Abstract

Erosion wear due to high-speed water droplets impact is a major reliability concern in several power generation and aerospace industries. The problem is synonymously referred to as water droplet erosion (WDE), liquid impingement erosion (LIE), rain erosion (RE), or leading-edge erosion (LEE). The present work addresses two important erosion responses; material endurance against erosion (known as the threshold condition) and the number of droplet impacts needed to initiate erosion damage (known as the incubation period). The objectives of the work are to develop a prediction model for the threshold condition, and to contribute to the understanding of the damage mechanisms in the incubation period.

The present work combines the theory of threshold crack propagation with dynamic wear and fracture properties of materials to arrive at a mathematical equation (model) for the threshold conditions. The developed model predicts the threshold impact velocity of metallic materials directly from their mechanical properties and impact conditions. The model is experimentally validated for five metallic alloys namely; Ti-6Al-4V alloy, 17-4 PH stainless steel, stainless steel (X22CrMoV12-1), 7075-T6 aluminum alloy, and 2024-T4 aluminum alloy. The developed model is also compared to the analytical model developed in the literature and found to predict threshold velocities with higher accuracy. The threshold velocity - simply calculated from the developed model - can directly evaluate the risk of developing erosion damage for specific material, and hence can be used as an effective material design and selection tool.

As for the incubation period, the influence of change in surface roughness (i.e., roughening) and the rate of change in hardness (i.e., hardening) on the damage accumulation process have experimentally and numerically been investigated in this work. Water droplet erosion tests were carried out on Ti-6Al-4V alloy and 17-4 PH stainless steel. Scanning electron microscopy (SEM), Confocal Laser Scanning Microscope (CLSM), Vickers Hardness measurement, Tensile Tests, and Finite Element Analysis (FEA) of Impact stresses were carried out. The results show that the dynamic surface roughening process results in higher and continuously-increasing impact stresses for the same impact pressure due to geometrical stress concentration. It is also found that the strain hardening exponent (n) influences the incubation period by controlling the rate at which the fracture of surface fragments is achieved as well as the rate with which mechanical properties change with the increase in number of droplet impingements. It is concluded that strain hardening exponent (n) and surface roughening are crucial parameters in the damage accumulation process during the incubation period.