Optimization Research on the Drilling Parameters of the RC-DTH Air Hammer by Employing Smoothed Particle Hydrodynamics

Abstract

Down the hole (DTH) air hammer is widely used in hard rock drilling for its high drill rate. Revealing the influence mechanism of drilling parameters by RC-DTH air hammer is of great significance to promote the application of RC-DTH air hammer impact rotary rock breaking technology in deep hard rock drilling. In this paper, based on LS-DYNA and Smoothed Particle Hydrodynamics, a three-dimensional numerical simulation model of piston-bit-rock in the RC-DTH air hammer impact rotary drilling is established to explore the propagation and action law of stress wave in rock and drilling tools, and to analyze the influence of different drilling parameters on the rock fragmentation effect. A testing facility, known as the RC-DTH air hammer impact energy test platform, has been constructed to confirm the accuracy of the numerical simulation results obtained. After a thorough analysis that accounts for the effects of stress concentration and drilling parameters on the efficiency of rock breaking, the identified optimal operating parameters are as follows: a drilling pressure of 4000 Newtons, a piston impact velocity ranging from 9 to 11 m per second, and a drill bit rotation speed of 30 rpm. Field tests confirmed that applying this optimal parameter set increased the single-impact energy of the hammer by approximately 32.5%, and optimized the energy transfer efficiency to around 17.5%, thereby achieving a quantifiable engineering balance between improving rock-breaking efficiency and reducing tool wear.

1. Introduction

The Reverse Circulation Down-the-Hole (RC-DTH) air hammer drilling method has been widely used in mineral exploration, water well drilling, and large-diameter hard rock pile foundation drilling, among other applications. − This method offers several advantages, including high penetration rates in hard rock drilling, the pro-duction of straighter holes, controlled dust pollution, and continuous sampling without lifting the drill. − The RC-DTH air hammer collaborates with the drilling rig and the corresponding drilling tool to achieve percussive-rotary drilling. The drilling rig rotates the drill pipe and applies weight on the bit (WOB), while com-pressed air is injected to drive the DTH air hammer through dual-wall drill pipes. This process breaks the rock under the combined action of drilling pressure, rotation, and impact load, significantly enhancing drilling efficiency compared to traditional rotation drilling in hard rock. − Li et al. investigated the energy consumption and mechanical properties of rock under dynamic and static combined loads, proposing the theory of rock incident wave and the theory of rock breaking under dynamic and static combined loads. Chiang and Elías employed the differential method to analyze the interaction process between the piston and the drill bit by dividing it into microsegments. Kivade et al. examined the drilling rate influence factors of the DTH air hammer drilling method through dimensional analysis and derived an empirical formula to estimate the drilling rate. Kwon et al. optimized the configuration of bit teeth using a hammer impact test system. The aforementioned studies have qualitatively summarized the phenomenological and wave transmission characteristics of rock breaking under dynamic combined with static loads. Simulating the physical process of hammer impact rock by finite element methods has been widely employed to characterize rock breaking under dynamic combined static loads quantitatively. − Zhu et al. used RFPA-Dynamics software to simulate the failure process of rock under dynamic and static loads and concluded that the rock strength increased significantly under dynamic loads. Bu et al. studied the stress wave propagation characteristics between the hammer and rock. Hashiba et al. surveyed the impact penetration behavior of a bit drilling into rock, and Tkalich et al. investigated the mechanical behavior of rotary-percussive drilling tools made of tungsten carbide (WC) hard metal in impact interaction with rock, respectively. However, field tests show that the piston of DTH hammer and drill bit have a short service life, and it is urgent to further improve their service life while ensuring drilling efficiency. The drilling parameters such as piston impact velocity, drilling pressure, and rotation speed are key factors relevant to DTH hammer service life.

This research introduces a three-dimensional numerical simulation model for the piston-bit-rock interaction in RC-DTH air hammer percussive rotary drilling, which is based on the Smoothed Particle Hydrodynamic. The model is used to conduct a quantitative investigation into how drilling parameters, such as pressure, piston impact velocity, and bit rotation speed impact the efficiency of rock breaking.

2. Numerical Simulation

To streamline the operational process of the RC-DTH hammer, the numerical model developed in this paper concentrates on simulating the single impact event of the RC-DTH hammer. The erosion of rock by drilling bit is a typical nonlinear dynamic process with high strain rate. To decrease the computational time required by the CPU, the model assumes the rock to be continuous, uniform, and isotropic. Additionally, it is assumed that there is no frictional interaction between the piston and drill bit with the outer wall of the RC-DTH air hammer. The research framework for the numerical simulation is depicted in Figure .

1.

Numerical simulation research framework.

2.1. The Basic Principle of SPH Method

The SPH (smoothed particle hydrodynamics) method is a grid-free Lagrangian technique designed for solving partial differential equations. It operates based on interpolation theory, where the solution is represented by a set of particles rather than a fixed grid. , The basic principle is to disperse the geometry into particles with physical quantities such as mass, density, internal energy and velocity to replace the mesh unit used in finite element method. To derive the physical quantities within a given space, we employ a method that involves interpolating and summing the properties of all particles within the vicinity. This methodology produces an approximation of the continuous field variables. Subsequently, a discrete field function is employed to resolve the pertinent partial differential equations. The SPH technique incorporates a kernel function to smooth the field function, and this approximation of the field is referred to as the kernel estimation of the field. The kernel estimate of the field function f at any point i < f(xi)> is equal to the sum of the interpolated field function values of all particles in a spherical (center) region with point i as the center and 2h as the radius (h is the smooth length).

$$<f(x_i)>=\sum _{\mathrm{j}}\frac{m_j}{p_j}f(x_i)W(x_i-x_j,h)$$ 1

where x _i is the position coordinate of point i, x _j is the position coordinate of particle j, which is in the space fixed coordinate system; f(x _j ) is the value of the field function at particle j; W is the kernel function, which reflects the distance and contribution degree of the point; h is a smooth length, and the particles with a distance of more than 2h from point i usually have no contribution to the physical quantity of point i j is the number of particles falling in the influence region of point i; m _j is the mass of particle j; ρ _j is the density of particle j. The kernel estimate of f in the differential form of point i is

$$<\mathrm{\nabla }f(x_i)>=-\sum _j\frac{m_j}{p_j}(f(x_i)-f(x_j))\mathrm{\nabla }W(x_i-x_j,h)$$ 2

Then the discrete form of the mass and momentum conservation equations is

$$\frac{\mathrm{d}{\rho }_i}{\mathrm{d}t}={\rho }_i\sum _j\frac{m_j}{{\rho }_j}[{(v_{\alpha })}_i-{(v_{\alpha })}_j]\frac{\partial W(x_i-x_j,h)}{\partial {(x_{\alpha })}_i}$$ 3

$$\frac{\mathrm{d}{(v_{\alpha })}_i}{\mathrm{d}t}=\sum _j{m}_j[\frac{{({\sigma }_{\alpha \beta })}_i}{{\rho }_i^2}+\frac{{({\sigma }_{\alpha \beta })}_j}{{\rho }_j^2}+\prod _{ij}{\delta }_{\alpha \beta }]·\frac{\partial W(x_i-x_j,h)}{\partial {(x_{\beta })}_i}$$ 4

$$\frac{\mathrm{d}{E}_i}{\mathrm{d}t}=\frac{P_i}{{\rho }_i^2}\sum _j{m}_j[{(v_{\alpha })}_i-{(v_{\alpha })}_j]\frac{\partial W(x_i-x_j,h)}{\partial {(x_{\alpha })}_i}$$ 5

where i and j are the numbers of rock particles; α and β are in the coordinate direction; ρ is particle density; m is the particle mass; x is the position coordinate of the particle in the global Cartesian coordinate system; W is a smooth kernel function; h is the smooth length; v is speed; σ_αβ is the stress tensor; ∏ _ij is artificial viscosity; δ_αβ is the Kronecker function; E is energy; P is pressure.

2.2. Model Setup

In this study, the SPH method is utilized to represent the rock as a conical structure with a specified radius of 65 mm and a height of 50 mm within the Ls-Dyna simulation framework. This geometric model is further divided into a total of 44,082 particles to capture the rock’s behavior. The failure criterion adopted for the rock in our analysis is the Mohr-Coulomb criterion, which is well-suited for characterizing the mechanical properties of geological materials under various stress conditions. − When the shear stress acting on a specific plane within the rock model attains its critical limit, the model undergoes yielding and subsequent failure. This threshold represents the point at which the internal structure of the rock can no longer sustain the applied stress, leading to a loss of structural integrity. Within the Ls-Dyna simulation software, the rock model is assigned the material property set designated by the keyword *MAT_MOHR_COULOMB. Additionally, the rock’s bottom boundary is fully constrained to simulate the fixed conditions typically encountered in drilling scenarios. In this paper, the rock is modeled using the Drucker-Prager criterion, which is suitable for granular materials such as rock, soil, and concrete. This criterion is based on the Mises criterion and incorporates the average principal stress, reflecting the intrinsic nature of shear-compression failure in rock materials. Its specific form is expressed as

$$f=\alpha I_1+\sqrt{J_2}-K=0$$ 6

where α and K are constants related to the internal friction angle and cohesion of the rock, and their expressions are as follows

$$\{\begin{array}{l}\alpha =\frac{\sin \phi }{\sqrt{3}\sqrt{3+{\sin }^2\phi }}\\ K=\frac{3C\cos \phi }{\sqrt{3}\sqrt{3+{\sin }^2\phi }}\end{array}$$ 7

In rock mechanics research, the Drucker-Prager (D-P) criterion is widely adopted due to its comprehensive consideration of the confining pressure effect and shear-induced dilatancy. To ensure that the numerical simulation results accurately reflect actual drilling conditions, the D-P criterion is selected as the basis for defining the constitutive relation of rock materials. The material density is set to 2.92 × 10^–9 t/mm³, and the Young’s modulus is set to 45,600 MPa, which are used to simulate hard rock materials such as granite during actual drilling operations. Material parameters are listed in Table . For the piston and drill bit of the positive circulation air DTH hammer, the solid steel material MATL20 from the HYPERMESH material library is selected. For components using this rigid body material model, all nodes will maintain constant relative displacement; the material density (Rho) is set to 7.85 × 10^–9 t/mm³; Young’s modulus (E) is set to 2.06 × 10⁵ MPa; Poisson’s ratio (PR) is set to 0.3. For the cemented carbide button teeth, the solid material MATL1 from the material library is selected, with a density of 14.5 × 10^–9 t/mm³, Young’s modulus (E) set to 5.88 × 10⁵ MPa, and Poisson’s ratio (PR) set to 0.22, as shown in Table . The constructed model diagram is diagramed in Figure .

1. Rock Material Parameters.

ρ (kg/m3)	E s (MPa)	μ	θ (rad)	C (MPa)
2920	45,600	0.20	0.75	12.5

2. Material Parameters of Piston, Drill Bit, and Hard Alloy Ball Teeth.

	ρ (kg/m3)	E (MPa)	μ
piston	7850	206,000	0.3
drill bit	7850	206,000	0.3
hard alloy ball teeth	14,500	588,000	0.22

2.

Model grid division diagram.

In the simulation setup, the attribute card is annotated with the keyword *SECTION_SOLID_TITLE for organizational clarity. The particle smoothing length is parametrized with a constant value of 1.2, while the minimum and maximum smoothing lengths are scaled by factors of 0.2 and 2.0, respectively. The contact surface characteristics are defined by a static friction coefficient of 0.15 and a dynamic friction coefficient of 0.12. The drilling simulation initializes with a 100 mm gap between the piston and the drill bit, culminating in a piston impact velocity of 10 m/s. The computational duration is fixed at 0.1 s, with a time step resolution of 0.001 s. To ensure numerical stability, mass scaling is employed, and the time step scaling factor is set to 0.9. The drill bit is subjected to a constant rotational speed of 30 rpm. gravitational acceleration is specified as 9800 mm/s², and a constant drilling force of 4000 N is applied to mimic the pressure exerted during the drilling process.

The simulation settings are as follows: The keyword *SECTION_SOLID_TITLE is added to the property cards of both the piston and the drill bit, and the finite element solution integration algorithm (ELFORM) is set to 1 (Constant stress solid element). The impact property card adopts Tools-INITIAL-*INITIAL_VELOCITY_GENERATION, and the static friction coefficient (FS) and dynamic friction coefficient (FD) are set to 0.15 and 0.12, respectively. For the contact part between the drill bit and the rock model, the property card *CONTACT_AUTOMATIC_NODES_TO_SURFACE is added; the preset contact points are incorporated into the set and entered into MSID and SSID, with the corresponding static friction coefficient (FS) and dynamic friction coefficient (FD) set to 0.2 and 0.18, the negative damping coefficient (VDC) set to 20, and the master-slave stiffness scaling factors (SFM, SFS) both set to 5 to avoid contact failure. Under the “load collectors” module, a surface constraint is applied to the bottom of the rock model via the “by face” method to limit the displacement of the rock formation. For the piston and the drill bit, surface mesh elements are selected and assigned the SECTSHELL element property; the CMO global constraint is set to 1, CON1 = 5 (restricting translation in the y/z directions), CON2 = 7 (restricting rotation in the x/y/z directions), and the thickness of the shell element is set to 0.1 mm to ensure axial movement. Meanwhile, gravity (9800 mm/s²) and a constant load of 4000 N (simulating drilling pressure) are added, both paired with “load curve” stress–strain curves.

3. Results and Discussion

3.1. Rock Intrusion Process

Figure illustrates the impact process of the model, depicting the interaction between the piston of the RC-DTH air hammer and the drill bit. At a time of 0.0099 s, the piston collides with the upper portion of the drill bit, subsequently reversing its direction. The drill bit initially engages with the rock surface due to gravitational forces at the onset of the impact. Upon the piston’s contact with the drill bit, the bit body commences downward penetration into the rock, halting its positive x-axis displacement at 0.1100 s. Following this, the drill bit reverses its movement along the negative x-axis and begins to rotate, facilitating the erosion of the rock model.

3.

Whole process of single impact motion of the model.

To investigate the failure mechanism of the rock model, the grid elements corresponding to various sections of the spherical tooth on the drill bit were selected. The X-axis displacement versus time curves for these elements are presented in Figure . The high degree of coincidence among the three displacement–time curves indicates that there is negligible deformation within the drill bit’s grid elements. The drill bit initially engages with the rock model due to gravitational forces and drilling pressure at 0.0019 s and subsequently collides with the piston at 0.0089 s. During this brief interval, the bit penetrates the rock along the X-axis, reaching its maximum depth of 2.5088 mm at 0.0109 s. Following this peak penetration, the bit then reverses its direction along the negative X-axis due to the impact reaction forces exerted by the rock model.

4.

Time curve of drill bit invasion depth diagram.

Figures and depict the progression of Von Mises stress distribution within the rock model. Immediately upon impact, an intrusion pit forms in the rock. This pit’s volume expands progressively as the drill bit penetrates until it reaches its maximum depth. Concurrently, the rotation of the bit causes the intrusion pit to enlarge along the YZ plane throughout the duration of the impact cycle, culminating at the termination of the drilling action.

5.

Top view: evolution of the rock intrusion pit along the X-direction over time (s).

6.

Section view: evolution of the rock intrusion pit over time (s).

3.2. The Influence of Drilling Parameters on the Fragmentation Effect of Rock

3.2.1. The Influence of Piston Impact Final Velocity

This section of the paper investigates the effect of the final impact velocity of RC-DTH air hammer piston on the stress it experiences. Numerical simulations are conducted at varying final impact velocities of 7, 9, 11, and 13 m/s. To analyze the stress distribution, Von Mises stress–time curves are generated by selecting elements in the stress concentration area near the piston hole. Figure presents these curves, showing that the equivalent stress values at the peaks of the stress waves corresponding to the different final impact velocities are 61.67, 103.86, 152.60, and 180.30 MPa, respectively. This indicates that an increase in the piston’s final impact velocity leads to a significant rise in both the peak equivalent stress and the average stress over one impact cycle. The results suggest that under actual working conditions, the final impact velocity of the RC-DTH hammer piston must be carefully considered to balance the rock-breaking effect with the stress concentrations within the internal components of the hammer. This is crucial to ensure the durability and performance of the drilling equipment.

7.

Von Mises stress variation curve of test elements under different piston impact end velocities: (a) 7 m/s; (b) 9 m/s; (c) 11 m/s; (d) 13 m/s.

The final impact velocity of the piston in the RC-DTH air hammer is a key factor that defines the impact energy of the system. To examine how the piston’s final impact velocity influences the efficiency of rock breaking, a series of numerical simulations are conducted with varying final impact velocities. For these simulations, the drilling pressure is maintained at 4000 N, and the drill bit’s rotation speed is fixed at 30 rpm.

Figure demonstrates a direct linear relationship between the drill bit’s penetration depth and the final impact velocity of the piston. The maximum penetration depth of 2.94 mm is observed at a piston velocity of 12 m/s. Concurrently, the piston’s rebound speed increases from 0.934 to 1.24 m/s as the final impact velocity is raised from 5 to 12 m/s. The data indicate that the piston’s final impact velocity has a minimal influence on the rebound speed, which is primarily governed by the valve mechanism within the RC-DTH hammer piston. This mechanism facilitates the reciprocating motion of the piston, thereby achieving a specific impact frequency.

8.

Intrusion depth and piston rebound speed diagram.

During the rock-breaking process involving the impact-rotational action of the RC-DTH air hammer, the impact energy is conveyed from the piston to the drill bit and subsequently from the bit’s ball teeth into the rock. To investigate the influence of the piston’s final impact velocity on the efficiency of this energy transfer within a three-dimensional numerical model, simulations are conducted under various final piston impact velocity scenarios. The simulations measure the piston’s impact energy, the rock’s internal energy, and the percentage efficiency of the impact energy transfer, as depicted in Figure .

9.

Impact energy, rock absorption of internal energy, and transfer efficiency.

The findings indicate that the impact energy of the piston rises considerably as the piston’s final impact velocity increases. Concurrently, the energy absorbed by the rock to facilitate breakage also increases with piston impact velocity, albeit at a slower rate. The model’s energy transfer efficiency exhibits a gradual decline as the piston impact velocity increases. During the impact sequence, the RC-DTH hammer’s piston strikes the drill bit, resulting in a conversion of impact energy.

A portion of this energy is transformed into the drill bit’s kinetic energy, another part becomes the potential energy associated with the piston’s rebound, and some is retained within the drill bit as internal energy. Additionally, a fraction of the energy is dissipated as thermal energy due to the collision and friction between the piston and the hammer’s outer wall. In the rock-breaking process, which involves both impact and rotation, the drill bit’s kinetic energy is partially converted into the energy required for rock fracturing. Some of this energy is stored in the rock as elastic deformation energy, while the remainder is lost as thermal energy due to the friction between the drill bit’s ball teeth and the rock surface.

Adjusting the impact velocity of the piston to an appropriate level can enhance the efficiency with which impact energy is transferred, while also mitigating the stress concentration in both the piston and the drill bit. Given the operational demands for the RC-DTH hammer’s drilling footage per hour in real-world scenarios, the optimal final impact velocity for the piston should be maintained within the range of 9–11 m per second. This range is determined to balance the effectiveness of rock breaking with the longevity and performance of the equipment.

3.2.2. Determination of Drilling Pressure

The drilling mechanism of the RC-DTH air hammer is primarily based on the piston’s impact on the drill bit, as opposed to increasing the bit’s penetration rate through drill pipe pressurization. , An optimal drilling pressure is essential for the efficient transfer of the hammer’s impact energy and for boosting the frequency of impacts. However, an excessively high drilling pressure can trigger a range of engineering challenges, including the loss of carbide teeth, erratic drill pipe vibrations, and rapid wear of the drill bit. Conversely, a drilling pressure that is too low may not guarantee the effective transfer of impact energy. Table presents the suggested working pressures for the RC-DTH hammer in practical operating scenarios. The numerical model was tested across a range of drilling pressures from 2.5 kN to 5.0 kN. The simulation results indicate that at a drilling pressure of around 4000 N, the drill bit approaches the rock surface closely before making contact, without any rebound, thus validating the model and the pressure setting for effective drilling operations.

3. Recommended Drilling Pressure for RC-DTH Air Hammer Drilling.

diameter of RC-DTH hammer/mm	minimum drilling pressure/KN	maximum drilling pressure/KN
76	0.1.5	3.0
102	2.5	5.0
127	4.0	9.0
152	5.0	15.0
203	8.0	20.0
305	16.0	35.0

3.2.3. The Influence of Rotation Speed of Drill Bit

To investigate the effect of various rotation speeds of the RC-DTH air hammer on rock breaking, seven distinct models with different rotation speeds were numerically simulated. The drilling pressure in these models was consistently set at 4000 N, while the final impact velocity of the piston was fixed at 10 m/s.

Figure illustrates that the maximum penetration depths for the seven model drill bits are 2.53 mm, 2.51 mm, 2.50 mm, 2.52 mm, 2.50 mm, 2.51 mm, and 2.46 mm, respectively. The data suggest that the rotation speed has a negligible effect on the depth of penetration during the impact breaking stage of the rock. However, during the rotary rock fragmentation stage, the rotation speed of the drill bit plays a significant role in the rebound height. At a rotation speed of 40 rpm, the rebound height of the bit is 2.06 mm, which is 1.37 mm higher than the 0.69 mm observed at a rotation speed of 10 rpm. This increase in rebound height is attributed to the fact that a higher rotation speed results in a larger impact reaction force at the point of contact between the ball teeth of the bit and the rock, leading to a greater rebound. Within an impact cycle, a higher rotation speed corresponds to a larger rotation angle of the drill bit per unit time, which in turn results in a greater amount of shear-induced rock fragmentation. This highlights the importance of rotation speed in the efficiency of rock fragmentation during the rotary stage of the drilling process.

10.

Drill bit penetration depth at different rotational speeds.

As the rotational speed of the RC-DTH air hammer bit rises, there is a notable increase in stress concentration within the drilling apparatus. To substantiate this finding, we sectioned the numerical model of the bit along the XZ biaxial plane. We then selected three specific elements to quantify the peak equivalent stress in the area of stress concentration, located between the bit and the carbide ball teeth. This analysis is illustrated in Figure .

11.

Position and Von Mises stress of the test particles diagram: (a) drill bit profile along the xy plane; (b) stress of elements A, B, and C versus rotating speed.

The maximum equivalent stress observed in elements A, B, and C increases at varying rates as the rotation speed of the bit increases. Notably, the peak stress at the cemented carbide ball teeth reaches a high of 851.0 MPa. Such excessive stress concentration can lead to accelerated wear or even the detachment of the carbide inserts from the drill bit, as well as severe vibration in the drill pipe, which can negatively impact the drilling process. To mitigate these issues and extend the working life of the bit while still achieving the necessary penetration rate with the RC-DTH air hammer, it is advisible to moderately reduce the rotation speed. Based on the findings, a bit rotation speed of approximately 30 rpm is deemed more reasonable under real-world working conditions, striking a balance between effective rock breaking and tool longevity.

3.3. Impact Performance Test of RC-DTH Air Hammer

To confirm the precision of the simulated impact energy of the RC-DTH air hammer piston, an experimental test platform was constructed specifically for measuring the impact energy of the RC-DTH hammer. Using this platform, the impact energy of a piston with an 89 mm inner diameter, which was also the size used in the numerical simulation, was accurately measured.

3.3.1. Test Principle

In the experimental setup, the impact energy of the RC-DTH air hammer piston is calibrated using the drop hammer method. The piston is elevated to a specific height, L, and then allowed to fall freely, striking a specialized drill rod. As the impact occurs, a strain waveform is generated and captured by the data acquisition and analysis system. The peak value of this strain waveform, denoted as μ₁, is directly proportional to the gravitational potential energy of the piston, establishing a linear relationship for calibration purposes.

$$L=\frac{1}{2}g{t}^2$$ 8

$$v=gt$$ 9

$$Q=\frac{1}{2}m{v}^2$$ 10

$$Q=a{\mu }_2+b$$ 11

where g is the gravitational potential energy, 9.81 m/s²; m is the piston mass, 4.86 kg; Q is the piston impact energy; a and b are constants. By substituting the mean value of the strain wave peak μ₂ measured by the RC-DTH air hammer piston during the working process into the linear equation, the impact energy is obtained.

3.3.2. Construction of DTH Hammer Impact Energy Test Platform

Figure a illustrates the setup where the test bit is positioned on a polyurethane plate and leveled. A 50 mm diameter PVC tube is chosen and fixed into the test bit’s through-hole. The piston, with an inner diameter of 52 mm, can slide freely within the PVC tube, and any frictional effects are not considered in this experimental setup. The strain gauge is linked to the data collection apparatus via a specialized cable. The gauge’s wiring is terminated at one end of the cable in a 1/4 bridge configuration, which is designed for straightforward tension and compression measurements. The connection utilizes a three-wire system, appropriate for 120 Ω resistance strain gauges. The strain gauge is attached to the force-measuring rod of the drill bit using AB glue, as depicted in Figure f. The test system, shown in Figure d, is then assembled. For each test, two channels are connected, and the device balance is cleared prior to sampling. The sampling frequency is set to 500 Hz. The test involves impacting the bit with the piston along the PVC tube from various heights. During this process, the oscilloscope of the sampling device is monitored for distinct peak signals. This procedure is repeated at different heights to ensure the collection of dependable data.

12.

RC-DTH hammer impact energy test platform.

Figure b illustrates the experimental setup, which utilizes the MGY-120 crawler-mounted hydraulic anchoring drill rig alongside a diesel-powered air compressor. The operational pressure for the system is established at 1.3 MPa. As depicted in Figure e, the damping plate assembly comprises a three-layer structure of polyurethane, rubber, and nylon plates, accompanied by two mounting frames. This configuration is designed to effectively absorb impacts, cushion vibrations, and mitigate stress concentrations. In Figure c, the components of the RC-DTH air hammer used in the testing are detailed. The hammer consists primarily of an inner cylinder, a valve rod, a piston, a drill bit, and an outer cylinder. The inner cylinder, piston, valve rod, and outer cylinder are interconnected. The drill bit is attached to the outer cylinder via a spline sleeve thread. The air intake head is linked to the outer cylinder through an upper joint thread, and a strain gauge is affixed to the force-measuring rod of the bit. Once the air pressure from the diesel air compressor reaches the required level for the test, the sampling equipment is used to monitor the test waveforms, ensuring that the data collected is reliable and stored for analysis.

3.3.3. Measurement Results

In the calibration test for the drop hammer, we conducted a thorough analysis of the data collected at various drop heights. We began by filtering out any peak values that were notably outside the expected range, either excessively high or low. For the remaining, more consistent peaks, we calculated the average values. Subsequently, we paired these values to solve the equations, resulting in a set of coefficients denoted as “a”. After averaging the “a” values, we introduced the initial coefficient, a ₀, back into the original equation to determine the value of b ₀. The detailed results of this calibration process are presented in Table .

4. Drop Hammer Test Record Table.

falling weight height/m	test times	gravitational potential energy/J	strain wave peak/με	a 0	b 0
1.0	1	47.67	174.91	0.85	–112.78
	2		176.97
	3		181.45
	4		180.52
1.4	1	66.74	235.89
	2		197.19
	3		186.32
	4		238.81
1.8	1	85.81	221.22
	2		245.65
	3		255.31
	4		195.31

Thus, the corresponding relationship between the strain peak and the impact energy is

$$Q=0.85{\mu }_2-112.78$$ 12

The peak value of shock wave is read by measured waveform. Outliers with significant error values at the wave peaks were excluded. The average value of the remaining valid wave peaks, as detailed in Table , was then used for further calculations. Based on this analysis, the impact energy of the RC-DTH hammer was determined to be approximately 331.94 J, with a final piston impact velocity of around 11.69 m per second. In the numerical simulation outcomes, the impact energy corresponding to the final impact velocity of the RC-DTH hammer piston, which falls between 11 and 12 m per second, was assessed using LS-PREPOST software. The measured impact energy ranged from 321.15 to 360.74 J. The discrepancy between the simulated and measured values is within an acceptable and controllable margin of error.

5. Piston Impact Performance Records.

waveform segment	stress wave peak/με	impact work/J
1	532	339.42
2	507	318.17
3	516	325.82
4	522	330.92
5	632	424.42
6	539	345.37

3.4. Comparison of Numerical Simulation with Experimental Results

To validate the reliability and accuracy of the numerical simulation model and experimental tests established in this study, two units were selected on the surface of the drill bit model from the aforementioned numerical simulation, both located at a specific height from the fixed reference plane. Their axial stress–time curves are plotted in Figure (a,b). The complete incident waveform of a single impact from the air DTH hammer described in this chapter is shown in Figure c, while a time-expanded view of the waveform in the impact region is presented in Figure d. A comparison was made between the incident waveform obtained from the numerical simulation and the measured incident waveform from the impact performance test of the air DTH hammer, the following conclusions can be drawn.

13.

Comparison of incident waveforms between numerical simulation and measured impact energy of pneumatic DTH hammer: (a,b) incident stress waveform diagram of bit unit in ls-prepost and its regional magnified view; (c,d) measured incident waveform diagram of pneumatic dth hammer impact on bit and its regional magnified view.

Under the condition of approximately equal piston mass in the numerical impact model and the actual test, the peak incident-wave values obtained from three sets of measurements on the bit model and the drill bit test rod were 144.89, 175.24, and 188.24 MPa, respectively. Multiple point measurements showed that the peak stress-wave values on the side surface of the bit model during impact ranged from 172.83 to 192.33 MPa, while the peak stress values of the measured waveforms ranged from 160 to 203 MPa. The difference between the simulated and measured values is relatively small, which can be explained by numerical simulation mesh calculation errors and experimental interference factors. Nevertheless, the stress peaks from both sources fall within similar ranges, confirming the rationality of the numerical model and the reliability of the experimental tests.

As illustrated in Figure d, after expanding the time axis of the incident waveforms, the measured and simulated waveforms exhibit nearly identical trends: a stress peak is generated at the instant of impact, followed by a fluctuating decay in stress. During the decay phase, multiple subsequent peaks occur at fixed intervals. This phenomenon is due to the continuous propagation and superposition of stress waves within the drill bit, forming periodic peaks that gradually diminish over time.

4. Conclusions

In the present study, we utilize the LS-DYNA and SPH methods to conduct a numerical simulation of the impact rotary rock breaking process employed by the RC-DTH air hammer. We have delineated the motion characteristics of the drill hammer during the percussive rotary rock breaking process. The investigation includes an analysis of how drilling parameters, such as drilling pressure, the final velocity of the piston, and the rotation speed of the bit, affect the rock breaking capabilities of the RC-DTH air hammer. To validate the numerical simulation findings, we have constructed a test platform specifically designed to assess the impact performance of the RC-DTH air hammer. Through a combination of numerical simulation results and experimental testing, the following conclusions have been drawn:

In the rock hammering process, the drill bit rapidly pierces the rock’s surface. After this quick insertion, it quickly recoils and rotates, exerting force to crush the rock, thereby accomplishing a full cycle of the hammering action.

The depth to which the drill bit penetrates the rock, the speed at which the piston rebounds, the energy imparted by the piston during impact, and the work done to break the rock all increase in a linear fashion with the final piston impact velocity. Conversely, the efficiency with which energy is transferred from the piston to the rock decreases linearly as the final piston impact velocity rises. Although the rotational speed of the drill bit does not significantly influence the impact rock breaking phase, enhancing the rotational speed during the rotary rock fragmentation phase can lead to a greater rebound height of the bit and an increased amount of rock shearing.

Traditional rotary drilling primarily relies on continuous compressive force from drill teeth to fragment rock, resulting in low efficiency and high energy consumption. In contrast, the RC-DTH hammer employs an impact-based rock-breaking process wherein the bit rapidly penetrates the rock surface, immediately rebounds, and simultaneously rotates. This action completes a full impact cycle by applying dynamic force to achieve rock fracture. Moreover, as the bit retracts from the crater, it effectively dislodges cuttings, thereby avoiding the “grinding phenomenon” commonly associated with conventional rotary drilling and significantly enhancing tool service life. Taking into account the rock breaking efficiency of the RC-DTH air hammer, as well as the stress concentration within the drilling tool, the optimal combination of working parameters has been determined to be as follows: a drilling pressure of 4000 N, a piston impact velocity ranging from 9 to 11 m per second, and a bit rotation speed of 30 rpm. This parameter set is believed to maximize the effectiveness of the rock breaking process while maintaining the integrity and performance of the drilling equipment.

Conceptualization and methodology, Jinlin QIAO; resources and data curation, Lei SHI; investigation, Lifang YAN; writingreview and editing, Yanwei HOU; supervision and project administration, Xiongwei LI; visualization and formal analysis, Tianhang WEI; funding acquisition, Jianlei GUO; software,validation and writingoriginal draft preparation, Zihao Liu. All authors have read and agreed to the published version of the manuscript.

The authors declare no competing financial interest.