Roland Hinterhölzl

Experimental Determination of the Tensile and Shear Behaviour of Adhesives under Impact Loading

A robust analysis of adhesively bonded joints requires valuable input data for simulation. Mechanical properties of adhesives in tensile and shear directions at high deformation rates are necessary, particularly for crash scenarios. Butt joint specimens and lap shear specimens were, therefore, tested under impact loading using a tensile split Hopkinson bar (SHB). In this study, the adhesive deformation was measured using a high-speed camera and digital image correlation (DIC). The method was compared to a measurement of the adhesive deformations using the classical SHB analysis. It could be shown that the accuracy of the deformation measurement was significantly increased using high-speed imaging and DIC. In the butt joint specimens, the adhesive stiffness was 12.28 times higher and the energy absorbed was 1.83 times smaller using the DIC measurement than when using the classical SHB measurement of the deformations. In the lap shear specimens, the adhesive stiffness was 6.13 times higher and the energy absorbed was 1.29 times smaller compared to a classical SHB measurement of deformations. Additionally, a 3D finite element analysis showed that the design of the specimens has a minor influence on the stress–deformation relation. Therefore, accurate stress–deformation relations of adhesives under impact loading can be obtained in SHB experiments using DIC measurement.

Forming Simulation of Thick AFP Laminates and Comparison with Live CT Imaging

Automated fiber placement (AFP) process can be used to manufacture laminates by laying up unidirectional slit tapes along a desired path and placing multiple layers on top of each other. Usually, the slit tapes are placed direct onto the tooling to attain the final part geometry. Alternatively, the laminate can be built up on a planar substrate and can be subsequently formed into the final shape. This kind of processing allows manufacturing highly curved parts, which may not be possible with the direct placement. In the present work a forming simulation of thick AFP laminates is developed to predict the tapes’ orientations and delamination as well as transverse tape spread-ups and separations during the forming process. The simulation model is built up through the material characterization experiments. Validation is performed comparing the results of the simulation vs. the experimental forming on two generic geometries. An optical inspection is made on the external layers of the laminates. In a second step, live computer tomography (CT) scans are used to inspect the tapes within an AFP laminate during forming of an L- and a Z-flange. Tapes re-orientation, gaps and tapes widening are observed experimentally and compared to the simulation results. The simulation is capable to predict the tows orientation and provides indicators concerning the tows spread-up and separation.

Out-of-autoclave prepreg consolidation: Coupled air evacuation and prepreg impregnation modeling

Out-of-autoclave prepregs produce low-porosity parts through a complex consolidation process that includes air evacuation through a partially impregnated microstructure and subsequent resin infiltration flow. In this article, we propose, develop, and validate the first model describing this consolidation process by computing the thickness change of the material during processing. The air evacuation period is first simulated as a function of fiber packing and pressure conditions. Then, the cure period is described by modeling flow and compaction phenomena. Model predictions are compared to several experimental case studies, which show that the effect of most material properties and process parameters is well captured, and used in parametric studies to identify key trends.

Simulation and experimental validation of gaps and bridging in the automated fiber placement process

Abstract With the increasing demand for carbon fiber-reinforced parts in the aerospace industry, automated manufacturing methods such as automated fiber placement (AFP) are being established. To utilize AFP’s strong potential on multiple-curved surfaces such as chamfered sandwich structures, it is important to analyze and control emerging effects like gaps, bridging, and steering. An analytical relationship between path generation and effects of the AFP manufacture of chamfered sandwich structures have been found with good agreement to experimental tests. The deformation behavior of an industrially used compaction roller has been analyzed numerically and experimentally. The results show a nonuniform deformation behavior on flat and curved surfaces and the limited deformability of the compaction roller.

An experimental technique to characterize interply void formation in unidirectional prepregs

Out-of-autoclave prepreg processing requires evacuation of volatiles in the early stages of processing to achieve an acceptable final void content. In this study, single prepreg plies were laid-up onto a glass tool to simulate a ply–ply interface, to gain an understanding of initial air entrapment and eventual removal mechanisms. The contact was recorded during processing with various edge breathing configurations to identify the relationship between evacuation pathways and contact evolution. The existence of preferential flow channels along the fibre direction of the material was demonstrated by characterizing the prepreg surface. Gas evacuation in those channels prevented contact during an extended ambient temperature vacuum hold. The contact between the prepreg and glass tool equilibrated around 80% during the ambient vacuum hold, and reached full contact at elevated temperature after a brief loss in contact due to moisture vaporization, when the resin pressure decreased to below the water vapour pressure.

Characterisation of Tensile Properties Perpendicular to Fibre Direction of a Unidirectional Thermoplastic Composite Using a Dynamic Mechanical Analysis System

The ability of a draping simulation to accurately predict the outcome of a forming process mainly depends on the accuracy of the input parameters. For pre-impregnated composites, material must be characterised in the same conditions as forming occurs, i.e. in temperature regulated environment. Given the issues encountered while testing specimens enclosed in a thermal chamber and mounted on a tensile testing machine, new test methods have to be developed. A new approach using a Dynamic Mechanical Analysis system is presented for the investigation of tensile properties perpendicular to fibre direction of unidirectional pre-impregnated composites. Analyses are focused on a unidirectional carbon fibre thermoplastic tape reinforced polyamide 6 in its molten state. Quasi-static tests are performed at forming temperature for different loading rates with specimens of different geometries in order to assess the reproducibility of the test method.

Thermoforming of glass fibre reinforced polypropylene: A study on the influence of different process parameters

The aim of this study was to analyse the forming behaviour of glass fibre reinforced polypropylene and to identify the influence of several process parameters on the resulting part quality. For this purpose, a complex forming tool was designed, consisting of several areas with single and double curvature. The specimens were produced from unidirectional (UD) tape using the Fiberforge RELAY2000® automated tape laying machine and a subsequent consolidation step. They were then fixed in a support frame, pre-heated in an infrared oven, and formed in the forming tool, which was mounted into a hydraulic heating press. The investigated process parameters were the number and force of the springs in the support frame, the tool temperature and the forming pressure and speed. The layups of the specimens were [0/90/0/90/0]s and [0/45/90/-45/0]s. After the forming process, the parts were analysed in terms of their quality, with a special focus on wrinkles, undulations, gaps and surface roughness. In addition to optic...

Finite element mapping for incompatible FE meshes of composite structures

A FE mapping algorithm for a data transfer between incompatible meshes is proposed.The structural analysis is based on process simulation data for composites.Only the transfer of fiber orientations is not sufficient for a structural analysis.Material properties such as FVC, permeability and stiffness are predicted.The method is capable of handling partially out-of-plane wrinkling defects. Finite element analysis (FEA) of structural composites is mostly based on an as-designed geometry and input data. As-designed input data do not consider the manufacturing processes. For an as-built structural simulation of composites, it is important to integrate manufacturing process data into the structural analysis. Therefore, mapping algorithms are needed to transfer and process data between different process and structural simulation steps considering the application of different finite elements and media discretization for the individual simulation steps. This paper considers a mapping algorithm based on a bucket sort algorithm, shape interpolation functions of finite elements and internal fiber architectures of composite materials with a subsequent material properties prediction. The proposed algorithm is applicable for unidirectional composites as well as for non-crimped, woven and braided fabrics. Particular, it is shown how fiber orientation, as vector value of finite elements, is sensible for a data transfer between meshes with out-of-plane material defects. This integrated simulation approach is applied on a generic demonstrator geometry and aerospace component geometries. The implementation is realized within a new developed simulation platform for composites structures, from process up to structural simulations.

Thermal dimensioning of manufacturing moulds with multiple resistively heated zones for composite processing

Multi-zonal, electrically heated moulds for composite processing offer the potential of a direct heat introduction with low thermal lag and high energy efficiency. However, appropriate thermal dimensioning of these tools requires the consideration of the thermal response of the tool itself as well as the thermal and cure behaviour of the part, which is to date mostly estimated based on experience. To realize the full potential of this tool class, a numerical method is presented to determine a sound partitioning of the designated heating area utilizing 3D finite element cure simulation. Further, a cure simulation model of an application case is set up and validated. The capability of the numerical method to significantly increase the temperature accuracy and the degree of cure homogeneity are demonstrated in an evaluation of the numerically improved application case. Finally, the effect of the tool material on the zone allocations and temperature accuracy is studied.

On the modeling of separation foils in thermoforming simulations

Composite forming simulations consist in modelling the forming process of composite components to anticipate the occurrence of potential flaws such as out-of-plane wrinkles and fibre re-orientation. Forming methods often consist of automated processes in which flat composite blanks are forced to comply with tool geometries. Although Finite Element forming simulations require the modelling of all stakeholders (blankholder, tooling and composite blank), consumables such as separation films are often not considered. Used in thermoforming processes, these films are placed between tooling and composite to ease part removal after forming. These films are also used to decrease tool/ply friction and thus, enhance forming quality. This work presents thermoforming simulations of pre-impregnated carbon fibre thermoplastic blanks in which separation films are modelled in the same manner as composite layers, i.e. by a layer of shell elements. The mechanical properties of such films are also characterised at the same t...