Frank Helbig

Numerical characterisation of the mechanical behaviour of a vertical spacer yarn in thick warp knitted spacer fabrics

Warp knitted spacer fabrics (WKSF) are technical textile structures that present special compression-elastic, air permeability, heat resistance and acoustic damping properties and because of that they are winning more application fields every day. In many of these fields, like the application for cushion structures in car seats and mattresses, the thick WKSF are of special interest and the mechanical behaviour of these structures needs to be known in detail but, although direct relationships exist between the textile technical and technological parameters and the mechanical behaviour of the mentioned structures, this characterisation is still being carried out experimentally. Analytical and numerical models have already been developed to study WKSF structures offering interesting contributions to a better knowledge of their behaviour and providing further research directions but there are some phenomena causing the particular and characteristic mechanical behaviour of these structures that have not been described yet. The aim of this paper is to give a better understanding of the mechanical behaviour of a single vertical spacer yarn of a thick WKSF under compression load by studying, with help of the finite elements method, the variations of the bending moment acting on the yarn during the compression process. In this article it has been found that these variations of the mentioned bending moment are the phenomena responsible for the characteristic mechanical behaviour of the WKSF structures.

Computational modelling of the complex dynamics of chemically blown polyurethane foam

This study presents computational analysis of the complex dynamics observed in chemically blown polyurethane foams during reaction injection molding process. The mathematical formulation introduces an experimentally motivated non-divergence free setup for the continuity equations which reflects the self expanding behaviour observed in the physical system. The foam growth phenomena which is normally initiated by adequate pre-mixing of necessary reactant polymers, leading to an exothermic polymerization reaction, bubble nucleation, and gas formation, is captured numerically. We assume the dependence of material viscosity on the degree of cure/polymerization, gas volume fraction, and temperature as well as non-dependence of mixture density on pressure. The set of unsteady nonlinear coupled partial differential equations describing the dynamics of the system are solved numerically for state variables using finite volume techniques such that the front of the flow is tracked with high resolution interface captu...

Mechanical properties of polymer melt-impregnated fiber tape sandwiches using injection molding technology

In this study, the direct melt impregnation of unidirectional glass fiber tapes in an injection molding process is investigated. The simple textile structures were used for a load-adapted reinforcement of injection-molded parts, determining the impregnation quality by mechanical tests. A sandwich layer construction was made with an outer unidirectional fiber layers and an inner injection-molded layer with pure or additionally short fiber-reinforced polypropylene (PP). The glass fiber tapes were produced in a continuously working fiber–foil process where the aligned fiber bundles have been fixed with one side on a PP foil under temporarily acting pressure and temperature. A special bundle spreading device reduced the number of individual fiber layers, which ensured the direct melt impregnation in the injection molding process. The mechanical properties of the sandwich structures were determined using a three-point bending flexural test as well as Charpy and puncture impact tests to investigate the energy absorption. The results were compared to unreinforced and globally short fiber-reinforced test samples. The local reinforcement, designed for bending stiffness and energy absorption, led to a considerable reinforcement effect with minimal mass increase in comparison with the short fiber-reinforced samples. The fiber masses required to achieve commensurable properties were significantly reduced. Thus, when using the fiber tapes, only one-third of the fiber mass necessary for reinforcement with short glass fibers was required.

Material Selection and Process Configuration for Free-Form, Voluminous and Textile-Based Multi-Material-Design by the Example of a Bucket Seat

Hybrid textile-based composites possess an enormous potential for energy and resource efficient large-scale production, with freedom in and high specific mechanical properties. This paper covers the connection of available and established production processes for textiles in a differential process chain for the manufacturing of complex shaped and elastic sandwich components. The technology enables both stiffness and comfort through elasticity.OLU-Preg®-organic sheets, polyurethane foam cores and 3D-spacer fabrics form the targeted properties of demonstrator models. This article refers to the demonstrator part “bucket seat”. To show the benefit of complex composite material, the lightweight and mechanical properties of the sandwich structures are tested in several variations of core and comfort shapes. Absolute and specific improvements of performance are shown in static and dynamic examinations. An Analysis of coupling effects, deformation and failure behavior of the multi-material design (MMD) complete the scientific approach of the structure-property relationships of hybrid composites.

Investigation of the specific adhesion between polyurethane foams and thermoplastics to suited material selection in lightweight structures

Lightweight construction combines various materials to create resource efficient components. Thermoplastics (TPs) combined with polyurethane (PUR) foams are increasingly used to create hybrid composites. Optimizing the energy efficiency is one of the main issues in the development of production processes of components. Reducing the number of process steps offers great potential in this respect. PUR foam develops a strong adhesive bond with most materials. This is used for the manufacturing of hybrid composite components by filling complex cavities with PUR foam simultaneously bonded with other TP polymer components. This way, one process step for joining is saved. The interfaces in this composite structures are critical points of the failure. A huge variety of TP is used for the production of hybrid composite components and PUR foam develops varying bonding strengths with all of them. Selecting the suitable TPs for a durable bonding with PUR foam in the desired production process necessarily requires information about the respective specific adhesion. In this investigation, different TPs were processed with PUR foams in order to manufacture sandwich composites. The TP facings are produced in the injection moulding process. Subsequently, the facings are combined with the foam core during reaction injection moulding. The wetting behaviour was examined using the contact angle measurement and the mechanical strength of the interface in the sandwich composite was determined using a tensile test. A precise order of the selected TPs concerning their specific adhesion to PUR foams was achieved with these investigative methods.

Torque-Fiber-Winding (TFW)-Procedure: Manufacturing of Textile-Based Unidirectional Prepreg for Raw Material and Material Development of Carbon Fibre Reinforced Thermoplastics

There is the need to determine the process capability of available and novel carbon fibre (CF) roving with minimal material and reproducible procedures in the field of research and development of continuous fibre reinforced composites and structural components, as well as to identify the power delivery in thermoplastic laminate constructions. The innovative TFW procedure with the appropriate system technology allows the production of piece size variable unidirectional (UD) prepreg in a continuous sequential process of spiral winding. A flexible surface design, resulting in the partial fixation of a single highly spread CF roving on fine nonwoven fabric. By defined accumulating of composite components, the fibre volume content (FVC) is adjustable and correspond to the level of spreading and to the grammage of nonwoven fabric. Minimum single layer thickness promote compound homogeneity and thereby allow the generation of greatest possible degrees of freedom in load-oriented structural design of CF-reinforced thermoplastic lightweight products in the laboratory staff.

Influence of the Chemical Functionalisation of Glass Fibre Surfaces on the Mechanical Properties of Continuous Fibre Reinforced Thermoplastics

Continuous fibre reinforcements in thermoplastic composites require an enhanced adhesion to the matrix component, in order to effectively divert external forces from the matrix. Therefore different silanes as adhesion promoters are used as a part of the sizing. They operate as a connector to the matrix component. These silanes affect the sliding properties of the sizing during the production of the glass filaments in a negative way, in which case the proportion of the adhesion promoter in the sizing must be kept at a low level in order to maintain the processing speed in the textile production process. With the immersion bath method, it was examined whether the treatment of the surface of textile fabric after the textile production process with a silane-containing aqueous solution could solve these problems. Different silane concentrations and solvents were considered. After drying the textiles were processed during a two-step pressing process directly into a multi-layer organic sheet with a textile-based polypropylene matrix. The successful layering of the adhesion promoter on the glass fibre surface was verified by Fourier transform infrared (FTIR) spectroscopy. With thermogravimetric analysis (TGA), the thermal resistance of the adhesion-promoting layer for the subsequent pressing process could be shown. In order to examine the influence of the layer on the fibre/matrix adhesion within the composites, the Young’s modulus and flexural modulus of the composite panels were determined. Impact experiments were made to measure the required penetration energy and the energy absorption capacity of the composite panels. An optimum for the amount of adhesion promoter could be found. Exceeding the optimum amount of adhesion promoter in the solution led to a decrease in the mechanical properties of the composite.