The effect of surface regular microtopography on fretting fatigue life

Transcription

1 Wear 253 (2002) The effect of surface regular microtopography on fretting fatigue life A. Volchok, G. Halperin, I. Etsion Shamban Tribology Laboratory, Department of Mechanical Engineering, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel Received 28 January 2002; received in revised form 23 May 2002; accepted 27 May 2002 Abstract The possibility of increasing fretting fatigue life of a tribo-pair by laser surface texturing technology is studied. An appropriate experimental fretting fatigue device is used to conduct comparative experiments with a cylinder-on-flat type of contact. A relatively small number of long fretting fatigue experiments under seemingly partial slip condition are described and analyzed. From these preliminary tests, it is found that regular micropores created by laser texturing in the fretted zone on the cylinder almost double the fretting fatigue life compared to a common non-textured fretting zone Elsevier Science B.V. All rights reserved. Keywords: Fretting; Fretting fatigue; Laser surface texturing 1. Introduction Fretting is the oscillatory sliding motion of small slip amplitude between two nominally stationary contacting surfaces [1]. It occurs when two quasi-static surfaces in contact are actually subjected to deformation and/or vibrations [1 3]. The damage caused by fretting is divided into fretting wear, when the contact load acts alone (both normal and tangential components), and fretting fatigue, when an external alternating load is superimposed on the contact load. It should be noted that both forms of damage often co-exist in the same contact [3]. Enhanced crack nucleation and propagation caused by fretting fatigue can significantly reduce the fatigue life of mechanical components [4 6]. Fretting fatigue commonly occurs in clamped joints and shrunk-on components, such as keyways, splines, wire strands, springs, dovetail joints, etc. [1 6]. It was found that fretting wear and fatigue are influenced in a consistent and analogous manner by controlled variations in experimental conditions [7]. That is, conditions that tend to accelerate fretting wear also accelerate fretting fatigue failures. Surface finish is one of the key factors affecting fretting behavior due to its strong relation to wear particles presence in the contact zone. On a rough surface, for example, debris can escape from areas of contact into adjacent hollows or depressions on the fretted surface [1]. The role played by wear debris presence in the contact zone is not yet com- Corresponding author. Tel.: ; fax: address: etsion@tx.technion.ac.il (I. Etsion). pletely clear. Some experimental results suggest that the best anti-fretting surfaces will be flat smooth and conformal. This decelerates the wear debris escape and keeps them in the areas of contact [8]. In many instances of dry friction, the protection afforded by the wear debris presence in the contact areas is greater than the damage caused by their presence [9]. On the other hand, it is a general observation that a high degree of surface finish accentuates fretting damage [1]. Fretting wear debris can cause severe abrasion, but on a rough surface there is a greater chance that wear debris will escape from the contact areas into the surface depressions, which will reduce the abrasive wear [10]. A literature survey revealed a multitude of surface modification methods that can be used to mitigate fretting damage [10 14]. A compressive residual stress inducement in the surface layer and a controlled change in the surface roughness were reported as two of the important mechanisms for the mitigation of fretting damage, especially for fretting fatigue. The compressive residual stress will reduce the tensile stress component of the fretting action. This will reduce the wear rate, close-up fretting fatigue cracks at the surface and prevent their propagation [11]. To minimize the fretting damage different treatments of surface roughening are sometimes adopted, e.g. shot peening. Most finishing methods (turning, grinding, lapping and honing) will yield a roughness pattern that can be described by some statistical height distribution of its peaks and valleys. A considerable advantage over conventional surface finishing techniques is reached by providing a uniform surface microtopography pattern. It was suggested [12] to produce regular net-pattern grooves in a fretted zone in order to increase fatigue resistance of /02/$ see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S (02)

2 510 A. Volchok et al. / Wear 253 (2002) machine parts, working under fretting conditions. In another investigation [13], experiments were conducted using instead of a plane fretting pad a grooved pad, which showed an increased fretting fatigue resistance of the assembly by a factor of more than two. Another simple and cost-effective machining method recommended for improving fretting fatigue performance is vibrorolling. Microgrooves, created by the vibrorolling technique, may substantially contribute to fretting resistance by both increasing fretting fatigue resistance and reducing fretting wear [14]. However, a rough surface containing potential stress raisers (e.g. microgrooves) is very dangerous, especially under fatigue conditions. The roll played by microsurface geometry of contacting surfaces on their fretting fatigue resistance is not yet clear enough. The objective of the present work, in light of the above uncertainty, is to investigate the effect of a regular surface roughness pattern on fretting fatigue behavior. This is examined by creating a regular micropores pattern, using a new technology of laser surface texturing, and allowing the escape of wear debris into the micropores. 2. Experiment details A simplified cylinder-on-flat contact geometry is chosen so that accelerated tests may be conducted while the same contact geometry is maintained for every test. The main effort in the present work was directed to obtain a fretting zone size that is comparable to the micropore scale. For this matter, a new experimental system was designed and constructed, which allows plane bending of a cantilever beam specimen with cylinder-on-flat type of fretting contact Test apparatus The conceptual scheme of the experimental apparatus, illustrated in Fig. 1, is prevalent and similar to those found in the literature [6,12]. A more detailed schematic diagram is shown in Fig. 2. A crank drive (1), consisting of a motor, eccentric with variable eccentricity and connecting rod, transforms the rotary motion of the motor shaft into a linear cyclic motion of the connecting rod. This applies an alter- Fig. 1. A conceptual scheme of the experimental apparatus. Fig. 2. Schematic diagram of the experimental apparatus.

3 A. Volchok et al. / Wear 253 (2002) nating bending load to the free end of the cantilever beam specimen (2). Cylindrical contact pads (3) are pressed on the beam specimen surface at its smallest cross-section using a closed loop loading frame (4). The pads loading frame, consisting of a spring (5), a loading screw (6) and a force transducer (7), is free to follow the beam motion only in the direction normal to its surface, thus permitting monitoring of the contact load. The self-aligning fretting pad holders secure full contact between the beam specimen and cylinders. The cyclic bending load causes longitudinal cyclic displacements due to tension/compression strains at the fretting zone. Hence, relative slip can occur at the interface between the beam specimen and the fretting cylindrical pads. The number of bending cycles can be monitored during the experiment with a photoelectric gate mounted near the eccentric shaft. An eddy current proximity probe (8) located between the fixed end and the smallest cross-section of the beam measures the displacement amplitude close to the smallest cross-section Test specimen and pads preparation The specimen, shown in Fig. 3, has a typical shape of symmetrical cantilever beam with a length of 165 mm, thickness of 2 mm, nominal width of 60 mm and only 8 mm width at its smallest cross-section. A DIN tool steel with hardness of HRC was used for the specimen material. The surface of the specimen in as received condition was ground to roughness average of and m parallel and perpendicular to the grinding direction, respec- tively. This surface finish allows keeping the fretting wear debris at the areas of real contact. Two SKF rollers RC 20 mm 20 mm with hardness of HRC were used as the fretting fatigue cylindrical contact pads. These rollers, in as received condition, have consistent surface roughness average of m and are suitable for conducting comparative experiments. These as received surfaces were used as counter faces against the textured specimens or cylinders, respectively Measurements and data processing The present test system allows online monitoring and measuring of the beam specimen displacement close to the fretting contact, the bending frequency and the average normal load. A 12-bit A-to-D converter of multifunctional I/O board Lab-PC-30GA is utilized for data acquisition and control in addition to sampling the output voltage signals from the respective probes. The sampling is performed with a multiplex technique at a rate much higher than the bending frequency, resulting in synchronous acquisition of the measured signals. The function used for the data sampling makes continuous time-sampled measurements of three channels, stores the data in a circular buffer and returns a specified number of scanned measurements on each call of this function. The digitized data received from each channel is low pass filtered to eliminate high frequency noise. Averaging and analyzing of the scan measurements returned on each call of the sampling function give the evolution of the beam specimen and contact pads response Fig. 3. (a) Microdimples on the beam specimen surface at the fretting zone; (b) textured surface profile with 31.9 m pore depth.

4 512 A. Volchok et al. / Wear 253 (2002) during the test. A data chart shows the time variation of the beam displacement amplitude and the average normal contact load. The bending frequency and the accumulated number of cycles from the test start are also presented. The proximity probe, which is used to measure the specimen s displacement amplitude near the fretting zone, helps in maintaining uniform test conditions from one test to another as well as during a given test. It also permits an automatic shut down of the rig s motor just a few cycles before final fracture of the specimen. The measured displacement amplitude reaches a certain average value in a few seconds from the test start. This value remains unchanged until the specimen starts fracturing at its smallest cross-section. As the critical crack propagates the beam portion between its smallest cross-section and the fixed end becomes less susceptible to the bending load applied on its free end. The proximity probe senses the decrease in the average value of the measured displacement amplitude and terminates the test when this amplitude declined to 85% of its initial value Experimental conditions and procedure The effect of either beam or cylinders surface topography on the fretting fatigue life was examined by creating a regular pattern of micropores using a technology of laser surface texturing. The textured surface, consisting of aligned rows and columns (see Fig. 3a), can be described by three parameters: pore s diameter, density and depth. In all the present experiments the micropores diameter was m, their density was 25 30% and their uniform depth was about 2.5, 5.5 and 32 m. Beam specimen texturing was applied to the smallest cross-section over its entire fretting zone (Fig. 3a), while cylinders texturing was applied over their entire cylindrical surface. A textured cylinder surface profile with 31.9 m pore depth is shown in Fig. 3b. The laser texturing provides easier escape of wear debris, associated with fretting motion, from the contact zone into the micropores. Two sets of preliminary tests were carried out to firstly obtain the pure fatigue (without fretting) characteristics of the beam specimen using ASTM standard procedure [15,16] and secondly to determine a proper normal contact load on the contact pads that will ensure fretting in the partial slip regime that is the most dangerous in fretting fatigue [2 6]. The first set of tests produced the pure fatigue limit of the beam specimen and allowed selecting a stress level just above this limit as a common basis for comparison in the series of fretting fatigue tests. The comparison was made regarding the mean number of bending cycles till fracture of the beam specimen (fatigue life). All the experiments were carried out under constant fully reversal imposed displacement amplitude of 7 mm, applied to the beam free end by the crank drive shown in Fig. 2, frequency of 25 Hz, in laboratory atmosphere with temperature of 23 ± 1 C and relative humidity of 45 ± 5%. A normal contact load of 350 N was selected following the second set of preliminary tests. A test was stopped either automatically, just prior to final fracture of the specimen, or manually in cases where cycles were reached without fracture of the specimen. A complete statistical analysis that accounts for both failed and unfailed tests requires the use of the maximum likelihood method [17]. However, because of the limited number of the present preliminary tests the unfailed specimens were excluded from the mean specimen s life comparison. The data was sampled at a rate of 2048 Hz resulting in about 82 data points per each bending cycle. Two thousand and forty-eight data points (25 cycles) were returned from the circular buffer (at a rate of 1 Hz) and analyzed for each call of the sampling function. Before each test and after being mounted, the beam specimen was cleaned with ethyl alcohol 95%. The cylinders were cleaned ultrasonically. After the test, the fretted surfaces were examined by optical microscopy to observe the surface damage. The damaged surfaces were also analyzed by two-dimensional profilometry. 3. Results and discussion The mean fatigue life for the pure fatigue tests as well as for the fretting fatigue tests with different pore depth and surface texturing location (either on the beam or on the cylinders) are summarized in Table 1. As can be seen the pure fatigue life of the beam specimen, based on five failed tests, was cycles. The mean fretting fatigue life (base line), also based on five failed tests, was only cycles, representing a 53% life reduction compared to the pure fatigue life. Table 1 Mean life and standard deviation in the fretting fatigue experiments Experiment type Pore depth ( m) Number of tested specimens (n) Number of failed specimens (k) Mean life Pure fatigue Base line Textured beam Textured beam Textured cylinder Textured cylinder Standard deviation

5 A. Volchok et al. / Wear 253 (2002) Fig. 4. Part of the fretted area on the non-textured beam specimen. Fig. 4 presents a portion of the fretted zone, on the base line beam specimen, which is typical for a partial slip regime. A central stick (dark) area is bounded by two slip (brighter) zones on both its sides. Wear marks having distinct directionality of the fretting motion are clearly shown in the slip zones and a fretting fatigue crack initiated on one of the boundaries of the stick area is also apparent. Such location of the fretting fatigue crack was also described in [4 6]. A profilometer scan of the fretted area, parallel to the fretting motion, is presented in Fig. 5 and is very similar to that shown in [18]. The width of the fretted area (between the two vertical dashed lines) is about 1.1 mm and hence is capable of containing several rows of micropores (see Fig. 3). The larger amount of wear in the two slip zones as compared to the central stick area is clearly evident from Fig. 5 and is typical of a partial slip regime. Fig. 6 presents the effect of regular microtopography on the mean fretting fatigue life normalized by the cycles of the base line. As can be seen the textured beam with 5.3 m depth micropores gave a 10% shorter mean fretting Fig. 6. The effect of microdimples on the mean fretting fatigue life. Fig. 5. Fretted area profilometry results of the non-textured beam specimen.

6 514 A. Volchok et al. / Wear 253 (2002) Table 2 Mean life and standard deviation of non-textured and textured beam specimens in the pure fatigue experiments Experiment type Pore depth ( m) Number of tested specimens (n) Number of fractured specimens (k) Mean life Pure fatigue Textured beam Textured beam Standard deviation fatigue life compared to the base line. On the other hand, when the pore depth on the beam was reduced to 2.6 m, a 51% longer mean fretting fatigue life was obtained. This behavior is attributed to the negative effect of laser texturing on the pure fatigue life of the beam specimen due to the creation of numerous stress concentration sites on the beam surface. The micropores are indeed weakening the beam and shorten its pure fatigue life as can be seen from Table 2 that presents test results of two textured beam cases under pure fatigue in comparison with the non-textured case. The pure fatigue life with 5.3 and 2.6 m deep pores was reduced by 50 and 16%, respectively. The very strong negative effect of the deeper pores on the pure fatigue life masks their positive effect on the fretting fatigue life and hence the end result is a shorter fretting fatigue life as shown in Fig. 6 for the 5.3 m deep pores. The opposite is true for the shallower pores of 2.6 m and hence the end result in this case is a longer fretting fatigue life. Texturing the cylinders, which are not susceptible to external alternating loading, provides the full positive effect of the micropores in the contact zone. The mean fretting fatigue life with textured cylinders having micropores of 5.9 m depth is 180% of the base line value (see Fig. 6). Increasing the micropores depth on the cylinders to 31.9 m, to accommodate a substantially larger volume of wear debris, increased the mean fretting fatigue life very marginally by only an additional 8%, to 188% of the base line value. A t-test [19], revealed that the difference in fretting fatigue life between the two cases of textured cylinders is statistically insignificant with 80% confidence level. This can be explained by the fact that the volume of a single micropore of either 5.9 or 31.9 m depth is several order of magnitude larger than the mean volume of a single wear particle having a typical diameter of 1 m [12]. Hence, the difference in the micropore depth is insignificant for the purpose of wear derbies entrainment. It is interesting to note (see Table 1) that the textured cylinders eliminate almost completely the negative effect of fretting on the pure fatigue life of the beam. With the easier escape of wear debris from the interface of the fretted area almost 90% of the beam pure fatigue life is regained. Wear debris presence in the interface may shorten the fretting fatigue life under partial slip condition by allowing higher fretting amplitudes. The laser texturing modified surface topography, by eliminating wear debris presence enhances adhesive interaction between the fretted surfaces. This may eventually reduce the fretting amplitude, which under partial slip conditions has a beneficial effect on fretting fatigue life [4,5]. 4. Conclusions The potential effect of surface regular microtopography on fretting fatigue life was demonstrated by conducting a limited number of fretting fatigue tests. The regular microtopography was obtained by using laser surface texturing to form micropores on one of the fretted surfaces. Laser texturing allowed an easier wear debris escape from the fretted zone into the micropores, improved the fretting fatigue resistance and showed a beneficial effect on fretting fatigue life. Texturing the surface of the tribo-pair component not subjected to fatigue almost doubled the fretting fatigue life of the mating component. The preliminary results show a potential of improving fretting fatigue resistance by optimization of the laser process parameters. Further investigation is required in order to evaluate optimum micropores parameters (diameter, density and depth) for their maximal beneficial effect on fretting fatigue resistance. Acknowledgements This research was partially supported by the German Israeli Project Cooperation (DIP). Help by Surface Technologies Ltd., in preparation of laser-textured specimens is gratefully acknowledged. References [1] R.B. Waterhouse, Fretting Corrosion, Pergamon Press, Oxford, [2] R.B. Waterhouse, Fretting Fatigue, Elsevier, London, [3] L. Vincent, Y. Berthier, M.C. Dubourg, M. Godet, Mechanics and materials in fretting, Wear 153 (1992) [4] D.A. Hills, D. Nowell, Mechanics of Fretting Fatigue, Kluwer Academic Publishers, Dordrecht, [5] T.C. Lindley, Fretting fatigue in engineering alloys, Int. J. Fatigue 19 (1997) S39 S49. [6] Y. Mutoh, Mechanisms of fretting fatigue, JSME Int. J. Ser. A 38 (4) (1995) [7] R.C. Bill, Fretting wear and fretting fatigue how are they related? Trans. ASME J. Lubric. Technol. 105 (4) (1983) [8] J. Warburton, The fretting of mild steel in air, Wear 131 (1989) [9] Y. Berthier, L. Vincent, M. Godet, Fretting fatigue and fretting wear, Tribol. Int. 22 (4) (1989)

7 A. Volchok et al. / Wear 253 (2002) [10] Y. Fu, J. Wei, A.W. Batchelor, Some considerations on the mitigation of fretting damage by the application of surface modification technologies, J. Mater. Process. Technol. 99 (2000) [11] R. Moobola, D.A. Hills, D. Nowell, Designing against fretting fatigue: crack self-arrest, J. Strain Anal. 33 (1) (1998) [12] N.L. Golego, A.Y. Aliabiev, V.V. Shevelia, Fretting Corrosion of Metals, Technika, Kiev, 1974 (in Russian). [13] R.B. Waterhouse, Physics and Metallurgy of Fretting, in: Proceedings of the AGARD Conference no. 161, (1975), pp Paper presented at the 39th Meeting of the Structures and Materials Panel in Munich, Germany, 6 12 October [14] V.P. Bulatov, V.A. Krasny, Y.G. Schneider, Basics of machining methods to yield wear and fretting-resistive surfaces, having regular roughness patterns, Wear 208 (1997) [15] ASTM E , Standard Practice for Presentation of Constant Amplitude Fatigue Test Results for Metallic Materials, American Standard for Testing of Materials (1998). [16] ASTM E , Standard Practice for Statistical Analysis of Linear or Linearized Stress-Life (S-N) and Strain-Life (ε-n) Fatigue Data, American Standard for Testing of Materials (1998). [17] N.C. Giri, Introduction to Probability and Statistics, 2nd Edition, Marcel Dekker, New York, [18] K. Nishioka, K. Hirakawa, Fundamental investigation of fretting fatigue. Part 2. Fretting fatigue test machines and some results, Bull. Jpn. Soc. Mech. Eng. 12 (50) (1969) [19] R.M. Bethea, B.S. Duran, T.L. Boullion, Statistical Methods for Engineers and Scientists, 3rd Edition, Marcel Dekker, New York, 1995.