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Comparison of liquid impingement results from whirling arm and water-jet rain erosion test facilities
Wear 271 (2011) 2625–2631
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Comparison of liquid impingement results from whirling arm and water-jet rain erosion test facilities E.F. Tobin a,∗ , T.M. Young a , D. Raps b , O. Rohr b a b
University of Limerick, Department of Mechanical, Aeronautical and Biomedical Engineering, Limerick, Ireland EADS Innovation Works, Metallic Technologies and Surface Engineering, 81663 Munich, Germany
a r t i c l e
i n f o
Article history: Received 1 September 2010 Received in revised form 2 February 2011 Accepted 2 February 2011
Keywords: Whirling arm Rain erosion Droplet impact Liquid impingement
a b s t r a c t A laboratory-scale Whirling Arm Rain Erosion test Rig (WARER) has been designed, developed and commissioned at the University of Limerick. The facility is capable of impact speeds of 178 ms−1 and a rainfall rate of 25.4 mm h−1 . Circular samples of 27 mm diameter (nominal) and a maximum thickness of 2 mm can be accommodated. The facility was developed to test the resistance of leading edge aircraft materials to the repeated impact of rain droplets in flight. A challenging feature of the design was to enclose the whirling arm within a chamber so that it could be operated in an open-plan laboratory. A design problem that arose was the effect of aerodynamic heating of the air within the chamber. This led to a warming of the water and consequently a change in the droplet formation, which in turn caused problems relating to the repeatability of the tests. A cooling system was thus incorporated to keep the temperature of the water droplets below the established threshold of 20 ◦ C. Calibration testing has been undertaken by conducting back-on-back testing of samples with a previously developed water-jet facility at EADS IW, Munich. This Pulsating Jet Erosion Test rig (PJET) is capable of 225 ms−1 droplet speed. Multiple impacts are produced on the sample at numerous locations, each location having an increased number of impacts. The impact frequency is equivalent of 25 mm h−1 rain fall. Clad aluminium alloy (AA2024-T3) was used with the soft AA1230 clad layer being removed during testing. Post impact evaluations were carried out using Confocal Laser Scanning Microscopy (CLSM) and surface roughness measurements. A cumulative erosion-time curve was produced from the WARER samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction 1.1. Background Rain erosion is a problem that is encountered on a day-to-day basis by aircraft in normal operations. It is mainly in the initial and final stages of a flight sector for transport aircraft that the problem arises. Aircraft that travel at high speeds (e.g. military aircraft) tend to have more significant rain erosion problems. Damage from the repeated impact of water droplets is seen mainly on leading edge structures of fixed-wing aircraft (e.g. wing, radome, engine nacelle), as well as on leading edges of propellers and helicopter rotor blades. The materials selected for wing and empennage leading edges must be capable of withstanding bird strikes. Aluminium alloys have been widely used for this application, and these materials also display acceptable erosion resistance. The substantially tighter tolerances for surface smoothness required to maintain
∗ Corresponding author. Tel.: +353 861928127 (mob.); fax: +353 61202944. E-mail address: [email protected] (E.F. Tobin). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.02.023
laminar flow on aerodynamic surfaces, however, poses a new challenge for these materials. Certain aircraft components, such as radomes, are made from materials that allow the transmission and reception of radar signals. These materials, however, tend to have a much lower erosion resistance, and it is for this reason that a lot of the pioneering research was conducted for this application. With the move towards more environmentally friendly, lighter aircraft structures and the extensive use of composite materials, there is a clear need for new erosion resistant materials and coatings, and for new techniques for their evaluation. At the University of Limerick, a Whirling Arm Rain Erosion test Rig (WARER) has been developed to compare the erosion resistance of different materials and coatings. Independently, a Pulsating Jet Erosion Test (PJET) was developed at EADS IW, Munich, for the same reason but using a different test methodology. A comparison of the results from these facilities is presented herein. It is noted that comparisons of the results from different test methods are generally regarded as problematic. Large variances have been shown in previous studies [1]. This is mainly due to the variances in the basic parameters of interest, which are the test speed, droplet size and rainfall rate.
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Table 1 Rain erosion test facilities. Test method
Examples
Maximum test speed (ms−1 )
Whirling arm
Royal Aircraft Establishment whirling arm rig [4,5] Wright-Patterson AFB rotating arm apparatus, UDRI [6] SAAB-SCANIA whirling arm rig [7] Dornier rotating arm apparatus [7,8] AS&T rain erosion rig [9] Single-Impact Jet Apparatus (SIJA) [10] Multiple-Impact Jet Apparatus (MIJA) [11] Micro Droplet Erosion Test (MDET) [12] Holloman High Speed Test Track [3] GRCI hydrometeor impact facility [3]
∼270 290 335 700 250 600 600 350 2800 1000
Water jet
Ballistic/rocket test
1.2. Objectives The objectives of this paper are as follows: • • • •
describe and compare alternative rain erosion test methods; describe the development of the WARER test rig; validate the performance of the WARER test rig; and correlate results from the WARER and PJET facilities.
1.3. Types of test apparatus There are a number of methods that have been used to test a material’s erosion resistance [2,3]. The most common types are shown in Table 1. The test facilities each have their own advantages and drawbacks. Generally speaking, it is hard to correlate results from one test facility to another. 2. WARER test facility (University of Limerick) The WARER test facility is a whirling arm design, with a test radius of 600 mm, measured to the centre of the single test coupon (Fig. 1). Circular coupons, with a thickness of no more than 2 mm, and a nominal diameter of 27 mm (test face diameter is 25 mm) can be accommodated. Stainless steel removable coupon holders are used to reduce downtime when changing coupons. The arm, manufactured from AA6082 T-6 marine grade aluminium, has an elliptical leading edge cross section with a flat mid section and a truncated triangular trailing edge. The hammerhead arm tapers from 150 mm to 50 mm over a distance of 925 mm. The arm is driven by an electric motor via pulleys and a shaft, mounted below the chamber. The vertical shaft, which runs on three deep groove roller bearings, is mounted within a housing that
can be aligned to the vertical plane via a series of adjustable links. This complex assembly was required due to the asymmetry of the attachments points on the base of chamber. The assembly, comprising clevises, rod ends and threaded bars, permit the length of each of the four links to be changed, thus enabling the arm to rotate in a horizontal plane. The shaft speed is recorded via a tachometer and the motor speed is controlled by an AC inverter speed control. The WARER is designed to operate between 128 and 178 ms−1 . The maximum speed equates to 300 kts calibrated airspeed (CAS) at an altitude of 10,000 ft (in the International Standard Atmosphere). This was selected by considering typical flight operations. In the majority of controlled civil airspaces around the world, aircraft are flown at speeds not exceeding 250 kts CAS below 10,000 ft. Above 10,000 ft commercial aircraft initially climb (or descend) at a constant CAS speed (usually about 290–310 kts). This condition equates to a common flight speed at which rainfall is encountered. A maximum level given for rainfall in the Tropics is freezing level, 4500 m (14,760 ft) [13]. The WARER is designed to simulate a rainfall of 25.4 mm h−1 , which is a widely used norm for rain erosion testing. This rate, however, is rarely seen in the temperate climates but with in the Tropics, it is a lot more common. A rain drop size of 2 mm diameter is frequently used for testing as this droplet size produces the highest volume of water within rainfall, along with one of the highest incidence rates [13]. The WARER water droplet system has 36 Luertype needles (gauge 30, with an inner diameter of 0.15 mm) equally spaced above the path of the moving coupon. Shrouds, made from acrylic tubing, protect the droplets from the swirling air. The water is supplied by adjustable header tank via a 19 mm circular pipe fitted inside the chamber. It was observed during development that when the pressure within the water droplet system exceeded a critical or threshold pressure (depending on needle geometry), jets of fine droplets were produced, rather than the desired 2 mm diameter droplets (thus rendering the affected tests worthless). It was also observed that this transition from droplets to fine water jets depends critically on the water temperature. The electric motor uses approximately 6.5 kW keeping the arm at the maximum test speed. Some energy is used to overcome friction in the shaft assembly, but the majority is used to overcome the drag on the arm. The stainless steel drum, with its safety lid, tends to retain most of the heat inside the chamber. A cooling system was thus incorporated to control the air temperature. The system consists of a 14 kW water chiller unit and four copper coils, with a total length of 50 m (Fig. 1). The cooling coils receive water at −1 ◦ C, which creates a sufficient heat transfer coefficient to keep the air in the chamber at a temperature of 18 ◦ C.
3. PJET test facility (EADS IW, Munich)
Fig. 1. Main components of WARER.
A Pulsating Jet Erosion Test rig (PJET), shown in Fig. 2, has been developed at EADS IW [14]. The principle of the PJET is based on a test method developed at Cavendish Laboratory, University of
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Table 2 WARER test conditions. Duration (min)
Velocity (ms−1 )
Rainfall rate (mm h−1 )
5 10 20 30 45 60 120
177 177 177 177 177 177 177
25.4 25.4 25.4 25.4 25.4 25.4 25.4
Table 3 Test parameters for PJET tests. Velocity 180 m/s Drop size 2 mm Frequency of the disc 20 Hz 0.8 mm Nozzle diameter 60 mm Sample-nozzle distance Test program 20, 50, 100, 250, 500, 1000, 2000, 3000, 6000 and 10000 impacts 5 repetitions per impact number at different sites
Fig. 2. (a) Principle of PJET and (b) picture of PJET.
Cambridge. The basic concept is that water jets can cause similar damage as water droplets if the front of the water jet, which impinges the surface, has a similar ball-shaped geometry compared to water droplets [15,16]. The PJET uses a high pressure pump feeding water through a nozzle, which generates a focused water jet. The continuous water jet is then separated into shorter jets by a rotating disc with two orifices (diameter 10 mm) on opposite sides of the disc. The disc rotates with a frequency of 20 Hz, resulting in an impact frequency of the water jets on the sample of 40 Hz. This impact frequency corresponds to the impingement frequency of rain drops which are striking the surface of an aircraft flying through heavy rain of 25 mm h−1 at a flight speed of 225 ms−1 . The velocity of the water jets can be controlled by adjusting the water pressure. A water jet velocity of 180 ms−1 was chosen for the tests of the present work. The nozzle diameter of 0.8 mm leads to a front diameter of the water jets of approximately 2 mm. The distance between the nozzle and sample is 60 mm. To prevent the formation of a residual water film on the sample, which protects the surface of the sample, an air nozzle is installed above the sample holder, which blows off the water film with pressurised air of 3 bar. A 40 m long hose with an internal diameter of 13 mm between the pump and the nozzle compensates the pressure variations, which can occur with a three cylinder high pressure pump.
The evaluation of the results from both test methods is done by surface roughness measurement and through examination of optical microscope and SEM images of the different samples. ASTM G73-10 [1] could not be followed as mass loss measurements could not be made for the PJET samples. 4.2. Test procedure for WARER The samples were punched, the initial mass recorded and inspected for scratches and imperfections. The coupon was accelerated to 177 ms−1 over a 30 s period. The cooling system was turned on and the temperature in the chamber checked. If the threshold temperature had not been exceeded, the water system was opened. The time of this action was recorded as the beginning of the test. The samples were tested for the prescribed time with each condition repeated five times. A 6 min period was used to indicate the rainfall rate over the whole test. For the shorter duration tests the rainfall rate was measured prior to testing. After each test, the coupons were dried with compressed air and the mass recorded. The surface roughness was then measured for each sample at three locations across the centre of the sample (Fig. 4). Metallurgical cross sections were also cut.
4. Experiments 4.1. Overview of tests conducted Comparative tests (see Tables 2 and 3) were conducted using the WARER and PJET test facilities. A clad aircraft grade aluminium alloy (AA2024-T3), with a thickness of 1.6 mm, was selected for the coupons. The clad material is AA1230 (approximately 40 m thick), which is rather soft and can be easily removed or scratched. This property of the material allowed for maximum results from short test durations. The material is also commonly found as aircraft skin material, making it a valid candidate material for this test program. Fig. 3 shows a cross section of the material before testing.
Fig. 3. Cross sectional view of 2024-T3 clad material. The approximate thickness is also shown.
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Mass loss was recorded after each test from the WARER. Changes in mass were not recorded for the PJET test due to the test being completed in a series (see Fig. 5). Furthermore, the mass change is very small due to the small area of the impact sites.
5. Results
Fig. 4. Example of a typical erosion pattern on a WARER test sample. The dashed white lines indicate approximate areas where roughness measurements were taken. The solid white line shows cross sectional cut.
4.3. Test procedure for PJET The test sample was mounted on a computer controlled x–ystage, which allows a programmable movement of the sample in a grid pattern, where the number of impacts of water jets can be set individually for each row. The test pattern is shown in Fig. 5. Each sample was tested with ten different impact series, corresponding to 20, 50, 100, 250, 500, 1000, 2000, 3000, 6000 and 10,000 impacts. Each impact series or impact number was repeated five times on different sites with a distance of 5 mm. The test parameters are summarized in Table 3. 4.4. Post impact evaluation of test coupons Roughness tests were carried out on the samples using two measurement techniques. A Hommel T500 surface tester was used on samples from the WARER. An Olympus Lext OLS3100 Confocal Laser Scanning microscope (CLSM) was used to measure the roughness of the PJET samples. The relatively small area of the jet impact site is below the ISO 4288 specifications for roughness measurement using a stylus profilometer. The WARER samples were also assessed using this method for direct comparison. The examined surface area was 192 m by 256 m.
Roughness measurements were used to correlate the results. The surface roughness measurements from the PJET and WARER are compared in Fig. 6. Values of arithmetic mean roughness (SRa ) and maximum height roughness (SRz ) were used to normalise the two sets of results. A correlation between SRa is shown in Fig. 6(a). The SRz results also show a trend when the same correlation factor is used (Fig. 6(b)). The ratio or correlation factor is approximately 1000 impacts (PJET) to 15 min test duration (WARER). Fig. 7 shows the surface of samples from both tests rigs. Similar roughness values were recorded from these images. Fig. 8 is a comparison of the 3D representation of the surface texture produced by the CLSM. It can be seen that the height between the highest and lowest peaks is of the same magnitude. The surface has degraded to the same extent in both these cases. Fig. 9 is a comparison of the surface under optical microscope. The impact site of the PJET can be seen to be approximately 1 mm in diameter. This is due to the impacts being concentrated on a single point. The area over which the droplet impacts occur in the WARER is almost 500 times larger and this explains the mass loss measured after each test. Cross sectional views (Fig. 10) of the damage from the PJET and WARER show that the surface features are of the same magnitude. The clad layer is indicated in these images by slight changes in the surfaces produced in each case. Lower magnification images (Fig. 11) of the PJET samples shows the clad layer clearly and the depth of damage. SEM images of the progressive degradation of the clad surface (Fig. 12) show the surface becomes highly irregular before the physical removal of material begins to occur. The SEM images do not correlate directly and this may be due to the difference in the dynamics of the impact of the droplet and the water-jet. The results of the erosion of the material from the WARER (Fig. 13) are presented in terms of mass of material removed. These results clearly indicate the incubation period, where the sample is under going damage without mass loss. After 30 min at the test conditions mass loss occurs. The largest pitting of the surface occurs between 45 and 60 min. The surface at 120 min has more numerous pits but the overall roughness is lower as the clad layer begins to be eroded down to the AA2024 substrate.
6. Discussion
Fig. 5. PJET test pattern. Cross sections were taken along the white line.
The correlation between the tests conducted at the two test facilities can be seen in Fig. 6. For SRa values of below 2 m, the correlation between the results is very clear. The ratio or correlation factor is approximately 1000 impacts (PJET) to 15 min test duration (WARER). The trend is seen to diverge beyond 2.5 m SRa , however, which is possibly due to the morphology of the surface increasing to the magnitude over which the actual roughness is being calculated. This divergence can also be seen in the SRz results. The standard deviations of the results are seen to increase with increasing time/impacts. As the incubation period is of most relevance with this method of correlation, the divergence of the results needs to be related to the cumulative mass loss curve in Fig. 13. The mass of the samples begins to change after 30 min testing. From this, the results beyond 30 min/2000 impacts are not reliable. This may also explain the divergence of the roughness values.
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Fig. 6. Comparison of roughness values taken from both sets of results using the CLSM: (a) SRa , (b) SRz .
Fig. 7. CLSM images at 500× magnification of surface used for roughness measurement (image size 256 m × 192 m): (a) 3000 impacts (PJET), (b) 45 min (WARER).
Fig. 8. 3D CLSM images of surface morphology used to calculate SRa (image size 256 m × 192 m): (a) 3000 impacts (PJET), (b) 45 min (WARER).
Fig. 9. 200× optical microscope images showing comparison of surface damage: (a) 3000 impacts (PJET), (b) 45 min (WARER).
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Fig. 10. Comparison of cross sectional images of samples (100×). PJET sample is taken with an optical microscope while the WARER sample is taken using SEM. Both have been set in resin: (a) 3000 impacts (PJET), (b) 45 min (WARER).
Fig. 11. 3000 and 6000 impacts on the PJET at 50× taken with an optical microscope. The depth of the pits can be clearly seen to not penetrate to the 2024 substrate: (a) 3000 impacts (PJET), (b) 6000 impacts (PJET).
Fig. 12. Comparison of SEM images of samples (300×): (a) 3000 impacts (PJET), (b) 45 min (WARER).
The volume of material removed can be approximated from the mass loss values as the cross sections show no penetration into the AA2024 layer (Fig. 11). This gives a volume removed of 9.5 mm3 after two hours of testing in the WARER which is approximately half the overall volume of clad (19.6 mm3 ). The full erosion time curve was not produced as it was irrelevant for the comparison of the samples. However, taking the slope of the line between 60 and 120 min as the maximum erosion rate [1], an incubation period of 51 min is seen for this material under this set of test conditions. CLSM imaging provided a method of correlating the results from both test facilities. The roughness measurements taken could not have been taken with contact-type stylus profilometer due to the evaluation length being much larger than the diameter of the impact site from the PJET. The results from the profilometry done at the University of Limerick followed the same trend as those of the CLSM but diverged at the higher roughness values. The stylus pro-
filometer also left a scratch mark on the soft clad which invalidated the results. Cross sectional images from both tests correlated with the CLSM results. The depth of damage created in both cases was of a similar magnitude. The cross sectional images of the PJET samples showed that the clad layer was still intact at 3000 impacts (PJET) or 45 min (WARER). The progression of the erosion from 3000 to 6000 impacts (Fig. 11) is shown clearly along side the undamaged clad layer. The SEM surface images, however, did not correlate the results of the CLSM. The samples can be seen to progressively degrade with large pits being formed over time. Although the roughness values for samples in question are similar, the physical features produced on the surface are different. The WARER sample has sharp peaks while the PJET sample has more rounded peaks. This may have been due to the difference in the impact dynamics of the droplet and the water-jet. Images beyond 60 min, however, showed an increase in the number of pits. This indicates that the clad material was being
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established to ensure that water droplets of the desired diameter are produced and not fine jets of water. Maintaining the water temperature below this value has been achieved using a 14 kW water chiller. The surface roughness of the sample has shown an obvious relationship between both test rigs. The use of both SRa and SRz values improve the correlation of results. Further testing is required using other materials in bulk form and with varying hardness values. This will provide a better understanding of the relationship between the two test methods. References
Fig. 13. Cumulative mass loss results from the WARER for AA2024-T3 clad tested at 177 ms−1 .
removed as the peaks became higher and were fractured by lateral jetting of the droplets. 7. Future work As part of future work to complement the results of these tests, bulk aluminium alloys will also be tested. AA1100-O alloy will be used as this material is one of the standard correlation materials used within ASTM G73-10. This material will allow extended testing without the issue of eroding through to the substrate of the layered AA2024 clad material. To confirm that the correlation factor remains constant at higher hardness values, AA2024 unclad will also be tested. 8. Conclusions A method of correlating results from two very different rain erosion test rig designs has been found. The correlation shows that the PJET can provide valuable data on the relative resistance of samples from comparison of results. The WARER has proven to be a valuable piece of equipment. The erosion curves produced from the results show that the rig is operating correctly. A threshold temperature of 20 ◦ C has been
[1] Standard Test Method for Liquid Impingement Erosion Using Rotating Apparatus, ASTM G73-10, April, 2010. [2] J.E. Field, ELSI conference: invited lecture: Liquid impact: theory, experiment, applications, Wear 233–235 (1999) 1–12. [3] W.F. Alder, Rain impact retrospective and vision for the future, Wear 233–235 (1999) 25–38. [4] T.J. Methven, B. Fairhead, A Correlation between Rain Erosion of Perspex Specimens in Flight and on a Ground Rig, ARC-TR-21-090, 1960. [5] G.F. Schmitt, C.J. Hurley, Development and calibration of a Mach 1.2 rain erosion test apparatus, AFML-TR-70-240, Air Force Materials Laboratory, WrightPatterson Air Force Base, Ohio, 1970. [6] C. Westmark, G.W. Lawless, A discussion of rain erosion testing at the United States Air Force rain erosion test facility, Wear 186–187 (1995) 384–387. [7] A.A. Déom, A. Luc, S. Amara, D.L. Balageas, Towards more realistic erosion simulation tests for high velocity EM and IR windows, Wear 233–235 (1999) 13–24. [8] H. Busch, G. Hoff, G. Langbein, et al., A discussion on deformation of solids by the impact of liquids, and its relation to rain damage in aircraft and missiles, to blade erosion in steam turbines, and to cavitation erosion, Philosophical Transactions of the Royal Society, Series A, Mathematical and Physical Sciences 260 (1110) (1966) 168–181. [9] T.M. Young, B. Humphreys, J.P. Fielding, Investigation of hybrid laminar flow control (HLFC) surfaces, Aircraft Design 4 (2001) 127–146. [10] T. Obara, N.K. Bourne, J.E. Field, Liquid-jet impact on liquid and solid surfaces, Wear 186–187 (1995) 388–394. [11] C.R. Seward, E.J. Coad, C.S.J. Pickles, J.E. Field, The liquid impact resistance of a range of IR-transparent materials, Wear 186–187 (1995) 375–383. [12] M. Grundwuermer, O. Nuyken, M. Meyer, J. Wehr, N. Schupp, Sol–gel derived erosion protection coatings against damage caused by liquid impact, Wear 263 (2007) 318–329. [13] Environmental Handbook for Defence Material, Part 4, Natural Environment, DEF STAN 00-35, Ch 6-01, November, 2006. [14] M. Grundwuermer, PhD thesis, VDI Verlag, No. 737, 2008. [15] F.P. Bowden, J.H. Brunton, Nature 181 (1958) 873–875. [16] F.P. Bowden, J.H. Brunton, Proceedings of the Royal Society of London: Series A 263 (1961) 433–450.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Comparison of liquid impingement results from whirling arm and water-jet rain erosion test facilities E.F. Tobin a,∗ , T.M. Young a , D. Raps b , O. Rohr b a b
University of Limerick, Department of Mechanical, Aeronautical and Biomedical Engineering, Limerick, Ireland EADS Innovation Works, Metallic Technologies and Surface Engineering, 81663 Munich, Germany
a r t i c l e
i n f o
Article history: Received 1 September 2010 Received in revised form 2 February 2011 Accepted 2 February 2011
Keywords: Whirling arm Rain erosion Droplet impact Liquid impingement
a b s t r a c t A laboratory-scale Whirling Arm Rain Erosion test Rig (WARER) has been designed, developed and commissioned at the University of Limerick. The facility is capable of impact speeds of 178 ms−1 and a rainfall rate of 25.4 mm h−1 . Circular samples of 27 mm diameter (nominal) and a maximum thickness of 2 mm can be accommodated. The facility was developed to test the resistance of leading edge aircraft materials to the repeated impact of rain droplets in flight. A challenging feature of the design was to enclose the whirling arm within a chamber so that it could be operated in an open-plan laboratory. A design problem that arose was the effect of aerodynamic heating of the air within the chamber. This led to a warming of the water and consequently a change in the droplet formation, which in turn caused problems relating to the repeatability of the tests. A cooling system was thus incorporated to keep the temperature of the water droplets below the established threshold of 20 ◦ C. Calibration testing has been undertaken by conducting back-on-back testing of samples with a previously developed water-jet facility at EADS IW, Munich. This Pulsating Jet Erosion Test rig (PJET) is capable of 225 ms−1 droplet speed. Multiple impacts are produced on the sample at numerous locations, each location having an increased number of impacts. The impact frequency is equivalent of 25 mm h−1 rain fall. Clad aluminium alloy (AA2024-T3) was used with the soft AA1230 clad layer being removed during testing. Post impact evaluations were carried out using Confocal Laser Scanning Microscopy (CLSM) and surface roughness measurements. A cumulative erosion-time curve was produced from the WARER samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction 1.1. Background Rain erosion is a problem that is encountered on a day-to-day basis by aircraft in normal operations. It is mainly in the initial and final stages of a flight sector for transport aircraft that the problem arises. Aircraft that travel at high speeds (e.g. military aircraft) tend to have more significant rain erosion problems. Damage from the repeated impact of water droplets is seen mainly on leading edge structures of fixed-wing aircraft (e.g. wing, radome, engine nacelle), as well as on leading edges of propellers and helicopter rotor blades. The materials selected for wing and empennage leading edges must be capable of withstanding bird strikes. Aluminium alloys have been widely used for this application, and these materials also display acceptable erosion resistance. The substantially tighter tolerances for surface smoothness required to maintain
∗ Corresponding author. Tel.: +353 861928127 (mob.); fax: +353 61202944. E-mail address: [email protected] (E.F. Tobin). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.02.023
laminar flow on aerodynamic surfaces, however, poses a new challenge for these materials. Certain aircraft components, such as radomes, are made from materials that allow the transmission and reception of radar signals. These materials, however, tend to have a much lower erosion resistance, and it is for this reason that a lot of the pioneering research was conducted for this application. With the move towards more environmentally friendly, lighter aircraft structures and the extensive use of composite materials, there is a clear need for new erosion resistant materials and coatings, and for new techniques for their evaluation. At the University of Limerick, a Whirling Arm Rain Erosion test Rig (WARER) has been developed to compare the erosion resistance of different materials and coatings. Independently, a Pulsating Jet Erosion Test (PJET) was developed at EADS IW, Munich, for the same reason but using a different test methodology. A comparison of the results from these facilities is presented herein. It is noted that comparisons of the results from different test methods are generally regarded as problematic. Large variances have been shown in previous studies [1]. This is mainly due to the variances in the basic parameters of interest, which are the test speed, droplet size and rainfall rate.
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Table 1 Rain erosion test facilities. Test method
Examples
Maximum test speed (ms−1 )
Whirling arm
Royal Aircraft Establishment whirling arm rig [4,5] Wright-Patterson AFB rotating arm apparatus, UDRI [6] SAAB-SCANIA whirling arm rig [7] Dornier rotating arm apparatus [7,8] AS&T rain erosion rig [9] Single-Impact Jet Apparatus (SIJA) [10] Multiple-Impact Jet Apparatus (MIJA) [11] Micro Droplet Erosion Test (MDET) [12] Holloman High Speed Test Track [3] GRCI hydrometeor impact facility [3]
∼270 290 335 700 250 600 600 350 2800 1000
Water jet
Ballistic/rocket test
1.2. Objectives The objectives of this paper are as follows: • • • •
describe and compare alternative rain erosion test methods; describe the development of the WARER test rig; validate the performance of the WARER test rig; and correlate results from the WARER and PJET facilities.
1.3. Types of test apparatus There are a number of methods that have been used to test a material’s erosion resistance [2,3]. The most common types are shown in Table 1. The test facilities each have their own advantages and drawbacks. Generally speaking, it is hard to correlate results from one test facility to another. 2. WARER test facility (University of Limerick) The WARER test facility is a whirling arm design, with a test radius of 600 mm, measured to the centre of the single test coupon (Fig. 1). Circular coupons, with a thickness of no more than 2 mm, and a nominal diameter of 27 mm (test face diameter is 25 mm) can be accommodated. Stainless steel removable coupon holders are used to reduce downtime when changing coupons. The arm, manufactured from AA6082 T-6 marine grade aluminium, has an elliptical leading edge cross section with a flat mid section and a truncated triangular trailing edge. The hammerhead arm tapers from 150 mm to 50 mm over a distance of 925 mm. The arm is driven by an electric motor via pulleys and a shaft, mounted below the chamber. The vertical shaft, which runs on three deep groove roller bearings, is mounted within a housing that
can be aligned to the vertical plane via a series of adjustable links. This complex assembly was required due to the asymmetry of the attachments points on the base of chamber. The assembly, comprising clevises, rod ends and threaded bars, permit the length of each of the four links to be changed, thus enabling the arm to rotate in a horizontal plane. The shaft speed is recorded via a tachometer and the motor speed is controlled by an AC inverter speed control. The WARER is designed to operate between 128 and 178 ms−1 . The maximum speed equates to 300 kts calibrated airspeed (CAS) at an altitude of 10,000 ft (in the International Standard Atmosphere). This was selected by considering typical flight operations. In the majority of controlled civil airspaces around the world, aircraft are flown at speeds not exceeding 250 kts CAS below 10,000 ft. Above 10,000 ft commercial aircraft initially climb (or descend) at a constant CAS speed (usually about 290–310 kts). This condition equates to a common flight speed at which rainfall is encountered. A maximum level given for rainfall in the Tropics is freezing level, 4500 m (14,760 ft) [13]. The WARER is designed to simulate a rainfall of 25.4 mm h−1 , which is a widely used norm for rain erosion testing. This rate, however, is rarely seen in the temperate climates but with in the Tropics, it is a lot more common. A rain drop size of 2 mm diameter is frequently used for testing as this droplet size produces the highest volume of water within rainfall, along with one of the highest incidence rates [13]. The WARER water droplet system has 36 Luertype needles (gauge 30, with an inner diameter of 0.15 mm) equally spaced above the path of the moving coupon. Shrouds, made from acrylic tubing, protect the droplets from the swirling air. The water is supplied by adjustable header tank via a 19 mm circular pipe fitted inside the chamber. It was observed during development that when the pressure within the water droplet system exceeded a critical or threshold pressure (depending on needle geometry), jets of fine droplets were produced, rather than the desired 2 mm diameter droplets (thus rendering the affected tests worthless). It was also observed that this transition from droplets to fine water jets depends critically on the water temperature. The electric motor uses approximately 6.5 kW keeping the arm at the maximum test speed. Some energy is used to overcome friction in the shaft assembly, but the majority is used to overcome the drag on the arm. The stainless steel drum, with its safety lid, tends to retain most of the heat inside the chamber. A cooling system was thus incorporated to control the air temperature. The system consists of a 14 kW water chiller unit and four copper coils, with a total length of 50 m (Fig. 1). The cooling coils receive water at −1 ◦ C, which creates a sufficient heat transfer coefficient to keep the air in the chamber at a temperature of 18 ◦ C.
3. PJET test facility (EADS IW, Munich)
Fig. 1. Main components of WARER.
A Pulsating Jet Erosion Test rig (PJET), shown in Fig. 2, has been developed at EADS IW [14]. The principle of the PJET is based on a test method developed at Cavendish Laboratory, University of
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Table 2 WARER test conditions. Duration (min)
Velocity (ms−1 )
Rainfall rate (mm h−1 )
5 10 20 30 45 60 120
177 177 177 177 177 177 177
25.4 25.4 25.4 25.4 25.4 25.4 25.4
Table 3 Test parameters for PJET tests. Velocity 180 m/s Drop size 2 mm Frequency of the disc 20 Hz 0.8 mm Nozzle diameter 60 mm Sample-nozzle distance Test program 20, 50, 100, 250, 500, 1000, 2000, 3000, 6000 and 10000 impacts 5 repetitions per impact number at different sites
Fig. 2. (a) Principle of PJET and (b) picture of PJET.
Cambridge. The basic concept is that water jets can cause similar damage as water droplets if the front of the water jet, which impinges the surface, has a similar ball-shaped geometry compared to water droplets [15,16]. The PJET uses a high pressure pump feeding water through a nozzle, which generates a focused water jet. The continuous water jet is then separated into shorter jets by a rotating disc with two orifices (diameter 10 mm) on opposite sides of the disc. The disc rotates with a frequency of 20 Hz, resulting in an impact frequency of the water jets on the sample of 40 Hz. This impact frequency corresponds to the impingement frequency of rain drops which are striking the surface of an aircraft flying through heavy rain of 25 mm h−1 at a flight speed of 225 ms−1 . The velocity of the water jets can be controlled by adjusting the water pressure. A water jet velocity of 180 ms−1 was chosen for the tests of the present work. The nozzle diameter of 0.8 mm leads to a front diameter of the water jets of approximately 2 mm. The distance between the nozzle and sample is 60 mm. To prevent the formation of a residual water film on the sample, which protects the surface of the sample, an air nozzle is installed above the sample holder, which blows off the water film with pressurised air of 3 bar. A 40 m long hose with an internal diameter of 13 mm between the pump and the nozzle compensates the pressure variations, which can occur with a three cylinder high pressure pump.
The evaluation of the results from both test methods is done by surface roughness measurement and through examination of optical microscope and SEM images of the different samples. ASTM G73-10 [1] could not be followed as mass loss measurements could not be made for the PJET samples. 4.2. Test procedure for WARER The samples were punched, the initial mass recorded and inspected for scratches and imperfections. The coupon was accelerated to 177 ms−1 over a 30 s period. The cooling system was turned on and the temperature in the chamber checked. If the threshold temperature had not been exceeded, the water system was opened. The time of this action was recorded as the beginning of the test. The samples were tested for the prescribed time with each condition repeated five times. A 6 min period was used to indicate the rainfall rate over the whole test. For the shorter duration tests the rainfall rate was measured prior to testing. After each test, the coupons were dried with compressed air and the mass recorded. The surface roughness was then measured for each sample at three locations across the centre of the sample (Fig. 4). Metallurgical cross sections were also cut.
4. Experiments 4.1. Overview of tests conducted Comparative tests (see Tables 2 and 3) were conducted using the WARER and PJET test facilities. A clad aircraft grade aluminium alloy (AA2024-T3), with a thickness of 1.6 mm, was selected for the coupons. The clad material is AA1230 (approximately 40 m thick), which is rather soft and can be easily removed or scratched. This property of the material allowed for maximum results from short test durations. The material is also commonly found as aircraft skin material, making it a valid candidate material for this test program. Fig. 3 shows a cross section of the material before testing.
Fig. 3. Cross sectional view of 2024-T3 clad material. The approximate thickness is also shown.
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Mass loss was recorded after each test from the WARER. Changes in mass were not recorded for the PJET test due to the test being completed in a series (see Fig. 5). Furthermore, the mass change is very small due to the small area of the impact sites.
5. Results
Fig. 4. Example of a typical erosion pattern on a WARER test sample. The dashed white lines indicate approximate areas where roughness measurements were taken. The solid white line shows cross sectional cut.
4.3. Test procedure for PJET The test sample was mounted on a computer controlled x–ystage, which allows a programmable movement of the sample in a grid pattern, where the number of impacts of water jets can be set individually for each row. The test pattern is shown in Fig. 5. Each sample was tested with ten different impact series, corresponding to 20, 50, 100, 250, 500, 1000, 2000, 3000, 6000 and 10,000 impacts. Each impact series or impact number was repeated five times on different sites with a distance of 5 mm. The test parameters are summarized in Table 3. 4.4. Post impact evaluation of test coupons Roughness tests were carried out on the samples using two measurement techniques. A Hommel T500 surface tester was used on samples from the WARER. An Olympus Lext OLS3100 Confocal Laser Scanning microscope (CLSM) was used to measure the roughness of the PJET samples. The relatively small area of the jet impact site is below the ISO 4288 specifications for roughness measurement using a stylus profilometer. The WARER samples were also assessed using this method for direct comparison. The examined surface area was 192 m by 256 m.
Roughness measurements were used to correlate the results. The surface roughness measurements from the PJET and WARER are compared in Fig. 6. Values of arithmetic mean roughness (SRa ) and maximum height roughness (SRz ) were used to normalise the two sets of results. A correlation between SRa is shown in Fig. 6(a). The SRz results also show a trend when the same correlation factor is used (Fig. 6(b)). The ratio or correlation factor is approximately 1000 impacts (PJET) to 15 min test duration (WARER). Fig. 7 shows the surface of samples from both tests rigs. Similar roughness values were recorded from these images. Fig. 8 is a comparison of the 3D representation of the surface texture produced by the CLSM. It can be seen that the height between the highest and lowest peaks is of the same magnitude. The surface has degraded to the same extent in both these cases. Fig. 9 is a comparison of the surface under optical microscope. The impact site of the PJET can be seen to be approximately 1 mm in diameter. This is due to the impacts being concentrated on a single point. The area over which the droplet impacts occur in the WARER is almost 500 times larger and this explains the mass loss measured after each test. Cross sectional views (Fig. 10) of the damage from the PJET and WARER show that the surface features are of the same magnitude. The clad layer is indicated in these images by slight changes in the surfaces produced in each case. Lower magnification images (Fig. 11) of the PJET samples shows the clad layer clearly and the depth of damage. SEM images of the progressive degradation of the clad surface (Fig. 12) show the surface becomes highly irregular before the physical removal of material begins to occur. The SEM images do not correlate directly and this may be due to the difference in the dynamics of the impact of the droplet and the water-jet. The results of the erosion of the material from the WARER (Fig. 13) are presented in terms of mass of material removed. These results clearly indicate the incubation period, where the sample is under going damage without mass loss. After 30 min at the test conditions mass loss occurs. The largest pitting of the surface occurs between 45 and 60 min. The surface at 120 min has more numerous pits but the overall roughness is lower as the clad layer begins to be eroded down to the AA2024 substrate.
6. Discussion
Fig. 5. PJET test pattern. Cross sections were taken along the white line.
The correlation between the tests conducted at the two test facilities can be seen in Fig. 6. For SRa values of below 2 m, the correlation between the results is very clear. The ratio or correlation factor is approximately 1000 impacts (PJET) to 15 min test duration (WARER). The trend is seen to diverge beyond 2.5 m SRa , however, which is possibly due to the morphology of the surface increasing to the magnitude over which the actual roughness is being calculated. This divergence can also be seen in the SRz results. The standard deviations of the results are seen to increase with increasing time/impacts. As the incubation period is of most relevance with this method of correlation, the divergence of the results needs to be related to the cumulative mass loss curve in Fig. 13. The mass of the samples begins to change after 30 min testing. From this, the results beyond 30 min/2000 impacts are not reliable. This may also explain the divergence of the roughness values.
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Fig. 6. Comparison of roughness values taken from both sets of results using the CLSM: (a) SRa , (b) SRz .
Fig. 7. CLSM images at 500× magnification of surface used for roughness measurement (image size 256 m × 192 m): (a) 3000 impacts (PJET), (b) 45 min (WARER).
Fig. 8. 3D CLSM images of surface morphology used to calculate SRa (image size 256 m × 192 m): (a) 3000 impacts (PJET), (b) 45 min (WARER).
Fig. 9. 200× optical microscope images showing comparison of surface damage: (a) 3000 impacts (PJET), (b) 45 min (WARER).
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Fig. 10. Comparison of cross sectional images of samples (100×). PJET sample is taken with an optical microscope while the WARER sample is taken using SEM. Both have been set in resin: (a) 3000 impacts (PJET), (b) 45 min (WARER).
Fig. 11. 3000 and 6000 impacts on the PJET at 50× taken with an optical microscope. The depth of the pits can be clearly seen to not penetrate to the 2024 substrate: (a) 3000 impacts (PJET), (b) 6000 impacts (PJET).
Fig. 12. Comparison of SEM images of samples (300×): (a) 3000 impacts (PJET), (b) 45 min (WARER).
The volume of material removed can be approximated from the mass loss values as the cross sections show no penetration into the AA2024 layer (Fig. 11). This gives a volume removed of 9.5 mm3 after two hours of testing in the WARER which is approximately half the overall volume of clad (19.6 mm3 ). The full erosion time curve was not produced as it was irrelevant for the comparison of the samples. However, taking the slope of the line between 60 and 120 min as the maximum erosion rate [1], an incubation period of 51 min is seen for this material under this set of test conditions. CLSM imaging provided a method of correlating the results from both test facilities. The roughness measurements taken could not have been taken with contact-type stylus profilometer due to the evaluation length being much larger than the diameter of the impact site from the PJET. The results from the profilometry done at the University of Limerick followed the same trend as those of the CLSM but diverged at the higher roughness values. The stylus pro-
filometer also left a scratch mark on the soft clad which invalidated the results. Cross sectional images from both tests correlated with the CLSM results. The depth of damage created in both cases was of a similar magnitude. The cross sectional images of the PJET samples showed that the clad layer was still intact at 3000 impacts (PJET) or 45 min (WARER). The progression of the erosion from 3000 to 6000 impacts (Fig. 11) is shown clearly along side the undamaged clad layer. The SEM surface images, however, did not correlate the results of the CLSM. The samples can be seen to progressively degrade with large pits being formed over time. Although the roughness values for samples in question are similar, the physical features produced on the surface are different. The WARER sample has sharp peaks while the PJET sample has more rounded peaks. This may have been due to the difference in the impact dynamics of the droplet and the water-jet. Images beyond 60 min, however, showed an increase in the number of pits. This indicates that the clad material was being
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established to ensure that water droplets of the desired diameter are produced and not fine jets of water. Maintaining the water temperature below this value has been achieved using a 14 kW water chiller. The surface roughness of the sample has shown an obvious relationship between both test rigs. The use of both SRa and SRz values improve the correlation of results. Further testing is required using other materials in bulk form and with varying hardness values. This will provide a better understanding of the relationship between the two test methods. References
Fig. 13. Cumulative mass loss results from the WARER for AA2024-T3 clad tested at 177 ms−1 .
removed as the peaks became higher and were fractured by lateral jetting of the droplets. 7. Future work As part of future work to complement the results of these tests, bulk aluminium alloys will also be tested. AA1100-O alloy will be used as this material is one of the standard correlation materials used within ASTM G73-10. This material will allow extended testing without the issue of eroding through to the substrate of the layered AA2024 clad material. To confirm that the correlation factor remains constant at higher hardness values, AA2024 unclad will also be tested. 8. Conclusions A method of correlating results from two very different rain erosion test rig designs has been found. The correlation shows that the PJET can provide valuable data on the relative resistance of samples from comparison of results. The WARER has proven to be a valuable piece of equipment. The erosion curves produced from the results show that the rig is operating correctly. A threshold temperature of 20 ◦ C has been
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