MEAM Seminar: “Mechanical Properties of Fibrous Network Materials”

Fibrous network materials are ubiquitous both in nature and in synthetic materials, and therefore it is important to understand the general properties of the materials and the physics and microstructures on which those properties depend. Specifically, since fibrous networks generally perform a structural function, their mechanical properties are of critical importance. We discuss here certain mechanical properties of specific fibrous network materials, including compression of pulmonary emboli and fracture of Whatman filter paper.

The first fibrous network material we consider is pulmonary emboli, which consist of a random network of fibrin fibers with pores filled with red blood cells (RBCs). We measure the stress-strain responses of human pulmonary emboli under cyclic compression which causes irreversible changes in the structure of the emboli. We describe the hysteretic response of emboli using a model of phase transitions in which the compressed embolus is segregated into coexisting rarefied and densified phases whose fractions change during compression. Our model takes into account the rupture of RBCs in the compressed emboli and stresses due to fluid flow through the emboli’s small pores.

The second fibrous network material we consider is Whatman filter paper, which consists of a network of cellulose fibers that typically interact through hydrogen bonding. The effect of humidity on the out-of-plane fracture toughness of Whatman filter paper is studied for a broad range of relative humidities using double cantilever beam (DCB) tests. Cohesive zone modeling and finite element simulations are used to model crack propagation in the cellulose network. We find that the force-displacement curves from the DCB experiments cannot be explained by a single cohesive zone model, so a novel model is developed which can capture the high peak and sudden drop in the force in the experimental data due to an initiation region. This new model agrees well with experimental data.