International Journal of Solids and Structures 59 (2015) 121–131

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International Journal of Solids and Structures

journal homepage: www.elsevier .com/locate / i jsols t r

Contact mechanics of fractal surfaces by spline assisted discretisation

http://dx.doi.org/10.1016/j.ijsolstr.2015.01.0210020-7683/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +61 404 188810.E-mail address: dorian.hanaor@sydney.edu.au (D.A.H. Hanaor).

Dorian A.H. Hanaor ⇑, Yixiang Gan, Itai EinavSchool of Civil Engineering, University of Sydney, Sydney, NSW 2006, Australia

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 June 2014Received in revised form 18 December 2014Available online 4 February 2015

Keywords:FractalContact complianceInterfaceMicro-mechanics

We present a newly developed approach for the calculation of interfacial stiffness and contact area evo-lution between two rough bodies exhibiting self-affine surface structures. Using spline assisted discret-ization to define localised contact normals and surface curvatures we interpret the mechanics ofsimulated non-adhesive elastic surface-profiles subjected to normal loading by examining discrete con-tact points as projected Hertzian spheres. The analysis of rough-to-rough contact mechanics for surfaceprofiles exhibiting fractal structures, with fractal dimensions in the regime 1–2, reveals the significanteffect of surface fractality on contact mechanics and compliance with surfaces having the same meanroughness but higher fractality showing lower contact stiffness in conditions of initial contact for a givenload. The predicted linear development of true contact area with load was found to be consistent withdiverse existing numerical and experimental studies. Results from this model demonstrate the applica-bility of the developed method for the meaningful contact analysis of hierarchical structures with impli-cations for modelling tribological interactions between pairs of rough surfaces

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction For two approaching bodies the transition from non-contact

Interfacial roughness plays a key role in the mechanical behav-iour of contacting bodies with significant implications forprocesses of force transfer, friction, wear and adhesion. Themulti-scale structures exhibited by material surfaces govern theextent and distribution of areas of real contact and force–displace-ment relationships under given conditions of loading. Conse-quently, the interplay of surface-structure and contact-mechanicshas been the subject of extensive research efforts with pioneeringstudies by Bowden and Tabor (Bowden and Tabor, 1950;Greenwood and Williamson, 1966) spawning a raft of further stud-ies over the past decades (Akarapu et al., 2011; Batrouni et al.,2002; Bhushan, 1998; Paggi and Zavarise, 2011; Patra et al.,2008; Tao et al., 2001). Contact mechanics of rough surfaces areof particular interest for the interpretation of tribological interac-tions across multiple scales (Buzio et al., 2003b; Carpinteri andPaggi, 2005; Grzemba et al., 2014; Misra and Huang, 2012) withvaluable engineering applications ranging from nano-electrome-chanical systems to tectonic dynamics. Additionally, the structuredependence of contact stiffness and frictional interactions consti-tutes valuable input in the construction of meaningful multi-bodymodels as used for the analysis of granular matter, geomaterialsand powders by discrete element method (DEM) models (Alonso-Marroquín et al., 2013; Luding, 2008).

conditions to bulk-type behaviour (elastic or otherwise) occursthrough the gradual deformation of asperities (Barber andCiavarella, 2000; Wriggers and Zavarise, 2002). Consequently thestress–strain behaviour of elastic bodies in contact at an interfacegenerally exhibits a form similar to that shown in Fig. 1, assumingthe system is unconstrained and exhibits bulk elastic behaviour inthe regime of interest.

Under subcritical normal stresses (r < rc), contact stiffnessthrough an interface between two rough surfaces is governed bythe geometry and distribution of asperities. Models of contact stiff-ness frequently employ simplified geometries to describe asperitystructures. Thus, the study of contact area and forces in regularwavy surfaces exhibiting sinusoidal profiles has been the subjectof several analytical interpretations (Gao et al., 2006; Johnsonet al., 1985, 1971) based on Hertzian contact mechanics. The earlierGreenwood and Williamson model (Greenwood and Williamson,1966) considered uniformly spherically capped asperities exhibit-ing a Gaussian distribution of heights. Similar quasi-random distri-butions of spherical elliptical and sinusoidal asperities wereconsidered in elastic and plastic contact mechanics models duringthe 1970s and 80s (Bush et al., 1975, 1976; McCool, 1986; Nayak,1973). Such surface structure simplifications are frequentlyemployed to model the resistance of rough surfaces to shearingforces (Ciavarella et al., 1999).

The field of fractal geometry, pioneered by Mandelbrot, hasrisen to prominence over the past several decades and findscontemporary applications in a broad range of disciplines

Fig. 2. Self-affine surfaces (a) Geometrically self-affine, (b) Statistically self affineacross all scales and (c) Statistically self affine with a highest level stochasticparameter.

122 D.A.H. Hanaor et al. / International Journal of Solids and Structures 59 (2015) 121–131

(Mandelbrot, 1983, 1985). In particular, the tendency for naturallyoccurring surfaces to exhibit self-affine fractal structures hasbrought about an increasing focus on the significance of fractali-ty-governed scaling behaviour of surfaces in fields of contactmechanics and tribology (Panagiotopoulos, 1992; Soare and Picu,2007). The importance of considering surface fractality stems fromthe tendency of conventional roughness parameters to bedominated by longest wavelength components while higher orderterms such as mean slope or curvature are dominated by shortestwavelength components.

A small range of significant experimental investigations into thesignificance of surface fractality on contact mechanics and tribol-ogy have been carried out (Buzio et al., 2003b; Panagouli andIordanidou, 2013; Sun and Xu, 2005). Computational studies ofthe contact mechanics of fractal rough surfaces are more numerousthan experimental ones and have frequently utilised geometricallyself similar structures such as the type described by Weierstrass-type functions, illustrated in Fig. 2(a) (Ciavarella et al., 2000,2004; Jackson, 2010; Warren and Krajcinovic, 1995). In suchprofiles identical surface structures exist at ever smaller scales,generated through iterative operations in similarity to a Kochcurve or Sierpinski triangle. In contrast to self-similar profiles, nat-ural surfaces are considered self-affine, which is to say they aresimilar at decreasing scales in a broader sense, exhibiting statisti-cally similar quantities rather than being identical upon magnifica-tion (Go and Pyun, 2006). In recent years researchers haveinvestigated more realistic simulated fractal rough surfaces, in2D or 3D, with random self-affine roughness (Campana et al.,2011; Izquierdo et al., 2012; Li et al., 2013), such as the profileillustrated in Fig. 2(b). In order to account for deterministic processgoverning higher level surface features, statistically self-affine sur-faces with a stochastic length parameter can be simulated (Hanaoret al., 2013; Izquierdo et al., 2012; Yan and Komvopoulos, 1998).This is illustrated in Fig. 2(c) and it can be seen that at the highestscale a characteristic wavelength is present such as that whichcould be expected from a particular mesoscopic surface process,giving an overall macroscopically flat surface.

Various models have been developed in order to study normalcontact between randomly rough surfaces exhibiting multiscalefeatures. Analytical and solutions for contact phenomena of theo-retical multiscale self-affine surfaces have been reported usingWeierstrass profiles and Archard surfaces (Ciavarella et al., 2000;

Fig. 1. Typical stress (r)-strain(e) curve showing transition to hard contact fo

Jackson, 2010). Additionally, several continuum numerical studieshave been undertaken, allowing the study of surface profiles

r elastic material occurring at a critical stress rC and transition strain eT.

D.A.H. Hanaor et al. / International Journal of Solids and Structures 59 (2015) 121–131 123

approximating natural surfaces. Notably these have included theuse of finite element methods (Hyun et al., 2004; Sahoo andGhosh, 2007; Yastrebov et al., 2011) and Boundary Element Meth-ods (BEM) (Pohrt and Popov, 2012; Putignano et al., 2012b). Withthe increase in available computational power, in order to accountfor the lower level interactions such as molecular scale friction,molecular dynamics based DEM models(Akarapu et al., 2011;Jerier and Molinari, 2012) have been employed. This is carriedout in an attempt to account for contact phenomena at increasinglyfiner scales.

Below a certain stress threshold, the real contact area betweenrough surfaces is predicted to exhibit a linear dependence onapplied load, an analytical result that is further supported byexperimental data confirming this tendency (Akarapu et al.,2011; Carbone and Bottiglione, 2008, 2011; Krick et al., 2012;Putignano et al., 2012b; Xu et al., 2008). The shearing of regionsof true contact, assumed to exhibit a constant shear strength, cou-pled with the linear dependence of contact area on applied normalload can be viewed as the origin of classical Amonton–Couloumbfriction and the validation of models of normal contact mechanicsof randomly rough surfaces is often carried out by testing for thislinear dependence and examining the mean contact segmentlength (Paggi and Ciavarella, 2010). As with most studies in contactmechanics, the contact of fractal surfaces is frequently representedby truncation or half-space approximation, in which a rough sur-face is in contact with a rigid flat. Further, such contact eventsare investigated as being non-adhesive and frictionless. This sim-plification is adequate for problems that neglect tangential forces.However, for models to be applicable for the study of tribologicalinteractions, it is essential to consider both normal and tangentialasperity–asperity interactions across a realistic range of scales thusrendering a rough to flat approach inapplicable.

In spite of the continual increase in available computationalcapabilities, new approaches are necessary in order to evaluatethe contact mechanics of forces acting at fractal surfaces. This is ofparticular significance to the study of situations involving simulta-neous interactions at multiple surfaces exhibiting morphing struc-tures such as frequently encountered in the analysis of granularmaterials, where processes of melting, weathering and plastic defor-mation give rise to evolving surface fractality and consequent devel-opment of new contact mechanics and continuum scale behaviour.

In the present work we investigate a computationally efficientmethod for the discretisation of fractal rough surface structuresand employ this tool to evaluate asperity interactions in normaland tangential orientations and thus re-examine the significanceof surface fractality on contact mechanics between pairs of simu-lated natural surface profiles.

2. Methods

2.1. Generation of fractal surface profiles

With the aim of generating representative surface profilesapproximating macroscopically flat real surfaces, fractal 2-D sur-face profiles with a higher level stochastic parameter were gener-ated over 1.0 � 106 (x,z) points using a variant of the previouslyreported method for generation of anisotropic 3-D surfaces. Thismethod, based on Weierstrass Mandelbrot functions, is describedby the following function (Ciavarella et al., 2006a; Hanaor et al.,2013; Mandelbrot, 1985; Yan and Komvopoulos, 1998):

z ¼ Lð4�2DÞ ln cXM

m¼1

Xn max

n¼0

c2nðD�2Þ

� cos /m;nþ1 � cos2pcnx

Lcos �p m

M

� �þ /m;nþ1

� �� �ð1Þ

For surface profiles in the 2D domain (length and height) asemployed here, the fractal dimension D is varied over the interval1–2, where 1 corresponds to a smooth continuous quasi-randomcurve and 2 corresponds to an area filling object within particularconstraints (of amplitude or mean surface roughness and resolu-tion). A stochastic length parameter, L, is included in Eq. (1) toaccount for stochastic higher level surface features, which are pres-ent with a given maximal spacing. In real surfaces the term L repre-sents a characteristic surface wavelength, i.e. macro-asperityspacing. Surfaces generated were scaled in the z-direction to yielda consistent mean roughness value RA across all profiles, equal to0.5% of the profile length. The scale of the lowest level features isdetermined by the resolution of the simulation and this can berelated to the scale of the finest roughness features in a real surface,limited by molecular scale parameters such as lattice spacing. Theparameter c represents the density of frequencies used to constructthe fractal profile. The randomised phase angle / is given by a uni-form distribution of size M x nmax. /m;n ¼ Uð0;2pÞ. In the presentwork M and nmax values of 20 were chosen as these were found togive sufficiently complex surfaces. Examples of simulated surfaceprofiles are shown in Fig. 3(a). For each contact scenario, two oppos-ing simulated surfaces were generated with separate randomised /sets. The equivalent surfaces in the 3D domain represented by theprofiles used in the present work are illustrated in Fig. 3(b). Herethe surface fractal dimension (D) varies over the interval 2–3,where2 corresponds to a stochastically generated smooth surface with asingle level of asperities and 3 corresponds to a quasi-volume-fillingobject (as far as resolution allows) with features at all scales.

2.2. Spline assisted discretization

Fractal surfaces are non-differentiable at all points (Sahoo andGhosh, 2007). For this reason in order to discretise simulated sur-faces in terms of surface normals and curvature values we applieda cubic spline for smoothing computationally generated fractalsurface profiles, with 1. . . imax surface points. This spline assisteddiscretisation yields a third order piecewise polynomial, f ðxÞ, suchthat the value of F, shown in Eq. (2), is minimised by varying overthe function f ðxiÞ:

F ¼ pXi max

i¼1

zi � f ðxiÞx0

� 2

þ ð1� pÞx0

Zð f 00ðxÞÞ2dx ð2Þ

For dimensional consistency here x0 corresponds to unit length(e.g. 1 lm). The value of smoothness parameter p is dependent onthe chosen simulation resolution and therefore was set by thetightness exponent a, using

p ¼ 1þ ðdx=x0Þa

10

� �1

ð3Þ

where dx is the spacing between adjacent x data. Thus, high valuesof tightness a yield closer fitting interpolations, while lower valuesgive a smoother spline. The dependence of spine fit on a is illus-trated in Fig. 4.

Obtaining a meaningful discretised representation of the fractalsurface necessitates the use of a tightness exponent sufficientlyhigh to represent the finest level of asperities, as determined bythe profile resolution. Excessively high tightness exponent values(�a > 3) may result in erroneous interpolation of localised surfacecurvature yielding radii of curvature smaller than the simulationresolution. For this reason we select the a tightness exponent suffi-ciently high to account for surface features at all scales while notresulting in features smaller than the simulation resolution. Valuesin the approximate regime 1.5 < a < 2.5 were found to satisfy thisrequirement as confirmed by consistent force displacementrelationships.

Fig. 3. (a) Representation of surface profiles in a 2D domain with varying fractal dimensions from D = 1 to D = 2 and (b) Surfaces in a 3D domain with varied fractality fromD = 2 to D = 2.5.

Fig. 4. Fractal surfaces, D = 1.4, with spine interpolation of varied tightness exponents, a.

124 D.A.H. Hanaor et al. / International Journal of Solids and Structures 59 (2015) 121–131

2.3. Surface sphere interpolation

By means of the spline interpolation as detailed in the previousstep, we describe the upper and lower fractal surfaces in terms oflocal tangential ‘spheres’, with determined surface normals, cen-tres and radii. These spheres are used only to convey local geomet-rical and material information to the contact problem, which is

thus converted to a series of local contact problems betweenHertzian spheres of the two surfaces.

At a given point, (xi,zi), we utilise the coefficients of the piece-wise polynomials yielded by the spline interpolation to assess localsurface curvature and normals. Surface normals (~ni) are given by

~nli ¼ �f 0ðxiÞðf 0ðxiÞ2 þ 1Þ

�0:5; ðf 0ðxiÞ2 þ 1Þ

�0:5n oTð4:1Þ

D.A.H. Hanaor et al. / International Journal of Solids and Structures 59 (2015) 121–131 125

~nui ¼ f 0ðxiÞðf 0ðxiÞ2 þ 1Þ

�0:5;�ðf 0ðxiÞ2 þ 1Þ

�0:5n oTð4:2Þ

where superscripts l and u, respectively indicate the upper andlower surface profiles.

Radii for upper and lower tangential spheres are found as theinverse of curvature values at discrete surface points:

Rli ¼

1jl

i

¼ ð1þ f 0ðxiÞ2Þ3=2

f 00ðxiÞð5:1Þ

Rui ¼

1ju

i

¼ �ð1þ f 0ðxiÞ2Þ3=2

f 00ðxiÞð5:2Þ

For surface points exhibiting local concavity, j (and R) assumenegative values while local convexity yields positive j values. Notethe special case with j ? 0 and R ?1 for a flat surface. Centres oflocal spheres are found by the following relationships:

~Oli ¼~al

i � Rli~nl

i ð6:1Þ

~Oui ¼~au

i � Rui~nu

i : ð6:2Þ

where~ali and~au

i are position vectors corresponding to (x,z) coor-dinates at individual points of lower and upper surfaces respectively.Surface spheres and surface normals interpolation of approachingasperity of representative profiles is shown in Fig. 5. The term surfacesphere is used here despite the 2D simplification applied.

2.4. Contact model

In the evaluation of contact events the upper surface is incre-mentally shifted downwards, in the global normal direction. Theinitial contact detection criteria (zu

i 6 zli) is applied to identify

points for localised contact force analysis. For a point in contact,the ‘local’ contact normal, ~nc

i , between the upper and lower sur-faces is a vector of unity magnitude and is given as:

~nci ¼

~Oui �~Ol

i

~Oui �~Ol

i

¼~Xi

~Xi

ð7Þ

Here ~Xi is the vector separation of the sphere centres. This con-tact normal usually differs from the surface normal calculated at

Fig. 5. Surface profiles with normals and spheres illustrated.

the grid x = xi. Similarly,~tci is defined as the local contact tangent,

a unity vector tangential to the ‘local’ contact.Owing to the presence of both positive and negative curvature

values and radii, contact detection involves the evaluation of localconvexity or concavity following the conditions formulated inTable 1. Following this procedure, illustrated in Fig. 6, contactevents are rejected for scenarios of concave to concave surface fea-tures as well as convex to concave scenarios where the concavefeature exhibits a smaller radius of curvature relative to the oppos-ing convex asperity. For accepted contacts the positions of contactcentres are determined as shown.

As surface displacement progresses an individual pair of inter-acting surface features may gradually involve an increasing num-ber of intruding profile points. Thus, to avoid discontinuitiesarising from individual asperities contacts being represented by afluctuating number of contact spheres converging at remotepoints, we verify the position of contact centres in the vicinity ofthe current meshing grid. Contact centre position is given as:

~cci ¼

~cui þ~cl

i

2¼~Xi

2� Ru

i � Rli

2~nc

i sgnðRui Rl

iÞ ð8Þ

With the assumption that grids are sufficiently fine to havelocally smooth topology, the number of actual contacts is lowerthan the number of grid points. We accept contact points onlywhen their respective centres are located inside the currentdomain. i.e.

xci 2 xi �

Dx2; xi þ

Dx2

� �ð9Þ

where xci is the global x-axis coordinate of the contact centre and Dx

is the grid spacing. Through this step we exclude cases wherespheres’ contacts occur at points distant from their respective sur-face profile point and thus eliminate the occurrence of contactsphere convergence at asperity peaks and consequent mesh-dependence and discontinuous force balances. In the absence of thiscriterion, a single peak to peak contact point may be represented bya large number of spheres corresponding to distant points, and con-sequently yield an erroneously high normal force contribution.

2.5. Force interpolation

In order to assess contact forces on the basis of spline discreti-sation of real or simulated surfaces, accepted contact points aremodelled as flat sections (assumed to be of 1 lm thickness) of elas-tically deforming Hertizan spheres (Popov, 2010; Takahashi, 1991).The cases for accepted contact events yield positive values foreffective radius, Reff used by the Hertzian contact problem as givenby:

Reffi ¼

2Rui Rl

i

Rui þ Rl

i

; di ¼ Rui þ Rl

i � j~XijsgnðRui Rl

iÞ;

~Fni ¼

p4

E�di~nci ; Ai ¼

ffiffiffiffiffiffiffiffiffiffiffidiR

effi

qð10Þ

where di corresponds to sphere intrusion and E⁄ is the effective elas-tic modulus. Following this approach we obtain local normal forces~Fn

i and individual contact areas Ai at discrete contact points presenton a surface profile element of unity thickness representing a sec-tion of a three dimensional surface of fractal dimension D + 1.

It should be noted that Spline assisted discretisation (SAD) canbe employed to study non-Hertzian contact by utilising alternative

functions of the form Fni ¼ f ðRl

i;Rui ; diÞ incorporating plastic, visco-

plastic or viscoelastic deformations. Additionally the representa-tion of functionally graded surfaces with varying elasticity maybe incorporated into SAD methods to capture the divergence

Table 1Contact selection/rejection criteria and contact position determination.

Contact Conditions Acceptance Contact position

Convex to convex Rli > 0; Ru

i > 0 Contact ~cui ¼ ~Ou

i � Rui ~n

ci ; ~c

li ¼ ~Ol

i þ Rli~n

ci

Convex to concave RliR

ui < 0; Rl

i þ Rui < 0 Contact ~cu

i ¼ ~Oui þ Ru

i ~nci ; ~c

li ¼ ~Ol

i � Rli~n

ci

RliR

ui < 0; Rl

i þ Rui > 0 No contact –

Concave to concave Rli < 0; Ru

i < 0 No contact –

Fig. 6. Configuration of accepted contact conditions (a) and (b), and rejectedcontact conditions (c) and (d).

Fig. 7. Diagram of forces, normals and incremental displacements at a single

126 D.A.H. Hanaor et al. / International Journal of Solids and Structures 59 (2015) 121–131

between bulk and nano-scale material properties (Paggi andZavarise, 2011).

Here we assume a uniform mode of deformation at all scales,here assumed to be elastic. In earlier work by Bhushan andMajumdar (1992), Majumdar and Bhushan (1991) and Morag andEtsion (2007), the significance of scale dependant deformationmodes were highlighted as smaller nano-scale asperities are likelyto undergo plastic deformation before the larger ones. The assump-tions in the present work are appropriate for conditions of low-surface penetration, such as those encountered in many-bodysystems.

Although, in the present work, incremental surface displace-ment is applied only in the global normal direction (jD~xT

i j ¼ 0),for each contact point we can evaluate the forces acting in the localtangential direction which in most cases will lead to a non-zeroglobal tangential net force. However, neglecting global tangentialdisplacement is appropriate for a laterally constrained system asfrequently encountered in conditions of normal contact betweenmacroscopically flat surfaces.

At each time step t of the simulation, the tangential force ateach accepted contact point i, modelled as two contacting spheres,is given as:

~Fti ðt þ DtÞ ¼~Ft

i ðtÞ þ G�D~xti ð11Þ

This is valid for the regime in which the magnitude of the fric-tional force does not exceed the magnitude of the maximum per-missible shear force at a point given by the molecular scalefriction Ff

Ff ¼ l0j~Fni j ð12Þ

Here G⁄ is the effective shear modulus and D~xti is the incremen-

tal displacement of point i in the local tangential direction, givenby the projection of the global incremental displacement on thelocal tangential direction, as illustrated in Fig. 7. In contrast tothe local normal force, the local tangential force is calculated fromincremental displacements relative to the previous positionsowing to the rotation and translation of asperities in the local tan-gential direction.

To account for sub-asperity-scale molecular surface interac-tions, we apply a so-called molecular friction coefficient l0 toaccount for asperity slip, which gives rise to a threshold force inthe local tangential direction as

If j~Fti jP Ff then ~Ft

i;new ¼ l0j~Fni j~tc

i ð13Þ

Finally the global coordinate system force balance, comprisingthe total forces acting in the global normal and tangential direc-tions (FN and FT), is determined by summing the components ofindividual local forces at contact points in the global coordinatesystem:

FN ¼XðFn;N

i þ Ft;Ni Þ; FT ¼

XðFn;T

i þ Ft;Ti Þ ð14Þ

Here Fn;Ni and Fn;T

i represent respectively the global normal andglobal tangential components of the local normal force acting atcontact point i.

3. Results

The presently reported method of spline assisted discretizationis applied to interpret the contact mechanics of simulatedroughness-normalised fractal surface profiles of unity thickness innormal contact with rough to rough and rough to rigid flat contact

contact point.

D.A.H. Hanaor et al. / International Journal of Solids and Structures 59 (2015) 121–131 127

conditions considered. Fig. 8 illustrates a contact event of opposingfractal surfaces while the discretised contact of individual surfacefeatures is shown in Fig. 9.

Using a fitness exponent of a = 2 for the spline fitting describedin Eq. (3), fractal profiles consisting of 106 data points were gener-ated with fractality varied over the interval D = 1–1.9 following Eq.(1). Relative to the stochastic length parameter L = 1 lm, fractalsurface profiles, with constant mean roughness (RA) values, weregenerated over a lengths L0 totalling 100 L. Surface profiles werescaled to give RA values proportional to 1/200 relative to L, or(RA = 5 nm). This value yields a typical aspect ratio for highestscale asperities in the simulated profiles representative ofmacroscopically smooth surfaces (Suh et al., 2003).

Initial conditions were controlled such that the minimal valueof zu

i � zli was zero at dN = 0. The upper surface was then displaced

downwards incrementally using 1000 evenly distributed incre-ments of dz reaching a consistent final displacement of 0.2A, whereA is the amplitude of the bottom surface profile. Simulated surfacesof high fractality exhibit sharp peaks that, owing to the limited res-olution of the simulation, may result in some local radii finer thanthe simulation resolution, consequently yielding erroneously lowstiffness at those points. For this reason, a minimal surface curva-ture radius is applied equal to half of the profile resolution (rmin =dx/2) equating to 0.5 Å, which is close proximity to a typical atomicradius, a scale below which the evaluation of Newtonian mechan-ical interactions is inapplicable. The use of fitness exponent a = 2further served to minimise the occurrence of this problem.

Fig. 8. Opposing fractal surface profiles

Fig. 9. SAD of contacting features of a fractal surface structure. Active contact spheres inreferred to the web version of this article.)

For an equivalent modulus of 10 GPa, by evaluating individualcontact events using the 2D Hertzian solution, results for the nor-mal force acting on the profile (FN in mN), the displacement (dN innm) and the total contact area (linear projection of area, AS in unitsof lm) were collected and averaged over 200 independently gener-ated surface profiles at each fractal dimension used.

The mean variation of normal force with displacement for eachfractal dimension studied is shown in Fig. 10(a). For a given surfacedeformation greater normal contact forces arise for surfaces withlower fractal dimensions while surfaces exhibiting greater fractal-ity are more compliant. The trend of mean surface separation, cor-responding to the displacement between the two mean surfaceplanes, with increasing force is shown in Fig. 10(b). Owing to thenormalisation of profile heights to yield constant mean roughnessvalues, surface pairs exhibiting larger fractal dimensions have agreater amplitude and initial mean surface separation with thisdisparity diminishing with increasing load.

Increasing contact stiffness with vertical displacement andapplied normal force is evident with all surfaces with this relation-ship following a power law form consistent with previous reports(Akarapu et al., 2011). The variation of non-dimensional stiffnesswith force, for a profile element of length/area L0/A0 as foundthrough SAD methods, shown in Fig. 11(a), is principally in agree-ment with reports obtained by Pohrt and Popov using a BoundaryElement Methods (Pohrt and Popov, 2012) and with numericalresults of Pastewka et al. (2013) showing higher fractal dimensionsexhibiting lower non-dimensional stiffness at a given load and a

at commencement of contact event.

red. (For interpretation of the references to color in this figure legend, the reader is

Fig. 10. (a) Overall global normal force as a function of normal displacement and(b) Mean surface separation as a function of applied normal force.

Fig. 11. (a) Dimensionless normal contact stiffness vs. dimensionless normal forceand (b) Contact area vs applied force.

128 D.A.H. Hanaor et al. / International Journal of Solids and Structures 59 (2015) 121–131

convergence of the stiffness/load curves for the surfaces of differ-ent fractal dimensions occurring at higher loads.

As shown in Fig. 11(b) true contact area increases in a near-linear fashion with increasing applied load, complying with Green-wood–Williamson and Archard models for contact mechanics atlow applied loads and exhibiting a similar trend with fractaldimension as that reported by Ciavarella et al. (2006a,b) andPutignano et al. (2012a). Here, for surfaces of constant meanroughness, higher fractality results in lower true contact area fora given load. Once again, this trend is theoretical at higher loadsas bulk compliance means that complete contact (AS = A0) is neverreached. At low loads, deviation from linearity arises as the resultof a limited number of accepted contact points in the presentsimulation.

3.1. Rough to rough/rough to flat

The present model allows the evaluation of rough to rough con-tact conditions in parallel to the rough to rigid flat model typicallyassumed. Further validation of the current SAD approach is carriedout by examining the dimensionless root-mean-squared-slope nor-malised variation of contact area with load (j), also known as thecoefficient of proportionality (Paggi and Ciavarella, 2010). Thisvalue is calculated and compared for both rough to rough and roughto rigid-flat scenarios with differing surface fractality following:

j ¼ RSqAE�F�1; where RSq ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffihjrhj2i

q¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXi¼n

i¼1

dzi

dx

� 2

n�1

vuut ð15Þ

It is shown in Fig. 12 that for surface profiles of fractal dimen-sions between 1 and 1.6 (0.4 6 H 6 1) the dimensionless valuesfor j fall within the range bounded by the values predicted byPersson (2001) and Bush et al. (1975) and are in close agreementwith results obtained using finite element methods by Hyunet al. (2004). In the absence of an applied global tangential load,values for the coefficient of proportionality are in agreement forboth rough to rough and rough to flat scenarios. For surfaces pro-files exhibiting higher fractal dimensions (H < 0.4), not typicallyencountered in natural surfaces, limitations exist in the applicabil-ity of contemporary surface mechanics models. As with earlierwork, the higher j values obtained in this regime may result fromthe exceedingly steep slopes (Rsq� 1) that are associated with the-oretical surface profiles as the fractal dimension approaches D = 2(or surfaces with D tending towards 3).

4. Discussion

The contact mechanics of rough surfaces exhibiting statisticalself affinity across a range of scales has been the subject of muchrecent research using a variety of analytical methods. In compari-son to FEA and BEM methods the discretization approachemployed in the present work has the advantage of enabling theanalysis of tangential and normal interactions between twomulti-scale randomly rough surfaces across in a numerical frame-work involving the use of reduced computational resources. Theefficacy of this approach is evident through the repeated simula-tion (with 100 repetitions) of the normal contact of different pairs

Fig. 12. Dimensionless product j plotted vs. Hurst exponent H showing comparison with FEA models and values predicted by Bush et al. and Persson (Greenwood andWilliamson, 1966; Hyun et al., 2004; Persson, 2006).

D.A.H. Hanaor et al. / International Journal of Solids and Structures 59 (2015) 121–131 129

of quasi-random fractal surface profiles consisting each of 106

points entailing 103 displacement steps per repetition and thedetermination of contact area and local and global normal and tan-gential contact forces at each step that was achievable in a desktopcomputer environment in an order of �6 h using a dual-core CPU.Results from the present method are validated by their consistencywith established relationships and experimental results withregards to the development of contact stiffness and true contactarea at rough interfaces.

While the fractal surfaces studied were computationally simu-lated, the discretization and force interpolation methods can justas readily be applied to profilometery derived surface data (usingatomic force microscopy, optical interferometry or stylus measure-ment). Within the framework of the presented methods a varietyof asperity-contact models can be applied to account for adhesion,plasticity and viscosity. Thus the Hertzian discretization used inthe presently reported analysis could be supplanted byJohnson–Kendall–Roberts (JKR), Maugis–Dugdale or Derjaguin–Muller–Toporov (DMT) to account for adhesive conditions orotherwise (Pietrement and Troyon, 2000) and further plasticityterms can readily be incorporated into the calculation of contactforces (Brake, 2012). Additionally, in contrast to the more compu-tationally intensive FEA methods, the presently reported analysisof normal contact does not account for the phenomenon of contin-uum mediated interactions between asperities within a single sur-face, which becomes more significant with increasingdisplacement. While this is not a trivial problem, in principal thepresent method of spline assisted discretisation can readily beextended to account for these interactions. Previously this has beenaddressed by increasing the inter-surface separation in proportionto the nominal pressure over the entire surface in order to moreaccurately capture the value of true contact area for homogenousflat surfaces (Ciavarella et al., 2008).

Comparable studies involving the sine-wave based discretisa-tion of mutli-scale surface profiles were carried out by Ciavarellaet al. (2006a) building on earlier studies (Nowell and Hills, 1989),yielding similar results to the present work with respect to load/deformation behaviour and contact area development. These stud-ies involved Weierstrass series made by the summation of up to 8sets with distinct amplitude/wavelength pairs yielding particularcontact radii. The current methods differ in their inclusion of tan-gential forces and the continuously scalable surface contact radii.

The topic of normal contact stiffness of fractal rough surfaces inthe elastic regime was recently examined by Pohrt and Popov in astudy which employed a Boundary Element type approach to studythe contact of a fractal surface with a rigid flat (Pohrt and Popov,2012; Pohrt et al., 2012). This study revealed a non-dimensionalisedstiffness following a power law with relation to applied normal loadand an inverse power law (negative coefficient) with relation to RMSroughness. Following logically from the positive correlation of sur-face fractality and RMS mean slope roughness, consistent with theresults of the present work, a decreasing stiffness was observed withincreasing surface fractality of roughness-normalised profiles, withthis trend diminishing at higher applied normal loads.

The majority of studies investigating normal contact stiffness,model a rough surface in contact with a rigid flat counter surface.At the micro/nano scale (below the scale of the roughness simu-lated) surfaces are often considered to be smooth. This approachallows the meaningful analysis of normal forces for contact prob-lems involving pairs of surfaces exhibiting different mechanicalproperties or structures by utilising a hybrid surface and constitu-tive material properties derived from the combined structures andproperties of the two surfaces of interest (Zavarise et al., 1995).Despite this, the use of a rigid flat counter surface does not allowthe comprehensive consideration of asperity–asperity interactionsincluding local normal and tangential forces and is thus of limitedapplicability for contact problems involving shear forces andmolecular scale frictional interactions.

Persson’s and Greenwood Williamson theories of contactmechanics predict a linear dependence of real contact area onapplied normal load at sufficiently low loads (Greenwood andWilliamson, 1966; Persson, 2006; Pohrt and Popov, 2012). Thisobservation has been noted as the physical origin of Colulomb fric-tion between flat surfaces and is frequently used as a benchmark totest numerical contact mechanics models. Thus both the aforemen-tioned BEM approach, molecular dynamics (Akarapu et al., 2011)and finite element analyses (FEA) (Batrouni et al., 2002; Hyunet al., 2004; Sahoo and Ghosh, 2007) of fractal surface structuresexamined the linearity of the dependence of non-dimensionalisedcontact area on load in order to validate the numerical methodsemployed. Experiments involving conductance phenomena havefurther demonstrated the dependence of true contact area on loadin fractal surfaces (Ciavarella et al., 2004). Results from the presentwork show that with the exception of very low loads, true contact

130 D.A.H. Hanaor et al. / International Journal of Solids and Structures 59 (2015) 121–131

area varies linearly with load as seen in Fig. 11(b). The non linearityin the AC/FN relationship at low loads results from the limited sim-ulation resolution which results in only a few contact points at lowloads. As shown by Greenwood and Williamson, a large number ofasperities of different heights needs to be considered in order toobtain the predicted linear relation between load and contact area(Greenwood and Williamson, 1966).

The normalisation of surface area by root mean squared slopeyields the value j, also known as the coefficient of proportionality.For fractal surfaces the value of kappa is reported as a constant.Results from the present work are in reasonable agreement withfinite element analysis which showed that the lower values ofthe Hurst dimension H, correlated to fractal dimension of 2D pro-files by H = 2 � D, exhibit increasing j values. The resulting j val-ues obtained in the present work by SAD are also found to largelylie between the constant values predicted by Bush et al. and Pers-son. The divergence observed at lower H values is likely the resultof rapidly increasing surface slope values towards high fractality(Zavarise et al., 2007). It should be noted that such steep featuresare seldom encountered in natural surfaces. Owing to the absenceof an applied global tangential load/displacement, in the presentwork j values are fundamentally in agreement between rough torough and rough to flat models. However, in contrast to rigid-flatcounter surface simplifications that are inadequate for the studyof shear interactions at rough interfaces, the current method is ofbroader utility and is applicable for the analysis of frictional inter-actions and their correlation to surface structure.

Experimental studies in the field of normal contact mechanicsof fractal surfaces confirm to a large extent the linear variation oftrue contact area with applied load and suggest normal surfacecompliance is reduced by decreasing the fractality of the contact-ing surfaces (Buzio et al., 2003a; Zahouani et al., 2009). Decreasingsurface fractality can occur through processes of weathering, melt-ing and abrasion and results in changes to the continuum scalemechanical behaviour exhibited by systems of multi-body contactssuch as granular matter where the transition regime from soft tohard contact is of significant consequence. The currently reportedmethod of spline assisted discretisation allows the evaluation ofcontact stiffness development in normal and tangential orienta-tions in multi-body systems exhibiting evolving surface structureswith potential applications for the interpretation of mechanicalbehaviour in granular systems.

5. Conclusions

A computationally efficient method for the spline assisted dis-cretisation of multiscale surface structures has been shown tofacilitate the evaluation of localised contact forces in surface-normal and surface-tangent orientations, allowing the evaluationof contact mechanics at real or simulated rough surfaces. By apply-ing the developed method to contact events between pairs of sim-ulated self affine rough surfaces of constant mean roughness,results showed a decreasing normal contact stiffness values withincreasing surface fractality and reproduced the reported lineardependence of true contact area on applied load.

By including asperity to asperity interactions, the developedmethods are of broader applicability relative to simplifications ofrigid flat counter surfaces and can be implemented for the analysisof contact and tribological interactions at interfaces in many-bodysystems exhibiting evolving surface structures.

Acknowledgements

Financial support for this research from the Australian ResearchCouncil through Grant No. DP120104926 is gratefully appreciated.

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