A New Methodology for Scaling the Mechanics of Pin-reinforcement in Composite Sandwich Structures under Compression using Digital Image Correlation

There is a great deal of interest in understanding the mechanics in pin-reinforced composite sandwich structures. In particular, characterizing the relationship of the mechanics in smaller to larger specimens due to changes in the pin response can expedite the development of more advanced models, as well as the assessment of new materials and processing conditions. In this investigation, specimens with a conventional symmetric pin configuration anchoring into each facesheet have been experimentally characterized under compressive loading. The effect of specimen size on the compressive response has been investigated through the number of active pins under loading. The stress-strain response of these structures exhibit initially high compressive stiffness followed by a significant load shedding and flow stress. The transition in the mechanical behavior is attributed to the failure of reinforced pins under a combination of axial compression and transverse bending due to their orientation relative to the facesheets. As the size of a specimen is decreased, the effective stiffness and strength decrease due to a decrease in effective pin stiffness. It was found that these effects on the compressive stress can be scaled through a stress scaling factor derived from the change in the structure of the specimen due to the number of active versus inactive pins per unit of area, which is scaled by the pin density related to the pin basis and spacing for the corresponding pin configuration. Using Digital Image Correlation (DIC), it was then possible to account for these size effects on compressive strain directly through the pin deformations. A stiffness scaling factor could then be determined from a model of a beam in bending under axial and transverse loading that was correlated to the DIC displacements to determine the effective pin stiffness and corresponding stiffness scaling factor for strains. A stretched exponential relationship was also found to describe the scaling of the effective pin stiffness with the number of active pins. The compressive stress-strain response was then corrected to show that similar scaled properties could be obtained for the compressive modulus. The Poisson's ratio was also obtained from the DIC strains averaged over the full specimen, and a scaling relation similar to that for the effective pin stiffness was developed.