Klaus Friedrich

The abrasive wear behaviour of continuous fibre polymer composites

The dry abrasive-dominant wear behaviour of several composite materials consisting of uni-directional continuous fibres and polymer matrices was investigated. Seven materials were examined: neat epoxy (3501-6), carbon fibre epoxy (AS4/3501-6), glass fibre/epoxy (E-glass/ 3501-6), aramid fibre/epoxy (K49/3501-6), neat polyetheretherketone (PEEK), carbon fibre/PEEK (APC2) and aramid fibre/PEEK (K49/PEEK). The wear behaviour of the materials was characterized by experimentally determining the friction coefficients and wear rates with a pin on-flat test apparatus. First, the effects of the operation variables apparent normal pressure, sliding velocity and apparent contact area were observed. The dimensionless wear rate increased linearly as the apparent normal pressure increased and decreased as the apparent contact area increased. Second, through microscopic observations of the worn surfaces and subsurface regions, basic wear mechanisms were identified as a function of fibre orientation. Observations of fibre-abrasive particle interactions allowed for the differentiation of the dominating wear mechanisms. Finally, a network of data was compiled on the wear behaviour in terms of the three material parameters: fibre orientation, fibre material and matrix material. This enabled the systematic selection of an ideal low wear composite material which would consist of a PEEK matrix reinforced with aramid fibres oriented normal to the contacting surface and carbon fibres oriented parallel to the contacting surface.

Evaluation of polymer composites for sliding and abrasive wear applications

Abstract The dry wear behaviour of several polymer composite materials sliding against smooth steel counterparts was investigated. Wear mechanisms were categorized after microscopic observation of the abraded surfaces into four main areas and their contribution towards the overall wear rate of a composite material was accounted for in a wear model. The contribution from the post-sliding wear mechanisms are not directly measurable and were estimated by employing the wear model. A general comparison was made between the sliding wear rates and previously ascertained severe abrasive wear rates of the materials under consideration. A greater increase in the wear resistances of the matrix materials due to fibre reinforcement was evident under the sliding wear conditions. Wear rate results were referenced to the wear rate of the unfilled epoxy. This database was then utilized in the design of an optimum wear resistant composite material for specified wear conditions.

Erosive wear of polymer surfaces by steel ball blasting

The erosion behaviour of a variety of polymeric materials has been studied using steel balls at 57 m sec−1 in an air-blast rig. It is shown that the softer polymers (polyethylene, polypropylene, polybutene-1) exhibit an incubation period prior to stabilizing to a linear erosion rate, here defined as reduction in thickness per testing time. The more brittle polymer, polystyrene, on the other hand, shows no incubation time and possesses the highest erosion rate. Further effects can arise from the morphology of semicrystalline polymers. In particular, it was found that a coarse spherulitic microstructure in polypropylene wears much faster than a fine spherulitic one. A decrease in testing temperature generally increases the wear rate. The individual mechanisms of erosive wear are illustrated by SEM micrographs of the worn surfaces. It is suggested that a “brittleness index” of the form (hardnessH/fracture energyGIc) is a good indicator for the erosion resistance of polymeric materials.

Effect of testing conditions and microstructure on the sliding wear of graphite fibre/PEEK matrix composites

The sliding friction and wear behaviour of unreinforced polyetheretherketone (PEEK) matrix and its unidirectional continuous and two-dimensional woven graphite fibre-reinforced composites were investigated. The operating wear mechanisms, as evinced by scanning electron microscopy of the worn surfaces, and the coefficients of friction and the wear rates changed considerably with the fibre reinforcement form and orientation. Sliding wear rates, on account of their extreme sensitivity to the microstructure of the interacting surfaces at the sliding interface, were found to be a function of not only the surface roughness, but also of the sliding time. Complex interactions arising due to the effects of the testing parameters such as fibre orientation, sliding velocity, contact pressure and interface temperature were characterized for the neat matrix and the two composite systems. The wear rates of the two-dimensional woven composites were almost an order of magnitude lower than those of the unidirectional fibre composite or the unreinforced matrix.

Composition-dependent properties of polyethylene/kaolin composites. Part II : Thermoelastic behaviour of blow-moulded samples

Samples of the blow-moulding grade HDPE filled with Kaolin were characterized by wide-angle X-ray scattering, microhardness and stretching calorimetry techniques. It is shown that crystallinity of the polymer matrix in the filled samples remains essentially the same as that in the neat polymer regardless of the filler content. The thermoelastic behaviour of all samples below the apparent yield point ∈* is quantitatively described by classical equations for elastic solids. The thermoelastic parameters of the boundary interphase (BI) are discussed in terms of predictions of the step-by-step averaging approach. The experimental values of the internal energy increment in the inelastic strain interval for unfilled and filled samples are analyzed in the light of the filler debonding processes in the latter.

Fatigue behaviour of aligned short carbon-fibre reinforced polyimide and polyethersulphone composites

Fibre reinforced plastics, with 50 to 55 vol % of aligned short carbon fibres of approximately 3 mm in length, have good mechanical properties and advantages in deformability during the manufacturing process of structural components. The mechanical properties and damage mechanisms of this kind of composite have not been investigated deeply in the past. In the present paper results of an examination programme on laminates with various stacking sequences and two thermoplastic matrix systems (polyimide and polyethersulphone) are given. It will be shown that composites reinforced with aligned discontinuous carbon fibres can be an alternative material to continuous-fibre reinforced composites when considering their static and fatigue properties.

Impact behaviour of short fibre/liquid crystal polymer composites

Abstract Impact behaviour of a liquid crystal polymer and its short glass and carbon fibre reinforced composites was investigated. The tests were carried out using an instrumented Izod impact tester and a modular falling weight instrument. The unfilled polymer exhibited a high degree of anisotropy in the mechanical properties and this anisotropy was drastically reduced by addition of fibres. Failure analysis performed by scanning electron microscopy revealed numerous failure mechanisms.

Temperature-dependent mechanical behaviour of PI and PES resins used as matrices for short-fibre reinforced laminates

Composite materials have increasing success in engineering application. To extend their usage to higher temperatures, specially adapted polymer resins must be used. Polyimide (PI) and polyethersulphone (PES) are among the candidate materials for service at high temperatures. The mechanical behaviour of the PI and PES matrix systems under different temperatures, the resulting properties and the damage and failure, under static as well as fatigue loading, are investigated in the present study together with unidirectional laminates of aligned short fibres in the 0° or 90° direction. The room-temperature results are compared to a conventional epoxy resin.

Effect of thermal treatments on the fracture behaviour of notched polyethylene terephthalate ribbons

The application of different thermal treatment procedures to thin polyethylene terephthalate (PET) sheets yields to microstructures of different molecular weight/ degree of crystallinity combination. As a consequence, variations in the mechanical properties, especially the fracture properties and the particular fracture mechanisms occur. This is demonstrated in this paper by measurements of elastic modulus, maximum stress, failure initiation energy, and total work to fracture of notched PET-ribbons. Failure mechanisms analysed by the use of optical and scanning electron microscopy vary between highly ductile via semi-brittle after crazing, to absolute brittle at very low stresses. The results are summarized in terms of a three-dimensional failure energy map divided into regions of particular failure behaviour for particular molecular weight/ degree of crystallinity combination. In addition, the typical values of material strength, defined as the product of resistance to damage initiation (maximum stress) and crack propagation (total work to failure) are given for each region. The optimum fracture resistance was achieved for PET material with moderately low molecular weight and moderately high degree of crystallinity.