Stefan Hackemann

Domain switching in process zones of PZT: characterization by microdiffraction and fracture mechanical methods

Ferroelastic domain switching in the vicinity of cracks in Soft-PZT was examined by X-ray microdiffraction. Preliminary experiments under electrical and mechanical load proved the ability of the method to quantitatively characterize domain switching in the process zone of cracks with sufficient local resolution. Using an in situ bending device the observation of domain switching under load was possible. The width of the process zone at a growing crack, measured by microdiffraction can be correlated with the increase of the crack resistance curve measured in fracture mechanical tests. The separation of mechanical and ferroelastic contributions to the R-curves was performed by determining R-curves before and after unloading the samples. Residual stresses caused by domain switching could not be detected.

Determination of elastic properties for a wound oxide ceramic composite

Abstract Thanks to its low cost and high flexibility, in the last few years the winding technique has been successfully adapted for the production of complex Ceramic Matrix Composite (CMC) components with load-oriented fibre alignment. Since the winding angle can be adjusted in any direction (from 0° to 90°) during the fabrication process, it is important for the design of components to evaluate the elastic properties of CMCs as a function of the winding angle. In this study, an inverse method based on the Classic Laminate Theory (CLT) has been used for the prediction of the elastic properties, i.e. Young’s modulus, shear modulus and Poisson’s ratio, for a wound oxide CMC material, called WHIPOX® (Wound HIghly Porous OXide ceramic matrix composite). For this purpose the characteristics of an equivalent unidirectional layer (UD-layer) with consideration of fibre volume content (FVC) and porosity were calculated. On the basis of microstructural analysis the computed WHIPOX®UDproperties have been divided into two sets of elastic properties for small (below 30°) and large winding angles (30° and above). Full coverage of the mechanical properties in different wound orientations, non-orthogonal with ±3°/±87°, ±15°/±75°, ±30°/±60° and orthogonal with ±45° and 0°/90°, were evaluated with in-plane tension, and Iosipescu-shear tests. A good correlation between experimental and analytically calculated results is shown in this paper.

Development and Test of Oxide/Oxide Ceramic Matrix Composites Combustor Liner Demonstrators for Aero-engines

Ceramic matrix composites (CMC) offer the potential of increased service temperatures and are thus an interesting alternative to conventional combustor alloys. Tubular combustor liner demonstrators made of an oxide/oxide CMC were developed for a lean combustor in a future aero-engine in the medium thrust range and tested at engine conditions. During the design various aspects like protective coating, thermo-mechanical design, development of a failure model for the CMC as well as design and test of an attachment system were taken into account. The tests of the two liners were conducted at conditions up to 80% take-off. A new protective coating was tested successfully with a coating thickness of up to t=1 mm. Different inspection criteria were derived in order to detect crack initiation at an early stage for a validation of the failure model. With the help of detailed pre- and post-test computer tomography scans to account for the micro structure of the CMC the findings of the failure model were in reasonable agreement with the test results.

DEVELOPMENT AND TEST OF OXIDE/OXIDE CMC COMBUSTOR LINER DEMONSTRATORS FOR AERO ENGINES

Ceramic matrix composites (CMC) offer the potential of increased service temperatures and are thus an interesting alternative to conventional combustor alloys. Tubular combustor liner demonstrators made of an oxide/oxide CMC were developed for a lean combustor in a future aero-engine in the medium thrust range and tested at engine conditions. During the design various aspects like protective coating, thermo-mechanical design, development of a failure model for the CMC as well as design and test of an attachment system were taken into account. The tests of the two liners were conducted at conditions up to 80% take-off. A new protective coating was tested successfully with a coating thickness of up to t=1 mm. Different inspection criteria were derived in order to detect crack initiation at an early stage for a validation of the failure model. With the help of detailed pre- and post-test computer tomography scans to account for the micro structure of the CMC the findings of the failure model were in reasonable agreement with the test results.

Interpreting high temperature deformation behavior of a ceramic matrix composite via high energy x-rays and numerical simulation

All-oxide Ceramic Matrix Composites (CMC), due to their damage tolerance and thermal stability, are promising candidates for high temperature applications, including combustion liners and thermal protection systems in aerospace. In these applications, mechanical loads are introduced at high temperatures up to 1200°C or even higher, which results in complex deformation behavior. For understanding the complex behavior of an all oxide CMC under such extreme environments, laboratory tests and numerical simulations have been performed. The material investigated in this study comprises Nextel R 610 alumina fiber bundles and a porous α alumina matrix, and the composite has been produced by a computer controlled winding process. Analytical and numerical work has been performed for developing a constitutive law describing the observed creep behavior of specimens with unidirectional fiber orientation under compressive load. While for fiber orientations parallel to the compressive load a model with isochoric matrix behavior captured the experimental results well, discrepancies occurred for other fiber orientations. Parameter studies indicated that depending on fiber orientation and matrix properties the composite deformation is due to a combination of matrix compaction and fiber rotation. In-situ synchrotron studies at Argonne National Laboratorys Advanced Photon Source have been conducted on unidirectional fibre reinforced CMC specimens at 1200°C while stepwise increasing compressive mechanical load. For investigating the local strain in the composite, diffraction measurements were conducted under representative loading, and transmission radiography was utilized to study the evolution of matrix deformation and fiber rotation. First results indicate that the strain in the fiber and matrix grains of the all alumina composites may be isolated during analysis, providing information on load transfer between fiber and matrix and on elastic and creep behavior of the composite. These results will be used to inform computational simulation to produce more accurate lifetime prediction in application.