Issue 27

H. Liu et alii, Frattura ed Integrità Strutturale, 27 (2014) 53-65; DOI: 10.3221/IGF-ESIS.27.07 55 channel (f). The high-pressure jet fluids push the impact body to return to its original position. The fluids in the upper chamber of the cylinder are extruded into the annular space between the hammer and outer sleeve through the vent (c). When the piston moves to the stopping end of the cylinder, the fluid pressure in the lower chamber of the cylinder increases. The pressure signal propagates along the feedback loop and reaches the control nozzle (b), which signals the jet fluids attached to the right sidewall to switch to the left sidewall [4,5]. The bistable fluidic amplifier controls the high-pressure drilling fluids to enter alternately in the upper and lower chambers, which causes the reciprocating motion of the impacting body. Impact force is generated periodically and impact energy is transferred to the anvil and tricone roller bit. Figure 3: Working principle of a liquid jet hammer and a fluidic amplifier. D ESCRIPTION OF FAILURE SCENARIO he fluidic amplifier is an automatic control device that uses several fluid properties [12] and is regarded as the most critical part in a liquid jet hammer. Therefore, the service life of a fluidic amplifier has an important effect on the normal work of a liquid jet hammer. The work condition of a fluidic amplifier is extremely complex because of high-velocity drilling fluids that contain abrasive particles. In previous oil-drilling applications, the velocity of the drilling fluids that enter the supply nozzle reaches 60 m/s to 100 m/s, and the failure of a fluidic amplifier is mainly due to abrasive erosion [13,14]. High-velocity solid particles in drilling fluids wear away the inner structures of a fluidic amplifier, and significant erosion pits can be seen on the inner surface of a fluidic amplifier. Abrasive erosion leads to the failure of the fluidic amplifier such that the service time of a fluidic amplifier ranges from a few hours to a dozen hours. The fluidic amplifier in this study was made of WC-11Co cemented carbide, as shown in Fig.4 (a), with an expected service life of 120 h in complex conditions. However, the accident occurred that the liquid jet hammer only worked within several seconds in the test. The fluidic amplifier from the YSC178 liquid jet hammer was taken out, and three fracture locations were found at the baseplates of the fluidic amplifier, as shown in Fig. 4(b). The appearance of the fractured section was smooth and no evident plastic deformation, such as elongation or bending, were observed. No signs of abrasive erosion were observed on the surface of the sidewalls and the supply nozzle. This form of failure was noticeably different from those in previous tests [13, 14]. This failure was evidently a brittle fracture, but not abrasive erosion. WC-11Co cemented carbide is a typical brittle composite material; fracture failure exhibits no distinct plastic deformation. The yield strength of this material is not easily determined by experimental tests. A sudden fracture failure may occur in T