Cellular materials are known to be useful in the application of designing light but stiff structures. This applies to
various components used in various industries such as rotorcraft blades, car bodies or portable electronic devices.
Structural application of the metal foam is typically confined to light weight sandwich panels, made up of thin solid
face sheets and a metallic foam core. The resulting high-stiffness structure is lighter than that constructed only out
of the solid metal material. The face sheets carry the applied in-plane and bending loads and the role of the foam
core is separate the face sheets to carry some of the shear stresses, while remaining integral with the face sheet.
Many challenges relating to the fabrication and testing of these metal foam panels continue to exist due to some
mechanical properties falling short of their theoretical potential. Hence in this study, a detailed three dimensional
foam structure is generated using series of 2D Computer Tomography (CT) scans, on Haynes 25 metal foam. Series
of the 2D images are utilized to construct a high precision solid model including all the fine details within the metal
foam as detected by the CT scanning technique. Subsequently, a finite element analysis is then performed on an as
fabricated metal foam microstructures to evaluate the foam structural durability and behavior under tensile and
compressive loading conditions. The analysis includes a progressive failure analysis (PFA) using GENOA code to
further assess the damage initiation, propagation, and failure. The open cell metal foam material is a cobalt-nickel-chromium-tungsten alloy that combines excellent high-temperature strength with good resistance to oxidizing
environments up to 1800 °F (980 °C) for prolonged exposures. The foam is formed by a powder metallurgy process
with an approximate 100 pores per inch (PPI).
Novel techniques for generating robust and accurate meshes based on 3-D imaging data have recently been developed which make the prediction of macro-structural properties of composite structures based on micro-structural composition straightforward. The accuracy of reconstructions is a particular strong point of these new techniques with geometric accuracy only contingent on image quality. Algorithms developed and used are topology preserving, volume preserving and multi-part geometric models can be handled straightforwardly. In addition to modeling different constituent materials as separate mesh domains, material properties can be assigned based on signal strength in the parent image thereby providing a way of modeling continuous variations in properties for an inhomogeneous medium. These new techniques have been applied to the analysis of a ceramic matrix composite which was micro-CT scanned and the influence of imaging parameters on both predicted bulk properties and localized stresses has been explored.
This paper utilizes the Computed Tomography (CT) as the NDE technique to characterize the initial matrix porosity's locations and sizes in a Ceramic Matrix Composites (CMC) test specimen. Further, the Finite Element (FE) method is applied to calculate the localized stress field around these pores based on the geometric modeling of the specimen's CT results, using image analysis, geometric modeling and meshing software, ScanIP/ScanFE [1]. The analyses will simulate experimental loading conditions where scanned specimens are then tensile tested to a 0.07 % total strain to identify the matrix cracking locations in relation to the original pores. Additional work is carried out combining the image processing and finite element to investigate the applicability of some novel meshing techniques. Finally, the calculated Finite Element [2-4] localized stress risers are compared with the observed matrix cracking locations. This work is expected to show that an FE model based on an accurate 3-D rendered model from a series of CT slices is an essential tool to quantify the effects of internal macroscopic defects of complex material systems such as CMCs.
Metal foams are expected to find use in structural applications where weight is of particular concern, such as space vehicles, rotorcraft blades, car bodies or portable electronic devices. The obvious structural application of metal foam is for light weight sandwich panels, made up of thin solid face sheets and a metallic foam core. The stiffness of the sandwich structure is increased by separating the two face sheets by a light weight foam core. The resulting high-stiffness structure is lighter than that constructed only out of the solid metal material. Since the face sheets carry the applied in-plane and bending loads, the sandwich architecture is a viable engineering concept. However, the metal foam core must resist transverse shear loads and compressive loads while remaining integral with the face sheets. Challenges relating to the fabrication and testing of these metal foam panels remain due to some mechanical properties falling short of their theoretical potential. Theoretical mechanical properties are based on an idealized foam microstructure and assumed cell geometry. But the actual testing is performed on as fabricated foam microstructure. Hence in this study, a high fidelity finite element analysis is conducted on as fabricated metal foam microstructures, to compare the calculated mechanical properties with the idealized theory. The high fidelity geometric models for the FEA are generated using series of 2D CT scans of the foam structure to reconstruct the 3D metal foam geometry. The metal foam material is an aerospace grade precipitation hardened 17-4 PH stainless steel with high strength and high toughness. Tensile, compressive, and shear mechanical properties are deduced from the FEA model and compared with the theoretical values. The combined NDE/FEA provided insight in the variability of the mechanical properties compared to idealized theory.
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