Progress in the study of dry shrinkage deformation and drying stress of raw bamboo

Zhu, J., and Yang, L. (2025). "Progress in the study of dry shrinkage deformation and drying stress of raw bamboo," BioResources 20(1), Page numbers to be added.

Abstract

Bamboo is a sustainable material that supports carbon sequestration and helps address the imbalance of timber supply vs. demand. Drying is a crucial step in bamboo processing, in the course of which shrinkage and stress accumulation can lead to defects such as cracking and deformation. Understanding stress and strain development during drying is critical for improving bamboo processing. This review paper explores bamboo’s gradient structure and moisture migration characteristics, focusing on the mechanisms behind shrinkage strain formation and the sources of stress. It reviews literature on bamboo drying and cellular structural changes, evaluating the evolution of stress and strain testing methods, from traditional sectioning techniques to advanced methods such as digital imaging and acoustic emission. The paper also summarizes progress in stress-strain research at both macroscopic and cellular scales. Current challenges include species-specific shrinkage variations, limitations in measurement techniques, and insufficient research on shrinkage above the fiber saturation point. To address these issues, the study recommends developing universal theoretical models, employing advanced detection technologies, comparing shrinkage patterns between bamboo culms and nodes, exploring drying stress composition, and adopting multi-scale research approaches. These strategies aim to enhance the quality of bamboo processing and promote higher-value applications within the industry.

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Progress in the Study of Dry Shrinkage Deformation and Drying Stress of Raw Bamboo

Jiayi Zhu,^a and Lin Yang ^a,b,*

DOI: 10.15376/biores.20.1.Zhu

Keywords: Bamboo drying; Raw bamboo; Shrinkage strain; Drying stress; Moisture content

Contact information: a: College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing 210037 China; b: Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China;

* Corresponding author: yanglin@nifu.edu.cn

INTRODUCTION

Raw bamboo is a material that retains its original form, including full cross-sectional tubes and partial cross-sectional slices (Tian et al. 2018). Regardless of origin, species, age, and harvesting site, issues related to drying and shrinkage are topics of importance for raw bamboo. It is susceptible to moisture and can easily interact with environmental humidity, resulting in dry shrinkage or wet swelling (Hone et al. 2020). This moisture sensitivity results in poor dimensional stability (Li et al. 2022). Thus, this review article considers studies aimed at improving raw bamboo’s drying and shrinking characteristics is crucial for its subsequent processing and utilization.

Drying is an essential step in bamboo processing, as proper drying treatments can improve its physical properties, mechanical properties, and dimensional stability (Li et al. 2003). Dry shrinkage is a critical indicator for assessing the performance and stability of bamboo. A greater degree of dry shrinkage increases the risk of cracking and warping after the bamboo is processed into panels (Cui et al. 2010). Studying bamboo shrinkage provides a scientific basis for optimizing drying parameters and improving processing and utilization methods. However, raw bamboo’s gradient structure and the uneven moisture content distribution during drying result in anisotropic drying strains, which generate drying stresses. These stresses vary in direction and can interact with one another, leading to defects such as cracking, warping, and crumpling upon accumulation, significantly affecting the efficiency of bamboo processing and utilization (Qi et al. 2023). Therefore, an in-depth study of the drying strain and drying stress of raw bamboo is of great theoretical and practical significance for optimizing the drying process (Wang et al. 2014; Dong et al. 2023).

Research on drying shrinkage strain and stress in raw bamboo is limited. While past studies have explored bamboo’s structure and its effect on drying strain, they have not fully addressed species differences, measurement limitations, or shrinkage beyond fiber saturation. These gaps hinder a complete understanding of bamboo drying and limit processing improvements.

This paper aims to fill these gaps by examining bamboo’s gradient structure, moisture migration’s effect on shrinkage, and the mechanisms behind drying stress. It will also review and compare the latest methods for measuring stress and strain. The goal is to provide insights and guidance to improve bamboo utilization and promote higher-quality, value-added products.

GRADIENT STRUCTURE AND WATER TRANSPORT MECHANISM OF THE ORIGINAL BAMBOO

Gradient Structure and Properties of Raw Bamboo

Bamboo material commonly used in the industry is primarily sourced from the culm sections. From a macro perspective, raw bamboo features staggered nodes and exhibits variable cross-sections that taper and curve. Each bamboo culm features a hollow interior surrounded by a three-layered bamboo wall structure. The outer layer, known as bamboo green due to its dense vascular bundles, contrasts with the inner layer, bamboo yellow, which is characterized by sparse vascular bundles (Zhou et al. 2024). Bamboo timber, positioned between bamboo green and bamboo yellow, serves as an intermediary layer (Chand et al. 2006; Zhiqian Li et al. 2014). As depicted in Fig. 1, studies have shown that bamboo nodes typically include sheath marks, nodal ridges, and diaphragms, which effectively prevent longitudinal splitting of the bamboo culm (Han et al. 2023; Jia et al. 2024).

Fig. 1. Macrostructure of bamboo

Microstructural observations show that bamboo is mainly composed of longitudinal tissues, including essential tissues and vascular bundles. The primary tissue consists of thin-walled cells for storage, while vascular bundles contain bamboo fibers and transport tissues including ducts and sieve tubes (Xian et al. 1990). Bamboo fibers are thick-walled cells that significantly influence the mechanical strength of bamboo and the transport of nutrients such as water and inorganic salts (Ray et al. 2005; Xu et al. 2007). Thus, vascular bundle distribution impacts the drying and shrinkage of bamboo. The gradient structure of bamboo results from differences between bamboo fibers and thin-walled cells.

The density of raw bamboo decreases from the bottom to the top, and its cellulose content varies with height, which contributes to differences in linear drying shrinkage at different heights (Guangrong Li et al. 2014). At the same height, the content of vascular bundles increases radially from the inside out, with fiber volume fraction following an exponential distribution (Ding et al. 2006). The variations in radial, tangential, and longitudinal shrinkage rates align with the distribution patterns of vascular bundles and thin-walled cells, as shown in Fig. 2. Li et al. (2021) noted the largest difference in shrinkage rates between radial and tangential directions in the bamboo yellow section, both in air-drying and full-drying conditions. This significant shrinkage difference makes the bamboo yellow prone to cracking and deformation, so controlling its shrinkage is a key focus. The functional gradient properties of raw bamboo arise from the anisotropic distribution of vascular bundles in longitudinal, radial, and tangential directions, leading to variations in mechanical properties.

Fig. 2. a) Chemical composition of raw bamboo layers; b) Relationship between gradient structure and dry shrinkage of raw bamboo

The anisotropy of raw bamboo significantly affects drying shrinkage and stress variations across different parts, influencing product properties. Drying is crucial in bamboo processing, as it changes the material from brittle to rigid as moisture content decreases (Xu et al. 2014). A comprehensive understanding of drying deformation and stress can improve dimensional stability and support industry advancements toward higher quality and greater value.

Moisture Transport Mechanisms

The deformation of raw bamboo is significantly influenced by moisture, which affects its entire life cycle. During changes in moisture content, defects such as cracking and deformation may occur. Changes in the dimensions of sieve tubes and conduits in bamboo are most pronounced in the saturated and completely dry states, with moisture content (MC) serving as a critical indicator of bamboo’s moisture characteristics. Inside the bamboo are numerous pores, including a sizeable capillary system and a microcapillary system, both of which serve as water storage spaces and channels for water movement.

Bamboo’s moisture is categorized into two types: bound water and free water (Lv et al. 2021). The fiber saturation point (FSP), typically between 25% and 30% (Guan et al. 2003), indicates the water content when free water has completely evaporated while bound water remains in the cell walls.

When the MC exceeds the FSP, free water evaporates first from the bamboo’s surface and moves through the large capillaries. This process is osmotic, driven by differences in capillary tension (Han et al. 2012). Since free water is not chemically bound to the cell walls, it evaporates mainly from the bamboo’s outer surface, influenced by environmental humidity. This rapid evaporation causes significant moisture migration and fluctuations in MC, creating a drying gradient as moisture moves from the inside to the exterior, affecting drying behavior.

Once the MC falls below the FSP, bamboo’s physical and mechanical properties change markedly; at this stage, accessible water is largely absent from the cell lumens, leaving only bound water in the cell walls. The internal moisture content remains higher than the external content, creating a gradient that drives water from high to low moisture areas. Bound water migrates through microcapillaries until equilibrium moisture content (EMC) is reached (Sun et al. 2006).

Raw bamboo’s moisture migration follows a hierarchical gradient structure, leading to varying moisture transport efficiency along the longitudinal and radial directions (Zhan et al. 2020). Moisture diffuses faster in higher or outer radial regions. The higher porosity (38.9%) and larger pore size (15.0 nm) of the inner internode allow moisture to enter the bamboo cavity through the inner surface. Radial moisture distribution is asymmetric, with higher moisture at the center and lower at the sides. During drying, most moisture migrates outward along the gradient through the outer surface, while a smaller amount moves inward through the inner surface and slowly passes through the bamboo diaphragm, influenced by temperature (Cheng et al. 2024). Understanding this moisture migration is key to optimizing drying methods and improving bamboo material properties.

FORMATION MECHANISM OF DRYING SHRINKAGE STRAIN OF RAW BAMBOO AND THE ROLE OF DRYING STRESSES

Principle of the Formation of Dry Shrinkage and Deformation of Raw Bamboo

Raw bamboo is a porous material characterized by an anisotropic structure closely related to water movement. It can absorb or release water in response to ambient temperature or humidity changes. During drying, cells initially saturated with water generate surface capillary tension as free water in the cell lumen moves outward through the striated pore membranes (Zhang et al. 2020). This process induces drying stress (Hunter 2001; Kang et al. 2011). When the sum of these tensions exceeds the lateral limit compressive strength of the cells, irregular collapse and contraction occur (Wang et al. 2013), resulting in shrinkage.

When the moisture content (MC) exceeds the fiber saturation point (FSP), the collapse of thin-walled tissues, reduction in cell wall thickness, and decrease in cell diameter occur due to the evaporation of free and bound water within the cell walls (Patera et al. 2013). This gradual free water loss leads to crumpling (Han et al. 2012) and drying strain (Vetter et al. 2015). At this stage, the outer layer of bamboo shrinks more than the inner layer (Zhong et al. 2009), making the shrinkage similar to that of wood (Liu et al. 2024b). The evaporation of free water from the cell lumen causes capillary tension to gradually decrease or disappear (Yan et al. 2022).

When the MC falls below the FSP, water movement primarily involves bound water, driven by the water content gradient. In this state, the microcapillary system in the cell wall voids facilitates water movement through diffusion. The macroscopic manifestation of shrinkage in bamboo is a change in size, resulting from the interaction of water inside and outside the cell walls, which leads to cell deformation (Patera et al. 2013). The primary cause of desiccation in raw bamboo is water loss from the cell walls (Vetter et al. 2015). At the cellular level, the shrinkage of bamboo results from the collective actions of multiple cells, with varying degrees of shrinkage among different cell types. Bamboo fibers play a decisive role in this process, exhibiting a significantly higher degree of dry shrinkage than thin-walled cells (Lou et al. 2021).

Cell displacement and deformation co-occur during the contraction process, with contraction stress primarily concentrated in the intercellular layer of the fibers (Yuan et al. 2023). Additionally, due to the gradient distribution of bamboo fibers in the vascular bundles, the tangential shrinkage stress on the green side of the culm wall is greater than that on the yellow side, increasing the likelihood of cracking in the bamboo material (Chen et al. 2019).

Measurement of drying shrinkage of raw bamboo

Strain is a physical quantity that reflects the degree of deformation, and studying the dry shrinkage strain of raw bamboo aims to understand its drying stress state. The stress-strain research methods for raw bamboo are analogous to those used for wood, although bamboo research has developed later. Due to the gradient structure of raw bamboo, measurement methods are less diverse compared to wood. The measurement techniques are primarily categorized into two main types: direct-contact and non-contact measurements. These include the slice, electrodiagnostic, digital scattering, and acoustic emission methods. The steps, advantages, and disadvantages of each measurement method are summarized in Table 1.

ANALYSIS OF DRY SHRINKAGE STRAIN PATTERN OF RAW BAMBOO

Relationship between drying strain and drying stress

The drying stress refers to the internal stress generated during the drying process due to the gradient differences in MC (Liu et al. 2023b; Zhang et al. 2024), which is the main factor causing drying defects such as cracking, internal fractures, and warping deformation. Given the challenges of directly measuring drying stress, it is typically analyzed and studied indirectly by measuring strain, which reflects the stress values. Throughout the drying process, the occurrence and development of stress are complex, with both the direction and magnitude undergoing dynamic changes. Additionally, different parts of the material experience significant variations in stress (Jiang et al. 2002).

Table 1. Measurement Methods for Stress and Strain in Raw Bamboo

From a structural mechanics perspective, raw bamboo is classified as a natural two-phase structural fiber-reinforced material, where the vascular bundles act as the reinforcing phase and the flexible thin-walled cells serve as the matrix (Tian et al. 2009). The multilevel weak interfaces present from tissues to cells contribute to bamboo’s anisotropic composite structure (Tian et al. 2012). These weak interfaces can lead to uneven stress distribution, a condition that persists throughout the drying process.

Research shows that drying stresses in bamboo walls vary throughout the drying process due to moisture content changes and internal gradients (Liu et al. 2023a). Initially, bamboo green, with higher moisture content, experiences compressive stresses due to the moisture gradient between the inside and outside. As drying progresses, these compressive stresses shift to tensile stresses, which increase until drying is complete.

During this phase, tensile stresses are higher in the outer layers, as they lose moisture faster. In contrast, bamboo yellow (partially dried or aged bamboo) primarily experiences compressive stresses, which increase as moisture content decreases, due to differential shrinkage between the inner and outer sections. This leads to a significant difference between actual tangential drying strain and free drying strain. In bamboo yellow, the actual drying strain (from both moisture loss and material properties) is higher than the free drying strain (from moisture loss alone). In bamboo green and bamboo timber, the opposite occurs, with free drying strain higher due to faster shrinkage of outer layers (Yan 2021).

The gradient structure of raw bamboo creates significant anisotropy, with differential drying shrinkage being the primary cause of drying stress. Both drying stress and capillary tension contribute to cell shrinkage (Lv 2018), and moisture content gradients further generate drying stresses (Behr et al. 2014). Thus, drying stress arises from bamboo’s two-phase structure and water content variations.

Research on drying strain in raw bamboo borrows from wood drying strain components, including free drying strain, elastic strain, viscoelastic strain, and mechanically adsorbed strain (Yin et al. 2021). Yan (2021) explored drying stress in raw bamboo, developing a framework with moisture content analysis, drying shrinkage strain composition, and drying stress analysis, along with a calculation model. However, most recent studies on drying shrinkage and stress focus on wood, with limited attention on bamboo, especially raw bamboo. Research has primarily focused on bamboo slices or strips, leaving questions about whether drying strain components are similar to those in wood and whether drying stress differs across raw bamboo parts. Further investigation and exploration are needed in these areas.

Manifestations of Drying Strain

Macro-level drying and shrinkage strains

The macroscopic structure of raw bamboo primarily consists of two major components: the culm and the node. Due to the spatial structure formed by alternating hollow internodes and nodes, the drying and shrinkage behaviors of the culm and node exhibit a cyclic pattern of alternating strengths and weaknesses (Yan 2021). Research has found that the volumetric shrinkage and tangential drying shrinkage of the internode are significantly greater than those of the node. Specifically, the volumetric shrinkage of the internode is 41.5%, much higher than the node’s 12.5%, while the tangential shrinkage of the internode is 6.2%, compared to 8.1% for the node (Chen et al. 2023). This indicates that the anisotropy of the node is more pronounced than that of the internode, making the node section more prone to cracking. These differences in shrinkage behavior are attributed to variations in structure and chemical content (Wahab et al. 2013).

Structurally, bamboo nodes have distinct shapes and unique features such as diaphragms, sheath scars, and nodal ridges, and they also contain a certain number of transverse and curved vascular bundles (Peng et al. 2014; Palombini et al. 2020; Xiang et al. 2021). In contrast, bamboo culms are primarily composed of longitudinal cells, with vascular bundles that show a parallel distribution from top to bottom and a gradient distribution along the radial direction. The vascular bundles on the outer side of the bamboo are smaller, with a high distribution density and a greater change in water content, resulting in more significant shrinkage.

On the other hand, the inner vascular bundles are larger, with a lower distribution density and less variation in water content (Zhang et al. 2024). The distribution of vascular bundles in bamboo affects its density, with the outer side being denser than the inner side. Since drying shrinkage is generally positively correlated with density, the outer side shrinks more. In bamboo timber, drying shrinkage increases by about 2%, while in the bamboo green section, it rises sharply. Specifically, the air-dry shrinkage rate of bamboo green increases by about 4%, and the full-dry rate increases by around 10% (Zhong 2011). During water loss, this gradient in water content between the inner and outer sides makes bamboo prone to cracking along the tangential direction (Yu 2003).

Yan (2021) found that while the tangential drying behavior of bamboo nodes and culms shares some similarities, the culms exhibited more shrinkage than the nodes. The complex joint structures of bamboo slowed down the tangential drying of the bamboo wall, and it was concluded that the difference in drying behavior between bamboo nodes and culms resulted from the combined effects of wall shrinkage and joint retardation.

The shrinkage in the thickness direction of the bamboo wall is mainly caused by radial shrinkage, and the decrease in diameter is a combined result of radial and tangential shrinkage (Han et al. 2012). Additionally, the microfibril angle and fiber length vary along different heights of bamboo, with less variation in the middle section (Jiang et al. 2000). This uniformity in the middle section leads to more synchronized drying and shrinkage behaviors (Yang et al. 2006). During water loss, water is primarily transported along the axial direction, and drying shrinkage decreases with height, with the bottom nodes experiencing significantly higher shrinkage strain than the middle and top nodes (Yan et al. 2020). At the same height, neighboring segments also show differences in shrinkage.

Bamboo’s dry shrinkage proportions vary significantly by direction, with tangential and radial shrinkage much higher than longitudinal shrinkage (Zhu et al. 2019). Tangential shrinkage ranges from 5.0% to 6.0%, slightly exceeding radial shrinkage, which falls between 4.4% and 5.1%. In comparison, longitudinal shrinkage is notably lower, measuring between 0.6% and 1.1% (Su et al. 2007). Since bamboo’s drying occurs primarily in the early stages, and because of the moisture content gradient within the bamboo, uneven tangential shrinkage between the inner and outer sides of the bamboo wall can result in cracking. Thus, bamboo cracking is closely related to tangential drying shrinkage.

Research on bamboo’s macro-scale drying behavior primarily examines differences between nodes and culms, axial variations at different heights, and tangential-radial variations at the same height. Most studies focus on specific bamboo sections (e.g., bamboo green, bamboo yellow) and static drying behavior, with less emphasis on dynamic changes in shrinkage, stress accumulation, and release during drying. Investigating the dynamic process of bamboo shrinkage strain and stress will provide a more comprehensive understanding of its drying behavior.

Drying and shrinkage strains at the micro level

Bamboo’s drying and shrinking behavior are interconnected at both macro and micro scales, with micro-scale studies essential for understanding the overall mechanism. Bamboo comprises essential tissues and vascular bundles; the basic tissues primarily consist of thin-walled cells, while the vascular bundles contain bamboo fibers (Jin et al. 2019). The basic tissue is primarily composed of thin-walled cells, and the vascular bundles also contain fibers and transport tissues such as ducts and sieve tubes (Xian et al. 1990; Abe and Yano 2010). These cells are bonded through the intercellular layer, sharing the load and stress.

Bamboo’s cell walls are rich in hydrophilic hydroxyl groups, making them susceptible to water absorption and loss, which leads to expansion and contraction. Water loss causes dry shrinkage, and advanced techniques such as nano-infrared spectroscopic imaging now allow for detailed studies of cell wall composition and changes during drying.

While microscopic mechanisms of drying and shrinking are studied, the link between macro and micro scales still lacks definitive conclusions (Nakano 2018; Gao et al. 2021). Research shows that sieve tubes and conduits in bamboo change significantly under different water conditions, with varying saturation points for different cell types affecting water movement. Wei et al. (2017) applied nano-infrared technology to overcome the limitations of traditional infrared spectroscopy, offering higher spatial resolution to study cellulose molecular arrangements (Feng et al. 2020).

Chen et al. (2019) used laser confocal techniques to study the wetting and swelling of bamboo cell walls under different humidity conditions. They found that the wetting and swelling strain of the cell wall was closely related to the cell type. Bamboo fibers exhibited greater absolute wetting and swelling than thin-walled cells. As the moisture content (MC) increases by 1%, the absolute swelling rate of fiber cells is higher than that of parenchyma and vessel cells, in the following order: fiber cells > parenchyma cells > vessel cells. However, in terms of relative swelling rate, the swelling rate of fiber cells (0.53%/%) is lower than that of parenchyma and vessel cells (both 1.1%/%). Zhang et al. (2024) studied changes in vascular bundles at different water contents. They showed that drying in the inner bamboo wall was primarily due to the contraction of thin-walled cells and a few vascular bundles while drying in the outer wall was dominated by a large number of vascular bundles and a small portion of thin-walled cells. The anisotropy of bamboo drying shrinkage was mainly influenced by density and, on the cellular scale, by the number of bamboo fibers surrounding the vascular bundles (Vetter et al. 2015; Yuan et al. 2021). The distribution of bamboo fibers across the vascular bundles resulted in a gradient of dimensional changes in drying shrinkage from the outer surface to the center of the bamboo (Azadeh and Ghavami 2018).

Feng et al. (2020) revealed the spatial distribution of cellulose linked to lignin in thin-walled cells using confocal microscopy Raman spectroscopy imaging. Liu et al. (2024b) studied the dynamic contraction behavior of individual bamboo cells with time-lapse microscopy. They combined it with the long short-term memory (LSTM) model to predict the morphological changes in individual thin-walled cells during contraction. Their findings revealed that fiber cells contracted at a rate of 2.53% per minute, significantly faster than the 0.58% per minute contraction rate observed in thin-walled cells. Consequently, the diameter of fiber cells decreased by about 30%, while thin-walled cells showed a smaller reduction of approximately 7%. Zhang et al. (2023) found that when the moisture content (MC) decreases from 64% to 0%, the shrinkage rate of bamboo fibers is 15%, which is higher than that of other cell types.

As shown in Fig. 3, bamboo microstructure changes when comparing the material before and after drying, with noticeable shrinkage of vascular bundles. Figures 3b–d show minimal change in fiber shape, with the tangential dimension decreasing from 202 μm to 193 μm, while parenchyma cells deform and collapse significantly (Yan et al. 2020). Yuan et al. (2023) using confocal laser scanning microscopy, revealed significant differences in deformation and stress between bamboo fibers and parenchyma cells. The wall thickness of fibers and parenchyma cells decreased by 24.7% and 25.4%, respectively. Contraction stress in fibers caused parenchyma cell areas to increase by 24.2%. Finite element simulations showed that shrinkage stress concentrated in fiber cell walls and intercellular layers, indicating the role of fiber wall shrinkage and intercellular stress in the drying and cracking of raw bamboo.

Fig. 3. Changes in the microstructure of raw bamboo before and after drying (Permission granted by Talor & Francis, Yan et al. 2020)

Bamboo fibers shrink more than thin-walled cells at the cellular scale. Although collapsed cells may recover some of their deformations (Liu et al. 2023c), strength loss is irreversible, and some deformations are complex to restore (Saifouni et al. 2016; Yang et al. 2018). Even in the late stages of drying, bamboo cell walls still contain bound water, exhibiting high plasticity and low energy storage modulus (Zhan et al. 2016). As the water content decreases, the thin-walled tissues continue to shrink and collapse, exacerbating the shrinkage of bamboo fibers, and these strains are hard to reverse (Yan et al. 2020). Future research should focus on the drying and shrinkage mechanisms of bamboo fibers and thin-walled cells in vascular bundles, further exploring the effects of intercellular layer stresses. Dynamic monitoring of the drying process and the establishment of quantitative models at both macro and cellular scales are needed to clarify their interactions and improve predictions and control of bamboo materials’ drying and shrinking behaviors.

SUMMARY AND OUTLOOK

In summary, raw bamboo retains its unique structural advantages and excellent physical and mechanical properties. However, drying deformation and stress during drying significantly affect the outcome, highlighting the need to study the drying and shrinkage mechanisms and stress interactions to improve product quality. Current research has limitations, including variations in physical properties, drying behavior, and stresses across species, processing methods, and bamboo parts, which require further exploration. Most studies rely on traditional methods with limited accuracy, and the focus has largely been on shrinkage below the fiber saturation point, while the effects of wrinkling at higher moisture content remain underexplored.

On the basis of work cited in this paper, five future research directions can be proposed:

(1) Establishment of a Universal Theoretical Model: Research should focus on bamboo in its original state, further studying the drying and shrinkage strain behavior across different bamboo species. Establishing a universal theoretical model will aid in systematically understanding the drying and shrinkage behavior of various bamboo materials, offering a theoretical foundation for a standardized drying process.

(2) Application of Advanced Detection Technology: Advanced technologies such as digital scattering and acoustic emission methods could enable real-time monitoring and analysis of bamboo drying strain and stress. Combined with numerical simulations, these technologies can enhance measurement accuracy and provide comprehensive data for modeling drying strain and stress.

(3) Comparative Study of Bamboo Culm and Node Drying and Shrinking: Future studies should compare bamboo culms and nodes’ drying and shrinking behaviors, as most current research focuses on internodal parts. A detailed analysis of nodes could reveal differences and commonalities in how each part responds to the drying process, guiding the creation of a more precise drying protocol.

(4) Study on the Composition of Drying Stress: In-depth research is needed to determine whether the shrinkage during bamboo drying, particularly above the fiber saturation point (FSP), is equivalent to wood wrinkling. Exploring the components of drying stress in raw bamboo will provide valuable data for optimizing the drying process and minimizing defects.

(5) Multi-Scale Research: Combining macro- and micro-scale studies is crucial for comprehensively understanding bamboo drying. Investigating the drying behavior at the cellular level, including the influence of intercellular stress, and constructing a multi-scale strain model could bridge the gap between cellular and macro-scale drying. This approach may offer new insights into bamboo drying and shrinkage mechanisms.

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Article submitted: October 20, 2024; Peer review completed: November 14, 2024; Revised version received and accepted: November 22, 2024; Published: December 10, 2024.

DOI: 10.15376/biores.20.1.Zhu