The elastic modulus E was measured by the slope of the conventional stress-strain curves tak-
ing the distance between grips as the gauge length. To measure the strength of fibres different
gauge lengths were used, in the range 10 – 50 mm; a minimum of 50 filaments was taken for each
gauge length to give data statistical meaning.
2
.
2
.
2
. Single fibre composite SFC
tests A silicon rubber mould was used to make dog-
bone shaped single fibre coupons approximately 60 mm long, 10 mm wide, 1 mm thick. Filaments
were selected as to assure their diameters were similar : 50 mm.
The epoxy resin was a bifunctional bisphenol-A type with an epoxy equivalent of : 195 Epikote
828 by Shell. The hardener was p-amine-dicy- cloexyl-methane, used at the content of 25 by
weight. Resin and hardener were intimately mixed at room temperature and freed from air bubbles
by degassing at 50°C for 10 min in a vacuum oven. The mould containing the filaments was
also equilibrated at 50°C prior to resin pouring. The casts were cured at 70°C for 2 h and post-
cured at 100°C for 3 h.
The coupons were slowly strained in an Instron tensile machine 1185 at the crosshead speed of
0.02 mmmin. The fragmentation of the fibre was observed by means of a microscope attached to
the machine, at magnification 40, both in natural and polarised light. The fragment lengths were
measured by the help of a calibrated eyepiece. Tests were repeated with identical coupons to get
at least 100 fragments for collecting a reasonable number of fragments.
The interfacial strength of HS-Carbon fibres and of E-Glass fibres was measured, for compari-
son, using the same epoxy resin as the matrix.
3. Results and discussion
3
.
1
. Agronomic data Ramie and Spanish Broom crops growth ceased
during the winter months. In spite of its tropical origin Ramie plant resists well the winter low
temperatures-such as the minimum temperatures below 0°C experienced in 1993 and 1995 winter-
thanks to the protection offered to rootstocks by a layer of dead leaves. Both species did not seem
to withstand prolonged waterlogging. Soils with poor drainage and high water retention must be
avoided, but at the same time the soil must have a good water retaining capacity. Spanish Broom
and Ramie are perennials, and the production cycle is up to 20 years Trotter, 1941; Bruno,
1951; Jarman et al., 1978, however this estimates is not supported by experimental evidence.
In Ramie vegetative regrowth generally began at the end of February Table 2 and stems are
sent up from underground rootstocks; during the following months the stems elong rapidly with few
branches. The species shows a good competition toward weeds due to a rapid interrow closure 30
days after regrowing. Stems ranged from 1.2 to 1.6 m high and about 0.8 to 1.2 cm in diameter,
depending on the climatic conditions experienced in the different years. Stems were cut during the
growing season and therefore the rootstocks send up new shoots, and consequently several crops
can be taken from the plant each year. According to Bally 1957, the first crop after planting is very
poor and unsuitable for fibre production. In each year, the time of harvest is very important; if
delayed the stems become lignified and it is difficult to remove the fibre. First-crop Ramie was
usually harvested before flowering June and the second crop during the peak of flowering Sep-
tember. In each case the correct harvesting time was when the stem bases had just turned brown
Iyengar and Bhujang, 1961.
Results demonstrate that Ramie grown in Cen- tral Italy under temperate environment can be
harvested three times a year. The third cutting is usually performed during late autumn and there-
fore is often at risk for the low temperatures and rainy conditions during this period.
During 1993 – 1998 trials, three crops were taken from the plant each year, with the exception
of 1995 and 1998 when the low temperatures of November below the long-period mean value, see
Table 1 andor rain distribution did not allow accomplishment of the last autumn harvest. The
annual total yield of green Ramie plants per hectare, as 1994 – 1998 mean value establishment
Fig. 1. Total dry yield a and stem dry yield b of Ramie and Spanish Broom in the different growing seasons and as overall
mean establishment year not included. Values are the mean of four replications and bars correspond to the mean standard
error.
cymes. The yield of dry Ramie plants per hectare per annum was 1905 g m
− 2
Fig. 1a with a dry stem yield of 1202 g m
− 2
Fig. 1b. The dry Ramie plant was composed by weight of 62 dry
stems and 38 dry leaves and cymes. The final product was obtained mainly from the first and
second harvest, which gave 93 of the yearly production as dry stems Fig. 2.
The crop reaches its full productivity in the year after planting; in the following years Ramie pro-
duction depended on climatic conditions rather than on the age of the crop. Rainfalls during the
summer months strongly affected the yield and higher production were observed during the most
rainy summer seasons such as 1996 735 mm from vegetative regrowth, VR, until third cutting, C3
and 1994 457 mm from VR to C3.
According to Iyengar and Bhujang 1961, Ramie can be harvested in the tropics three or
four times a year when grown under favourable conditions; however, with supplementary irriga-
tion and fertiliser application more than six har- vests a year are possible Dempsey, 1975.
According to Dempsey 1975 the number of crops is reduced to two or three annually as the
climate
becomes more
temperate. Although
Ramie can be harvested more frequently in the tropics yields per crop are lower, annual yields per
hectare in the tropic and in the temperate zones are consequently similar.
The yield dry fibre was almost 3 Dempsey, 1975; Jarman et al., 1978 of the green stems,
giving a total fibre yield of about 200 g m
− 2
as 1994 – 1998 mean value.
In Spanish Broom vegetative regrowth gener- ally began at the beginning of April Table 2.
Plants reached up to 2 m high, with long, slender, leafless or few-leaved, green, rushlike branchlets.
The above ground plants were cut at the end of the growing season Table 2.
This crop performed well after the establish- ment year 1992, reaching in 1993 a total fresh
biomass of about 4700 g m
− 2
corresponding to 2200 g m
− 2
dry yield Fig. 1a. The dry yield was composed of 56 new branches, representing the
economic yield. When established, Spanish Broom can reach a fresh biomass yield about of 4000 g
m
− 2
per annum as 1993 – 1998 mean value, repre-
Fig. 2. Partition of Ramie stem dry yield among first, second and third cutting in the different years of the production cycle.
The establishment year 1993 was not included in the mean value.
year was not included, is about 10 000 g m
− 2
. The fresh Ramie plant was composed, by weight,
of 70 green stems and 30 green leaves and
sented by 53 of long slender terete green branches. The dry yield per annum was about
1800 g m
− 2
Fig. 1a with a dry branchlets yield of 900 g m
− 2
Fig. 1b. The moisture content of branchlets averaged 60, while Ramie stems pre-
sented over 80 of moisture. The useful dry yield to total fresh biomass ratio was higher in Spanish
Broom than in Ramie Fig. 3 and this fact is important for the necessity to storage raw mate-
rial with low moisture content and to maximise the marketable products.
Spanish Broom was disease resistant, exception given by a fungal attack tracheomycosis in the
fourth growing season, that decreased the yield. This species appeared also drought and heat toler-
ant; moreover it is a nitrogen-fixing plant and therefore could be cultivated on marginal lands
due to its low input requirements.
3
.
2
. Morphological and chemical aspects Ramie and Spanish Broom cortical fibres are
multiple elementary fibres ultimates arranged in bundles. In Ramie, the elementary fibres are
bound by gums and pectins Jarman et al., 1978; Batra, 1981, while in Spanish Broom they are
bound together
by lignin
Trotter, 1941;
Fontanelli, 1998. Bundles of ultimates are clearly visible in SEM
micrograph Fig. 4a, b: Spanish Broom con- tained many more ultimates than Ramie. Both
species showed a thick secondary cell wall indicat- ing a high cellulose content, a defined lumen
sometimes collapsed Morton and Hearle, 1993. The diameter of the ultimates varied from 10 to
25 mm in Ramie, while the Spanish Broom ulti- mates ranged from 5 to 10 mm. Ramie ultimate
fibres Fig. 4a are flat and irregular in shape, with a thick cell wall, and taper to rounded ends,
while Spanish Broom fibres Fig. 4b are more regular in shape, as observed also by Trotter
1941. The primary cell wall is often lignified and this aspect is responsible of the low hygroscopicity
of the fibres.
The longitudinal SEM view of the Ramie and Spanish Broom bundles Fig. 5a, b, shows irregu-
larities and defects. In Ramie the analysis revealed cross markings or dislocation characteristics of
flax fibres. The influence of these defects on the fibres mechanical characteristics is discussed later.
The chemical composition of the whole stem Table 3 showed a significant difference between
Fig. 3. Stem dry yield to total fresh yield ratio for Ramie and Spanish Broom in the different growing seasons. Values are the mean of four replications and bars correspond to the mean standard error.
Fig. 4. Cross section micrography of Ramie a and Spanish Broom b fibres.
tallinity reported in Table 3 is given by the ratio to the area of the 002 peak to the intensity of the
amorphous background. The FTIR spectra of bundles are shown in Fig.
7a, b: both fibres showed the same absorption bands with significant differences in intensity and
width at half height. The intensity of signal at 1373 cm
− 1
lignin, related to the signal intensity at 2916 cm
− 1
as the internal standard, was lower in Ramie fibre than in Spanish Broom 10.8 and 17.0,
respectively, in agreement with the chemical re- sults. The signal of the non-cellulosic components
at 1734 cm
− 1
hemicellulose esters and pectic substances appeared slightly more pronounced
for Ramie than for Spanish Broom; due to the prevailing pectic substances in Ramie Jarman et
al., 1978; Batra, 1981.
Another very important difference between the species is the polymerisation degree that was about
3000 in Ramie and about 1700 in Spanish Broom Table 3.
3
.
2
.
1
. Fibre stiffness and strength Both types of fibres were very rigid when loaded
in tension. The load-elongation diagrams were almost linear up to fracture Fig. 8. Occasionally,
irregularities were observed in the curves which were attributed to a failure of some of the
individual fibrils, of which filaments are made up, prior to the final cumulative rupture; and b
internal rearrangement of fibre subunits under the action of the tensile load. The mean elastic moduli
were 65 9 18 GPa and 21.5 9 5 GPa for Ramie and Spanish Broom, respectively. These values
place the examined fibres among the stiffest veg- etable fibres such as, for example, cotton, pineap-
ple and sisal. Although the standard deviation for Ramie was greater than that for Spanish Broom,
the coefficients of variability, i.e. the standard deviation to the mean value ratio, were substan-
tially the same for both fibres. The modulus of Ramie was very high; it approached that of E-glass
fibres : 72 GPa making this fibre very attractive for mechanical purposes. Although the stiffness of
Spanish Broom is 13 than that of Ramie, it is yet greater than that of most rigid non-oriented poly-
mers 1 – 3 GPa making it possible to stiffen commodity plastics such as, e.g. polyolefins.
the two species. Ramie has an higher lignin content and a lower level of pentosans and extractives than
Spanish Broom. More interesting is the difference in the compositional analysis of the two fibres
Table 3. Ramie fibre showed a content of lignin, pentosans and extractives lower than Spanish
Broom. Both fibres had a high content of cellulose, no less than 70. The highest X-ray index of
crystallinity obtained for Ramie, Table 3, confirms this. In fact, while cellulose is a highly crystalline
polymer, hemicelluloses and lignin are amorphous. In the diffractograms in Fig. 6, three peaks are
shown, at : 22° and 14 – 18°, two of them almost superimposed, corresponding to the 002, 101
and 10-1 planes of cellulose. The index of crys-
The brittle behaviour of both fibres allowed their strength to be analysed in terms of Weibull’s
statistics. Due to the intrinsic variability of prop- erties that characterises natural products, the ten-
sile data of vegetable fibres are rather variable. The situation, on the other hand, appears to be
the same with man-made fibres, too. Broad distributions in tensile strength of fibres
is usually attributed to flaws or defects that can be naturally exist or be introduced during handling
or processing or, finally, resulting from surface ageing. It is widely accepted that these defects are
Fig. 5. Longitudinal view of Ramie a and Spanish Broom b fibres obtained by SEM.
Table 3 Composition , degree of polymerisation and cristallinity of Ramie and Spanish Broom dry stems and fibres
Compounds Spanish Broom
Ramie Fibre
Stem Stem
Fibre 0.2
0.7 0.9
0.3 Lignin acid soluble
20.6 Lignin acid insoluble
1.3 13.7
6.2 21.5
Total lignin 1.5
14.4 6.6
7.0 15.3
10.5 9.5
Pentosans Extratives
8.1 1.0
16.2 3.3
3.6 5.1
Ash Degree of polymerisation
3000 1700
X-ray index of cristallinity 58
52
Fig. 6. X-ray diffractogram of Ramie and Spanish Broom fibres.
the main cause of premature failure of the fibre under tensile load Curtin, 1994. Since the occur-
rence of flaws is random in nature, the tensile strength is to be characterised by a statistical
model, the most widely used being the Weibull’s distribution function Weibull, 1951. In the two-
parameter model, the cumulative probability of failure P
n
s, i.e. the fraction of filaments having tensile strength not exceeding s, is given by:
P
n
s = 1 − e
− lsg
a
1 where a and g are the parameters that characterise
the fibre, s is the stress at break and l is the gauge length.
Eq. 1 can be cast as: fP
n
,l = ln[ln1 − P
n
s
− 1
] − ln l = a
ln s − a ln g 2
Fig. 7. FTIR Spectra of Ramie a and Spanish Broom b fibres.
Fig. 7. Continued
so that a plot of fP
n
s, l versus lns is linear; a
and g are thus obtained by the slope and the intercept, respectively.
Fig. 9 gives the plots of fP
n
, l versus lnstress at break, at 50 mm gauge lengths, for both types
of filaments. Similar plots for the other gauge lengths were also drawn.
Once a and g are known, the mean fibre strength, s
m
, at a given gauge length can be calculated by the following equation:
log s
m
l = a
− 1
logl + logg +
log[G1 + aa
− 1
] 3
Where G is the complete Gamma function. A plot of log s
m
versus log l is again expected to be
Fig. 10. Comparison of the influence of gauge length on strength of Spanish Broom, Ramie, carbon and glass fibres
according to Eq. 3.
Fig. 8. Typical stress-strain diagrams for Spanish Broom and Ramie filaments at 10 mm gauge length.
linear. The mean tensile strength at the gauge lengths required in the fragmentation analysis is
experimentally inaccessible and is evaluated by extrapolation of such plots.
Fig. 10 gives the plots of logmean stress ver- sus log gauge length for Ramie and Spanish
Broom fibres. The solid line in each figure repre- sents the regression line. In both cases it is ob-
served that the fibre strength increases with decrease of gauge length. The strength of Ramie
fibres was rather high, in the range 800 – 1000 MPa. These values, that agree with those found
by Ho¨ck 1995, are not far from the strength of E-glass; only flax is reported to be stronger Flem-
min et al., 1995. Spanish Broom fibres are some- how weaker; the strength varied, in fact, from 400
to 700 MPa. No data on this fibre are known in literature for comparison.
Both elastic modulus and strength data are large enough for present fibres to be utilised as
reinforcing means of low stiffness matrices such as non oriented glassy or crystalline polymers typi-
cal strength in the range 20 – 80 MPa, provided the transverse properties are not of primary
importance.
It is interesting to compare the properties of Ramie and Spanish Broom fibres and of the two
most important synthetic types, E-glass and car- bon Fig. 10. As expected, the artificial fibres are
stronger than the vegetable counterparts whose
Fig. 9. Distribution of fibre strength according to Eq. 2.
strength, however, approaches the GPa range. It has to be noted that a comparison made on the
ground of specific properties would appreciably reduce the distance between glass specific gravity
:
2.6 gml and the two natural fibres specific gravity : 1.45 gml. This confirms that both
types of vegetable fibres can stay beside artificial reinforcements,
at least
in non-structural
applications. It is also interesting to note that the slopes of
lines in Fig. 10, are substantially the same for all fibres. This was somehow stunning since one
would expect natural fibres to exhibit a much wider variability and a more pronounced effect of
filament length on fracture stress. All fibres ap- peared to be very similar in this respect, instead.
To be mentioned is however the fact that glass fibres are isotropic in nature, whereas the veg-
etable filaments, due to the alignment of cellulose micro fibrils along the axis, are not. Moreover,
vegetable filaments may be split if high transverse stress are applied. All this would adversely affect
the properties of unidirectional composites but might
have positive
effect on
the impact
behaviour. It is interesting to note that the elongation at
break, o
B
, of both fibres was influenced by the gauge length the same way the strength was Fig.
11. The behaviour of fibres was almost linear so that s
B
: Eo
B
. Since there is no influence of gauge length on E fluctuation of modulus values was
independent of gauge length, it follows that the gauge length dependence of strength has to be
paralleled by that of elongation.
3
.
2
.
2
. Interface strength The chief function of the interface in composite
materials is to transmit stresses from the weak polymer matrix to the high strength fibres. The
stress transfer efficiency depends on the mechani- cal properties of the matrix, the load bearing
capacity of the fibre and the strength of the fibre-matrix interface. A good adhesion is also
required to prevent environmental agents from impairing the interface. In case of lignocellulosic
fibres the degradation caused by water at the interface is of primary concern because the fibres
are highly hygroscopic. The assessment of inter- face soundness is consequently of primary impor-
tance. Out of the several methods devised for characterising the stress transmission capability
across the interface, the most elegant is the single fibre composite SFC test. This technique, first
proposed by Kelly and Tyson 1965, has been widely used to study the interfacial adhesion of
synthetic fibres Fraser et al., 1983; Di Benedetto and Lex, 1989; Di Benedetto, 1991; Curtin, 1994;
Levita et al., 1997 and only recently extended to the natural fibres Van Den Oever and Bos, 1998.
When a single fibre coupon is loaded in tension, fragmentation of the fibre occurs, Fig. 12, pro-
vided the ultimate elongation of the matrix is higher than that of the fibre. Fragmentation con-
tinues until all the segments are shorter than a critical length. Beyond this point, the stress trans-
fer is no longer high enough to cause fibre break- age. The maximum shear stress the interface can
bear t, is given by the following equations:
t = s
c
d 2l
c
= 3s
c
d 8l
m
4 where s
c
is the strength of the fibre at the critical length l
c
, l
m
is the observed mean fragment length and d is the fibre diameter. The results
from SFC tests are collected in Table 4. It is interesting to note that the present data of the
critical length is close to the pull-out fibre length reported by Wollerdorfer and Bader 1998 for
Ramie and other vegetable fibres.
Fig. 11. Influence of gauge length on elongation at break of Spanish Broom and Ramie.
Fig. 12. Multiple fragmentation in a single fibre coupons loaded in tension view in polarised light. Table 4
Diameter d, mean fragment length l
m
, critical length l
c
, fibre strength s
c
and interface strength t for Ramie, Spanish Broom, carbon and glass fibres
a
l
m
mm Fibre type
l
c
mm d mm
s
c
MPa t
MPa 0.35
0.47 Ramie
1480 :
50 79
Spanish Broom :
50 0.32
0.43 1325
77 0.55
0.73 7430
Carbon 36
: 7
0.34 0.46
: 11
4740 Glass
56
a
All fibres embedded in the same resin and tested in identical conditions.
It has to be pointed out that the assumptions on which the SFC method is based upon are the
brittle behaviour of the fibres requirement fully fulfilled by present fibres and that the probability
to find defects along the filaments only depends on the gauge length. The latter assumption is
satisfied in artificial fibres whose diameter is fairly constant. In the case of natural fibres there can be
differences in diameter among filaments. A differ- ence in diameter brings about a change in surface
extension even at constant gauge length that scales with the square of diameter. When compar-
ing strength data pertaining to different gauge lengths, one has either to limit the diameter vari-
ability or to assure that diameters are evenly distributed.
The method usually adopted in the Weibull’s analysis for determining s
c
at fibre lengths of 1 mm, or less, is the extrapolation of the log s
m
versus log l plots. The fragment length distribu- tion, i.e. plots of cumulative probability versus
fragment length, are obtained from the SFC tests. From such plots, l
m
at Pl = 0.5 were obtained and from Eq. 4 the t values were calculated.
A point of weakness of this approach is the fact that the strength of fibres are determined at gauge
lengths in the 10
1
mm order of magnitude and estimated in the 10
− 1
mm order of magnitude. It
is doubtful such a long extrapolation would accu- rately estimate s
c
. Although doubts can be cast on the real meaning of t values, particularly
when, as in the present case, they exceed the yield strength of the matrix, they can be safely used to
compare the interfacial properties of similar com- ponents
Di Benedetto
and Lex,
1989; Di
Benedetto, 1991; Levita et al., 1997. High t val- ues are generally taken as indicative of good
adhesion between resin and fibre. One of the reasons for the efficiency of epoxy
resins as adhesives is the formation of polar groups -OH that strongly interact with high
surface energy solids. A high concentration of -OH groups, on the other hand, characterises the
surface of cellulosic materials so that a strong interface can readily develop. Besides chemistry,
other effects that may contribute to the stress transfer mechanism are: 1 the irregularity of the
surface; and 2 the variation of fibre diameter. The simple stress analysis embodied in equation 4
assumes the fibres to be circular. As shown in Fig. 5a,b, the actual fibres have rather rough surfaces
so that the real extension of the interface is higher than computed assuming a circular cross-section.
The values of t should accordingly be lowered.
A further contribution to the shear strength of the interface comes from the longitudinal fluctua-
tion of diameter because in the region in which the diameter changes the shear component of the
stress acting upon the interface decreases and normal components develop. This provides an
effective locking mechanism since the fibre would be held in place even in the absence of adhesion.
4. Conclusions
