Materials Science Forum Vols. 825-826

Abstract: The fatigue-life prediction of continuously fiber reinforced carbon/epoxy composites is of importance in order to support or partially replace the extensive amount of mechanical testing necessary for safe structural applications. However, the factors influencing the damage behaviour and the degradation of mechanical properties under real applications are numerous. To be able to predict fatigue-life of composites in an application-oriented way in the future, two novel approaches towards fatigue-life predictions have been studied by the authors in the last years. In this work, the promising approaches based on fatigue stiffness and fatigue strength and their potentials are introduced briefly.

Abstract: In order to meet the increasing lightweight requirements, the application of fiber reinforced plastics is indispensable. To ensure the structural durability of the car or machine under operational conditions, it is essential to know the long term behavior of carbon fiber reinforced plastic material (CFRP) under the numerous influencing factors under fatigue loading. For a reliable safety assessment of the car structure under operational conditions, the degradation of the stiffness and of the static strength after a certain damage due to cyclic loading is of particular importance. The paper covers the loss of stiffness and remaining strength as a function of fatigue damage for specimen and components under membrane, shear and bending loading. The tests are done using different layer set-ups (unidirectional, angle ply, quasiisotropic) and various loading conditions (membrane, shear and bending loading). In order to account for the transferability, the tests are carried out using specimen and components (hat sections). Both specimen and components show a significant loss in strength and stiffness.

Abstract: In this paper a new metal - CFRP (Carbon Fibre Reinforced Polymer) hybrid concept is introduced. The new aspect of this hybrid is a thermoplastic polymer component inserted between the metal and CFRP structure to reduce the gap of the different stiffnesses in each structure. To characterise defects in the hybrid, both destructive and nondestructive testing (NDT) approaches are shown. Especially the interface between the metal structure and the thermoplastic polymer component as well as the interface between the thermoplastic polymer component and the CFRP structure, where is most-likely the starting point for cracks, are investigated. An additional probability of detection (POD) approach enables to connect fracture mechanics with crack values of NDT. In consequence it is possible to achieve the assessment of defect-caused damage progress, so called effects of defects.

Abstract: In the present study the high cycle fatigue behaviour of multilayered Cu films with different thicknesses and microstructure on silicon substrate was investigated. An ultrasonic resonance fatigue testing systems was used to study the isothermal mechanical fatigue behaviour of the Cu multilayers at room temperature and 150°C. Investigations of fatigue damage on the surface of the samples showed distinct slip band formation on preferentially oriented grains of the Cu metallization. The degree of fatigue damage, which could be related to the thickness, grain size and orientation of the Cu films, was evaluated by determination of the slip band density as a function of loading cycles. It was found that with increasing film thickness and grain size the density of deformed area strongly increases, with the majority of surface deformation occurring in the grains with ⟨111⟩ orientation with respect to the film surface. Furthermore increasing the testing temperature resulted in a significant degradation ofthe multilayered film stack due to a considerable increase in the degree of plastic deformation of theCu metallization layer.

Abstract: In this work 3D visualization of wear in milling inserts has been investigated by Focused Ion Beam tomography. It has been observed that the morphology of the cracks differs in the z-axis direction, allowing particular characteristics of the microstructure and wear evolution to be visible. Two types of cracks develop: principal and lateral cracks. The formation of lateral cracks is strongly influenced by the degradation of the binder phase in the regions surrounding the principal crack. The lateral cracks and the deflection of the main cracks present a particular semi-elliptical geometry, which correlates with the stress field originated during the input of a cycling load in a fatigue condition.

Abstract: Reinforcement of powder metallurgy steels with fused tungsten carbide (FTC) has been shown to improve the wear resistance under certain loading conditions. A weakness is however the low hardness of the matrix, which results in selective wearing of the matrix between the carbide particles, i.e. “washing-out” effects. In the present study, in a first round fine metallic or carbidic particles were added to the iron-graphite powder mix, and the blends were die compacted and sintered in protective atmosphere. The specimens were then tested under different wear loads. It showed that also metallic powders added were transformed into carbides during sintering, thus resulting in reinforcement by fine carbides. Since Mo alloying proved to be most effective with regard to hardness and strength, specimens Fe-3%Mo-0.8%C reinforced with 20% coarse FTC (all in mass%) were sintered and tested according to ASTM G65, Continuous Impact Abrasion and dry sliding against ball bearing steel. It showed that Mo addition had quite pronounced positive effect on the G65 erosion resistance and, somewhat less, on dry sliding, while wear resistance in CIAT was less affected; the basic wear mechanisms however remained unchanged.

Abstract: The formation of a surface layer of cobalt on cemented carbides which occurs on cooling during sintering is an often observed phenomenon which has been discussed in the recent literature. The presented work shows different factors which influence the formation of the layer and proposes factor-related mechanisms. For this purpose cemented carbide samples with different compositions, WC grain sizes and carbon contents were produced and studied. The results reveal that besides the cooling conditions also the variations in composition and microstructure of the material play an essential role for the formation of a surface layer.

Abstract: Hardmetals, manufactured from powders by pressing and sintering, are the most important tool materials in service today. In many applications, such as milling or percussion drilling, they are subjected to fatigue with considerable loading cycle numbers. In the present study, the fatigue behaviour of hardmetals in push-pull loading was investigated up to N_max = 10¹⁰ using ultrasonic resonance fatigue testing. It showed that with all hardmetal grades investigated there is no fatigue “limit”, i.e. a horizontal branch of the S-N curve, but a consistent drop of the curve up to maximum N. Crack initiation was found to occur predominantly microstructure-controlled, as compared to defect controlled as typical for powder metallurgy tool steels.Keywords: gigacycle fatigue, WC-Co hardmetals, ultrasonic fatigue testing, fatigue limit

Abstract: The application-oriented distribution of wood chips is a major obstacle in the production of 3D molded composite parts made of plastics and wood. Similar to fibers in conventional fiber-reinforced plastic components long chips can be used for force transmission. Short chips and wood powder can be distributed homogeneously between longer chips to increase the wood volume percentage and to allow post-processing-free edge areas. The applied process parameters during the incorporation of the matrix material determine the filling of the cavity and the impregnation of the chips. Different formaldehyde-free resin and adhesive systems, as well as thermoplastic powders are used as matrix material. By controlling the subsequent pressing process of the oriented and impregnated chips the density and wall thickness of the molded part can be influenced. In this work different production process variants and a concept for a modular 3D experimental mold are investigated. The flowability and bulk density of the wood chip fractions are characterized and the compression pressure required in the production process is determined. Initial tests with a panel mold are carried out and the bending strength of the manufactured specimen is examined.

Abstract: Fiber reinforced plastics, due to their good mechanical properties and simultaneously low density, are very attractive for many uses. They are also gaining more importance in the civil engineering applications. Bio composites based on sustainable raw materials are becoming much more attractive, especially in construction industry, due to their recyclability and eco balance benefits. In this paper, a method to develop a façade profile from regenerated viscose filament and bio resin system (PTP^®) using pultrusion process is discussed. The PTP^® resin system from Bio-Composites And More GmbH comprises of 60% epoxidized vegetable oil. The pultrusion process is optimized to reach 60 cm/min speed from the initial speed of 4 cm/min by modifying the resin mixture. Though the production speeds are below the industrial standards, economic production scale could be still reached.