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Paper des Monats
- ja (1)
Eingeladener Vortrag
- nein (88)
Untersuchungs des Ermüdungsverhaltens glasfaserverstärkter Thermoplaste mittels Schwingungsmessungen
(1990)
Fatigue Behaviour of High-Performance Fibre-Reinforced Composites with a Thermoplastic Matrix
(1990)
An Advanced Equipment for Single-Fibre Pull-Out test designed to monitor the fracture process
(1993)
Investigations on the cold crystallization of pure and filled PETP by dielectric measurements
(1993)
The cold crystallization of pure and filled poly(ethylene terephthalate) (PETP) after quenching from the melt has been investigated by dielectric measurements in two different modes: The first one using constant heating-rate (in the temperature range from 320 to 480 K) and the second one under isothermal conditions (at 378 K). The complex dielectric constant (ε´, tan δ) is influenced by crystallization. Two α-relaxation regions are identified: The αa-relaxation region from the pure amorphous phase and the α´-relaxation region from the spherulitic phase. The investigations on the filled systems (PETP + 10% aramid puple, PETP + 30% short glass fibres) prove that an influence of the fillers on the time delay and rate of crystallization and on the geometry of the growing spherulites exists.
Pull-out of Single Fibres
(1994)
Hier wird der chronologische Verlauf der Kristallisation verfolgt. Sie beginnt mit den thermodynamischen Grundlagen der Keimbildung und des Wachstums, stellt anschließend Wachstumsformen in der Frühlphase der Kristallisation vor und behandelt schließlich die spärolithische Hauptkristallisation sowie die sogenannte Nachkristallisation.
Energy Dissipation of the Fibre-Matrix Debonding Process Investigated with the Indentation Technique
(1995)
An advanced equipment for single-fibre pull-out test designed to monitor the fracture process
(1995)
After a delamination process in a C-fiber reinforced polymer composite, the identification of the polymer residue of poly-(phenylene sulphide) on carbon fiber surfaces gives important information on the composite failure mechanism. Using scanning force microscopy (SFM) in its lateral force imaging mode or with the stiffness imaging mode, it is almost impossible to distinguish the two composite components owing to a very low material-based contrast of the local friction and compliance. The new technique using contrast based on a local AC current measurement allows a clear identification of the conducting carbon fiber and the non-conducting polymer coverage.
A mathematical model for the description of the overall crystallization of polymers has been developed which separates the pure geometric spreading of the semicrystalline superstructures from the increasing crystallinity inside these entities. The model uses the Avrami theory for the geometric part and a freely optional function for the increasing local crystallinity. The two functions are combined by a mathematical procedure called convolution. The free option of the local crystallinity function enables this model to permit calculation of this function, which is not accessible to direct measurement. Thus with given Avrami parameters and measured values of the time-dependent overall crystallinity, this function of the local crystallinity (secondary crystallization function) can be calculated. The model has been applied to test data produced by computer simulations.
The effect of transcrystallinity in carbon fiber reinforced poly(p-phenylene sulfide (PPS) composites on the apparent shear strength was investigated with the single fiber pull-out test. Transcrystalline zones around the reinforcing fibers do not seem to improve the adhesion level significantly. Neighbor fibers hinder the formation of the transcrystalline zone and a ductile fracture behavior can be observed. However, the apparent strength level is slightly higher for composites containing such reinforcing neighbor fibers compared with single fiber composite samples. During annealing a brittle interface can be formed in the multifiber composite yielding a higher level of the apparent shear strength.
Energy based characterization of the fibre-matrix interface using the pull-out and push-in test
(1997)
Characterisation of the fibre/matrix interface in reinforced polymers by the push-in technique
(1997)
The push-in test is the only micromechanical test method that is not restricted to artificial fibre/matrix arrangements, but allows the in situ characterisation of interfaces in composites fabricated and stressed under realistic conditions. However, with the application of this method to reinforced polymers some problems arise both in the mathematical model for evaluating test data and in the practical performance of the test. Because in some cases the deformation of the relatively compliant polymeric matrices cannot be neglected, an extension of the existing model is required. For this purpose, the elastic energy of the material around the debonded part of the fibre is estimated and included in the energy-balance analysis. Because of the small diameter of the fibres usually used for reinforcing polymers, a test apparatus was designed which ensures a high positioning accuracy in the xy plane as well as in the z direction. In order to minimise thermal and mechanical influences, the microscope for fibre selection and the force sensor/indenter are directly connected together and the apparatus is designed to be stiff in all components. A solid-state bending joint guarantees very precise control of the axial movement. Three examples of the application of this easy-to-handle and low-cost test apparatus are presented briefly in the paper: assess fibre/matrix combinations, measures to improve the interfacial adhesion and the influence of water on the interface.
Friction Measurement on fiber reinforced polymer composites with using the indentation technique
(1998)
Influence of water on the fibre-matrix adhesion investigated by the single fiber indentation method
(1998)
Viscoelastic properties of the interphase in fibre reinforced polymers - measurement and simulation
(1999)
The strength of composite reinforcement fibres is normally measured on samples of much greater length than the actual residual fibre lengths found in many composite materials. This is due to a number of limitations of the standard techniques which are employed. We present a description of a technique which enables values for the tensile strength of composite reinforcement fibres at short gauge lengths to be obtained. The technique is based on an adaptation of a micro-mechanical test apparatus for fibre pullout measurements. Data is presented which was obtained at gauge lengths of 180380 µm on E-glass and S-2 glass® fibres taken from different chopped reinforcement products. The technique can be used at gauge lengths as short as 20 µm. The data indicates that the values of average fibre strength in these products are significantly below the pristine glass strength values.
Jahresbericht 2002
(2003)
Mikromechanik
(2003)
Bestimmung der Haftfestigkeit in Verbundwerkstoffen mittels Indentations- und Durchdrückexperimenten
(2004)
Mikromechanische Testmethoden zur Bestimmung von Haftung und Reibung in Faserverbundwerkstoffen
(2004)
Jahresbericht 2004
(2005)
Mapping Epoxy Interphases
(2005)
The epoxy/metal interface regions are recognized as crucial for the mechanical reliability of composite materials. In particular, the spatial stress distribution is governed by modulus variations, which may result from the physical-chemical interactions between epoxy and the metal surface. The properties of the interphase between bulk polymer and metal were analyzed in nanometer scale. It was aimed to characterize the stiffness gradient of the polymeric interphase, which was formed by a thermoset being cured in contact with a metal surface.
Jahresbericht 2006
(2007)
Mikromechanische Methoden zur Messung der Faser-Matrix-Haftung in Polymermatrix-Verbundwerkstoffen
(2007)
Atmospheric-plasma fluorination was used to introduce fluorine functionalities onto the surface of carbon fibers without affecting their bulk properties. The interfacial adhesion between atmospheric-plasma-fluorinated carbon fibers and poly(vinylidene fluoride) (PVDF) was studied by means of direct wetting measurements and single fiber pullout tests. Measured contact angles of PVDF melt droplets on modified carbon fibers show that short exposure times of carbon fibers to atmospheric-plasma fluorination (corresponding to a degree of surface fluorination of F/C = 0.01 (1.1%)) leads to improved wettability of the fibers by PVDF melts. The apparent interfacial shear strength as a measure of practical adhesion, determined by the single-fiber pullout test, increases by 65% under optimal treatment conditions. The improved practical adhesion is not due to the formation of transcrystalline regions around the fibers or a change of the bulk matrix crystallinity or to an increased surface roughness; it seems to be due to the compatibilization of the interface caused of the atmospheric-plasma fluorination of the carbon fibers.
The interaction between direct fluorinated carbon fibres and various fluoropolymers (ethylene-chlorotrifluoroethylene, poly vinylidene fluoride, fluorinated ethylene propylene copolymer and tetrafluoroethylene-perfluoro alkoxy vinyl ether copolymer) was studied by means of direct wetting measurements between fibres and the polymer melts and single fibre pull-out tests. The results of both techniques allow the adhesion behaviour between the fibres and the matrices to be predicted. The results obtained show that a low degree of surface fluorination of carbon fibres leads to an improved wettability between the fibres and fluoropolymer melts and this is an indicator for an improved thermodynamic work of adhesion. The apparent interfacial shear strength as measure of practical adhesion, determined by the single fibre pull-out test, increases with increasing degree of surface fluorine content up to a maximum, which depends on the degree of fluorination of the matrix used. The improved interaction between the fibre and the matrix is due to an enhanced compatibility at the fibre/matrix interface.
Jahresbericht 2007
(2008)
The quality of interfacial interaction is dictated by the surface chemistry of the carbon fibres and the composition of the matrix. The composition of poly(vinylidene fluoride) (PVDF) was modified by the addition of maleic anhydride grafted PVDF. The surface properties of the various matrix formulations were characterised by contact angle and electrokinetic measurements. Carbon fibres were modified by industrial electrochemical oxidation and oxidation in nitric acid, or the use of a traditional epoxy-sizing of industrially oxidised fibres. The surface composition, morphology and wetting behaviour of the carbon fibres was characterised. The interaction between modified PVDF and the carbon fibres was studied by direct contact angle measurements between PVDF melt on single carbon fibres and by single fibre pull-out tests. The best wetting and adhesion behaviour was achieved between PVDF containing 5 ppm grafted maleic anhydride (MAH) and epoxy-sized carbon fibres. The addition of MAH-grafted PVDF to the unmodified PVDF caused the apparent interfacial shear strength to increase by 184%. The apparent interfacial shear strength of this fibrematrix combination allowed for the utilisation of 100% of the yield tensile strength of PVDF.
Triggered biodegradable composites made entirely from renewable resources are urgently sought after to improve material recyclability or be able to divert materials from waste streams. Many biobased polymers and natural fibers usually display poor interfacial adhesion when combined in a composite material. Here we propose a way to modify the surfaces of natural fibers by utilizing bacteria (Acetobacter xylinum) to deposit nanosized bacterial cellulose around natural fibers, which enhances their adhesion to renewable polymers. This paper describes the process of modifying large quantities of natural fibers with bacterial cellulose through their use as substrates for bacteria during fermentation. The modified fibers were characterized by scanning electron microscopy, single fiber tensile tests, X-ray photoelectron spectroscopy, and inverse gas chromatography to determine their surface and mechanical properties. The practical adhesion between the modified fibers and the renewable polymers cellulose acetate butyrate and poly(l-lactic acid) was quantified using the single fiber pullout test.