Kaiserslautern - Fachbereich Maschinenbau und Verfahrenstechnik
Refine
Year of publication
- 2005 (17) (remove)
Document Type
- Doctoral Thesis (17)
Has Fulltext
- yes (17)
Keywords
- Flüssig-Flüssig-Extraktion (2)
- Rotordynamik (2)
- Stoffübergang (2)
- Abgasnachbehandlung (1)
- Ammoniumcarbamat (1)
- Armierung (1)
- Axialschub (1)
- Diffusionskoeffizient (1)
- Diffusionsmessung (1)
- Diffusionsmodell (1)
Faculty / Organisational entity
Carbon-fibre reinforced plastics have been widely used in the aerospace industry as
materials for structural components. During recent years, the focus has been on
preform/RTM materials with the aim of improving material properties and reducing
costs. Harnessing the full potential of these materials requires a model for assessing
the properties and in particular long-term behaviour. Such a model needs to take into
account the special conditions of these materials. Basic failure mechanisms have to
be analysed in order to develop this kind of model.
Consequently, the aim of the work was to investigate the fatigue phenomenon in
preform-CFRP materials with thermoset matrices on a microstructural level. The
influence of the dynamic loading and the temperature on the emerging fracture
phenomena should be identified. Based on the results, a common fracture mechanism
should be found. The failure should be described on a mesoscopic level so that
it is not restricted on the fatigue failure at a single crack front.
To achieve this aim, different preform materials with EP matrix (some of which had
been subjected to impact) were loaded with dynamic compression load and high
frequent alternate bending. The fatigue behaviour of the matrix systems was investigated
by CT tests.
By means of microfractography, the only method for detecting fatigue failure as such,
the failure mechanisms were analysed at submicroscopic level. The results showed
correlations between microstructure and failure.
It became apparent that what in the technical literature has been given as an explanation
for the appearance of the fatigue striations in the scanning electron microscope
had to be corrected. As undercuts are not reflected in the SEM as dark striations,
the appearance of the striations must be based on different inclinations of the
local fractured surface to the primary electron beam.
On the basis of this result the shape and the formation of the fatigue striations could
be shown in resin pockets and fibre imprints. Fatigue striations have a shape which
sticks out from the fracture plane, preferably in the form of steps.
There was no proof for an influence of the high frequent load on the formation of
fatigue striations. However, it was possible to find lamellar fracture phenomena which
have not been described in the technical literature yet. Due to their shape and their occurrence these can be understood rather as a sign of a dynamic load then as a
fracture phenomenon of a high frequent cyclic loading.
The examinations of the high frequent loaded samples, where temperatures up to
120°C occurred, as well as in the CT tests with elevated temperatures (60% Tg)
yielded no proof that the temperature has an influence on the mechanical failure
behaviour. However, the formation of the fatigue striations in high frequent loaded
specimens leads to the deduction that adiabatic heating exists at the crack tip which
leads to large plastic deformations because the glass transition temperature is exceeded
locally.
The microfractographic investigations showed that the fatigue striations appear as
separate static fractures. On account of their shape and in relation to the matching
fracture surfaces plastic processes can be held responsible for the formation of the
striations. Altogether this leads to a modification of the models for the origin of fatigue
striations prevalent in the technical literature. The suggested model associates the
real fracture growth under fatigue loading only with a small part of the loading cycle.
Crack propagation only occurs when the maximum stress intensity is reached in the
area of the upper loading of the cycle. Microplastic processes by molecular rearrangement
in the stress field ahead of the crack tip lead to the blunting of the crack
tip, which is reflected as fatigue striations on the fracture surface. Simultaneously, the
cyclic loading causes damages in the molecular network of the thermoset. This leads
to the possibility of fracture formation below the static stress at break.
On the basis of the model and of fatigue crack growth diagrams it is possible to
establish thresholds for the stress intensity necessary for crack propagation under
cyclic load. The upper threshold of the stress intensity corresponds to KC, because it
marks the transition to unstable crack growth. The lower threshold is determined by
the value of the cyclic stress intensity factor where crack growth has just ceased to
be ascertainable.
With the existing model of local crack growth under fatigue loading and the results of
the chronological course of failure from the microfractographic investigations of the
different materials it was possible to detect a general failure mechanism for the
preform-CFRP materials.
When an external alternating load is applied, an inhomogeneous stress field forms in
the composite material. In areas stressed within the growth stress, fatigue growth occurs in the form of secondary fractures within the matrix. The primary crack front
runs along these damaged points in the material until global failure occurs. This leads
to a discontinuous, stepwise failure expiration under fatigue loading. This general
mechanism permits assessment of the damage behaviour and the progression of
failure in various types of fibre reinforcement.
This thesis deals with the development of thermoplastic polyolefin elastomers using recycled polyolefins and ground tyre rubber (GTR). The disposal of worn tyres and their economic recycling mean a great challenge nowadays. Material recycling is a preferred way in Europa owing to legislative actions and ecological arguments. This first step with worn tyres is already done in this direc-tion as GTR is available in different fractions in guaranteed quality. As the traditional applications of GTR are saturated, there is a great demand for new, value-added products containing GTR. So, the objective of this work was to convert GTR by reac-tive blending with polyolefins into thermoplastic elastomers (TPE) of suitable me-chanical and rheological properties. It has been established that bituminous reclamation of GTR prior to extrusion melt compounding with polyolefins is a promising way of TPE production. By this way the sol-content (acetone soluble fraction) of the GTR increases and the GTR particles can be better incorporated in the corresponding polyolefin matrix. The adhesion be-tween GTR and matrix is given by molecular intermingling in the resulting interphase. GTR particles of various production and mean particle size were involved in this study. As polyolefins recycled low-density polyethylene (LDPE), recycled high-density polyethylene (HDPE) and polypropylene (PP) were selected. First, the opti-mum conditions for the GTR reclamation in bitumen were established (160 °C < T < 180 °C; time ca. 4 hours). Polyolefin based TPEs were produced after GTR reclamation in extrusion compounding. Their mechanical (tensile behaviour, set properties), thermal (dynamic-mechanical thermal analysis, differential scanning calorimetry) and rheological properties (both in low- and high-shear rates ) were determined. The PE-based blends contained an ethylene/propylene/diene (EPDM) rubber as compatibilizer and their composition was as follows: PE/EPDM/GTR:bitumen = 50/25/25:25. The selected TPEs met the most important criterion, i.e. elongation at break > 100 %; compression set < 50%. The LDPE-based TPE (TPE(LDPE)) showed better me-chanical performance compared to the TPE(HDPE). This was assigned to the higher crystallinity of the HDPE. The PP-based blends of the compositions PP/(GTR-bitumen) 50/50 and 25/75, whereby the ratio of GTR/bitumen was 60/40, outperformed those containing non-reclaimed GTR. The related blends showed also a better compatibility with a PP-based commercial thermoplastic dynamic vulcanizate (TDV). Surprisingly, the mean particle size of the GTR, varied between < 0.2 and 0.4-0.7 mm, had a small effect on the mechanical properties, however somewhat larger for the rheological behaviour of the TPEs produced.
The scientific and industrial interest devoted to polymer/layered silicate
nanocomposites due to their outstanding properties and novel applications resulted
in numerous studies in the last decade. They cover mostly thermoplastic- and
thermoset-based systems. Recently, studies in rubber/layered silicate
nanocomposites were started, as well. It was presented how complex maybe the
nanocomposite formation for the related systems. Therefore the rules governing their
structure-property relationships have to be clarified. In this Thesis, the related
aspects were addressed.
For the investigations several ethylene propylene diene rubbers (EPDM) of polar and
non-polar origin were selected, as well as, the more polar hydrogenated acrylonitrile
butadiene rubber (HNBR). The polarity was found to be beneficial on the
nanocomposite formation as it assisted to the intercalation of the polymer chains
within the clay galleries. This favored the development of exfoliated structures.
Finding an appropriate processing procedure, i.e. compounding in a kneader instead
of on an open mill, the mechanical performance of the nanocomposites was
significantly improved. The complexity of the nanocomposite formation in
rubber/organoclay system was demonstrated. The deintercalation of the organoclay
observed, was traced to the vulcanization system used. It was evidenced by an
indirect way that during sulfur curing, the primary amine clay intercalant leaves the
silicate surface and migrates in the rubber matrix. This was explained by its
participation in the sulfur-rich Zn-complexes created. Thus, by using quaternary
amine clay intercalants (as it was presented for EPDM or HNBR compounds) the
deintercalation was eliminated. The organoclay intercalation/deintercalation detected
for the primary amine clay intercalants, were controlled by means of peroxide curing
(as it was presented for HNBR compounds), where the vulcanization mechanism
differs from that of the sulfur curing.
The current analysis showed that by selecting the appropriate organoclay type the
properties of the nanocomposites can be tailored. This occurs via generating different
nanostructures (i.e. exfoliated, intercalated or deintercalated). In all cases, the
rubber/organoclay nanocomposites exhibited better performance than vulcanizates
with traditional fillers, like silica or unmodified (pristine) layered silicates.The mechanical and gas permeation behavior of the respective nanocomposites
were modelled. It was shown that models (e.g. Guth’s or Nielsen’s equations)
developed for “traditional” vulcanizates can be used when specific aspects are taken
into consideration. These involve characteristics related to the platy structure of the
silicates, i.e. their aspect ratio after compounding (appearance of platelet stacks), or
their orientation in the rubber matrix (order parameter).
The use of polymers subjected to various tribological situations has become state of
the art. Owing to the advantages of self-lubrication and superior cleanliness, more
and more polymer composites are now being used as sliding elements, which were
formerly composed of metallic materials only. The feature that makes polymer composites
so promising in industrial applications is the opportunity to tailor their properties
with special fillers. The main aim of this study was to strength the importance of
integrating various functional fillers in the design of wear-resistant polymer composites
and to understand the role of fillers in modifying the wear behaviour of the materials.
Special emphasis was focused on enhancement of the wear resistance of
thermosetting and thermoplastic matrix composites by nano-TiO2 particles (with a
diameter of 300nm).
In order to optimize the content of various fillers, the tribological performance of a
series of epoxy-based composites, filled with short carbon fibre (SCF), graphite,
PTFE and nano-TiO2 in different proportions and combinations, was investigated.
The patterns of frictional coefficient, wear resistance and contact temperature were
examined by a pin-on-disc apparatus in a dry sliding condition under different contact
pressures and sliding velocities. The experimental results indicated that the addition
of nano-TiO2 effectively reduced the frictional coefficient, and consequently the contact
temperature, of short-fibre reinforced epoxy composites. Based on scanning
electron microscopy (SEM) and atomic force microscopy (AFM) observations of the
worn surfaces, a positive rolling effect of the nanoparticles between the material pairs
was proposed, which led to remarkable reduction of the frictional coefficient. In particular,
this rolling effect protected the SCF from more severe wear mechanisms, especially
in high sliding pressure and speed situations. As a result, the load carrying
capacity of materials was significantly improved. In addition, the different contributions
of two solid lubricants, PTFE powders and graphite flakes, on the tribologicalperformance of epoxy nanocomposites were compared. It seems that graphite contributes
to the improved wear resistance in general, whereas PTFE can easily form a
transfer film and reduce the wear rate, especially in the running-in period. A combination of SCF and solid lubricants (PTFE and graphite) together with TiO2 nanoparticles
can achieve a synergistic effect on the wear behaviour of materials.
The favourable effect of nanoparticles detected in epoxy composites was also found
in the investigations of thermoplastic, e.g. polyamide (PA) 6,6 matrix. It was found
that nanoparticles could reduce the friction coefficient and wear rate of the PA6,6
composite remarkably, when additionally incorporated with short carbon fibres and
graphite flakes. In particular, the addition of nanoparticles contributed to an obvious
enhancement of the tribological performances of the short-fibre reinforced, hightemperature
resistant polymers, e.g. polyetherimide (PEI), especially under extreme
sliding conditions.
A procedure was proposed in order to correlate the contact temperature and the
wear rate with the frictional dissipated energy. Based on this energy consideration, a
better interpretation of the different performance of distinct tribo-systems is possible.
The validity of the model was illustrated for various sliding tests under different conditions.
Although simple quantitative formulations could not be expected at present, the
study may lead to a fundamental understanding of the mechanisms controlling friction
and wear from a general system point of view. Moreover, using the energybased
models, the artificial neural network (ANN) approach was applied to the experimental
data. The well-trained ANN has the potential to be further used for online monitoring and prediction of wear progress in practical applications.Die Verwendung von Polymeren im Hinblick auf verschiedene tribologische Anwendungen
entspricht mittlerweile dem Stand der Technik. Aufgrund der Vorteile von
Selbstschmierung und ausgezeichneter Sauberkeit werden polymere Verbundwerkstoffe
immer mehr als Gleitelemente genutzt, welche früher ausschließlich aus metallischen
Werkstoffen bestanden. Die Besonderheit, die polymere Verbundwerkstoffe
so vielversprechend für industrielle Anwendungen macht, ist die Möglichkeit ihre Eigenschaften
durch Zugabe von speziellen Füllstoffen maßzuschneidern. Das Hauptziel
dieser Arbeit bestand darin, die Wichtigkeit der Integration verschiedener funktionalisierter
Füllstoffe in den Aufbau polymerer Verbundwerkstoffe mit hohem Verschleißwiderstand
aufzuzeigen und die Rolle der Füllstoffe hinsichtlich des Verschleißverhaltens
zu verstehen. Hierbei lag besonderes Augenmerk auf der Verbesserung
des Verschleißwiderstandes bei Verbunden mit duromerer und thermoplastischer
Matrix durch die Präsenz von TiO2-Partikeln (Durchmesser 300nm).
Das tribologische Verhalten epoxidharzbasierter Verbunde, gefüllt mit kurzen Kohlenstofffasern
(SCF), Graphite, PTFE und nano-TiO2 in unterschiedlichen Proportionen
und Kombinationen wurde untersucht, um den jeweiligen Füllstoffgehalt zu optimieren.
Das Verhalten von Reibungskoeffizient, Verschleißwiderstand und Kontakttemperatur
wurde unter Verwendung einer Stift-Scheibe Apparatur bei trockenem
Gleitzustand, verschiedenen Kontaktdrücken und Gleitgeschwindigkeiten erforscht.
Die experimentellen Ergebnisse zeigen, dass die Zugabe von nano-TiO2 in kohlenstofffaserverstärkte
Epoxide den Reibungskoeffizienten und die Kontakttemperatur
herabsetzen können. Basierend auf Aufnahmen der verschlissenen Oberflächen
durch Rasterelektronen- (REM) und Rasterkraftmikroskopie (AFM) trat ein positiver
Rolleffekt der Nanopartikel zwischen den Materialpaaren zum Vorschein, welcher zu
einer beachtlichen Reduktion des Reibungskoeffizienten führte. Dieser Rolleffekt
schützte insbesondere die SCF vor schwerwiegenderen Verschleißmechanismen,
speziell bei hohem Gleitdruck und hohen Geschwindigkeiten. Als Ergebnis konnte die Tragfähigkeit dieser Materialien wesentlich verbessert werden. Zusätzlich wurde
die Wirkung zweier fester Schmierstoffe (PTFE-Pulver und Graphit-Flocken) auf die tribologische Leistungsfähigkeit verglichen. Es scheint, daß Graphit generell zur Verbesserung
des Verschleißwiderstandes beiträgt, wobei PTFE einen Transferfilm bilden
kann und die Verschleißrate insbesondere in der Einlaufphase reduziert. Die
Kombination von SCF und festen Schmierstoffen zusammen mit TiO2-Nanopartikeln
kann einen Synergieeffekt bei dem Verschleißverhalten der Materialien hervorrufen.
Der positive Effekt der Nanopartikel in Duromeren wurde ebenfalls bei den Untersuchungen
von Thermoplasten (PA 66) gefunden. Die Nanopartikel konnten den Reibungskoeffizienten
und die Verschleißrate der PA 66-Verbunde herabsetzen, wobei
zusätzlich Kohlenstofffasern und Graphit-Flocken enthalten waren. Die Zugabe von
Nanopartikeln trug offensichtlich auch zur Verbesserung der tribologischen Leistungsfähigkeit
von SCF-verstärkten, hochtemperaturbeständigen Polymeren (PEI)
insbesondere unter extremen Gleitzuständen, bei. Es wurde eine Methode vorgestellt,
um die Kontakttemperatur und die Verschleißrate mit der durch Reibung dissipierten
Energie zu korrelieren. Diese Energiebetrachtung ermöglicht eine bessere
Interpretation der verschiedenen Eigenschaften von ausgewählten Tribo-Systemen.
Die Gültigkeit dieses Models wurde für mehrere Gleittests unter verschiedenen Bedingungen
erklärt.
Vom generellen Blickpunkt eines tribologischen Systems aus mag diese Arbeit zu
einem fundamentalen Verständnis der Mechanismen führen, welche das Reibungsund
Verschleißverhalten kontrollieren, obwohl hier einfache quantitative (mathematische)
Zusammenhänge bisher nicht zu erwarten sind. Der auf energiebasierenden
Modellen fußende Lösungsansatz der neuronalen Netzwerke (ANN) wurde darüber
hinaus auf die experimentellen Datensätze angewendet. Die gut trainierten ANN's
besitzen das Potenzial sie in der praktischen Anwendungen zur Online-
Datenauswertung und zur Vorhersage des Verschleißfortschritts einzusetzen.
Die vorliegende Arbeit beschäftigt sich mit dem Reibungs- und Verschleißverhalten
Polytetraßuorethylen-basierender Verbundwerkstoffe (PTFE) bezogen auf eine Anwendung
als tribologisch beanspruchte Maschinenelemente im Temperaturbereich
zwischen Raumtemperatur und Temperaturen in kryogenen Medien. Dieser Temperaturbereich
ist relevant für eine Reihe neuer, innovativer Technologien, allen voran die
Wasserstofftechnologie als Alternative zu fossilen Energieträgern.
Der Ausgangspunkt dieser Arbeit ist eine auf bekannten Erfahrungen und entsprechenden
Publikationen aufbauende Werkstoffauswahl. Daher wurde PTFE als Matrixwerkstoff
ausgewählt, da es sich bereits in Tieftemperaturanwendungen bewährt hat.
Zur Verstärkung der PTFE-Matrix wurden ein polymerer Füllstoff, Polyetheretherketon
(PEEK) beziehungsweise ein aromatisches Polyester, und kurze Kohlenstofffasern
ausgewählt. Diese Werkstoffkomponenten wurden zu einer Reihe von Verbundwerkstoffen
mit systematisch variierendem Faser- und Füllstoffgehalt zusammengesetzt.
Der experimentelle Teil beschäftigt sich schwerpunktmäßig mit tribologischen Untersuchungen
bei Raumtemperatur mit Hilfe einer Eigenbau-Stift-Scheibe-Prüfapparatur.
Als Gegenkörper kommen geschliffene Laufringe aus 100 Cr6 Stahl zum Einsatz.
Alle Verbundwerkstoffe wurden bei Standard-Testbedingungen von 1 m/s, 1 MPa sowie
Raumtemperatur getestet. Einer der verschleißbeständigsten Verbundwerkstoffe
wurde auch bei verschiedenen Geschwindigkeiten und Belastungen geprüft. Das Reibungsverhalten
dieser Werkstoffe zeigte sich anders als erwartet, weshalb zusätzliche
Versuche erforderlich waren, um den TransferÞlmbildungsprozess beobachten zu können.
Zur Einordnung der tribologischen Ergebnisse werden auch reines PTFE und
einige lediglich partikelgefüllte PTFE-Compounds hinsichtlich Reibung und Verschleiß
getestet. Weiterhin werden die für tribologische Anwendungen wichtigen mechanischen
und thermischen Werkstoffeigenschaften untersucht.
Im Diskussionsteil werden die Einßüsse der Füllstoffe und Fasern auf die resultierenden
mechanischen, thermischen und tribologischen Werkstoffeigenschaften bewertet.
Die im Rahmen dieser Arbeit beschafften bzw. hergestellten Werkstoffe wurden parallel
an der Bundesanstalt für Materialforschung und -prüfung, BAM, Berlin, tribologischen
Beanspruchungen in verschiedenen kryogenen Medien, insbesondere auch in ßüssigem
Wasserstoff, unterworfen. Auf die dort erarbeiteten Ergebnisse wird ebenfalls kurz
eingegangen.
Externe elektrische Gleichspannungsfelder können sowohl den physikalischen, als auch den reaktiven Stoffaustausch bei der Flüssig-Flüssig Extraktion signifikant beeinflussen, wodurch eine Steigerung des Stoffüberganges im elektrischen Feld erzielt werden kann. Die Gründe hierfür sind im elektrischen Feld gesteigerte Grenzflächenturbulenzen und feldinduzierte Konzentrationspolarisationen im Phasengrenzflächenbereich, welche durch Migrationswechselwirkungen verursacht werden. Das elektrische Feld hat bezüglich des reaktiven Stoffübergangs sowohl in Einzel- als auch im Mehrkomponentensystem keinen Einfluss auf das chemische Gleichgewicht. Jedoch wird durch das Feld die Kinetik beschleunigt und das Gleichgewicht schneller erreicht. Auch die maximale Trennselektivität im Mehrkomponentensystem, welche im Gleichgewicht erreicht wird, wird nicht durch das Feld verändert. Diese ist primär von der Konzentration und der Säurestärke abhängig. Lediglich im Falle sehr schwacher Säuren ist das Gleichgewicht über das natürliche hinaus verschiebbar. Diese Stoffaustauscherhöhung ist durch die im elektrischen Feld erhöhte Dissoziation der Übergangskomponente gemäß dem 2. Wien’schen Effekt erklärbar. Zudem ist die feldinduzierte Stoffaustauscherhöhung stark von der Feldwirkrichtung abhängig. Der Feldeinfluss ist dann maximal, wenn das Feld direkt in Stoffübergangs-richtung wirkt. Dies ist bei unbewegten (z. B. planaren) Grenzflächen erreichbar. So konnte in planaren Stoffübergangszellen und am hängenden Tropfen eine starke Stoffaus-tauschbeschleunigung in der Größenordnung von ca. 1000 % erzielt werden. Am bewegten Tropfen konnte zwar eine Stoffaustauscherhöhung durch die im Feld geänderten hydrodynamischen Betriebsgrößen (wie Tropfengröße und Verweilzeit) erzielt werden, jedoch konnte darüber hinaus keine weitere Stoffaustauschbeschleunigung erzielt werden. Dies kann damit erklärt werden, dass bei bewegter sphärischer Grenzflächengeometrie das Feld nicht nur in Stoffübergangsrichtung wirkt und feldinduzierte Polarisations-erscheinungen sich weitgehend kompensieren. Daher gelingt in klassischen Extraktionsapparaten, welche mit Dispergierung und Tropfen-bildung arbeiten, die Verfahrensumsetzung der kontinuierlich betriebenen Extraktion im Hochspannungsfeld nicht effizient. Diese gelingt in einem speziellen Zentrifugalextraktor, dem Taylor-Couette Elektroextraktor, in wessen Ringsspalt zwischen zwei als Elektroden fungierenden, konzentrischen Zylindern auf Grund der Rotationsbewegung sich eine planaranaloge, zylindrische Phasengrenzfläche ausbildet und das Feld somit direkt in Stoffübergangsrichtung wirken kann. Auch wird der stationäre Betriebszustand binnen weniger Minuten erreicht. Zudem entstehen im Phasengrenzbereich Taylorverwirblungen, welche ebenfalls den Stoffaustausch erhöhen. Zur theoretischen Beschreibung konnten Stoffübergangsmodelle entwickelt werden, welche die feldinduzierten Polarisationseffekte berücksichtigen. So gelingt die Berechnung des reaktiven Stoffaustauschs über ein elektrostatisch erweitertes Kinetikmodell, welches neben der chemischen Reaktion, der Grenzflächenadsorption des Ionenaustauschers und dem Reaktionsgleichgewicht, auch die Migration über die Nernst Planck Gleichung, sowie auch die Elektrodissoziation über einen Ansatz nach Onsager berücksichtigt. Die Berücksichtigung der im elektrischen gesteigerten Grenzflächenturbulenz gelingt über einen elektrostatisch erweiterten Ansatz nach Maroudas und Sawistowski. Auch wurde ein Modell zur Berechnung des Stoffübergangs im Taylor-Couette Extraktors vorgestellt. Die Berechnung der anliegenden elektrischen Felder gelingt über die Finite Elemente Methode basierend auf den Maxwell’schen Gleichungen oder vereinfacht über die Laplace Gleichung. Wesentlich ist, dass nicht die Elektrodenpotentialdifferenz, sondern das berechnete Potential an der Phasengrenzfläche den Stofftransfer im elektrischen Feld bestimmt, was durch die Simulationsrechnungen bestätigt wurde.
Zur Eigenspannungsausbildung bei der wickeltechnischen Verarbeitung thermoplastischer Bandhalbzeuge
(2005)
Filament winding is today a well established production technique for fiber reinforced
pressure vessels. Most of the parts are still made using thermosets as matrix material,
but parts with thermoplastic matrices are today on the edge to mass production.
Usually these parts are made from fully consolidated unidirectional fiber reinforced
thermoplastic tapes. During processing the matrix material is molten and the tapes
are placed on the substrate where they re-solidify. A wide range of material combinations
are available on the market. The materials used in the present investigation are
semi-crystalline thermoplastics and glass or carbon fiber i.e. carbon fiber reinforced
Polyetheretherketone, glass fiber reinforced Polyetheretherketone and glass fiber
reinforced Polypropylene.
Applications can be found in the field of medium and high pressure vessels like they
are used for natural gas and hydrogen storage or for tubes and pipes for their transport.
During the design of such parts mostly idealized properties as for example tensile
strength are used. Residual stresses which are inherent for composite materials
are only considered as part of the safety factor.
The present work investigates the generation of residual stresses for in-situ consolidation
during filament winding. Within this process consolidation of the tape material
and the substrate takes place immediately after the tapes are placed. This is contrary
to the normal curing of thermoset materials and has a large influence on the generation
of the residual stress. The impact of these stresses on the behavior of the produced
parts during service is one of the topics of this investigation. Therefore the
background of thermal residual stresses in semi-crystalline thermoplastic parts is discussed
and a closer look on the crystallization behavior of the matrix materials was
taken. As the beginning of the crystal growth is a major point in the generation of thermal residual stress.
The aim of the present work is to find process parameter combinations that allow to
compensate the thermal residual stresses and to generate a residual stress profile
that – unlike the thermal residual stresses - brings about structural benefit. Ring
samples with a defined geometry were made to measure the generated stresses.
The geometry of the samples was chosen in a way that prevents influences of the
boundary conditions of the free edges on the measuring point.
In the investigations the residual stresses were measured in circumferential direction
using a method where the ring samples were cut in radial direction and the deformation
was measured using strain gages. From the strain the local stress can be determined.
It was tried to minimize the number of experiments. Therefore the influence of filament
winding process parameters on the residual stress were investigated using a
Design of Experiments approach where the main influences on the residual stress
generation can be found from a relatively small number of experiments such as 8
instead of 128. As a result of these experiments it was found that the winding angle,
the mandrel temperature, the annealing, the wall thickness and the tape tension have
a significant influence on residual stresses. With increasing winding angles the influence
on the measured circumferential stresses increase regardless to kind of residual
stresses. The mandrel temperature has a large influence on the temperature difference
that causes the stress between fiber and matrix. They are caused by different
thermal expansion coefficients of fiber and matrix. Structural benefit through annealing
is only theoretically possible because the required outside temperatures
along with internal cooling of the parts can not be realized within an industrial processes.
Increasing wall thickness leads to also increasing residual stress but it can not
be the aim to build oversized parts for the sake of residual stresses. The applied tape
tension was identified as a parameter that can be used to achieve the desired residual
stress state with reasonable efforts.
Different ways of varying the tape tension with increasing wall thickness were investigated.
The tape tension was increased with every layer to a chosen maximum value or, after half of the layers were placed, in one step to the maximum value. Furthermore
a continuously high tape tension and a variant without tape tension was investigated.
The experiments led to the conclusion that increasing tape tension with increasing
wall thickness is a viable way to have structural benefit from residual stress.
The increasing in one step gave the best results.
The impact of the thermal history during production is discussed as well. Temperatures
must not exceed the softening point of the matrix. Otherwise a part of the tape
tension gets lost by relaxation. In a particular case the relaxation reached an amount
where the compensation of the thermal stresses failed. Thermodynamic calculations
led to the conclusion that the energy transfer into the material by mandrel heating
and melt energy caused a temperature above the softening point.
The impact of tape tension on material quality is documented. Very low tape tension
can not guarantee a proper consolidation. On the other hand excessive tape tension
can lead to matrix squeeze out and in particular cases to cracks due to too high residual
stresses. Therefore the tape tension profile should be well adapted to work
load, the composite and its properties.
Investigations on the relaxation behavior of the residual stresses showed that relaxation
occurs and that a part of the residual stress relaxes when the samples were exposed
to higher temperatures. Test at room temperature showed no significant sign
of relaxation. When the temperature was raised – in this case to 80 °C - the samples
clearly relaxed. The amount of induced residual stress sank to half of its initial value.
Investigations on the structural benefit showed that material savings of up to 23 % of
weight are possible for high pressure applications and fiber reinforcements with relatively
low fiber volume content. Higher fiber volume contents which also mean higher
strengths reduce the benefit. As the strength of the material increases the benefit
reduces in relation to it.
Nevertheless there is a potential in material saving and one should keep in mind that
the costs to establish the equipment to control the tape tension is cheap in comparison
to the achievable result.