Jan Quintelier

Effect of Graphite Powder on Sliding Performance of Sintered Polyimide Composites at Various Normal Loads and Sliding Velocities

Polymer composites with graphite additives are often used to control sliding properties. The lubricating action of graphite, however, strongly depends on the working conditions of normal load and sliding velocity. Sintered polyimide with 15 wt.-% graphite powders is worn under 50 to 200 N normal loads and 0.3 to 1.2 m/s sliding velocities in line contact with a steel counterface. The tribological properties of graphite are unfavourable under low sliding velocity and high normal loads, causing high friction and wear rates. The transfer film then mainly consists of rough graphite flakes, and large wear debris particles indicate overload, as the loading capacity of graphite is limited. Graphite powders reduce coefficients of friction from 0.45 to 0.12 under medium to high sliding velocities and normal loads. The transfer films are rendered smooth by mixing polyimide and graphite fillers. Stabilisation in friction and wear as a function of normal load and sliding velocity (pv-value) coincides with levelling of the maximum polymer surface temperature of 115 °C. The favourable action for graphite under sliding, i.e. mechanical shear, and the improved thermal conductivity of the polymer bulk, control the sliding mechanisms of polymer composites under certain sliding conditions.

Influence of Re-adhesion on the Wear and Friction of Glass Fibre–Reinforced Polyester Composites

Based on the well-known pin-on-disc test rig, a new test setup for online measuring of wear and friction behaviour of polymer matrix composites has been developed. In contrast to a traditional friction-and-wear test rig, a steel pin and composite disc are used for studying the influence of wear debris and fibre orientation. During sliding, a thin adhesive film is possibly formed on the wear track of a composite disc, consisting of wear debris that is squeezed under the steel pin and that finally smoothens onto the composite surface. By optical microscopy, it was observed that most of the debris particles originate from the edges of the wear track. The thin film deforms continuously, with large and dark wear particles observed at the edge of the wear track. A lower coefficient of friction is achieved when the particles are re-adhered to the mating surface. The film formation mechanism depends on the normal force, sliding velocity, and bulk composite structure: because pultruded composite profiles are presently used with a layered structure, a change in film properties is observed depending on the wear depth.

Classification of wavelet decomposed AE signals based on parameter-less self organised mapping

Composite materials are characterised by different types of failure mechanisms which are typically associated with matrix cracking, fibre-matrix debonding and fibre breakage. These three mechanisms result in a different AE signature, which can be often recognised. In certain cases it is necessary to classify and map the damage types so as to be able to evaluate the accumulated damage and remaining strength of the material. In this framework, neural networks are widely used for damage characterisation. The classical approach involves recording waveform features and tries to associate them with the underlying damage source. Nevertheless, very often, it is very hard to draw definite conclusions based on these features. In this study, we have used a new type of a neural network which is called parameter-less self organised mapping. It is based on Kohonen neural networks but it is not bound to the naturally subjective learning rate, neighbourhood function and their annealing with the training progress. Moreover, for the training of the NN and the subsequent classification, we have successfully used wavelet decomposed AE signals.

Online Wear Monitoring of Polymer Matrix Composites

Polymer matrix composites are widely used as bearing materials for heavy load applications. Still fundamental knowledge about the wear mechanisms of these materials and the evolution in time of these mechanisms is lacking. Currently these mechanisms are only analyzed by post mortem analysis. The Laboratory Soete, based on the well-known pin-on-disc test rig, has developed a new test-setup. Instead of the standard composite specimen and steel disc, a rotating composite disc and steel pin is used to be able to have a visible composite wear track. Standard wear and friction measurements will be further combined with vibration measurements. The vibration measurements give valuable information about the pin-disc contact. The combination of all these measurements should yield valuable information on the active wear mechanisms, the occurrence of fiber and matrix fracture, fiber pull out, generated frictional heat, formation mechanisms of wear particles, and the interaction between all these effects.

Patellofemoral Contact during Simulated Weight Bearing Squat Movement: A Cadaveric Study

The Ghent Knee Rig was built in 2006 for studying the biomechanical behavior of post-mortem human knees. To validate this test rig the patellofemoral contact pressures and areas were investigated in 3 post-mortem knees tested under the same circumstances and compared to results in literature. To load the quadriceps, the vastus intermedius and rectus femoris were separated and clamped together. The pulling cable was aligned according to the shaft of the femur to keep the Q-angle at physiological values. A pressure film was inserted in the patellofemoral joint to measure the patellofemoral contact area and pressure. The results follow the general accepted trends of patellofemoral contact during knee flexion and extension; when the patella enters the trochlear groove at approximately 20 degrees of knee flexion, the intra articular contact pressure and area start to build up and the contact area on the patella shifts from distal to proximal. Though working with cadaveric specimens remains a simulation of in vivo situations with well known limitations, the test rig shows a good repeatability and reliability. The next stage of this research project is a comparison of normal with pathological knees.

Atomic Force Microscopy of Sintered and Thermoplastic Polyimide Surfaces after Macroscopic Wear Tests

Polyimide surfaces after macroscopic wear at 80 to 260°C are studied by atomic force microscopy to give additional insight in the tribophysical and -chemical processes during sliding. Three sliding regimes are distinguished with hydrolysis resulting in rough surfaces, imidisation resulting in orientation of polymer molecules and melting resulting in short-range arrangements.

Contact pressure distribution in postmortem human knee during dynamic flexion-extension movement

A test rig for the evaluation of the biomechanical behavior of post mortem human knees during squatting has been constructed based on the accepted Oxford knee rig. Post mortem human knees were used to simulate their biomechanical behavior during a squatting movement.

Acoustic Emission as Analyzing Tool for Wear Mechanisms of Composite Materials

This article summarizes the efforts done for using acoustic emission as wear mechanisms monitoring technique for wear testing in a pin-on-disc setup. The basic characteristics of the mechanisms were previously determined via controlled tensile testing (fiber breakage, debonding …). The knowledge of these basic characteristics then resulted in an easier classification of wear mechanisms related to wear testing of pultruded glass fiber reinforced polyester.

A relation between laboratory and full-scale testing of polyester/polyester composites under static and dynamic load

The tribological behaviour of a polymer composite is compared during small-scale and large-scale sliding tests and it is observed that test results strongly depend on the fibre orientation and test configuration. Different wear mechanisms are evaluated by optical microscopy and finite element modelling in relation to a real application of polyester/polyester discs as bearing elements.

Frictional behavior of glass fiber reinforced polyester under different loads

Pultruded glass fibre reinforced polyester where used to investigated the frictional behaviour of the 45° fibre orientation. Therefore, on a rebuild Pin-on-disc test rig, using composite discs and steel pins, the frictional behaviour of these materials is investigated. Due to the indicated fibre orientation, and a simple trigger mechanism, it was possible to distinguish the friction force for different fibre orientations. The classical know orientations, parallel and perpendicular to the direction of sliding provided expected results, for both cases. The ~45° orientation, which is extremely important regarding filament wounded bearings, behaved in both cases similar -- regarding friction force-- in an unexpected way. The friction force of the 45° orientation was higher in all cases, and the amount was similar to the difference between the parallel and perpendicular case. These findings yield the conclusion that the 45° orientations cannot be neglected in frictional studies, and the behaviour of weft-warp structures also determines performance.