MECHANICAL PROPERTIES OF SWITCHGRASS AND MISCANTHUS B. Liu, A. B. Koc
ABSTRACT. The mechanical properties of energy crops in the longitudinal and transverse directions are necessary for modeling and simulation of biomass stems. Modeling of biomass stems would help in analyzing the interactions between processing equipment and biomass material before building physical systems. While some of the mechanical properties of switchgrass and miscanthus stems are available in the literature, these properties are not complete for modeling and simulation of these materials. Therefore, the objective of this research was to determine the mechanical properties of switchgrass and miscanthus stems by using compressive, tensile, and shearing tests in the longitudinal and transverse directions. Tensile, compressive, and shear strengths and modulus of elasticity of switchgrass and miscanthus tended to decrease with decreasing stem diameter in both the longitudinal and transverse directions. Tensile and compressive strengths of the first internode of switchgrass were 178.0 and 27.3 MPa in the longitudinal direction and 0.7 and 4.1 MPa in the transverse direction. Shear strength for the first internode of switchgrass was 2.2 and 21.1 MPa in the longitudinal and transverse directions. Tensile and compressive strengths of the first internode of miscanthus were 373.1 and 56.9 MPa in the longitudinal direction and 1.8 and 6.3 MPa in the transverse direction. Shear strength for the first internode of miscanthus was 94.4 and 8.7 MPa in the transverse and longitudinal directions. The experimental data collected in this research would be useful for the development of simulation models for investigating the interactions between shearing tools and energy crops and in designing harvest and particle reduction equipment. Further research would be useful for determining the effects of moisture content, growth conditions, and maturity stage on the mechanical properties of these crops. Keywords. Biofuel, Compressive strength, Miscanthus, Shear strength, Switchgrass, Tensile strength.
T
he renewable energy share of total energy consumed in the U.S. is projected to increase from 9% in 2012 to 12% in 2040, and approximately 2% of this renewable energy is expected to come from biomass sources (DOE, 2014). Switchgrass (Panicum virgatum L.) and miscanthus (Miscanthus × giganteus) are two of the warm-season energy crops that are considered as biofuel feedstocks (DOE, 2006). Switchgrass and miscanthus have similar cellular structures and C4 photosynthesis mechanisms, requiring comparable amounts of inputs, and the production of these crops would require similar equipment and machinery (Dolginow and Massey, 2013; Lee et al., 2014). The low energy and low bulk density of energy crops like switchgrass and miscanthus require efficient harvesting and processing equipment to overcome the major challenges of harvesting, transport, and storage of biomass feedstocks. The mechanical properties of biomass materials must be known for the design and development of optimal harvest and size reduction equipment for low energy and time inputs
Submitted for review in May 2016 as manuscript number ES 11925; approved for publication by the Energy Systems Community of ASABE in February 2017. The authors are Bo Liu, ASABE Member, Assistant Professor, Department of BioResource and Agricultural Engineering, California Polytechnic State University, San Luis Obispo, California; A. Bulent Koc, ASABE Member, Assistant Professor, Department of Agricultural Sciences, Clemson University, Clemson, South Carolina. Corresponding author: A. Bulent Koc, 252 McAdams Hall, Clemson University, Clemson, SC 29634; phone: 864-656-0496; e-mail:
[email protected].
(Sharma et al., 2011; Yu et al., 2006). Unlike most metals, plastics, and other synthetic materials, biomass materials have anisotropic structures, and their mechanical properties are different in different directions. Therefore, the mechanical properties of these materials must be determined in longitudinal (along the fibers) and transverse (across the fibers) directions. In addition, plant variety, growth and environmental conditions, harvest time, maturity stage, and moisture content also affect biomass properties. Several researchers have conducted experiments to determine the mechanical properties and shearing characteristics of biomass materials including switchgrass and miscanthus (Johnson et al., 2012; Kaack and Schwarz, 2001; Liu et al., 2012; Sharma et al., 2011; Yu et al., 2006). For example, Yu et al. (2006) measured ultimate tensile and shear stress at the second internodes of Alamo and Kanlow varieties of switchgrass. They recommended that size reduction equipment that reduces particle size by shear failure would be more energy efficient than using tensile failure. Recommendations for similar size reduction equipment were made by Yu et al. (2014) for big bluestem, corn stalk, intermediate wheatgrass, and switchgrass. Switchgrass and miscanthus can be harvested in the fall or spring seasons. Harvesting in spring, after cold temperatures, affects the mechanical properties of energy crops (Sharma et al., 2011). Sharma et al. (2011) reported that the mean tensile strength was significantly greater before frost than after frost (p < 0.0001), and the average tensile strength was approximately three times the shear strength. In addi-
Transactions of the ASABE Vol. 60(3): 581-590
© 2017 American Society of Agricultural and Biological Engineers ISSN 2151-0032 https://doi.org/10.13031/trans.11925
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tion, internode 2 had significantly higher shear strength values than internode 3 (Sharma et al., 2011). Closely packed stem cells and more mature fibers near the soil were the reasons for internode 2 being more difficult to bend, shear, and break than internode 3 (Sharma et al., 2011). Blade type, blade oblique angle, and direction of cutting also affect the cutting force and energy of biomass materials. Liu et al. (2012) measured the cutting force at the first internode of miscanthus samples and determined the shear, tensile, and bending strengths at internodes 1 through 7. Flat blades compressed the plant stem before cutting, but serrated blades penetrated the plant stem and initiated separation during the initial compression (Liu et al., 2012). Because of this penetration, the serrated blades consumed less cutting energy and required less cutting force than the flat blades (Liu et al., 2012). Johnson et al. (2012) studied the effects of blade oblique angle and cutting speed on cutting energy of miscanthus stems. A 60° oblique cut needed the least average specific energy of 741.9 J m-1 at an average cutting speed of 12.9 m s-1 compared to a 30° oblique angle and straight cuts (Johnson et al., 2012). Igathinathane et al. (2010) investigated the effects of corn stalk orientation on mechanical cutting. The specific energy needed for dry corn stalk internodes ranged from 11.3 to 23.5 kN m-1 (Igathinathane et al., 2010). Cutting corn stalks in the longitudinal direction reduced the specific energy significantly compared to the transverse direction (Igathinathane et al., 2010). The harvesting and processing equipment used for biomass is mostly adapted from the agriculture, food, feed, and pharmaceutical industries (Tumuluru et al., 2011). Machines developed for harvesting and processing conventional crops may not be suitable or efficient for energy crops such as switchgrass and miscanthus. Because the stem length, diameter, thickness, and hardness of energy crops are different from conventional crops, there is a need for comprehensive mathematical models to describe biomass particle size reduction operations (Igathinathane et al., 2010; Yu et al., 2006). Modeling and simulations require the mechanical properties of energy crops. For example, the finite element method models a structure by using a mesh of elements that are connected with nodes. The elements can have simple or complex material properties to characterize the behavior of the structure under investigation (Rieben et al., 2005). Modeling of biomass stems would help in determining the interactions between processing equipment and biomass materials, which would allow studying the behavior of the processing system before building it. While modeling of biomass materials is a difficult task because of the inhomogeneous and anisotropic structures of biomass materials, modeling would allow researchers to implement various designs before manufacturing physical components. Some experimental data on the morphological and mechanical properties of miscanthus (Johnson et al., 2012; Kaack and Schwarz, 2001; Liu et al., 2012) and the tensile strength and shear strength of switchgrass (Sharma et al., 2011; Yu et al., 2006) are available in the literature. However, a complete set of the anisotropic mechanical properties of miscanthus and switchgrass that could be used for the development of three-dimensional
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finite element models is not available in the literature. Therefore, the objective of this research was to determine the mechanical properties of switchgrass and miscanthus stems in longitudinal and transverse directions using compressive, tensile, and shearing tests.
MATERIALS AND METHODS Switchgrass (Panicum virgatum L.) and miscanthus (Miscanthus × giganteus) samples were obtained from the University of Missouri Bradford Research and Education Center (Columbia, Mo.). The geographic coordinates of the field plots where the samples were collected are 38° 53′ 55.66″ N and 92° 12′ 31.18″ W. Cave-in-Rock switchgrass and Miscanthus × giganteus samples were collected from a fiveyear-old planting by manually cutting the plants close to the ground. The sample stems were brought to the laboratory and stored at 20°C before the experiments. Tensile, compressive, and shearing tests for the determination of mechanical properties were conducted using a universal testing system (model TA-HDi, Texture Technologies Corp., Scarsdale, N.Y.). Similar methodology was used by other researchers for determining the mechanical properties of various crops (Liu et al., 2012; Sharma et al., 2011; Yu et al., 2014, 2006). Using special fixtures, the biomass samples were subjected to enough force to cause material failure. Failure was defined as the fracture point of the material at which the biomass stem was separated into two or more pieces. The universal testing system continuously recorded the applied force and displacement data for analysis. The universal testing system has the ability to measure forces up to 7500 N. The accuracy of the load cell used to measure the forces is within ±0.5% of the full scale. The crosshead speed, on which the test fixtures were mounted, ranged from 0.01 to 20 mm s-1. A digital caliper with an accuracy of ±0.01 mm was used to measure the dimensions of the test specimens. All specimens were prepared using an ultrasonic cutter (USW-334, Honda Electronics) to create smooth edges and avoid cracks on specimens. The setup of the experiments is shown in figure 1. The moisture contents of the biomass samples were determined by measuring the mass of the samples before (>30 g) and after drying them in an oven for 24 h at a constant temperature of 103°C, as specified by ASABE Standard S358.2 (ASABE, 2008). Sample stems at all seven internodes for switchgrass and miscanthus were used in the tests at the same strain rate. Nodes and internodes of miscanthus and switchgrass stem samples were numbered 1 through 7 starting from the stem base. TENSILE TESTS For tensile tests, switchgrass and miscanthus specimens were subjected to tensile forces in longitudinal or transverse directions until failure occurred. Test clamps in the universal testing system had to be used carefully to ensure that gripping and holding the biomass materials would not damage the sample ends and affect the measurements. Specialized attachments and grip mechanisms were designed and fabricated for the tensile tests.
TRANSACTIONS OF THE ASABE
Figure 3. Specimen preparation for tensile tests in the longitudinal direction (along the fibers).
Figure 1. Universal testing system used for the measurement of mechanical properties.
For tensile tests in the longitudinal direction, the specimens were cut in a dumbbell shape, and the grips of the testing apparatus held the specimen firmly at the wider ends (fig. 2). The dumbbell shape allowed the stress to concentrate in the test area and then fracture where most of the strain occurred. The test results were neglected if a fracture occurred outside of the narrow area at which the strain was measured. To prevent the biomass specimen from slipping because of the smooth surfaces of the metal grips during the tests, the wider ends of the samples (outside of the test section) were glued and clamped to the sample holder in all of the tests. The edges of the test specimens were also smoothed to avoid off-target concentrated stresses. Another consideration was that the biomass stems have different stem diameters; hence, the curvature might cause the specimens to crack when they were glued and clamped to the metal grips. Therefore, steel bars with different diameters were fabricated and used for various specimens to minimize cracking and sliding. For tensile tests in the transverse direction, the sliding and curvature issues still existed. To avoid these issues, two steel plates were fabricated, and each of these plates had small bumps that fit the curvatures of the specimens. In addition to
Figure 2. Dumbbell-shaped specimen used for tensile tests.
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the clamps, super glue was used to bond the specimen and the metal plates. Figures 3 and 4 show the gripping and attachment tools that were used to conduct the longitudinal and transversal tensile tests. Tensile forces were applied to the specimens longitudinally and transversely to cause specimens to fail. The displacement and force data were recorded on a PC for further analyses. The crosshead speed was set at 1 mm min-1 for all tests. The tensile strength of the samples was computed from the maximum load acting normal to the cross-sectional area where the sample failure occurred. Tensile strength was calculated using equation 1:
δt =
Ft × 10 − 6 At
(1)
where δt is the tensile strength at failure (MPa), Ft is the tensile force at failure (N), and At is the failure area of the sample in the narrow section (m2). The dimensions of the stem in the narrow section were measured using a digital caliper. Young’s modulus or the modulus of elasticity is used to describe the elastic properties of linear objects that are stretched or compressed and is defined as the ratio of the stress to the strain. Strain (ε) was calculated using equation 2, stress (σ) is defined by equation 3, and Young’s modulus (E) was calculated using equation 4: ε=
ΔL L
(2)
Figure 4. Specimen preparation for tensile tests in the transverse displacement direction (across the fibers).
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F A
(3)
σ F L = ε A ΔL
(4)
σ= E=
where L is the sample length (m), ΔL is the change of length (m), F is the force (N), A is the area where the force is applied (m2), and E is Young’s modulus (N m-2). The results for tensile strength and Young’s modulus were obtained from the tests of switchgrass and miscanthus at internodes 1 to 7 (from root to tip). COMPRESSIVE TESTS The apparatus used for compressive strength measurements was the same as the apparatus used for tensile tests. However, compressive forces were applied to the specimens. Under compression force, the length of a specimen is shortened. The material tends to spread in the lateral direction and hence increase in cross-sectional area. Equation 5 was used to determine the compressive strength of the specimens:
δc =
Fc × 10 − 6 Ac
(5)
where δc is the compressive strength at failure (MPa), Fc is the compressive force at failure (N), and Ac is the failure area of a sample (m2). Compressive stress measurements were conducted by applying compressive stress to cause the sample to fail. Physical dimensions, compressive force, and displacement curves for each biomass stem sample were recorded. The crosshead speed was set at 1 mm min-1 for the compressive tests. The experimental setups for compressive tests in the longitudinal and transverse directions are shown in figures 5 and 6, respectively. For compressive tests in the longitudinal direction, two metal blocks were fabricated for each type of bio-
mass. Each block had nine holes in which the biomass stem ends were placed. The diameters of the holes were 2.2 and 2.3 mm through 4.5 mm in steps of 0.3 mm for switchgrass and 4.5 mm through 12.5 mm in steps of 1.0 mm for miscanthus. The holes were 5 mm deep. The stems were placed in the holes in which they best fit, and the upper and lower blocks were aligned. For compressive tests in the transverse direction, the blocks had grooves of different widths to hold the sample piece (fig. 7). To reduce the impact of stem curvature on the tests, the samples were cut into small sizes. SHEARING TESTS Shearing tests are performed to determine the shear strength of a material. The tests measure the maximum shear stress that may be sustained before a material fails. Shearing is typically reported in MPa based on the area of the sheared edge. The shearing tests in this study provided information about the fundamental shear characteristics of the biomass stems. Ultimate shear stress or shear strength was calculated using equation 6:
δs =
Fs × 10 − 6 As
(6)
where δs is the shear stress at failure (MPa), Fs is the shearing force at failure, and As is the failure area of sample. Shear stress measurements were carried out by applying force in the longitudinal and transverse directions to cause the samples to fail. Physical dimensions, shearing force, and displacement curves were determined for each sample. A fixed crosshead speed of 1 mm min-1 was selected for the tests. The experimental setup used for shearing tests in the longitudinal direction is shown in figure 8. Sample holders were designed and fabricated to hold a whole stem sample vertically, and a metal plate was used to shear the stem along its fiber direction. For shearing tests in the transverse direction (fig. 9), only a small piece of stem was used, clamped by four screws, and a metal plate was used to shear the stem across its fibers.
Figure 5. Specimen tightly fitted in a hole of the sample holder for compressive test in the longitudinal direction (along the fibers).
DATA ANALYSIS Linear regressions involving the independent variable x (stem diameter) and dependent variable y (mechanical property) were carried out to fit the experimental data points. The 95% confidence intervals (CI) on the mean response were calculated using equation 7, and the prediction interval (PI) values were calculated using equation 8 (Montgomery and Runger, 2007):
Figure 6. Specimen tightly fitted in a groove on the sample holder for compressive test in the transverse direction (across the fibers).
Figure 7. Specimen under compressive testing in transverse direction.
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12
Miscanthus Switchgrass
Stem Diameter (mm)
10 8 6 4 2 Figure 8. Specimen preparation for shearing test in the longitudinal direction (along the fibers). Shearing direction
1 + n
PI = y ± t (α, df )S yx 1 +
( xi − x ) 2
i −1 ( xi − x )2
1 + n
n
( xi − x ) 2
i −1 ( xi − x )2 n
(7)
(8)
where t is the critical t-statistic, α is the 5% probability of error, df is the degrees of freedom, Syx is the standard error of the estimate, xi is the given value of x (stem diameter), x is the average of the xi (stem diameter) values, and n is the number of data points used in the regression analysis. The confidence intervals and prediction intervals were calculated using Microsoft Excel. The mechanical properties measured in transverse and longitudinal directions were compared using t-test statistics.
RESULTS AND DISCUSSION The average moisture contents of the unconditioned switchgrass and miscanthus stems were 9.8% (w.b.) and 10.2% (w.b.), respectively. These moisture contents were close to the typical moisture contents of switchgrass and miscanthus (10% to 15%) at harvest; at these moisture levels, field drying of crops is not necessary before baling and storage (Mathanker and Hansen, 2014). The switchgrass and miscanthus internodes were numbered from bottom (internode 1) to top (internode 7). The diameters of the switchgrass and miscanthus stems were measured at the center of each internode. The average diameters of internodes 1
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0
1
2
3 4 5 Internode Number
6
7
Figure 10. Average diameters of miscanthus and switchgrass stems measured at each internode. Error bars show ±1 standard deviation.
Figure 9. Specimen preparation for shearing test in the transverse direction (across the fibers).
CI = y ± t (α, df )S yx
0
to 7 for switchgrass stems were 4.0, 3.80, 3.81, 3.5, 3.2, 3.0, and 2.7 mm, respectively. The average diameters of internodes 1 to 7 for miscanthus stem were 10.1, 8.8, 8.1, 7.3, 6.4, 6.0, and 5.2 mm, respectively. The maximum and minimum stem diameters of miscanthus measured in this study were 10.1 and 5.2 mm, respectively, whereas the maximum and minimum diameters of miscanthus internodes reported by Kaack and Schwarz (2001) were 9.2 and 8.8 mm, respectively, for different harvest periods. Figure 10 shows the change in the average diameter of switchgrass and miscanthus stems by internode number. The average diameters of miscanthus stems measured at internodes 1 through 4 were 10.1, 8.8, 8.1, and 7.3 mm, respectively. These values are larger than the average diameters for the same internodes (9.2, 8.6, 8.0, and 7.2 mm) reported by Liu et al. (2012). However, the average diameters of miscanthus measured at internodes 5 through 7 (7.0, 6.8, and 6.6 mm) reported by Liu et al. (2012) were slightly larger than the diameters (6.4, 6.0, and 5.2 mm) measured in this study. The miscanthus samples used in this study were harvested in November, whereas the miscanthus samples used by Liu et al. (2012) were harvested after overwintering in February. The maximum and minimum diameters of switchgrass stems (5.207 and 2.426 mm) reported by Yu et al. (2006) were larger than the maximum and minimum diameters (4.29 and 2.09 mm) measured in this study. These variations in stem diameter could be due to differences in plant species, harvest time, maturity level, moisture content, and growing conditions.
SWITCHGRASS STEM PROPERTIES The tensile, compressive, and shear strengths of switchgrass and miscanthus stems measured in the longitudinal and transverse directions are shown in figure 11. The average tensile strength decreased with decreasing internode diameter (fig. 11a). The average tensile strength measured at internode 1 was more than twice (2.17 times) the value at internode 7 in the longitudinal direction. A similar result between the tensile strengths at internodes 1 and 7 was observed in the transverse direction (fig. 11b). However, this difference (1.75 times) was not as large as that measured in the longitudinal direction. The average tensile strength of
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Figure 11. Average tensile, compressive, and shear strengths of switchgrass and miscanthus stems in the longitudinal and transverse directions.
switchgrass measured in the transverse direction was 0.46% of the value in the longitudinal direction. The overall mean tensile strength in the longitudinal direction was 133.3 MPa for Cave-in-Rock switchgrass, as compared to 97.8 MPa for Alamo switchgrass and 89.7 MPa for Kanlow switchgrass, as reported by Yu et al. (2006). The compressive strength in the longitudinal and transverse directions ranged from 20.4 to 32.3 MPa and from 1.4 to 4.1 MPa, respectively (figs. 11c and 11d). In the longitudinal direction, the compressive strength values measured at internodes 2 and 3 were larger than at internode 1. This
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might be due to structural variations and deformations taking place earlier at internode 1, which was closer to the soil. Similarly, the compressive strength values measured at internode 7 were larger than at internodes 4, 5, and 6 in the longitudinal direction. The average compressive strength decreased with increasing internode number in the transverse direction except for internode 6, which was larger than internode 5. The measured shear strength of switchgrass was much smaller in the longitudinal direction than in the transverse direction (figs. 11e and 11f). This was expected, as shearing
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Figure 12. Modulus of elasticity (Young’s modulus) of switchgrass and miscanthus in (a) longitudinal and (b) transverse directions.
in the longitudinal direction occurs parallel to the fibers, whereas shearing in the transverse direction occurs in the cross-fiber direction. For both shearing directions, shear strength followed a decreasing trend from internode 1 to internode 7. Shear strength in the transverse direction decreased almost linearly with increasing internode number. The mean shear strength for Cave-in-Rock switchgrass stems at 9.8% moisture content was measured as 16.3 MPa. Yu et al. (2006) reported mean shear strength values of 20.5 MPa for Alamo switchgrass and 17.9 MPa for Kanlow switchgrass. When the tensile strength and shear strength values were compared, the shear strength was significantly lower than the tensile strength in the longitudinal direction (p < 0.0001); however, the shear strength was significantly higher than the tensile strength in the transverse direction (p < 0.0001). The average modulus of elasticity decreased from 13362.2 to 7120.8 MPa in the longitudinal direction as internode number increased, except for internode 3, which was larger than internode 2 (fig. 12). The modulus of elasticity decreased from 63.7 to 41.2 MPa with increasing internode number in the transverse direction. The modulus of elasticity values were significantly lower in the transverse direction than in the longitudinal direction (p < 0.00001). MISCANTHUS STEM PROPERTIES The tensile, compressive, and shear strengths of miscanthus stems measured in the longitudinal and transverse directions are shown in figure 11. The tensile strength of miscanthus in the longitudinal direction ranged from 198.9 to 373.1 MPa (fig. 11a). The tensile strength in the transverse direction (fig. 11b) was approximately 0.47% of the value in the longitudinal direction. A similar ratio (0.4%) of tensile strength values in the transverse and longitudinal directions was reported by Liu et al. (2012). In their study, the tensile strength of the stem cortex in the longitudinal direction increased for the first four internodes and then decreased for the fifth and sixth internodes (Liu et al., 2012). However, the tensile strengths of miscanthus stems in this study decreased with increasing internode number. The variations in tensile strength values between the two studies might be due to the harvest time and
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growing conditions of the miscanthus samples. The compressive strength of miscanthus measured in both directions decreased with increasing internode number, as shown in figures 11c and 11d. The shear strength of miscanthus in both directions followed a decreasing trend with increasing internode number. The shear strength values decreased for internodes 1 through 4, increased for internode 5, and then decreased for internodes 6 and 7 in the longitudinal direction (fig. 11e). In the transverse direction, the shear strength was slightly larger at internode 2 than at internode 1 and then decreased almost linearly with increasing internode number (fig. 11f). The highest shear strength values were measured at internodes 1 and 2 in the transverse direction as 94.4 and 97.0 MPa, respectively. The average shear strength of miscanthus stems in the longitudinal direction was about 8.0% of the shear strength measured in the transverse direction. The shear strength of miscanthus stems ranged from 53.9 to 97.0 MPa in this study, while the shear strength of miscanthus stems reported by Liu et al. (2012) ranged from 12 to 42 MPa in the transverse direction. In both studies, the shear strength was higher at internode 2 than at internode 1, and it decreased with increasing internode number in the transverse direction. The modulus of elasticity (Young’s modulus) of miscanthus is shown in figure 12. The modulus of elasticity of miscanthus stems decreased from 12117.4 MPa for internode 1 to 2683.8 MPa for internode 7 in the longitudinal direction. The modulus of elasticity of miscanthus stems in the transverse direction was 8.8% of the value measured in the longitudinal direction. However, the modulus of elasticity values reported by Liu et al. (2012) increased with increasing internode number. This difference might be due to the absence of pith in miscanthus internodes, as reported by Liu et al. (2012). The average modulus of elasticity measured in this study was 6528.9 MPa, which is greater than the average modulus of elasticity of miscanthus (4500 MPa) reported by Kaack and Schwarz (2001). LINEAR REGRESSIONS BETWEEN MATERIAL PROPERTY AND STEM DIAMETER Linear regressions between tensile strength and stem di-
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ameter and between modulus of elasticity and stem diameter for switchgrass and miscanthus are shown in figures 13 through 16. The confidence interval is the space between the two inner curves (dashed lines), and the 95% prediction interval is shown by the outer dotted lines around the regression line. Figures 13 and 14 show the tensile strength and Young’s modulus of switchgrass samples with 9.8% moisture content in the longitudinal and transverse directions, respectively. Figures 15 and 16 show the tensile strength and Young’s modulus of miscanthus samples with 10.2% moisture content in the longitudinal and transverse directions, respectively. The results for each mechanical property and the linear regression results for switchgrass and miscanthus stems are summarized in table 1. The tensile tests on switchgrass and miscanthus stems showed that the tensile strength and Young’s modulus had much higher values in the longitudinal direction than in the transverse direction. One of the possible reasons is that the fibers grow in the longitudinal direction, which makes them more resistant to failure under tensile stress. The mechanical properties of both switchgrass and miscanthus had higher values at internodes with larger stem diameters than at internodes with smaller diameters. This y = 70.952x − 101.66 R² = 0.6053
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might be because stems with larger diameters have more mature fibers and packed cells. The experimental results indicated that the mechanical properties of biomass materials could not be measured as accurately as the properties of more uniform, synthetic materials, such as metals and plastics. In addition to the structural differences, the moisture content, maturity level, time of harvest, plant variety, and geographic conditions where the plants are grown could contribute to the variations in mechanical properties of biomass plants. Scatter plots of the mechanical properties versus stem diameter showed the trends between stem diameter and material properties. Material properties increased proportionally with stem diameter. Linear regression analysis of miscanthus and switchgrass mechanical properties showed relatively low coefficient of determination (R2) values for the mechanical property and stem diameter correlations. The R2 values between switchgrass tensile strength and stem diameter and between Young’s modulus and stem diameter were 0.60 and 0.54, respectively. This indicates that 60% of the change in tensile strength and 54% of the change in Young’s modulus can be explained by stem diameter. Other variables or factors would be needed to explain the rest of the changes in tensile strength and Young’s modulus. The R2 values were relatively low. While the diameters of the plant stems were
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Figure 13. (a) Tensile strength and (b) Young’s modulus of switchgrass (9.8% moisture content) in the longitudinal direction. The confidence interval is the space between the two inner curves (dashed lines), and the 95% prediction interval is shown by the outer dotted lines. 0.9
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Figure 14. (a) Tensile strength and (b) Young’s modulus of switchgrass (9.8% moisture content) in the transverse direction. The confidence interval is the space between the two inner curves (dashed lines), and the 95% prediction interval is shown by the outer dotted lines.
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Figure 15. (a) Tensile strength and (b) Young’s modulus of miscanthus (10.2% moisture content) in the longitudinal direction. The confidence interval is the space between the two inner curves (dashed lines), and the 95% prediction interval is shown by the outer dotted lines. 4.0
50
Tensile Strength (MPa)
3.5 3.0
Young;'s Modulus (MPa)
y = 0.2406x − 0.5262 R² = 0.512
2.5 2.0 1.5 1.0 0.5 0.0
y = 2.3817x − 3.6572 R² = 0.3166
40 30 20 10 0
-0.5 -10
-1.0 3.0
5.0
7.0
9.0
11.0
Stem Diameter (mm) (a)
3.0
5.0
7.0
9.0
11.0
Stem Diameter (mm) (b)
Figure 16. (a) Tensile strength and (b) Young’s modulus of miscanthus (10.2% moisture content) in the transverse direction. The confidence interval is the space between the two inner curves (dashed lines), and the 95% prediction interval is shown by the outer dotted lines.
similar, the cell structure and fiber maturity might be different among the internodes. In addition, the inhomogeneous and anisotropic structures of plant stems as well as the crop variety, moisture content, maturity stage, and growth conditions are among the factors that affect the material properties, causing relatively lower R2 values. Modeling of biomass materials would be useful for designing new machinery and optimizing the efficiency of current machinery. The materials that are considered in designing machinery tend to have homogeneous and isotropic characteristics. The properties of isotropic materials are not direction dependent. Biomass materials, on the other hand, have inhomogeneous structures and anisotropic characteristics. Once determined, the anisotropic mechanical properties of biomass materials in the longitudinal and transverse directions can be used for numerical modeling of biomass crops.
CONCLUSIONS The effects of stem diameter and internode number on the tensile, compressive, and shear strengths and modulus of
60(3): 581-590
elasticity of switchgrass and miscanthus were determined in the longitudinal and transverse directions using a universal testing machine. Special gripping or sample-holding mechanisms that account for the curvature of the stems must be developed for such tests. Bonding the sample ends to the sample holder with an adhesive might also be necessary to prevent the samples from slipping during the tensile tests. The mechanical properties of miscanthus showed a decreasing trend with increasing internode number from the stem base in both the longitudinal and transverse directions. However, the mechanical properties of switchgrass varied with increasing internode number and did not follow a clear trend in either direction. Failures in stems caused by tensile stress in the longitudinal direction required the highest force compared to the other failure tests for both switchgrass and miscanthus. The mechanical properties reported in this research are for switchgrass and miscanthus stems at moisture contents that are desirable for harvest. Further research might be necessary to determine the effects of moisture content, growth conditions, and maturity stage on the mechanical properties of switchgrass and miscanthus crops. The me-
589
Table 1. Linear regression analysis of mechanical properties stem diameter (x) of switchgrass and miscanthus. Mechanical Property Linear Regression Switchgrass (9.8% MC) Longitudinal Tensile strength y = 70.95x − 101.66 Young’s modulus y = 4853.1x − 6321.1 Compressive strength y = 7.8683x − 0.1709 Shear strength y = 0.5874x − 0.281 Shear modulus y = 17.237x − 5.292 Transverse Tensile strength y = 0.1807x − 0.035 Young’s modulus y = 16.143x − 0.314 Compressive strength y = 1.5063x − 2.1722 Shear strength y = 6.6849x − 5.6311 Shear modulus y = 245.98x − 160.88 Miscanthus (10.2% MC) Longitudinal Tensile strength y = 38.822x − 25.191 Young’s modulus y = 1702.5x − 6186 Compressive strength y = 5.36x + 0.08 Shear strength y = 0.65x + 1.35 Shear modulus y = 7.9782x + 13.102 Transverse Tensile strength y = 0.2406x − 0.5262 Young’s modulus y = 2.3817x − 3.6572 Compressive strength y = 0.4962x + 0.7111 Shear strength y = 8.99x + 10.533 Shear modulus y = 77.62x − 7.8387
(y) and 2
R
0.61 0.54 0.34 0.39 0.59 0.54 0.48 0.51 0.46 0.55 0.67 0.60 0.37 0.35 0.36 0.51 0.32 0.42 0.43 0.41
chanical properties determined in this research would be useful for the development of simulation models of miscanthus and switchgrass stems and in designing harvest and particle reduction equipment.
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