Showing papers by "Maik Gude published in 2020"

Joining of Thermoplastic Composites with Metals Using Resistance Element Welding

Abstract: Joining is a key enabler for a successful application of thermoplastic composites (TPC) in future multi-material systems. To use joining technologies, such as resistance welding for composite-metal joints, auxiliary joining elements (weld inserts) can be integrated into the composite and used as an interface. The authors pursue the approach of embedding metal weld inserts in TPC during compression moulding without fibre damage. The technology is based on the concept of moulding holes by a pin and simultaneously placing the weld insert in the moulded hole. Subsequently, the composite component can be joined with metal structures using conventional spot welding guns. For this purpose, two different types of weld inserts were embedded in glass fibre reinforced polypropylene sheets and then welded to steel sheets. A simulation of the welding process determined suitable welding parameters. The quality of the joints was analysed by microsections before and after the welding process. In addition, the joint strength was evaluated by chisel tests as well as single-lap shear tests for the different weld insert designs. It could be shown that high-quality joints can be achieved by using the innovative technology and that the load-bearing capacity is significantly influenced by the weld inserts head design.

Influence of component design on features and properties in thermoplastic overmoulded composites

Abstract: Thermoplastic composite overmoulding is an integrated process to manufacture components with combined continuous and short fibre reinforcements. These components benefit from high intrinsic mechanical properties, geometric complexity and low production cycle times. In this study, ribbed plates were manufactured by overmoulding short-fibre CF/PPS (carbon fibre/polyphenylene sulphide) material onto continuous woven-fibre CF/PPS flat preforms. Specifically, the effects of the rib geometry and flow length on the process-induced features were investigated using optical microscopy. The bonding at the overmoulded interface was evaluated via quasi-static tensile rib pull-off tests. Results indicate that both the bond strength and corresponding failure type vary with rib geometry. However, the effects of the specimen position along the flow length are only significant towards the end path. The implications of certain rib designs are discussed both qualitatively and quantitatively, based on feature development at the overmoulded interface during manufacture.

Experimental and Numerical Impact Analysis of Automotive Bumper Brackets Made of 2D Triaxially Braided CFRP Composites

Abstract: The impact behavior of carbon fiber epoxy bumper brackets reinforced with 2D biaxial and 2D triaxial braids was experimentally and numerically analyzed. For this purpose, a phenomenological damage model was modified and implemented as a user material in ABAQUS. It was hypothesized that all input parameters could be determined from a suitable high-speed test program. Therefore, novel impact test device was designed, developed and integrated into a drop tower. Drop tower tests with different impactor masses and impact velocities at different bumper bracket configurations were conducted to compare the numerically predicted deformation and damage behavior with experimental evidence. Good correlations between simulations and tests were found, both for the global structural deformation, including fracture, and local damage entities in the impact zone. It was proven that the developed phenomenological damage models can be fully applied for present-day industrial problems.

Multi-material adhesively bonded structures: Characterisation and modelling of their rate-dependent performance

Abstract: The rate-dependent failure response of multi-material adhesive joints for three deformation modes is investigated. A combination of carbon fibre reinforced polymers (CFRP) and titanium alloy Ti-6Al-4V is employed. The experiments provide important information about the failure sequence of a multi-material adhesive joints, which depends upon the loading rate regime. This is the first time that dynamic fracture mechanics experiments are performed in multi-material adhesive structures. The observed experimental results suggest a rate-dependent failure sequence for mode I dominated fracture. Simulations of the experiments are used to predict and rationalise the failure performance of the multi-material adhesive joint. The numerical analysis highlighted the importance of the individual knowledge of the rate-dependent mechanical performance of adhesive and composite to fully understand the fracture sequence of multi-material joints under impact.

Clinching in in-situ CT—A numerical study on suitable tool materials

Abstract: In lightweight design, clinching is a cost efficient joining method, especially for different joined materials and thicknesses. The quality of such a joint is usually evaluated by ex-situ destructive testing methods. These methods, however, do not yield the detection of phenomena occurring during the joining process such as elastic deformations and cracks that close after unloading. Alternatively, in-situ computed tomography (in-situ CT) can be used to investigate the manufacturing process of the clinch point depending on varying boundary conditions regarding process and material parameters. As the use of a classical clinching tool made of steel leads to strong artefacts and to a high attenuation, the image quality of CT scans is limited so far. The aim of this work is the identification of materials or material combinations which allow CT measurements and are suitable as a clinching tool material. Therefore, mechanical requirements on the tool materials are identified, potentially CT-suitable tool materials are selected and numerically investigated. In the investigations, two lubricated aluminium sheets are single-step clinched to form a rotationally symmetric joint without cut section. The simulation model is verified by applying common quality criteria on the simulation results (undercut, bottom thickness, neck thickness). It is shown that e.g. silicon nitride and titanium β alloys are best suited for the punch, cermets and titanium α+β are suitable for the die, and fibre reinforced plastics can be selected for the blank holder tip.

The Impact of Draping Effects on the Stiffness and Failure Behavior of Unidirectional Non-Crimp Fabric Fiber Reinforced Composites.

Abstract: Unidirectional non-crimp fabrics (UD-NCF) are often used to exploit the lightweight potential of continuous fiber reinforced plastics (CoFRP). During the draping process, the UD-NCF fabric can undergo large deformations that alter the local fiber orientation, the local fiber volume content (FVC) and create local fiber waviness. Especially the FVC is affected and has a large impact on the mechanical properties. This impact, resulting from different deformation modes during draping, is in general not considered in composite design processes. To analyze the impact of different draping effects on the mechanical properties and the failure behavior of UD-NCF composites, experimental results of reference laminates are compared to the results of laminates with specifically induced draping effects, such as non-constant FVC and fiber waviness. Furthermore, an analytical model to predict the failure strengths of UD laminates with in-plane waviness is introduced. The resulting stiffness and strength values for different FVC or amplitude to wavelength configurations are presented and discussed. In addition, failure envelopes based on the PUCK failure criterion for each draping effect are derived, which show a clear specific impact on the mechanical properties. The findings suggest that each draping effect leads to a “new fabric” type. Additionally, analytical models are introduced and the experimental results are compared to the predictions. Results indicate that the models provide reliable predictions for each draping effect. Recommendations regarding necessary tests to consider each draping effect are presented. As a further prospect the resulting stiffness and strength values for each draping effect can be used for a more accurate prediction of the structural performance of CoFRP parts.

Determining the Damage and Failure Behaviour of Textile Reinforced Composites under Combined In-Plane and Out-of-Plane Loading.

Abstract: The absence of sufficient knowledge of the heterogeneous damage behaviour of textile reinforced composites, especially under combined in-plane and out-of-plane loadings, requires the development of multi-scale experimental and numerical methods. In the scope of this paper, three different types of plain weave fabrics with increasing areal weight were considered to characterise the influence of ondulation and nesting effects on the damage behaviour. Therefore an advanced new biaxial testing method has been elaborated to experimentally determine the fracture resistance at the combined biaxial loads. Methods in image processing of the acquired in-situ CT data and micrographs have been utilised to obtain profound knowledge of the textile geometry and the distribution of the fibre volume content of each type. Combining the derived data of the idealised geometry with a numerical multi-scale approach was sufficient to determine the fracture resistances of predefined uniaxial and biaxial load paths. Thereby, Cuntze’s three-dimensional failure mode concept was incorporated to predict damage and failure. The embedded element method was used to obtain a structured mesh of the complex textile geometries. The usage of statistical and visualisation methods contributed to a profound comprehension of the ondulation and nesting effects.

Non-destructive testing of a rotating glass-fibre-reinforced polymer disc by swept source optical coherence tomography

Abstract: Composite materials are used for high-performance rotating blades, e.g. in turbines and wind power plants. Here, optical coherence tomography was used to visualize large areas of fibre-reinforcedpolymer discs at rotation speeds of up to 1200 rpm. These measurements allowed to visualize the fibre structure over large areas of the disc. By recording the front and back reflex, the wobble of the disc was measured precisely. Additionally, the recorded structure was used to detect even small deviations from a uniform rotation.

Motion blur suppression by using an optical derotator for deformation measurement of rotating components

Abstract: With increasing requirements on the efficiency of aircraft engines and the application of advanced materials, the non-contact evaluation of the deformation and the vibration behaviour is of increasing interest. For optical deformation measurements at high surface speeds, motion blur becomes a critical uncertainty factor. A full-field measurement method for rotating components is proposed, in which in-situ high-resolution images can be acquired for digital image correlation. The measurements are carried out through an optical derotator, which allows rotational motion blur to be largely avoided. Optical image distortions due to the use of a dove-prism are considered and the effects are attempted to be minimized by an angle-precise triggering of the digital single-lens reflex camera. The measured deformations on a generic bladed disc under different rotational speeds up to 2500 RPM show a good correlation compared both to 3D digital image correlation measurements from ARAMIS as well as to numerical predictions. The results demonstrate that the approach is sound for measuring the deformation, whereas measurements at the circumference reveal the image distortion effects, posing that further steps are necessary for an investigation of the vibration behavior.

Influence of Ice Accumulation on the Structural Dynamic Behaviour of Composite Rotors

Abstract: The implementation of wind turbines as a source of sustainable, renewable energy is increasing. Although the prospects of renewable energy development are promising, ice accumulation on turbine blades still stands as a major operational issue. Excessive ice mass on turbine blades can lead to damage or total failure of the blades but also to the nacelle gearbox and to the generator. Therefore, a detailed understanding of the ice accumulation on the composite blades and the effect on their modal properties can be beneficial and give an insight before catastrophic failure occurs. On the one hand, it can be understood how ice accumulation affects the profile of the composite surface to consequently identify the relationships between ice accumulation and mass, stiffness, as well as damping distribution. On the other hand, by mapping these relationships, the first step is performed towards solving the inverse problem, which is to identify critical ice accumulation at an early stage based on modal properties. In this way, ice detection and identification can provide significant savings in time and costs. To investigate the basic relationships between ice accumulation and structural dynamic behaviour, an experimental rotor test rig is developed, combining an electromotor with a climate chamber. The test rig simulates various environmental conditions under different rotational speeds and ice distributions. The first experimental tests are performed on a glass-fibre reinforced epoxy rotor, and several measurements are conducted deploying different kinds of icing and temperature conditions. Various sensors are applied to characterise the vibration response as well as mass, type, and spatial distribution of the ice. The results are evaluated with regard to identifying unknown relations between ice accumulation and the structural dynamic behaviour of composite rotors.

Characterization and Numerical Modelling of Through-Thickness Metallic-Pin-Reinforced Fibre/Thermoplastic Composites under Bending Loading

Abstract: Through-Thickness Reinforcement (TTR) technologies are well suited to improving the mechanical properties in the out-of-plane direction of fibre-reinforced composites. However, besides the enhancement of delamination resistance and thus the prevention of overall catastrophic failure, the presence of additional reinforcement elements in the composite structure affects also the mechanical properties in in-plane direction. In this work, the flexural behaviour of a glass-polypropylene (GF/PP) hybrid yarn-based composite with TTR in form of metallic pins has been investigated experimentally and numerically. The insertion of the metallic pins is realized via thermoactivated pinning technology (TAP). In four-point-bending tests, it is shown that the flexural stiffness and strength decreases with an increase of the overall pin density. Hereby, it is observed that the pins act as crack initiators. For numerical modelling on specimen level, a continuum damage mechanic (CDM) model is used to predict the nonlinear deformation response of the composite, as well as fibre fracture and matrix cracking. A debonding and slipping phenomena of the pin in the composite is modelled by a cohesive zone modelling approach for the interface between pin and composite.

Lastangepasste Generierung von irregulären Gitterstrukturen auf Basis von Voronoi-Diagrammen

Abstract: Die Verwendung von Gitterstrukturen innerhalb eines additiv gefertigten Bauteils kann bei gezielter Anpassung und Dimensionierung der Gitterstege belastungs- und gewichtsoptimiert erfolgen Dies ist von Vorteil, da bei gleichbleibender Ausengeometrie das Gewicht eines Bauteils deutlich reduziert werden kann