55
Tribology Letters, Vol. 23, No. 1, July 2006 (Ó 2006)
DOI: 10.1007/s11249-006-9113-9
A novel test rig for in situ and real time optical measurement
of the contact area evolution during pre-sliding of a spherical contact
A. Ovcharenkoa, G. Halperina, I. Etsiona,* and M. Varenbergb
b
a
Mechanical Engineering Department, Technion, Haifa 32000, Israel
Max Planck Institute for Metals Research, Heisenbergstr. 3, Stuttgart 70569, Germany
Received 31 May 2006; accepted 18 July 2006; published online 22 August 2006
An experimental test rig was developed in order to investigate elastic–plastic single micro-spherical contact under combined
normal and tangential loading. This novel apparatus allows in situ and real time direct optical measurement of the real contact area
(RCA) evolution in pre-sliding. It also allows relative displacement measurements under very low rates of tangential loading (down
to 0.01 N/s) to capture accurately the fine details at sliding inception. This is achieved by piezoelectric actuation in closed loop
feedback control in addition to synchronization with data and image acquisition to obtain real time measurement. The RCA
measurement is realized by direct optical observation technique, whereas two different image processing algorithms were
implemented for the elastic and the elastic–plastic contact regimes. The various features and capabilities of the test rig are presented
along with some preliminary experimental results of RCA and friction behavior to assess its performance.
KEY WORDS: spherical contact, elastic–plastic contact, real contact area, in situ real time measurement, image processing
1. Introduction
Contacting surfaces are rough on micro-scale and
consist of micro-asperities that, under normal load,
form contact junctions at their summits. The elastic–
plastic deformation behavior of such contacting micro-asperities plays an important role in several contact mechanics problems such as friction, adhesion,
wear, electrical and thermal contact conductance to
name a few. A key parameter in all theses problems is
the real contact area (RCA). An accurate knowledge
of the RCA and its evolution under varying normal
and tangential loading is important for studies
involving many tribology and contact mechanics
issues [1].
Two review papers [2,3] that were published in 1980
and 1985, respectively, describe various experimental
techniques to measure contact area, which include both
indirect and direct methods. Of all the various techniques mentioned in Refs. [2] and [3] the most practical
ones are: the electrical contact resistance, optical, and
ultrasonic techniques. All these three techniques are
considered in situ measurements of the RCA but only
the optical and ultrasonic methods are direct ones. The
measurement of electrical contact resistance can be used
indirectly [4] for estimation of contact area evolution
under combined normal and tangential loading of
contacting surfaces. However, this method requires
perfectly clean noble metals, because oxide and con*To whom correspondence should be addressed.
E-mail: etsion@tx.technion.ac.il
taminant layers can distort significantly the calculated
contact area.
The widely used ultrasonic technique is based on
measuring the ultrasonic wave reflection coefficient from
the interface of two contacting surfaces [5–6]. Ultrasonic
waves are reflected from solid/air boundaries but pass
through solid contacts. Hence, this difference can be
used to distinguish air gaps from RCA in rough surfaces
contact. The main advantage of this method is the fact
that it is not limited to conducting or transparent
surfaces as in the case of electrical resistance or optical
methods. However, the lateral resolution of the acoustic
reflection measurement with conventional transducers
and equipment is relatively low (about 0.2 mm) [7]. This
poor resolution can be improved by deconvolution
signal processing techniques but this adds complexity to
the measurement. In addition, this technique requires a
coupling medium, like water, between the acoustic
transducer and specimen, which further complicates the
measurement. Moreover, the RCA is obtained by time
consuming scanning of the contact zone with the ultrasonic probe. Therefore, this technique is suitable for
static loading only and does not permit real time
dynamic measurements.
The optical observation technique is a prevalent
method for RCA estimation. It requires that one of the
surfaces will be transparent to light but contrary to the
ultrasonic technique the integration of optics in an
experimental setup is quite simple. The optical method
can be used discretely or in real time, according to the
optical apparatus.
1023-8883/06/0700–0055/0Ó 2006 Springer Science+Business Media, Inc.
56
A. Ovcharenko et al./Real time optical measurement
Perhaps the first publication on discrete optical
measurement of single spherical contact is from 1950 by
Parker and Hatch [8]. The contact area images at various normal loads were still photographed on film
through a microscope at regular discrete increments of
tangential loading until gross sliding begun. The tangential load was applied through a string, which pulled a
glass flat by means of suspended beaker that was gradually filled with water. No image processing technique
was used and hence, only the apparent area of the
contact could be estimated from the still photographs.
Two more publications [9,10] from 1981 and 1984,
respectively, describe optical measurements of RCA by
still photography under normal load only. In Ref. [9] the
spherical contact was limited to elastic deformation
whereas in Ref. [10] the deformation was purely plastic.
Reference [11] describes a pin on disc apparatus that was
used to study the contact area between polymer layers
and glass hemispheres during sliding. As in the previous
cases, the contact images were still photographed
through a microscope.
An experimental setup for the RCA evolution in presliding regime of spherical and conical contacts of
metals and polymers is described in [12]. The experimental system is similar to that employed in [8]. The
contact images were still photographed throughout a
microscope and analyzed graphically taking into consideration non-contact areas within the contact zone.
Since the tangential load was applied discretely and not
continuously, it was not possible to obtain contact
images in real time and to identify properly the instant
of sliding inception. Laser profilometry [13] is yet
another optical method for RCA measurement that,
similar to the acoustic method, consumes relative long
time due to the required scanning of the contact zone.
Real time optical observation of RCA in sliding
regime using CCD camera for digital acquisition of
contact images is described in Ref. [14]. A fixed rough
surface of metallic specimen was loaded against a moving glass flat. The glass flat was pulled by a servomotor at
pre-selected relatively large velocities (above 10 mm/s).
Image processing tools were employed to estimate the
RCA evolution during the sliding. Another real time
investigation of initiation, propagation and distribution
of micro-slip at the contact interface of rubber spherical
specimen against glass flat is described in [15]. The CCD
acquired images were analyzed using correlation method
of image processing technique, in order to identify microslip in the contact interface during pre-sliding. The tangential force was gradually increased by pouring water
into a container (see also [8]) until gross sliding begun.
A very recent publication [16] describes real time optical
observation of contact area evolution under normal loading only. The normal load was increased continuously by a
loading system that was synchronized with a CCD camera.
Image processing was used to obtain the RCA.
As can be seen from the above, no published literature exists so far on in situ and real time direct measurement of RCA evolution during pre-sliding.
Consequently, the main goal of the present work is to fill
this gap by developing an apparatus which will allow
such direct optical measurements. The novel apparatus
should also allow simultaneous measurements of RCA
and friction force, during continuous and smooth
application of tangential loading at pre-selected desired
low rates. It should also allow RCA measurements in
both elastic and elastic–plastic contact regimes by means
of appropriate image processing techniques.
2. Description of the test rig
In general the test rig allows studying the evolution of
RCA between a rigid flat and a deformable sphere under
combined normal and tangential loading in both elastic
and elastic–plastic contact modes. It was designed to
fulfill the goals mentioned in the introduction, i.e.:
Variable normal loads to achieve pure elastic or
elastic–plastic contact mode.
Smooth and continuous application of tangential load
at low rates to accurately detect fine details during
pre-sliding and sliding inception.
Synchronization of data and video acquisition with
piezoelectric actuation to allow real time measurement of normal and friction force, RCA, relative
displacement, and instant of sliding inception.
The test rig consists of four main modules that are
shown in figure 1: An actuation module that holds a
transparent rigid flat and applies the tangential load to
the spherical contact. A tangential force module (5),
which holds a sphere and measures the tangential friction force applied to it by the flat. A normal force
Optical
module
7
Direction of actuation
5
6
1
2
F
3
P
4
Thin plate s
Figure 1. Schematic representation of the experimental setup with 4
modules: (I) Actuation module consisting of: parallelogram frame (1),
mechanical lever (2), piezoelectric actuator (3), and proximity probe
(4). (II) Friction force measurement module (5). (III) Normal force
module (6). (IV) Optical module (7).
57
A. Ovcharenko et al./Real time optical measurement
module (6), which applies and measures the normal
loading of the contact, and an optical module (7) for
accurate measurement of the contact area evolution.
The actuation module consists of four main components: a frame (1), a mechanical lever (2), a piezoelectric
actuator (3), a proximity probe (4), and a motion control board (National Instruments PCI-7352, USA). The
frame, in the form of a parallelogram, includes a rigid
upper member that holds the rigid sapphire flat, and is
supported on two vertical thin plates (see figure 1). This
parallelogram frame ensures horizontal motion of the
rigid flat with respect to the sphere as indicated by the
‘‘direction’’ arrow shown in the figure. Moreover, such
frame excludes any undesired vertical micro-vibrations
that can exist in experimental setups that are based on
linear bearings support.
The horizontal actuation is obtained by means of a
mechanical lever (see figure 1) that is pushed by a piezoelectric actuator (Piezomechanik PSt 150/7/100
VS12 VbS, Germany). The maximum possible
displacement of the actuator is only about 130 lm,
however, the mechanical lever provides a gain of four
times which is sufficient to compensate for various
clearances in the system and ensure sliding inception.
The piezoactuator is controlled through a closed PID
feedback loop to provide a desired displacement of the
rigid sapphire flat at a pre-selected constant velocity in
spite of the lump non-linear stiffness, c, of the system
and the contact. It was found that during pre-sliding the
rate Q_ of the tangential force behaves according to the
_
relation Q½N=s
¼ c V½lm=s, where V is the pre-selected velocity (in the range 0.05–220 lm/s), and
c = 0.184[N/lm]. Hence, Q_ could be varied between
0.01 and 40 N/s. A proximity probe (4) (SKF CMSS60,
USA), with 0.1 lm resolution, was mounted on the
sapphire holder while its target was fixed to the lever
holding the sphere (see picture in figure 2) in order to
obtain the exact relative displacement between the
sphere specimen and the sapphire flat. It should be
noted that during horizontal displacement of the parallelogram frame the upper member with the sapphire
flat moves down a few micrometers. This vertical
displacement is accommodated by the normal loading
module, which allows free vertical displacement of the
sphere while maintaining a constant normal force.
Figure 2 shows a picture and a corresponding scheme
of the friction force measurement module. It consists of
a lever, piezoelectric friction force sensor, and a holder
with a sphere specimen. Because of the required low rate
of the tangential loading a sensitive piezoelectric friction
force sensor (Kistler 9215, Switzerland) was used. It
provides quasistatic measurement of the friction force
without any built-in pre-amplifier and hence, has negligible drift of the measured signal in time. This friction
force sensor can operate in two distinct ranges of tangential loads: 0–20 N and 0–200 N with resolutions of
10 and 50 mN, respectively. The friction force sensor
was aligned with the contact interface plane (see the
dashed line in the scheme of figure 2) in order to eliminate any possible bending moment due to the friction
force. This arrangement ensures very accurate measurement of the true friction force.
The normal force loading module, shown in figure 3,
consists of a double-arm loading lever, a load cell and an
eccentric loading mechanism. The load cell (Entran
ELFS-3M-250N, France) was used for measurement of
the normal load with a resolution of 0.2 N. Dead
weights can be put on either guide (a), (b) or both for
any desired normal load up to 250 N, which can cause
plastic deformation of the sphere contact zone. The
eccentric loading mechanism allows smooth manually
controlled application and release of the normal force.
a
b
Ball joint
Load cell
Counterweight Ball joint
Friction force sensor
Contact
interface line
Guides
Dead weight
Target of
proximity probe
Normal load, P
Sapphire
a
Tangential load, Q
F
Lever
Sphere specimen
Holder
Normal load, P
Figure 2. Picture and scheme of the friction force measurement
module.
Lever
b
Eccentric mechanism
Figure 3. Picture and scheme of the normal loading module.
58
A. Ovcharenko et al./Real time optical measurement
The last module, shown in figure 4, is the optical
module which allows in situ observation of the contact
zone through a small circular window in the upper part
of the frame. The optical module includes: a microscope
(Navitar Zoom 6000, USA) with changeable zoom and a
built-in beam splitter (BS), color CCD camera (Pulnix
TMC-6i, USA), illumination source, and an image
acquisition board (National Instruments PCI-1411,
USA). The stand of the optical system allows preliminary rough focusing, whereas the optical microscope
permits the final fine focusing onto the plane of contact
in order to obtain sharp images. Since each sphere
specimen replacement may deviate from the previous
position of the contact zone, an accurate horizontal
adjusting mechanism of drive screws was used to ensure
that the contact area will always be located at the CCD
field of view. The zoom of the optical system was then
adjusted for best lateral resolution (from 1.2 to 3.6 lm
corresponding to magnification range of 45 to 7,
respectively) of the contact area image (maximum possible CCD pixels) according to the contact size at different normal loads. Further slight improvement of the
image contrast (especially in the elastic regime of
deformations) is obtained by application of polarized
light.
The assembly of all four modules is presented in
figure 5. It can be seen that the normal loading module
is perpendicular to the friction force measurement
module in order to provide optimal modules arrangement and proper functioning. It is important to note
that these two modulus are also autonomous so that
normal and tangential force measurements are decoupled and do not affect each other.
3. Signal and image processing
A data acquisition board (National Instruments PCI6221, USA) was used to acquire the measured signals
(forces and displacement) from the various sensors. A
CCD camera
Microscope
Illumination
BS
Tangential load, Q
Normal load, P
Microscope
Illumination
Figure 4. Picture and scheme of the optical module.
Figure 5. Picture of the test rig assembly: Parallelogram frame (1),
Mechanical lever (2), Piezoelectric actuator (3), Proximity probe (4),
Friction force measurement module (5), Normal loading module (6),
Optical module (7).
low pass filter was used to remove high frequency noise
in the above time domain signals. Signal and image
processing along with full control of the test rig was
realized on Labview 7.0 software.
During the application of combined normal and
tangential loading, contact area images were acquired at
a constant video rate of up to 25 fps for short experiments (a few seconds corresponding to high Q_ values),
or at a lower rate down to 0.1 fps for longer experiments
(tens of minutes corresponding to low Q_ values). Then
all images were saved to a computer memory for further
analysis by image processing tools. A data file of time
dependent normal load, friction force, relative displacement and RCA was created for each experiment
for further analysis.
Two image processing algorithms were used to estimate the RCA from the contact area images.
The first method, suitable for mostly elastic contact,
is based on analyzing interferometric patterns of
Newton’s rings formed due to reflected light from the air
gap between the sapphire flat and the lightly deformed
sphere. The central dark spot of the Newton’s rings
pattern (see figure 6a) represents the RCA since a
destructive interference occurs at the points of contact
[9,11]. Monochromatic and polarized light was used to
enhance the sharpness and contrast of the interferometric images. The procedure of the RCA estimation
consists of a few stages. First, the centroid [17] of the
light intensity distribution of the Newton’s rings is
determined (see figure 6b1). Then, two concentric circles
are arbitrarily drawn with respect to the centroid such
that one is smaller and the other is larger than the
estimated RCA border. These two circles contain
between them the region of interest where the sought
border of RCA is located. Radial searching lines, originating from the centroid, are used to obtain the radial
intensity distribution between the above two concentric
59
A. Ovcharenko et al./Real time optical measurement
RCA
(a)
(c)
(b)
Original image
Circular edge detection
function
RCA estimation
Detected boundary
points of RCA and
doted circular white
line of their mean
radius
Region of interest
is between these
two concentric
circles
Centroid
(b1)
Zoom of the image in (b)
Figure 6. Image processing algorithm for real contact area (RCA) estimation from Newton’s rings pattern in elastically loaded sphere.
circles. The points of the RCA boundary are determined
when the intensity increases sharply and exceeds a preselected threshold level. These boundary points of the
RCA are then fitted by a circle (using circular edge
detection function [17]) whose radius represents the
RCA mean radius (figure 6c).
In the case of moderate and high normal loads, a
relatively sharp edge is formed at the circumference of
the contact area due to plastic deformations. Incident
light outside the contact area reflects mostly in a wide
angle and, thus, a very small amount of this light returns
into the optical module (see figure 7a). As a result, there
is a high contrast between the reflections form inside and
outside the contact area as can be seen in figure 7b. For
this case a quite simple image processing algorithm is
used for RCA estimation by setting a proper threshold
level [17] for image intensity distribution. Consequently,
the pixels of the original contact area images are binarized (assigned 0 or 1) according to their intensity level
with respect to the threshold and the total number of
pixels of RCA (assigned 1) is calculated. Then, by using
calibration coefficients for different optical magnifications, the total number of RCA pixels is converted to
RCA in squared millimeters. An example of two different
copper spheres in contact with the sapphire flat is presented in figure 8. A 5 mm mirror polished sphere and a
2.38 mm rough sphere are shown in figures 8a, b,
respectively. It can be seen from figure 8b1 that the
above image processing algorithm filters out non contact
spots, and provides an accurate RCA.
4. Preliminary experimental results
To assess the performance of the developed test rig,
some preliminary experiments were carried out. All
experiments were done in a configuration of a deformable sphere loaded against a rigid flat. The sphere
specimens were made of bearing steel (AISI 52100) or
copper (UNS C10200) and the rigid flat was of sapphire.
The self measured (or reported in the literature)
mechanical and geometrical properties of the sphere
specimens and the sapphire flat, along with the critical
normal load, Pc, at plastic yield inception of the spheres
[18], are summarized in table 1.
All the experiments were carried out at room temperature of 19–23 °C and relative humidity of 40–60%.
Each experiment was performed on a new area of contact and both the sphere and sapphire surfaces were
(b)
Light beams do
not enter the
optical module
(a)
Optical
module
Sapphire
Sphere
Normal load, P
Figure 7. The scheme of image formation of the contact area for
elastic–plastic loaded sphere.
60
A. Ovcharenko et al./Real time optical measurement
100µm
(a)
(a1)
Threshold
application
70µm
(b)
(b1)
Original images
Binarized images
Figure 8. Contact area original images of smooth (a) and rough (b) surfaces before, and corresponding binarized images (a1) and (b1) after
threshold application for real contact area (RCA) estimation.
Table 1.
The mechanical and geometrical properties of bearing steel and copper spherical specimens, and sapphire flat.
No
1
2
3
Material
Bearing steel
Copper
Sapphire
D (mm)
4.5
5, 10
Flat
H (Gpa)
Y (Gpa)
9.47
1.15
19*
*
2
0.35
2.95*
E (Gpa)
m
*
0.30
0.33*
0.27*
*
205
139
435*
Ra (nm)
200
100
5
Pc (N)
184
0.7, 2.9
–
D – diameter, H – hardness, Y – yield strength, m – Poisson’s ratio, E – Young’s modulus, Ra – roughness average, Pc – critical normal load at
plastic yield inception [18].*Values obtained from the literature.
cleaned in acetone before each loading. The normal load
was applied for 30 s before the measurements were
taken. At least three repetitions of each experiment were
made and data repeatability was better than 90%.
Figure 9 shows a comparison between the measured
contact area, A, and the Hertz analytical solution [19]
for different normal loads, P, in the elastic regime of
deformation (P<Pc, see Table 1). The experiment was
carried out with a bearing steel ball of 4.5 mm. The
slight deviation (less than 5%) of the measured RCA
from the Hertz analytical solution, especially at low
loads, is probably due to the accuracy of fitting
boundary points by a circle, according to the image
processing algorithm for analyzing Newton’s rings (see
Section 3).
Figure 10 shows a similar comparison between the
measured RCA and the results from a numerical model
by Kogut and Etsion [18] for different normal loads, P,
in the elastic–plastic regime of deformation (P>Pc). For
this case the experiment was carried out with a 5 mm
diameter copper sphere. The model in [18] provides
simple dimensionless expressions for the elastic–plastic
RCA vs. applied normal load at a Poisson’s ratio of 0.3.
It can be seen that the experimental results correlate
quite well (less than 5% deviation) with the model
results.
Figure 11 shows the possible effect of dwell time (time
under steady load) on RCA. The test was done with a
10 mm copper sphere under a relatively large normal
load of 160 N, deep into the elastic–plastic regime of
deformation. The measured RCA is normalized by A0,
corresponding to the RCA at a dwell time of 40 s. As
can be seen from the figure, the normalized RCA
increases by no more than 1.2% even for a relatively
long dwell time of 300 s (note that the resolution of the
RCA measurement is better than 0.1%). It is interesting
to note the linear relation between RCA and the logarithm of the dwell time as represented by the dashed line
which fits well (R2 = 0.997) the normalized RCA
results. Therefore, it can be safe to assume that contact
creep has negligible effect on the RCA measurements
even at long period of time corresponding to the lowest
Q_ value.
Figure 12 shows the effect of tangential loading on
the RCA, relative displacement and friction evolution in
time. In this experiment a steady normal load of 135 N
61
A. Ovcharenko et al./Real time optical measurement
2
Real Contact Area, A [mm ]
0.06
0.05
0.04
0.03
0.02
0.01
0.00
0
50
100
150
200
Normal Load, P [N]
Figure 9. Real contact area (RCA) experimental results (circles) in comparison with the Hertz analytical solution (line) for an elastic contact of a
4.5 mm bearing steel ball (Pc = 184 N).
2
Real Contact Area, A [mm ]
0.20
0.16
0.12
0.08
0.04
0.00
0
50
100
150
200
Normal Load, P [N]
Figure 10. Real contact area (RCA) experimental results (triangles) in comparison with the KE model [18] (line) for an elastic–plastic contact of a
5 mm copper sphere (Pc = 0.7 N).
1.012
Normalized RCA, A/A0
1.010
1.008
1.006
1.004
1.002
1.000
10
100
1000
Dwell Time [sec]
Figure 11. Normalized real contact area (RCA) as a function of dwell time for 10 mm copper sphere (P = 160 N, Pc = 2.9 N).
was first applied to a 5 mm copper sphere in order to
cause plastic deformation of the contact zone. Then, a
tangential load, Q, was applied gradually at a constant
rate of Q_ = 2.25 N/s until sliding incepted between the
sphere and the flat. A monotonic, non linear, increase in
RCA of about 40% is observed in figure 12 as the
friction force, F, increases linearly from zero to its
maximum value (static friction) at the instant of sliding
inception. The sliding inception, that occurred about
17.5 s after activation of the piezoelectric actuator, can
be easily identified in the figure by the simultaneous
maximum in the friction force and sharp transition in
the slopes of both the RCA and relative displacement.
Figure 13 presents images of the RCA corresponding to
62
0.23
40
0.22
35
30
0.21
25
0.20
Friction Force
Relative
Displacement
RCA
20
0.19
15
0.18
10
0.17
5
0.16
Friction Force, F [N]
Relative Displacement [ µm]
2
Real Contact Area, A [mm ]
A. Ovcharenko et al./Real time optical measurement
0
0
5
10
15
20
25
30
Time, T [sec]
Figure 12. The time evolution of friction and real contact area (RCA) for a 5 mm copper sphere (Pc = 0.7 N) under combined normal and
tangential loading (P=135 N, Q_ = 2.25 N/s).
F = 0 (normal load only) and sliding inception, as obtained
in the experiment of figure 12. The arrow in figure 13b
indicates the direction of friction on the sphere. The
effect of the tangential loading on the size and shape of
the RCA is clearly observed by comparing the two
images in the figure. Figure 13c shows a superposition
of the two images to facilitate the visualization of the
RCA non-axisymmetric growth evolution. The results
are very different from these presented in Ref. [20] where
RCA of a copper spherical specimen was measured after
the loads (normal and tangential) were removed and the
residual contact trace on the sphere was examined with a
remote optical microscope. It was concluded in [20]
from the residual diameter of the contact area that RCA
(a)
(b)
100µm
(c)
Contact area at
instant of sliding (b)
is essentially unaffected by the tangential loading. This is
clearly in contrast to our present more accurate in situ
and real time observations that show ‘‘junction growth’’.
The concept of ‘‘junction growth’’, which was introduced in 1959 by Tabor [21], was explained by the need
to maintain a constant von Mises stress at yielded
contact points. According to this requirement a contact
area, which has already yielded plastically under a given
normal load, must grow when subjected to additional
tangential loading, in order to reduce its mean contact
pressure and be able to accommodate the additional
shear stresses.
Finally, experiments that were performed over a wide
range of tangential loading rates (0.03–37 N/s) showed
100µm
Original contact
area under normal
load alone (a)
Fig. 13. The contact area images obtained from the experiment in figure 12 under: (a) normal load alone, (b) at the instant of sliding inception
(the direction of rigid flat motion is indicated by the arrow), and (c) their superposition to demonstrate the real contact area (RCA) growth
evolution.
A. Ovcharenko et al./Real time optical measurement
basically very similar results to these presented in
figure 12. Hence, it can be safely assumed that the fine
details of RCA and friction evolution during pre-sliding
are well captured with the current rate of tangential
loading.
5. Conclusion
A novel test rig, which allows accurate, real time and
in situ direct RCA measurement, which is synchronized
with motion and data acquisition, was described. A
closed loop feedback motion control of a piezoelectric
actuator allows accurate smooth and continuous application of tangential loading, differently from other
existing setups that employ similar direct optical
observation techniques. It was found that tangential
loading at relatively low rate of 2.25 N/s is sufficient to
accurately capture the fine details of RCA and friction
evolution in pre-sliding and at sliding inception. The
preliminary results showed good resolution (better than
0.1%) of RCA measurement with the test rig, using
intensity threshold concept for image processing
algorithms. The RCA evolution under normal loading,
as measured by the test rig, correlates well with existing
theoretical results for both elastic and elastic–plastic
regimes of deformation. The test rig was found capable
of capturing directly, in situ and in real time the exact
size and shape evolution of RCA in pre-sliding.
References
[1] P.W. O’Callaghan and S.D. Probert, Wear 120 (1987) 29–49.
[2] K.L. Woo and T.R. Thomas, Wear 58 (1980) 331–340.
[3] B. Bushan, ASLE Trans. 28 (1985) 75–86.
63
[4] J. Cortney-Prat and E. Eisner, R. Soc. Lond. 238A (1957)
529–549.
[5] F. Aymerich and M. Pau, J. Tribol. 126 (2004) 639–645.
[6] M. Pau, F. Aymerich and F. Ginesu, Wear 253 (2002) 265–274.
[7] R. Dwyer-Joyce and B. Drinkwater, Tribol. Lett. 14 (2003) 41–52.
[8] R. Parker and D. Hatch, Proc. Phys. Soc. (Lond.) B63 (1950)
185–197.
[9] M. Chaudhri, Philos. Magaz. A 44 (1981) 667–675.
[10] M. Chaudhri, I. Hutchings and P. Makin, Philos. Magaz. A 49
(1984) 493–503.
[11] K. Tanaka, Wear 100 (1984) 243–262.
[12] C. Constantinou and M. Chaudhri, J. Mater. Sci. 24 (1989) 4279–
4292.
[13] E. Diaconescu, M. Glovnea, Evaluation of contact area by
reflectivity 3rd Aimeta International Tribology Conference, Italy
2002.
[14] S. Lo and S. Tsai, J. Tribol. 124 (2002) 229–238.
[15] K. Adachi, K. Kato, J. Liu, and H. Kawamura, The effect of
contact morphology on initiation and propagation of micro-slip
at contact interface ASME/STLE International Joint Tribology
Conference, California USA 2004.
[16] A. Azushima, S. Kuba, S. Tani and D. Olsson, Wear 260 (2006)
258–264.
[17] T. Klinger, Image Processing with LabVIEWä IMAQä Vision
(Prentice Hall PTR, 2003).
[18] L. Kogut and I. Etsion, J. Appl. Mech. Trans. ASME 69 (2002)
657–662.
[19] S. Timoshenko and J. Goodier, Theory of Elasticity 3rd ed.
(McGraw-Hill, New York, 1970).
[20] I. Etsion, O. Levinson, G. Halperin and M. Varenberg, J. Tribol.
127-1 (2005) 47–50.
[21] D. Tabor, Proc. R. Soc. A 251 (1959) 378.