Untersuchungen des Schadensverhaltens dreidimensional verstärkter Faserverbundwerkstoffe im Rasterlektronenmikroskop

Investigation of the Fracture Behaviour of Three-Dimensionally Reinforced Composites by Electron Scan Micorscopy By the use of woven or braided textile performs it is possible to reduce manufacturing costs and to improve damage tolerance of composite materials due to a three-dimensional fibre reinforcement. An important requirement for the application of these new technology is a basic understanding of the fracture behaviour because the unique fibre architecture can cause totally different effect. In an experimental program composite specimen with 3D woven and braided fibre reinforcement have been tested in the special fixture of an electron microscope, allowing the detection and monitoring of smallest damages (fibre-fracture, matrix-fracture, interphase-fracture, delaminations notch-growth) and the investigation of damaging effects directly during static and dynamic tests. The loading condition of the notched and un-notched specimen was three-point bending. The Electron Scan Microscope in combination with the in-situ loading fixture has been prooven to be a very interesting tool to investigate micromechanical damaging effects. It shows that the 3D reinforcement, especially o the woven material, leads to a significant improvement o damage tolerance resulting for example in a drastical limitation of notch growth in static and dynamic tests.     

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.     

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.     

Untersuchung des Schädigungsablaufes in kohlenstoffaserverstärkten Kunststoffen

Acoustic Emission Investigations on Failure Mechanisms in Carbon Fibre Reinforced Plastics For a reliable application of modern high-strength materials such as carbon fibre reinforced plastics the knowledges of failure mechanisms as well as detailed informations about the influence of defects occuring during the manufacturing process are necessary. To study the damage progression in fibrous composites a non destructive manner, a very sensitive test failure processes in the laminate have only microscopic dimensions. In our investigations the damage progression was studied by means of acoustic emission analysis (AEA) for different types of ply orientated containing several artifical defects, as cut-through fibres, disbonded fibres and porous matrix. In a further step internally pressurized tubes were tested to investigate the applicability of AE-results to real components. Additionally, fractographic investigations improve the interpretation of the acoustic emission data. It is shown, that AEA succeeds in monitoring damages already at a very early state of damage. At increasing loads the different types of defect emit characteristic AE, which is correlated with a void-type dependent damage progression in the laminate. These knowledges of the failure mechanisms and their typical acoustic emission allow a classification of the different types of manufacturing defects concerning their influence. So, especially the porous matrix could be shown to be the most significant failure type as it causes the largest decrease of strength. The onset of AE at porous materials, however, was found at very low load levels, so that this type of defect can be detected properly.     

Thermisches Spritzen vanadiumkarbidverstärkter Eisenbasisschichten

Thermal spraying of vanadium carbide reinforced iron based HVOF sprayed cermet coatings for wear protection like WC‐Co(Cr) and Cr₃C₂‐NiCr have found a broad range of applications in the past. By using chromium containing matrices, they exhibit good corrosion resistance along with outstanding wear resistance. In present research in the area of powder metallurgy and PTA welding iron based alloys with high content of chromium and vanadium have been developed, revealing similar properties and therefore being a cost efficient alternative to established cermet coatings. HVOF sprayed coatings of these iron based alloys are investigated regarding their economic applicability and their corrosion and wear properties.     

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.     

Mechanisches Verhalten von kohlenstoffaserverstärkten Duromeren im Tieftemperaturbereich

Investigations on the suitability of carbon fiber reinforced epoxy for using at cryogenic temperatures The following investigations show the mechanical properties of some carbon fiber reinforced epoxies under quasistatic load. Both prepregs and wet laminates are tested to get a suitable fiber/matrix-combination for using at cryogenic temperatures. Therefore the material research is done at room temperatures. Therefore the material research is done at room temperature and at liquid nitrogen atmosphere. The characteristics, which serve for the basis of valuation are ultimate strength, elongation at fracture and modulus of elasticity. At liquid nitrogen temperature ultimate tensile strength and elongation at fracture show a decrease in comparison to room temperature, but Young's Modulus increases insignificantly with tensile loading. In the compression test all the characteristics grow larger. Furthermore the influence of the testing temperature on the mechanical characteristics of prepregs and wet laminates is pronounced differently. The experiments also show, that the dependence of the temperature on the mechanical properties may be influenced by the kind of the reinforcing fibers. This statement applies especially to the value of the obtainable interlaminar shear strength.     

Untersuchungen zum Mindestversuchsaufwand bei Lebensdaueruntersuchungen an einem Wälzgetriebe

Investigations Concerning the Minimal Experimental Expenditures for Fatigue Tests on Wheels for Friction Drives Results of fatigue tests on wheels for friction drives show always great scatter. The reliable experimental evaluation of the interesting scatterregions (scatter at constant stress in the range of finite life or the width of the range of transition) demands a great number of samples. In practice minimal experimental costs are aimed at and therefore the general possibilities of lowering the necessary number of samples are discussed on the example of a friction drive. The reduction of the numbers of samples, levels of load and the limiting value of stress cycles endured are especially investigated in the range of transition. Considerations concerning the reduction of the levels of load to minimal two lead to a method of testing known as ?boundary technique”? from the literature. Some remarks are made to the statistical background of this method.     

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.     

Zyklische Beanspruchung drahtverstärkter Verbundstrangpressprofile mit Aluminiummatrix

Cyclic loading of wire‐reinforced aluminium matrix composite extrusions Aluminium matrix composite extrusions reinforced with wires featuring high strength and stiffness represent an innovative materials concept for lightweight structures. The use of reinforcing elements should improve the mechanical properties and hence enhance the performance of the lightweight structures. Composite extrusions made from the aluminium alloy EN AW‐6060 reinforced with reinforcing elements made from the spring steel 1.4310 and the cobalt‐base alloy Haynes 25 were examined under cyclic loadings which are of vital importance for the desired applications. Initially, load controlled multiple step tests at a load ration of R = ‐1 allowed for the determination of the cyclic stress‐strain‐curve. Afterwards, lifetime predictions were determined from these results by using mechanical models proposed by Morrow and Basquin, which were reviewed Woehler tests without mean stress. Furthermore, light and electron microscopy served for the clarification of damage and failure mechanisms. The investigations have been carried out with varying materials, configurations and surface treatments of the reinforcing elements. The investigations strived for the identification of the parameters’ influence on the lifetime behaviour to optimize the materials systems regarding the fatigue behaviour.     

Mikromechanisches stochastisches Versagensmodell uniaxial faserverstärkter Verbundwerkstoffe

Micromechanical stochastic failure model of uniaxial fibre-reinforced composites A theoretical model of stress transfer between a transversal isotropic fibre and the surrounding matrix material in a uniaxially fibre-reinforced composite near a single matrix flaw is discussed including friction controlled fibre-matrix interface debonding. The rise of fracture toughness due to frictional fibre sliding is studied accounting for Weibull strength distribution of fibres. The total dissipative work may be used as figure of merit regarding the damage tolerance. A critical evaluation is presented concerning some previous models of local failure probabilities. Numerical results are demonstrated. Conditions for an optimized C/Al-composite are presented.     

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.     

Untersuchungen zum Abrasiv-Gleitverschleiß nach dem Schleifpapierverfahren

In this paper the grinding paper technique as a method to examine the abrasive resistance of different materials is described. Pins made from the test materials have been pressed on a roller by a normal force. The roller is covered with grinding paper. Due to the relative movement between roller and pin the pin is abraded. The specimen materials are chromium white cast iron G-X 300 Cr Mo 15 3 with different heat treatments, five Headfield steels with different chemical compositions as well as one ferritic-perlitic steel C 45. The response of each specimen of the chromium white iron to the heat treatment was investigated. Furthermore the effect of the proportion of the alloy elements and their influence on the abrasion resistance of the Headfield steel was tested. The ferritic-perlitic steel were used as reference material. The results of the grinding paper rig were compared to results of tests with the REVETEST-scratch-tester. A combination of both testing methods offers the possibility to improve the abrasion resistance of materials or the use of machine parts made from the tested materials.     

Thermometrische Untersuchungen zum Schädigungsablauf bei einstufigen und einfachen nicht-einstufigen Schwingbeanspruchungen

Thermometric Investigations of the Damage Process under Single Stage Loading and Simple Program Loading History A critical view of the actual cumulative damage theories shows the necessity to develop a large physical interpretation of the damage process. Thermometric investigations in connection with deformation measurements and microstructure analysis enable a physical grounded assessment of fatigue behavior under single stage loading and simple program loading history. Relations between temperature changes and cycles to failure are discussed by investigating the normalized steel C 15 and the high-strength aluminium alloy 2024-T3.     

Numerische Simulation der Faserbewegung beim Strangpressen kurzfaserverstärkter Glasschmelzen

Numerical Simulation of the Fibre-Motion during the Extrusion of Short-Fibre-Reinforced Glass-Melts Fibre-reinforced materials are characterized by an anisotropic behaviour of the mechanical properties, which is caused by the alignment of the embedded fibres. In the case of short-fibre-compounds this behaviour is strongly influenced by the mechanism of flow during the manufacturing process. Numerical simulation Methods are preferentially used to get informations about the orientation of the reinforced fibres at the end of the molding-process and to improve the properties of the compound. For that, a model is developed, which calculates the motion of the short-fibres in the area of flow, basing on a three-dimensional finite-element-computation. Thereby the interaction between the Particular fibres during the orientation process is considered by using an interaction coefficient. Examplified at the extrusion of short fibre reinforced glass-melts, the fibre orientation is determined at models with different geometries of the pressing tool and variable boundary conditions. This procedure allows to determine the influence of the process-parameters On the expected quality of the composite. The represented simulation-model can also be used for other molding- and extrusion-processes of fibre reinforced materials.     

Analyse der Mikrostruktur und des Eigenspannungszustandes seilverstärkter Aluminiumstrangpressprofile

Analysis of the microstructure and the residual stress state of aluminium extrusions reinforced with ropes Closed profiles of the aluminium wrought alloy EN AW‐6060 have been in‐situ‐reinforced during the extrusion process with ropes built by wires drawn of the nickel‐based alloy Inconel 601 (2.4851). These profiles have been investigated with non‐destructive and semi‐destructive testing methods regarding the residual stresses evolving during cooling from extrusion temperature to room temperature. The results obtained via X‐ray diffraction and hole drilling method have been compared with simulations using a simplified model of the compound investigated. Thereby, compressive residual stresses appearing during the cooling process have been detected parallel to the rope axis nearby the rope‐matrix‐interface because of plastic flow of the matrix material. Additionally, the microstructure of the extruded composite has been analysed focussing on the contingent formation of intermetallic phases at the rope‐matrix‐interface and on the influence of the reinforcements on the longitudinal weld.     

Untersuchungen zum Erosionsverschleiß an schweißtechnisch hergestellten Beschichtungen

Erosion resistance of hard‐facing deposits Erosive wear is inflicted by flying, bumping and furrowing particles inside the gaseous medium. Especially air vents – fan blades are the principal victims ‐ and conveying systems are attacked by solid particle impingement if fly ash, raw‐meal, cement or clinker are involved. High load of particles in combination with high circular velocities cause enormous loss of material on exposed components. Hard‐facing deposits as produced by flux‐cored arc and plasma trans‐arc welding in form of wear plates or local overlays at pertinent places are able to diminish this effect. Secure dimensioning of above mentioned systems during project engineering status only is hardly possible. So far solutions for wear protection are mainly based on time‐ and cost‐intensive field tests on the part of the manufacturer or user of affected plants and equipment. Within the framework of presented investigations experiences in laboratory testing of hard‐facing materials for attacked components are discussed. Wear tests on platings by practically relevant media give information about influencing factors as well as wear‐ and damage‐mechanism.