Issue 27

H. Liu et alii, Frattura ed Integrità Strutturale, 27 (2014) 53-65; DOI: 10.3221/IGF-ESIS.27.07 56 WC-11Co cemented carbide if the actual local stress exceeds the material strength limit. In addition to different unexpected failure scenarios, the manufactured material and loading situation of the fluidic amplifier in this study are different from those in previous tests. (a) (b) Figure 4: Intact and damaged baseplates of the fluidic amplifier. (a) Intact fluidic amplifier; (b) three fracture locations on the baseplates. (1) New material. WC-11Co cemented carbide was used to manufacture a fluidic amplifier instead of 35CrMo alloy steel in the previous test to reduce abrasive erosion. This material exhibits several good material properties, such as high hardness, high strength, good wearing resistance, and great corrosion resistance [14]. (2) No disc springs. A set of disc springs used in the previous test was installed at the top of the fluidic amplifier. These disc springs were adjusted to provide a small fitting allowance among the components inside a liquid jet hammer; in this way, sealing problem could be solved and cylinder deformation could be avoided. However, the disc springs with an unreasonable design and unpredictable fitting allowance can lead to more serious leakage and abrasive erosion. Therefore, the disc springs in this test were removed from a liquid jet hammer [13, 14]. In this study, the following failure analyses were comprehensively investigated: (1) strength analysis with numerical simulations; (2) fractographic analysis; (3) metallographic analysis; (4) and processing defect analysis. N UMERICAL SIMULATIONS FOR STRUCTURE STRENGTH ANALYSIS OF THE FLUIDIC AMPLIFIER n this study, the simplified physical process explains that before the liquid jet hammer has applied work on the drilling fluid pressure and the axial static pressure caused by threaded connection have generated pre-stress on the fluidic amplifier, the impact force caused by the backward stroke of the impacting body is exerted at the bottom of the fluidic amplifier. The force analysis diagram of the fluidic amplifier is shown in Fig. 5. However, the effect of the static and dynamic loads on the material strength of the fluidic amplifier remains unknown. The “implicit-explicit sequence solving” analysis method [15] is employed to investigate the reason for fracture failure. The following is the procedure of the numerical analysis: (1) the drilling fluid pressure on the sidewalls of the baseplate and the terminal velocity of the backward stroke of the impacting body can be calculated using the CFD method; (2) the axial static pressure on the fluidic amplifier can be theoretically estimated; (3) the calculated drilling fluid and axial static pressures are set at the boundary conditions in the implicit static calculation analysis, and pre-stress static results could be obtained by an implicit solver; (4) based on the implicit results, the terminal velocity of the backward stroke of the impacting body is set to an initial condition in the explicit solver; (5) the dynamic load results can be obtained using the explicit solver of ANSYS/LS-DYNA. The effect of the static and dynamic loads on the material strength of the fluidic amplifier can be determined by comparing the first-principle and equivalent stresses to the strength limit. I