Abstract
Parallel strand bamboo (PSB) is a new type of bio-composite. In the present study, three-point bend end-notched flexure (3ENF) tests of PSB were conducted to analyze the fracture behavior including fracture process, resistant curves, and critical energy release rate in the longitudinal (L) system. The results show that transverse-longitudinal (TL) and thickness-longitudinal (ZL) specimens had the same fracture process including the stages of fracture process zone (FPZ) development and opening crack propagation but different mode II critical energy release rate (GIIc), and they indicate that the fiber bridging had a significant influence. The fracture processes suggest the PSB specimens of which the initial crack length was 0.5 times the half span had a stable crack propagating process, and the crack propagation length was wide enough to evaluate GIIc, which was 5.02 N∙mm-1 of TL specimens and 2.71 N∙mm-1 of ZL specimens. Besides, there was no obvious influence of span/depth ratio on the fracture resistance of both ZL and TL specimens when the ratio was larger than 15.
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Mode II Fracture Behavior of Parallel Strand Bamboo Measured by 3-Point End-Notched Flexure Tests
Zhen-wen Zhang,a Jia-liang Zhang,b Yu-shun Li,d,* and Rui Liu c
Parallel strand bamboo (PSB) is a new type of bio-composite. In the present study, three-point bend end-notched flexure (3ENF) tests of PSB were conducted to analyze the fracture behavior including fracture process, resistant curves, and critical energy release rate in the longitudinal (L) system. The results show that transverse-longitudinal (TL) and thickness-longitudinal (ZL) specimens had the same fracture process including the stages of fracture process zone (FPZ) development and opening crack propagation but different mode II critical energy release rate (GIIc), and they indicate that the fiber bridging had a significant influence. The fracture processes suggest the PSB specimens of which the initial crack length was 0.5 times the half span had a stable crack propagating process, and the crack propagation length was wide enough to evaluate GIIc, which was 5.02 N∙mm-1 of TL specimens and 2.71 N∙mm-1 of ZL specimens. Besides, there was no obvious influence of span/depth ratio on the fracture resistance of both ZL and TL specimens when the ratio was larger than 15.
Keywords: Parallel strand bamboo; Mode II fracture; Critical energy release rate; Fracture resistant curve
Contact information: a: School of Civil and Environmental Engineering, Ningbo University, Ningbo, P.R. China, 315211; b: College of Science & Technology, Ningbo University, Ningbo, P.R. China, 315300; c: College of Architecture and Environmental Design, Kent State University, Kent, OH, USA, 44242; d: Construction Engineering College, Qingdao Agricultural University, Qingdao, P.R. China, 266109;
* Corresponding author: lys0451@163.com
INTRODUCTION
Parallel strand bamboo (PSB) is a new type of bio-composite that can be used as an excellent sustainable construction material because of its outstanding attributes (Li et al. 2015, 2017; Zhang et al. 2019). To effectively use PSB as a structural material, a better understanding of its mechanical properties, including the fracturing properties, is required.
PSB is usually fabricated by gluing of bamboo strands through high pressure and then forming the material into boards. The bamboo strands are layered and aligned with each other in the parallel-to-grain direction, and therefore the bamboo fibers of PSB distribute in the same direction. As a result, PSB can be treated as a wood-like material and be considered as an orthotropic material, with a local symmetry material coordinate (Fig. 1). The longitudinal axis (L) is parallel to the direction of fibers; the transverse axis (T) is perpendicular to the fibers; and the thickness axis is (Z). The crack propagation in the L system was investigated in the present study. It is one of the most frequent crack propagation systems in timber beams as well as PSB beams (Smith and Vasic 2003; Huang et al. 2015). Therefore, a fundamental requirement is to measure mode I and mode II fracture properties.
To obtain the fracture properties of solid wood, the double cantilever beam (DCB) and three-point bend end-notched flexure (3ENF) tests have been used in recent years as the principal methods (De Moura et al. 2009; Yoshihara and Satoh 2009; Xavier et al. 2015). Moreover, Yoshihara (2010) adopted these methods to wood-like materials. Therefore, these methods are expected to be extended to measure the fracture properties of PSB, and several studies have reported the investigation of the fracturing properties of PSB (Huang et al. 2018a; Wang et al. 2019).
In this study, 3ENF tests of PSB were conducted, and the fracture behaviors including fracture process, resistant curves (R-curves,) and critical energy release rate were analyzed. Fracture properties of both TL and ZL system were examined, and the compliance-based beam method (De Moura et al. 2009) was employed to obtain R-curves.
Fig. 1. PSB boards and the coordinate system (L, T, and Z represent the longitudinal axis parallel to the direction of fibers, the transverse axis perpendicular to the fibers and the thickness axis respectively)
EXPERIMENTAL
Materials
A PSB board was used to get the specimens for testing and was fabricated by Phyllostachys, a common bamboo genus in China. All specimens were cut from this same PSB board. According to the PSB fabricating process, it is natural to define the parallel-to-grain, perpendicular-to-grain and thickness directions of the PSB boards as the L-axis, T-axis, and Z-axis, respectively (Fig. 1), and the elastic properties of PSB in this coordinate system are given in Table 1.
3ENF Tests
Figure 2 shows the configuration of 3ENF specimens and the diagram of the tests. Table 2 lists the dimensions tested in this study. There were 6 groups with 5 specimens each according to the span-to-depth ratio (2L/2h) and the initial crack length, which are two constraint parameters of the specimen dimensions.
(a) Specimen configuration (a presents initial crack length)