Sandip Haldar

Mechanics of Curved Pin-Reinforced Composite Sandwich Structures

Pin-reinforced sandwich composites have recently attracted the attention in lightweight structural applications where it substantially improves out-of-plane and shear properties for sandwich composites. However, there is not a great deal of understanding in regards to how shaping these composites affects their mechanical performance when the orientation of the pin-reinforcement may change due to the shaping process. In this investigation, singly curved pin-reinforced sandwich composites using K-Cor have been fabricated using a bend fixture to curve the specimen and heat treatment to soften the core during before bonding with the composite face sheets in order to prevent any damage to the core during the shaping process. Experiments were then performed on the curved sandwich specimens using different boundary conditions on the edges. The boundary conditions were found to result in increased load bearing capacity when supported at the edge compared to support at the bottom due to increased lateral constraint that delayed the onset of bending shear failure. Digital Image Correlation (DIC) was also used to determine the deformation fields from the images captured during deformation to quantify the effects of boundary conditions on the onset of failure initiation, and the results were used to develop a new Finite Element Analysis (FEA) model that is capable predict the mechanical behavior of the curved K-Cor sandwich composites.

Characterization of Mixed-Mode Energy Release Rates for Carbon Fiber/Epoxy Composites Using Digital Image Correlation

There is great interest in using advanced mechanical characterization techniques, such as Digital Image Correlation (DIC), to investigate complex fracture behavior of fiber-reinforced composites. In this study, unidirectional carbon fiber/epoxy composites were fabricated from the pre-preg carbon fiber epoxy tapes for the characterization of their energy release rates. Quasi-static mixed-mode fracture experiments were performed using a Wyoming Test Fixture. The crack tip response of the composite was determined by using the displacement fields obtained from DIC based on LEFM. The fracture parameters obtained from the DIC analysis have been compared to those determined from the global load displacement response using ASTM standard. The results show that the experimental investigation of the energy release rates using more local, near field displacements around the crack tip obtained by DIC provide accurate quantification of the crack tip response relative to the global loading conditions. Effects of global bending and fixture compliance on the global load–displacement response can also be eliminated by using full-field DIC measurements.

Mechanics of Fiber-Reinforced Porous Polymer Composites

Fiber reinforcement has been found in natural porous structures, such as Palmetto Wood. These reinforcements appear to improve the mechanical behavior of these porous structures to enhance strength and energy absorbing capability. Similar use of fiber reinforcement has yet to be pursued in synthetic porous materials to form composites. Using multi-scale DIC, we previously investigated the macroscale and microscale mechanisms responsible for the mechanical behavior of Palmetto wood, which we used to formulate new mechanical models of the effects of fiber reinforcement on the evolution of damage and plastic strain. We have now formulated synthetic fiber-reinforced porous materials with fiber reinforcement to determine if these effects translate under quasi-static flexure using multi-scale DIC and the new mechanical models.

Mechanical Behavior of Bio-inspired Sandwich Composites

Nature is a popular source of inspiration for the development of new materials and structures. Palmetto wood has been found to be a potential bio-inspiration due to its historically successful mechanical performance to develop materials with enhanced mechanical properties. To understand the basis of mechanical performance of Palmetto wood, its failure mechanism and energy absorbing capacity is elucidated at multiple length scales. The quasi-static and dynamic three-point bend tests has been used to reveal the leading failure mechanisms like shear dominated debonding and pore collapse. The damage evolution under quasi-static and dynamic impact is determined. The sandwich material systems are yet to be investigated in detail to utilize its potential in advanced engineering applications. We present some work on development of sandwich composites with improved mechanical behavior using Palmetto wood as a biological template. The quasi-static and dynamic characterization of nano-enhanced sandwich materials is presented. The leading failure mechanism in Palmetto wood are found to be shear dominated delamination at the macrofiber-matrix interface caused by shear strain concentration and shear cracking in the porous cellulose matrix caused by pore collapse. Reinforcement in the bio-inspired core and the nano-enhancement in the adhesive increase the mechanical properties of the sandwich structure.