Romana Piat

Micromechanical Modeling of Porous Carbon/Carbon Composites

A procedure to model fiber-reinforced composites containing pores of irregular shapes is presented. Closed-form expressions for contributions of fibers and pores into effective elastic moduli are provided. The procedure is applied to predict the transverse elastic properties of unidirectional carbon/carbon composites (carbon fibers in pyrolytic carbon matrix) densified by chemical vapor infiltration. Infiltration treatment results in the formation of irregularly shaped pores randomly oriented in the plane perpendicular to the direction of fiber (transverse plane). These pores are analyzed using a numerical conformal mapping technique, and their contribution to the effective elastic properties is expressed in terms of the cavity compliance contribution tensor. Components of this tensor are found for a variety of typical pore shapes.

Hierarchical material modeling of carbon/carbon composites

Unidirectional, long fiber carbon/carbon composites fabricated by chemical vapor infiltration (CVI) consisting of carbon fibers in a pyrolytic carbon matrix are anisotropic materials. It is practically impossible to identify experimentally the elastic properties (modules) of this anisotropic material. The aim of this investigation is to predict the elastic properties of this composite theoretically. The study of this material with the help of microscopy gives information about the very complicated anisotropic structure of this composite at each length scale. That is the reason that a hierarchical model for this material is developed, which consists of four length levels. A methodology for identification of the elastic properties for such composites is proposed. The problem is solved with the help of a homogenization procedure for each level.

Application of the Micro-Computed Tomography for Analyses of the Mechanical Behavior of Brittle Porous Materials

Micro Computed Tomography (μCT) can be applied for three-dimensional characterization of structural features like pores in a non-destructive way. The resolution of the volumetric data depends upon the size of the specimen, its x-ray absorption coefficient and the tomography system used. With a commercial desktop μCT system we achieved a Voxel size of 14.7 μm on a carbon/carbon composite specimen, which was further loaded until fracture in a four point cyclic bending test. Based on this investigation we present a methodology for brittle materials to determine the initial pore distribution (pores down to a size of about 50 μm), the three dimensional stress-state and the fracture surface corresponding to the non-deformed microstructure.

On contribution of pores into the effective elastic properties of carbon/carbon composites

In this paper, we discuss the effect of porosity on the effective elastic properties of unidirectional carbon/carbon composites (carbon fibers in pyrolytic carbon matrix) densified by chemical vapor infiltration (CVI). It is shown that CVI treatment results in formation of irregularly shaped pores randomly oriented in the plane perpendicular to the direction of fiber (transverse plane). These pores are analyzed using numerical conformal mapping procedure, and their contribution to the effective elastic properties is expressed in terms of the cavity compliance contribution tensor. Components of this tensor are found for a variety of typical pores shapes.

Entwicklung des hierarchischen Materialmodells von CVI‐CFC‐Werkstoffen

Mittels der chemischen Gasphaseninfiltration (CVI) hergestellte kohlefaserverstarkte Kohlenstoffe (CFC-Werkstoffe) weisen eine sehr komplexe Struktur auf. Ein hierarchisches Materialmodell, das aus 4 Hierarchieebenen besteht, wurde entwickelt: Nanoebene, Mikroebene, Mesoebene, makroskopisches Bauteil. Bei der Entwicklung des Modells wurde fur jede Hierarchieebene ein reprasentatives Volumenelement (RVE) definiert, das die mechanischen Eigenschaften des Materials auf dieser Ebene charakterisiert. Mit Hilfe der Multi-Inclusion-Methode und der selbstkonsistenten Methode wurden die Homogenisierungen auf den verschiedenen Hierarchieebenen durchgefuhrt und damit die gemittelten Elastizitatstensoren berechnet.

Influence of the Crack Widening on the Transverse Cracking in Lamellar Metal-Ceramic Composites

Stress field in the ceramic layer containing multiple transverse cracks is determined using a modified 2-D shear lag approach and a finite element method.

Stress analysis of cracked MMCs with Lamellar microstructure

Stress fields in a single-domain sample of MMC containing multiple cracks in the ceramic layer are investigated using a direct analytical method and finite element modeling.

Micro-Computed Tomography Image Based Numerical Elastic Homogenization of MMCs

Properties of an interpenetrating metal–ceramic composite with freeze-cast preforms are investigated. For the estimation of elastic properties of the composite numerical homogenization approaches for 2D and 3D finite element models are implemented. The FE models are created based on micro-computed tomography (μCT) images. The results of the numerical 2D and 3D modeling coincide and are in good agreement with available experimental measurements of elastic properties.

Numerische Modellierung des Rissfortschrittes in porösen CVI-CFC-Verbundwerkstoffen

Kurzfassung Zur Beschreibung der Rissausbreitung in spröden Materialien wurde in der Literatur eine Kombination von Spannungs- und Energiekriterien vorgeschlagen. Ausgehend von der inkrementellen Energiebilanz während des Rissfortschritts wird unter Einbeziehung der dort formulierten Kriterien ein Algorithmus zur Berechnung des Rissfortschritts hergeleitet. Dieser wird zur Simulation der Rissausbreitung in porösen Materialien mit folgenden Mikrostrukturen verwendet:1. Modellierung der typischen CVI-CFC-Mikrostruktur auf Basis von Querschnittsaufnahmen einer Probe mit anschließendem Vergleich der Resultate aus der Simulation und dem 4-Punkt-Biegeversuch,2. idealisiertes, poröses Material mit kreisförmigen Poren. Ausgehend von den Resultaten der durchgeführten Untersuchungen sind Aussagen über die Rissentwicklung in einem realen Material möglich.

Aspects of Material Modelling of Pyrolytic Carbon

Carbon/carbon composites fabricated by chemical vapor infiltration (CVI) consisting of carbon fibers in a pyrolytic carbon (pyro-C) matrix are particularly interesting for industrial application due to their high strength and light weight. One of the difficulties in hierarchical modelling of this composite [1] is that the material constants of pyrolytic carbon are unknown. The pyro-C shows four forms with a different polycrystalline structure: isotropic, low textured (LT), medium textured (MT) and highly textured (HT) [2]. The pyro-C is modelled as a system of ellipsoidal single crystals of graphite embedded in an infinite homogeneous matrix with unknown effective overall parameters of the polycrystal [3]. The local average stress and strain in the embedded crystals are then calculated and used to obtain the overall moduli. A probability density function is introduced to simulate spatial crystal orientations. The different forms of the pyro-C are described by this density function. The calculation of the overall elasticity and compliance tensors for each pyro-C modification is reduced to a system of equations, which are solved numerically. The theoretical results are compared with experimental data. A system of parallel cracks is embedded in this brittle homogenized material. The overall elasticity and compliance tensors for the material with cracks are also calculated.