Research On Crack Propagation Law Of Fracturing Pumphead Based On Cohesive Zone Model

At present,70%of the proven oil and gas reserves in China are unconventional resources every year.Horizontal well and hydrofracture technology is widely used in unconventional oil and gas exploitation because the characteristics of low porosity,low permeability and low pressure of the unconventional oil and gas.As a key component of hydrofracture technology,fracturing pump needs to provide large displacement and high pressure fracturing fluid.Pumphead often breaks down under the action of cyclic high pressure fracturing fluid which greatly slows down the unconventional oil and gas development process,which makes the improvement of the fatigue life of the fracturing pumphead become a hot research topic.Therefore,it is of great theoretical and engineering significance to study the crack propagation law of fracturing pumphead.In this paper,the fatigue crack growth of 5ZB-2800 type fracturing pump was studied the cohesive zone model is chosen as the foundation.Based on the result of the fracture toughness tests,the key parameters of cohesive zone model are calibrated,then the cracking liad,crack front shape and damage distribution under monotonic loading are systematically studied,the cyclic cohesive zone model subroutine was programmed and a simplified finite element model of the pump head was proposed,according to the working characteristics modes of the fracturing pump,the fatigue load spectrum and the initial crack form were determined.The fatigue crack growth rate and fatigue damage accumulation of the pumo head were deeply studied.The main research contents and achievements are as follows:（1）Based on the compliance curve of the crack propogation process,the bilinear traction separation criterion is chosen,the cohesive zone modek parameters of30CrNi₂MoV are:fracture energy G_c=142.43 kJ·m^-2,interface strength T_max=4019.59MPa,interface stiffness K≥10⁶ N/mm³.（2）Under the monotonic load,the cracking load of the pump head material is negatively correlated with the initial crack length and positively correlated with the thickness of the model.The shape of the crack front is directly related to the thickness of the model,with the increase of the thickness,the shape of the crack leading edge gradually changes from a straight line to an elliptical shape;As the crack continues to growth,the crack leading edge gradually evolves into a straight line;the damage area of the crack leading edge is about 1.63 mm,and the damage accumulatin velocity in the middle of the crack front is faster than the crack front.（3）The cyclic cohesive subroutine was programmed by introducing the fatigue damge evolution equation,and the reliability of the subroutine was verified by comparing the experimental results.The results shows that the cumulative rate of fatigue crack damage and the rate of fatigue crack growth are negatively correlated with the stress ratio and positively correlated with the peak load.（4）The fatigue crack propagation velocity in the discharge chamber is faster than the crack in the plug cavity,and the growth speed in the middle of the crack front is the slowest;the larger the crack radius,the faster the fatigue crack growth rate;When the initial crack size is smaller than the critical dimension under the corresponding working conditions,the pressure of the fracturing medium is not the main factor driving the fatigue crack growth.