Single particle mechanical characterization of ground switchgrass in air dry and wet states using a microextensometer

To understand the mechanical process of pelletization, it is critical to study the particle level interactions that cause particles to bind together and form a pellet. A micromechanical extensometer device, inspired by the MEMS technology, was developed and used to perform tensile experiments to deduce the stress-strain response of single particles of ground biomass. The effect of moisture, which has a significant role in forming pellets, was examined based on the micromechanical characterization of moisture conditioned and unconditioned (control; air dried) switchgrass particles. Conditioned particles exhibited three phases sigmoidal shaped stress-strain response. The three phases include the first linear elastic zone, where the particle behaved linearly up to approximately 1.6% strain, the second linear elastic zone (strain ranging from 1.6 to 2.1%) with significantly increased elastic modulus (168.9–223.7%), and the third zone (strain beyond 2.1%), where elastic modulus declined sharply (down to 90.9% of the second zone). The modulus of elasticity up to 1.5% strain for unconditioned and conditioned switchgrass particles were 1.60 ± 0.33 GPa and 6.99 ± 1.66 GPa, respectively (p = 0.00). The nominal fracture strength of unconditioned (6.2%, w.b.) and conditioned (17.5%, w.b.) switchgrass particles were determined as 35.77 ± 14.99 MPa and 130.42 ± 87.56 MPa, respectively (p = 0.08). The nominal fracture strain of unconditioned and conditioned switchgrass particles were determined as 2.43 ± 0.70% and 1.51 ± 0.66%, respectively (p = 0.06). Increase in the stiffness of switchgrass particles is contributed to the bundling of fibers promoted by the activation of binders due to increased moisture content.