A fatigue-based model for the droplet impingement erosion incubation period

For the lifetime assessment of surfaces affected by droplet impingement erosion, only droplet impact test facilities were available at the start of this research to evaluate and rank materials and coatings.
In this thesis, a fatigue-based analytical model for the prediction of the droplet impingement erosion incubation period (timespan before erosion by detachment of wear particles starts) has been developed with the intention of creating a science-based alternative that allows for the selection and development of materials and/or material properties for an enhanced resistance to droplet impingement erosion.

The impact of a water drop on a solid surface at high velocity results in a high water pressure in the contact area due to the local compression of water in a part of the drop (water hammer pressure). This pressure depends partly on the physical properties of the solid material. Upon drop impact, three different waves start travelling in the solid material. The Rayleigh surface wave has been identified as producing the highest stress cycles in the region around the drop impact contact area.
Depending on the type of material and material properties, different types of wear modes have been identified: brittle fracture, surface fatigue and a mixture of the two. Surface fatigue, without any local brittle fracture, can be considered as the wear mode with the highest erosion lifetime. Thus, in the predictive model, surface fatigue has been considered as the wear mechanism, provided that the fracture toughness of all microstructural phases is above a certain threshold value. This depends, however, on the drop impact velocity and the drop size.
As the maximum stress cycle at the surface follows from the Rayleigh surface wave, fatigue properties of the material as given by standard S-N curves, and fatigue damage accumulation based on the Palmgren-Miner, theory have been used for this predictive model for the droplet impingement erosion incubation period.

For an accurate model validation, the predictive model has first been applied to the thermoplastic material PBT, produced by injection moulding and compression moulding, and compared with drop impact erosion results on the same PBTs. For both PBTs a good similarity between test results and model predictions for the injection moulded PBT was found (deviation of 29%). However, the absolute value of the incubation period predicted by the model for the compression moulded PBT differed by a factor of 15.
In a second validation, the analytical model for the prediction of the incubation period of metal surfaces was compared with a wide range of liquid droplet erosion incubation period tests. The model was extended for the use of S-N curves for non-ferrous and ferrous metals –aluminium and stainless steel respectively – by including the effects of additional surface hardening and residual compressive stress at the surface due to a water drop peening effect. Model predictions were performed for stainless steel AISI 316 and aluminium 6061-T6, using S-N fatigue curves from different literature sources, and including the defined additional surface hardening and a residual compressive stress state at the surface due to “water drop peening effect”. For the droplet impact velocity range of 140 to 400 m/s they showed excellent agreement with the multi-regression equation as determined from an ASTM interlaboratory test program. Nearly all incubation period predictions were within the 95% confidence limits of the aforementioned multi-regression equation.

In this research, a strongly enhanced understanding of the relationship between the physical and mechanical properties and the drop impact erosion incubation period of metals, thermoplastics and elastomers has been obtained. The physical and metallurgical mechanisms resulting in the degradation process of the metal surface during the incubation period were identified. These consisted of 1) surface plastic deformation and formation of dents; 2) surface hardening and residual compressive stress as a result of these surface plastic deformations; 3) fatigue crack initiation; and 4) fatigue crack growth.

The selected wave properties (dynamic impedance) and fatigue properties of the metals, thermoplastics and elastomers used in the presented analytical model were identified with respect to developing guidelines for enhanced droplet impingement erosion incubation life. The relative impact pressure (pwh/vd) can be used to identify to what extent certain material classes reduce the water hammer pressure and corresponding maximum stress due to the Rayleigh wave.