Research Article  Open Access
Zhiqiang Huang, Xun Peng, Gang Li, Lei Hao, "Response of a TwoDegreeofFreedom Vibration System with Rough Contact Interfaces", Shock and Vibration, vol. 2019, Article ID 1691582, 13 pages, 2019. https://doi.org/10.1155/2019/1691582
Response of a TwoDegreeofFreedom Vibration System with Rough Contact Interfaces
Abstract
This paper is focused on the influence of the rough contact interfaces on the dynamics of a coupled mechanical system. For this purpose, a twodegreeoffreedom model of a coupled seismicvibratorroughground system is proposed with which the nonlinear vibration properties are analyzed. In this model, the forcedeflection characteristic of the contact interfaces is determined by finite element analysis. By analyzing the undamped free vibration, it was found that the variation of the secondorder natural frequency with amplitude increases with rougher contact interfaces; however, the amplitude has little influence on the firstorder natural frequency of the system. For the harmonic excited analysis, the jump frequencies and hysteretic region both decrease with rougher contact interfaces. Moreover, it is inferred from the bifurcation diagrams that, increasing the excitation force, the system can bring about chaotic motions on rough contact interfaces.
1. Introduction
Contact interfaces exist in a wide range of mechanical systems and play an important role in the overall static and dynamic characteristics of such systems. The rough surface topography of the contact interfaces affects the wear, friction, and normal contact stiffness of the contact mechanics, which has a great influence on the dynamic properties, vibration noise, and energy transfer of the whole mechanical system [1–3]. In order to describe the rough contact interfaces, the fractal geometry is applied to construct the rough surface topography [4, 5]. Berry and Lewis [6] formed the initial basis for a fractal surface roughness description using the Weierstrass–Mandelbrot fractal function. Then, Majumdar and Bushan [7] developed the firstcontact models (the MB model) for the rough surface using the Weierstrass–Mandelbrot function. For the last twenty years, analytical approach has been of interest to many researchers. Ciavarella et al. [8] applied the fractal model for the investigation of contact stiffness and contact resistance of rough surfaces. Yan and Komvopoulos [9] extended the MB model to threedimensional fractal surface and investigated the actual contact area and interfacial contact force of elasticplastic rough surface. Several researchers have undertaken the study of the normal contact stiffness of rough surfaces [10, 11]. To further consider the impact of friction on the normal contact stiffness, Liu et al. [12] introduced a friction factor into the contact model of elasticplastic rough surfaces. Due to the shortcomings of analytical approach, such as neglecting the bulk deformation, the finite element analysis has become an efficient way of contact analysis. For example, Komvopoulos et al. [13], Pei et al. [14], and Sahoo et al. [15] adopted finite element analysis for contact between rough surfaces.
Hertzian contact theory is generally employed for modeling the dynamic contact interactions. Nayak [16] introduced a theoretical groundwork to study dynamic contact interactions based on a singledegreeoffreedom (SDOF) system model by using the harmonic balance method. After that, several researchers studied the vibration characteristics of sphereplane contact [17–20], but such contacts cannot reflect the actual topography of a rough surface. To construct the actual contact interfaces of rough surfaces, Xiao et al. [21] studied the forcedeflection characteristics of a rough solid bodyrigid flat surface model and analyzed the free vibration and forced damped vibration of the SDOF system. Tamonash et al. [22] studied how the material properties of rough surfaces influence the forcedeflection curves and vibration characteristics of a rough contact system. All the above studies involving SDOF systems are limited to ignore effects of contact interfaces on coupled mechanical system. Twodegreeoffreedom (TDOF) systems are important in engineering analysis because many practical mechanical components involve coupled vibrating systems that can be modeled using TDOF systems [23–25].
To investigate the influence of rough contact interfaces on engineering components, the present investigation takes as its example a seismic vibrator, which is a typical TDOF system. Castanet and Sallas et al. [26, 27] proposed the weightedsum theory by assuming the vibrator as a TDOF system. After that, several researchers studied the dynamic response of the vibrator by describing it as a TDOF system. Lebedev et al. [28] analyzed the radiation of seismic waves by proposing a theoretical model to account for the baseplate flexure. Liu et al. [29] established the kinetic equations of the vibratorground coupled system based on the theory of soil dynamics and analyzed the influence of the system parameters on the vibrator output signal. In its working process, the vibrator radiates signals into the earth by being in contact with the ground surface. Hence, the contact interfaces have important effects on the dynamic response of the vibrator. However, the previous studies lack the influence of the ground surface topography on the free and forced vibrations of the vibrator system, making it necessary to establish a vibratorrough ground coupled system.
The aim of the present investigation is to study the nonlinear dynamic behavior of a TDOF vibration system that accounts for the influence of rough contact interfaces. As a typical TDOF system, how the contact interfaces influence the free vibrations and damped forced vibrations of the seismic vibrator are analyzed. A modified twovariable Weiestrass–Mandelbrot fractal function is used to construct the ground surface topographies, and the forcedeflection characteristic of the vibrator in contact with the rough ground is determined by finite element contact analysis. The powerlaw function determined by the forcedeflection relationship is used to describe the nonlinear contact stiffness between the vibrator and ground. A nondimensional kinetic equation of the TDOF system is developed to study the effect of the rough contact interfaces on the vibration response, and a field experiment is carried out to verify the dynamic model. By using numerical methods, the natural frequency, frequency response, and bifurcation diagram for different rough surface topographies are illustrated. The present research results will assist the study of the effects of contact interfaces on the engineering mechanical systems.
2. Description of the TDOF System
The vibrator is the key component in the seismic vibrators that are used widely in oil and gas exploration. The geometry of the vibrator is shown in Figure 1, the main components of which are the reaction mass and the baseplate (including top plate, supporting columns, piston, and baseplate pad). The reaction mass surrounds the piston, and the weight of former supported by two air suspensions is loaded on the baseplate as a static pressure. When the vibrator is in operation, the vehicle is lifted up and its weight (the holddown load) is loaded on the vibrator baseplate. The vibrator radiates signals into the earth by being in contacting with the ground; therefore, the vibrator and ground constitute a coupled TDOF system, as shown in Figure 2.
To consider the nonlinear contact characteristics of rough surfaces, a restoring force given by f (z) is applied to the system. The kinetic equation about the static equilibrium position of the TDOF system in Figure 2 is given bywhere is the mass of the reaction mass, is the mass of the baseplate, and are the vibration displacements, is the hydraulic stiffness, and are the linear damping coefficients of the hydraulic oil and the ground, respectively, is the static load, including the holddown load and the weight of the vibrator, is the static displacement due to the static load, and is the excitation force.
3. Model and Verification
3.1. Fractal Surface Modeling
To establish the contact interfaces between the vibrator and ground, it is necessary to construct the surface topography of ground. Due to the selfsimilarity and scaledependence of the ground surface, a fractal model can be used to describe the surface geometry [30, 31]. The threedimensional modified ground surface topography is characterized using a modified twovariable Weierstrass–Mandelbrot fractal function [7] given bywhere L is the sample length, D is the fractal dimension that determines the relative contributions of the high and lowfrequency components of the surface profile, G is a characteristic length scale of the surface that is independent of the frequency, M is the number of superposed ridges used to construct the surfaces, n is a frequency index (), is a random phase in the range [0, 2], and is a scaling parameter. Consideration regarding surface flatness and frequency distribution density suggests that = 1.5 is typical for most surfaces [32]. The ground surface height z(x, y) at different points can be evaluated in MATLAB, as shown in Figure 3.
3.2. Finite Element Contact Model
To determine the normal contact stiffness between the contact interfaces, static analysis of rigid flat padrough surface is carried out using finite element contact analysis. Because the vibrator is in contact with the rough ground through its baseplate pad, a rough surface with sufficient area for the contact of the baseplate pad must be constructed, and the surface heights were generated from Equation (1) in MATLAB. The surface points were then imported into ABAQUS, and a rough deformable solid body was generated with the generated rough surface profile. The rough ground solid body was meshed using 3D solid element C3D8R, and the mesh size was refined in and near the contact region. The contact interaction between the rough ground and the baseplate was surfacetosurface contact, and the rough surface of the ground solid was defined as the slave surface while the bottom surface of the baseplate pad was the defined as master surface. The bottom of the ground solid was constrained from moving in any directions. The baseplate pad was defined as a rigid body, the centroid of which was chosen as the reference point. As shown in Figure 4, a displacement in the z direction was applied to the reference point to move the baseplate pad incrementally to contact with the rough ground surface. The curves of normal reaction force versus normal displacement were fitted in a powerlaw function as follows:where k and n are coefficients determined by surface parameters including the surface topography and material properties.
3.3. Dynamic Model
Substituting the forcedisplacement equation (Equation (3)) into the kinetic equation of the TDOF system, the equation is obtained as follows:
This is normalized using the following nondimensional variables:
Equation (4) is expressed in a nondimensional form as
To simplify the nondimensional equation, the restoring force is approximated by a thirdorder Taylor series expansion, givingwhere .
Equation (7) can be solved numerically using the fourthfifth order Runge–Kutta method to obtain the dynamic response of the TDOF system. To validate the dynamic model, a field experiment was developed in which the test vibrator was a 249 kN (55977 lbf) vibrator mounted on an EV56 seismic vibrator as shown in Figure 5(a). The acceleration of the reaction mass was obtained by an accelerometer located on the top of the reaction mass, as shown in Figure 5(b). As given in Table 1, the system parameters used for the following simulation were determined from the field experiment, which comprised a 5.905 t reaction mass, a 1.923 t baseplate, and 32.5 t vehicle. Furthermore, the test vibrator was operated under sweep excitation with a frequency of 3–120 Hz, and the excitation amplitude is 99,600 N. The profile of the test field ground surface on which the vibrator was located was measured by a threedimensional laser scanner, and the parameter of Equation (3) for the test field ground surface was determined using the finite element model as above. The acceleration of the reaction mass calculated by Equation (7) is shown in Figure 6, along with the results measured by the accelerometer. From Figure 6, the numerical results clearly agree with the experimental results. Therefore, this vibratorrough ground coupled model can be used in further research to study how the rough surface topography influences the dynamic response of the vibrator.
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4. Results and Discussion
4.1. ForceDeflection Characteristics of Rough Surfaces
To establish the forcedeflection characteristics of different rough grounds, different rough surface fractal parameters are chosen to construct different rough surface topographies. The fractal parameters are (i) D = 2.3, 2.4, 2.5, and 2.6 for G = 6e − 4 m and (ii) G = 6e − 3 m, 6e − 5 m, and 6e − 6 m for D = 2.4. The elastic modulus of the baseplate pad is 2.12e5 MPa and that of the ground is 290 MPa. The equivalent elastic modulus can be calculated, where are elasticity modulus and Poisson’s ratio of baseplate pad and ground, respectively [33]. The variations of the forcedeflection characteristics for the different values of D and G are shown in Figure 7.
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As can be seen in Figure 7, the rougher the surface topography (lower D and higher G), the stronger the nonlinearity, and vice versa. The values of k and n of Equation (3) for the different fractal parameters are given in Table 2. The value of n clearly increases with rougher ground surface topography.

4.2. Undamped Free Vibrations
The undamped free vibration of the TDOF system can be analyzed by setting the damping factor and excitation force to zero. Equation (7) then becomes
Because of the nonlinear restoring forces among the contact interfaces, the initial displacement of the vibrator can affect the natural frequency of the system. The initial conditions are for differing initial displacement of the baseplate. The nondimensional natural frequencies for differing initial displacement avoiding contact loss are calculated by using the fourth and fifthorder Runge–Kutta method and are shown in Figure 8. The maximum initial displacement avoiding contact loss decreases with increasing n values, which corresponds to rougher contact interfaces. The maximum values of initial displacement in Figure 8 are 0.9771, 0.7946, and 0.6114 for n = 1.046, 1.653, and 2.333, respectively. It is noteworthy that the minimum values of initial displacement x_{2} of the TDOF system differ from those of the SDOF system [21]. The minimum initial displacement of SDOF system for different contact interfaces is −1, while the range of minimum initial displacement is . As can be seen from Figure 8(a), changing the initial displacement clearly has little influence on the firstorder natural frequency of the system and mainly affects the secondorder natural frequency under the present initial condition. The secondorder natural frequency reaches the maximum value at the static equilibrium position and then decreases with the increase or decrease of the initial displacement from the static equilibrium position. In addition, the maximum reductions of the secondnatural frequency are 8.54%, 5.66%, and 0.35% for n = 1.046, 1.653, and 2.333, respectively. According to the numerical results, it is inferred that the natural frequency of the baseplate decreases with the increase of the excitation force when the vibrator is in contact with rough ground surfaces. The result is similar to the research of Rik and Guy [34]. Meanwhile, the decrease rate of natural frequency for rougher ground surface is larger.
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4.3. Harmonic Excited Vibration Results
To further understand how the rough contact interfaces influence the vibrator, the damped forced vibration characteristics of the system under external harmonic excitation are investigated. In order to obtain the effects of and on the frequency response of the system, several groups of various values are used and the other system parameters are taken as above. Figure 9 shows the frequency response curves of the reaction mass and baseplate for different combinations of and . It can be seen that have obvious effects on the amplitude reduction of the reaction mass and baseplate especially the peak amplitude. Therefore, in the following study, are taken as to reduce the suppression of nonlinearity of the system.
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Figure 10 compares the forced damped characteristics for different values of n. It can be observed that, with increasing n, which corresponds to rougher surface topography, the amplitudes of the reaction mass and baseplate decrease. The frequency response curves of the baseplate exhibit a jump phenomenon for n = 1.653 and 2.333, as do the frequency curves of the reaction mass at the same frequencies. However, there is no jump phenomenon observed in the resonance curves of the reaction mass. With the increase of the roughness of the surface topographies, the jump frequencies decrease as well as the hysteretic region between the jumpup and jumpdown frequencies. It can also be observed that the frequency response curves for n = 1.653 and 2.333 bend to the left, the system exhibiting softeningspringtype behavior. Hence, when the vibrator is in contact with rough ground surface, due to the sweep excitation, the displacement response amplitude of the baseplate may jump to a larger value suddenly at the same drive level.
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In the following section, the nondimensional frequency and nondimensional excitation Y are taken as bifurcation parameters to obtain the bifurcation diagram of the baseplate in contact with the rough surface. The other parameters of the system are taken as . Figure 11 shows that, with increasing nondimensional excitation Y, the motion of the baseplate becomes more complicated. Therefore, the nondimensional excitation Y can affect the nonlinear oscillations of the coupled vibratorrough ground system.
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Cross sections of the threedimensional bifurcation diagram for n = 2.333 are shown in Figure 12. For Y = 0.3178, the motion of the system is periodic for (approximately), then the system response jumps to another period motion. When Y = 0.943, the motion of the system is a periodic motion before about . At , the system enters chaotic motion, after which it returns to periodic motion through perioddoubling bifurcation. Therefore, the system exhibits perioddoubling bifurcations. Based on the bifurcation diagrams, the relative Poincaré map and power spectrum are shown in Figures 13–18, where (a) and (b) depict the Poincaré map on plane () and the power spectrum, respectively. Poincaré map and power spectrum are efficient ways to distinguish the periodic motions and chaotic motions. At (Figure 13), period1 motion is mainly based on the fundamental frequency vibration with the same frequency as the excitation, accompanied by highorder harmonics. Figure 14 shows that the Poincaré map has the appearance of a strange attractor typical of chaos, and the spectrum is a continuous curve. Then, the system turns into a period8 response at (Figure 15). The Poincaré map has eight points, and the spectrum has 1/8 subharmonic. With the increase of the exciting frequency, the eight points combine into four points, and then four points combine into two points (Figures 16 and 17). The spectrum has 1/4 subharmonic and 1/2 subharmonic as , respectively. After (Figure 18), the system enters into a periodic motion. According to the above results, the vibrator system may exhibit bifurcation and chaotic motion under high drive level. These motions will affect the accuracy of vibrator output signal. Using multiple seismic vibrators to work at the same time rather than one seismic vibrator under high drive level is an efficient way to avoid chaotic motion.
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5. Conclusions
Herein, the influence of the rough contact interfaces on a TDOF coupled vibratorrough ground system was studied. The main conclusions are as follows:(1)The forcedeflection characteristic of the contact interfaces between the vibrator and ground was determined by finite element analysis according to the fractal contact model. The restoring force of the nonlinear contact interfaces was described using a powerlaw function that depends on the finite element analysis results.(2)With the increase or decrease of the initial displacement, the secondorder natural frequency of the system decreases and the reduction of the secondorder natural frequency increases with rougher contact interfaces. However, the initial displacement has little influence on the firstorder natural frequency of the system.(3)The harmonic response analysis shows that the jump phenomenon occurs in the amplitude curves of the reaction mass and baseplate at the same frequencies. The jump frequencies and the response amplitude of the system both decrease with rougher contact interfaces. According to the threedimensional bifurcation diagram, it is inferred that larger nondimensional excitation can cause the chaotic motions of the system with rough contact interfaces. It is found that, when a vibrator is used on rougher ground surface, it is not suitable to use excessive excitation force.
Data Availability
The numerical data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this article.
Acknowledgments
This work was supported by the National Science Foundation of China (Grant nos. 50474040 and 50674078) and the National High Technology Research and Development Program of China (Grant no. 2012AA061201).
References
 K. L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, UK, 1985.
 O. E. Lundberg, A. Nordborg, and I. Lopez Arteaga, “The influence of surface roughness on the contact stiffness and the contact filter effect in nonlinear wheeltrack interaction,” Journal of Sound and Vibration, vol. 366, pp. 429–446, 2016. View at: Publisher Site  Google Scholar
 I. I. Argatov and Y. A. Fadin, “Mathematical modeling of the periodic wear process in elastic contact between two bodies,” Journal of Friction and Wear, vol. 29, no. 2, pp. 81–85, 2008. View at: Publisher Site  Google Scholar
 B. Mandelbrot, “How long is the coast of britain? Statistical selfsimilarity and fractional dimension,” Science, vol. 156, no. 3775, pp. 636–638, 1967. View at: Publisher Site  Google Scholar
 B. B. Mandelbrot, The Fractal Geometry of Nature, W.H. Freeman, New York, NY, USA, 1982.
 M. V. Berry and Z. V. Lewis, “On the weierstrassmandelbrot fractal function,” in Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 370, no. 1743, pp. 459–484, 1980. View at: Publisher Site  Google Scholar
 A. Majumdar and B. Bhushan, “Role of fractal geometry in roughness characterization and contact mechanics of surfaces,” Journal of Tribology, vol. 112, no. 2, pp. 205–215, 1990. View at: Publisher Site  Google Scholar
 M. Ciavarella and G. Demelio, “Elastic multiscale contact of rough surfaces: archard's model revisited and comparisons with modern fractal models,” Journal of Applied Mechanics, vol. 68, no. 3, pp. 496–498, 2001. View at: Publisher Site  Google Scholar
 W. Yan and K. Komvopoulos, “Contact analysis of elasticplastic fractal surfaces,” Journal of Applied Physics, vol. 84, no. 7, pp. 3617–3624, 1998. View at: Publisher Site  Google Scholar
 S. Jiang, Y. Zheng, and H. Zhu, “A contact stiffness model of machined plane joint based on fractal theory,” Journal of Tribology, vol. 132, Article ID 011401, 2010. View at: Publisher Site  Google Scholar
 X. Miao and X. Huang, “A complete contact model of a fractal rough surface,” Wear, vol. 309, no. 12, pp. 146–151, 2014. View at: Publisher Site  Google Scholar
 P. Liu, H. Zhao, K. Huang, and Q. Chen, “Research on normal contact stiffness of rough surface considering friction based on fractal theory,” Applied Surface Science, vol. 349, pp. 43–48, 2015. View at: Publisher Site  Google Scholar
 K. Komvopoulos and N. Ye, “Elasticplastic finite element analysis for the headdisk interface with fractal topography description,” Journal of Tribology, vol. 124, no. 4, pp. 775–784, 2002. View at: Publisher Site  Google Scholar
 L. Pei, S. Hyun, J. Molinari, and M. Robbins, “Finite element modeling of elastoplastic contact between rough surfaces,” Journal of the Mechanics and Physics of Solids, vol. 53, no. 11, pp. 2385–2409, 2005. View at: Publisher Site  Google Scholar
 N. Ghosh and P. Sahoo, “Finite element contact analysis of fractal surfaces,” Journal of Physics D, vol. 40, pp. 4245–4252, 2007. View at: Google Scholar
 P. R. Nayak, “Contact vibrations,” Journal of Sound and Vibration, vol. 22, pp. 297–322, 1972. View at: Google Scholar
 J. Sabot, P. Krempf, and C. Janolin, “Nonlinear vibrations of a sphereplane contact excited by a normal load,” Journal of Sound and Vibration, vol. 214, no. 2, pp. 359–375, 1998. View at: Publisher Site  Google Scholar
 Q. L. Ma, A. Kahraman, J. PerretLiaudet, and E. Rigaud, “An investigation of steadystate dynamic response of a sphereplane contact interface with contact loss,” Journal of Applied Mechanics, vol. 74, no. 2, pp. 249–255, 2007. View at: Publisher Site  Google Scholar
 J. PerretLiaudet and E. Rigaud, “Experiments and numerical results on nonlinear vibrations of an impacting Hertzian contact. Part 2: random excitation,” Journal of Sound and Vibration, vol. 265, no. 2, pp. 309–327, 2003. View at: Publisher Site  Google Scholar
 H. Xiao, M. J. Brennan, and Y. Shao, “On the undamped free vibration of a mass interacting with a Hertzian contact stiffness,” Mechanics Research Communications, vol. 38, no. 8, pp. 560–564, 2011. View at: Publisher Site  Google Scholar
 H. Xiao, Y. Shao, and M. J. Brennan, “On the contact stiffness and nonlinear vibration of an elastic body with a rough surface in contact with a rigid flat surface,” European Journal of MechanicsA/Solids, vol. 49, pp. 321–328, 2015. View at: Publisher Site  Google Scholar
 T. Jana, A. Mitra, and P. Sahoo, “Dynamic contact interactions of fractal surfaces,” Applied Surface Science, vol. 392, pp. 872–882, 2017. View at: Publisher Site  Google Scholar
 A. D. Dimarogonas and S. Haddad, Vibration for Engineers, PrenticeHall, Englewood Cliffs, NJ, USA, 1992.
 L. Cveticanin, “The motion of a twomass system with nonlinear connection,” Journal of Sound and Vibration, vol. 252, no. 2, pp. 361–369, 2002. View at: Publisher Site  Google Scholar
 A. F. Vakakis and R. H. Rand, “Nonlinear dynamics of a system of coupled oscillators with essential stiffness nonlinearities,” International Journal of NonLinear Mechanics, vol. 39, no. 7, pp. 1079–1091, 2004. View at: Publisher Site  Google Scholar
 C. Alain and M. Laycock, “Vibrator controlling system,” 1965, U.S. Patent 3208550. View at: Google Scholar
 J. J. Sallas, “Seismic vibrator control and the downgoing Pwave,” Geophysics, vol. 49, no. 6, pp. 732–740, 1984. View at: Publisher Site  Google Scholar
 A. V. Lebedev and I. A. Beresnev, “Nonlinear distortion of signals radiated by vibroseis sources,” Geophysics, vol. 69, no. 4, pp. 968–977, 2004. View at: Publisher Site  Google Scholar
 J. Liu, Z. Q. Huang, and G. Li, “Dynamic characteristics analysis of a seismic vibratorground coupling system,” Shock and Vibration, vol. 2017, Article ID 2670218, 12 pages, 2017. View at: Publisher Site  Google Scholar
 A. C. Armstrong, “On the fractal dimensions of some transient soil properties,” European Journal of Soil Science, vol. 37, pp. 641–652, 2010. View at: Google Scholar
 Z. F. Hou, Z. Chen, and L. Li, “Fractal analysis of soil profile roughness,” Advanced Materials Research, vol. 383–390, pp. 4944–4948, 2012. View at: Google Scholar
 A. Majumdar and C. L. Tien, “Fractal characterization and simulation of rough surfaces,” Wear, vol. 136, no. 2, pp. 313–327, 1990. View at: Publisher Site  Google Scholar
 S. Jiang, Y. Zheng, and H. Zhu, “A contact stiffness model of machined plane joint based on fractal theory,” Journal of Topology, vol. 132, no. 1, 2010. View at: Publisher Site  Google Scholar
 R. Noorlandt and G. Drijkoningen, “On the mechanical vibratorearth contact geometry and its dynamics,” Geophysics, vol. 81, no. 3, pp. 23–31, 2016. View at: Publisher Site  Google Scholar
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Copyright © 2019 Zhiqiang Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.