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Erosion modelling

The modelling of material response is a fundamental step in the prediction procedure. Impact loads are classified according to their amplitude with respect to material yield strength and ultimate strength.

For the present application, mean impact load lies between both limits. Hence, the material surface is progressively hardened by successive impacts. The work hardening process was characterized by LTPCM from microhardness measurements on cross sections of eroded samples. A major parameter of the model is the thickness of the hardened layer.

Using the impact load model, a relationship was derived between pit depth and impact load. It was systematically used to analyse pitting tests and determine the amplitude of the hydrodynamic load (typically in MPa) responsible for each pit. The distribution of impact loads is considered as the signature of the cavitating flow.

In practice, the information on cavitating flow aggressiveness was reduced to three integral parameters: pitting rate, mean diameter and mean amplitude of impact loads. This basic description of flow aggressiveness was used to estimate mass loss.
The erosion model developed in the framework of the PREVERO project allowed us to compute incubation time and mean depth of penetration rate MDPR. An equation has been derived to predict each of them as a function of flow aggressiveness and material properties measured by LTPCM.

The model points out a characteristic time and a characteristic length for cavitation erosion. The characteristic time is the covering time i.e. the time required for the material surface to be entirely covered by impacts without overlapping. As for the characteristic length, it is the thickness of the hardened layer.

The erosion rate MDPR under steady state conditions (measured typically in µm/h) is scaled by the ratio of this characteristic length to this characteristic time, with a multiplicative factor, which depends mainly upon the average amplitude of impact loads.

The incubation time is proportional to the covering time with a coefficient which also depends upon load amplitude and which tends to unity when mean load approaches material ultimate strength.

The values predicted by the model proved to be in satisfactory agreement with the experimental ones obtained from mass loss tests.

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