Title Page Mechanical and Morphological Characterization of Full-Culm Bamboo by Yusuf Akintayo Akinbade BEng, University of Southampton, UK 2010 MSCE, Purdue University, USA 2011 Submitted to the Graduate Faculty of Swanson School of Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2020 Committee membership Page UNIVERSITY OF PITTSBURGH SWANSON SCHOOL OF ENGINEERING This dissertation was presented by Yusuf Akintayo Akinbade It was defended on March 6, 2020 and approved by Dr. Christopher M. Papadopoulos, Ph.D., Professor, Department of Engineering Sciences and Materials, University of Puerto Rico, Mayagüez Dr. John Brigham, Ph.D., Professor, Department of Engineering, Durham University, UK Dr. Amir H. Alavi, Ph.D., Assistant Professor, Department of Civil and Environmental Engineering Dr. Ian Nettleship, Ph.D., Associate Professor, Department of Mechanical Engineering and Materials Science Dissertation Director: Dr. Kent A. Harries, Ph.D., Professor, Department of Civil and Environmental Engineering ii Copyright © by Yusuf Akintayo Akinbade 2020 iii Abstract Mechanical and Morphological Characterization of Full-Culm Bamboo Yusuf Akintayo Akinbade, Ph.D. University of Pittsburgh, 2020 Full-culm bamboo that is bamboo used in its natural, round form used as a structural load- bearing material, is receiving considerable attention but has not been widely investigated in a systematic manner. Despite prior study of the effect of fiber volume and gradation on the strength of bamboo, results are variable, not well understood, and in some cases contradictory. Most study has considered longitudinal properties which are relatively well-represented considering bamboo to be a unidirectional fiber reinforced composite material governed by the rule of mixtures. Despite the dominance of transverse failure (splitting) of bamboo in load-bearing applications, very little study of bamboo transverse properties has been conducted. The objective of this work is therefore to develop a framework and the tools required to evaluate the material and mechanical properties of full-culm bamboo. The study focuses on transverse properties and recognizes that bamboo is a heterogeneous highly orthotropic functionally graded material rather than a homogeneous fiber- reinforced composite as is often assumed. This framework brings together work conducted in the area of bamboo geometric, morphologic and material characterization to develop a correlation with mechanical properties. The effect of fiber volume ratio and gradation in the bamboo cross-section in the characterization is studied and used as a basis to establish materials- and mechanics-based constitutive models for the behavior of full-culm bamboo. The impact of material variability and uncertainty in the mechanical behavior of the full-culm is investigated and included in the presented models. Experimental, imaging and numerical results from this study indicate that considering the iv transverse behavior of bamboo as a fiber-reinforced material, governed by the rule of mixtures, is not appropriate. The scope of the work focuses on materials test specimens. This is believed to be the scale at which internal heterogeneity of the bamboo effects experimentally determined data and is also a scale at which complex modeling is still appropriate. The models developed in this work have two primary and related uses: 1) providing a platform for researchers to better understand the results of bamboo material property tests; and 2) providing a platform against which to validate macroelement models suitable for structural evaluation and design. v Table of Contents Acknowledgement ..................................................................................................................... xvii 1.0 Introduction ............................................................................................................................. 1 1.1 Motivation ....................................................................................................................... 1 1.2 Bamboo as a Natural Resource for Construction ........................................................ 2 1.3 Taxonomy of Bamboo .................................................................................................... 3 1.4 Bamboo Morphology and Microstructure ................................................................... 5 1.5 Numerical Study and Modelling of Bamboo ................................................................ 8 1.6 Objective and Organization of Study ........................................................................... 9 1.7 Nomenclature ................................................................................................................ 11 2.0 Mechanical Characterization of Full-Culm Bamboo ........................................................ 14 2.1 Materials Tests for Bamboo ........................................................................................ 14 2.2 Specimen Preparation .................................................................................................. 17 2.3 Test Methods ................................................................................................................. 20 2.3.1 Full-Culm Compression Parallel to Fibers ..................................................... 20 2.3.2 Full-Culm Shear Parallel to Fibers ................................................................. 22 2.3.3 Flat Ring Flexure............................................................................................... 23 2.3.4 Circumferential Compression Test ................................................................. 25 2.3.5 Digital Image Correlation................................................................................. 28 2.4 Mechanical Test Results ............................................................................................... 31 2.4.1 Full-Culm Test Results ..................................................................................... 31 2.5 Flat Ring Flexure Test Results with DIC ................................................................... 33 vi 2.6 Circumferential Compression Test Results with DIC .............................................. 41 2.7 Summary ....................................................................................................................... 44 3.0 Bamboo Culm Wall Structure and Image Analysis........................................................... 47 3.1 Background ................................................................................................................... 47 3.2 Application of Rule of Mixtures to Bamboo Culm Wall Properties ........................ 54 3.3 Material Properties of Fibers and Matrix .................................................................. 60 3.4 Digital Imaging to Obtain Fiber Volume Distribution ............................................. 62 3.4.1 Background ........................................................................................................ 62 3.4.2 Image Analysis ................................................................................................... 63 3.4.3 Fiber Volume Distribution ............................................................................... 65 3.5 Clipped Flat Ring Flexure Testing and Results ......................................................... 66 3.5.1 Material and Specimen Preparation ............................................................... 67 3.5.2 Clipped Flat-Ring Flexure Test Results .......................................................... 69 3.6 Shaved Flat Ring Flexure ............................................................................................ 73 3.7 SEM Imaging of Flat-Ring Flexure Failure Surfaces ............................................... 76 3.7.1 Specimen Preparation for Imaging ................................................................. 77 3.7.2 Microscopic Images and Observations............................................................ 78 3.8 Summary ....................................................................................................................... 87 4.0 Characteristics of Bamboo Culm Wall ............................................................................... 89 4.1 Background ................................................................................................................... 89 4.2 Bamboo Material .......................................................................................................... 91 4.3 Image Extraction .......................................................................................................... 92 4.4 Image Analysis .............................................................................................................. 93 vii 4.5 Summary ....................................................................................................................... 97 5.0 Uncertainty in Bamboo Materials Characterization ......................................................... 98 5.1 Random field Methodology and Application ........................................................... 102 5.2 Implementation ........................................................................................................... 104 5.3 Summary ..................................................................................................................... 110