Chapter 28 Compression Testing of Micro-Scale Unidirectional Polymer Matrix Composites Torin Quick, Sirina Safriet, David Mollenhauer, Chad Ryther, and Robert Wheeler Abstract This project builds on work done by Lu et al. An experimental study is carried out to characterize the failure behavior of a fiber reinforced polymer matrix composite at the micro-scale using the same test methodology. In order to address the issue of catastrophic failure observed in the previous effort, a physical stop for the indenter that limits maximum displacement to a predetermined value is integrated into the specimen design. Micron-sized specimens of IM7/BMI unidirectional composite with an integrated indenter displacement control were fabricated using Focused Ion Beam (FIB) milling. The specimens were compression tested using a custom built, SEM-based in-situ micro-testing device. During compression, SEM images are acquired continuously between displacement intervals so the deformation phenomena can be observed. Initial results showed that the integrated indenter displacement control prevents complete destruction of the specimen after the onset of failure. Damage observed includes interface failure, broken fibers, and general crushing. Parallel efforts on larger-scale compressive testing are conducted on millimeter-sized specimens using an in situ mechanical test frame located in an X-ray micro computed tomography ( CT) system. Failure response includes longitudinal splitting or brooming and kinking. A quantitative comparison of the compressive strength and modulus obtained from the two size scales specimen shows that there is no indication of a size effect. The experimental results will be used to validate the numerical models of micro-compression behavior. Keywords Failure mechanism • Composite • In-situ compression testing • Kink bands • X-ray micro computed tomography 28.1 Introduction In spite of significant improvements in composite resins and fibers during the last few decades, the compressive strength of polymer matrix composites still remains at 50–60 % of their tensile strength. This significantly reduces the advantage position of these materials in structures in which compressive strength is a primary design requirement. The mechanisms of composite failure in compression are not fully understood. Therefore, there is the need for a better understanding of the physics and mechanics of compressive failure, which can be achieved by micro-scale experimental studies. The failure mechanisms in unidirectional composites under compression load have been examined extensively [1–5]. Four common failure mechanisms are micro-buckling, kinking, fiber failure and splitting [6]. Each of these failure modes may be observed in a single specimen, or a specific mode may dominate for the same composite material tested under different conditions. Under the USAF Summer Faculty Fellowship Program, a methodology for performing in-situ compression test to characterize the failure behavior of a fiber reinforced composite at the micro-scale was pioneered by Lu et al. [7]. The material investigated was unidirectional carbon fiber reinforced polyimide matrix composite (IM7/BMI). The specimens were prepared by two steps in the micro fabrication process: ion sputtering (coarse cutting) and ion milling FIB (fine cutting). Speckle patterns were created on the specimen surface for further use in Digital Image Analysis (DIC). In-situ compression tests were conducted using a custom micro-mechanical testing device placed inside the SEM. The micron-size specimens T. Quick • D. Mollenhauer • C. Ryther U.S. Air Force Research Laboratory, WPAFB, Dayton, OH 45433, USA S. Safriet ( ) University of Dayton Research Institute, Dayton, OH 45469, USA e-mail: Sirina.Safriet.ctr@us.af.mil R.Wheeler Micro Testing Solutions LLC, Hilliard, OH 43026, USA © The Society for Experimental Mechanics, Inc. 2016 A.M. Beese et al. (eds.), Fracture, Fatigue, Failure and Damage Evolution, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-21611-9_28 225
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