Microstructure and Mechanical Properties of Aligned Natural Fibre Composites

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Microstructure and Mechanical Properties of Aligned Natural Fibre Composites
Rask, Morten
Publication date:
2013
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Rask, M. (2013). Microstructure and Mechanical Properties of Aligned Natural Fibre Composites. Technical
University of Denmark.

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Microstructure and Mechanical
Properties of Aligned Natural Fibre
Composites
Morten Rask
September 2012
Ph.D. Thesis
DTU Wind Energy PhD-0009 (EN)
Cover image: Left: A small test specimen used for synchrotron X-ray tomographic
microscopy to visualize damage mechanisms during loading. The specimen is made
from a flax fibre yarn/polypropylene composite. The red box identifies the scanning
area. Right: Reconstructed 3D visualization of the specimen. Flax fibres (5-10
times thinner than a human hair) and flax fibre yarns are identifiable.
Department of Wind Energy
Section of Composites and Materials Mechanics
Technical University of Denmark
Risø Campus
Building 228, DK-4000 Roskilde, Denmark
Phone [phone], Fax [phone]
www.vindenergi.dtu.dk
DTU Wind Energy PhD-0009 (EN)
ISBN 978-87-92896-11-7
Summary
Recently, there has been a great interest in developing and maturing natural
fibre composites for structural applications. Natural fibres derived from plants
such as flax and hemp have the potential to compete with traditional glass
fibres as reinforcements in polymer matrices, due to good specific properties
(stiffness-to-density ratio). The perspective of using natural fibres is to have a
sustainable, biodegradable, CO2-neutral alternative to glass fibres.
However, so far, it has not been possible to take full advantage of the natural
fibre properties when using them for composite applications. Several challenges
havetobeaddressedandsolved,manyofwhichpertaintothefactthatthefibres
are sourced from a natural resource: 1) Inconsistent properties, depending on
plantspecies, growthandharvestconditions, andfibreextractiontechniques. 2)
Strength values of composites are lower than expected based on tests of single
fibres. 3) Compared to continuous glass fibres, natural fibres are relatively
short, which makes it difficult to achieve an optimized fibre architecture. 4)
Naturalfibresarehydrophilic,meaningthattheydonotbondwellwithstandard
polymer matrix systems, most of which are hydrophobic.
The present ph.d. thesis is primarily concerned with challenges 2 (unexpected
low strength of composites) and 3 (optimization of fibre architecture).
Reasons for the lower than expected strength of natural fibre composites are
investigated by performing X-ray tomographic microscopy during tensile tests
of small composite specimens. With this technique, 3D images can be obtained
with spatial resolution < 1 µm. By studying the 3D microstructure of the
composite specimens at a number of arrested load steps, a number of damage
ii Summary
mechanisms have been identified: (i) Interface splitting cracks typically seen
at the interfaces of bundles of unseparated fibres, (ii) matrix shear cracks, and
(iii) fibre failures typically seen at fibre defects. The three damage mechanisms
initiated at about 50, 75 and 90% of the failure stress, respectively.
After harvesting the plants, the fibre bundles in the plants are extracted, and
separated into individual fibres. If this separation is not complete, bundles
consisting of 5-15 fibres will remain among the fibres. Important insight was
gained on the significance of avoiding bundles of unseparated fibres. It was
found that such bundles are likely to result in fibre/matrix debonding cracks,
which can lead to ultimate failure by large splitting cracks. Also, the fibre
bundles were observed to have a tendency to fail across the entire cross section
of the bundle. This will lead to a large stress concentration, which can result in
specimen failure.
Since individual natural fibres are relatively short (50-70mm), they are tradi-
tionally spun into fibre yarns in order to be able to handle the fibres. However,
spinningthefibreseffectivelyequatestointroducingalargeamountoffibremis-
alignment, which decreases the composite stiffness properties. Through devel-
opment of a model based on the geometry of a yarn with fibre twisting and yarn
helicity, the relation between fibre misalignment and composite tensile stiffness
was examined. The model incorporates a ±-stiffening effect, similar to what is
used in laminate theory. Experimental studies were performed with composites
fabricated from yarns with different amounts of fibre twist and yarn helicity.
By fitting the proposed model to the experimental data, good agreement was
obtained. Fromthemodelpredictions, itwasfoundthatyarnhelicityisactually
more detrimental than fibre twisting with regards to composite stiffness.
Finally, studiesareperformedonthefracturetoughnessofnaturalfibrecompos-
ites. Initially, a novel approach is proposed for calculating the fracture tough-
ness from data obtained from double cantilever beam tests. The developed
approach is based on determination of the curvatures of the beams during the
tests and it is not necessary to have any knowledge of the layup sequence, or
stiffness and thickness of individual layers. This is especially beneficial for com-
plicated/unknown beam layups. It was proposed that the beam curvatures are
determined using strain gauges. After developing the approach, it was used to
determine the fracture toughness of flax/PLA (polymer based on lactic acid)
specimens made from yarns with different twisting angles. It was found that a
high twisting angle greatly decreases the fracture toughness of the composite,
such that specimens made with yarns with no fibre twisting were more than 10
times tougher than specimens with a high degree of twisting.
Thus, based on the work in the present ph.d. thesis, it is found that achieving
a method for separating the fibres completely without damaging them, is im-
iii
portant for optimizing the composite strength. Furthermore, it is found that
achieving a good fibre alignment is important for both the composite stiffness
and the composite fracture toughness. These suggestions for manufacturers of
natural fibre composites, are presented with an overall purpose of contributing
to optimizing natural fibre composites for load-bearing usage.
iv
Resum´e
Over den seneste tid, har der være stor interesse i at udvikle og modne natur-
fiberkompositter til strukturelle anvendelser. Naturfibre fra planter s˚asom hør
og hamp har potentiale til at konkurrere med traditionelle glasfibre som for-
stærkningipolymermatricer,grundetdegodespecifikkeegenskaber(stivhed-til-
densitet forhold). Perspektivet i at anvende naturfibre er at have et vedvarende,
bionedbrydeligt, CO2-neutralt alternativ til glasfibre.
Hidtil har det dog ikke været muligt at drage fuldt udbytte af egenskaberne
af naturfibre, n˚ar de anvendes til kompositter. En række udfordringer skal
adresseres og løse, hvoraf mange er relaterede til det faktum at naturfibre ud-
vindes af en naturlig ressource: 1) Inkonsistente egenskaber, der afhænger af
planteart, vækst- og høstforhold, og fiberekstraktionsteknik. 2) Styrkeværdier
afkomposittererlavereendhvadforventesbaseretp˚aenkeltfiberforsøg. 3)Sam-
menlignet med kontinuerlige glasfibre er naturfibre relativt korte, hvilket gør
det vanskeligt at opn˚a en optimeret fiberarkitektur. 4) Naturfibre er hydrofile,
hvilketbetyder,atdeikkebindergodtmedalmindeligepolymermatrixsystemer,
hvoraf de fleste er hydrofobe.
Denneph.d. afhandlingomhandlerprimærtudfordring2(uventetlavkomposit-
styrke) og 3 (optimering af fiberarkitektur).
˚Arsager til at naturfiberkompositter har lavere styrke end forventet undersøges
ved at udføre R¨ontgen tomografisk mikroskopi under træktest af sm˚a komposi-
teemner. Ved denne teknik kan man opn˚a 3D billeder med en rumlig opløsning
< 1 µm. Ved at studere 3D mikrostrukturen af kompositemner ved en række
belastningstrin, identificeres en række skadesmekanismer: (i) Grænseflade split-
revner, som typisk ses ved grænsefladerne af bundter af ikke-adskilte fibre, (ii)
vi Resum´e
matrix forskydningsrevner, og (iii) fiberbrud, som typisk ses ved fiberdefekter.
De tre nævnte skadesmekanismer begynder ved henholdsvis 50, 75 og 90% af
brudlasten.
Efter høst af planterne, bliver fiberbundterne udvundet og adskilt i individuelle
fibre. Hvis denne adskillelse ikke er komplet, vil bundter best˚aende af 5-15 fibre
findes blandt fibrene. Vigtig indsigt opn˚as om betydningen af at undg˚a bundter
af ikke-adskilte fibre. Det blev konstateret, at s˚adanne bundter er tilbøjelige til
at resultere i fiber/matrix grænsefladerevner, som kan føre til emnesvigt p.g.a.
store splitrevner. Det blev desuden observeret at fiberbundterne har en tendens
til at svigte p˚a tværs af hele tværsnittet af bundtet. Dette vil medføre en stor
spændingskoncentration, hvilket kan resultere i emnesvigt.
Eftersom individuelle naturfibre er forholdsvis korte (50-70mm), bliver de tra-
ditionelt spundet til fibergarner, med henblik p˚a at kunne h˚andtere fibrene.
Imidlertid svarer spinding af fibrene faktisk til at indføre en stor mængde mis-
orientering, hvilket nedsætter stivhedsegenskaberne af kompositterne. Gennem
udvikling af en model baseret p˚a geometrien af et garn med spundne fibre og
garnhelicitet, undersøges forholdet mellem fibermisorientering og trækstivhe-
den af kompositter. Modellen indeholder en ±-afstivningseffekt, svarende til
hvad der anvendes i laminatteori. Eksperimentelle undersøgelser udføres med
kompositter fremstillet af garner med forskellige mængder af fibersnoning og
garnhelicitet. Ved at fitte den foresl˚aede model til de eksperimentelle data,
blev god overensstemmelse opn˚aet. Ud fra modellens forudsigelser blev det kon-
stateret, at garnhelicitet faktisk er mere skadelig end fibersnoning med hensyn
til kompositstivhed.
Endelig udføres undersøgelser af brudsejheden af naturfiberkompositter. Først
udvikles en ny fremgangsm˚ade til beregning af brudsejhed fra data opn˚aet ved
s˚akaldte double cantilever beam forsøg. Fremgangsm˚aden er baseret p˚a bestem-
melse af krumninger af bjælkerne under test, og det er ikke nødvendigt at
have kendskab til oplægsekvensen, eller stivhed og tykkelse af de individuelle
lag. Dette er specielt fordelagtigt for komplicerede/ukendte bjælkeoplæg. Det
foresl˚as, at krumningerne af bjælkerne bestemmes ved hjælp af tøjningsfølere
(strain gauges). Efter at have udviklet fremgangsm˚aden, blev den anvendt til
at bestemme brudsejhed af hør/PLA (polymer baseret p˚a mælkesyre) emner
fremstillet af garner med forskellige snoningsvinkler. Det konstateres, at en høj
snoningvinkel i høj grad nedsætter brudsejhed af materialet, eksempelvis blev
det fundet at emner fremstillet med garner uden fibersnoning var mere end 10
gange sejere end emner med en høj grad af fibersnoning.
Baseret p˚a arbejdet udført i denne ph.d. afhandling, er det s˚aledes blevet kon-
stateret, at udvikling af en fremgangsm˚ade til fuldstændig adskillelse af fibrene
uden at beskadige dem, er vigtigt for at optimere kompositstyrken. Endvidere
vii
blev det fundet, at en god fiber ensretning er vigtig for b˚ade stivhed og brud-
sejhed af kompositten. Disse forslag til producenter af naturfiberkompositter er
fremsat med det overordnede form˚al at bidrage til optimering af naturfiberkom-
positter til lastbærende anvendelser.