Mechanical Properties and Stress Strain Relationship Models for Bamboo Scrimber

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More and more scientists are interested in researching on bamboo scrimber both for the manufacturing technology and mechanical properties.He et al. [15] investigated the mechanical performance of bamboo scrimber made from moso bamboo (Pyllostachys pubescens) treated by heat oil.Naresworo Nugroho et al. [16,17] chosen zephyr strand from moso bamboo (Pyllostachys pubescens Mazel) to made the structural composite board.Huang [18] examined how the accelerated aging method and aging resistant influence the performance of bamboo scrimber.The physical and mechanical properties of bamboo scrimber have been studied by Ahmad et al. [19].Pannipa et al. [20] examined the physical and mechanical properties of bamboo scrimber made from an Asian bamboo (Dendrocalamus asper Backer).Sharma et al. [21] compared the mechanical properties for bamboo scrimber with that for laminated bamboo lumbers.Huang et al. [22] and Zhou et al. [23] also investigated the failure mechanisms for bamboo scrimber.Considering the elevated temperatures, Xu et al. [24] examined the compressive and tensile properties of PBSL.Li et al. [11,25] investigated the eccentric compression properties of bamboo scrimber columns produced both by cold press technology.Zhong et al. [26] studied how the temperature influence on the compressive strength parallel to the grain for bamboo scrimber.Wei et al. [27] investigated the properties for bamboo scrimber beams.Zhang et al. [28] studied the mechanical properties of AFRP bamboo scrimber beams.
The former studies are mainly on the bamboo scrimber made by the cold press method, however hot press technology becomes more and more popular.Considering the bamboo bundle preparation, glue immersion, forming and hot pressing process, the manufacturing technology for bamboo scrimber were studied by Liang Cheng [29].Yu et al. [30] carried out an experimental study on the preparation, physical, mechanical, and interfacial morphological properties of bamboo scrimber.Shangguan et al. [31] investigated the compressive performance of bamboo scrimber and proposed strength models.Li et al. [32] investigated the compression properties of bamboo scrimber manufactured by hot press technology and proposed a stress strain relationship model.As for the stress strain relationship model under compression, Huang et al. [22] also proposed one for bamboo scrimber made by the cold press method.
As discussed above, even though both the manufacturing technology and mechanical properties for bamboo scrimber have been investigated considering many influencing factors, the research on mechanical performance of bamboo scrimber manufactured by hot press technology is limit and good constitutive models for bamboo scrimber are not yet available.A systemic study on the mechanical properties and models is needed.The constitutive model is the fundamental theory for the further mechanical properties' study.Thus, this paper perform a study on the mechanical performance and stress strain relationship for bamboo scrimber manufactured based on a new technique.
2 Materials and Test Methods [32] With the harvest age of 3-4 years, Moso bamboo (Phyllostachys pubescens) were chosen from Yichun City in the Jiangxi province.Natural bamboo culms were cut into tubes (Fig. 1a) with the length of 2000 mm.All tubes were cut into 20 mm wide strips (Fig. 1b), and the outer skin (epidermal) and inner cavity layer (pith peripheral) were removed by a planer.All strips were split into bamboo bundles by passing through a roller press crusher.Then these bamboo strand bundles (Fig. 1c) were dried and charred with the temperature of 165°C and the air pressure of 0.3 MPa.Phenol glue was used to manufacture the bamboo scrimber specimens.All treated bamboo bundles were immersed in glue pool (Fig. 1d) and then dried in the air (Fig. 1e).Finally the bamboo bundles were put into moulds (Fig. 1f) and these were then pressed into bamboo scrimber by the pressing machine (Fig. 1g).A transverse compression of 10.3 MPa was applied for the blocks under the hot pressing temperature with the value of 160°C.Gross bamboo scrimber boards could be seen from Fig. 1h and the cross section of bamboo scrimber were shown in Fig. 1i.
In order to investigate the tensile, compression, bending and shear properties as well as the stress strain relationship model for bamboo scrimber manufactured based on a new technique, five groups of specimens were manufactured.Each group consisted of 36 identical specimens.The sizes for the compression specimens and bending specimens are 50 mm × 50 mm × 150 mm and 50 mm × 50 mm × 760 mm respectively.The sizes and shape for other specimens could be seen from Fig. 2. As for the naming rules for the specimens, "TS" and "TH" stand for the tensile parallel to grain group and tensile perpendicular to grain group respectively."CS", "B660" and "SS" stand for the compression group, bending group and shear group respectively.Four side surfaces of the compression specimens were named as A, B, C, D.
Two strain gauges were pasted on each surface for two wide side surfaces of the tensile specimens and four side surfaces of the compression specimens.The test was performed using a microcomputer-controlled electro-hydraulic servo universal testing machine and a Data Acquisition System.The total loading duration is controlled between 6 and 10 min.The load was applied initially through load control in the elastic stage, and then was changed to displacement control before the proportional limit.The frequencies of all strains were 1 values per second.The test was halted until the specimen had sustained significant damage.The final moisture content was 8.22% and the density was 1254 kg/m 3 for bamboo scrimber with the standard deviation and coefficient of variation of 80.4 kg/m 3 , 0.064 respectively.Fig. 3 shows the test photos.Figure 4 shows the stress strain curves for the tensile specimens.Overall speaking, the stress strain curves are linear with the increasing of loading until the final failure.The tensile specimen split suddenly at the ultimate state.None clear crack or failure phenomenon could be seen during the loading process.The slopes for the curves vary among all tensile specimens and it means that the test results for elastic modulus have a certain discreteness.
According to the failure process for all specimens, two main failure phenomena series could be classified as can be seen from Fig. 5.The fibers around the natural nodes of bamboo are weak points and the specimens were broken firstly around these points.Detailed failure modes information for tensile specimens can be found in Tab. 1.Most of the specimens failed with the mode A. The total number for mode B is 5 which is 13.89% of 36 specimens.The mean values for strength, elastic modulus and Poisson's ratios for the specimens under tensile parallel to grain are 156.2MPa, 15649 MPa and 0.441, with the standard deviation of 25.1 MPa, 1586 MPa, 0.055 and with the coefficient of variation of 0.161, 0.101, and 0.124 respectively.The characteristic values for them are 114.9MPa, 13039 MPa and 0.35 respectively.

Tensile Perpendicular to Grain
Figure 6 plots the stress strain relationship for the specimens.Compared with the specimens under tensile parallel to grain, some of the specimens under tensile perpendicular to grain have nonlinear stage

Compression
Figure 8 plots the strain-stress curves for the test specimens under compression both parallel to grain and perpendicular to grain.Compared the two series of the test curves with each other, each specimen experienced elastic stage and elastic-plastic stage.However, the specimens under compression parallel to

Bending
None clear phenomenon could be seen before 80% of the ultimate load.Small cracks appeared in the middle part of the specimens around the ultimate load.With increase of loading, more and more cracks could be seen until the final failure.Fig. 10 shows the typical failure model for bending specimens.The mean values for bending strength and elastic modulus are 144.3MPa, and 9199 MPa, with the standard deviation of 12.03 MPa and 766 MPa, and with the coefficient of variation of 8.34 and 7.72 respectively.The characteristic values for them are 124.5 MPa and 8658 MPa respectively.

Shear
The load displacement curves could be seen in Fig. 11.The discreteness for both the slope values of the curves and the ultimate load values for the specimens is very big.None clear phenomenon could be seen in  the initial stage.Slight noise could be heard around 85% of the ultimate load, and then small cracks could be seen along the main shear line close to the ultimate load.Finally, the specimen split into two main parts and Fig. 12 shows the typical failure model for shear specimens.The mean value for shear strength is 26.7 MPa, with the standard deviation of 5.58 MPa and the coefficient of variation of 20.93.The characteristic value is 17.5 MPa.

Test Results
Table 1 presents the test results for all bamboo scrimber specimens and other results for the materials obtained from literatures.As can be seen from Tab. 1, with the COV values of 28.64 and 25.72 respectively, f th and E th have the largest discreteness among the values for the strength and elastic modulus for bamboo scrimber.Except E b , all other test values are for bamboo scrimber manufactured based on the new technique are bigger than the test values for bamboo scrimber (Sharma et al.) [21] produced by the traditional method.
The elastic modulus for the bamboo scrimber specimens under tensile parallel to grain is bigger than that for GB and LG but smaller than that for GLG.The elastic modulus for the bamboo scrimber under compression parallel to grain is also smaller than that for GLG but bigger than that for LG.Except the two elastic modulus values mentioned above, all other values for bamboo scrimber specimens are bigger than the corresponding values for other materials listed in Tab. 1.It means that the bamboo scrimber material manufactured based on the new technique mentioned above has very good mechanical properties and could be used in construction area.From the point of CHV values, the relationship among the   Even though four main stages could be summarized for the whole loading process for bamboo scrimber specimens under compression parallel to grain [32], it is not necessary to consider the descent stage from the design point of view.The elastic modulus value E ts for bamboo scrimber (made of Phyllostachys pubescens) under tension parallel to grain is 10.5% bigger than E cs for bamboo scrimber under compression parallel to grain for the mean values.However, the elastic modulus value E cs is 75.9% bigger than E ts for GLG made of Guadua angustifolia Kunth.The values for E cs and E ts are not very close to each other for bamboo scrimber made of by every bamboo species.Based on the discussion above, the four-linear model for stress strain relationship was proposed as shown in Fig. 13a.As can be seen from Fig. 13b, the proposed model gives good agreement with the test data.(2-1) (2-3) where r is the stress value of the bamboo scrimber under compression parallel to grain; E cs is the modulus of elasticity for bamboo scrimber under compression parallel to grain; E ts is the modulus of elasticity for bamboo scrimber under tensile parallel to grain; E p is the secant modulus for peak point (e c0 ,f c0 ) ; e is the strain value of the PBSL; e cy is the strain for the yield point; e c0 is the compression peak strain value; f c0 is the compression peak stress value; e cu is the ultimate maximum compression strain value.

Quadratic Function Model for Stress Strain Relationship
A curve could be chosen for the elastic-plastic stage and the curve could be expressed as the quadratic function [32].As for other two stages, same lines were chosen as the tri-linear model.This model could be called as quadratic function model for stress strain relationship (Fig. 13b) which is finer than the tri-linear model.
The quadratic function model for stress strain relationship could be expressed as follows:

Cubic Function Model for Stress Strain Relationship
The curve for the elastic-plastic stage could also be expressed as the cubic function.Thus the cubic function model for stress strain relationship could be chosen which can be seen from Fig. 13c.
The cubic function model for stress strain relationship for bamboo scrimber can be expressed as follows: All three proposed models could predict the relationship between the stress and the strain.The main difference is that the latter two models are finer than the tri-linear model (Fig. 13d).In addition, if the values for E cs and E ts are close to each other, the smaller value could be chosen as the elastic modulus both for the tension and compression, and all three models proposed above could be simplified.

Conclusions
In order to investigate the basic mechanical properties for bamboo scrimber manufactured based on a new technique, a large quantities of experiments have been carried out.Based on the analysis of the test results, the following conclusions can be drawn.
1. Two main typical failure modes were classified for bamboo scrimber specimens both under tension parallel to grain and tension perpendicular to grain.Brittle failure happened for all tensile tests.The slope values for the elastic stages have bigger discreteness compared with those for the specimens under tensile parallel to grain.The failure modes for bamboo scrimber specimens under compression parallel to grain could be divided into four.Only one main failure mode happened both for the bending specimens and the shear specimens.2. With the COV values of 28.64 and 25.72 respectively, f th and E th have the largest discreteness among the values for the strength and elastic modulus for bamboo scrimber.From the point of CHV values, the relationship among the mechanical parameters for bamboo scrimber were proposed based on the test results.Compared with other green building materials, bamboo scrimber manufactured based on a new technique has better mechanical performance and could be used in construction area.3. Three stress strain relationship models which are four-linear model, quadratic function model, and cubic function model were proposed for bamboo scrimber specimens manufactured based on a new technique.The latter two models gives better prediction for stress strain relationship in elastic plastic stage.

Figure 4 :
Figure 4: Stress strain curves for tensile specimens parallel to grain

Figure 5 :
Figure 5: Typical failure modes for tensile parallel to grain specimens

Figure 6 :
Figure 6: Stress strain curves for tensile specimens perpendicular to grain

Figure 7 :
Figure 7: Typical failure modes for tensile perpendicular to grain specimens

Figure 8 :
Figure 8: Stress train curves for compression specimens

Figure 11 :
Figure 11: Load displacement curves for shear specimens

Figure 12 :
Figure 12: Typical failure mode for shear specimens

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Stress Strain Relationship Model 4.1 Four-Linear Model for Stress Strain Relationship

Table 1 :
Test results for all bamboo scrimber specimens and other materials