Kamen Tushtev

Evaluation of Mechanical Properties and Comprehensive Modeling of CMC with Stiff and Weak Matrices

The mechanical properties of ceramic matrix composites (CMC) depend on the individual properties of fibers and matrix, the fiber-matrix interface, the microstructure and the orientation of the fibers. The fiber-matrix interface of ceramics with stiff matrices (e.g. CVI-derived SiC/SiC) must be weak enough to allow crack deflection and debonding in order to achieve excellent strength and strain to failure (weak interface composites WIC). This micromechanical behavior has been intensively investigated during the last 20 years. With the development of CMC with weak matrices (weak matrix composites WMC) as e.g. oxide/oxide composites or polymer derived CMC the mechanical response can not be explained anymore by these models as other microstructural mechanisms occur. If the fibers are oriented in loading direction in a tensile test the WMC behave almost linear elastic up to failure and show a high strength. Under shear mode or if the fibers are oriented off axis a significant quasiplastic stress-strain behavior occurs with high strain to failure and low strength. This complex mechanical behavior of WMC will be explained using a finite element (FE) approach. The micromechanical as well as the FE models will be validated and attributed to the different manufacturing routes.

Elastic properties of braided ceramic matrix composites

Abstract Fibre reinforced ceramic components are most efficiently produced by the braiding technique. Since the braiding angle can be adjusted during the preform fabrication process in a wide range as required by the loading situation of the composite, the evaluation and prediction of the elastic properties as a function of variable braiding angles is of high importance. Based on the classical laminate theory an inverse method for the determination of the elastic properties of an equivalent unidirectional ply is elaborated and applied to predict the effect of variable braiding angles. Orthogonal and non-orthogonal braided carbon/carbon composites were tested under in-plane tensile and shear loading in order to validate the analytical concept. A very good correlation between theoretical and experimental results was obtained.

Mechanismen und Modellierung der Verformung und Schädigung keramischer Faserverbundwerkstoffe

Auf der Basis der Kontinuumschädigungsmechanik wird das mechanische Verhalten eines bidirektional verstärkten Carbon/Carbon-Verbundwerkstoffs in einem experimentell gestützten Modell beschrieben. Der betrachtete Werkstoff repräsentiert eine Gruppe keramischer Faserverbundwerkstoffe, deren mechanische Eigenschaften von einer porösen und schwachen Matrix stark beeinflusst sind. Mit dem Modell, das in das kommerziell verfügbare Finite-ElementeProgramm MARC implementiert wurde, ist es möglich, das Werkstoffverhalten unter quasistatischer Zug-, Scherund Druckbelastung für beliebige Belastungsrichtungen relativ zur Faserorientierung in sehr guter Übereinstimmung mit den experimentellen Ergebnissen vorherzusagen.

Improvement of Oxide/Oxide CMC and Development of Combustor and Turbine Components in the HiPOC Program

Three different oxide/oxide ceramic matrix composite (CMC) materials are described. Design concepts for the attachment of the CMC component to the metal structure of the gas turbine are developed in a first work stream focused on the combustion chamber and the turbine seal segment. Issues like environmental barrier coating (EBC)/thermal barrier coatings (TBC), application and volatilization, allowance for the different thermal expansion and the mechanical fixation are addressed. The design work is accompanied by CFD and FEM simulations. A variation of the microstructural design of the three oxide/oxide CMC materials in terms of different fiber architecture and processing of matrix are considered. Also, mechanical properties of these variations are evaluated. The material concepts are developed further in a second work stream. The CMCs are tested in various loading modes (tension, compression, shear, off-axis loading) from room temperature to maximum application temperature focusing on tensile creep behavior. By modification of the matrix and the fiber-matrix interface as well as EBC coatings, the high temperature stability and the insulation performance are enhanced. An outline of the “High Performance Oxide Ceramic”-program HiPOC for the following years is given, including manufacturing of a high-pressure tubular combustor and turbine seal segments from the improved materials as technology samples, for which validation testing up to technology readiness level 4 is scheduled for 2011.Copyright

Development and Validation of Oxide/oxide CMC Combustors within the HiPOC Program

In the framework of the High Performance Oxide Ceramics program (HiPOC), three different oxide/oxide ceramic matrix composite (CMC) materials are studied for a combustion chamber application in continuation of the work reported in Gerendas et al. [1]. A variation in the micro-structural design of the three CMC materials in terms of different fiber architecture and matrix processing are considered in a first work stream. By modification of the matrix and the fiber-matrix interface as well as the application of an environmental barrier coating (EBC), the high temperature stability is enhanced. Furthermore, design concepts for the attachment of the CMC component to the metal structure of the engine are finalized in a second work stream. Issues like sealing of cooling leakage paths, allowance for the different thermal expansion and the mechanical fixation are addressed. An interim standard of the mechanical attachment scheme is studied on a shaker table. Also the friction coefficient between the metallic and ceramic components is analyzed in order to set the proper tightening torque. The manufacturing of the CMC combustor is improved in several iterations in order to achieve a high quality material with optimized fiber architecture. Afterwards, two CMC materials are selected for the combustion testing and the finalized design of the metallic and CMC components is manufactured. A fit check is performed prior to EBC application and laser drilling of the effusion holes in order to evaluate the impact of the manufacturing tolerances on the function of the sealing and attachment scheme and to correct small issues at this stage. First results from the validation testing in a high-pressure tubular combustion rig up to a Technology Readiness Level 4 (TRL4) are reported.Copyright

5.5 Oxide/Oxide CMCs – Porous Matrix Composite Systems; Composites With Interface Coatings

This chapter compiles information about ceramic matrix composites (CMC) based on all-oxide materials (Ox-CMCs). The development of oxide–oxide composites started effectively in the 1990s, aiming for applications under oxidizing environments. Nowadays, these composites have reached a good level of maturity, showing a damage tolerant behavior by either porous matrix systems or fiber coatings, tensile strength of about 200 MPa, and excellent chemical stability due to their oxide nature. This chapter is then focused on presenting an overview about the developments and processing methods of Ox-CMCs and its constituents, fibers and matrices systems. Their key properties are addressed in details with examples of commercially available composites. Importance is also given to the main limitation of oxide systems in relation to thermal degradation and creep behavior. For a better understanding of Ox-CMCs capacity, actual industrial applications are reported in the fields of aviation, aerospace, heat-treatment furnaces, etc. Additionally, comments about the future development of this class of material are given in the end.

Characterization of Mechanical Properties under Shear Load of a Short-Carbon-Fiber-Reinforced C/SiC Ceramic

The main objective of this work is the evaluation of the mechanical shear properties of a short-carbon-fiber-reinforced ceramic, which shows strong non-homogeneity in its microstructure and anisotropy through different fiber orientations. In this work, the shear modulus (G-modulus) and shear strength of this material were determined with the Iosipescu shear test and the Asymmetric-Four-Point-Bend shear test (AFPB test) at room temperature. Both test methods provide a nearly pure shear stress state in the shear plane and are therefore suitable for determination of the mechanical properties under shear load. Different notch opening angles with θ = 0° or θ = 110° and sample sizes for both methods are discussed. For strain measurement, strain gauge rosettes are applied on two sides of the test specimens. Because of the limited size of basic material, for the Iosipescu test small specimens were bonded onto aluminum tabs, which induced different failure mechanisms. Therefore the Iosipescu results are only valid for determination of shear modulus but not for evaluation of shear strength.

A New Clinically Relevant T-Score Standard to Interpret Bone Status in a Sheep Model

Background Osteoporosis is diagnosed by bone loss using a radiological parameter called T-score. Preclinical studies use DXA to evaluate bone status were the T-score is referenced on bone mineral density (BMD) values of the same animals before treatment. Clinically, the reference BMD represents values of an independent group of healthy patients around 30 years old. The present study established a clinically similar T-score standard to diagnose osteoporosis in a sheep model. Material/Methods We used 31 female merino land sheep (average 5.5 years old) to study osteoporosis. The following groups were compared using DXA measurement: 1) control; 2) ovariectomized (OVX); 3) OVX combined with a deficient diet (OVXD); and 4) OVXD combined with methylprednisolone administration (OVXDS). Further, an independent group of 32 healthy sheep (4–6 years old) were measured as an independent baseline. BMD was measured at 0 months, 3 months, and 8 months after treatment. Results The same significance pattern between the treated groups and either baseline groups was seen. However, using an independent baseline changed the “clinical” interpretation of the data from an osteoporotic bone status (T-score <−2.5) after 3 months of OXDS treatment into an osteopenic bone status (T-score <−1.5 to −2.4). Conclusions Using an independent baseline enhanced the statistical significance and showed the clinical relevance. Furthermore, an independent baseline is a reliable alternative to use of a new control group for future experiments and thus reduces the number of animals needed by eliminating the need for a control and corresponding to clinical practice.

A novel, hydroxyapatite-based screw-like device for anterior cruciate ligament (ACL) reconstructions

BACKGROUND Rupture of the anterior cruciate ligament (ACL) is one of the most common injuries of the knee. Common techniques for ACL reconstruction require a graft fixation using interference screws. Nowadays, these interference screws are normally made of titanium or polymer/ceramic composites. The main challenge of application of a fixation device made entirely of bioactive ceramic is in relation to the low strength of such materials. The purpose of this study was to evaluate a novel geometry for a fixation device made of pure hydroxyapatite for ACL reconstructions that can overcome some problems of the titanium and the polymer/ceramic screws. METHODS Finite Element Analysis was used for optimization of the stress distribution in conventional interference screw geometry. For experimental evaluation of the new fixation device, ex vivo tests were performed. RESULTS The innovative screw-like fixation device is characterized by multiple threads with a large thread pitch. The novel design enabled the insertion of the screw into the bone without the application of an external torque or a screwdriver. In turn, it also allowed for the use of low-strength and high-bioactivity materials, like hydroxyapatite. Ex vivo tests showed that the novel screw can sustain pull-out forces up to 476 N, which is comparable to that of the commercially available BioComposite™ interference screws (Arthrex Inc., Germany), as a reference. CONCLUSIONS In summary, the novel screw design is a promising strategy to develop all-ceramic fixation devices for ACL reconstructions, which may eliminate some drawbacks of the current interference screws.