Image-Guided Failure Assessment of Porous Materials
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25 MaL Res. Soc. Symp. Proc. Vol. 5910 2000 Materials Resarch Society
Image-guided failure assessment has been used in another study to assess local bone failure [1]. For the purpose of this study we hypothesized that image-guided failure assessment could also be used to nondestructively monitor and evaluate fracture initiation, fracture progression, and damage accumulation of aluminum porous materials on the microscopic level as well as to determine how architecture influences microstructural failure. The aim of this study was to implement image-guided failure assessment for aluminum foams and evaluate resulting fracture patterns. EXPERIMENT The micro-tomographic system described in this work is based on a compact fanbeam type tomograph (jtCT 20, Scanco Medical, Bassersdorf, Switzerland), also referred to as desk-top
1 iCT
[5]. A microfocus X-ray tube (Kevex PXS5) with a focal spot of 10
pim is used as a source. The filtered 40 kVp X-ray spectrum is peaked at 25 keV. The imaging chain includes a detector with integrated acquisition electronics to measure the intensity of the transmitted X-rays. The detector consists of a linear CCD-array with 1024 elements and a pitch of 25 jgm. To perform a measurement, the object is mounted on a turntable that can be shifted automatically in the axial direction. Rotation and axial shift are effected with stepping motors. In the high-resolution mode, 600 projections are taken over 2160 (1800 plus half the fan angle on either side). A standard convolutionbackprojection procedure with a Shepp and Logan filter is used to reconstruct the CT images in either 512x512 or 1024x1024 pixel matrices. The whole system is housed in a standard 19-inch rack allowing an extremely compact design. In order to be able to select different fields of view from 8 mm to a maximum of 18 mm, the turntable is mounted on a linear slider, which permits to position the turntable freely between source and detector. The spatial resolution of the system was defined by the 10% contrast-level of the modulation transfer function (MTF) resulting in a spatial resolution of 28 jim in plane [5]. In order to visualize material failure on a microstructural level, a novel MCD was devised to measure unloaded and loaded aluminum specimens directly in the gCT. The device includes two steel load platens, a moment-relief bearing, a cap screw and an integrated load cell. Since micro-tomographic imaging utilizes low-energy X-rays, the measurement window in the micro-compression device (12 mm in diameter) is made of a radiolucent, highly stiff plastic in order to enable X-rays to penetrate the device. The micro-compressor has an outer diameter of 19 mm and a total length of 65 mm in order to fit in the jtCT opening. The internal load chamber fits specimens with maximal diameters of 9 mm and maximal lengths of 22 mm. For the actual experiment, specimens are first placed in the MCD and imaged using the jiCT system. The alloy is then compressed using a servo-hydraulic testing machine (Model 8511, Instron Corporation, Canton, MA
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