28 Compression Testing of Micro-Scale Unidirectional Polymer Matrix Composites 227 Fig. 28.1 Schematic illustration of the in-situ micro-load fixture used in the experiments 28.2.2 In-Situ Micro-Compression Set Up A custom built, SEM-based loading fixture was used to conduct the compression tests on the micron-size specimens. The schematic illustration of the fixture is shown in Fig. 28.1. A piezoelectric actuator was used to impose sub-nanometer displacement over a total stroke of 40 m. A strain-gage load cell, with a maximum load capacity of 100 g, was used to measure the forces applied to the sample. An alignment flexure was employed to ensure uniform axial motion of the load train. A polished 130 m sapphire fiber was used as the compression platen. The contact surface was polished with FIB milling to ensure the surface was perpendicular to the loading direction. At the other end of the test device, a piezoelectric controlled slip-stick motion x-y-z positioning stage was used to precisely place the specimen underneath the compression platen. This stage based its movement on a piezoelectric inertial force mechanism that provided nanometer scale positioning resolution with zero backlash. The fixture is 50 mm wide 50 mm tall 150 mm long and thus fit well inside the SEM chamber without disturbing or inhibiting the SEM’s function. Instrumentation control and data acquisition were achieved by using the Labview software from National Instrument (NI). The compression tests were conducted in a quasi-static manner, with a typical displacement rate of 100 nm/s. The test was separated by periods where the piezoelectric actuator was held at a constant voltage in order to optimize the collection of high-resolution SEM images. 28.2.3 In-Situ CT Compression Set Up Small cylindrical composite rods were used for in-situ compression testing inside the X-ray CT. The Deben CT5000 testing stage was used in the Xradia X-ray microscope Versa XRM-520 for a real time observation of the damage evolution of the materials which were subjected to load. The specimens were positioned perpendicularly to the beam direction on the hardened steel disks (M50). The built-in controller software Scout and Scan was used for setting the imaging parameters. A scintillator CCD detector (obj 4 ) was used. The specimen was first scanned without load with a voxel size of 1.5 m. Then, the specimen was loaded in compression with a loading speed of 0.1 mm/min. The projection (2D X-ray) images were continuously acquired to observe damage of the specimen while under compressive loading. They were taken at 1 s intervals of exposure time. The load and displacement were displayed and automatically recorded via Microtest software. After the specimen failed, CT scan was performed to obtain volumetric images of damaged specimen. A source voltage of 60 kV was used at a power of 5 W. Thousand six hundred and one projections were taken over a rotation of 360ı, and the exposure time for each projection was 45 s.
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