In-situ micro-compression testing for characterizing failure of unidirectional fiber composites

A novel in-situ compression test has been developed to characterize the failure behavior of a fiber reinforced composite at the micro-scale. The material used was carbon fiber reinforced bismaleimide matrix unidirectional composite with a fiber volume fraction of 60%. Micron-size specimens were prepared from the bulk composite through a two-step micro-fabrication process. First, a pre-patterned stencil mask was placed on the surface of the composite and co-sputtered using a broad ion beam milling system which produced an array of small pillars. Secondly, these pillars were transferred into a scanning electron microscope (SEM) and serially milled using the focused ion beam (FIB) into various micron-size specimens. In-situ compression tests were conducted using a custom micro-mechanical testing device placed inside the SEM chamber. The micron-size specimens were compressed by a conical indenter with a flat end surface. During compression, high resolution SEM images were acquired continuously between displacement intervals to analyze the deformation phenomena. The images were further analyzed in conjunction with digital image correlation (DIC) technique to compute quantitative strain values. Results show that, under compression, high shear strains developed in polymer matrix between the fibers. The high shear strains caused micro-cracking of the matrix. Once the matrix failed, the fibers were seen to bend in shear mode, which ultimately led to the failure of the composite.