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International Journal of Impact Engineering 57 (2013) 17e26

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International Journal of Impact Engineering

journal homepage: www.elsevier .com/locate/ i j impeng

Behaviour of luffa sponge material under dynamic loading

Jianhu Shen a, Yi Min Xie a,*, Xiaodong Huang a, Shiwei Zhou a, Dong Ruan b

aCentre for Innovative Structures and Materials, School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne 3001, Australiab Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, John Street, Hawthorn, Victoria 3122, Australia

a r t i c l e i n f o

Article history:Received 11 July 2012Received in revised form7 January 2013Accepted 13 January 2013Available online 5 February 2013

Keywords:Luffa spongeEnergy absorptionStrain rateBiological materialSustainability

* Corresponding author. Tel.: þ61 3 9925 3655.E-mail address: mike.xie@rmit.edu.au (Y.M. Xie).

0734-743X/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.ijimpeng.2013.01.004

a b s t r a c t

Luffa sponge is a light-weight natural material which has the potential to be used as an alternativesustainable material for various engineering applications such as packaging, acoustic and vibrationisolation, and impact energy absorption. The strain rate effect is an important material property for suchapplications. In the present study, compressive tests at different strain rates on luffa sponge materialwere conducted over a wide density range from 24 to 64 kg m�3. A photographic technique was appliedto measure the sectional area which has an irregular shape. The stressestrain curves of luffa spongematerial at various strain rates were calculated based on this measurement. When the dynamic data arecompared with those of quasi-static experiments, it is found that the compressive strength, plateaustress and specific energy absorption of the luffa sponge material are all sensitive to the rate of loading. Itis also found that the dynamic enhancement for the compressive strength was more prominent than thatfor the plateau stress. The underlying mechanism was discussed and clarified. Empirical formulae wereproposed for the macroscopic strength, densification strain and specific energy absorption at variousstrain rates. A comparison study shows that the luffa sponge has better energy absorption capacity perunit mass than other cellular materials with similar plateau stress at various strain rates.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Cellular materials are commonly used as packaging and clad-ding materials to protect products and personnel during collisionand impact events [1]. The mechanical characterization of thosecellular materials at varying strain rates is essential for their effi-cient use in dynamic loading applications. As we approach the limitof non-renewable natural resources, there is an increasingly urgentneed to find alternativematerials that fulfil not only themechanicalstability but also some integrated multi-functional properties withlow environmental impact and renewable source of reproduction[2]. The fruits of Luffa Cylindrica (LC) have a netting-like fibrousvascular system. When they are dried, the fibrous network struc-ture serves like an open cell foam material. In a recent research [3],it has been proved that the luffa sponge is potential to be used as analternative sustainable material for various practical applicationssuch as packaging, acoustic and vibration isolation, and impactenergy absorption. The luffa sponge material can be derived fromfruit of the LC plant and has recycling capability and triggeredbiodegradability [4,5]. The importance of the luffa sponge material

All rights reserved.

is growing in our society because of the search for sustainable so-lutions using new materials. In a recent study [3], the authorsdiscovered that under quasi-static loading the luffa spongematerialexhibits remarkable stiffness, strength and energy absorption ca-pacities that are comparable to those of a variety of metallic cellularmaterials.

A limited amount of research has been conducted on the luffasponge as a source of bio-fibres and bio-composites in the last tenyears. The previous research indicated that it is a potential alter-native material for water absorption [6,7], and waste water treat-ment [8,9]. The luffa fibres were also used as reinforcement fibre forother materials [10e14] and cell immobilization for biotechnology[15e18]. At the same time, the sponge gourd of LC, the origin of luffaspongematerial, has not yet had their potential fully explored.Withregard to the industrial and technological development, the cost offuel is on the increase. Oil is extracted from seeds for industrial use[6]. The seed of LC is used as oil sources. The oil extracted from LC isfinding increasing use in the production of biodiesel which is nowgaining acceptance because of low CO2 emission and other con-siderations [19].

However, there is a lack of scientific data on the mechanicalproperties of luffa sponge material because up to now its mainpractical use has been as a body scrub in the bathroom. The luffasponge material and luffa fibres have so far found very limited

J. Shen et al. / International Journal of Impact Engineering 57 (2013) 17e2618

practical applications due to a lack of experimental data. To the bestknowledge of the authors, no experimental investigation has everbeen conducted on the dynamic properties of the luffa spongematerial.

In this paper, dynamic mechanical properties of luffa spongecolumns were tested to investigate performance of this light-weight material. Uniaxial dynamic compressive tests were con-ducted at strain rates of 100 and 102 s�1 by using a High Rate Instronmachine to study the dynamic strength and energy absorption ofthe luffa sponge material. Cylindrical specimens with differentrelative densities were tested at a room temperature of 22 �C anda humidity of around 40%. An energy efficiency method wasadopted to obtain the values of the densification strain and plateaustress, and thus the energy absorption capacity per unit volume. Theexperimental results were discussed together with quasi-static datafrom our previous research [3] and other popular cellular materials.

2. Experiments

2.1. Specimens

The luffa sponge material used in our experiments is froma pharmacy in Australia which is sold as a bath sponge. A brieftreatment procedure for manufacturing these bath sponges fromnatural luffa fruits was provided by the supplier. The luffa fruitswere harvested after they were fully mature with their skin turningbrown. The dried luffa fruits were slightly squashed laterally tocrack and remove the skin. Then the two ends of luffa fruits werecut and the seeds were removed. The original luffa sponges werebleached using liquid chlorine bleach (4%) for about 1 h to improvetheir appearance by making them whiter. After that, they weresoaked in cleanwater for half an hour and then dried in the sun. Thechemical composition of the luffa sponge depends on several fac-tors, such as plant origin, weather condition, soil, pre-treatment,etc. A set of reference values for the chemical composition of LCfoam can be found in a previous research [20].

In the present study, dynamic uniaxial compressive tests wereconducted at two strain rates, namely, 1 and 102 s�1 to obtain theproperties of luffa sponge material under dynamic loading withdifferent strain rates. For each strain rate, about 30 specimens were

Fig. 1. A luffa sponge specimen with a diameter of 64 mm and illustration of its hierarchicafibre; and the photographs at the right present the microstructures of luffa fibres. Those p

tested with different density range from 25 to 65 kg m�3. All thetests were performed at a room temperature of 22 �C and humidityof approximately 40%.

According to the standard of compressive tests and recentresearch [21] on other cellular materials, the specimen size effect isnegligible for foams when the dimension of a specimen is suffi-ciently larger than the cell size, i.e. 7 times of the cell size formetallic foams. Thus the specimen for luffa sponge should be largeenough to eliminate the specimen size effect. The available max-imum size along radial direction for luffa sponge is the wholesection of the luffa sponge column. It was used as the cross sectionof the specimen which ranges from 55 mm to 86 mm in diameter.The height of the specimen was selected as around 50 mm in ourtests. Fig. 1 shows a typical specimen for a dynamic compressivetest. The luffa sponge column is composed of luffa fibres. Thosefibres interconnect with each other and form networks with micro-trusses. Fig. 1 also shows the principal orientation of the luffa fibresvary with the location in the specimen in a regular pattern. On theinner surface, the thickest luffa fibre grows along longitudinal di-rection which is along the dynamic compressive direction. Whileon the outer surface, the thickest luffa fibre grows along circum-ferential direction. In the interlayer between the inner surface andouter surface, the fibre grows in all three directions. In the coreregion, the strongest luffa fibre is along radial direction. A furthermicroscopic view of the cross section of the luffa fibre reveals thatthe luffa sponge is a material with hierarchical architectures asshown in Fig. 1.

A bandsaw was used to cut the specimen from an initial driedluffa bath sponge. After the initial cutting, the specimens weremilled further to make the two cutting surface smooth and parallel.For most of the specimens, the cross sectional area at two ends ofthe specimen are different, thus besides the specimen, two thinslices (about 5 mm thick) were cut from the two ends of the luffasponge to be used as samples to measure the cross sectional area ofa luffa sponge specimen. Because the cross section for most of thespecimens is not perfect circular, a photograph for each slice wastaken and Photoshop was used to get the area of the cross section.Then the sectional area of the specimen is the average of these twoslices. The volume of the luffa sponge specimen is the product ofthis cross sectional area and the height of the specimen.

l microstructures. The photographs in the middle indicate the orientation of the luffahotographs were taken from different specimens.

0.02 0.04 0.06

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Effective region for compressive testsSee Figure 4 (b)

Nor

mal

ised

dat

a

Time (s)

Displacement (u/150 mm) Velocity (v/5 ms-1) Drive (V/2.88 v) Compressive force (F/1.6 kN)

Fig. 3. Illustration of the overall compression process of high strain rate tests on luffasponge specimen (All signals were normalised by an average value for better visualeffect).

J. Shen et al. / International Journal of Impact Engineering 57 (2013) 17e26 19

2.2. Test equipment

The dynamic tests were performed using a High Rate InstronTest System with two different drive methods. A low rate methodwas selected for strain rate around 1 s�1 and a high rate methodwith a drive profile was adopted for the other strain rate, i.e. 102 s�1.A photograph of the dynamic experimental setup is shown inFig. 2(a). The specimen was placed on the bottom compressionplaten which was positioned at a considerable long distance fromthe top platen at the beginning of each test. The specimen wasaccelerated to the required velocity across that distance before thetop surface of the specimen touched the top platen. Double sidedsticky tape was used to retain the specimen on the bottom platenand avoid any possible slippage of the specimen. The surface fric-tion effect is an important factor for most compression test. Tominimize this effect, the double sided glue was only applied to thecentre region of the cross section for most of the specimens exceptfor the very first several trial specimens. Moreover, it should benoted that the friction effect on the uniaxial plateau stress is not sosignificant for crushable cellular materials, because the Poisson’sratio during the compression in plateau region of a specimen isclose to 0.

When the top surface of the specimen touched the top platen,the bottom platen and specimen began to decelerate, whichresulted in a dropping strain rate during the compression process.High Rate Instron Test System provides a feedback mechanism byits FastTrack� VHS 8800 controller to change the drive profile ofthe hydraulic system according to the force history from exper-imental data. Thus a relatively constant loading rate can be ach-ieved during later experiments. The initial drive profile is a constantvalue. It can be increased according to the force history obtained bythe load cell to obtain the new drive profile. The consistency of thestrain rate during compression of the specimen can be improvedfurther by iterating the procedure several times. However, due tothe variation of the compressive force of luffa sponge specimenscaused by variation of density, little improvement was achievedafter practically two iterations. Thus drive profiles obtained fromspecimens with density around 40 kgm�3 after two iterations wereselected for all the rest tests at the strain rate of 102 s�1 as shown inFig. 3(a). The effect of drive profiles after iteration on force dis-placement curves was similar to that for aluminium foams in ourprevious study [22].

Fig. 2. Experimental setup for dynamic compressive tests on luffa sponge material (thespecimens shown are only for the test group with a strain rate of 1 s�1).

A constant speed of 5 m s�1 with a sampling rate of 500 kHz wasused for dynamic compression tests at a strain rate of 102 s�1. Theload history was measured by a Kistler load cell 9071Awithout datafilter. The displacement history was measured by an inherent LVDTwith data filter by setting cut off frequency of 1000 Hz.

The displacement and load curves were obtained and theaverage compressive force, densification strain and energy ab-sorption were worked out accordingly. The humidity during thetest was monitored by a humidity meter as shown in Fig. 2. Thespecimens were placed in the test room for at least 2 h before thecompressive test to eliminate the difference in temperature andhumidity. The temperature in the test room was from 20 to 28 �C,but the temperature variation during each test was less 1 �C. Thehumidity was between 30% and 60%, but during each test its var-iation was less 1%.

3. Experimental results

3.1. Deformation features

Force and displacement histories were obtained from the Ins-tron machine (VHS 8800). A typical force displacement curves areshown in Fig. 4(a). Similar to other cellular materials, the overallcompressive behaviour can be characterised by a steep elastic re-gion up to compressive strength, a plateau collapse region up todensification, and a densification region with a sharp increase offorce over displacement. Despite of trivial differences, the curvealso shows clearly a fairly constant compressive force over a longstroke, which represents an ideal energy absorption feature.Nominal stress (defined as force over original cross sectional area)and nominal strain (defined as displacement over original thick-ness of the luffa sponge specimen) were calculated and shown inFig. 4(b) for a different specimen in Fig. 4(a). The deformationpatterns at high strain rate of 102 s�1 are shown in Fig. 5. Thisconfirms that the overall compression of the specimen is axial(along the longitudinal direction) rather than folding of the wall ofthe specimen under dynamic loading. Localised crushing band wasobserved in plateau collapse region which is similar to the quasi-static compressive behaviour of luffa sponge material.

The deformation was uniform in the elastic range before itreached compressive strength. Localised crushbandswere observedin plateau collapse region, and the initial strain for localised

(a) a typical force-displacement curve with three regions for luffa sponge material.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0

10

20

30

40

50

60

Energy efficiency (%)

Com

pres

sive

stre

ss (M

Pa)

Strain

Compressive stress (MPa)

Dynamic compressive strength0.67 MPa

Plateau stress0.52 MPa

Energy efficiency

Densification strain0.578Energy absorption (J/m3)

Energy absorption

(b) the illustration of using energy efficiency method to determine compressive strength andcalculate plateau stress, densification strain and energy absorption capacity.

Peak force to calculate compressive strength

Plateau force to calculate plateau stress

Fig. 4. Dynamic compressive history data for luffa sponge material.

J. Shen et al. / International Journal of Impact Engineering 57 (2013) 17e2620

deformation varied with different specimens. The strain remainsuniform until the strain reached about 0.2 as shown in Fig. 5. Thenlocalise deformation occurred in the weakest layer of the specimen.As the crushing proceeded, further deformation still occurred in thislayer but with other layers collapsing simultaneously which formedseveral crushing bands. Sometimes, the latter collapsing bandsdeveloped at a higher degree of distortion than the previous ones.Deformation kinematics as well as the high strain rate effect due tolocalizeddeformation resulted in subsequent hardening so as to lockthe collapse band before densification, which allowed other bandsto collapse sequentially as well.

3.2. Dynamic compressive strength

The stressestrain curves were further calculated and a typicalstressestrain curve is shown in Fig. 4(b) together with the illus-tration of energy efficiencymethod to calculate densification strain,

plateau stress and energy absorption capacity. The dynamic com-pressive strength of the luffa sponge material is taken to be theinitial peak stress if it exists. If there is no such peak stress, thestress at the intersection of two slopes is taken to be the com-pressive strength, namely, the slope for the initial loading and thatfor the stress plateau region. In general, the compressive strength ofthe luffa specimen increases with the density. However, for similardensity, the strength varies greatly as well. This is expected,because other research [3] indicated that the component materialof luffa sponge varied with the weather, maturity, soil etc. Fora traditional rate sensitive material, the strain rate effect is inde-pendent on the density of the material. In similar research formetallic foams [22], it was found that the strength enhancementratio with respect to strain rate exhibited a power law relationship.This relationship is difficult to verify for luffa sponge material dueto the scatter in its mechanical properties. Thus the following for-mula similar to that for metal foams [22] will be adopted for the

Fig. 5. Deformation patterns of luffa sponge under dynamic impact (V0 ¼ 5.0 m s�1, L ¼ 52.0 mm; High Speed Cameral settings: frame rate: 10,000 frame per second, shutter speed:1/50,000, resolution: 512 � 512 pixels).

J. Shen et al. / International Journal of Impact Engineering 57 (2013) 17e26 21

compressive strength, plateau stress, and energy absorption of luffasponge material.

s0sf

¼ A�1þ Cε

$P� r0

rf

!B

(1)

where, sf is the yield stress of the base material, rf is the density ofthe base material, s0 and r0 are the compressive strength anddensity of the luffa sponge, _ε is the strain rate, C and P are twoconstants determined by the strain rate effect of cellular material, Aand B are two constants determined by the topology and failurepatterns of cellularmaterial. However, as a natural cellularmaterial,luffa sponge has hierarchical microstructures. As mentioned pre-viously, the base solid material of luffa sponge remained unknownat the present due to the lack of scientific data on the mechanical

properties of luffa sponge material. The luffa sponge columnspecimen in our experiment is composed of luffa fibres. Our tensiletest results on luffa fibres indicated that the density and tensilestrength of luffa fibres varied with their orientation in a luffasponge. For the same orientation at the similar location, they alsovaried with different specimens. It cannot be used as an effectivebase material at one hierarchical level. Thus the empirical formulaeare given in the following format:

s0 ¼ A�1þ Cε

$P�rB0 (2)

The unknown base material properties will be included in thecoefficients A. Coefficients C, B and P are non-dimensional valuesand are determined by the microstructures and strain rate effect ofthe luffa sponge material. By fitting the quasi-static data, the

J. Shen et al. / International Journal of Im22

coefficient B can be obtained as 1.28. Then, we fix B and use theexperimental data obtained from strain rate 10�3, 1 and 102 s�1 todetermine the other three different coefficients: A, C and P, whichare 1946.8, 0.43 and 0.15 respectively.

The calculated values of the term inside the first set of brackets,1þ 0:43 _ε0:15, are from 1.15 to 1.86 corresponding to strain ratesfrom 10�3e102 s�1. The dynamic enhancement for the compressivestrength is more prominent than other cellular materials. Fig. 6shows a favourable comparison between the compressivestrength from the empirical formula and corresponding exper-imental data, for luffa sponge with different relative densities andat three strain rates.

3.3. Densification strain, plateau stress and energy absorptioncapacity using energy efficiency method

The stress level for energy absorption against impact is one of thekey parameters in designing the protective structures. It is normallydefined as the average stress over the plateau region. The com-pressive strength, s0, corresponds to the collapse of the weakestlayer of cells which is crushed first in compression and it is under-estimated the stress level as indicated in Fig. 4(a). Or it is the initialdynamic buckling load due to the orientation of luffa fibre alongloading directionwhichmay overestimate the stress level as shownin Fig. 4(b). Hence, the compressive strength is not applicable torepresent the stress level for energy dissipation of luffa spongematerial in compression. The plateau stress, spl, obtained using anenergy efficiency method [23] is more appropriate and accurate tobe used as the stress level during energy dissipation process.

The energy dissipation efficiency, Ed, of the luffa sponge ata particular strain, εa, is defined as

Ed�εa� ¼

Zεa0

sðεÞdε

sa; 0 � εa � 1 (3)

where sa is the stress at εa. The densification strain, εd, is defined asthe maximumvalue of εi which satisfies the condition of maximumefficiency,

dEdðεaÞdε

����εa¼εi

¼ 0; 0 � εi � 1 (4)

20 25 30 35 40 45 50 55 60 65 700.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80 Strain rate =102 s-1

Strain rate =100 s-1

Strain rate =10-3 s-1

Eq. (2) (102 s-1) Eq. (2) (100 s-1) Eq. (2) (10-3 s-1)

Com

pres

sive

stre

ngth

(MPa

)

Density (kg/m3)

Fig. 6. Strain rate effect on the dynamic compressive strength of luffa sponge material.

Then, the plateau stress spl is calculated as

spl ¼

Zεd0

sðεÞdε

εd(5)

The densification strain corresponds phenomenally to the startpoint from which the stress starts rising sharply and physically tothe end of compression of all large voids in the material. From thispoint onwards, the luffa sponge can still dissipate energy by plasticdeformation, but its dissipation efficiency will start to decrease asshown in Fig. 4(b). Fig. 7 presents the trend of densification strainwith respect to increasing strain rate. In general, the densificationstrain decreases slightly with strain rate. Compared with the effectof density, this drop is negligible and will be ignored in our currentanalysis.

Based on the present experimental data, the densification strainis linearly related to the density of the luffa sponge. Hence

εd ¼ 0:68� 1:6� 10�3r0 (6)

Following the similar argument for compressive strength, theempirical formula for plateau stress is obtained as follows.

spl ¼ 2328�1þ 0:375 _ε0:14

�r1:220 (7)

The calculated values of the term inside the bracket,1þ 0:375 _ε0:08, are from 1.14 to 1.71 corresponding to strain ratesfrom 10�3e102 s�1. The dynamic enhancement for the compressivestrength is more prominent than the plateau stress. Fig. 8 showsa favourable comparison between the plateau stress from empiricalformula and corresponding experimental data, for luffa spongewith different relative densities and at several strain rates.

As shown in Fig. 2, the load cell was fixed on the top platen.Therefore, the measured force included the inertial force (the forcerequired to stop the specimen from moving at 5 ms�1) of thespecimen. The average stress enhancement in the plateau collapseregion caused by the inertia of specimen can be estimated using thetheorem of momentum as

Ft ¼ mv0F ¼ mtv ¼ r0AL

Lεdv

v ¼ Ar0v

2

εdand

siA ¼ F ; thus; si ¼r0v

2

εd

(8)

pact Engineering 57 (2013) 17e26

20 30 40 50 60 700.0

0.1

0.2

0.3

0.4

0.5

0.6

Strain rate = 102 s-1

Strain rate = 100 s-1

Strain rate = 10-3 s-1

Trend line for 102 s-1

Trend line for 100 s-1

Trend line for 10-3 s-1

Den

sific

atio

n st

rain

Density (kg/m3)

Fig. 7. The strain rate effect on the densification strain of luffa sponge material.

20 25 30 35 40 45 50 55 60 650.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65 Strain rate=10-3 s-1

Strain rate=100 s-1

Strain rate=102 s-1

Eq. (7) (10-3 s-1) Eq. (7) (100 s-1) Eq. (7) (102 s-1)

Plat

eau

stre

ss (M

Pa)

Density (kg/m-3)

Fig. 8. Strain rate effect on the plateau stress of luffa sponge material.

J. Shen et al. / International Journal of Impact Engineering 57 (2013) 17e26 23

where si is the nominal stress enhancement caused by the inertiaeffect, A is the cross sectional area, L is the initial length, r0 is thedensity of luffa sponge specimen, v is the compression velocity, t isthe duration of the inertial force, and εd is the densification strain.The expression is identical to that obtained from 1D structuralshock model which was put forward by Reid et al. [24] and furtherdeveloped by Harrigan et al. for cellular materials [25]. It should benoted that the deformation pattern of the luffa sponge specimensdoes not satisfy the requirements for 1D structural shock model inthe current loading velocity, i.e. 5 m s�1. For example, there is noobvious shock front and the crush process of the sponge is notsequential from the impact end etc. However, this estimation givesan upper bound for the effect of inertial force on the plateau stressof cellularmaterial under compression. Consider two extreme caseswith maximum inertial force for our tests, i.e. r0 ¼ 60 kg m�3,εd ¼ 0.6, V0 ¼ 5 m s�1 and r0 ¼ 20 kg m�3, εd ¼ 0.6, V0 ¼ 5 m s�1 thevalue of stress si is only 0.8% and 0.59% of the measured staticplateau stress, 0.424 MPa and 0.104 MPa respectively. Thus thisinertia effect is small and can be neglected in our analysis for dif-ferent strain rates.

The energy dissipation capacity of luffa sponge material can becharacterised by the energy absorption per unit initial volume, w,during the compression process before densification occurs. This

20 30 40 50 60 700.05

0.10

0.15

0.20

0.25

0.30

0.35 Strain rate = 102 s-1

Strain rate = 100 s-1

Strain rate = 10-3 s-1

Ener

gy a

bsor

ptio

n pe

r uni

t vol

ume

(MJ/

m3 )

Density (kg/m3)

Fig. 9. Strain rate effect on the energy absorption capacity of luffa sponge material.

value is equal to the area under the stressestrain curve. Fig. 9 showsthat the energy dissipation capacity per unit volume increasesfavourably with the strain rate. It indicates that luffa sponge ma-terial will dissipate more energy under dynamic loading. Thus theenergy dissipation capacity for the range of strain rates consideredcan be written as,

w ¼ 1583�1þ 0:375_ε0:14

��r0 � 2:35� 10�3r2:220

�(9)

3.4. Stress enhancement over strain rate

The possible mechanisms for strain rate effect of luffa spongematerial are entrapped gas in the luffa fibre, viscosity of the basecomposite material of luffa sponge, collapse pattern change of luffafibres, initial inertia of the specimen, and inertia effect of luffa fibres(type II structure [26e28]). The inertia effect of luffa fibres refers tothe effect that the fibres along impact direction will undergo moreaxial deformation under dynamic loading than that under quasi-static loading because of the constraint provided by the lateralinertia of those fibres and their surrounding fibres. Similar to othercellular materials, the first three mechanisms will contribute to thestrain rate effect of plateau stress of the luffa sponge. However,because the complexity of topology of luffa sponge and dimen-sional variation of luffa fibres, it is very difficult to quantify thecontribution of air flow, viscosity of the base material of luffasponge, collapse pattern change of luffa fibres to the stressenhancement over strain rate. As for the last two mechanisms,there was evidence from our experimental results to prove theirexistence.

It is observed from Figs. 6 and 8 with corresponding empiricalformulae that the stress enhancement caused by strain rate effectfor the compressive strength is more than that for the plateau stressfor all specimens. For a similar average value (trend line) for com-pressive strength and plateau stress at quasi-static loading rate,a direct comparison of compressive strength and plateau stress athigh strain rate in Fig. 10(a) confirms this trend because the all datafor compressive strength is higher than the data for the plateaustress. It is also found that strength enhancement is more promi-nent for specimens whose stressestrain curves exhibit larger initialpeak behaviour as shown in Fig. 10(b). In the case of stressestraincurves shown in Fig. 10(b), the compressive strength is taken asthe first peak stress of the stressestrain curves. As the strain in-creases, the stress difference between quasi-static and dynamicloading becomes smaller towards a fixed value determined by thedifference of plateau stresses under two loading conditions. Onlythe last two mechanisms will cause these phenomena. By changingthe duration, t, for inertial force in Eq. (8) from the overall defor-mation time, t ¼ Lεd/v, to that during which the stress increasesfrom zero to first peak stress (compressive strength), the initialinertia enhancement on stress history can be estimated by addingthis inertial force to the plateau stress as is shown by the modifiedplateau stress in Fig. 10(a). The duration, t, was measured directlyfrom corresponding stress-history data recorded in the experi-ments. It can be seen that this initial inertia effect plays an insig-nificant role in the compressive strength enhancement. To illustratethis calculation, the specimen in Fig. 4(a) was taken as an example.The corresponding deformation at the first peak load is about2 mm. Thus the duration is t ¼ 0.002/5 ¼ 4 � 10�4 s. The inertialforce to stop the specimenwith amass of 10.5 g from 5m s�1 to 0 insuch a short period can be calculated as Fi ¼mv/t¼ 10.5�10�3 � 5/4 � 10�4 ¼ 131 N. Therefore this effect only results in a slightincrease of the peak force, 2500 N.

From the stressestrain curves in Fig. 10(b), the stress enhance-ment pattern is similar to that of a typical type II structure studied by

20 30 40 50 60 700.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Compressive strength Modified plateau stress (MPS) Plateau stress (PS) Linear fit of strength Linear fit of MPS Linear fit of PS

Com

pres

sive

Stre

ss (M

Pa)

Density (kg/m )

(a) more prominent stress enhancement for strength than plateau;

0.0 0.2 0.4 0.6 0.80.0

0.2

0.4

0.6

0.8

Plateau stress 0.266 MPa

Com

pres

sive

stre

ss (M

Pa)

Strain

Density = 41.6 kg·m , strain rate = 10 s Density = 42.4 kg·m , strain rate = 10 s

Plateau stress 0.447 MPa

(b) stress-strain curves for dynamic and quasi-static tests of luffa sponge with type II microstructures

Fig. 10. Comparison of the stress enhancement features over strain rate.

J. Shen et al. / International Journal of Impact Engineering 57 (2013) 17e2624

Calladine [26] and Su et al. [27,28]. It can be seen from Fig. 1 that theorientations of the fibres vary with their locations on the luffasponge specimen and the majority of the fibres on the inner surfaceof the specimen are along the loading direction. We believe thatthose fibres contribute more to the larger enhancement in thecompressive strength than that in the plateau stress for the luffasponge material. A special experiment was conducted in which thefinal crushing length of specimen was controlled to a strain of 30%using an aluminium tube as a stopper. After that, the specimenwascut to review the deformation of longitudinal fibres on the innersurface of the specimen as shown in Fig. 11(a). It can be seen clearlythat some of the fibres deform outwards, which is similar to thedeformation mode of a half of the type II structure shown inFig. 11(b). Other fibres serve as an additional mass along the longi-tudinal fibres. The extra energy dissipated in the dynamic loadingprocess is caused by the axial plastic deformation of the fibres alongthe loading direction rather than by the bending of fibres in thequasi-static loading process. The length scale of fibres with type IIbehaviour varies with the imperfection in the alignment of theselongitudinal fibres. Fig. 11(a) also shows the bending length is muchsmaller on the other side of the luffa sponge specimen. Furthermore,the photographic examinations performed on the inner surface ofluffa sponge specimens with various densities shows that themaximum fibremisalignments are highly scattered. Apart from thisimperfection atmacroscopic level, individualfibrewalls as shown inFig. 1 possess further imperfections at smaller length scales that aredifficult to quantify such as the waviness or non-uniform thickness

distribution along the fibre walls. Thus it will be very difficult toquantify the strength enhancement theoretically.

4. Comparison and discussion

As a cellular material, the luffa sponge is extremely light as hasbeen shown in our previous paper [3]. It is also at the lowest regionin the density range of natural biological cellular material cellularmaterials previously studied [1]. However, our research resultsindicate that, when the luffa column is compressed longitudinallyat a high strain rate, the amount of energy it can absorb per unitmass is higher than many commonly used metallic foams [21,22]and the polymer foams at similar strain rates and plateaustresses, as shown in Fig. 12. It even exceeds the upper bond pre-dicted by axial crushing of tube structures [29]. The excellentspecific energy absorption of luffa sponge is attributed partially toits light base material and microstructures resulting in a higherdensification strain. Due to the high strength-to-weight ratio ofcellular materials and favourable enhanced energy absorption ca-pacity at high strain rate, the luffa sponge can be used as a goodpackaging material and an excellent energy dissipation material.This special feature provides an opportunity to widen the appli-cations of natural cellular materials in general and the luffa spongein particular, especially when light weight is a key design require-ment. At the same time, when the density of the cellular materialsdecrease to a certain range, other interesting functional propertieswill emerge such as good damping and reversible properties [30].Luffa sponge also exhibits several interesting functional featuressuch as water healing, recovery, and shape memory properties. Weare currently conducting further investigation on these veryimportant properties.

It is also worth noting that human beings have been able todiscover and develop many materials, notably various metals,metallic alloys, ceramics, plastics, as well as their composites, withperformances significantly surpassing biological materials [31,32].When light weight of the material is considered, various metallic/ceramic/polymeric foams, honeycombs, and microlattices, aredesigned and fabricated using those materials as base materials.However, most of these man-made materials are not environ-mentally friendly and have not been designed with genuine con-cerns over their long-term sustainability [32]. One possible solutionis to find an alternative biological counterpart for those materials inuse if the sustainability is a dominant factor to be considered. Ourresults confirm luffa sponge is such an alternative material. Thesustainability of the farming and preparation of luffa spongesshould be investigated further before this material becomesa widely used commodity.

However, there are several limitations of the luffa sponge ma-terial. The luffa sponge material is not an isotropic material. Themechanical properties at the lateral direction should be lower thanthat at longitudinal direction. The energy absorption capacity perunit mass of luffa sponge is lower than that of aluminium honey-combs and balsa woods which are highly anisotropic as shown inFig. 12. However there is no big block of luffa sponge available toeliminate the possible size effect to obtain representative com-pressive properties along lateral direction. An approach has beendeveloped to manufacture large block of luffa spongematerial fromavailable luffa sponge columns. The anisotropic properties of luffasponge material will be further investigated and the results will bepublished in a separate paper.

It should be noted that the empirical formulae in this paperare presented in a dimensional format which is a limitation of ourresearch. This is due to the lack of the mechanical properties ofthe base composite materials as well as its specific compositepatterns of those base ingredients. The data and corresponding

Fig. 11. Mechanism of the stress enhancement over strain rate for luffa sponge material.

J. Shen et al. / International Journal of Impact Engineering 57 (2013) 17e26 25

prediction formulae for other hierarchical levels are not available.As mentioned previously, the luffa sponge material is a cellularmaterial with structural hierarchy as shown in Fig. 1. The luffasponge column specimen is composed of luffa fibres. According to

Fig. 12. Comparison with other cellular materials. (Strain rate for luffa sponge: 102 s�1;Expanded polypropylene [34]: 60 s�1; Balsa wood [35]: 3000 s�1; Alporas foam [22]:10�3 to 102 s�1; Honeycombs [36]: þ for 10�3 s�1 and * for 102 s�1; others are quasi-static data [29]. The data are marked by its density in kg m�3).

other research on the microstructures of luffa sponge material[10e14], the luffa fibres are hollow with many micro-tunnels.Those microstructures have also observed in our own in-vestigations. Similar to other natural cellular material with sim-ilar components, the cell walls of those micro-tunnels may befibrous themselves and consist of oriented cellulose nanofibrils ina hemicellulose and lignin matrix. Those hierarchical levelsrequired further investigation.

As mentioned in Section 2.1, the luffa sponge used in our ex-periments was obtained from pharmacies in Australia which wassold as a bath sponge. There were two treatments in manufacturingthe bath sponge from natural luffa fruits which may influence themechanical properties of luffa sponge, namely, bleaching and lat-eral squashing. Similar to other cellulose based natural fibres suchas cotton [33], bleach would result in slight loss of mass so as to itsmechanical strength of untreated luffa sponge. The lateralsquashing would result in damage of the original luffa fibres. Theexperimental data and empirical formula from our experiment willunderestimate the strength of untreated luffa sponge material. Thedetailed influence of those pre-treatments will be investigatedfurther in our future work.

5. Conclusions

The strain rate effect of the luffa sponge material was inves-tigated in this paper by conducting experiments at different strainrates on cylindrical specimens of the luffa spongewith awide range

J. Shen et al. / International Journal of Impact Engineering 57 (2013) 17e2626

of density. The mechanical properties of the luffa sponge materialat various strain rates were obtained at the macroscopic level. Thefollowing conclusions can be drawn from our investigations.

(1) Compressive strength, plateau stress and specific energy ab-sorption of luffa sponge material are all sensitive to the rate ofloading;

(2) Empirical formulae are obtained for compressive strength,densification strain and specific energy absorption at variousstrain rates at the macroscopic level;

(3) The dynamic enhancement for compressive strength is moreprominent than that for the plateau stress. The underlyingmechanism can be explained by the type II structure responseof luffa fibres;

(4) The energy absorption capacity per unit mass at high strainrates of luffa sponge is higher than that ofmany commonly usedmetallic foams;

(5) Luffa sponge has the potential to be used as an alternativesustainable material for various engineering applications suchas packaging, acoustic and vibration isolation, and impact en-ergy absorption.

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