Materials Science Forum Vols. 825-826

Abstract: The interlaminar shear strength is a characteristic value describing the mechanical behavior of composite materials such as laminates. Several methods for the determination of the interlaminar shear strength are described in open literature by several authors. Among these methods, the ILSS test (DIN EN ISO 14130) measuring the apparent interlaminar shear strength by using a modified bending test is the state of the art technique, as both the necessary testing equipment and the sample geometry are quite common. However, the ILSS tests implements shear loads indirectly by bending often leading to sample failure which is then not solely initiated by shear loads. Particularly for ductile matrices or those showing pronounced elastic behavior under bending, no interlaminar shear failure can be implemented and the interlaminar shear strength can not been determined or – if the user is not sensitized to the identification of non-shear failure behavior – the determined value is not correct.Up to now, alternative methods for determining the interlaminar shear strength implementing a shear load directly to the sample are quite elaborate regarding the test equipment to be used or the specimen preparation and geometry. In this contribution the authors present a novel test setup for an edge shear test which allows both a direct shear load and at the same time a reduced complexity of the specimen geometry which is comparable to those used in the ILSS test. The authors present results based on this novel testing method in comparison to conventional ILSS tests.

Abstract: X-ray refraction is based on optical deflection of X-rays, similar to the well-known small angle X-ray scattering, but hundreds of times more intense, thus enabling shorter measurement time. We show that X-ray refraction techniques are suitable for the detection of pores, cracks, and in general defects. Indeed, the deflected X-ray intensity is directly proportional to the internal specific surface (i.e., surface per unit volume) of the objects. Although single defects cannot be imaged, the presence of populations of those defects can be detected even if the defects have sizes in the nanometer range.We present several applications of X-ray refraction techniques to composite materials:- To visualize macro and microcracks in Ti-SiC metal matrix composites (MMC);- To correlate fatigue damage (fibre de-bonding) of carbon fibre reinforced plastics (CFRP) to X-ray refraction intensity;- To quantify the impact damage by spatially resolved single fibre de-bonding fraction as a function of impact energy in CFRP laminates.An example of classic high-resolution computer tomography of an impact-damaged CFRP will also be presented, as a benchmark to the present state-of-the-art imaging capabilities. It will be shown that while (absorption) tomography can well visualize and quantify delamination, X-ray refraction techniques directly yield (spatially resolved) quantitative information about fibre de-bonding, inaccessible to absorption tomography.

Abstract: The resin transfer molding (RTM) offers great conditions for mass production of fiber reinforced plastics. In this process, preformed fiber textiles are infiltrated with matrix material (for example: epoxy resin). During the infiltration, the matrix material starts a curing process until the complete consolidation. After the de-molding and a short post-processing step, the final part is ready to use. To reduce the cycle time for the RTM manufacturing, it is necessary to model and predict the flow behavior of the matrix material in a realistic way. An important parameter is the preform permeability, which characterizes the flow resistance of fibers against the flowing matrix material.In this study a new measurement setup is presented, which is able to determine the permeability directly during the manufacturing process, with integrated pressure and temperature sensors. This approach has many advantages against conventional measurement setups, that try to recreate the RTM process with a simple replication. With these replicas, it is only possible to simulate low flow velocities and pressures. Dynamic effects that occur at higher velocities cannot be regarded. Furthermore, the new setup has the advantage that measurement artifacts, like capillarity, have a lower impact. In addition to that, the infiltration can be done with a constant viscosity test fluid as well as with reactive matrix material. Thus, it allows further determination of the time depending viscosity.

Abstract: The fatigue behaviour of short fibre reinforced thermoplastics is highly dependent on the morphological conditions. The capability of simulating fatigue behaviour and damage mechanisms of fibre reinforced polypropylene increasingly interests industrial partners of the presented study. For the morphological analysis both destructive and non-destructive methods such as computed Xray tomography and a combination of polishing and microscopic methods were applied. The determination of the composite morphology (fibre distribution, orientation tensor, fibre length distribution) was assisted by several methods of specific automatic data evaluation concepts. Fatigue testing was done on a servohydraulic dynamic testing machine using a testing frequency of 10 Hz, which is in the range of the real loading in application of the components. The analysis included thermal investigations (IR camera) and local strain measurements.

Abstract: Chromium and molybdenum exhibit continuous solubility in the solid phase region at temperatures of 908°C and above [1]. At lower temperatures, the system exhibits a miscibility gap. Furthermore a congruent minimum in the liquidus boundary exists at 1854°C. Chromium and molybdenum powders with different particle morphologies were mixed and porous green parts were produced by pressing. Sintering experiments were performed at different temperatures and for different chromium to molybdenum ratios. To investigate the evolution of the microstructure, sintering was interrupted at different temperatures and points in time. The microstructure and morphology of the sintered parts was investigated by scanning electron microscopy as well as light optical microscopy. It was found that during sintering, a Cr-Mo solid solution is formed. Depending on the molybdenum content, this induces either shrinking or swelling of the porous parts. Samples exhibited a linear expansion of up to 10% and final porosities of up to 65%.

Abstract: Hybrid ball bearings, consisting of metallic washers in combination with ceramic bearing balls, feature a variety of significant advantages in comparison to standard steel bearings, including mechanical properties and reduced friction during operation. Key aspects for a successful operation are a prevention of defects of both balls and washers, as well as the knowledge of critical and optimal operation parameters. This relevant information can be obtained through test rig trials, where vibration analysis has found to be a versatile and efficient tool for the characterization of the operational status. In this contribution, hybrid thrust ball bearings with Si₃N₄ balls are investigated. After an introduction of defined damages in different parts of the bearing, test rig trials were conducted, and the vibration behavior during operation was compared to new, unused bearings. The characteristic vibrational frequencies, obtained through a variety of software-based filter and analysis algorithms, were correlated with materialographic investigations of failed bearings. The proposed method was shown to yield valuable information about damage morphologies and, subsequently, about the status of the bearing during operation.

Abstract: The Damascus technique is a manufacturing process where steels with different compositions, in particular the carbon content, are forge welded. Materials with contrary properties are generated by different carbon additions: e.g., substantial toughness and elongation are combined with high tensile strength and hardness. Since the 3^rd century AD blades and weapons have been produced by this technique in Europe.In this work various Damascus steels with different compositions were investigated by means of metallographic methods. The focus is set on the interface between individual steel layers. While the majority of interfaces look uniform and are influenced only by carbon diffusion, some areas show the enclosement of oxides and slag stringers as a result of faulty workmanship during the forge welding process.After metallographic preparation the various microstructures were characterised by light optical microscopy and confirmed by Vicker´s microhardness measurements.

Abstract: The manufacture of a rotational shaft seal (soft component) adhesively bonded to a polymeric housing (hard component) is not state of the art. The paper approaches the feasibility to shape such hard-soft composite employing assembly injection molding technique.

Abstract: In this work carbon fibre-reinforced polymer (CFRP) laminates and short glass fibre- reinforced polymers (sGFRP) were investigated by means of X-ray scatter dark field imaging (SDFI) using Talbot-Lau grating interferometer computed tomography. For the characterisation of the laminate structures of CFRP the anisotropic properties of the small angle scattering signal was used to image fibre bundles running in different directions. SDFI allows the visualisation of the weave pattern structure of a carbon fibre bundle in three dimensions, even if the individual fibres cannot be separated or the absorption contrast between the carbon fibres and the epoxy resin matrix is very low. For the investigated sGFRP samples qualitative information about local fibre anisotropies within a specimen were obtained by SDFI. Due to the complex behaviour of fibre alignment during the injection process a clear interpretation of the SDFI signals was difficult. As a reference method, all samples were scanned by means of high resolution cone beam Computed Tomography (µXCT). For the sGFRP the combination of µXCT and appropriate software tools provides local fibre orientations and allows three-dimensional visualisation by colour coding each extracted fibre.

Abstract: In this study an experimental investigation was undertaken to characterize the influence of twisted yarns on the mechanical and fatigue behavior of 2D braided composites with a braiding angle of ±45°. Rectangular specimen made of untwisted and twisted carbon yarns are tested in the braiding (±45° fiber orientation) and in the yarn direction (0/90° fiber orientation). Experimental results will include modulus and strength values based on tensile and compression tests for both test directions. In addition, the effect of fiber twisting on S/N-curves are discussed.