When the pump head body of the water injection pump is subjected to an alternating load during the operation process, the fatigue crack is generated. The fracture failure caused by the pump head body, the defects of low stress should suddenly brittle fracture reason and the pump head body is closely related to the inner defects may be, smelting, casting, forging production of inclusions, pores, white spots caused by.
The fatigue failure of the pump head undergoes 3 stages, namely the incubation period, the crack expansion period and the final fracture.
1. Incubation period: the pump head under the alternating load, to produce the time required for crack and alternating cycle, regardless of whether the pump body material local defects, there are incubation period. The degree of small defects of raw materials is different, the length of incubation period is quite different.
2. Expansion period: under the action of alternating load, from the initiation of the crack initiation to crack, the crack from the micro to the macro expansion. This expansion is the slow expansion of the crack size under the critical crack size condition of the final fracture.
3. Fracture: the external load is constant, with the subcritical crack growth, the crack length increases, the stress intensity factor of the crack tip increases with the increase of K value. When the crack length reaches a critical crack size, crack tip stress intensity factor value will reach the fracture toughness under fatigue conditions, pump head rapid crack instability expansion, pump head end fracture.
It can be known that the fatigue life is determined by the formation of the crack initiation period, the subcritical expansion rate of the fatigue crack, and the critical dimension of the final fracture.
