A new device for fretting fatigue testing

Transcription

1 Transactions of The Indian Institute of Metals A new device for fretting fatigue testing G.H. Majzoobi 1, R. Hojjati 1, M. Nematian 1, E. Zalnejad 1, A.R. Ahmadkhani 2 and E. Hanifepoor 1 1 Faculty of Mechanics, Azad University of Takestan, Takestan, Iran 2 Mechanical Engineering Department, Faculty of Engineering, Bu-Ali Sina University, Hamadan, Iran gh_majzoobi@yahoo.co.uk Received 26 June 2008 Revised 11 May 2009 Accepted 11 May 2009 Online at TIIM, India Keywords: Fretting fatigue testing device; load cell; variable crank system; Al7075-T6 Abstract Fretting fatigue damage occurs in contacting parts when they are subjected to fluctuating loads and sliding movements at the same time. Fretting fatigue can reduce the fatigue life of materials by half or even more. Fretting fatigue tests are usually performed using universal hydraulic testing devices. The contact pressure is produced by a fixture, typically designed and manufactured by researchers. In this investigation, a new device is introduced in which the fluctuating loading is supplied by a variable crank system (VCSD). The device called VCSD for abbreviation is basically a position control machine in which displacements can be imposed with an accuracy of 0.01 mm. The axial and contact loads are measured by load cells. The friction load is also measured by using foil strain gauges using a Wheatstone bridge configuration. The functionality of the device is examined by making a comparison between fretting fatigue lives of a number of Al7075-T6 specimens tested on a universal testing machine and VCSD. The results show a very close agreement between the functionality of the two testing rigs. The main advantages of VCSD are its higher frequency with respect to universal devices, simplicity, and cheapness. It can be developed further for high and low temperature tests in future. 1. Introduction Fretting fatigue damage occurs in contacting components when they are subjected to oscillating loads and sliding movements at the same time. This phenomenon may occur in many applications such as bearings shafts, bolted and riveted connections, steel cables, steam and gas turbines [1,2]. Fretting fatigue may reduce the endurance limit of a component by half or more, in comparison to the normal fatigue conditions. Unfortunately, there is not a universal machine available in the market for fretting fatigue testing. Generally, the universal hydraulic fatigue testing devices are used for this sort of testing. However, these universal machines require accessories for producing contact pressure needed for fretting fatigue. Additionally, the frequency of these types of machines is low and consequently, the fatigue tests are time consuming. Arora et al [3] designed a fretting fatigue test rig in such a way that the initiated small cracks at the contact edge could be directly observed and the crack propagation could be recorded using the video microscope. The fretting fatigue test jig is mounted on a 810 MTS material test system of 100 kn capacity. On the loading jig two frictional force load cells are mounted. Normal force to the specimen pad interface is applied through the normal force load cell by turning the loading bolt. Matlik et al [4] designed and constructed a hightemperature, high-frequency fretting rig and mounted on a custom built machine base. Matlik et al [4] employed a piezoelectric stack actuator to generate the tangential loading, Q, at the contact [5]. The contact region is heated via radiant heating using Norton Crystar igniters. More details are given in references [6] and [7]. Majzoobi and Jaleh [8], Noovin Rooz et al [9] and Majzoobi et al [10] used a universal fatigue testing machine along with a special jig for conducting fretting fatigue tests. In this special jig, the pressure required to produce fretting is transmitted to the contact area, via a calibrated proving ring, shown in Fig. 1, to the contact area through the loading pads. Each pad (see Fig. 2) has two bases (contacting surfaces) through which the load is exerted on the specimen. The normal load is induced by an adjusting screw and is measured using a Wheatstone bridge circuit. Four strain gauges are bonded to the proving ring and wired to create a Wheatstone bridge circuit to measure the elastic strains induced by loading the ring through the load adjusting screw. Gutkin, and Alfredsson [11] studied the crack growth in a shrink-fit assembly subjected to rotating bending. They observed a significant reduction in the fatigue crack propagation life, up to 50%, as compared to an equivalent Fig. 1 : The ring assembly used for fretting fatigue tests [9].

2 Fig. 2 : The pads used for transmitting the contact pressure. pure bending case. Rajasekaran, and Nowell [12] developed a biaxial fatigue experiment capable of simulating the loading experienced by a dovetail blade root in an aircraft gas turbine. The device can simulate the effects of centrifugal loading, disk expansion force and blade vibration. Ebara and Fujimura [13] used a fretting fatigue specimen with bolt tightened shoe on both sides of the plate. The contact load was gained from the strain measured by a strain gauge fixed at the bolt centre. A fretting test apparatus has been designed at Purdue University in USA to study fretting fatigue at elevated temperature. The fretting fatigue specimen is clamped between two contact pads and then loaded with a remote cyclic load that leads to eventual specimen cracking and failure. The main motivation behind the development of this device was to determine conditions that cause frettinginduced cracks to form in advanced turbine engine materials such as the rash of high cycle fatigue (HCF) engine failures that led to the grounding of many military aircraft during the 1990s. 2. Test rig A new fretting fatigue testing device has been developed by the authors in this work. A general view and a schematic of the device (called in this paper as VCSD) are depicted in Figs. 3 and 4, respectively. The device consists of four parts: Fig. 4 : A schematic of VCSD (i) the chassis and the structure of the apparatus, (ii) the contact pressure unit through which the pressure is transmitted by two pads to the specimen; (iii) the variable crank system which produces the oscillatory axial displacement; and (iv) the instrumentation which consists of load cells for measuring the contact and axial forces imposed on the specimens, the inverter for varying the frequency, and fatigue cycles counter. The chassis and the structure of the device have been designed to be sufficiently rigid to resist the fatigue consequences due to oscillations of the variable crank system and also to damp out the fluctuations transmitted to the various components of the device. 2.1 Variable crank system The axial fluctuating load is applied to the specimen by a variable crank mechanism shown in Fig. 5. The mechanism consists mainly of a stepped eccentric shaft and two suspension plates. The stepped shaft is made of two segments of 35 and 45 mm in diameters (see Fig. 6(b)). The two segments have an eccentricity of 1 mm with respect to Fig. 3 : A general view of the VCSD Fig. 5 : The assembly of eccentric shaft-bush bearing-inner ring of the ball bearing.

3 Fig. 6 : (a) section of the eccentric bush bearing and (b) section of the eccentric shaft. Fig. 7 : Contact loading system of the VCSD. each other. An eccentric bush bearing with 1 mm eccentricity is also used in the variable crank system. As shown in Fig. 6(a), the inner and the outer diameters of the bush bearing are eccentric with respect to each other as much as 1 mm. The bush bearing is mounted on the stepped shaft by some mechanical connections. If the shaft is kept fixed (OA is fixed in Fig. 6(b)) and the bush bearing is turned on the shaft (OB rotates with respect to OA in Fig. 6(b)), a total variation of 0 to 2 mm is obtained for eccentricity of the system. This means that during each revolution of the shaft, a completely reversed fluctuating axial displacement of ±1mm is obtained for the maximum eccentricity of 2 mm. This amount of displacement is sufficient for most of fretting fatigue tests and in particular if the deformations are supposed to remain within the elastic range. The bush bearing is shrunk onto a ball bearing through which the oscillatory displacement is transmitted to the two upper and lower suspension plates. The inner ring of the bearing can freely rotate with rotation of the bush bearing. The outer ring, however, is settled and remains continuously in contact with the upper and the lower plates. In fact, the rotation of the eccentric shaft-bush bearing-inner ring assembly is converted to a vertical oscillating displacement which is transmitted to the plates by the outer ring of the ball bearing. The assembly of eccentric shaft-bush bearing-inner ring is illustrated in Fig. 6. As the figure indicates, the two upper and lower plates are suspended by two vertical columns which can freely slide inside the hole ways made into the upper cross head of the device. The vertical displacement of the upper and lower plates gives rise to the tension and compression of the specimen, respectively. The angular eccentricity of the bush bearing, ϕ (see Fig. 6(a)), can be varied using two rotating disks. The disks are serrated in one face in the form of male and female. One disk being connected to the shaft is settled and the other being connected to the bush can be adjusted by turning it on the shaft. The shaft periphery is scaled for ±1 mm fluctuation amplitude with accuracy of 0.01 mm. The desired amplitude is set up by changing the angular eccentricity of the bush bearing. 2.2 Contact loading system The contact loading system, shown in Fig. 7, is completely embedded in the lower cross head which is supported by the two main columns of the device and can be moved vertically. The contact loading system consists of two pads, two adjusting screws and two load cells. The contact load is induced by two adjusting screws and is measured using two load cells. The load cell readings can ensure that the contact loads produced by tightening screws in two opposite directions are equal. The pads are placed in solid disks which in turn are located between the specimens and the load cells. Two types of pads geometries, flat and cylindrical with various width and radius are manufactured for enabling the flat on flat and cylinder on flat fretting fatigue tests to be carried out. A number of flat and cylindrical pads are shown in Fig. 2. The pads are constrained to displace vertically as this may affect the sliding oscillations between specimen and pads. 2.3 Instrumentation The axial load is measured by a DSCK type 20kN load cell (see Fig. 4) manufactured by BONGSHIN. The variation of load monitored by a digital display can be transferred to a computer for further processing. Two load cells of the type CDES mounted between the adjusting screws and the pads (see Fig. 7) monitor the contact load on separate digital displays. The variation of the load cells can also be recorded on a computer. The frequency of the device can vary using an inverter up to a maximum of 25 Hz. The friction force, created by normal force and sliding movement between the specimen and pads, is measured by a Wheatstone bridge consisting of four strain gauges, two in direction of friction and two at right angle to this direction. Two strain gauges are bonded to the pads to measure the elastic strain induced by friction force. The pads are clamped to the specimen by the same normal loads as was to be used in real experiments. The specimen is then pulled by constant load using the universal tensile testing machine. The strain induced by this load is measured by Wheatstone Circuit Bridge attached to the pads. This procedure is repeated for different loads and the calibration curve of friction force versus strain is obtained. The friction coefficient can be determined from the relation μ = F/P in which P is the contact load measured from the load cells and F is the friction force measured from the calibration curve of the Wheatstone Circuit Bridge output. 3. Experimental results 3.1 Test specimens Aluminum alloy 7075-T6 was used in this investigation. From a number of tensile tests, the yield stress and ultimate strength of Al7075-T6 were obtained as: σ y =520MPa and σ u/t =590 MPa, respectively. The flat specimens used in this work were prepared in accordance with ASTM standard. The

4 Fig. 8 : Flat specimens used for fretting fatigue tests. specimen had a width of 14.5 mm, a thickness of 4.5 mm and a gauge length of 70 mm. The material s composition obtained using EDXRF apparatus is Al 91%, Cu 1.9%, Mg 2%, Cr 0.25, Zn 4.8% and Mn 0.7%. The specimen geometry is shown in Fig. 8. A number of specimens were shot-peened under the following conditions: the average diameter of the shots was 0.5 mm, the velocity of shots was 80 m/sec and the time duration of exposure was 8 minutes. Fig. 10 : A deep pitting in the contact zone of a specimen under fretting fatigue conditions. 3.2 Fretting fatigue tests In order to examine the functionality of the fretting fatigue device, VCSD, two types of specimens, virgin and shotpeened were tested in this investigation. The experiments were conducted for stress ratio of R=0.1, frequencies of 5 Hz (for universal testing device) and 20 Hz (for VCSD) at a constant contact force of 1200 N, and working stress amplitudes of 180, 200, and 280 MPa. A number of tests were also performed using the universal fatigue testing machine Table 1 : Fatigue lives obtained from the experiments for virgin and shot-peened specimens AppliedStress Fatigue life Fatigue life Error% (MPa) (UNTM) VCSD Virgin specimens Shot-peened specimens UNTM stands for univer Fig. 11 : Microcracks propagation in the contact area of specimen under fretting fatigue conditions. under the same conditions as stated above for comparison purposes. The arrangement of the fretting fatigue test was explained in section 1. The results are given in Table 1. As the results indicate, the fatigue lives obtained from the universal testing machine and VCSD are very close. The differences can be attributed to the scatters which are normally observed in S-N curves of materials obtained by experiment. Fig. 9 : Fracture surfaces of two specimens under fretting fatigue conditions. 3.3 Optical microscopy Fracture surfaces of tested specimens were examined using optical microscopy. Two typical results are illustrated in Fig. 9. The figure clearly indicates that the fracture surface consists of two quite distinct regions; a fatigue zone created by crack propagation and a tensile region which gives rise to fracture of specimen when it is sufficiently weakened by the crack zone development. The two wear zones which are created in the contact area between pads and specimens are depicted in Figs. 10 & 11. Apart from the dimple type roughness produced by shot-peening seen in the figures, the deep pits which are direct consequences of fretting fatigue damage are clearly observed in the figures.

5 4. Conclusions A new fretting fatigue device is developed by the authors in this work. The device is capable of exerting adjustable axial and contact loads. The frequency of the device can be varied from 1 to 25 Hz. The functionality of the rig is validated by fretting fatigue test results using the universal fatigue testing machine. The device has a simple variable crank system for axial fluctuating loading. The system is pure mechanical with no need for hydraulic systems which are sophisticated, expensive and slow. The device is well instrumented for varying and measuring axial and contact loads, fatigue cycles, test frequency and friction force. References 1. Hills D A and Nowell D, Mechanics of fretting fatigue, Kluwer Academic Publisher, (1994). 2. Anderson T L, Fracture Mechanics: Fundamentals and Application, Second Edition, CRC Press, (1994). 3. Prithvi Raj Arora, Jacob M S D, Mohd. Sapuan Salit, Elsadig Mahdi Ahmed, Saleem M, Prasetyo Edi, Experimental evaluation of fretting fatigue test apparatus, International Journal of Fatigue 29 (2007) Matlik J F, Farris T N, Haake F K, Swanson G R and Duke G C, High-frequency, high-temperature fretting-fatigue experiments, Wear, 261 (2006) Matlik J F, Farris T N, High-frequency fretting fatigue experiments, in: Mutoh Y, Kinyon S E, Hoeppner D W (Eds.), Fretting Fatigue: Advances in the Basic Understanding and Applications, ASTM STP 1425, American Society of Testing and Materials, West Conshohocken, PA, (2003) Hills D A, Nowell D, The development of a fretting fatigue experiment with well-defined characteristics, in: Attia, M.H, Waterhouse R.B. (Eds.), Standardization of Fretting Fatigue: Test Methods and Equipment, ASTM STP 1159, American Society of Testing and Materials, Philadelphia, PA, (1992) Murthy H, Rajeev P, Okane M and Farris T N, Development of test methods for high temperature fretting of turbine materials subjected to engine type loading, in: Mutoh Y, Kinyon S E, Hoeppner D W (Eds.), Fretting Fatigue: Advances in the Basic Understanding and Applications, ASTM STP 1425, American Society of Testing and Materials, West Conshohocken, PA, (2003) Majzoobi G H and Jaleh M, Duplex surface treatments on AL7075-T6 alloy against fretting fatigue behavior by application of titanium coating plus nitriding, Materials Science and Engineering: A, (2007) Novin Rooz A., Majzoobi G.H., Nemati J. 2, Farrahi G.H.., The Effects of Chrome and Titanium Coating on Fretting Fatigue Resistance of Aluminum Alloy 7075-T6, Steel research int. 79 (2008), Special Edition Metal Forming Conference 2008, 2 (2008) Majzoobi G.H., Nemati J., Novin Rooz A.J., Farrahi GH., Modification of Fretting Fatigue behavior of AL7075-T6 alloy by application of Titanium Coating and shot Peening, Tribology International, 42 (2009) Gutkin R and Alfredsson B, Growth of fretting fatigue cracks in a shrink-fitted joint subjected to rotating bending, Engineering Failure Analysis, 15 (2007) Rajasekaran R, Nowell D, Fretting fatigue in dovetail blade roots: Experiment and analysis, Tribology International, 39 (2006) Ryuichiro Ebara and Masanori Fujimura, Fretting fatigue behavior of Ti 6Al 4V alloy under plane bending stress and contact stress, Tribology International, 39 (2006) 1181.