Faserinstitut Bremen

Lightweight design is an essential part of the overall Volkswagen strategy for reducing the CO2 emission. The use of carbon fiber reinforced polymers (CFRP) offers an enormous lightweight potential in comparison to aluminum, enabling a weight reduction, if a load-adapted (unidirectional) CFRP-design is used, of up to 60% in automobile parts without a degradation of the functionalities. Today, the use of CFRP is limited in mass series applications of the automotive industry by the cost of the conventional carbon fiber precursor Poly-Acrylic-Nitrile (PAN). Fifty percent of the cost of a conventional carbon fiber already belongs to the cost of the PAN precursor. The analysis of lignin as an alternative precursor shows clearly a significant reduction in the cost of CFRP and reduction of CO2 emission during carbon fiber production. This fact is essential to make carbon fibers ready for a mainstream use within the automotive industry. Key aspects are: the examination and quantification of lignin as an alternative precursor, the optimization of the manufacturing processes, the characterization and quantification of the properties of the novel carbon fibers within an established material pre-validation process and a final economic efficiency and sustainability analysis. Furthermore, the process ability and demonstrators as well as the suitability for high volume production of the developed processes are main issues for successful implementation in future lightweight vehicle concepts.

Ultrasonic guided waves have been used successfully in structural health monitoring systems to detect damage in isotropic and composite materials with simple and complex geometry. A limitation of current research is given by a lack of freely available benchmark measurements to comparatively evaluate existing methods. This article introduces the extendable online platform Open Guided Waves ( http://www.open-guided-waves.de ) where high-quality and well-documented datasets for guided wave-based inspections are provided. In this article, we describe quasi-isotropic carbon-fiber-reinforced polymer plates with embedded piezoelectric transducers as a first benchmark structure. Intentionally, this is a structure of medium complexity to enable many researchers to apply their methods. In a first step, ultrasound and X-ray measurements were acquired to verify pristine conditions. Next, mechanical testing was done to determine the stiffness tensor and sample density based on standard test procedures. Guided wave measurements were divided into two parts: first, acoustic wave fields were acquired for a broad range of frequencies by three-dimensional scanning laser Doppler vibrometry. Second, structural health monitoring measurements in the carbon-fiber-reinforced polymer plate were collected at constant temperature using a distributed transducer network and a surface-mounted reversible defect model. Initial results serving as validation are presented and discussed.

Abstract Summary: The degree of plasma‐induced hydrophilization and the stability of the treatment are closely linked to the textile structure and the weave construction. This study investigates the plasma activation of polyester (PES) fabric structures to improve their wettability, as well as the aging effects. The hydrophilic modification is carried out by low pressure plasma treatments using oxygen containing gaseous mixtures (Ar/O 2 and He/O 2 ), and it is characterized by calculating contact angles (static and advancing) from capillary rise tests with water. In all cases, the wettability of plasma‐treated PES fabrics is improved significantly due to the formation of polar groups on the surface. In particular, the hydrophilicity of looser structured fabrics is improved remarkably as compared to tightly woven fabrics. Furthermore, the capillary phenomenon in fibrous assemblies is also described in this study. SEM image of a tightly woven fabric and the schematic representation of the liquid flow within a plasma‐hydrophilized fabric by capillary forces. magnified image SEM image of a tightly woven fabric and the schematic representation of the liquid flow within a plasma‐hydrophilized fabric by capillary forces.

The influence of temperature is regarded as particularly important for a structural health monitoring system based on ultrasonic guided waves. Since the temperature effect causes stronger signal changes than a typical defect, the former must be addressed and compensated for reliable damage assessment. Development of new temperature compensation techniques as well as the comparison of existing algorithms require high-quality benchmark measurements. This paper investigates a carbon fiber reinforced plastic (CFRP) plate that was fully characterized in previous research in terms of stiffness tensor and guided wave propagation. The same CFRP plate is used here for the analysis of the temperature effect for a wide range of ultrasound frequencies and temperatures. The measurement data are a contribution to the Open Guided Waves (OGW) platform: http://www.open-guided-waves.de . The technical validation includes initial results on the analysis of phase velocity variations with temperature and exemplary damage detection results using state-of-the-art signal processing methods that aim to suppress the temperature effect.

This chapter contains sections titled: Introduction Fibre Properties Characteristic Values and Statistics Significance of Fibre Testing Methods Suitability of the Measurement Method Recommended Methods Properties of Natural Fibres Fibre Tables and Summary References to Fibre Tables

Abstract For hemp cultivar Fedrina 74 we investigated the influence of different sowing dates in 1997 and 1998. The highest yields occurred with early sowing dates, at the middle of April. The pure fibre content of the plants of the last sowing date increased rapidly, reaching the same amounts as earlier sown plants. We measured the influence of harvesting technique and retting duration on strength and fineness. Comparison of the results of the purely visual assessment of the degree of retting with the Near Infrared Spectroscopy (NIRS) results shows a good correlation. The NIRS method can supply a reproducible method of analysis for retting. In 1998 we examined damage to the fibres due to the wet conditions during retting. In 1997 an increase in retting time led to a decrease of fibre bundle width. In the North German region, with high humidity in autumn, early sowing is advantageous. For both years early sowing in combination with early harvesting proved the optimal way to get a good yield and a secure collection of the stems after retting.

During the last several years, the demand of animal natural fibers for the production of high-quality textiles has increased worldwide, especially in Europe, Japan and North America. Due to the restricted availability and the high prices, adulteration and false declaration are common. Usually, fiber analysis is done by scanning electron microscopy, but this method is very time-consuming and expensive, and results strongly depend on processing stage and expertise of the microscopist. For these reasons, reliable methods are required to determine the composition and the proportion of distinct animal fibers in yarns, fabrics and garments, irrespective of their processing conditions. In this study, a DNA-analytical method is presented to unequivocally identify cashmere/cashgora, fine wool, yak and camel hair (Bactrian camel, dromedary) in untreated and treated (washed, bleached, dyed) fibers samples, as well as in fiber blends.

This article describes the improvement of the mechanical characteristics, in particular impact properties, of hemp fiber reinforced poly(lactic acid) (PLA) composites by the admixture of man-made cellulose fibers (Lyocell). It was examined to what extent these characteristics can be influenced by the mixture of hemp and Lyocell fibers. For this purpose, composites of hemp fibers, man-made Lyocell fibers and hemp/Lyocell fiber mixtures were produced with a PLA matrix and fiber mass proportions of 40% by compression molding. The composites were investigated for their tensile and impact properties. The results show that combining hemp and Lyocell in a composite can improve the impact strength by 160% compared to hemp fiber reinforced composites. Thus, it is possible to adjust the properties of composites, optimizing them for certain requirements.

Abstract The surfaces of polyester (PET) fabrics and foils were modified by low‐pressure RF plasmas with air, CO 2 , water vapor as well as Ar/O 2 and He/O 2 mixtures. To increase the wettability of the fabrics, the plasma processing parameters were optimized by means of a suction test with water. It was found that low pressure (10–16 Pa) and medium power (10–16 W) yielded a good penetration of plasma species in the textile structure for all oxygen‐containing gases and gaseous mixtures used. While the wettability of the PET fabric was increased in all cases, the Ar/O 2 plasma revealed the best hydrophilization effect with respect to water suction and aging. The hydrophilization of PET fabrics was closely related to the surface oxidation and was characterized by XPS analysis. Static and advancing contact angles were determined from the capillary rise with water. Both wetting and aging demonstrated a good comparability between plasma‐treated PET fabrics and foils, thus indicating a uniform treatment. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1452–1458, 2006

Abstract Hemp (Cannabis sativaL.) and nettle (Urtica dioicaL.) are both attractive candidates for high fibre yields with little or no biocide requirement. Separation of fibre fine enough for quality yarns to make hemp fabric or blends has been achieved in Western Europe in the last decades only on a laboratory scale because process costs are high. In Hungary, Romania, the Ukraine and Poland a hemp processing industry has continued retting mainly by water processes. Search for a commercially and environmentally viable method led us also to explore enzymatic separation, which was initiated by various researchers in the late 1960s and 1970s. This involves the use of various enzymes that dissolve pectin and hemicellulose between the cell walls thus freeing the fibre bundles and fibres. We tested various commercial and non commercial products (Röhm Enzyme GmbH and Novozymes AS/Bayer AG) and methods and then measured our results against samples of fibre separated by other methods using a Stelometer to determine tensile strength of fibre bundle collectives and OFDA (Optical Fiber Diameter Analyzer) to analyze fibre bundle width. Our results showed enzymatic separation capable of producing comparably fine and strong fibre suitable for quality textiles. These studies open the way for sustainable and local production of high value fibre with low impact on the environment.

The third dataset dedicated to the Open Guided Waves platform aims at carbon fiber composite plates with an additional omega stringer at constant temperature conditions. The two structures used in this work are representative for real aircraft components. Comprehensive measurements were recorded in order to study (I) the impact of the omega stringer on guided wave propagation, and (II) elliptical reference damages of different sizes located at three separate positions on the structure. Measurements were recorded for narrowband excitation (5-cycle toneburst with varying carrier frequencies) and broadband excitation (using chirp waveforms). The paper presents the results of a technical validation including numerical modelling, and enables further research, for example related to probability of detection (POD) analysis.

The design of antenna coils for low frequency and high frequency bands for the integration in fiber reinforced composite materials to achieve wireless power and data transfer has been studied. These systems are to be implemented in structural health monitoring, which use Lamb waves and piezo-wafer-actives Sensors (PWAS). This research focuses on the experimental evaluation of the possible problems in the realization of such system. There are concerns regarding the electrical anisotropy of carbon fiber reinforced polymer (CFRP), which might render it impossible to transmit energy into the material and data out of it. Experimental demonstrations of coil coupling for the inductive power transfer in such surrounding are presented here, indicating that it is feasible to wirelessly transmit enough power to excite the PWAS to a receiver covered with one layer of the CFRP. The quality of such transmission will vary depending on layer geometry, frequency used, and range of transmission desired.

This article presents investigations into the weldability of high-performance polymers and carbon fiber reinforced thermoplastics (CFRP) using laser transmission welding techniques. Fundamental studies of the absorption characteristics of CFRP as joining partner and the heat distribution within the surface due to laser impact have been performed. The welding process has been analyzed with respect to the properties of the weld seam and lap shear strength tests have been realized for different material combinations.

The current state of the art in joining of carbon-fibre reinforced composites (CFRP) to metals such as aluminium is - for the case of aircraft structures, e.g.- riveting or bolting. However, to reduce structural weight and improve structural performance, integral, load-bearing aluminium-CFRP-structures are desirable. To produce such structures, a novel joint configuration together with an appropriate thermal, laser-based joining process is suggested by the authors. In this paper, the joint configuration (based on CFRP-Ti-aluminium joints) and the laser beam conduction welding process will be presented, and first specimens obtained will be discussed with respect to their properties. It will be shown that the novel approach is in principle suitable to produce load-bearing CFRP-aluminium structures.

Aerospace carbon fibre-reinforced components are cured under high pressure (7 bar) and temperature in an autoclave. As in an industrial environment, the loading of an autoclave usually changes from cycle to cycle causing different thermal masses and airflow pattern which leads to an inhomogeneous temperature distribution inside the carbon fiber-reinforced plastic part. Finally, the overall process can be delayed and the part quality can be compromised. In this paper, the heat transfer in a small laboratory autoclave has been investigated using calorimeter measurements and a fluid dynamic model. A complex turbulent flow pattern with locally varying heat transfer coefficient has been observed. Especially, the pressure and the inlet fluid velocity have been identified as sensitive process parameters. Further finite element simulations with adjusted boundary conditions provide accurate results of the curing process inside of the components for selective process control. The heat transfer coefficient has been found to be almost stationary during the observed constant pressure autoclave process allowing a separated investigation of the heat transfer coefficient and the curing of the components. The presented method promises therefore a detailed observation of the autoclave process with reduced computational effort.

Stinging nettle ( Urtica dioica L.) is a bast fibre plant ideally suited to cultivation in central Europe, producing fibres of remarkable high tensile strength and fineness. Only little literature is available about nettle-reinforced standard plastics. The present study represents a first approach to produce nettle-reinforced poly(lactic acid) (PLA) with fibre loads of 20–40 wt-% to assess the technical potential of this material compared to 30 wt-% nettle/polypropylene. The tensile strength could only be increased in case of 30 wt-% nettle/poly(lactic acid) from 52 of the pure PLA to 59 MPa. This is far away from the real potential of the nettle fibres used here with a single element tensile strength of 930 ± 500 MPa. Concerning the Young’s and flexural modulus, a clear reinforcement effect was found for all poly(lactic acid) composites. The effect was strongest in case of 30 wt-% nettle/PLA: both moduli increased from <3500 MPa of poly(lactic acid) to >5,000 MPa. This is as well far below the single element value of the pure fibres (26,451 ± 14,445 MPa). As known from PLA reinforced with other bast fibres, the unnotched Charpy impact strength is lower than that of the pure polymer. The nettle-reinforced samples were found to have Charpy impact values <50% of the pure PLA. In general, the results show a good potential for nettle as reinforcement for PLA. The crucial point for the future development will be to improve the fibre–matrix interaction in order to increase especially the tensile strength of the composites by closing the large gap between fibre and matrix strength.

A micromechanical implicit finite element analysis using Samcef/Mecano was performed to analyze the effect of process-induced deformations and stresses on a unidirectional carbon fiber–epoxy composite material (G1157/RTM6). During the manufacturing process, chemical and thermal shrinkage deformations occur and lead to internal stresses. In this article, several effects on the level of stress on the microscale will be discussed, which address the nonlinear behavior of the polymer matrix material. A parametric study was done on the influence of these effects on residual stresses including temperature dependency of the young’s modulus, thermal expansion coefficient, nonlinear thermomechanical stress–strain behavior, microyielding, microdegradation, and viscoelasticity. Several experiments have been carried out to investigate the thermomechanical behavior of the matrix material and to derivate constitutive equations. The derived equations and the discussed effects are integrated into an analysis model of a squared unit cell and used to perform a coupled curing thermomechanical simulation. As a result, the development of process-induced stresses is presented with the integration of nonlinear local material effects. Assessment of these effects is one of the key aspects for interpretation of process-induced stresses on the macroscale.

Although composite materials have numerous advantages, some disadvantages, including high manufacturing costs, are relevant. In particular, if the material is applied to large structural components, such as the wings, flaps or fuselage of an airplane, efficient manufacturing processes are required to generate products that are both high quality and cost effective. Therefore, monolithic designs often become integral due to the lower overall part count and simplified designs (e.g. reducing the number of joints and fasteners significantly). For highly integrated monolithic structures, developing a robust manufacturing process to produce high quality structures is a major challenge. An integral structure must conform to the tolerance requirements because those requirements may change. Process-induced deformations may be an important risk factor for these types of structures in the context of the required tolerances, manufacturing costs and process time. Manufacturing process simulations are essential when predicting distortion and residual stresses. This study presents a simulation method for analysing process-induced deformations on the structure of a composite multispar flap. The warpage depends on the thermal expansion and shrinkage of the resin. In this study, a sequentially coupled thermo-mechanical analysis of the process will be used to analyse temperature distribution, curing evolution, distortion and residual stresses of 7.5 m long composite part.

Easily available commercial enzymes currently have great potential in bast fibre processing and can be modified for different end uses. There are several new technologies using enzymes that are able to modify fibre parameters, achieve requested properties, improve processing results and are more beneficial to the ecology in the area of bast fibre processing and fabrics finishing. Enzymatic methods for retting of flax, "cottonisation" of bast fibres, hemp separation, and processing of flax rovings before wet spinning, etc., fall into this group of new technologies. Such enzymatic biotechnologies can provide benefits in textile, composite, reinforced plastic and other technical applications. Laboratory, pilot and industrial scale results and experiences have demonstrated the ability of selected enzymes to decompose interfibre-bonding layers based on pectin, lignin and hemicelluloses. Texazym SER spray is able to increase flax long fibre yields by more than 40%. Other enzymes in combination with mild mechanical treatment can replace aggressive and energy-intensive processing like Laroche "cottonisation". Texazym SCW and DLG pretreatments of flax rovings are presented.

Abstract The plasma‐assisted process presented in this paper facilitates the deposition of nitrogen containing functional coatings on textiles. Low pressure RF plasma was used to deposit multi‐functional thin films, which have a high amine content. Either acetylene or ethylene were mixed with ammonia to obtain a crosslinked structure that contained functional groups, which were accessible for dye molecules throughout the film volume. Varying deposition conditions were used in order to compare hydrocarbon gas mixtures in terms of deposition rates, water contact angles, aging, dyeability, yellowness index, and rub/wash fastnesses. The deposition rates were found to be higher for the acetylene discharges, but decreased with increasing ammonia‐to‐hydrocarbon ratios for both gas mixtures. Although this indicates etching effects, a permanent hydrophilization could still be obtained. These findings demonstrate that plasma polymerization provides an eco‐friendly multi‐functionalized surface modification, since the use of chemicals, waste water etc. can be eliminated. Dyeing of the plasma coatings by acid dyestuffs showed that the relative color strength value, i.e. amine functionalities, can be noticeably enhanced, while being strongly influenced by the energy input and by the gas ratio. It was evident that the coating quality could be improved significantly using an ammonia/ethylene plasma due to reduced unsaturated bonds, the latter being investigated by the CIELAB color spaces. A high dyeing fastness indicated a strong dye‐molecule bonding, which in turn, is an indication of permanency of the amine groups. A pilot‐scale web coater at Empa was used to demonstrate the feasibility of industrial scale‐up. magnified image