226 T. Quick et al. were compressed by a conical indenter with a flat end surface. High resolution SEM images and DIC were used to monitor specimen deformation, strains, and fracture during loading. While this methodology demonstrated the feasibility of the testing of micro-compression specimens, significant limitations were encountered. Excessive compliance of the micro-load frame in the SEM leads to difficulties in precisely controlling the applied loads and strains during and immediately following the onset of failure. The result is a nearly instantaneous destruction of the specimen. This study aims to address this challenge by the integration of a physical stop for the indenter that limits maximum strain to a predetermined value. Micro-compression specimens with integrated indenter displacement control will be fabricated and compression tested. With the integrated indenter displacement control, it is expected that the onset and propagation of damage can be observed without the catastrophic failure of the specimen. Parallel efforts on larger-scale compressive testing will be conducted on millimeter-sized specimens using a Deben mechanical test frame within the Xradia X-ray microscope, similar to work accomplished by Wang et al. [8]. In-situ micro tomography allows detailed observation of the local internal deformations at different strain levels. The X-ray CT data sets will be complimented with digital volume correlation (DVC) to obtain a full-strain data at the loading increments to hopefully provide insight into the sequence of events that lead to the failure of the specimen. Comparison of quantitative force/displacement data and failure processes observed in micro- and meso-scale specimens will help in the understanding of “size” effect and the role of imperfections/defects in the compressive failure of composite laminates. The experimental results will be used to validate numerical models for micro-compression behavior. The goals are: (1) to understand the failure mechanism of composites at the micro-level by observing the sequence of events of deformation phenomena, and (2) to use U.S. Air Force Research Laboratory composite analysis codes to model micro-compression behavior. 28.2 Experimental 28.2.1 Materials and Sample Preparation The material used in this study is a carbon fiber reinforced bismaleimide composite (IM7/BMI) from Cytec Engineering Materials. This unidirectional prepreg tape has a ply thickness of 125 m and a fiber volume fraction of 60 %. Composite panels of 1 ply and 24 plies of the material were manufactured and autoclave cured following the company recommended cure cycle [9]. For fabrication of a micro-compression specimen, a small piece (1 1 cm) of material was cut from the single ply composite panel. The specimen surfaces were carefully polished using a series of silicon-carbide paper and diamond lapping pads to thin the bulk material down to reduce the FIB milling time. The specimen was mounted to an SEM stub and sputter coated with a carbon layer to prevent electrical charging. Focused ion beam milling was used to create the pillar and arresting shoulders that act as the physical stop for the indenter. This was conducted in a TESCAN LYRA 3 FEG-SEM FIB. The apertures and beam currents were adjusted depending on the process steps. Material was removed on all four sides of the pillar to create an approximately 20 15 60 m pillar. The top surface of the material was cut independent of the pillar to create the arresting shoulders for the 130 m diameter indenter. The pillar was cut to its final height in order to set the vertical offset between the top of the pillar and shoulders. For this effort, the ideal difference in height between the two surfaces is the amount the material would strain before failure. This height can be adjusted depending on the desired outcome of the test. The specimen was prepared for DIC analysis by sputter coating a thin layer of Cr followed by Pt. The speckle pattern was created on the front surface by FIB milling away small areas of the Pt coating such that the underlying material was not affected. The specimens used in the meso-scale testing were small cylindrical solid rod specimens (8 mm long 1.4 mm diameter) and would be used for in-situ micro tomography. They were fabricated following these steps. A rectangular section (8.5 25 3.4 mm) of the 24 ply panel was cut and polished on the top and bottom surfaces. Care was taken to ensure that the top and bottom of the specimens were parallel and flat. A special aligning jig was fabricated to hold the specimen during machining using a diamond core-drill specially designed for preserving the core. Multiple cylindrical solid rod specimens of uniform cross section were obtained. Their ends were surface ground parallel to within 2 m tolerance. The rod specimens were ultrasonically cleaned for 5 min to remove debris. They were air-dried and oven-dried at 38ıC overnight. The specimens were kept in a desiccator until ready to test.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==