# Vibration Damage Analysis of Bottom Hole Assembly Under Axial Impact Based on Dynamic Analysis
## 2 Impact Simulation Model
### Geometric Models, Boundary Conditions, and Material Properties  
#### 2.2.1 Model Construction  
Based on the working principle of the downhole axial impactor, a vertical wellbore section model of the percussive drilling assembly was established using the finite element software ABAQUS v.2024. The BHA configuration consisted of PDC bit + impactor + stabilizer + non-magnetic drill collar (NMDC) + heavy weight drill pipe (HVDP). The model adopted improved tetrahedral elements (C3D10M) for meshing, with distortion control enabled. The central region was meshed using C3D8R elements, incorporating element deletion and hourglass control to simulate rock fragmentation and failure during the drilling process. To enhance the computational accuracy, mesh refinement was applied near the center of the bottom rock formation. Since the deformation and stress distribution of the borehole wall were not considered, the borehole wall was defined as SR4 shell elements and treated as a discrete rigid body. To improve the computational efficiency, several simplifying assumptions were made, including treating the bit and PDC cutters as rigid bodies. The material parameters of the BHA components are listed in Table 1. The geometric model and boundary conditions are illustrated in Figure 1a to ensure the applicability of the simulation materials in the response analysis; the parameters of each component in the drill string were defined based on the typical physical and mechanical properties of downhole tools. The impact components adopt the material properties of alloy steel, while the drill bit is modeled using PDC composite material, ensuring the engineering representativeness and numerical stability of the simulation model. The number of mesh elements is 45,773 for the rock formation and 17,421 for the PDC BHA components. The final mesh division of the rock-breaking model is shown in Figure 1b.  
\begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Component**} & **Elastic Modulus** & **Density** & \multirow{2}{*}{**Poisson’s Ratio**} & **Tensile Strength** \\  & **(GPa)** & **(kg/m\({}^{3}\))** & & **(MPa)** \\ \hline Impactors & 207 & 7850 & 0.30 & \(1.08\times 10^{3}\) \\ Drill Collar & 207 & 7850 & 0.30 & \(1.05\times 10^{3}\) \\ Stabilizer & 220 & 7850 & 0.30 & \(1.03\times 10^{3}\) \\ Drill Bit & 579 & 15,000 & 0.22 & \(3.54\times 10^{3}\) \\ \hline \end{tabular}
\end{table}
Table 1: Drilling tool combination material parameters.  
During the simulation process, the rock-breaking mechanism induced by the drill bit impact can be divided into the following steps: (1) The drill bit is subjected to a specified WOB and rotates along the Z-axis at a given rotational speed. (2) Under the coupled action of periodic impact loading, the drill bit fractures the rock. The rock failure process is modeled using the Drucker-Prager (D-P) failure criterion, and the bit advances as rock elements are progressively removed due to failure. According to the constitutive model of the rock, an explicit dynamic analysis step is selected. Geometric nonlinearity and large deformation control are enabled to ensure small step sizes and stable displacement variations throughout the simulation process.