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The determination of stress waveforms produced by percussive drill pistons of various geometrical designs
Abstract
Control of the impact energy delivered to the bit of a percussive rock drill is determined by the piston impact velocity and the piston geometry. The basic equations describing generation, transmission, and reflection of the stress wave are defined and a computer program is described by means of which pistons may be designed to produce waveforms of desired characteristics.
... Dutta et al. [13] established a basic equation describing the generation, transmission, and reflection of stress waves and compiled a calculation program. Using the finite difference method and the transmission-reflection relationship of stress waves, they calculated the stress wave generated by the impact hammer and drill rod. ...
To optimize and improve the impact performance of a hydraulic rock drill, it is helpful to test the stress waves of the drill and analyze the impact energy, impact frequency, and energy utilization rate. For this study, a stress wave test bench was designed and built, according to international standards, in order to study the impact process of a hydraulic rock drill under the working pressures of 18 MPa and 23 MPa. The impact energy, impact frequency, and energy utilization rate of two different hydraulic rock drill pistons in low, middle, and high gear were analyzed using a control variable method. The results demonstrate that the impact stress waves of the rock drill periodically occur in the drill rod, and then decay exponentially until they become close to zero. Moreover, the amplitude of the incident stress wave determines the rock-breaking ability of the drill. The impact energy of the short piston is greater than that of the long piston, with a maximum average value of 346.1 J; the impact frequency of the long piston is higher than that of the short piston, with a maximum average value of 62 Hz; and the energy utilization rate of the short piston is higher than that of the long piston, with a maximum average value of 56.92%, which is close to the theoretical ideal efficiency. Therefore, it can be concluded that the impact performance of a hydraulic rock drill can be effectively tested using the proposed horizontal bench, and that piston characteristics and the working pressure are the main factors affecting impact performance. Accordingly, when developing a hydraulic rock drill, it is advisable to select a shorter piston and a higher working pressure, thus allowing the drill to provide good impact performance.
... Examples are given in text books [8] and research papers. Some examples related to percussive drilling were presented by Dutta [9] and by Luciano & Dante [10]. In inverse problems of longitudinal impact, the impact force history and the impact velocity are prescribed, and the axial variation of characteristic impedance is sought. ...
Rock breaking by double hydraulic hammers has a wide application prospect in hard-rock roadway excavation because of its environmental protection and strong rock-breaking ability, but the optimal spacing under double-indenter synchronous impact is not clear. Based on the rock-breaking experimental system under double-indenter synchronous impact established by combining a detachable double-indenter device and Hopkinson pressure bar, the impact tests of three kinds of rocks are carried out, and a theoretical model of specific energy consumption is derived to determine the optimal spacing. The results show that when the indenter spacing is 28 mm, 42 mm, and 14 mm, respectively—that is, when the cross section of the fragmentation crater is in the shape of a “peanut” with a fine middle and two large ends—the concrete, limestone, and sandstone specimens reach the minimum specific energy consumption, which is reduced by 22.55 %, 30.60 %, and 11.85 %, respectively, compared with single-indenter impact. The theoretical model, which explains and verifies the experimental phenomena and results well, can be divided into four zones, and the optimal spacing is in Zone III. The research can be helpful for actual hard-rock tunneling under double hydraulic hammers.
Rock breaking under impact load has a wide application prospect in hard‐rock roadway excavation because of its environmental protection and strong rock‐breaking ability, but the key factors affecting the rock‐breaking effect are not clear enough. Based on a percussive penetration experimental system (PHPB) into rock by combining the Hopkinson Pressure Bar and a detachable indenter, the effects of impact velocity, waveform and rock strength on the force‐penetration curves, percussive efficiency, and crushing work ratio are studied by model simplification, experimental analysis, and theoretical verification. The results show that the peak percussive force and percussive depth are linearly related to the impact velocity; the penetrative coefficient and crushing work ratio are decayed exponentially with the increase of the thickness‐width ratio of the specimens; the rectangular wave can significantly improve the penetration efficiency; the newly established shear‐tensile model with a double‐V structure can accurately predict each stage of rock breaking, which is consistent with the experimental results. The conclusions can be used to speed up the project progress in mechanical excavation of hard‐rock roadway. The PHPB system is established to replace rock breaking by a hydraulic breaker. The shear‐tensile breaking model is proposed to predict each stage of rock breaking. The theoretical analysis in the paper can explain the experimental results well.
A framework for modeling the transient response of percussive drilling systems is presented. The proposed approach is based on the Distributed Transfer Function Method (DTFM), which is a semi-analytical modeling technique. Experimental results obtained from a percussion testbed for The Regolith and Ice Drill for the Exploration of New Terrains (TRIDENT) were incorporated into this modeling technique. DTFM is shown to be a convenient, modular modeling approach, capable of handling complex boundary conditions and drill rod geometries. Moreover, this technique is computationally simple and allows for straightforward incorporation of experimentally measured boundary forcing via numerical convolution, as well as control of the frequency content in the transient response. An experimental study is used to demonstrate the ability of the proposed approach to characterize unknown boundary conditions.
Axial impact between two elastic rods with piecewise constant characteristic impedance is considered. The distribution of characteristic impedance of the impacted rod and the impact velocity are given. By use of a 1D recursive algorithm, the characteristic impedances of the impacting rod are determined as long as they are all positive so that a prescribed impact force is realized. A condition for positivity of the characteristic impedance is derived in terms of transmission coefficients for wave energy. At the time from which positivity of all characteristic impedances cannot be maintained, or earlier, the characteristic impedance of the impacting body is continued, e.g., at the level of the last positive characteristic impedance or at level zero corresponding to cutting off the impacting rod. Examples of prescribed exponentially decreasing and linearly increasing impact forces are presented for a uniform impacted rod with constant characteristic impedance. In these examples, there is good agreement between the prescribed impact force and the impact force obtained from 3D FE simulation with a piecewise linear diameter of the impacting body that approximates the piecewise constant diameter obtained by use of the 1D recursive algorithm.
Traditional impact pile drivers are designed in a serial space style and are unsuitable for narrow and small construction sites, owing to their inflexible adjacent structure. In this work, we develop a new structure with a parallel arrangement between the pull rods and precast pile, which is lighter and more efficient compared with traditional pile structures. The simulated integrated system is based on a finite element model and includes a novel impact pile driver, the pile itself, and the surrounding soil. We also use an artificial neural network to obtain a model of the stress wave pattern and allow a genetic algorithm program to be modelled as a target function for optimisation, in order to achieve the lightest total mass. Our proposed integrated framework for this compact impact pile driver demonstrates the feasibility of building flexible equipment for the future civil engineering industry.
The reasons why oscillation exists in the measurement of dynamic stress-strain curves of the SHPB subjected to a rectangular loading wave have been analyzed and a conclusion of that the oscillation can be controlled by the loading stress waveforms was made. Then, the Impact Discrete Inversion Technique proposed by the authors was introduced to design the rams based on expected stress waveforms. The confirmative experimentation results obtained using the designed ram which can produce an approaching sine wave showed that the oscillations were effectively controlled.
As percussive drills are largely built up of rods which are subjected to axial impact at moderate velocities, elastic wave propagation in such rods, and associated phenomena, are fundamental and theory of elastic waves in rods is presented. As the designs of percussive drills are generally such that 1-D theory is adequate for engineering purposes, the presentation is limited to such a theory. Numerical examples are given. It is pointed out that models of existing or conceivable percussive drills can be considered to be built up of members which may or may not coincide with components of the real drills. Different types of such members are segment members, interface members and end members. Finally, some features of simulation programs developed, tested and used by the author and his coworkers are presented. -from Author
A possibility of the efficient use of rotary percussive drilling to provide drilling smaller diameter holes (40–70 mm) both in mining and prospecting is disclosed herein. A new construction designed for the nipple thread connection is described. The relative amplitude variation, change of power pulse time and energy in their propagation throughout the drilling tool are determined. A possibility of the efficient power pulse transfer along the drill string to the rock destruction tools with new nipple connections which allow automating the make-up and breakout system of drill pipe was supported by experiments.
Extension drilling of rock is simulated numerically using a Victor desk-top computer. The hammer, adapter and extension rods are uniform and have the same characteristics impedances. The same is true for the joints and the bit. Two efficiencies are determined as functions of six dimensionless parameters which represent the number of joints, the joint-to-hammer length ratio, the joint-to-rod characteristic impedance ratio, the initial gap between bit and rock and the bit/rock interaction (two parameters). The first efficiency is defined as the ratio of the work performed on the rock to the impact kinetic energy of the hammer. The second is based on the sum of the work performed on the rock and the rebound kinetic energy of the hammer, which is assumed to be fully retrieved. Both efficiencies generally decrease with the number of joints and the joint-to-hammer length ratio. Exceptionally, however, the efficiencies may increase slightly with the number of joints. The difference between the second and first efficiency, which is due to hammer rebound, generally increases when the rock becomes increasingly hard. This dependence on rock hardness is strongest when the number of joints and the joint-to-hammer length ratio are low. In the absence of joints the efficiencies have an oscillatory dependence on a small initial gap. When one or several joints are used, however, the efficiencies essentially decrease with the initial gap.
Over the last several decades, homogenous single-layer armor has been replaced by multi-layer integral armor to improve ballistic penetration resistance. This has led to better attenuation of shock wave energy by multiple interface reflections and transmissions. Efforts have been reported to improve the penetration resistance by providing higher energy dissipation at higher levels of impedance mismatch. However, high stress concentrations and stress reversals have made these interfaces the primary sources of failure. This paper discusses a concept for a new class of blast and penetration resistant (BPRM) materials which are layer-less but designed to have a continuous gradient of impedance that can dissipate the shock energy without material failure. In a simplistic approach by applying the classical theory of uniaxial stress propagation, it has been shown that attenuation of the stress wave energy would be possible by controlling the impedance distribution within the body of such a material. The development of such material to resist blast or impact will overcome the current common difficulty of interfacial delamination failure in any protective barrier system or armors.
A method for synthesizing non-uniform, linearly elastic impactors which generate given impact forces on given targets is presented. The impactors are assumed to obey Webster's equation, and the targets are assumed to be linear with time-invariant properties. The method is illustrated by solving two synthesis problems concerned with the generation of exponential and ramp impact loads. The results are verified by solving the corresponding analysis problems.
Simple models representing churn drilling, down-the-hole drilling and hammer drilling are established and analyzed with respect to the efficiencies (work done on rock/ impact energy) of these percussive methods. Hammers and rod are treated according to one- dimensional stress wave theory, whereas the bits are treated as rigid masses. The bit-rock interaction is represented by a force-displacement relationship which is linear during loading as well as unloading. Physical insight and a clear overall picture are emphasized.For churn drilling an efficiency close to 100 per cent is attainable, and for down-the-hole and hammer drilling the maximum efficiencies are slightly higher than 90 per cent. For hammer drilling this means that the maximum efficiency is shown to be about 10 per cent higher than according to previous theories in which the bit mass has not been considered. This maximum efficiency appears to be obtainable with standard equipment for normal rocks.The efficiency in down-the-hole drilling is generally greater than or equal to the efficiency in hammer drilling, while the efficiency in churn drilling may be higher or lower than the efficiencies of the two other methods. The efficiencies are all quite insensitive to variations in rock-drilling parameters.Stress waves predicted theoretically in hammer drilling agree well with measured stress waves for different bits.
Wave propagation and efficiency in percussive rock drilling with a bent rod is studied theoretically using a combined approach of FFT and matrix method. The bent drill rod is represented by straight uniform elements. The extensional and flexural stress waves in the drill rod are described by the one-dimensional wave equation and by Euler-Bernoulli theory, respectively. The conditions at the bit and rock during loading are represented by a penetration resistance, a lateral stiffness and a rotational stiffness. Numerical results are given for two cases; the first involves a drill rod with a constant radius of curvature through a certain bend angle, and the second a drill rod with a sharp bend defined by the same bend angle. It is found that for slightly bent rods the efficiency is practically the same as for straight rods. Thus, slight bends, which are always present in real drill rods, do not appear to be significant from the point of view of efficiency. Furthermore, it is found that the efficiency can remain high even if the rod is significantly bent. Thus, percussive rock drilling systems with special designs for drilling in curved paths are feasible from the point of view of efficiency.
An analytical model has been developed that describes the motion of the bit of a pneumatic jackhammer and the forces exerted at its tip during penetration of a target. This model assumes that the energy is transmitted in the form of a one-dimensional longitudinal wave without inclusion of the effect of lateral inertia. The piston and the bit can be of arbitrary shape, but are approximated here by uniform stepped sections.
A computer program was developed to provide numerical results for the operation of the jackhammer/target system. The analysis includes the pressures that act on both the top and bottom of the piston and additional thrusts that are applied by the operator, an external thrust device, due to the weight of the jackhammer, or as the result of a combination of these effects. The program includes (1) the incident wave for an arbitrarily shaped piston/bit system, (2) the force history at any position of this unit, (3) the force profile along the bit at any instant, (4) the indentation history of the jackhammer bit when the force-indentation relation for the bit/target combination is knowna priori, (5) the rebound velocity of the piston, and (6) the energy transfer from the piston to the target. Numerical examples indicating the results of the calculations are presented and compared with data from corresponding experiments that involved the penetration of an Ingersoll-Rand jackhammer with five different tip shapes into Sierra granite. Both the force histories and indentation histories at the bit tip were measured; correspondence with predicted values was very good.
The effects of variations in air pressure and applied thrust as well as the character of the force-indentation relation on the energy transferred to the target are also discussed.
This paper highlights an improved experimental approach for eliminating oscillation that exists in the dynamic stress–strain response of rocks and other brittle materials obtained from tests using a split Hopkinson pressure bar (SHPB). Both analytical and experimental results for a number of rock types are presented to verify the idea. Results from the investigation indicate that oscillation in the dynamic stress–strain response of these materials originates from the Pochhammer–Chree dispersion of the loading incident wave, and more evidently from the relatively low strength and elastic modulus of the samples compared with metallic materials. In order to control the oscillation effectively, it is proposed that a half-sine loading waveform should be used instead of the conventional rectangular loading waveform in SHPB tests. Experimental results obtained from both the conventional and the improved methods are presented, including dynamic complete stress–strain curves for granite, sandstone and limestone. The improved method eliminates oscillation in the tests, provides better stability of strain rate and more representative results than those obtained from the conventional rectangular loading waveform shape.
This paper describes how longitudinal and bending stress waves in percussive drill rods generate noise. It is shown that in practical drills the largest noise source is the bending waves, which are mainly generated because of imperfections in the piston, chuck, and drill rod alignment. Equations are presented which enable the dominant vibration frequencies of the noise spectra generated by the bending and longitudinal waves to be precisely predicted. Means for reducing the amplitude of the bending waves are discussed.
Dynamic fracture and delamination of unidirectional graphite/epoxy composites are investigated for end-notched flexure (ENF) and center-notched flexure (CNF) pure mode II loading configurations using a modified split Hopkinson pressure bar. Results show that delamination and energy absorbed in fracture increase with impact energy with CNF>ENF. A power law analytical model reasonably describes the variation of energy release rate with delamination and energy absorbed. A crack embedded deeper in a specimen (as in CNF) contributes more to dynamic fragmentation than cracks at the surface or near the edge (as in ENF). Hackle features on mode II fracture surfaces decrease with impact energy.
A method is described for computing the efficiency of conversion of the energy in the stress wave in the drill steel produced by the striker impact into work done by the bit on the rock. This method is applicable to any arbitrary but physically realizable waveform of the stress wave in combination with any arbitrary but physically realizable force-displacement characteristic for the motion of the bit into and out of the rock. The results of representative computations show that fairly good efficiencies (> 50 per cent) are obtained over a fairly wide range of values of a dimensionless parameter that incorporates quantitative measures of the stress waveform, the force-displacement characteristic, and the material properties and geometry of the drill steel. Although efficiency values of 100 per cent are theoretically possible, they may be obtained only under certain special conditions that apparently are not physically realizable with percussive drilling systems as presently conceived. The optimum value of the applied thrust force on a percussive drill is shown to be essentially equal to twice the average rate of transfer of momentum from striker to drill steel. Hence, the maximum benefit with respect to raising the power output of a percussive drill by increasing blow frequency is obtainable only if the thrust force is correspondingly increased.
Energy Transmission in Percussive Drilling
- Ramanathan
On longitudinal impact—I–VI
- Fischer
Digital Machine Computations of the Stress Waves Produced by Striker Impacts in Percussion Drilling Machines
- Simon