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Mechanical properties of luffa sponge
journal of the mechanical behavior of biomedical materials 15 (2012) 141–152
Available online at www.sciencedirect.com
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Research Paper
Mechanical properties of luffa sponge Jianhu Shena, Yi Min Xiea,n, Xiaodong Huanga, Shiwei Zhoua, Dong Ruanb a
Centre for Innovative Structures and Materials, School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne 3001, Australia b Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, John Street, Hawthorn, VIC 3122, Australia
art i cle i nfo
ab st rac t
Article history:
The paper presents the first scientific study of the stiffness, strength and energy absorption
Received 3 April 2012
characteristics of the luffa sponge with a view to using it as an alternative sustainable
Received in revised form
engineering material for various practical applications. A series of compression tests on
25 June 2012
luffa sponge columns have been carried out. The stress–strain curves show a near constant
Accepted 6 July 2012
plateau stress over a long strain range, which is ideal for energy absorption applications. It
Available online 17 July 2012
is found that the luffa sponge material exhibits remarkable stiffness, strength and energy
Keywords:
absorption capacities that are comparable to those of some metallic cellular materials in a
Luffa sponge
similar density range. Empirical formulae have been developed for stiffness, strength,
Mechanical properties
densification strain and specific energy absorption at the macroscopic level. A comparative
Energy absorption
study shows that the luffa sponge material outperforms a variety of traditional engineering
Sustainability
materials. & 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Biological materials and structures have distinguished them from traditional human developed counterparts because of their unique characteristics (Meyers et al., 2011). The most attractive one is their long-term sustainability to the natural environment. The performance of the traditional man-made materials, notably various metals, metallic alloys, ceramics, plastics, as well as their composites, significantly surpass the biological materials (Bonderer et al., 2008; Zhang et al., 2011). However, most of the man-made materials are not environmentally friendly and little has been concerned with their sustainability (Zhang et al., 2011). By contrast, those biological materials are based on relatively weak base materials such as minerals and proteins which are easily degradable, bio-compatible, pollution-free, recyclable and energy-efficient (Zhang et al., 2011). Furthermore, over millions of years n
Corresponding author. Tel.: þ61 3 9925 3655. E-mail address: [email protected] (Y. Min Xie).
1751-6161/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2012.07.004
to accommodate the natural environment to which they were exposed, a large number of biological systems evolve periodic cells with self-similar hierarchical microstructures. Similar to turtle’s shell (Damiens et al., 2012), jellyfish mesogloeas (Zhu et al., 2012), wood (Stanzl-Tschegg et al., 2011; Wegst, 2011), and E. aspergillum sponge (Mayer, 2011), those hierarchical architectures are optimised or partially optimised and can achieve multi-functions with high toughness and efficiency. As we approach the limit of non-renewable natural resources, these properties are essential for the long-term sustainability of our habitat, and is becoming increasingly significant to human civilisations (Zhang et al., 2011). Luffa sponge is one of such commercially viable and environmentally acceptable biological material derived from fruit of Luffa cylindrica (LC) plant and having recycling capability and triggered biodegradability (John and Thomas, 2008; Oboh and Aluyor, 2009). It is relatively stable in their
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journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
intended lifetime but would biodegrade after disposal in composting conditions. At the same time, the fruits of LC have a netting-like fibrous vascular system. When they are dried, the fibrous network structure serves like an open cell foam material. It has the potential to be used as an alternative material for man-made cellular materials. The importance of biological materials such as the luffa sponge is growing as we search for sustainable solutions using new materials. Only a limited amount of research has been conducted on the luffa sponge as a source of bio-fibres and bio-composites in the last ten years. Those researches indicated that it was a potential alternative material for packaging (Mazali and Alves, 2005), water absorption (Bal and Bal, 2004; Demir et al., 2006), and waste water treatment (Laidani et al., 2001; Oboh et al., 2011). The luffa fibres were also used as reinforcement fibre for other materials (Boynard and D’Almeida, 2000; Ghali et al., 2009; Laranjeira et al., 2006; Paglicawan et al., 2005; Tanobe et al., 2005) and cell immobilisation for biotechnology (Chen and Lin, 2005; Roble et al., 2002; Tavares et al., 2008; Zampieri et al., 2006). At the same time, sponge gourd (LC), the origin of luffa sponge material, have not yet had their potentialities fully explored. With regard to industrial and technological development, the cost of fuel is on the increase. Oil can be extracted from seeds for industrial use (Bal and Bal, 2004). The oil extracted from LC is finding increasing use in the production of biodiesel which is now gaining wide acceptance because of low CO2 emission and other considerations (Ajiwe et al., 2005). However, there is a lack of scientific data on the mechanical properties of luffa sponge material because up to now its main practical use is a body scrub in the bathroom. Due to the lack of experimental proofs, their complex hierarchy microstructures and other common limitations of biological materials, potential applications have not been implemented in practice for luffa sponge material as well as luffa fibres. To this end, the mechanical properties of luffa sponge columns were tested and compared with other cellular materials to check the performance of this light weight material. Uniaxial quasi-static compressive tests were conducted at a strain rate of 103 s1 by using an Instron machine to study the mechanical properties of luffa sponge material. Cylindrical specimens with different relative density were tested at a room temperature of 251 C and a humidity of approximately 40%. An energy efficiency method was adopted to obtain the values of the densification strain and plateau stress, and thus the energy absorption capacity per unit volume. The experimental results were also discussed together with test results published by other researchers for other celluar materials.
2.
Experiments
2.1.
Specimens
The luffa sponge used in our experiments was obtained from pharmacies in Australia which was sold as a bath sponge. A brief treatment procedure for manufacturing these bath sponges from natural luffa fruits was provided by the
supplier. The luffa fruits were harvested after they were fully mature with their skin turning brown. The dried luffa fruits were slightly squashed laterally to crack and remove the skin. Then the two ends of luffa fruits were cut and the seeds were removed. The original luffa sponges were bleached using liquid chlorine bleach (4%) for about 1 h to improve their appearance by making them whiter. After that, they were soaked in clean water for half an hour and then dried in the sun. The chemical composition of the luffa sponge depends on several factors, such as plant origin, weather condition, soil, pre-treatment, etc. A set of reference values for the chemical composition of LC foam can be found in a previous research (Siqueira et al., 2010). The measured maximum diameter for each specimen of dry luffa sponges tested was within the range of 55–86 mm. According to the standard of compressive tests and recent research (Ashby et al., 2000) on other cellular material, the specimen size effect is negligible for foams when the dimension of specimen is sufficiently larger than the cell size, i.e., 7 times of the cell size for metallic foams. Thus the specimen for the luffa sponge should be large enough to eliminate the specimen size effect. The available maximum size along radial direction for luffa sponge is whole section of the luffa sponge column. For most of the luffa sponge column, the section area at two ends of the specimen is different, thus two thin layers of luffa sponge slices were cut at each end. From a single luffa sponge, several cylindrical test specimens (approximately 50 mm high) and two measurement slices (approximately 5 mm thick) from both ends were cut using a bandsaw cutting machine. After the initial cutting, the specimens were milled further to make the two end surfaces smooth and parallel. For calculation purpose, the cross sectional area was taken as the enclosed section (ignoring any internal voids). Because the cross section for most specimens was not a perfect circle, the actual cross sectional area was determined from photographs taken of each measurement slice and processed using image software— Photoshop. The actual cross sectional area from the two measurement slices was then averaged. An equivalent diameter was calculated from the average area. This equivalent diameter was within the range of 42–81 mm. A total of 26 specimens were tested. Fig. 1(a) shows a typical cylindrical specimen for a compressive test. The volume of the luffa sponge specimen is the product of this cross sectional area and the height of the specimen.
2.2.
Geometry features and standard variation of density
After the specimens were cut, three different core topologies were observed from our luffa sponge specimens as shown in Fig. 2(a). The variation in specimen diameter and cross sectional area is shown in Fig. 2(b). The luffa sponge column is composed of luffa fibres. Those fibres interconnect with each other and form networks with micro-trusses. The length of those micro-trusses represents the cell length of the luffa sponge material. A rough measurement indicates that the average length of those microtrusses is in the scale of millimetre (1–5 mm). The major orientation of these luffa fibres in the luffa sponge columns exhibits a regular pattern. According to different orientation
journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
143
Inner surface
Core region
Interlayer
Outer surface Longitudinal direction Circumferential direction
Radial direction
Fig. 1 – A luffa cylindrical specimen for a compressive test: (a) Different regions. (b) Orientation of luffa fibres (left: inner surface; middle: outer surface; right: cross section). (c) Microstructures of luffa fibres (left: luffa fibre in 1 mm; middle: cross section of luffa fibre in 101 mm; right: cross section of luffa fibre in 102 mm). of the luffa fibre, the luffa sponge cylinder can be divided into four regions, namely, the inner surface, the outer surface, interlayer and core as shown in Fig. 1(a). On the inner surface, the thickest luffa fibre grows along longitudinal directions. While on the outer surface, the thickest luffa fibre grows along circumferential directions. In the core region, the strongest luffa fibre is along radial direction. In the interlayer between inner surface and outer surface, the fibre grows in all three directions as shown Fig. 1(b). It can be seen from Fig. 3, the variation of density of the luffa sponge material is irrelevant to the core topology and the size of the luffa sponge columns. Luffa sponge is a material with hierarchical architectures at several length scales. At each hierarchical level, there is a corresponding density. We observed three levels, namely, the luffa sponge column (50 mm), the luffa fibre (1 mm) and the cell wall of luffa fibre (0.01 mm) as shown in Fig. 1. The density of luffa sponge column is 25–65 kg/m3. The density for luffa fibre is
350–650 kg/m3. A previous research also showed that the density of the luffa fibre is approximately 353 kg/m3, with a Young’s modulus of 1332 MPa (735%) and tensile strength of 11.1 MPa (785%) (Paglicawan et al., 2005). And the density for cell wall of luffa fibre is 820– 920 kg/m3 (Siqueira et al., 2010). The microstructures of cell wall of luffa fibre and their mechanical properties remained unknown at present. For this reason, the empirical formulae in this paper will be given on the macroscopic level. Similar to other cellular materials, the mechanical properties of the luffa sponge material are closely related to its density rather than other geometric features mentioned above. Thus the specimens were grouped only according to their densities in order to obtain a reliable empirical formula to represent their mechanical performance. The effect of specimen size and core topology on its mechanical properties was disregarded in the current paper.
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journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
2.3.
Fig. 2 – Different core topologies and sizes for luffa sponge specimens: (a) core topology (three different patterns are shown, namely two links, three links and four links); (b) variation of diameter of cross section (from 46 mm to 81 mm).
65 60 Effect of outer diameter
Density (Kg/m3)
55 50 45 40 35
Experimental procedures
Uniaxial compressive tests were conducted at very low strain rates, namely, 103 s1, to obtain the mechanical properties under quasi-static loading. An Instron machine was used to conduct most of these tests as shown in Fig. 4. A Shimaszu machine was also used for several trail tests. Both machines were calibrated using the same load-cell to ensure the consistency of the experiment results. The displacement and load curves were recorded and the average compressive force, densification strain and energy absorption can be worked out using an energy efficiency method. A temperature and humidity metre was used to monitor the temperature and humidity around the test specimens during the tests. Like other natural sponges such as the sponge Euplectella aspergillum (Johnsona et al., 2010), the mechanical properties of luffa sponge are also influenced by the moisture. Thus the humidity during the test was monitored. The specimens were placed in the test room for at least 2 h before the compressive test to eliminate the difference in temperature and humidity. The temperature in the test room was from 201 to 281, but during each test its variation was less than 11. The humidity was between 30% and 44%, but during each test its variation was less than 1.0%. Photographs were taken during the quasistatic compressive test at every 30 s which corresponded to a 1.5 mm displacement interval for the test velocity used. It should be noted that for the second specimen, a strong light was used to illuminate the specimen to get better photographs. The consequence of this action was that the temperature increased from 21.5 1C to 28.5 1C and the humidity dropped from 39% to 33% during this test. No obvious variation of stress–strain curve was found for this test as shown in Fig. 5(a).
30 25
3.
Experimental results
3.1.
Deformation features
20 60
55
65
75
70
80
85
90
Diameter of luffa sponge specimen (mm) 65
64.1 Core topology 2 Core topology 3 Core topology 4
60
Density (Kg/m3)
55
57.4
Typical force–displacement curves are given in Fig. 5(a) with similar density. It shows clearly a fairly constant compressive force over a long stroke, which represents an ideal energy
54.1
50 45 40
Humidity and temperature metre
35 30 28.5
25
Load cell Top platen
27.3
20 0
5
10
15
20
25
30
35
40
45
50
55
Specimen number
Fig. 3 – Variation of density of luffa cylinders: (a) with respect to equivalent diameter (no particular trend was found); (b) with respect to core topology (no particular trend was found).
v0
Luffa specimen Bottom platen
Fig. 4 – Experimental set-up of a quasi-static test of a luffa sponge column.
journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
145
Fig. 5 – Deformation features of luffa sponge columns crushed axially: (a) force–displacement curves of specimens of similar density; (b) micro-truss level rotation and bending of luffa fibre. (Dotted lines represent the original fibre orientation, and the dash dot lines in the right photograph represent rotated original fibres. It can be seen that the fibre has also bending deformation.) absorption feature. Generally, the deformation can be divided into three regions, namely, a rapid increasing elastic region, a relative smooth plateau collapse region, and a densification region with sharp increase of force over displacement as shown in Fig. 5(a). The deformation process is shown in Fig. 6. Those photographs confirm that the overall compression strain of the specimen is axial along the longitudinal (loading) direction rather than folding of the wall of the specimen. The deformation is uniform in the elastic region. Similar to metallic foams, the compressive deformation process for luffa sponge material under quasi-static loading was not
uniform in the plateau collapse region. Localised crushing band was observed which was similar to the static compression behaviour of metallic foams described by Bastawros et al. (2000) and Shen et al. (2010). At a smaller scale, luffa fibres exhibited axial compression and tension as well as micro-truss-level distortions and rotations which were similar to the open-cell metallic foams as shown in Fig. 5(b). Nominal stress (defined as force over original cross sectional area) and nominal strain (defined as displacement over original thickness of the foam specimen) were calculated to get other mechanical properties.
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journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
Fig. 6 – Typical deformation process of a luffa sponge material (Density: 52.3 kg/m3, Length: 51.3 mm). The deformation was uniform until the deformation reach 3.5 mm, then localised crush band occurred at the middle part of the specimen.
3.2.
Compressive strength
Similar to other cellular materials (Gibson et al., 2010), the compressive strength of the luffa sponge material is taken to be the initial peak stress if it exists. If there is no such peak stress, the stress at the intersection of two slopes is taken to be the compressive strength. The two slopes are the slope for the initial loading and that for the stress plateau. It has been found by other researchers (Ashby et al., 2000; Lu et al., 1999; McCullough et al., 1999; Miyoshi et al., 1999) that compressive strength of cellular materials obeys a power law with the relative density
s0 r ¼A 0 syf rf
!B ð1Þ
where syf is the yield stress of the base material, rf is the density of the base material, s0 and r0 are the compressive strength and density of the luffa sponge, A and B are two constants determined by the topology and failure patterns of cellular material. As mentioned previously, the base solid material of luffa sponge and its mechanical properties remained unknown at the present due to the lack of scientific data on the mechanical properties of luffa sponge material. The luffa sponge column specimen in our experiment was composed of luffa fibre. Our preliminary tensile test results on luffa fibres indicated that the density and tensile strength of luffa fibre varied greatly with different orientations in the same luffa sponge and with different specimens for the same orientation. A large quantity of tests should be conducted to get an accurate average values for mechanical properties of luffa fibres. Thus the empirical formulae are given in the
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journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
Table 1 – Fitting parameters for empirical formulae of strength, densification strain and plateau stress. Mechanical parameter
A(C) error
B(D) error
Correlation coefficient
Standard derivation
Strength Plateau stress Densification strain
0.38 0.18 0.02172
0.24 0.11 5.38 104
0.74 0.89287 0.48172
0.095 0.0587 0.02467
Linear fit (y ¼ AþBx) was used for all experimental data. Data for strength and plateau stress were converted to logarithmic scales before fitting. ‘‘A error’’ is the standard error for A.
following format
0.50 Experimental data
0.45
The unknown base material properties are included in the coefficients A and B. By fitting to Eq. (2) using the quasi-static data in our experiment, the parameters A and B are 2.25 103 and 1.28, respectively if the unit of stress is Pa and that of density is kg/m3. The fitting parameters are listed in Table 1. For truss material with compression and stretching deformation pattern, an exponent of 1 is expected for plastic yielding and 2 for elastic buckling (Mostafa and Damiano, 2010). For open cell foams with bending dominant deformation pattern, an exponent of 1.5 is usually found for plastic collapse strength and 2 for elastic collapse strength (Gibson et al., 2010). A lower exponent indicates fewer penalties for strength and energy absorption with decreasing density for designing very light weight cellular materials. The exponent, 1.28 from our test results, compares favourably with that for open-cell foams. But it is larger than that for truss materials. However, for solid material based metallic foams, the exponent within the range of 1.5–3 is found in other experimental work (Gibson et al., 2010). The value from our experiment suggests that the collapse of the luffa sponge is determined by the coupling of axial yielding and bending of the luffa fibre. The experimental results and empirical prediction are shown in Fig. 7. It should be noted that the derivation of luffa sponge material is large as shown in Fig. 7 and discussed in Section 2.1. The scattering effect may also contribute to the low exponent. If the bending is the dominant deformation mechanism for luffa fibres, more experimental data will increase the value of this exponent. On the other hand, if the axial stretching and compressing of luffa fibre is the dominant deformation mechanism, more experimental data will decrease the value of this exponent.
Compressive strength (MPa)
ð2Þ
Empirical formula, Eq. (2)
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 20
25
30
35
40
45
50
55
60
65
70
Density (kg/m3)
Fig. 7 – Compressive strength of luffa sponge material. The square symbols represent the experimental data, the error bars show the derivation of experimental data from their empirical values. Young's Modulus
12
Young's moduls (MPa)
s0 ¼ Ar0
B
Empirical formula
10 8 6 4 2 0 20
3.3.
25
Elastic modulus
Similar to metallic foams (Ashby et al., 2000), the slope of the initial loading portion of the curve is lower than that of the unloading curve. It indicates that there is localised plastic deformation in the specimen at stress levels well below that of the compressive strength of the luffa sponge material reducing the slope of the loading curve. However, the reloading curves after unloading roughly is close to the initial loading curves. As a result, measurement of Young’s modulus was made from the slope of the initial loading curve in the current study. Thus, the slope between 25% and 75% of the compressive strength is taken as the elastic modulus of luffa sponge material. Following the same argument for the compressive strength, for Young’s modulus, the empirical
30
35
40
45
50
55
60
Density (kg/m3)
Fig. 8 – Young’s modulus of luffa sponge material. The square symbols represent the experimental data, the error bars show the derivation of experimental data from their empirical values. formula is E ¼ 1:48 106 r0 1:16
ð3Þ
where E is the Young’s modulus of luffa sponge; the unit of stress is Pa, and that of density is kg m3. The experimental results and empirical predictions are shown in Fig. 8. Similarly to the compressive strength, the exponent, 1.16, is compared favourably with those from
journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
Gibson et al. (2010), 1.5. However, for solid material based metallic foam, this exponent is within the range of 1–2 found in other experimental work (Gibson et al., 2010). It should be noted that the standard derivation of luffa sponge material is fairly large as discussed in Section 2.1. This may also contribute to the low exponent.
3.4. Unloading and reloading features of luffa sponge material
1.0 0.9 0.8 Stress (MPa)
148
0.7 0.6 0.5 0.4 0.3
3.5. Energy efficiency method to determine densification strain, plateau stress and specific energy absorption The compressive strength, s0, corresponds to the collapse of the weakest layer of cells which was crushed first in compression. Hence, this stress is not applicable to representing the energy dissipation of foams in compression. It is the plateau stress, spl, which is closely related to the energy dissipation capacity of foams and its value is obtained using an energy efficiency method (Li et al., 2006). The energy dissipation efficiency, Ed, of the foam is defined as (Li et al., 2006) R ed sðeÞde ; 0rea r1 ð4Þ Ed ðea Þ ¼ 0 sa and the densification strain, ed, is defined as the maximum value of ei which satisfies the condition of the maximum efficiency dEd ðea Þ 9ea ¼ ei ¼ 0; de
0rei r1
ð5Þ
0.2 0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
Strain 35 Density=41.1 kg/m
30 Loading modulus (MPa)
For any energy absorption devices, the bounce-back behaviour is critical to some of their applications. The bounceback behaviour is dominated by the stored elastic strain during collapse (Lu and Yu, 2003). This feature can be captured by the unloading and reloading curves for a specimen at different collapse strains. Typical unload–reload characteristics at different compressive strains are presented in Fig. 9(a). The relatively high gradient of unloading curves indicates the stored elastic energy is relatively small for luffa sponge material. The unloading and reloading curves do not overlap with each other. Due to the steep gradient the dissipated energy in the hysteresis loop is very small compared to the area under the entire stress–strain curve. Thus luffa sponge can be used in those situations where a once-off, non-reversible energy absorption is required. The unloading curve is not linear and is steeper at the beginning of the unloading. The reloading curve, on the other hand, is almost linear. This phenomenon indicates the hysteresis of the luffa sponge material at a strain rate of 103 s1. The hysteresis is more prominent at larger compressive strain. For a given temperature and humidity, it is observed that the unloading and reloading curves intersect at exact unloading point as shown in Fig. 9(a). The slope of reloading curve, reloading modulus, varies with the strain level at the unloading point as shown in Fig. 9(b). The reloading modulus is very similar to the Young’s modulus measured in Section 3.3 at the plateau collapse region ranging from the first peak stress to the densification strain.
25 20 Young's Modulus from non-cyclic loading
15
6.45MPa
10 5 0 0.0
0.2
0.6 0.4 Unloading strain
0.8
1.0
Fig. 9 – Unloading and reloading features of luffa sponge material: (a) stress–strain curves with 9 unloading points and the total unloading strain is about 5% (an enlarged view of unloading features in the elastic deformation region is provided); (b) loading modulus in each of the unloading circle versus loading strain at the unloading point.
Then, the plateau stress spl is calculated as R ed sðeÞde spl ¼ 0 ed
ð6Þ
An example is shown in Fig. 10. The densification strain physically corresponds to the start point from which the stress starts to rise sharply. From this point onwards, the luffa sponge can still dissipate energy by plastic deformation, but its dissipation efficiency will start to decrease. Based on the present experimental data, we can propose that the densification strain is linearly related to the density of the luffa sponge. Hence ed ¼ CDr0
ð7Þ
By fitting to Eq. (7) using the quasi-static data in our experiment, the parameters C and D can be obtained as 0.68 and 1.62 103, respectively. The unit of density is kg/m3. The fitting parameters are listed in Table 1. The empirical prediction and experimental data are shown in Fig. 11.
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journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
Strain 1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 0.60
Compressive Stress
0.9
0.54 Energy Efficiency
0.48
0.7
0.42 Plateau stress 0.31MPa
0.6 0.5
0.36 0.30
Compression strength 0.27MPa
0.4
0.24
0.3
Energy efficiency
Compressive stress (MPa)
0.8
0.18 Densification strain 0.57
0.2
0.12
0.1
0.06
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.00 1.0
0.9
Strain Fig. 10 – Illustration of energy efficiency method. Energy absorption per unit volume (N/mm2)
0.7
Densification strain
0.6 0.5 0.4 0.3 0.2
Experimental data for luffa sponge material Empirical formula
0.1
Aluminium foam
0.0 0.06
0.08
0.10
0.12
0.14
0.16
0.25
0.20
0.15
0.10
0.05 25
30
35
40
45
50
55
60
Density (kg/m3)
Relative density
By fitting to Eq. (2) using the quasi-static data for plateau stress in our experiment, the parameters A and B can be obtained as 8.65 103 and 1.22, respectively if the unit of stress is Pa and that of density is kg/m3. The fitting parameters are listed in Table 1. They are slight different from these for compressive strength. Similar to the compressive strength of the luffa sponge, the exponent, 1.22, is compared favourably with those from Gibson et al. (2010), 1.5. However, for solid material based metallic foam, this figure should be within the range of 1.5–3 found in other experimental work (Gibson et al., 2010). Similar to strength of luffa sponge material, the scattering effect may also contribute to the low exponent. The energy dissipation capacity of luffa sponge material can be characterised by the energy absorption per unit initial volume, w, during the compression process before densification occurs. This value is equal to the area under the stress–strain curve. Thus the energy dissipation capacity for
Experimental data Empirical formula
20
0.18
Fig. 11 – Densification strain for luffa sponge specimens.
0.30
Fig. 12 – Energy dissipation per unit initial volume versus relative density. The square symbols represent the experimental data, the error bars show the derivation of experimental data from their empirical values.
the density range in our experiment can be written as w ¼ 8:65 103 ð0:68r0 1:22 1:62 103 r0 2:22 Þ
ð8Þ
Fig. 12 shows that the energy dissipation capacity per unit volume relatively linearly increases with relative density in the density range tested.
4.
Comparison with other materials
As a biological material, the experimental data in this paper about luffa sponge material may significantly expand the density range of previously studied natural cellular materials (Gibson et al., 2010) with respect to the strength, as shown in Fig. 13. This special feature provides an opportunity to widen
150
journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
1000
Energy dissipation per unit mass (J/kg)
Strength (MPa)
100
Natural cellular materials such as trabecular bones, palms, and wood Luffa sponge (results from present study)
10 1 0.1 Other biological materials such as tomato, potato, and apple
0.01 10
100
1000
8000
Aporas® aluminium foam Cymate aluminium foam
7000
Polymer foams Luffa sponge from present study
6000 5000 4000 3000 2000 1000 0
Density (kg/m3)
0
50
100
150
200
250
300
Density (kg/m3)
Fig. 13 – Comparison between luffa sponge and other natural materials (Gibson et al., 2010). 1000
Luffa sponge (density: 44 ~ 47 kg/m )
900 Compressive stress (kPa)
the applications of natural cellular materials in general and the luffa sponge in particular, especially when the light weight is a key design requirement. It is worth noting that human beings have been able to discover and develop many materials, notably various metals, metallic alloys, ceramics, plastics, as well as their composites, with performances significantly surpassing biological materials (Bonderer et al., 2008; Zhang et al., 2011). When light weight of the material is considered, various metallic/ ceramic/polymeric foams, honeycombs, and microlattices, are designed and fabricated using those materials as base materials. However, most of these man-made materials are not environmentally friendly and have not been designed with genuine concerns over their long-term sustainability (Zhang et al., 2011). An efficient solution is to find an alternative biological counterpart for those materials in use. For this purpose, a comparative study has been carried out below to show the merit of the luffa sponge material besides its sustainable features. As a cellular material, the luffa sponge is extremely light as shown in Fig. 13. At less than 50 kg/m3, its density is about one fifth of that of a typical aluminium foam (Alporass). For a similar density (25–65 kg/m3), very few materials currently exist such as Cymat aluminium foam (Z70 kg/m3), Ni–P microlattices (Z0.9 kg/m3), silica aerogels (Z1.0 kg/m3), carbon nanotube aerogels (Z4.0 kg/m3), and polymer foams (Z8.0 kg/m3). However, our research results indicate that, when the luffa column is compressed longitudinally, the amount of energy it can absorb per unit mass is comparable to that of the aluminium foam (Idris et al., 2009; Shen et al., 2010) and most of the polymer foams (Avalle et al., 2001), as shown in Fig. 14(a). It should be noted that Alporass aluminium foam has superior specific energy absorption than other metallic foams when their density is lower than 300 kg m3 (Ashby et al., 2000). The good specific energy absorption of luffa sponge is attributed partially to its light base material as well as a higher densification strain. Due to the high strength-to-weight ratio of cellular materials, luffa sponge can be used as a good packaging material and an excellent energy dissipation material. It should be seen that the average plateau stress of aluminium foam is 4–5 times higher than the luffa sponge material. To show the efficiency of the
Nickel microlattice (density: 43 kg/m )
800 700 600 500 400 300 200 100 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Strain
Fig. 14 – Comparison of energy absorption capacity per unit mass between luffa sponge and: (a) other man-made foams; (b) Ni–P microlattice (Schaedler et al., 2011).
microstructures of luffa sponge material, we have also compared the performance of the luffa sponge with other cellular materials with similar density mentioned above. Cymat foam has a strength of 40 kPa at a density of 70 kg m3 (Ashby et al., 2000). Aerogels exhibit a continuous increasing of stress over strain under compression which is not comparable to luffa sponge material. Expended Polystyrene foams have a strength of 200 kPa at a density of 45 kg m3 (Avalle et al., 2001). The polyamide reinforced with modified polyphenylene and polystyrene foam has a strength of 800 kPa at a density of 45 kg m3 (Avalle et al., 2001). As an important example, the comparison with a recently invented metallic microlattice of a similar density (Schaedler et al., 2011) shows that the average plateau stress of the luffa sponge (about 350 kPa) is significantly higher than that of the Ni–P microlattice (about 120 kPa), as shown in Fig. 14(b). It should be noted that the strength decreases with density for any kind of cellular materials. These comparisons indicate that the luffa sponge material has better compressive strength than most of other available cellular material of similar density such as Cymat aluminium foam, expended Polystyrene foams and Ni–P microlattices.
journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
It should be noted that the luffa sponge material is a cellular material with structural hierarchy as indicated in Fig. 1. As mentioned previously, the luffa sponge column specimen is composed of luffa fibres. According to other research on the microstructures of luffa sponge material (Boynard and D’Almeida, 2000; Ghali et al., 2009; Laranjeira et al., 2006; Paglicawan et al., 2005; Tanobe et al., 2005), the luffa fibres are hollow with many micro-tunnels. Those microstructures have also been proven by our own investigations. Similar to other natural cellular material with similar components, the cell walls of those micro-tunnels may be fibrous themselves and consist of oriented cellulose nanofibrils in a hemicellulose and lignin matrix. Those hierarchical levels required further investigation. However, a rough estimation can be made for contributions of topology at different hierarchical levels to the strength, plateau stress and energy absorption capacities of luffa sponge material using the available material properties from others research (Paglicawan et al., 2005). The density is approximately 353 kg/m3 with a Young’s modulus of 1332 MPa (735%) and tensile strength of 11.1 MPa (785%). The obtained relative plateau stress versus relative density was compared with other polymeric materials (Gibson and Ashby, 1982) as shown in Fig. 15. The experimental data and empirical prediction exhibit a similar trend with that of other polymeric materials. It indicates that the relative strength are not superior to others cellular material if only one level of structural hierarchy is considered. The prediction formulae for other hierarchical levels are not available because there is a lack of the mechanical properties of the base composite materials as well as its specific composite patterns of those base ingredients. It is worth further research effort to understand and bio-mimic the luffa sponge material with superior strength and energy absorption capacities. We also find that the severely crushed luffa sponge column (up to a nominal final compressive strain of 95%) is able to recover to its original length and shape (up to 98%) after it is submerged in water and then dried. We are currently conducting further investigation on this very interesting phenomenon.
151
As mentioned in Section 2.1, the luffa sponge used in our experiments was obtained from pharmacies in Australia which was sold as a bath sponge. There were two treatments in manufacturing the bath sponge from natural luffa fruits which may influence the mechanical properties of luffa sponge, namely, bleaching and lateral squashing. Similar to other cellulose based natural fibres such as cotton, bleach would result in slight loss of mass so as to its mechanical strength of untreated luffa sponge. The lateral squashing would result in damage of the original luffa fibres. The experimental data and empirical formula from our experiment will underestimate the strength of untreated luffa sponge material.
5.
Conclusions
In this study, a series of compressive tests were conducted to examine the stiffness, strength and energy absorption characteristics of the luffa sponge material under quasi-static compressive load. The Young’s modulus, compressive strength, densification strain, plateau stress and energy absorption capacity of the luffa sponge material have been obtained for the first time. A set of empirical formulae have been proposed to predict the mechanical properties of luffa sponge material on the macroscopic scale. It should be noted that there is a limitation of our current work, i.e., the luffa sponge material used in our experiment was bleached and laterally squashed initially. The effect of such pre-treatments on the mechanical properties of the luffa sponge is not available and is under further investigation. From our investigation, the following conclusions can be drawn. (1) The stress–strain curves show a near constant plateau stress over a long strain range, which is ideal for energy absorption application. It could be used as an alternative packaging material. (2) The exponent from our experiments suggests that the deformation of the luffa sponge is determined by the coupling of axial compression/tension and bending of luffa fibres. (3) It is found that the luffa sponge material exhibits remarkable stiffness, strength and energy absorption capabilities that are comparable to those of some metallic cellular materials such as aluminium foams and Ni–P microlattices. The strength of luffa sponge is better than most of other available cellular materials in the similar density range such as expended Polystyrene foams and Ni–P microlattices. As an ultra-light cellular material, it has great potential to be used as an environmentally friendly engineering material.
r e f e r e nc e
Fig. 15 – Comparison of relative plateau stress between luffa sponge and other polymeric materials (Gibson and Ashby, 1982).
Ajiwe, V., Ndukwe, G., Anyadiegwu, I., 2005. Vegetable diesel fuels from Luffa cylindrica oil, its methylester and ester-diesel blends. Chemistry Class Journal 2, 1–4.
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/jmbbm
Research Paper
Mechanical properties of luffa sponge Jianhu Shena, Yi Min Xiea,n, Xiaodong Huanga, Shiwei Zhoua, Dong Ruanb a
Centre for Innovative Structures and Materials, School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne 3001, Australia b Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, John Street, Hawthorn, VIC 3122, Australia
art i cle i nfo
ab st rac t
Article history:
The paper presents the first scientific study of the stiffness, strength and energy absorption
Received 3 April 2012
characteristics of the luffa sponge with a view to using it as an alternative sustainable
Received in revised form
engineering material for various practical applications. A series of compression tests on
25 June 2012
luffa sponge columns have been carried out. The stress–strain curves show a near constant
Accepted 6 July 2012
plateau stress over a long strain range, which is ideal for energy absorption applications. It
Available online 17 July 2012
is found that the luffa sponge material exhibits remarkable stiffness, strength and energy
Keywords:
absorption capacities that are comparable to those of some metallic cellular materials in a
Luffa sponge
similar density range. Empirical formulae have been developed for stiffness, strength,
Mechanical properties
densification strain and specific energy absorption at the macroscopic level. A comparative
Energy absorption
study shows that the luffa sponge material outperforms a variety of traditional engineering
Sustainability
materials. & 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Biological materials and structures have distinguished them from traditional human developed counterparts because of their unique characteristics (Meyers et al., 2011). The most attractive one is their long-term sustainability to the natural environment. The performance of the traditional man-made materials, notably various metals, metallic alloys, ceramics, plastics, as well as their composites, significantly surpass the biological materials (Bonderer et al., 2008; Zhang et al., 2011). However, most of the man-made materials are not environmentally friendly and little has been concerned with their sustainability (Zhang et al., 2011). By contrast, those biological materials are based on relatively weak base materials such as minerals and proteins which are easily degradable, bio-compatible, pollution-free, recyclable and energy-efficient (Zhang et al., 2011). Furthermore, over millions of years n
Corresponding author. Tel.: þ61 3 9925 3655. E-mail address: [email protected] (Y. Min Xie).
1751-6161/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2012.07.004
to accommodate the natural environment to which they were exposed, a large number of biological systems evolve periodic cells with self-similar hierarchical microstructures. Similar to turtle’s shell (Damiens et al., 2012), jellyfish mesogloeas (Zhu et al., 2012), wood (Stanzl-Tschegg et al., 2011; Wegst, 2011), and E. aspergillum sponge (Mayer, 2011), those hierarchical architectures are optimised or partially optimised and can achieve multi-functions with high toughness and efficiency. As we approach the limit of non-renewable natural resources, these properties are essential for the long-term sustainability of our habitat, and is becoming increasingly significant to human civilisations (Zhang et al., 2011). Luffa sponge is one of such commercially viable and environmentally acceptable biological material derived from fruit of Luffa cylindrica (LC) plant and having recycling capability and triggered biodegradability (John and Thomas, 2008; Oboh and Aluyor, 2009). It is relatively stable in their
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journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
intended lifetime but would biodegrade after disposal in composting conditions. At the same time, the fruits of LC have a netting-like fibrous vascular system. When they are dried, the fibrous network structure serves like an open cell foam material. It has the potential to be used as an alternative material for man-made cellular materials. The importance of biological materials such as the luffa sponge is growing as we search for sustainable solutions using new materials. Only a limited amount of research has been conducted on the luffa sponge as a source of bio-fibres and bio-composites in the last ten years. Those researches indicated that it was a potential alternative material for packaging (Mazali and Alves, 2005), water absorption (Bal and Bal, 2004; Demir et al., 2006), and waste water treatment (Laidani et al., 2001; Oboh et al., 2011). The luffa fibres were also used as reinforcement fibre for other materials (Boynard and D’Almeida, 2000; Ghali et al., 2009; Laranjeira et al., 2006; Paglicawan et al., 2005; Tanobe et al., 2005) and cell immobilisation for biotechnology (Chen and Lin, 2005; Roble et al., 2002; Tavares et al., 2008; Zampieri et al., 2006). At the same time, sponge gourd (LC), the origin of luffa sponge material, have not yet had their potentialities fully explored. With regard to industrial and technological development, the cost of fuel is on the increase. Oil can be extracted from seeds for industrial use (Bal and Bal, 2004). The oil extracted from LC is finding increasing use in the production of biodiesel which is now gaining wide acceptance because of low CO2 emission and other considerations (Ajiwe et al., 2005). However, there is a lack of scientific data on the mechanical properties of luffa sponge material because up to now its main practical use is a body scrub in the bathroom. Due to the lack of experimental proofs, their complex hierarchy microstructures and other common limitations of biological materials, potential applications have not been implemented in practice for luffa sponge material as well as luffa fibres. To this end, the mechanical properties of luffa sponge columns were tested and compared with other cellular materials to check the performance of this light weight material. Uniaxial quasi-static compressive tests were conducted at a strain rate of 103 s1 by using an Instron machine to study the mechanical properties of luffa sponge material. Cylindrical specimens with different relative density were tested at a room temperature of 251 C and a humidity of approximately 40%. An energy efficiency method was adopted to obtain the values of the densification strain and plateau stress, and thus the energy absorption capacity per unit volume. The experimental results were also discussed together with test results published by other researchers for other celluar materials.
2.
Experiments
2.1.
Specimens
The luffa sponge used in our experiments was obtained from pharmacies in Australia which was sold as a bath sponge. A brief treatment procedure for manufacturing these bath sponges from natural luffa fruits was provided by the
supplier. The luffa fruits were harvested after they were fully mature with their skin turning brown. The dried luffa fruits were slightly squashed laterally to crack and remove the skin. Then the two ends of luffa fruits were cut and the seeds were removed. The original luffa sponges were bleached using liquid chlorine bleach (4%) for about 1 h to improve their appearance by making them whiter. After that, they were soaked in clean water for half an hour and then dried in the sun. The chemical composition of the luffa sponge depends on several factors, such as plant origin, weather condition, soil, pre-treatment, etc. A set of reference values for the chemical composition of LC foam can be found in a previous research (Siqueira et al., 2010). The measured maximum diameter for each specimen of dry luffa sponges tested was within the range of 55–86 mm. According to the standard of compressive tests and recent research (Ashby et al., 2000) on other cellular material, the specimen size effect is negligible for foams when the dimension of specimen is sufficiently larger than the cell size, i.e., 7 times of the cell size for metallic foams. Thus the specimen for the luffa sponge should be large enough to eliminate the specimen size effect. The available maximum size along radial direction for luffa sponge is whole section of the luffa sponge column. For most of the luffa sponge column, the section area at two ends of the specimen is different, thus two thin layers of luffa sponge slices were cut at each end. From a single luffa sponge, several cylindrical test specimens (approximately 50 mm high) and two measurement slices (approximately 5 mm thick) from both ends were cut using a bandsaw cutting machine. After the initial cutting, the specimens were milled further to make the two end surfaces smooth and parallel. For calculation purpose, the cross sectional area was taken as the enclosed section (ignoring any internal voids). Because the cross section for most specimens was not a perfect circle, the actual cross sectional area was determined from photographs taken of each measurement slice and processed using image software— Photoshop. The actual cross sectional area from the two measurement slices was then averaged. An equivalent diameter was calculated from the average area. This equivalent diameter was within the range of 42–81 mm. A total of 26 specimens were tested. Fig. 1(a) shows a typical cylindrical specimen for a compressive test. The volume of the luffa sponge specimen is the product of this cross sectional area and the height of the specimen.
2.2.
Geometry features and standard variation of density
After the specimens were cut, three different core topologies were observed from our luffa sponge specimens as shown in Fig. 2(a). The variation in specimen diameter and cross sectional area is shown in Fig. 2(b). The luffa sponge column is composed of luffa fibres. Those fibres interconnect with each other and form networks with micro-trusses. The length of those micro-trusses represents the cell length of the luffa sponge material. A rough measurement indicates that the average length of those microtrusses is in the scale of millimetre (1–5 mm). The major orientation of these luffa fibres in the luffa sponge columns exhibits a regular pattern. According to different orientation
journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
143
Inner surface
Core region
Interlayer
Outer surface Longitudinal direction Circumferential direction
Radial direction
Fig. 1 – A luffa cylindrical specimen for a compressive test: (a) Different regions. (b) Orientation of luffa fibres (left: inner surface; middle: outer surface; right: cross section). (c) Microstructures of luffa fibres (left: luffa fibre in 1 mm; middle: cross section of luffa fibre in 101 mm; right: cross section of luffa fibre in 102 mm). of the luffa fibre, the luffa sponge cylinder can be divided into four regions, namely, the inner surface, the outer surface, interlayer and core as shown in Fig. 1(a). On the inner surface, the thickest luffa fibre grows along longitudinal directions. While on the outer surface, the thickest luffa fibre grows along circumferential directions. In the core region, the strongest luffa fibre is along radial direction. In the interlayer between inner surface and outer surface, the fibre grows in all three directions as shown Fig. 1(b). It can be seen from Fig. 3, the variation of density of the luffa sponge material is irrelevant to the core topology and the size of the luffa sponge columns. Luffa sponge is a material with hierarchical architectures at several length scales. At each hierarchical level, there is a corresponding density. We observed three levels, namely, the luffa sponge column (50 mm), the luffa fibre (1 mm) and the cell wall of luffa fibre (0.01 mm) as shown in Fig. 1. The density of luffa sponge column is 25–65 kg/m3. The density for luffa fibre is
350–650 kg/m3. A previous research also showed that the density of the luffa fibre is approximately 353 kg/m3, with a Young’s modulus of 1332 MPa (735%) and tensile strength of 11.1 MPa (785%) (Paglicawan et al., 2005). And the density for cell wall of luffa fibre is 820– 920 kg/m3 (Siqueira et al., 2010). The microstructures of cell wall of luffa fibre and their mechanical properties remained unknown at present. For this reason, the empirical formulae in this paper will be given on the macroscopic level. Similar to other cellular materials, the mechanical properties of the luffa sponge material are closely related to its density rather than other geometric features mentioned above. Thus the specimens were grouped only according to their densities in order to obtain a reliable empirical formula to represent their mechanical performance. The effect of specimen size and core topology on its mechanical properties was disregarded in the current paper.
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journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
2.3.
Fig. 2 – Different core topologies and sizes for luffa sponge specimens: (a) core topology (three different patterns are shown, namely two links, three links and four links); (b) variation of diameter of cross section (from 46 mm to 81 mm).
65 60 Effect of outer diameter
Density (Kg/m3)
55 50 45 40 35
Experimental procedures
Uniaxial compressive tests were conducted at very low strain rates, namely, 103 s1, to obtain the mechanical properties under quasi-static loading. An Instron machine was used to conduct most of these tests as shown in Fig. 4. A Shimaszu machine was also used for several trail tests. Both machines were calibrated using the same load-cell to ensure the consistency of the experiment results. The displacement and load curves were recorded and the average compressive force, densification strain and energy absorption can be worked out using an energy efficiency method. A temperature and humidity metre was used to monitor the temperature and humidity around the test specimens during the tests. Like other natural sponges such as the sponge Euplectella aspergillum (Johnsona et al., 2010), the mechanical properties of luffa sponge are also influenced by the moisture. Thus the humidity during the test was monitored. The specimens were placed in the test room for at least 2 h before the compressive test to eliminate the difference in temperature and humidity. The temperature in the test room was from 201 to 281, but during each test its variation was less than 11. The humidity was between 30% and 44%, but during each test its variation was less than 1.0%. Photographs were taken during the quasistatic compressive test at every 30 s which corresponded to a 1.5 mm displacement interval for the test velocity used. It should be noted that for the second specimen, a strong light was used to illuminate the specimen to get better photographs. The consequence of this action was that the temperature increased from 21.5 1C to 28.5 1C and the humidity dropped from 39% to 33% during this test. No obvious variation of stress–strain curve was found for this test as shown in Fig. 5(a).
30 25
3.
Experimental results
3.1.
Deformation features
20 60
55
65
75
70
80
85
90
Diameter of luffa sponge specimen (mm) 65
64.1 Core topology 2 Core topology 3 Core topology 4
60
Density (Kg/m3)
55
57.4
Typical force–displacement curves are given in Fig. 5(a) with similar density. It shows clearly a fairly constant compressive force over a long stroke, which represents an ideal energy
54.1
50 45 40
Humidity and temperature metre
35 30 28.5
25
Load cell Top platen
27.3
20 0
5
10
15
20
25
30
35
40
45
50
55
Specimen number
Fig. 3 – Variation of density of luffa cylinders: (a) with respect to equivalent diameter (no particular trend was found); (b) with respect to core topology (no particular trend was found).
v0
Luffa specimen Bottom platen
Fig. 4 – Experimental set-up of a quasi-static test of a luffa sponge column.
journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
145
Fig. 5 – Deformation features of luffa sponge columns crushed axially: (a) force–displacement curves of specimens of similar density; (b) micro-truss level rotation and bending of luffa fibre. (Dotted lines represent the original fibre orientation, and the dash dot lines in the right photograph represent rotated original fibres. It can be seen that the fibre has also bending deformation.) absorption feature. Generally, the deformation can be divided into three regions, namely, a rapid increasing elastic region, a relative smooth plateau collapse region, and a densification region with sharp increase of force over displacement as shown in Fig. 5(a). The deformation process is shown in Fig. 6. Those photographs confirm that the overall compression strain of the specimen is axial along the longitudinal (loading) direction rather than folding of the wall of the specimen. The deformation is uniform in the elastic region. Similar to metallic foams, the compressive deformation process for luffa sponge material under quasi-static loading was not
uniform in the plateau collapse region. Localised crushing band was observed which was similar to the static compression behaviour of metallic foams described by Bastawros et al. (2000) and Shen et al. (2010). At a smaller scale, luffa fibres exhibited axial compression and tension as well as micro-truss-level distortions and rotations which were similar to the open-cell metallic foams as shown in Fig. 5(b). Nominal stress (defined as force over original cross sectional area) and nominal strain (defined as displacement over original thickness of the foam specimen) were calculated to get other mechanical properties.
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Fig. 6 – Typical deformation process of a luffa sponge material (Density: 52.3 kg/m3, Length: 51.3 mm). The deformation was uniform until the deformation reach 3.5 mm, then localised crush band occurred at the middle part of the specimen.
3.2.
Compressive strength
Similar to other cellular materials (Gibson et al., 2010), the compressive strength of the luffa sponge material is taken to be the initial peak stress if it exists. If there is no such peak stress, the stress at the intersection of two slopes is taken to be the compressive strength. The two slopes are the slope for the initial loading and that for the stress plateau. It has been found by other researchers (Ashby et al., 2000; Lu et al., 1999; McCullough et al., 1999; Miyoshi et al., 1999) that compressive strength of cellular materials obeys a power law with the relative density
s0 r ¼A 0 syf rf
!B ð1Þ
where syf is the yield stress of the base material, rf is the density of the base material, s0 and r0 are the compressive strength and density of the luffa sponge, A and B are two constants determined by the topology and failure patterns of cellular material. As mentioned previously, the base solid material of luffa sponge and its mechanical properties remained unknown at the present due to the lack of scientific data on the mechanical properties of luffa sponge material. The luffa sponge column specimen in our experiment was composed of luffa fibre. Our preliminary tensile test results on luffa fibres indicated that the density and tensile strength of luffa fibre varied greatly with different orientations in the same luffa sponge and with different specimens for the same orientation. A large quantity of tests should be conducted to get an accurate average values for mechanical properties of luffa fibres. Thus the empirical formulae are given in the
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Table 1 – Fitting parameters for empirical formulae of strength, densification strain and plateau stress. Mechanical parameter
A(C) error
B(D) error
Correlation coefficient
Standard derivation
Strength Plateau stress Densification strain
0.38 0.18 0.02172
0.24 0.11 5.38 104
0.74 0.89287 0.48172
0.095 0.0587 0.02467
Linear fit (y ¼ AþBx) was used for all experimental data. Data for strength and plateau stress were converted to logarithmic scales before fitting. ‘‘A error’’ is the standard error for A.
following format
0.50 Experimental data
0.45
The unknown base material properties are included in the coefficients A and B. By fitting to Eq. (2) using the quasi-static data in our experiment, the parameters A and B are 2.25 103 and 1.28, respectively if the unit of stress is Pa and that of density is kg/m3. The fitting parameters are listed in Table 1. For truss material with compression and stretching deformation pattern, an exponent of 1 is expected for plastic yielding and 2 for elastic buckling (Mostafa and Damiano, 2010). For open cell foams with bending dominant deformation pattern, an exponent of 1.5 is usually found for plastic collapse strength and 2 for elastic collapse strength (Gibson et al., 2010). A lower exponent indicates fewer penalties for strength and energy absorption with decreasing density for designing very light weight cellular materials. The exponent, 1.28 from our test results, compares favourably with that for open-cell foams. But it is larger than that for truss materials. However, for solid material based metallic foams, the exponent within the range of 1.5–3 is found in other experimental work (Gibson et al., 2010). The value from our experiment suggests that the collapse of the luffa sponge is determined by the coupling of axial yielding and bending of the luffa fibre. The experimental results and empirical prediction are shown in Fig. 7. It should be noted that the derivation of luffa sponge material is large as shown in Fig. 7 and discussed in Section 2.1. The scattering effect may also contribute to the low exponent. If the bending is the dominant deformation mechanism for luffa fibres, more experimental data will increase the value of this exponent. On the other hand, if the axial stretching and compressing of luffa fibre is the dominant deformation mechanism, more experimental data will decrease the value of this exponent.
Compressive strength (MPa)
ð2Þ
Empirical formula, Eq. (2)
0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 20
25
30
35
40
45
50
55
60
65
70
Density (kg/m3)
Fig. 7 – Compressive strength of luffa sponge material. The square symbols represent the experimental data, the error bars show the derivation of experimental data from their empirical values. Young's Modulus
12
Young's moduls (MPa)
s0 ¼ Ar0
B
Empirical formula
10 8 6 4 2 0 20
3.3.
25
Elastic modulus
Similar to metallic foams (Ashby et al., 2000), the slope of the initial loading portion of the curve is lower than that of the unloading curve. It indicates that there is localised plastic deformation in the specimen at stress levels well below that of the compressive strength of the luffa sponge material reducing the slope of the loading curve. However, the reloading curves after unloading roughly is close to the initial loading curves. As a result, measurement of Young’s modulus was made from the slope of the initial loading curve in the current study. Thus, the slope between 25% and 75% of the compressive strength is taken as the elastic modulus of luffa sponge material. Following the same argument for the compressive strength, for Young’s modulus, the empirical
30
35
40
45
50
55
60
Density (kg/m3)
Fig. 8 – Young’s modulus of luffa sponge material. The square symbols represent the experimental data, the error bars show the derivation of experimental data from their empirical values. formula is E ¼ 1:48 106 r0 1:16
ð3Þ
where E is the Young’s modulus of luffa sponge; the unit of stress is Pa, and that of density is kg m3. The experimental results and empirical predictions are shown in Fig. 8. Similarly to the compressive strength, the exponent, 1.16, is compared favourably with those from
journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
Gibson et al. (2010), 1.5. However, for solid material based metallic foam, this exponent is within the range of 1–2 found in other experimental work (Gibson et al., 2010). It should be noted that the standard derivation of luffa sponge material is fairly large as discussed in Section 2.1. This may also contribute to the low exponent.
3.4. Unloading and reloading features of luffa sponge material
1.0 0.9 0.8 Stress (MPa)
148
0.7 0.6 0.5 0.4 0.3
3.5. Energy efficiency method to determine densification strain, plateau stress and specific energy absorption The compressive strength, s0, corresponds to the collapse of the weakest layer of cells which was crushed first in compression. Hence, this stress is not applicable to representing the energy dissipation of foams in compression. It is the plateau stress, spl, which is closely related to the energy dissipation capacity of foams and its value is obtained using an energy efficiency method (Li et al., 2006). The energy dissipation efficiency, Ed, of the foam is defined as (Li et al., 2006) R ed sðeÞde ; 0rea r1 ð4Þ Ed ðea Þ ¼ 0 sa and the densification strain, ed, is defined as the maximum value of ei which satisfies the condition of the maximum efficiency dEd ðea Þ 9ea ¼ ei ¼ 0; de
0rei r1
ð5Þ
0.2 0.1 0.0 0.0
0.2
0.4
0.6
0.8
1.0
Strain 35 Density=41.1 kg/m
30 Loading modulus (MPa)
For any energy absorption devices, the bounce-back behaviour is critical to some of their applications. The bounceback behaviour is dominated by the stored elastic strain during collapse (Lu and Yu, 2003). This feature can be captured by the unloading and reloading curves for a specimen at different collapse strains. Typical unload–reload characteristics at different compressive strains are presented in Fig. 9(a). The relatively high gradient of unloading curves indicates the stored elastic energy is relatively small for luffa sponge material. The unloading and reloading curves do not overlap with each other. Due to the steep gradient the dissipated energy in the hysteresis loop is very small compared to the area under the entire stress–strain curve. Thus luffa sponge can be used in those situations where a once-off, non-reversible energy absorption is required. The unloading curve is not linear and is steeper at the beginning of the unloading. The reloading curve, on the other hand, is almost linear. This phenomenon indicates the hysteresis of the luffa sponge material at a strain rate of 103 s1. The hysteresis is more prominent at larger compressive strain. For a given temperature and humidity, it is observed that the unloading and reloading curves intersect at exact unloading point as shown in Fig. 9(a). The slope of reloading curve, reloading modulus, varies with the strain level at the unloading point as shown in Fig. 9(b). The reloading modulus is very similar to the Young’s modulus measured in Section 3.3 at the plateau collapse region ranging from the first peak stress to the densification strain.
25 20 Young's Modulus from non-cyclic loading
15
6.45MPa
10 5 0 0.0
0.2
0.6 0.4 Unloading strain
0.8
1.0
Fig. 9 – Unloading and reloading features of luffa sponge material: (a) stress–strain curves with 9 unloading points and the total unloading strain is about 5% (an enlarged view of unloading features in the elastic deformation region is provided); (b) loading modulus in each of the unloading circle versus loading strain at the unloading point.
Then, the plateau stress spl is calculated as R ed sðeÞde spl ¼ 0 ed
ð6Þ
An example is shown in Fig. 10. The densification strain physically corresponds to the start point from which the stress starts to rise sharply. From this point onwards, the luffa sponge can still dissipate energy by plastic deformation, but its dissipation efficiency will start to decrease. Based on the present experimental data, we can propose that the densification strain is linearly related to the density of the luffa sponge. Hence ed ¼ CDr0
ð7Þ
By fitting to Eq. (7) using the quasi-static data in our experiment, the parameters C and D can be obtained as 0.68 and 1.62 103, respectively. The unit of density is kg/m3. The fitting parameters are listed in Table 1. The empirical prediction and experimental data are shown in Fig. 11.
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Strain 1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 0.60
Compressive Stress
0.9
0.54 Energy Efficiency
0.48
0.7
0.42 Plateau stress 0.31MPa
0.6 0.5
0.36 0.30
Compression strength 0.27MPa
0.4
0.24
0.3
Energy efficiency
Compressive stress (MPa)
0.8
0.18 Densification strain 0.57
0.2
0.12
0.1
0.06
0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.00 1.0
0.9
Strain Fig. 10 – Illustration of energy efficiency method. Energy absorption per unit volume (N/mm2)
0.7
Densification strain
0.6 0.5 0.4 0.3 0.2
Experimental data for luffa sponge material Empirical formula
0.1
Aluminium foam
0.0 0.06
0.08
0.10
0.12
0.14
0.16
0.25
0.20
0.15
0.10
0.05 25
30
35
40
45
50
55
60
Density (kg/m3)
Relative density
By fitting to Eq. (2) using the quasi-static data for plateau stress in our experiment, the parameters A and B can be obtained as 8.65 103 and 1.22, respectively if the unit of stress is Pa and that of density is kg/m3. The fitting parameters are listed in Table 1. They are slight different from these for compressive strength. Similar to the compressive strength of the luffa sponge, the exponent, 1.22, is compared favourably with those from Gibson et al. (2010), 1.5. However, for solid material based metallic foam, this figure should be within the range of 1.5–3 found in other experimental work (Gibson et al., 2010). Similar to strength of luffa sponge material, the scattering effect may also contribute to the low exponent. The energy dissipation capacity of luffa sponge material can be characterised by the energy absorption per unit initial volume, w, during the compression process before densification occurs. This value is equal to the area under the stress–strain curve. Thus the energy dissipation capacity for
Experimental data Empirical formula
20
0.18
Fig. 11 – Densification strain for luffa sponge specimens.
0.30
Fig. 12 – Energy dissipation per unit initial volume versus relative density. The square symbols represent the experimental data, the error bars show the derivation of experimental data from their empirical values.
the density range in our experiment can be written as w ¼ 8:65 103 ð0:68r0 1:22 1:62 103 r0 2:22 Þ
ð8Þ
Fig. 12 shows that the energy dissipation capacity per unit volume relatively linearly increases with relative density in the density range tested.
4.
Comparison with other materials
As a biological material, the experimental data in this paper about luffa sponge material may significantly expand the density range of previously studied natural cellular materials (Gibson et al., 2010) with respect to the strength, as shown in Fig. 13. This special feature provides an opportunity to widen
150
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1000
Energy dissipation per unit mass (J/kg)
Strength (MPa)
100
Natural cellular materials such as trabecular bones, palms, and wood Luffa sponge (results from present study)
10 1 0.1 Other biological materials such as tomato, potato, and apple
0.01 10
100
1000
8000
Aporas® aluminium foam Cymate aluminium foam
7000
Polymer foams Luffa sponge from present study
6000 5000 4000 3000 2000 1000 0
Density (kg/m3)
0
50
100
150
200
250
300
Density (kg/m3)
Fig. 13 – Comparison between luffa sponge and other natural materials (Gibson et al., 2010). 1000
Luffa sponge (density: 44 ~ 47 kg/m )
900 Compressive stress (kPa)
the applications of natural cellular materials in general and the luffa sponge in particular, especially when the light weight is a key design requirement. It is worth noting that human beings have been able to discover and develop many materials, notably various metals, metallic alloys, ceramics, plastics, as well as their composites, with performances significantly surpassing biological materials (Bonderer et al., 2008; Zhang et al., 2011). When light weight of the material is considered, various metallic/ ceramic/polymeric foams, honeycombs, and microlattices, are designed and fabricated using those materials as base materials. However, most of these man-made materials are not environmentally friendly and have not been designed with genuine concerns over their long-term sustainability (Zhang et al., 2011). An efficient solution is to find an alternative biological counterpart for those materials in use. For this purpose, a comparative study has been carried out below to show the merit of the luffa sponge material besides its sustainable features. As a cellular material, the luffa sponge is extremely light as shown in Fig. 13. At less than 50 kg/m3, its density is about one fifth of that of a typical aluminium foam (Alporass). For a similar density (25–65 kg/m3), very few materials currently exist such as Cymat aluminium foam (Z70 kg/m3), Ni–P microlattices (Z0.9 kg/m3), silica aerogels (Z1.0 kg/m3), carbon nanotube aerogels (Z4.0 kg/m3), and polymer foams (Z8.0 kg/m3). However, our research results indicate that, when the luffa column is compressed longitudinally, the amount of energy it can absorb per unit mass is comparable to that of the aluminium foam (Idris et al., 2009; Shen et al., 2010) and most of the polymer foams (Avalle et al., 2001), as shown in Fig. 14(a). It should be noted that Alporass aluminium foam has superior specific energy absorption than other metallic foams when their density is lower than 300 kg m3 (Ashby et al., 2000). The good specific energy absorption of luffa sponge is attributed partially to its light base material as well as a higher densification strain. Due to the high strength-to-weight ratio of cellular materials, luffa sponge can be used as a good packaging material and an excellent energy dissipation material. It should be seen that the average plateau stress of aluminium foam is 4–5 times higher than the luffa sponge material. To show the efficiency of the
Nickel microlattice (density: 43 kg/m )
800 700 600 500 400 300 200 100 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Strain
Fig. 14 – Comparison of energy absorption capacity per unit mass between luffa sponge and: (a) other man-made foams; (b) Ni–P microlattice (Schaedler et al., 2011).
microstructures of luffa sponge material, we have also compared the performance of the luffa sponge with other cellular materials with similar density mentioned above. Cymat foam has a strength of 40 kPa at a density of 70 kg m3 (Ashby et al., 2000). Aerogels exhibit a continuous increasing of stress over strain under compression which is not comparable to luffa sponge material. Expended Polystyrene foams have a strength of 200 kPa at a density of 45 kg m3 (Avalle et al., 2001). The polyamide reinforced with modified polyphenylene and polystyrene foam has a strength of 800 kPa at a density of 45 kg m3 (Avalle et al., 2001). As an important example, the comparison with a recently invented metallic microlattice of a similar density (Schaedler et al., 2011) shows that the average plateau stress of the luffa sponge (about 350 kPa) is significantly higher than that of the Ni–P microlattice (about 120 kPa), as shown in Fig. 14(b). It should be noted that the strength decreases with density for any kind of cellular materials. These comparisons indicate that the luffa sponge material has better compressive strength than most of other available cellular material of similar density such as Cymat aluminium foam, expended Polystyrene foams and Ni–P microlattices.
journal of the mechanical behavior of biomedical materials 15 (2012) 141 –152
It should be noted that the luffa sponge material is a cellular material with structural hierarchy as indicated in Fig. 1. As mentioned previously, the luffa sponge column specimen is composed of luffa fibres. According to other research on the microstructures of luffa sponge material (Boynard and D’Almeida, 2000; Ghali et al., 2009; Laranjeira et al., 2006; Paglicawan et al., 2005; Tanobe et al., 2005), the luffa fibres are hollow with many micro-tunnels. Those microstructures have also been proven by our own investigations. Similar to other natural cellular material with similar components, the cell walls of those micro-tunnels may be fibrous themselves and consist of oriented cellulose nanofibrils in a hemicellulose and lignin matrix. Those hierarchical levels required further investigation. However, a rough estimation can be made for contributions of topology at different hierarchical levels to the strength, plateau stress and energy absorption capacities of luffa sponge material using the available material properties from others research (Paglicawan et al., 2005). The density is approximately 353 kg/m3 with a Young’s modulus of 1332 MPa (735%) and tensile strength of 11.1 MPa (785%). The obtained relative plateau stress versus relative density was compared with other polymeric materials (Gibson and Ashby, 1982) as shown in Fig. 15. The experimental data and empirical prediction exhibit a similar trend with that of other polymeric materials. It indicates that the relative strength are not superior to others cellular material if only one level of structural hierarchy is considered. The prediction formulae for other hierarchical levels are not available because there is a lack of the mechanical properties of the base composite materials as well as its specific composite patterns of those base ingredients. It is worth further research effort to understand and bio-mimic the luffa sponge material with superior strength and energy absorption capacities. We also find that the severely crushed luffa sponge column (up to a nominal final compressive strain of 95%) is able to recover to its original length and shape (up to 98%) after it is submerged in water and then dried. We are currently conducting further investigation on this very interesting phenomenon.
151
As mentioned in Section 2.1, the luffa sponge used in our experiments was obtained from pharmacies in Australia which was sold as a bath sponge. There were two treatments in manufacturing the bath sponge from natural luffa fruits which may influence the mechanical properties of luffa sponge, namely, bleaching and lateral squashing. Similar to other cellulose based natural fibres such as cotton, bleach would result in slight loss of mass so as to its mechanical strength of untreated luffa sponge. The lateral squashing would result in damage of the original luffa fibres. The experimental data and empirical formula from our experiment will underestimate the strength of untreated luffa sponge material.
5.
Conclusions
In this study, a series of compressive tests were conducted to examine the stiffness, strength and energy absorption characteristics of the luffa sponge material under quasi-static compressive load. The Young’s modulus, compressive strength, densification strain, plateau stress and energy absorption capacity of the luffa sponge material have been obtained for the first time. A set of empirical formulae have been proposed to predict the mechanical properties of luffa sponge material on the macroscopic scale. It should be noted that there is a limitation of our current work, i.e., the luffa sponge material used in our experiment was bleached and laterally squashed initially. The effect of such pre-treatments on the mechanical properties of the luffa sponge is not available and is under further investigation. From our investigation, the following conclusions can be drawn. (1) The stress–strain curves show a near constant plateau stress over a long strain range, which is ideal for energy absorption application. It could be used as an alternative packaging material. (2) The exponent from our experiments suggests that the deformation of the luffa sponge is determined by the coupling of axial compression/tension and bending of luffa fibres. (3) It is found that the luffa sponge material exhibits remarkable stiffness, strength and energy absorption capabilities that are comparable to those of some metallic cellular materials such as aluminium foams and Ni–P microlattices. The strength of luffa sponge is better than most of other available cellular materials in the similar density range such as expended Polystyrene foams and Ni–P microlattices. As an ultra-light cellular material, it has great potential to be used as an environmentally friendly engineering material.
r e f e r e nc e
Fig. 15 – Comparison of relative plateau stress between luffa sponge and other polymeric materials (Gibson and Ashby, 1982).
Ajiwe, V., Ndukwe, G., Anyadiegwu, I., 2005. Vegetable diesel fuels from Luffa cylindrica oil, its methylester and ester-diesel blends. Chemistry Class Journal 2, 1–4.
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