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J. He et alii, Frattura ed Integrità Strutturale, 34 (2015) 564-573; DOI: 10.3221/IGF-ESIS.34.62 566 Elasto-plastic Deformation Theory The Elasto-plastic Deformation Theory are proposed by Marshall and Evans, it insists that the indentation area will be developed once the solid particles in drilling muds scour through the surface of fluidic amplifiers. Elasto-plastic deformation domain will be subsequently formed underneath the indentation area, and the axial fracture induced by external loads will be transformed into radial fracture, where after the volumetric wearing capacity V can be derived from formula 1. 3.2 3.7 1.58 1.3 0.62 0 c V V r k H     (1) where, 0 V denotes the velocity of solid particles, r denotes the radius of particle,  denotes the density of particle, H denotes the strength of particles, and c k denotes the stress factor of materials. Micro-cutting Theory The erosion of solid particles with sharped corner on ideal plastic materials has been studied by Finnie, and the micro- cutting theory was proposed according to a series of analysis and experiments. The volumetric wearing capacity V can be derived from formula 2, which is closely related to the incision angle.   mv V P   (2) where, m denotes the mass of a single particle, v denotes the velocity of moving particles, P denotes elastic fluid pressure of flow domain and  denotes the incision angle of solid particles. The micro-cutting theory is feasible to the erosion of solid particles on elastic materials while the incision angle is plenty small. While the incision angle of solid particles is approximate to 90˚, large deviation of erosion volume on brittle materials will be obtained. Secondary Erosion Theory There are two stages for the erosion process of typical materials, which is proposed by Tilly with his research on abrasive materials. First of all, the solid particles washout the surface of abrasive material, then the abrasive material will be rewashed by some solid particles with minor size. The presence of secondary erosion theory is essentially according with micro-cutting theory. No. Particle size  m Velocity m/s Solid content % 1# 20 68.9 2 2# 45 68.9 2 3# 75 68.9 2 4# 45 61.3 2 5# 45 76.6 2 6# 45 68.9 4 7# 45 68.9 8 Table 1 : The properties and obtained acoustic velocities of prepared samples with water-saturated sand. Each theory of anti-erosion property has its own features and specific feasibility, whereas the limitation of each theory is still existed. In the study process of anti-erosion property of a fluidic amplifier, a single theory should not merely be considered as the theoretical basis, and the practice value of each theory should be taken into consideration, which