot of experience in the design of rotor blades and damage during operation over the past decades. Nevertheless, cracks in the rotor blade structure are still causes of costly repairs and operational failures. This indicates that there are still knowledge gaps in the design of fatigue damage in rotor blades, which is especially (but not exclusively) true for rotor blade bonding. To address these knowledge gaps and increase rotor blade reliability, the Add2RelaiBlade project was established in 2021 as a complementary addition to the ReliaBlade project. The focus of the Add2ReliaBlade project is on the development, characterization and validation of simulation methods and models for the description of the (fatigue) damage behavior of (short fiber reinforced) adhesive joints in rotor blades. The focus is on trailing edge bonding, as this is particularly susceptible to damage. This sub-project provides substantial contributions in the context of extended material testing and non-destructive imaging for short fiber reinforced adhesives, modeling of the spatial distribution of fiber orientation in short fiber reinforced trailing edge bonding, continuum mechanical and energy based modeling of the fatigue damage behavior of (short fiber reinforced) bonded joints as well as the establishment of data based methods of numerical mechanics for the analysis of fatigue damage of short fiber reinforced bonded joints.Led by: Prof. Dr.-Ing. habil. Raimund RolfesTeam:Year: 2021Funding: Federal ministry for Economic Affair and EnergyDuration: 01.05.2021-30.4.2024
Structures
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Development of a safety cockpit for gliders (CraCpit)In the joint project "CraCpit", the partners are developing solutions for a safety cockpit for gliders. The LCC and the Akaflieg München at the TU Munich are concentrating on the new development of a cockpit structure, while the ISD and the Akaflieg Hannover at the Leibniz Universität Hannover (LUH) are addressing a retrofit solution for existing aircraft types. The goal of the partners at the LUH is the design and the certifiable draft of retrofittable structural elements for the production of a safety cockpit in older, existing glider types according to current requirements. In the event of a crash, cockpit structures are subjected to high stresses that lead to large deformations and material failure. The retrofitted structural components take on different (opposing) functions, such as ensuring the pilot's survival space or energy dissipation to reduce the impact. The effectiveness of the individual components can be assessed and optimised using FEM simulations. For this purpose, appropriate material formulations are required that are able to physically represent the structural response from the onset of loading far into the post-failure range with sufficient accuracy. The focus of the work at the ISD lies in the material modelling, the simulation and the evaluation of the component function of the retrofit elements. The validation of the material models is carried out through tests at sub-component level. After the optimisation of the components through simulation, a test at the structural level (simulation of the entire fuselage and crash test of an example fuselage (prototype)) concludes the project. The experimental work is carried out in close cooperation with the partner Akaflieg Hannover e.V.. For this purpose, various test specimens and prototypes will be built and tested. In addition, an industrial partner will be involved to ensure commercial exploitation.Led by: Prof. Dr-Ing habil. Raimund RolfesTeam:Year: 2017Funding: Federal Ministry for Economic Affairs and Energy – 20E1703DDuration: 2018-2021
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Improved structural performance through the use of random field analysisThe research performed within this project uses the effect of random variations in structure’s geometry and/or material to get information on local sensitivity of structures to deviations from their baseline value. This information cannot only be useful in quality assurance, by finding areas most sensitive to deviations, but can also be used to improve the design. This approach can load to an increase in structural parameters such as buckling load, fatigue life and others.Led by: Prof. Dr-Ing habil. Raimund RolfesTeam:Year: 2019Funding: SE²A excellence, Cluster of DFGDuration: 2019-2022
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Multistable Morphing Structures using Variable Stiffness CompositesThe research project aims at developing multistable structures with morphing capabilities. A variable stiffness composite is used which allows stiffness tailoring with much larger design space. The developed semi analytical method is validated well within a Finite element framework. In this work, the concept of static, smart and dynamic actuations are exploited on bistable laminates to reduce the snap-through requirements.Led by: Prof. Dr-Ing habil. Raimund RolfesTeam:Year: 2019Funding: Deutscher Akademischer Austauschdienst (DAAD)Duration: 2019-2021
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FANFOLD – Fast nonlinear machine learned analysis for rotor bladesThe performance and reliability of the rotor blade is crucial for the efficiency of a wind turbine. The blades account for a large part of the turbine costs - their repair and maintenance costs are comparatively high. Rotor blades need to be less prone to failure and less often in need of repair. Concepts of predictive maintenance and the digital twin, which will probably account for a significant part of the profits in the rotor blade market in the future, also point in this direction of reducing repair and maintenance costs. A prerequisite for the implementation of the above concepts is a fast analysis method for fibre composite structures (prediction and evaluation of damage progression and service life). For the overall simulation of the rotor blade, FE analyses using linear-elastic material models are used today. Non-linear effects due to damage or even continuous damage evolution must be investigated on a smaller scale by means of experiments or detailed simulations. In order to go one step further and, for example, to record the influence of non-linear, progressive damage processes on the aeroelasticity and service life, or in order to be able to base the quasi-static simulation in the design on less conservative reduction factors, the overall simulation on the rotor blade would have to be carried out directly taking progressive damage processes into account (load redistribution effects). Obstacles so far are the too high calculation effort and the costly experimental characterisation of existing material models. Two main topics are to be addressed in order to meet these challenges: 1. development of a new, non-linear and fast structure simulation at blade level 2. reduction of the material characterisation effort through machine learning The aim of this sub-project is a valid, efficient and cost-effective non-linear rotor blade simulation as mentioned under point one.Led by: Prof. Dr-Ing. habil. Raimund RolfesTeam:Year: 2020Funding: Federal Ministry for Economic Affairs and Energy – FKZ 03EE3028ADuration: 2020 –2023
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Nanocomposites
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Functionalized, multi-physically optimized adhesives for inherent structural health monitoring of rotor blades (Func2Ad)The performance and reliability of the rotor blade is crucial for the efficiency of a wind turbine over its entire life cycle. The blades make up a large part of the equipment cost - their manufacturing and maintenance costs are extremely high. The adhesive technology in the rotor blade is a key technology for achieving competitive advantages in the wind industry. The processing and curing properties (processability) of the adhesives as well as their operational stability (fatigue strength) in the cured state are two key parameters with regard to the system economy and the return on investment. A third would be the remote diagnosis of the glued joints of the rotor blade (Structure Health Monitoring). The research project proposed here on particle-modified adhesive systems for the wind industry starts with the three points mentioned. A main innovation is the functionalization of the adhesive resin through particle modification to implement a structural monitoring system inherent in the adhesive connections on the rotor blade. This is said to be done by modifying the electrical properties of the adhesive resin. At the same time, the processability and fatigue strength of the adhesive should be positively influenced by the modification. If the modified resin system is only optimized for one of the three aspects mentioned, there is a risk of poor performance with regard to the other. The physical properties of the adhesive must therefore not be separated for the three requirement areas, but must be considered and optimized together in their interaction and their interrelationships. In order to optimize this and increase the efficiency of the multiphysical material models, machine learning methods are used within the simulation framework.Led by: Prof. Dr-Ing. habil. Raimund RolfesTeam:Year: 2023Funding: Bundesministerium für Wirtschaft und Klimaschutz, FKZ 03EE3069 A-FDuration: 01.01.2023-31.12.2026
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Material Modeling
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Abstract modelling of the nonlinear mechanical response of joints in fiber reinforced composite assembliesIn Aviation, Automobile and Wind turbine industries, there are large composite structures which are connected using thousands of mechanical joints or adhesives. For the efficient construction of such composite structures, it is essential to evaluate the behavior of such composite joints, which is usually very complex due to the presence of non-linearities and the joint has distinct failure modes. Accurate simulation of composite joints using detailed models gives a good estimate of the joint behavior and its failure properties but it comes at the cost of computational time. The project aims to reduce the computational cost involved in the simulation of composite joints by developing an abstract model with reduced degrees of freedom. The reduction in degrees of freedom of the model is sought by using structural elements such as Shells and Beams. The project aims to create a model which will capture the behavior of the joint in the finite strain regime, such that the anisotropy of composite material and various non-linearities such as plasticity, damage, contact, friction etc. can be simulated efficiently.Led by: Prof. Dr-Ing. habil. Raimund RolfesTeam:Year: 2022Funding: DFG, German Research FoundationDuration: 01.07.2022-30.06.2025
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Functionalized, multi-physically optimized adhesives for inherent structural health monitoring of rotor blades (Func2Ad)The performance and reliability of the rotor blade is crucial for the efficiency of a wind turbine over its entire life cycle. The blades make up a large part of the equipment cost - their manufacturing and maintenance costs are extremely high. The adhesive technology in the rotor blade is a key technology for achieving competitive advantages in the wind industry. The processing and curing properties (processability) of the adhesives as well as their operational stability (fatigue strength) in the cured state are two key parameters with regard to the system economy and the return on investment. A third would be the remote diagnosis of the glued joints of the rotor blade (Structure Health Monitoring). The research project proposed here on particle-modified adhesive systems for the wind industry starts with the three points mentioned. A main innovation is the functionalization of the adhesive resin through particle modification to implement a structural monitoring system inherent in the adhesive connections on the rotor blade. This is said to be done by modifying the electrical properties of the adhesive resin. At the same time, the processability and fatigue strength of the adhesive should be positively influenced by the modification. If the modified resin system is only optimized for one of the three aspects mentioned, there is a risk of poor performance with regard to the other. The physical properties of the adhesive must therefore not be separated for the three requirement areas, but must be considered and optimized together in their interaction and their interrelationships. In order to optimize this and increase the efficiency of the multiphysical material models, machine learning methods are used within the simulation framework.Led by: Prof. Dr-Ing. habil. Raimund RolfesTeam:Year: 2023Funding: Bundesministerium für Wirtschaft und Klimaschutz, FKZ 03EE3069 A-FDuration: 01.01.2023-31.12.2026
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