Microstructure characterization of CVI-densified carbon/carbon composites with various fiber distributions

Mechanical behavior of multi-phase composites is crucially influenced by volume fractions, orientation distributions and geometries of microconstituents. In the case of carbon–carbon composites manufactured by chemical vapor infiltration, the microconstituents are carbon fibers, pyrolytic carbon matrix, and pores. The local variable thickness of the pyrolytic carbon coating, distribution of the fibers and porosity are the main factors influencing the properties of these materials. Two types of fiber arrangements are considered in this paper: 2D laminated preform and random felt. The materials are characterized by determining their densities and their fiber distribution functions, by establishing types of pyrolytic carbon matrix present in the composites, and by studying the porosity. A technique utilizing X-ray computed tomography for estimation of the orientation distribution of the fibers and pores with arbitrary shapes is developed. A methodology based on the processing of microstructure images with subsequent numerical simulation of the coating growth around the fibers is proposed for estimation of the local thickness of the coating. The obtained information is appropriate for micromechanical modeling and prediction of the overall thermo-mechanical properties of the studied composites.     

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.     

Effects of defects in hybrid sheet moulding compound – Evaluation of defects and the impact on mechanical properties

Sheet moulding compound is a widely used fibre‐reinforced material. Generally, it consists of discontinuous glass fibres in a thermoset matrix system. Due to the finite fibre length, mechanical properties of structural components are limited. To overcome this drawback, sheet moulding compound is locally reinforced with a unidirectional carbon fibre sheet moulding compound material in the approach presented in this contribution. The manufacturing of this hybrid material consisting of discontinuous glass fibre sheet moulding compound and continuous carbon fibre sheet moulding compound can result in different defects, such as folds or fibre misalignments. These defects may affect mechanical properties of the hybrid material. Consequently, this article deals with the investigation and analysis of defective hybrid sheet moulding compound components, which were examined by means of tensile tests. Results point out that investigated defects have different effects on mechanical properties. However, independent from the type of defect, mechanical properties were reduced. With a reduction of 68.86 %, folds have one of the greatest influences on tensile strength. In addition, depending on the angle deviation, even greater reductions can occur. Furthermore, the reduction of the mechanical properties can be identified clearly with increasing angle deviation.     

Characterization of Elastic Properties in Porous Silicon Carbide Preforms Fabricated Using Polymer Waxes as Pore Formers

Thorough elastic analysis of porous silicon carbide preforms fabricated using two different polymer waxes as pore formers is carried out. Both the amount and the mixture ratio of the waxes were varied to fabricate preforms with different pore morphologies and porosities in the range of 27–67 vol%. Results show that both the longitudinal and the shear elastic constants decrease with increasing porosity. The rate of decrease in the elastic constants follows a model based on minimum solid area up to an intermediate porosity level. Uniaxial pressure applied prior to cold isostatic pressing and sintering significantly reduces the stiffness along the press direction. For the same initial powder mixture type, the elastic anisotropy of the preforms increases with an increase in the applied uniaxial pressure. The extent of anisotropy is strongly dependent on both the SiC/wax ratio as well as the mixture ratio between the two wax types. At low pore volume fractions a higher volume content of the smaller diameter wax and at high pore volume fractions a higher volume content of larger diameter wax lead to preforms with lowest anisotropy. A map is finally proposed to describe the dependence of the preform elastic properties on the type of initial powder mixture used.     

Rigidity of diamond reinforced metals featuring high particle contents

Diamond/metal composites with diamond contents between 57 and 72 vol% have been produced by gas pressure assisted liquid metal infiltration using Ag–3 wt% Si and Al–2 wt% Cu as matrix. The experimental data cover a range of Young’s moduli from 300 to 425 GPa and 245 to 370 GPa for the Ag–3Si and the Al–2Cu-based composites, respectively. Experimental Young’s moduli are compared to the Mori–Tanaka mean field scheme (MTM), the generalized self-consistent scheme (GSCS), the bimodal hard sphere model (TBHS), and the differential effective medium scheme (DEM). At the lower end of volume fractions investigated, the predictions by the GSCS, the TBHS, and the DEM are very close to each other and to the experimental results while the MTM is clearly lower. With increasing volume fraction the differences between the models accentuate and the data up to 72 vol% of diamond are best described by the DEM.     

A Field Study on Distance Education and Communication: Experiences of a Virtual Tutor

It is clear that the Internet has the capacity to change how individuals interact with others as well as increase access to information. Whether either one of these factors affects the social landscape has yet to be determined. This fact has not kept many from anticipating the effects of the technology on society. In this paper, we contextualize some of the main issues of discussion regarding the Internet, describing these positions in terms of utopian and dystopian perspectives. By resurrecting William Ogburn's theory of the cultural lag (1964), we present a framework for understanding the extreme responses to the technology. The lag suggests that the effects of a technology will not be apparent to social actors for some time after it is introduced to a society. As such, much of the discourse concerning the Internet is ideologically charged, filled as much with the hopes and fears of individual authors as with the reality of the medium's effects.     

Analysis of the elastic properties of an interpenetrating AlSi12-Al2O3 composite using ultrasound phase spectroscopy

An interpenetrating composite fabricated by squeeze-casting a eutectic aluminium-silicon alloy into a porous alumina preform is studied in this work. The preform was fabricated by pyrolysis of cellulose fibres used as pore forming agent, pressing of the green ceramic body and subsequent sintering of alumina particles. The resulting preform had both micropores within the ceramic walls and macropores between those walls, which were infiltrated by the liquid metal. Composites with alumina contents varied in the range of 18-65 vol.% were studied. Three longitudinal and three shear elastic constants of the composites were determined using ultrasound phase spectroscopy on rectangular parallelepiped samples. Complete stiffness matrix of one sample was determined by modifying the sample geometry by cutting at the corners of the sample and subsequent ultrasonic measurements. All composites exhibit a moderately anisotropic behavior, which can be attributed to a non-random pore orientation distribution caused by uni-axial pressing of the preforms prior to sintering. The experimental results are compared with several theoretical micromechanical models.     

Complete determination of elastic moduli of interpenetrating metal/ceramic composites using ultrasonic techniques and micromechanical modelling

The complete stiffness matrices of several metal/ceramic composites were analysed using the complementary ultrasonic spectroscopic techniques ultrasound phase spectroscopy (UPS) and resonant ultrasound spectroscopy (RUS). Three different aluminum/alumina composites having complex interpenetrating architectures were studied: a composite based on freeze-cast ceramic preform, a composite based on open porous ceramic preform obtained by pyrolysis of cellulose fibres, and a composite based on discontinuous fibre preform. Six of the nine independent elastic constants describing orthotropic elastic anisotropy were pre-determined by ultrasound phase spectroscopy and used as initial guess input for resonant ultrasound spectroscopy analysis, making the final results of all nine elastic constants more reliable. In all cases, consistent and reproducible results are obtained. Finally the experimental results were compared with effective elastic constants calculated using micromechanical modelling and a good correspondence between both is obtained.