Time-temperature-stress equivalence in compressive creep response of Chinese fir at high-temperature range

Construction and Building Materials 235 (2020) 117809

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Time-temperature-stress equivalence in compressive creep response of Chinese fir at high-temperature range Junfeng Wang a,b, Xuan Wang a, Qian He a, Yaoli Zhang a, Tianyi Zhan a,⇑ a b

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, PR China Guangxi Zhuang Autonomous Region Forestry Research Institute, Nanning 530002, PR China

h i g h l i g h t s
TSSP and TTSSP were verified to be applicable to predict viscoelasticity of wood.
The up-threshold temperature when applying TSSP and TTSSP was 180 °C.
The reasons why TSSP and TTSSP failed above 200 °C were explained from chemical and anatomical points of view.

a r t i c l e

i n f o

Article history: Received 3 July 2019 Received in revised form 8 November 2019 Accepted 5 December 2019

Keywords: Creep Time-stress superposition principle (TSSP) Time-temperature-stress superposition principle (TTSSP) Chemical components Anatomical features

a b s t r a c t Predicting long-term viscoelasticity properties of wood is important for designing dimensions for structure-used wood and optimizing thermo-mechanical treatment technique. In this study, compressive creep of Chinese fir wood was tested at a series of temperature (140–220 °C) and stress (0.03–0.15 MPa) conditions. The time-stress superposition principle (TSSP) and time-temperature-stress superposition principle (TTSSP) were applied to predict long-term creep behavior. Changes of chemical components and anatomical features of wood cell wall after creep test were investigated by Raman spectroscopy and scanning electron microscopy. The results showed that quasi-linear change of strain was acquired when temperature at or below 180 °C. TSSP and TTSSP were feasible to predict compressive creep at an up-threshold temperature of 180 °C. When temperature was 200 and 220 °C, obvious non-linear strain was obtained, and TSSP and TTSSP failed to construct master curves. Much significant degradations of carbohydrates was found at 200 °C, which could explain why TSSP failed to construct master curves from a chemical point of view. Crack and rupture on the cell walls and among the cell corners were observed when temperature at or above 200 °C and stress at or above 0.09 MPa. From an anatomical point of view, the destruction of wood cell structures could explain why TSSP failed to construct master curves. The finding in this studies broads the knowledge of the viscoelasticity of wood, and is helpful for further investigations on the thermo-mechanical manufacturing techniques. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Wood has been widely utilized with a historical background, while the utilization is restricted by its dimensional instability and poor mechanical properties. Thermo-mechanical densification is one of the most effective approaches to improve the mechanical properties of wood, especially plantation-grown wood [1–5]. Intrinsic porous and plasticity of wood cell provides the possibility to be compressed and densified under the combination of heat and pressure [4].

⇑ Corresponding author. E-mail address: [email protected] (T. Zhan). https://doi.org/10.1016/j.conbuildmat.2019.117809 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

Densification is a time-dependent behavior, i.e., densification extent increases with prolonged treatment time. Moreover, densification performance is closely related to treatment temperature and pressure [6]. Greater densification can be commonly obtained at higher temperatures and/or pressures. While, it is demonstrated that at higher temperature with long processing time, the wood constituents can be seriously degraded, and the mechanical and fracture strength of treated wood is lowered as a result [7,8]. Among the main chemical components in wood cell wall, the hemicelluloses are most thermally affected. The acetic acid releases in the degradation of hemicelluloses acts as a depolymerization catalyst that further increases polysaccharide decomposition [9], and a part of the paracrystalline cellulose is degraded, and the cross-linking density of the lignin is elevated [10–12].

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However, it is proved that the crystalline index of the cellulose could increase at proper temperature and duration ranges [11]. Selecting appropriate temperature, pressure and duration parameters are important for optimizing thermo-mechanical technique and improve related products’ properties. As a viscoelastic material, deformation of wood increases with loading time. In addition, deformation of wood is positively related to temperature and load level [13], that is to say, increasing temperature, load level and prolonging loading time have similar effects on the increment of deformation. Time-temperature superposition principle (TTSP) is a useful method to predict long-term mechanical property of material according to short-term data at high-temperature range [14,15]. There have been many efforts to apply TTSP to the study on the creep, relaxation and dynamic viscoelasticity of wood and its composites [16–22]. However, the applicability of TTSP to viscoelastic properties of wood is still controversial, because wood has multiple transition regions [23]. Analogous to TTSP, time-stress superposition principle (TSSP) is also applied to predict long-term mechanical behaviors of wood and other polymers [24–28]. TSSP is verified by a stepwise increase of stress level at a constant temperature. When applied stresses are sufficiently small within the linear viscoelastic region, its effect on the materials’ properties is negligible [29]. One of the advantages of TSSP than TTSP is the non-uniform heating of thick polymeric materials with low thermal conductivity is prevented [24]. Time-stress superposition at a series of temperatures can be combined to give the time–temperature-stress superposition principle (TTSSP) [27,29–31]. TTSSP is capable to characterize deformation of wood and predict the long-term creep and relaxation behaviors, while little related information is available. The presented work focused on the validation of TSSP and TTSSP on the deformation of Chinese fir at high-temperature range. Moreover, from the chemical and anatomical point of view, applicabilities of TSSP and TTSSP were explored. The main objective of this study is to broaden the knowledge of the viscoelasticity of wood and to drive conclusions for further investigations on the thermomechanical manufacturing techniques.

2. Materials and methods 2.1. Materials Plantation Chinese fir trees (29-year-old) with straight boles were selected within a stand at the Forestry Station in Zhejiang, China. Samples with a dimension of 10  10  10 mm3 (longitudinal  radial  tangential) were obtained successively from heartwood part. Before testing, all samples were conditioned in sealed containers over P2O5 for 9 weeks. MC and raw density of

the conditioned samples was 0.6% and 0.39 ± 0.02 g/cm3, respectively. 2.2. Creep test The schematic illustration of the whole test is displayed in Fig. 1. Creep test was carried out by a Dynamic Mechanical Analyzer (Q800, TA Instruments, New Castle, DE, USA). After the P2O5 conditioning, the specimens was placed into the device. A compression mode was selected, and the compressive loading was applied in the tangential direction. The top plate imposed the static force of 3, 6, 9, 12 or 15 N, due to the maximum force the device could provide was 18 N. Correspondingly, the compressive stress was 0.03, 0.06, 0.09, 0.12 or 0.15 MPa. Five test temperatures were set as 140, 160, 180, 200 and 220 °C, and the test time was 60 min. Creep strain was determined by the compressive deformation, automatically recorded through the device. MC of the conditioned wood sample was as low as 0.6%, in hence water-loss caused shrinkage of the sample could be negligible. Additionally, thermal expansion in high temperatures was not taken into account. Mass loss of sample was determined by weighing specimens before (m0) and after (mi) the test, and calculated as:

Mass lossð%Þ ¼ ðm0  mi Þ=m0  100

ð1Þ

Six replicates for each condition were performed and the results plotted in the corresponding figures are the average values of the six replicates. 2.3. Characterization After the creep test, the specimens was smoothened in three (cross, radial and tangential) sections. 15-um-thick sample was cut in the cross sections on a sliding microtome (Leica 2010R, Leica Microsystems, Wetzlar, Germany). The sample was then placed on a glass slide with a drop of H2O, covered by a coverslip (0.17 mm thickness), and sealed with nail polish. The 15-um-thick sample for Raman spectroscopy characterization was performed by a LabRam Xplora confocal Raman microscope (Horiba Jobin Yvon, Paris, France) equipped with a confocal microscope (Olympus BX51, Tokyo, Japan). The Raman light was detected by an air-cooled front-illuminated spectroscopic charge-coupled device and the laser power on the sample was approximately 8 mW. The spectrum from each location was obtained by averaging 2 s cycles. Within the chosen areas (secondary cell wall and cell corner of tracheid), the filter calculated the intensities, and the background was subtracted by taking the baseline from the first to the second border. The spectra were baseline-corrected using the Savitsky-Golay algorithm method. Typical chemical changes after compressive

Fig. 1. Schematic illustration of the creep test and characterization.

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Fig. 2. Compressive creep strain as a function of time (left side) and master curve (right side) at different temperatures.

creep tests at 160, 180 and 200 °C are displayed and were discussed in Section 3.2. After sectioning for Raman test, the remaining creep-tested specimens was coated with gold sputtering for about 20 s.

Anatomical features of the specimens were characterized using a scanning electron microscopy (SEM, Hitachi S-3400, Hitachi Ltd., Tokyo, Japan). Anatomical features in different sections were observed using an acceleration voltage of 5 kV. Typical features

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after compressive creep tests at 180 and 200 °C are displayed and were discussed in Section 3.3. 3. Results and discussion 3.1. Compressive creep Strain e increased as a function of compressive time, in the left side of Fig. 2, irrespective of tested temperature and stress. At a given temperature, more e can be found at greater stresses. Comparing to applied stress at 0.03 MPa, the final e (i.e., e with a duration of 60 min, e60 min) increased about 37% at 0.15 MPa when the temperature was 180 °C. In addition, the higher the temperature, the more the e at any given time. e60 min was 1.94, 2.34, 2.63, 2.68 and 2.73%, respectively at 140, 160, 180, 200 and 220 °C when stress was 0.09 MPa. When temperature below or at 180 °C, quasilinear change of e in a logarithmic timescale could be found. However, at temperatures of 200 and 220 °C changing rate of e increased with compressive time, which manifested the onset of non-linear creep behavior [29]. With increasing temperature, chemical constituents obtained more heating energies. When obtaining heating energies, wood and other polymers exhibited as decrement of stiffness and increment of viscosity [32–34]. It was obviously found that the increasing temperature and stress had similar effects on the e increment of Chinese fir during creep test. TTSP is a widely accepted concept, which connects time- and temperature-scale rheological properties of wood and other polymers [16,35]. Various model have been proposed to shift the time- and temperature –scale properties. The most commonly used is the Williams-Landel-Ferry (WLF) model [36]. According to WLF model, the temperature (T) dependence of e can be expressed as [29,37]: 

eðt; TÞ ¼ eðt þ logUT ; T 0 Þ ¼ eðf; T 0 Þ logUT ¼

ð2Þ

C 1 ðT  T 0 Þ C 2 þ ðT  T 0 Þ

ð3Þ

Similar to TTSP, TSSP was also proposed to explore the intrinsic viscoelastic behavior and predict the life-time of viscoelastic materials and structures [38]. It was rationalized to display TSSP based on TTSP. Hence, the stress (r) dependence of e and the shift factor can be rewritten as: 

eðt; rÞ ¼ eðt þ logUr ; r0 Þ ¼ eðn; r0 Þ logUr ¼

ð4Þ

C 01 ðr  r0 Þ C 02 þ ðr  r0 Þ

ð5Þ

where t is the creep time, log UT and log Ur refer to temperature and stress shift factors, f (f = t + log UT) and n (n = t + log Ur) are 



the corresponding prolonged times, eðf; T 0 Þ and eðn; r0 Þ are master curves at reference temperature (T0) and reference stress (r0), and C1, C2, C 01 and C 02 are material constants [36]. In this study, Eqs. (4) and (5) were used to construct TSSP master curves at a reference stress and different temperatures, and Eqs. (2) and (3) were used to further study the feasibility of TTSSP based on TSSP master curves When r0 selected as 0.03 MPa, master curves were constructed from 140 to 220 °C, shown in the right side of Fig. 2. Smooth master curves could be developed at temperatures of 140, 160 and 180 °C, while master curves at 200 and 220 °C were failed to construct. This non-smooth viscoelastic behavior may be explained by the non-linear creep behavior of Chinese fir. At high-temperature range, transverse compression strength and plastic deformability of wood reduced [39,40], attributed to the degradation of chemical

Fig. 3. Mass loss after creep test at different temperatures.

components in wood cell wall [41,42]. In Fig. 3, the mass loss of specimens after creep test is exhibited. The mass loss data at each temperature was calculated as the average value from different stress conditions. According to Fig. 3, pronounced mass loss could be found when temperature at or above 200 °C. Moreover, combination treatment of stress and high-temperature resulted in crack and even destruction of wood cell structures [43,44], which destroyed the integrity of anatomical structure of wood. Detailed investigations of chemical compositions and anatomical features are displayed and were discussed in Sections 3.2 and 3.3. Master curves at 140, 160 and 180 °C indicated an accelerated creep characterization of approximately 2 centenaries beyond the test duration. In other word, to predict the compressive behavior in a 10-year duration at high-temperature (140, 160 and 180 °C) and 0.03 MPa, one needs only to perform creep tests at stresses up to 0.15 MPa with 3-min duration. Fitted with Eq. (5), the corresponding values of log Ur can be calculated as shown in Fig. 4. The experimental value of log Ur as a function of stress was highly linear, at all the three temperatures (140, 160 and 180 °C). The similar linear behaviors has also been reported by some previous studies [17–19,45], and attributed to intermolecular cooperativity within polymeric materials in the wood cell walls. According to Figs. 2 and 4, TSSP was feasible to construct master curves of compressive creep response of Chinese fir up to 180 °C. Furthermore, to verify the TTSSP on creep behavior, a secondary master curve (T0 = 140 °C, r0 = 0.03 MPa) was developed by Eq. (2). In Fig. 5, the secondary master curve is displayed. Additionally, log UT calculated by Eq. (3) is shown as the inset in Fig. 5. Based on the results in Fig. 5, we concluded that TTSSP was feasible to predict compressive creep of wood at high-temperature range up to 180 °C. The verifications of TSSP and TTSSP on rheological properties of wood were helpful to optimize the manufacturing involved in the thermo-mechanical treatment of wood. 3.2. Raman spectroscopic interpretation To explain why TSSP failed to predict creep behavior when temperature exceeded 180 °C, the chemical changes after compressive creep tests were investigated. Raman spectra at and nearby the upthreshold temperature for applying TSSP (160, 180 and 200 °C) with a 0.09 MPa stress were compared with that of the original sample. Fig. 6 shows the average Raman spectra extracted from secondary wall and cell corner in the cross-section of creeptested wood. The cellulose, lignin and hemicelluloses behaved

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Fig. 4. Variation of shift factor log Ur with stress difference for r0 = 0.03 MPa.

Fig. 5. Master curve of strain constructed by the temperature- time-strain equivalence principle. Inset shows variation of shift factor log UT at different temperatures (T0 = 140 °C, r0 = 0.03 MPa).

Fig. 6. Average spectra acquired from secondary wall (a) and cell corner (b) of original and creep-tested wood.

different Raman bands. On the basis of previous studies [46–48], band assignments for different chemical components have been elucidated. The typical evident peaks at 1603 and 1656 cm1 were attributed to aromatic ring symmetric stretching vibration. The peak at 2889 cm1 was assigned to C–H stretching of carbohydrates. In addition, the bands at 1095 and 1123 cm1 were attributed to the asymmetric and symmetric stretch of the C-O-C linkages, respectively. The average Raman spectra recorded from different positions (secondary wall and cell corner) had almost the same peak position but different intensities, regardless of creep temperature. In particular, the intensity of carbohydrates at 2889 cm1 was more pronounced in the spectra of secondary wall

(Fig. 6a) than in that of cell corner (Fig. 6b), while the intensity of lignin peaks at 1603 and 1656 cm1 showed an opposite trend. This is consistent with the structure of the wood cell that has been previously reported [49,50]. For further visualizing the distribution of chemical components, Raman mapping technique was used. The distributions of lignin and carbohydrates and their overlay images are exhibited in Fig. 7. Specifically, the spatial distributions of carbohydrates, cellulose and lignin was visualized based on the peak at 2889, 1163 and 1603 cm1, respectively. Slight decrements of the intensities at 1095, 1123 and 2889 cm1 could be found in both secondary wall and cell corner regions when temperature was 160 or 180 °C, while

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Fig. 7. Raman images of original and creep-tested wood that were calculated by integrating from 2768 to 2920 cm1 (e-h, carbohydrates), from 1103 to 1220 cm1 (i-l, cellulose), from 1543 to 1660 cm cm1 (m–p, lignin), and overlay images (a–d).

Fig. 8. Anatomical features of Chinese fir after compressive creep tests at 180 (a, b and c) and 200 °C (d, e and f) (applied stress = 0.09 MPa). (a, d) cross sections. (b, e) radial sections. (c, f) tangential sections. Scale bars = 20 lm (a–f).

J. Wang et al. / Construction and Building Materials 235 (2020) 117809

more pronounced decrements were observed at 200 °C (Fig. 7h). The degradation of carbohydrates occurred in the amorphous region through b-(1, 4)-bonds breaking between the saccharide units of cellulose and hemicelluloses [51]. In addition, the intensities of the lignin signals (1603 cm1) of the creep-tested woods were increased in the cell corner regions, and reduced in the secondary wall regions (Figs. 6 and 7m–p), indicating that the relative concentration of lignin was higher in the cell corner regions. This variation was associated with that the different lignin units in different regions. In the secondary wall, lignin was more of a linear type with less branching [52]. According to Raman spectra results, we assumed that the server degradation of chemical components, especially carbohydrates, at 200 °C could explain why TSSP failed to construct master curves. 3.3. Anatomical features To explain why TSSP failed to predict creep behavior when temperature exceeded 180 °C, the typical anatomical variations after creep tests were studied. Features of latewood tracheid cells at 180 (the up-threshold temperature for applying TSSP) and 200 °C with a 0.09 MPa stress are exhibited in Fig. 8. The reason for exhibiting latewood was that latewood cell had thicker cell wall and larger stiffness, and was more representative than earlywood cell to elucidate the combination influence of heat and pressure. Three sections (cross, radial and tangential) of latewood were shown in Fig. 8a–f. When creep temperature was 180 °C, basically no crack or rupture was found on the cell walls or among cell corners (Fig. 8a–c). However, crack among cell corners (Fig. 8d) and rupture on the tracheid cell walls (Fig. 8e and f) were found at 200 °C. Similar results were also observed when stress exceeded 0.09 MPa or temperature at 220 °C. According to SEM experiments, we concluded that the destruction of wood cell structures was another reason for explaining why TSSP failed to construct master curves at or above 200 °C. 4. Conclusions (1) Quasi-linear change of strain could be observed when temperature at or below 180 °C regardless of stress level, while obvious non-linear creep behavior was obtained at 200 and 220 °C. TSSP and TTSSP were feasible to predict compressive creep of Chinese fir wood at high-temperature range. The up-threshold temperature was 180 °C when applying TSSP and TTSSP. (2) Slight degradations of carbohydrates occurred when temperature was below 180 °C, and much significant degradations of carbohydrates were found at 200 °C. The relative concentration of lignin was higher in the cell corner region than in the secondary wall, regardless of compressive temperature. From a chemical point of view, server degradation of chemical components, especially carbohydrates, at 200 °C could explain why TSSP failed to construct master curves. (3) Combination influence of heat and stress would induced crack and rupture on the cell walls and among cell corners when temperature at or above 200 °C and stress at or above 0.09 MPa. From an anatomical point of view, the destruction of wood cell structures could explain why TSSP failed to construct master curves.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgement This work was supported by the National Key Research and Development Program of China (2017YFD0600202), the National Natural Science Foundation of China (No. 31700487), the Natural Science Foundation of Jiangsu Province (CN) (No. BK20170926), the Science and Technology Major Project of Guangxi Zhuang Autonomous Region (GUIKE AA17204087-13), the Forestry Science and Technology Project of Guangxi Zhuang Autonomous Region (GUILINKEZI 2016-19), the Fundamental Research Funds of Guangxi Forestry Research Institute (LINKE 201819), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Advanced analysis and testing center of Nanjing Forestry University. References [1] P. Bekhta, E.-A. Salca, Influence of veneer densification on the shear strength and temperature behavior inside the plywood during hot press, Constr. Build. Mater. 162 (2018) 20–26. [2] I. El-Houjeyri, V.D. Thi, M. Oudjene, M. Khelifa, Y. Rogaume, A. Sotayo, Z. 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