Date of Award

Spring 5-5-2023

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

Advisor

Eric N. Landis

Second Committee Member

Bill Davids

Third Committee Member

Stephen Shaler

Additional Committee Members

Roberto Lopez-Anido

Edwin Nagy

Abstract

Due to its excellent structural qualities and accessibility, wood is among the most often utilized structural materials. Despite its ubiquity, wood poses numerous challenges. It is heterogeneous and anisotropic. It has a complex hierarchical ultrastructure, and the properties can have wide variation within a species, and indeed within an individual tree. This work aims to improve our understanding of the strength and fracture behavior of spruce-pine-fir (south) (SPFs), particularly in cross-grain direction. This study’s primary goal is to examine the relationship between crack propagation and cross grain morphology under the following loading configurations: compact tension, compression, and rolling shear. The broader goal is to be able to use this information to improve our ability to predict the performance of mass timber structures. In order to better characterize micromechanical processes and damage progression, acoustic emission (AE) techniques were applied.

In this investigation, fracture in compact tension specimens was characterized by both R-curve and bulk fracture energy approaches. Our results show that the fracture follows a distinct route that deviates from the initial crack direction depending on the end-grain angles. This deviation is driven by a competition between maximum strain energy release rate and minimum crack resistance. For crack propagation in the tangential direction, cracks are confined to an earlywood region, which corresponds to the direction of least resistance. This pattern continues even as the end-grain shifts until an angle of about 40°, when the crack begins to jump across earlywood/latewood rings. At roughly 45°, the crack path shifts to a strictly radial direction, corresponding to a path of least resistance. In order to further quantify different micromechanical mechanisms, acoustic emission monitoring was used to track the propagation of damage. To identify different damage sources, an artificialneural network (ANN) technique was used to detect, classify, and quantify the AE energy sources. Results showed that earlywood cell wall tearing, dominant at 0°, produced higher energy release than cell wall separation, which dominates 90°crack propagation. Fiber bridging was also identified as another damage mechanism that occurs in the later stages of the crack growth, but in cross-grain fracture, it produces minimal AE energy. The same ANN approach was used to identify the damage mechanisms in specimens under rolling shear. Cross-laminated timber’s (CLT) mechanical performance is greatly influenced byrolling shear characteristics. In this work, the impact of end-grain orientation on rolling shear strength and modulus was evaluated. AE signal classification was applied to separate the associated damage modes and to determine the AE energy sources. Macroscopically, damage typically initiates along the glue line, but further crack growth is highly dependent on end grain morphology. Specimens with end-grain parallel to the axis of shear showed tangential propagation along an earlywood line, but as the dominant grain angle changes, cracks jump across growth rings, or if the angle is high enough, shift to a radial direction. AE results showed cell wall tearing to be the dominant energy dissipation mechanism, but cell wall peeling and bridging have significant contributions at higher end-grain angles.

Through this research, we are better able to link damage sources to particular micro mechanical energy dissipation. This information is in a suitable form for inclusion in computational models that can be used to simulate structural performance as a function of material morphology.

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