Entwicklungs- und Einsatzmöglichkeiten faserverstärkter Verbundwerkstoffe im Vergleich mit konventionellen Werkstoffen

Future Development and Application of Fibre Reinforced High-Performance Composites in Comparison with Conventional Materials . A review is given about the nowadays employed and new developed reinforcements and matrix systems including a critical comparison between advantages and disadvantages of each of the possible components. Special emphasis is given regarding mutual compatibility and processability into high-strength composites. The characteristics of technically producible composites as well as theoretically possible combinations are being critically compared with those of conventional materials and especially examined regarding its economical use. Several examples for a cost/benefit analysis and an optimal choice of materials are being stated and explained on special ranges of applications. Operational possibilities and limits in view of technique composites are being described which give a survey on the future chances of this category of materials.     

Charakterisierung faserverstärkter Leichtmetalle

Metal-matrix composites combine the properties of metals with those of ceramic fibres. Pistons of highly stressed Diesel engines are reinforced in their bottom zones by the implantation of a preform of fibres. Investigations concentrate on the nondestructive detection of inclusions and defects of fibre distribution. Due to the very small dimensions of the defects test methods capable of responding sensitively to such small defect dimensions have to be applied. Good prerequisites to solve the task of defect detection are provided in first line by ultrasonic inspection within the high-frequency range in connection with scanning methods. Natural defects up to a size of 120 μm could be detected by ultrasonics. With artificial test defects, the limit was reached at 100 μm. Eddy-current inspection, another test procedure, was applied for the detection of fibreless zones, as there exist differences of conductivity between fibreless and fibre-reinforced zones. The use of scanning methods with special probes allows to depict fibreless zones up to a size of 100 μm on the specimen surface.     

Ein stochastisches,schadensmechanisches Versagensmodell für Komposite

A stochastic damage mechanics failure model of composites This paper focuses on the theoretical simulation of strength and failure of specimens with nonlocal damage. A concept is proposed for quantitative interpretation and prediction of nonlinear. nominal stress-strain curves of damaged materials like fine ceramics or intermetallic alloys. For that reason methods of damage mechanics and of probability theory are combined.     

Herstellung faserverstärkter,reaktionsgebundener Siliziumcarbidkeramiken unter Verwendung intermetallischer Siliziumlegierungen

Manufacturing of reaction bonded, carbon-fiber reinforced SiC-composites using intermetallic silicon alloys An advanced manufacturing technology for reaction bonded, carbon-fiber reinforced SiC-composites is shown using intermetallic silicon alloys as reactant. By infiltration of pure silicon in porous preforms, the carbon fibers are converted to siliconcarbide. A toughening effect is only possible, if the C-fiber strands are preserved during the melt infiltration. A screening in the FeSi-system was performed to control the reactivity of silicon with the free carbon, contained as powder as well as fibers in the preform. The chemical reactivity of FeSi alloys with 10–15 wt% Fe provide both, fiber preservation and optimized microstructural properties. The results of tribological tests show long term stable friction behaviour with asymptotic μ-values corresponding to a contact pattern, where the Si(Fe)SiC matrix is bearing and the friction is controlled by the carbon (C-fiber)/SiC-ceramic ratio in the surface. The low specific weight and the low wear of the CMC material open up the possibilities to use it for car lifetime brake disks.     

Aussichten für die Entwicklung technisch nutzbarer faserverstärkter Hochtemperaturwerkstoffe

Prospects for the Development of Useful High Temperature Resistant Fibre-Strengthened Materials. New and better materials are still in demand in the field of high temperature technology. High creep rupture strengths and resistance to thermal shock are required when these materials are to be used at temperatures above 1000°C in the presence of combustion gases and flowing and/or static air with high oxygen content. Production methods are reviewed and a critical discussion is given on present research results as well as possible matrix combinations for fibre materials. The chemical compatibility between the composite components is thoroughly discussed. It is stressed that the problems of chemical compatibility and oxidation resistance need special consideration in the development stages of these materials.     

Faserverstärkte Verbundwerkstoffe

Fibre Reinforced Composite Materials. The basic principles of the reinforcing and stiffening effect of fibers in composite materials as well as the failure behaviour of composites are discussed for composites which are reinforced with unidirectionally arranged continuous fibers, with short fibers and with multidirectionally arranged fibers. The most candidate continuous fiber materials for high performance composites which are presently available such as carbon fibers, boron- and silicon carbide fibers, organic fibers and fiberglass are compared with respect to properties and price. Whiskers are considered to have only very limited significance as reinforcing material. Furthermore the paper presents the most candidate fiber/matrix-combinations such as boron-, carbon-, organic- and glassfiber reinforced polyimide- and phenolic resin, boron- and carbon fiber reinforced aluminium, carbon- and SiC-fiber reinforced glass as well as carbon/carbon composites. The problems of chemical compatibility between fiber and matrix and the mechanical properties of these composites at room temperature and at elevated temperature are shown and discussed.     

Dynamisch-mechanische Analyse naturfaserverstärkter Kunststoffe

Dynamic-mechanical analysis of natural fiber reinforced plastics Recently natural fibers are increasingly used as reinforcement in plastics. These materials are distinguished by their high tensile strength and stiffness as well as their low density at the same time. For technical applications the material behaviour in addiction of temperature is very interesting. Useing the example of flax fiber reinforced polypropylen it is shown, that the dynamic-mechanical analysis can be a meaningful test to describe the behaviour of natural fiber composites by different temperatures.     

Schwingbruchstrukturen an faserverstärkten Kunststoffen

Structures of Fatigue Fractures in Fibre-Reinforced Plastics The report deals with macrofractographic and microfractographic structures in non-reinforced epoxide resins as well as in glass-fibre reinforced plastics and carbon-fibre reinforced plastics after fatigue stresses. Epoxide resins show fatigue striations and fracture lines similar to those in metals. The distance between the fatigue striations amounts to between 0.70 and 150 m?m and increases continuously in the direction of fracture propagation. The instantaneous fractures in non-reinforced resin show parabolic cusps besides Wallner-lines and lance-shaped structures. Surfaces of delamination fractures of glass-fibre reinforced plastics show fatigue striations which are characteristic for fibre-reinforced composites, but their morphology differs from the fatigue striations in non-reinforced resin. This was the first time that fatigue striations could be discovered in fibre-reinforced composites of a fibre content of 70 per cent by volume. These fatigue striations are strongly marked at the positions of the fibres and will probably arise only in case of a sufficiently high local stress intensity. Moreover, the amounts of the local slicing and shearing stress components, of which is composed the stress causing the fracture, could be partly responsible for the formation of fatigue striations. The formation of the fatigue striations allows to draw conclusions with respect to the fracture-causing local stress distribution and the local crack propagation in fibre-reinforced plastics. The surfaces of instantaneous fractures in glass-fibre reinforced plastics and carbon-fibre reinforced plastics show parabolic cusps. This is another distinguishing characteristic as compared with the surfaces of fatigue fractures.     

Transluzente oxidfaserverstärkte Glasmatrix‐Verbundwerkstoffe

Translucent oxide fiber reinforced glasmatrix composites The aim of the work is the developement of transparent glass matrix composites. Therefore besides the mechanical properties also the optical properties of the components have to be adapted. In this study the influence of different fiber coatings (boron nitride, titanium oxide and a boron nitride/titanium oxide double coating) on the mechanical and optical properties of Nextel 440‐fiber reinforced glass was investigated. Micromechanical investigations (push‐in‐tests) and 3‐point‐bending tests have shown the best improvement of the fracture toughness for the binary boron nitride/titanium oxide coating. For single coatings of boron nitride or titanium oxide the transparency was characterized by the transmission spectra.     

Fraktographie an faserverstärkten Kunststoffen

Fractography of Fibre Reinforced Plastics If damage such as cracks of fractures occur in fibre reinforced plastic components, the fractography, that is the examination of the fracture surface, is the most important part of the failure analysis. If this investigation is done in the scanning electron microscop (SEM), it is called microfractography. Successful application of microfractography on metals has been made for more than 20 years in elucidating material failure. On the subject of microfractography on fibre reinforced plastic, basic knowledge has been worked out but is known to day only to a few specialists. This field is in development in the USA and in Western-Europe A comprehensive and definitive fracture atlas, as available for metals, is not existing. The completion of such an atlas will take about five to ten years. The US-Air Force, however, has the intention to produce a handbook ?Fractography of Composite Structures”? in a shorter time. The results presented in this paper have been obtained from 1973 to 1984. Proceedings from defined fractures following static and dynamic loading the microfractography features are characterised and documented with pictures. The results are compared with those published in the literature.     

Mikroanalytische untersuchungen Faserverstärkter keramischer Werkstoffe

Microanalytical investigations have been made on samples of ceramic fibers (SiC-fibres (Nicalon), C-fibre coated with TiN) and fibre-reinforced ceramics (SiC- and glass-matrices). High resolution Auger electron spectroscopy (HRAES), electron probe microanalysis (EPMA) and scanning electron microscopy were employed for these examinations. Analysis was best performed with HRAES on account of its lateral and depth resolution. AES depth profiles of ceramic fibres are reported and compared with the surface analysis of fibers in the composites after being broken in situ.     

Untersuchungen zum Erosionsschutz kohlenstoffaserverstärkter Duroplaste

Investigations on erosion protection of carbon-fibre-reinforced thermosetting plastics A double layer system of polyurethane rubber and a metal foil was described by calculation and experiment. There is a significant increase in erosion resistance of a carbon-fibre-reinforced plastic component by application of this system and the experimental results correlate with theoretical estimations. Among the investigated materials surface layers of titanium and a titanium aluminium alloy respectively possess the best resistance in the range of all angles of incidence. The polyurethane rubber also exhibits a very good erosion behaviour under conditions of normal impact as well as at shallow impact angles at room temperature. So the rubber takes care of a good emergency running quality after a destruction of the metal foil. It was found that the relative erosion rate of the protective metal layer decreases with increasing thickness of the rubber. A thickness of about 0,5–0,7 mm of the protective metal layer is sufficient in practice. The titanium foil should have a thickness of about 100–200 m?m for the design life because if the metal foil is too weak it will undergo destruction rather fast under laboratory conditions. There is no change of the well-known erosion mechanisms with the formation of walls around the places of normal impacts as well as grooving and machining at shallow impact angles by application of the double layer system. An increase in temparature up to 80 °C doesn't cause any change of the erosion rate of the investigated metal foils in the double layer system but will influence the erosion of the polyurethane rubber alone. The galvanic deposition of nickel is another way for the use of the double layer system especially for components with a complex geometric shape.     

Aufbau und Eigenschaften glasfaserverstärkter Eisenverbundwerkstoffe

Properties and Microstructure of Iron-Glassfiber Composites It is possible to produce powdermetallurgically glasfibre reinforced iron composites after high deformation by extrusion. These materials represent some interesting technological aspects because an anomalous increasing in tensile strength and other mechanical properties can be found, although the Young's-Modulus of the additive is lower than of the matrix. On the other hand electrical conductivity and Young's Modulus of the composites show a linear decreasing with the volume fraction of the second phase. This behaviour is reasoned in the fibre structure of the composites. Other physical properties such as thermal conductivity or the internal friction show an anomalous behaviour because of an excursive phononscattering at the interphases, which the curled microstructure possess in multiple forms.     

Zur exakten und näherungsweisen Berechnung unidirektionalverstärkter Kunststoffe

Exact and Approximate Calculations of UD-Layers It is shown that the quasi-elastic solution of layers nearly almost used in practical viscoelastic calculations is an excellent approach. The exact solution achieved with simple integral equations are compared with the quasi-elastic solution and the occuring deviations are described. The creep functions of layers are given the form of the uniaxial creep function of the isotropic viscoelastic material. This makes the creep behaviour of layers very clear and symplifies practical calculations. The given solutions are under no restriction with regard to the chosen form of the uni-axial creep function.     

Die Schwingfestigkeit von Bauteilen aus faserverstärkten Werkstoffen

Fatigue Strength of Components of Fibre Reinforced Materials The constant and variable amplitude fatigue strength as well as the tensile and compressive strength of a composite material with carbon/HT fibre and epoxy matrix were experimentally determined. Unnotched dogbone shaped specimens of multidirectional CFRP with 0°, ± 45° and 90° layers were used for the tests. The results of constant amplitude testing with the stress ratios R = +0.1; ?0.5; ?1.0; ?1.66; and ?5.0 are presented. The variable amplitude testing was done using a flight-by-flight loading sequence which is typical for the wing root area of a fighter airplane. The life was also calculated and the outcome was compared with the test result. The influence of loading modifications regarding the high- and the low load-cycle ranges of the load spectrum was also examined. The fatigue behaviour of the composite material was compared with the fatigue behaviour of metals.     

Einfluß verschiedener thermomechanischer Belastungen auf das Schädigungsverhalten faserverstärkten Aluminiums

Influence of various thermomechanical loadings on the damaging of fibre‐reinforced aluminium Under cyclic thermomechanical loading fatigue of metal matrix composites is strongly affected by the phasing between the mechanical and the thermal load. Therefore the current study focusing on an Al/SiC‐composite aims at numerically analysing the influence of different thermomechanical loadings on the inelastic matrix deformation and damage. Thereby the non linear finite element method (FEM) which is applied here allows a good access to the problem.