Characterization and modeling of biobased composites and structural insulated panels [electronic resource]

Biobased materials have been proposed as suitable replacements for a variety of synthetic engineered materials, with applications ranging from biomedical devices to automotive components to construction supplies. These materials are attractive for sustainable design and engineering because of their rapid renewability, low embodied energy, competitive mechanical properties, and closed-loop lifecycle potential. Biobased composites, which in the context of this research refer to natural fiber reinforced biopolymers, represent a high volume subset of the broader field of biobased materials that have potential for use in construction applications. Before biobased composites can be considered viable replacements for conventional building materials, several unanswered questions must first be addressed. Can the same design principles and analytical models used for traditional materials be extended to biobased ones? Are the structural properties of biobased composites sufficient to replace ubiquitous construction materials, such as plywood and timber? Does exposure of biobased materials to weathering and hostile environmental conditions impact their mechanical properties and appearance, and if so, by how much? The research presented herein addresses these questions through the examination of biobased composites composed of poly-hydroxyalkanoates (PHAs), a class of bacterially synthesized biopolymers, and natural fiber reinforcements made from hemp and jute. This research addresses fundamental questions about the viability of biobased materials as primary structural elements through the development and application of practical design tools and the identification of principal environmental degradation mechanisms. To advance biobased composites in structural applications, contributions have been made in the areas of (1) biobased composite constitutive modeling, (2) biobased composite sandwich panel design and characterization, and (3) biobased polymer and composite environmental degradation characterization. A phenomenological constitutive model, calibration procedure, and finite element implementation are developed, and demonstrated to capture the tension-compression anisotropy exhibited by biobased composites. Blind predictions of biobased composites evaluated in tension and flexure are shown to capture the elastic and nonlinear inelastic response of experimental tests within 10%. Sandwich panels are proposed for improving the structural and thermal efficiency of biobased composites. Partially and fully biobased sandwich panels are designed for different failure modes using classical elastic methods and evaluated in four-point bending. The sandwich panel bending properties are compared with wood and engineered wood and are found to be mechanically competitive on the basis of initial stiffness and thermally competitive on the basis of thermal resistance. A service and strength design method is proposed wherein the flexural behavior of the sandwich panels is predicted using four distinct yielding limit states. Finite element simulations and the proposed service and strength model are demonstrated to capture the elastic stiffness and ultimate strength of the panels within 13% and 21%, respectively. Environmental degradation mechanisms for biobased composites and neat biobased polymers are characterized using two proposed accelerated weathering and mechanical testing protocols. Composite and polymer specimens are subjected to accelerated UV, heat and moisture and tested at regular intervals to determine changes in physical, visual, and mechanical properties. Following weathering, composite specimens exhibit lower modulus, lower ultimate strength, and higher strain at failure in addition to fading, mass loss, and cracking. Polymer specimens exhibit higher modulus, lower ultimate strength, and lower strain at failure in addition to fading, mass loss, and cracking. The observed changes in physical and mechanical properties are attributed to photo-oxidation and hydrolytic degradation of the PHB bio-polyester and cyclic hygrothermal expansion and contraction of the natural reinforcing fibers. The implications of this research for designing, testing, and modeling biobased composites and sandwich structures are discussed. Composite manufacturing and simplified stiffness and strength design recommendations are also provided. Future research topics are suggested with the goal of facilitating the acceptance of biobased composite materials in industry and increasing their accessibility to practitioners.

Keywords: Civil and Environmental Engineering.
Advisor: Billington Sarah L. (Sarah Longstreth)
Borja Ronaldo I. (Ronaldo Israel)
Deierlein Gregory G. (Gregory Gerard)