Characterisation and validation of a pulsating jet erosion test

This study focuses on the further development of a new method of liquid erosion testing by means of a pulsating jet erosion test (PJET). The method was validated in comparison to existing test procedures. Parameters regarding velocity, length and diameter of the jet were evaluated by optical observation through a real-time triggered camera with a high-speed shutter. For the method of the optical parameterisation, good conditions for correct framing of the water jets, were achieved by high illumination and short shutter times. The influence on the jet’s front shape and rounding was included in the parameterisation and investigated in terms of air resistance, diameter and shape of nozzles, the propulsion system of the water, the pump, the hose, and the rotating disc. The jet front plays an important role for the simulation of raindrops by means of water jets. A preliminary study on the measurement of the water hammer pressure of the water jet segments was carried out with a piezo-sensor. The results showed a rapid rise of the pressure, corresponding to the impact pressure and duration. The results presented 100 times lower values when compared to the theoretical water hammer pressure, indicating a lack of water hammer pressure at the moment of impact. However, it is more likely that the piezo-sensor was unable to capture the fast appearing and disappearing water hammer pressure, thus measuring only the transmitted impulse. The optical observation showed the presence of a water-film on the samples. The theory states a diminishment of erosion damage due to such film, which was verified by the material test. The problem was solved by installing an air nozzle to blow-off the water-film from the samples with compressed air. The first step in the validation of the test procedure was taken by comparing the apparent failure mechanisms of the PJET with those obtained with the Multiple Impact Jet Apparatus (MIJA) of the Cavendish Laboratory, University of Cambridge. A comparability of the damage mechanisms was possible to observe regarding both non-destructive surface deformations and destructive damage to the coatings. The second step in the validation was the actual material tests of coatings, which were used to evaluate the specific materials. The coatings examined were polysiloxane coatings, applied to an aluminium substrate, with a CAA surface treatment, and an epoxy primer as adhesion promoter. The tests were also the base for the verification of the PJET procedure. The material evaluation was correlated to parameters of the samples such as hardness, coating thickness and erosion resistance. From the results and the validation of the material testing it can be concluded that the PJET is a working test procedure for liquid impact. The erosion resistance was evaluated with both the PJET and the MIJA by two independent methods. The results of the evaluations presented a practically complete coherence in question of the mutual erosion resistance between the samples pertaining to each evaluation method. It was seen that the harder the sample, the better the erosion resistance, at the same time as the higher the cross-linking, the better the erosion resistance, although a higher cross-linking decreases the hardness. As conclusion it can be stated that the PJET complies with existing test standards. The erosion damage obtained is comparable within proper test series and with other erosion test methods and finally that the validation of PJET test procedure was successful.<p>Validerat; 20101217 (root)