Materials Science Forum Vols. 825-826

Abstract: A novel composite-composite joining technology based on metal pins oriented in through thickness direction of the composites is presented. A defined pin geometry, which is capable of establishing a through-thickness form-fit connection between composites and the metal reinforcement, is created on thin metal sheets in an automated pin production process. Based on numerical simulations of the fracture of unreinforced single lap shear (SLS) composite specimens, optimum locations for the pin reinforcement were found. Tests on reinforced SLS specimens proved that an enhanced damage tolerance can be achieved by the use of cold metal transfer welded pins (CMT pins) as through-the-thickness reinforcement of the joint area. This paper investigates the mechanisms responsible for the load transfer and failure of such through-the-thickness reinforced composite-composite joints during monotonic loading.

Abstract: Until the present days, in discontinuous conveyor systems, e.g. cranes and elevators, steel wire ropes and steel chains are the exclusively used tension members. Nevertheless, these tension members are exhibiting essential disadvantages, e.g. high dead weight, low bending flexure and high susceptibility to corrosion. For fulfilling the steadily increasing requirements especially in running ropes, alternative tension members have to be developed and approved concerning their series-production readiness by utilization of new technologies and new materials. High-strength fiber ropes are demonstrating promising mechanical properties, regarding tensile strength, vibration behavior and bending fatigue life. Still, the application of such high-strength fiber ropes is limited, due to unavailable systematic investigations and insufficient guidelines for dimensioning and processes for determination of discard criteria. With the help of new technologies and procedures, e.g. multi-zone cyclic bend-over-sheave (CBOS) test machinery and computer-assisted tomography, new knowledge concerning the wear behavior of fiber ropes is to be gained and new discard criteria are to be developed.

Abstract: Ceramic particle reinforced metal matrix composites (PRMMCs) combine the strength and brittleness of ceramics with the toughness of a metallic matrix. In order to use these materials in construction and operational design their fracture mechanical behavior must be evaluated. In this study, a 30 vol.-% Al₂O₃ reinforced austenitic TRIP steel processed by powder metallurgical technique was investigated using precracked miniature SENB-specimens in 3-point-bending. An elastic-plastic analysis by means of the J-integral method in combination with optical crack observation showed the materials ability of stable crack growth, i. e. R-curve behavior. In addition to the mechanical tests microstructural studies were performed, whereby particle debonding and fracture as well as martensitic phase transformation and crack bridging within the matrix were identified as fracture energy dissipating mechanisms.

Abstract: A novel methodology for the characterization of short fibre reinforced polymers using Xray computed tomography is introduced and evaluated. Scans with high resolution lead to data quality that allows for single fibre examination. Data analysis takes the information of local direction into account to determine centre lines and start- and endpoints. Reproducibility and accuracy are determined for glass fibre filled Polypropylene. Nine analyses of the same specimen show very small standard deviation for fibre length and orientation distribution. Accuracy of up to 97 % is determined by comparing the XCT result with semi-automatically generated ground truth. Variation of resolution shows no influence on fibre orientation but a decrease in mean fibre length for lower resolution. The proposed method leads to accurate results at 2 μm voxel edge length or below for the investigated material system.

Abstract: For composite design, it is desirable to have data covering the failure envelope from Mode I to Mode II. The existing standard procedures for quasi-static testing (ISO 15024 for Mode I and ISO DIS 15114 for Mode II) have recently been shown to be adaptable for the respective fatigue tests under displacement control. The Calibrated End-Loaded Split (C-ELS) test set-up developed for Mode II further allows performing a Fixed-Ratio Mixed Mode I/II (FRMM) test by simply inverting the loading direction compared to mode II.

Abstract: The properties of fibre-reinforced composites depend on the interface between the matrix and the fibre. The surface of the fibres plays a key role in the load transmission. In this article, a method based on scanning electron microscopy images of fibre cross sections is introduced to quantify the surface roughness. By utilising an image editing software to get a digital profile of the fibre contour, the arithmetic roughness R_a of a carbon fibre was calculated. The method was tested on a C-fibre with a diameter of 7 microns and compared with the images of an atomic force microscopy (AFM). The method allows a comparison of different types of fibres as well as an investigation regarding the influence of fibre treatments.

Abstract: The use of micro test specimens is a good way to characterize micro injection molding processes and the resulting material properties. The material properties of microparts may differ from standard injection molding parts, due to an overrepresentation of the surface layers with high fiber orientation and divergent morphology. In order to characterize the distribution and agglomeration of fibers and particles for the manufacturing of micro injection molding parts of functionalized polymer compounds, it is essential to manufacture the test specimens and the part using the same process. The distribution and size of these particles e.g. Carbon-Nano-Tubes (CNT) or piezo ceramic particles is dependent on the polymer plastication process during injection molding. Therefore the use of micro test specimens is a requirement for precise material selection and engineering.Due to the minimum material needs, micro test specimens are also useful for the comparison of the material properties of new polymers and compounds, which were produced in amounts of 20 g to 100 g. Another application is the testing of highly elastic and ductile materials with strains over 100%. By using micro test specimens it is possible to test high strains with low elongations in a short time.A new innovative micro test specimen has been developed at the Technische Universität Chemnitz in cooperation with the Kunststoff-Zentrum in Leipzig, that is especially designed for the testing and dimensioning of plastic microparts with weights less than 0.1 g. The main feature of the new specimen and testing process is the combined positive and force-fitted locking, which enables a precise positioning of the micro specimen and an even application of the clamping force. In order to achieve reproducible clamping, testing and handling of the sample, the clamping and testing process are spatially separated. The shape of the test specimen enables a parameter optimization for the micro injection molding process.

Abstract: An online monitoring scheme to determine viscosity and permeability for a composite main spar infusion is presented. During the manufacturing process, defects such as dry spots and pores can occur and lead to poor part quality. In this study, dielectric sensors are used to create the information needed to model the process. Several experiments have been carried out to investigate the correlation between the sensors signal and the viscosity and to derivate constitutive equations. The provided data and the derived permeabilities are integrated into a simulation model of a cross section of the composite main spar. As a result, the real permeability can be identified. The knowledge of these parameters is one of the key aspects for a prediction of the resin flow and therefore for an improved process control.

Abstract: In this study online-capable dielectric analysis techniques were investigated to show its potential on the example of detecting phase transitions of polyamide 6 (PA6) and its composites. The differential scanning calorimetry (DSC), a standard testing method commonly applied in thermophysical analysis, is used as a reference method throughout this work. Dielectric measurement techniques are introduced as a means for providing an online measuring concept that is able to monitor phasechanges in both neat, as well as glass and carbon reinforced PA6. Whereas a simple sensor setup has proven to be adequate for dielectric measurements of neat and glass reinforced PA6, the insertion of an additional insulation layer between sample and sensor was necessary to overcome short circuit problems induced through the conductive nature of carbon in carbon fiber reinforced PA6.Results show that crystallization and melting can be successfully identified using dielectric analysis and can be compared directly with results from the DSC. Analysis in the dielectric method is based on relative permittivity ϵ′, loss factor ϵ′′ and ion viscosity ρ. Here phase changes can be observed as a frequency dependent step in the measurement signal, which becomes increasingly apparent with increasing frequency. Plotting the first derivative of ϵ′, ϵ′′ and ρ relative to the temperature the phase change can be depicted in form of a peak, similar to the case of DSC. The derivative signals can be used as a direct means for monitoring in manufacturing processes.

Abstract: The long term behaviour of fibre reinforced polymers is important for many uses. In this publication experiments are described in which three different carbon fibre epoxy compound specimens, with different fibre orientations, are compared. For a comparison to metallic materials, as well as to get a benchmark, steel and aluminium specimens are also tested. All specimens are loaded with constant loads; the investigated load types are bending and torsion. In the experiments two different scenarios are investigated. The first scenario compares the creep behaviour under uniform start deflections and the second scenario compares the creep behaviour under uniform load conditions.