Effects of Water Absorption on Mechanical Propertiesof Hemp Fiber Composites

Asim ShahzadMaterials Research Centre, School of Engineering, Swansea University, Swansea SA2 8PP, UK

Effects of water absorption on mechanical propertiesof hemp fiber composites have been studied. Finite dif-ference method modeling of water absorption in thesecomposites showed the diffusion to be Fickian in na-ture. The residual tensile and flexural propertiesshowed immediate decline following immersion inwater, the loss in modulus being more pronouncedthan the loss in strength. The fracture of tensile testedcomposites following immersion in water was found tobe more ductile than brittle, which showed the evi-dence of shear stresses being more significant in theseconditions. For impact damaged composites immersedin water, most of the degradation in residual tensileproperties occurred within first 100 h of immersion inwater at same impact energy level and further immer-sion in water for up to 400 h did not result in any fur-ther degradation. No deterioration in fatigue strengthand fatigue sensitivity of composites following immer-sion in water was observed. POLYM. COMPOS., 33:120–128, 2012. ª 2011 Society of Plastics Engineers

INTRODUCTION

One of the main advantages composite materials have

over conventional materials is their excellent corrosion re-

sistance. However, exposure to different environments can

affect the properties of composite materials. Ambient

moisture, water, chemicals, sunlight, heat, and radiation

all cause changes in the microstructure and the chemical

composition of polymer matrix composite materials.

These changes in turn cause changes in properties such as

strength, modulus, impact, and fatigue.

Natural fibers are increasingly replacing synthetic

fibers in composite materials. Natural fiber composites are

more susceptible to environmental attack than synthetic

fiber composites because of various reasons. Natural fibers

are susceptible to environmental degradation because of

their structure, which consists of cellulose, hemicellulose,

lignin, and pectin. Biodegradation by microorganisms

occurs because they hydrolyze the carbohydrate polymers,

especially hemicelluloses, into digestible units [1]. Photo-

chemical degradation by ultraviolet (UV) light occurs pri-

marily in lignin which is also responsible for color

change. Cellulose is less susceptible to UV degradation.

Hemicellulose and cellulose are degraded by heat much

before the lignin.

Natural fibers are hydrophilic and readily absorb mois-

ture. It can cause fibers to swell and ultimately rot

through attack by fungi. As soon as the natural fibers are

exposed to moisture, hydrogen bonds are formed between

the hydroxyl groups (��CH2OH) of the cellulose mole-

cules and water. Water uptake in the cell walls causes

fibers to swell until the forces of water absorption are

counterbalanced by the cohesive forces of the cell walls.

The swelling of fibers is found to be directional with the

maximum swelling occurring in the lateral direction and

minimum in the longitudinal direction. The amorphous

part of native cellulose, with all hydroxyl groups accessi-

ble to water, contributes more to swelling than the crys-

talline part, where only the surfaces are available for

water sorption [2]. Therefore, increasing the crystallinity

of the fiber reduces the swelling capacity. The hydrophilic

hemicellulose also contributes greatly to fiber swelling.

Reducing the amount of hemicelluloses also reduces the

swelling capacity of fibers. Thus, the swelling of fibers as

a result of moisture absorption can affect the dimensional

stability of composites and the shear stresses thus gener-

ated can weaken the interfacial bonding of composites.

These can have adverse effects on the mechanical proper-

ties of composites. A possible solution is to improve

fiber–matrix interface by using compatibilizers and adhe-

sion promoters. With better adhesion, the moisture sensi-

tivity is usually reduced. Also, surface treatments of fibers

with silanes can make the fibers more hydrophobic.

Jimenez and Bismarck [3] exposed different natural

fiber to 100% relative humidity and determined the equi-

librium moisture uptake. Cornhusk, hemp, and sisal fibers

had equilibrium moisture uptake of 33% whereas abaca,

flax, luffa, henequen, lechuguilla, and lyocell fibers had

equilibrium moisture uptake of 23–30%.

The moisture absorption in natural fiber reinforced

composites is found to increase with increase in fiber con-

tent. This has been found to be true for the following

Correspondence to: Asim Shahzad; e-mail: mr_asim_shahzad@yahoo.com

DOI 10.1002/pc.21254

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2011 Society of Plastics Engineers

POLYMER COMPOSITES—-2012

composites: jute-epoxy [4], jute-polyester [5], hemp-poly-

ester [6], pineapple-polyester [7], banana-polyester [8],

woodflour-polyester [9], ramie/cotton-polyester [10], sisal-

epoxy [11], sisal-polypropylene [12], rice husk-polypro-

pylene [13], pineapple-polyethylene [14], cellulose-poly-

propylene [15], and rice hull-HDPE [16].

Increasing the water temperature accelerates the water

sorption but does not affect the saturation uptake. This

has been shown to be true for sisal-polypropylene compo-

sites for water temperatures of 508C and 708C [12].

The purpose of this study was to investigate the effect

of water immersion on the mechanical properties of hemp

fiber reinforced polyester composites. These composites

were immersed in water for varying lengths of time and

the effects of immersion on their mechanical properties

were observed.

EXPERIMENTAL WORK

Laminates were made from non-woven short hemp

fiber mats in unsaturated polyester resin, using hand lay-

up and compression molding techniques. The mean fiber

weight fraction of laminates was 56%. Samples of nomi-

nal dimensions of 125 3 20 3 2.5 mm3 were immersed

in distilled water. The water absorption of composites fol-

lowing immersion in water was determined as per BS EN

ISO 62:1999, Plastics-determination of water absorption.

A total of 57 samples were used for this purpose. Before

immersion, the samples were first conditioned to a con-

stant mass at 238C and 50% relative humidity (RH). Sam-

ples were taken out of the container after pre-determined

intervals of time, wiped with clean cloth and weighed in

a balance having an accuracy of 0.1 mg. The samples

were weighed within one minute of taking out of the con-

tainer. The samples were tested for their mechanical prop-

erties by using the standard methods.

Static tensile properties of the composite samples were

determined by using Hounsfield testing machine. A total

of 34 samples were used for this testing. All tensile tests

were performed as per BS EN ISO 527: 1996, Plastics-

determination of tensile properties, parts 1 and 4. Fatigue

testing was carried out as per BS ISO 13003:2003, fiber-

reinforced plastics – determination of fatigue properties

under cyclic loading conditions. A total of 20 samples

were used for this testing. Fatigue tests were performed

using pneumatic fatigue machines at a frequency of 1 Hz

and a stress ratio (R value) of 0.1 for tension–tension fa-

tigue. The choice of frequency ensured that the heating

effect due to hysteresis was minimal. The R value of 0.1

in tension–tension was chosen to maximize the cyclic

effects without invoking the complications of compressive

stresses and the likely variations in failure mechanisms.

The maximum stresses during cyclic loading were

recorded as stress level of fatigue. Tests were carried out

until either complete failure of the specimen occurred or

until 106 cycles, the latter being designated the upper

boundary of the low cycle fatigue regime. The number of

cycles to failure was recorded for each specimen and

these data were plotted in the form of S–N curves.

Low velocity impact tests were performed in the pur-

pose-designed and assembled falling weight impact rig.

All the testing was carried out as per BS EN ISO 6603-1:

2000, Plastics – determination of puncture impact behav-

ior of rigid plastics; part1: non-instrumented impact test-

ing. A total of 40 samples were used for this testing. Af-

ter setting-up the impact rig, the sample was tightly

clamped between two steel rings of internal diameter 18

mm by four nuts. By adding steel weights on the impac-

tor, the weight of the impactor was adjusted to obtain the

required levels of impact energy. For most of the tests the

height of the impactor was kept constant at 0.1 m and

value of the weight was changed according to required

impact energy. The head of the impactor was a hemi-

spherical steel nose with a diameter of 12.7 mm. The

impactor was held and released using an electromagnet.

The impactor was captured after impact to prevent sec-

ondary strikes. The flexural properties were determined in

three point bending test as per BS EN ISO 14125:1998,

fiber-reinforced plastic composites-determination of flex-

ural properties.

RESULTS AND DISCUSSION

Water Absorption

Water absorption behavior of composites over 3700 h

of immersion in water is shown in Fig. 1. As anticipated

the water absorption is quite rapid initially, reaching

almost 15% after about 400 h of immersion. Further

immersion increases the weight gain to about 16% after

3700 h of immersion but the error bars lie within the error

bars of weight gain after 400 h of immersion. This sug-

gests that the samples have reached their equilibrium

water intake after about 400 h of immersion.

Compared to natural fiber composites, the saturation

water intake for CSM glass fiber reinforced polyester

composites has been reported to be 0.6% at fiber weight

FIG. 1. Water absorption behavior of hemp fiber reinforced polyester

composites.

DOI 10.1002/pc POLYMER COMPOSITES—-2012 121

fraction of 44% [17] and 3.5% at fiber weight fraction of

65% [18].

The diffusion of a liquid into another material is com-

monly described by the well-known Fick’s law [19],

@C

@t¼ D

@2C

@x2ð1Þ

where C is the concentration of the liquid, D is diffusion

coefficient of the material absorbing the liquid, t is time,

and x is a distance term.

For Fickian diffusion, mass uptake versus time is

expressed as the following equation for thin plate samples

like the one used in this research [19],

Mt ¼ M116tD1=2

ph2

� �1=2ð2Þ

where Mt is the mass uptake at time t, M1 is the equilib-

rium mass uptake, and h is the sample thickness. The

mass uptake is therefore proportional to square root of

time. The moisture absorption curve should be linear up

to and exceeding about 60% of the equilibrium moisture

content and this fact is used to determine whether Fickian

behavior is observed or not [20].

To correlate the experimental behavior of water uptake

of this material with the theoretical behavior, computer

modeling by 3D Finite Difference Method based on Fick-

ian Diffusion [21] was used and compared with the exper-

imental results. The results are shown in Fig. 2. The curve

based on experimental results is in agreement with the

condition about the linear portion of the graph. The

curves also show that absorption of water in this material

is in excellent agreement with theoretical behavior pre-

dicted by the model. The water absorption behavior is

thus demonstrated to be Fickian in this material.

Apart from determining whether the moisture absorp-

tion behavior is Fickian or not, the moisture absorption

graph is a very useful tool in determining various absorp-

tion parameters of the material. These are diffusion

coefficient D, sorption coefficient S, and permeability

coefficient P.

The value of diffusion coefficient D is obtained from

the slope of weight gain versus square root of time curve,

and is given by,

D ¼ pkh

4M1

� �2ð3Þ

where k is the slope of the linear portion of the Mt versus

t1/2 curve, h is the sample thickness, 2.5 mm in this case,

and M1 is the maximum weight gain due to water absorp-

tion. The slope of the linear portion of the curve was cal-

culated to be 0.11 g/h1/2. The value of M1 was 2.18 g.

Using these values in Eq. 3 above, the value of the diffu-

sion coefficient was calculated to be 8.7 3 10213 m2/s.

The sorption coefficient or solubility S is defined as

S ¼ M1=Mi ð4Þwhere M1 is the weight of water taken up at equilibrium

and Mi is the initial weight of the sample. The values of

S for individual samples were calculated and their average

value was found to be 0.16.

Similarly permeability coefficient P, which represents

the combined effects of sorption and diffusion, is defined

as

P ¼ DS ð5Þwhere D is diffusion coefficient and S is sorption coeffi-

cient. Using the values of D and S given above in Eq. 5,the value of P was calculated to be 1.32 3 10213 m2/s.

The values of absorption parameters determined for

hemp-polyester composites in this research are compara-

ble to the ones reported in literature for natural fiber com-

posites [7].

Tensile Properties

The effects of water immersion on the normalized ten-

sile properties of the composites are shown in Fig. 3. The

normalized values were obtained by dividing the tensile

FIG. 2. Experimental and theoretical comparison of water uptake

behavior in hemp fiber composites.

FIG. 3. Effects of water immersion on the normalized tensile properties

of the composites.

122 POLYMER COMPOSITES—-2012 DOI 10.1002/pc

properties following immersion in water by the properties

of dry composites. The loss in strength is only about 10%

of the original strength following immersion for 400 h

whereas the loss in modulus is about 60% of the original

modulus following same time of immersion. However, the

loss in strength continues at further immersion in water

and following immersion for 3700 h, the loss in strength

is about 35% of the original strength and the loss in mod-

ulus is about 60% of the original modulus. The decline in

tensile modulus is greater compared to decline in tensile

strength which is expected because of the weakening of

the fiber/matrix interface and the plasticization of the ma-

trix and fiber. It has been shown [22] that moisture

absorption in natural fibers results in greater reduction in

modulus than strength, and the composites made of these

fibers will also be expected to show similar behavior.

The comparison of stress–strain graphs of one of the

typical tensile tested sample after 3700 h in water with

that of dry sample is shown in Fig. 4. The graph shows

the decrease in modulus of the composites and the corre-

sponding increase in the strain to failure following immer-

sion in water.

One significant effect of immersion in water was the

change of mode of fracture of these composites from brit-

tle to more ductile failure even after about 50 h of immer-

sion. The dry samples exhibited completely brittle frac-

ture. For samples immersed in water for 100 h and above,

the mode of failure was almost entirely ductile. This tran-

sition from brittle to more ductile fracture is also shown

in Figs. 5 and 6. It was observed that for most samples

that failed in ductile mode, the fracture plane was inclined

at an angle of typically about 6458 to the tensile axis as

shown by the arrow in Fig. 5b. The plasticization of poly-

ester resin and hemp fibers, the swelling of fibers and

weakening of fiber/matrix interface seem to have contrib-

uted to this transition from brittle to ductile fracture. The

shear stresses induced because of swelling of fibers seem

to have resulted in debonding and sliding of matrix and

fibers resulting in ductile failure. The shear stress in any

material in uniaxial tension is maximum at 458 plane and

this is the plane at which these samples tended to fail.

This suggests that as against dry samples where normal

stresses are the dominant cause of fracture, shear stresses

also become dominant following immersion in water and

contribute significantly to the fracture of samples.

Figure 7a and b shows SEM micrographs of fracture

surfaces of the tensile tested samples after 3700 h of

immersion in water. These samples surfaces had to be

dried before their SEMs could be taken. The dried surfa-

ces still showed the damage that immersion in water had

done to the composites. Figure 7a shows the normal

modes of failure like matrix fracture and fiber fracture are

present and there is evidence of increased fiber/matrix

debonding because of the weakening of the interfacial

bonding, as shown by arrowhead in Fig. 7b.

Impact Properties

The effect of water immersion on impact damage toler-

ance of hemp fiber composites was evaluated. Since it

was not possible to do the impact testing with the samples

immersed in water, the samples were first subjected to

impact testing and then immersed in water for up to 400

FIG. 4. Comparison of stress–strain graphs of composites immersed in

water and dry sample.

FIG. 5. Fracture modes of tensile tested hemp fiber composites. (a) Brittle fracture of dry samples and (b) ductile fracture of samples following

immersion in water for 3700 h. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—-2012 123

h, which is the average time these composites took to

reach their saturation level.

The residual tensile properties of the impact-tested

composites following immersion in water are shown in

Figs. 8–10. The comparison of residual tensile strength of

impacted samples following immersion in water for up to

400 h is shown in Fig. 8. Immersion in water for non-

impacted samples resulted in 10% reduction in tensile

strength and 50% reduction in tensile modulus following

immersion in water for 100 h. Immersion in water of

samples following impact for the same time period results

in further degradation of residual tensile properties. The

residual strength of 2J impacted samples is reduced by

30% and that of 3J impacted samples is reduced by 60%

compared to non-impacted samples, following the same

immersion time of 100 h. As shown in Fig. 1, these com-

posites attain their saturation water uptake following

immersion in water for 400 h and Fig. 2 shows that most

of the degradation in tensile properties takes place by this

time. However, Fig. 8 shows that immersion in water for

up to 400 h does not result in further degradation in resid-

ual strength compared to immersion in water for 100 h.

More importantly the graph shows that the composites do

not suffer any appreciable deterioration in strength follow-

ing impact and immersion in water for the same impact

energy level.

The comparison of residual tensile stiffness of

impacted samples following immersion in water for up to

400 h is shown in Fig. 9. The graph shows similar deteri-

oration in residual stiffness of composites following

immersion in water to strength. The residual stiffness of

2J impacted samples is reduced by 30% and that of 3J

impacted samples is reduced by 60% compared to non-

impacted samples following immersion in water for

100 h. Just like strength, the stiffness does not suffer

much degradation following immersion in water for up to

400 h.

Figure 10 shows the strain to failure of impacted sam-

ples following immersion in water for up to 400 h. The

strain to failure of all the samples increases following

immersion in water. However most of the increase occurs

within fist 100 h of immersion and any further immersion

does not result in any difference in strain to failure.

These results show that poor impact damage tolerance

of this material is further degraded when they are

immersed in water. However, at same impact energy

level, most of the degradation occurs within first 100 h of

immersion in water and further immersion in water for up

FIG. 6. Sectional views of the samples (a) and (b) shown in Fig. 5. [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

FIG. 7. SEM micrographs of fracture surfaces of the tensile tested sample following 3700 h immersion in water.

124 POLYMER COMPOSITES—-2012 DOI 10.1002/pc

to 400 h does not result in any further degradation. Com-

pared to non-impacted and non-immersed composites, the

composites lost almost 62% of their original strength and

almost 77% of their original stiffness following impact at

3J and immersion in water for 400 h.

Fatigue Properties

The study of fatigue properties in water is important

because of widespread use of composite materials in

aqueous environments where they can be exposed to con-

siderable fatigue loading. Generally the synthetic fiber

composites have been found to show loss of fatigue

strength when immersed in water [23–27]. The reduction

in strength has been attributed to ingress of moisture in

cracks and plasticization of polymer matrix, which

reduces the stiffness of composites.

For fatigue testing in water, a specially designed water

chamber was used. The samples were almost completely

immersed in water throughout the fatigue testing. Two

different kinds of testing regimes were used. In the first,

the composites were immersed in water straightaway

without any pre-conditioning. In the second, the compos-

ite were first immersed in water for an average of 400 h

to allow the water to ingress the composites. As shown in

Fig. 1, the water absorption in these composites is close

to their equilibrium saturation level after about 400 h of

immersion in water.

The comparison of S–N curves of dry and wet compo-

sites is shown in Fig. 11. The S–N curves of the non-con-

ditioned and pre-conditioned wet composites have similar

slopes as dry composites, suggesting good fatigue proper-

ties of these composites in water. The deterioration in ten-

sile properties because of immersion in water does not

seem to have any adverse effect on their fatigue perform-

ance. The pre-conditioned composites start having lower

tensile strength than dry composites because of immersion

in water for 400 h, they still show similar decline in fa-

tigue strength with increase in fatigue cycles as those of

dry composites or the composites immersed in water

without pre-conditioning. The endurance limit also

remains unaffected at 20 MPa for dry as well as wet com-

posites, as shown by the arrowheads. These results seem

to suggest that the fatigue strength of these composites is

not affected by immersion in water. Fatigue is a fiber

stiffness sensitive property and the unaffected fatigue

FIG. 8. Comparison of residual tensile strength of impacted samples

following immersion in water for up to 400 h. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 9. Comparison of residual tensile stiffness of impacted samples

following immersion in water for up to 400 h. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 10. Comparison of strain to failure of impacted samples following

immersion in water for up to 400 h. [Color figure can be viewed in the

online issue, which is available at wileyonlinelibrary.com.]

FIG. 11. Comparison of S–N curves of dry and wet composites. [Color

figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—-2012 125

properties of these composites points at very good fatigue

properties of hemp fibers in water.

One possible reason for good fatigue resistance of

these composites in water could be the closure of micro-

cracks due to swelling of fibers developing in different

parts of the composite because of fatigue loading. Natural

fibers can swell considerably more than synthetic fibers

and can help to close these microcracks. However this as-

pect needs to be explored further.

Figure 12 presents an SEM micrograph of the fracture

surface of a sample fractured after 861,000 cycles at ten-

sile stress of 18 MPa that was pre-immersed in water for

400 days. The figure shows that pre-immersion in water

for 400 h and subsequent fatigue loading in water for

about 240 h seems to have had little effect on fiber–ma-

trix interfacial bonding. The fibers seem to be sticking to

the matrix quite well. This structural integrity of the com-

posites in water is also expected to be one of the reasons

for their good fatigue performance in water.

These studies have shown good fatigue performance of

hemp fiber composites in water despite the reduction in

their tensile properties. The comparable fatigue properties

of these composites to synthetic fiber composites in water

can have useful implications for the use of these materials

in aqueous media.

Flexural Properties

The flexural properties of the composites in water were

determined by immersing them in water for varying

lengths of time and evaluating their flexural properties

using three point bending test. The effects of water

immersion on normalized flexural properties are shown in

Fig. 13. Similar to tensile behavior, these composites rap-

idly lost about 25% of their intrinsic flexural strength af-

ter 50 h immersion in water. However, this reduction

soon stabilized. Prolonged immersion of almost 2000 h of

exposure, the composites lost about 30% of their original

flexural strength.

Similar to reduction in tensile modulus, immersion in

water seems to have had more adverse effect on flexural

modulus than flexural strength of these composites. The

decline in flexural modulus was again quite rapid after 50

h immersion in water and the composites lost almost 65%

of their intrinsic flexural modulus. However, this loss in

flexural modulus stabilized and no further appreciable

decline in flexural modulus was observed after immersion

for about 2000 h in water.

Composites with Sealed Edges

The samples used in the study of water absorption

behavior had cut edges that resulted in direct exposure of

fibers and the fiber/matrix interface to water. To deter-

mine the effect of preventing the ingress of water through

cut edges, composite samples with their cut edges sealed

with water-resistant silicone sealant were immersed in

water and their water absorption and tensile properties

were investigated.

Water Absorption. The water absorption behavior of

the composites with sealed edges compared to samples

with cut edges is shown in Fig. 14. There was only mar-

ginal difference in the absorption behavior of composite

with sealed and non-sealed edges. The water absorption in

composites with sealed edges was slightly lower but within

the error bars for non-sealed edges which shows that the

difference was not significant. This is not unexpected

because the surface of area of cut edges was only about

12% of the total surface area of the samples. The maxi-

mum difference at any point in water absorption is only

about 1.5%. However even this difference was observed to

be decreasing with increase in water immersion time.

Tensile Properties. The effect of water immersion on

tensile strength of composites with sealed edges compared

FIG. 12. SEM micrograph of fracture surface of fatigue tested sample

pre-immersed in water.

FIG. 13. Effects of water immersion on normalized flexural properties

of composites.

126 POLYMER COMPOSITES—-2012 DOI 10.1002/pc

with that of non-sealed edges is shown in Fig. 15. It is

quite clear that the reduction in water absorption immedi-

ately after immersion due to sealed edges has no appreci-

able positive effect on tensile strength of these compo-

sites. The composites with non-sealed edges have slightly

higher tensile strength than those with sealed edges for

same immersion time in water, but the difference is

within error bars of the two, suggesting that the difference

is not significant.

The comparison of the effect of water immersion on

tensile modulus of composites with sealed- and non-

sealed edges is shown in Fig. 16. Again composites with

sealed edges are observed to have similar reduction in

their tensile modulus to composites with non-sealed edges

for same immersion time. The values of tensile modulus

are within error bars at same water immersion times, indi-

cating that the difference is not significant.

It was thus observed that sealing the cut edges of the

samples for the same immersion time does not lead to

any significant reduction in water absorption of these

composites. Therefore the reduction in tensile properties

of composites with sealed and non-sealed edges following

immersion in water is quite similar. It has been shown by

Ellis and Found [27] for CSM glass fiber/polyester com-

posites that sealing the sample edges does not have signif-

icant effect on water absorption or tensile properties of

the laminates. Hemp fiber composites are shown to

behave similarly to glass fiber composites in this respect.

Therefore, immersing the hemp fiber composite samples

with exposed cut edges in water can give a good indica-

tion of their overall properties in water.

CONCLUSIONS

Studies on effects of water immersion on water uptake

and mechanical properties of hemp fiber composites have

been performed. Hemp fiber composites showed a satura-

tion water uptake of about 16% following immersion in

water for 3700 h. Tensile properties of hemp fiber compo-

sites showed immediate decline following immersion in

distilled water, the loss in modulus being more pro-

nounced than the loss in strength. However, the loss in

properties stabilized after prolonged immersion. Following

immersion in water for 3700 h, the composites lost almost

35% of original strength and 60% of original modulus.

Plasticization of polyester matrix and hemp fiber and loss

in stiffness of hemp fibers following immersion in water

were considered to be the major factors.

Immersion in distilled water resulted in degradation of

impact damage tolerance of hemp fiber composites. Com-

pared to non-impacted and non-immersed composites, the

composites lost almost 62% of their intrinsic strength and

almost 77% of their intrinsic stiffness following impact at

3J and immersion in water for 400 h.

Hemp fiber composites showed no deterioration in fa-

tigue properties following immersion in water. Compo-

sites fatigue tested in water without conditioning

and those pre-conditioned in water for 15 days and then

fatigue tested showed similar fatigue strength to dry

composites.

FIG. 14. Comparison of water absorption behavior of composites with

sealed and non-sealed edges.

FIG. 15. Comparison of effect of water absorption on tensile strength

of composites with sealed edges and non-sealed edges.

FIG. 16. Comparison of effect of water absorption on tensile modulus

of composites with sealed and non-sealed edges.

DOI 10.1002/pc POLYMER COMPOSITES—-2012 127

Similar to the decline in tensile properties, hemp fiber

composites showed immediate decline in flexural proper-

ties, the decline in modulus being more pronounced than

decline in strength, following immersion in water which

later stabilized for prolonged immersion times. The com-

posites lost almost 30% of their intrinsic strength and

65% of their intrinsic modulus following immersion in

water for 2000 h.

Composites immersed in water with their cut edges

sealed did not show any appreciable difference in terms

of water absorption and reduction in tensile properties

compared to composites immersed in water with cut

edges.

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128 POLYMER COMPOSITES—-2012 DOI 10.1002/pc