Testing of rotor blade erosion protection systems

Transcription

1 RECOMMENDED PRACTICE DNVGL-RP-0171 Edition February 2018 Testing of rotor blade erosion protection systems The electronic pdf version of this document, available free of charge from is the officially binding version.

2 FOREWORD DNV GL recommended practices contain sound engineering practice and guidance. February 2018 Any comments may be sent by to This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of this document. The use of this document by others than DNV GL is at the user's sole risk. DNV GL does not accept any liability or responsibility for loss or damages resulting from any use of this document.

3 CHANGES CURRENT This is a new document. Changes - current Recommended practice DNVGL-RP Edition February 2018 Page 3

4 CONTENTS Changes current... 3 Section 1 General Introduction Objective Scope Application References Definitions and abbreviations... 9 Contents Section 2 Test procedure Test procedure Section 3 Rotating arm test rig Outline Rotating carrier arm Number of carrier arms Radial position of specimen Distance from origin of droplet to centre of specimen in rotor plane Angle of incidence Distance of test specimen to side wall Section 4 Specimens Geometry Material Specimen preparation Accelerated ageing Tapes as erosion protection...20 Section 5 Test parameters Test condition parameters Derived test parameters Section 6 Calibration General Calibration intervals Calibration specimens Evaluation of calibration results Recommended practice DNVGL-RP Edition February 2018 Page 4

5 Section 7 Inspection parameters Overview of inspection parameters Inspection interval and time Cleaning method Inspection method...25 Contents Section 8 Result parameters Overview of result parameters Mass loss Failure modes Stages of erosion progress End of incubation period Breakthrough Section 9 Displaying results Displaying results...30 Section 10 Test report Test report...32 Section 11 Summary Summary...36 Appendix A Specimen geometry A.1 Specimen geometry Appendix B Derived test parameters B.1 Droplet velocity B.2 Rain intensity B.3 Droplet impact velocity...39 B.4 Specific impact frequency...39 Appendix C Results from round robin tests C.1 Parameter overview...43 C.2 Reference curve for calibration specimens C.3 Test results on coating systems Appendix D Influences to be considered...61 D.1 Overview D.2 Shadowing effect...61 Recommended practice DNVGL-RP Edition February 2018 Page 5

6 Changes historic...63 Contents Recommended practice DNVGL-RP Edition February 2018 Page 6

7 SECTION 1 GENERAL 1.1 Introduction This recommended practice (RP) provides technical recommendations to support the execution of rain erosion tests on rotating arm test rigs. The intention of the RP is to reach a position where results from different rotating arm test rigs are comparable. 1.2 Objective The objective of this recommended practice is to: specify a detailed test procedure for rain erosion tests (RET) performed with a rotating arm test rig to ensure comparable results when using different test rigs specify the main influencing parameters for the assessment of the rain erosion. These include: mechanical properties of the tested system (protection system + laminate) substrate preparation method of application for leading edge protection system curing conditions of the coating testing temperature accumulated number of droplet impacts to reach a pre-defined erosion stage test rig parameters, e.g.: droplet size droplet distribution impact speed. specify the geometry and material of a calibration specimen provide guidance for defining a calibration reference band provide guidance for the testing of coating systems and tapes and how to document the results provide anonymised test results of the round robin tests. 1.3 Scope The rain erosion performance of the test specimens is dependent on many parameters which are not directly connected to the erosion protection system itself such as, the substrate below the protection system (laminate and filler) and the test rig parameters. This recommended practice provides guidance as to which parameters will influence the test results, and therefore shall be monitored and controlled during erosion testing, to ensure comparable results when using different test rigs. As far as applicable, the parameters are set to represent the environmental conditions that a leading edge of a rotor blade on a wind turbine is exposed to. In addition, guidance is provided for the selection of a calibration specimen. The results from a round robin test on calibration specimens and on three coated specimens are anonymised and provided in App.C. The designs of all three test rigs used for these tests were very similar. An evaluation of the erosion test results with regards to the erosion performance, lifetime, outliers or the required number of specimens, is not within the scope of this RP. This RP was developed as an extension to the requirements specified in the ASTM G73-10 standard. 1.4 Application In the following paragraphs the application of this RP compared to other publications connected to rain erosion at DNV GL is clarified. Recommended practice DNVGL-RP Edition February 2018 Page 7

8 1.4.1 This RP supports the execution of rain erosion tests on rotating arm test rigs. It specifies the boundary conditions to ensure comparable test results on different test rigs. In addition to that, guidance for representative test parameters is provided. This RP does not specify requirements, such as minimum survival times, for the certification of an erosion protection system. The objectives of this RP are especially important for blade manufacturers who aim to improve the performance of their erosion protection system based on different test campaigns. Also for comparisons of test results with the erosion performance on the turbines, it is essential to have a basis of reliable and well aligned test results Coatings for protection of fibre reinforced plastic structures with heavy rain erosion loads Class programme DNVGL-CP-0424 defines a test matrix to acquire a type approval for a coating system. The class programme specifies a minimum quality level and in this way helps filter out unsuitable and low performing materials when considering loads and ageing effects such as temperatures and climatic influences. The objective of the class programme and the certification of materials, is to ensure that the coating system will be produced with a constant quality and ensures that changes in formulation and properties are correctly documented. The class programme DNVGL-CP-0424 is not limited to erosion tests, but also covers tensile and gloss tests at different temperatures, with and without UV exposure. It must be emphasized that material qualifications do not consider the materials survivability under operational loads for the turbine life Rotor blades for wind turbines Rotor blade component certification is based on DNVGL-ST-0376 in combination with DNVGL-SE When considering an erosion protection system within the context of a blade certification, the following additional considerations shall be made: it shall be shown that the specimens are representative for the specific blade production considering the following items: leading edge lay-up materials substrate production method application method and quality of leading edge protection system. appropriate maintenance intervals and maintenance measures shall be defined. 1.5 References Table 1-1 Normative DNV GL documents Document code DNVGL-CP-0424 DNVGL-SE-0441 DNVGL-ST-0376 Title Coatings for protection of FRP structures with heavy rain erosion loads Type and component certification of wind turbines Rotor blades for wind turbines Recommended practice DNVGL-RP Edition February 2018 Page 8

9 Table 1-2 Normative external documents Document code ASTM G73-10 ISO 2808 ISO 4618:2014 ISO :2005 ISO/IEC Title Standard Test Method for Liquid Impingement Erosion Using Rotating Apparatus Paints and varnishes - Determination of film thickness Paints and varnishes - Terms and definition Metallic materials - Vickers hardness test - Part 1: Test method General requirements for the competence of testing and calibration laboratories 1.6 Definitions and abbreviations For the purposes of this document, the terms and definitions given in ISO 4618 and the following apply Definition of verbal forms Table 1-3 Definition of verbal forms Term Definition shall should may verbal form used to indicate requirements strictly to be followed in order to conform with the document verbal form used to indicate that among several possibilities one is recommended as particularly suitable, without mentioning or excluding others, or that a certain course of action is preferred but not necessarily required verbal form used to indicate a course of action permissible within the limits of the document Definition of terms Table 1-4 Definition of terms Term angle of incidence breakthrough droplet concentration end of incubation period exposure zone failure mode gauge zone length number of specific impacts rain intensity Definition impact angle of the rain drop on the specimen surface point in time when the erosion progress breaks through the protective layer to the underlying substrate number of droplets per cubic meter exposure time until the first mass loss or damage is visually detectable the area the rain is distributed on e.g. cracking, peeling, abrasion the area on the specimen where the erosion performance will be evaluated number of impacts per projected unit area perpendicular to the impact velocity height of raining water accumulated per unit of time Recommended practice DNVGL-RP Edition February 2018 Page 9

10 Term specific impact frequency stage of erosion progress terminal velocity water volume concentration Definition number of specific impacts per unit of time reference point in time: end of incubation period or breakthrough highest droplet falling velocity due to air resistance cumulated volume of water when considering all droplets contained in a unit volume of space Definitions of symbols and equations Table 1-5 Symbols Symbol Unit Definition A [m 2 ] area covered with rain b [m] distance of test specimen to side wall COV [-] coefficient of variation d [mm] mean diameter of a droplet g [m/s²] gravitational acceleration I [m/s] rain intensity k [-] constant value for power law equation K [1/s] constant l [m] length L [m] length m [-] exponent for power law equation [#Impacts/m 2 ] [#Impacts/m 2 ] [#Impacts/(s m 2 )] specific number of impacts specific number of impacts N following the best fit reference line for the data points of sample j specific number of impacts per unit time specific impact frequency P [m 3 /s] water volumetric flow rate q [#Droplets/m 3 ] droplet concentration r [m] radius Ra [ μm] average surface roughness s [#Impacts/m 2 ] alternatively [m/s] standard deviation for specific number of impacts N (alternatively standard deviation for v s ) t [s] exposure time v [m/s] velocity Recommended practice DNVGL-RP Edition February 2018 Page 10

11 Symbol Unit Definition [m 3 ] volume of a droplet x [m] [#Impacts/m 2 ] alternatively [m/s] distance from origin of droplet to centre of specimen in rotor plane mean value for specific number of impacts N (alternatively mean value for v s ) [ ] angle of incidence [ ] half angle between rows of rain dispensers [rad] coverage angle [-] ratio of coverage angle and 2 [-] water volume concentration [rad/s] angular velocity centre point of specimen outer point of specimen inner point of specimen index for number of test sample gauge zone rotor plane (impact velocity of) the sample with the drops (impact velocity of) the sample with the drops at the centre position of specimen reference (impact velocity of) the sample with the drops maximum specimen (impact velocity of) droplets at the outer position of specimen minimum specimen (impact velocity of) droplets at the inner position of specimen droplet falling (velocity) terminal droplet falling (velocity) droplet falling (velocity) when reaching the rotor plane where impacts with the specimen occur (distance of) influence Recommended practice DNVGL-RP Edition February 2018 Page 11

12 Symbol Unit Definition specimen distance Abbreviations Table 1-6 Abbreviations Abbreviation Description FRP RP RET fibre reinforced plastics recommended practice rain erosion test Recommended practice DNVGL-RP Edition February 2018 Page 12

13 SECTION 2 TEST PROCEDURE 2.1 Test procedure Testing laboratories should comply with the requirements of ISO The erosion damage is reproduced on specimens mounted on an arm which rotates horizontally, through an artificial rain field. The rain impacts the surface of the test specimen and erodes the surface, which is protected with the coating or tape, to be tested. The degree of erosion damage caused by the droplet impacts shall be inspected and documented. This shall be performed by visual inspection and picture documentation at defined intervals. Detailed picture documentation enables the investigation of the initial damage at the end of the incubation period, as well as the damage progress. The time needed to erode the surface to a specified limit, is the measure which is used to compare the performance of the protections systems with each other. There are two erosion stages which are commonly used to specify the survival time of the specimens: 1) end of incubation period 2) breakthrough to the underlying substrate. It is essential to monitor and control all parameters which influence the test result. The test apparatus, test procedures and the substrates are not fully standardized, the parameters listed in Sec.10 shall as a minimum, be controlled and monitored. The relationship between accelerated erosion tests to real-life erosion is part of current research and, cannot yet be quantified. It is currently state of art to use accelerated erosion tests with high impact speeds to assess the performance of rain erosion protection systems. Recommended practice DNVGL-RP Edition February 2018 Page 13

14 SECTION 3 ROTATING ARM TEST RIG 3.1 Outline An outline of the rotating arm test rig is shown in Figure 3-1. An artificial rain field may be generated over the entire swept area of the specimen, or a part of it. Figure 3-1 Rotating arm test rig The test parameters relating to the rig design are shown in Table 3-1. Table 3-1 Test rig parameters Test parameter Unit Nominal condition rotating carrier arm [-] aerofoil shaped with an integrated specimen Recommended practice DNVGL-RP Edition February 2018 Page 14

15 Test parameter Unit Nominal condition number of specimen carrier arms [-] max. 3 radial position for the centre of the specimen, r c [m] min. 1.0 vertical distance from origin of droplet (needle) to centre of specimen in rotor plane, x [m] min. 0.2 angle of incidence, α [ ] 90 distance of test specimen to side wall, b [m] to be documented 3.2 Rotating carrier arm The aerofoil contour of the carrier arm reduces the influence of the support structure on the test result. The influence of any uneven air flow within the test chamber, on the test results is not fully established, therefore this influence is one of the main design drivers for the test rig. As an aerofoil contour for the carrier arm, the specimen geometry may be used. The specimen geometry is specified in App.A. 3.3 Number of carrier arms A maximum of three carrier arms should be used to avoid an influence of the turbulence of one specimen on the preceding to avoid any shadowing effect (see [D.2]). 3.4 Radial position of specimen The radial distance from the rotor centre to the centre of the test specimen shall be at least 1.0 m in order to reduce the aerodynamic influence of the support structure (considering a constant rotational speed). The influence of the centrifugal forces and the resulting longitudinal stresses on the test results is unknown. Thus, a minimum radius of 1.0 m shall be specified to limit the centrifugal forces on the specimen compared to a set impact velocity. Recommended practice DNVGL-RP Edition February 2018 Page 15

16 3.5 Distance from origin of droplet to centre of specimen in rotor plane Figure 3-2 Distance from origin of droplet (needle) to centre of specimen in rotor plane The falling distance, x, from the needle to the specimen centre plane should be at least 200 mm as shown in Figure 3-2. One reason for specifying a falling distance above 200mm is the decreasing risk of influences from shadowing effect when the droplet falling speed is increased (see [D.2]). 3.6 Angle of incidence The angle of incidence α is defined as shown in Figure 3-3: Figure 3-3 Angle of incidence Recommended practice DNVGL-RP Edition February 2018 Page 16

17 3.7 Distance of test specimen to side wall The minimum distance between the test specimen and the side wall shall be determined based on the individual test rig and rain field. An influence of the side wall onto the test result shall be avoided. Recommended practice DNVGL-RP Edition February 2018 Page 17

18 SECTION 4 SPECIMENS 4.1 Geometry The relevant geometry parameters for the specimens are listed in Table 4-1: Table 4-1 Parameters related to the specimen geometry Specimen geometry parameters Unit Nominal condition cross-sectional shape of specimen [-] U-shaped and integrated in the aerofoil design of the carrier arm. Leading edge curvature shall be measured exposure zone [m] length of exposure zone shall be larger than gauge zone gauge zone length of specimen l gz [m] min. 0.2 m Cross-sectional shape A U-shaped cross section is considered most representative for rotor blade leading edges. A standard specimen geometry is described in App.A Exposure zone The exposure zone is the area the rain is distributed on. It may be smaller or larger than the specimen length. To avoid edge effects, the exposure zone shall be larger than the gauge zone. An illustration of the exposure zone is shown in App.A Gauge zone length The gauge zone is the area on the specimen where the erosion performance is evaluated. To avoid edge effects, the gauge zone shall be smaller than the exposure zone. In App.A a sketch of the gauge zone is shown. 4.2 Material The specimens typically consist of two main components, the substrate and the protection system which shall be tested. If the test is referencing a particular blade or blade family, the test specimen substrate should be built with the same materials as the leading edge in the blade production. The same is valid for the protection system, e.g. coating or tape. Any deviation from the rotor blade production shall be documented and evaluated. 4.3 Specimen preparation Production methods, manufacturing tolerances and materials have a large influence on the test results. The test specimens should be built with the same production methods as the leading edges in the blade production. Any deviation from the rotor blade production process shall be documented and the influence of the deviations on the test results shall be evaluated. The following parameters shall be documented: Recommended practice DNVGL-RP Edition February 2018 Page 18

19 Table 4-2 Specimen parameters Specimen parameter Unit Nominal condition identification number of specimen [-] to be documented materials (fibres, resins, filler, coating etc.) and material suppliers [-] as in blade production, to be documented lay-up [-] as in blade production, to be documented surface preparation [-] as in blade production, to be documented curing cycles, temperatures and duration for substrate and coating [-] as in blade production, to be documented all layer thicknesses (filler, primer, coating etc.) [μm] minimum values of blade production, to be documented coating / tape application method [-] as in blade production, to be documented coating application quality [-] as in blade production, to be documented Production method and coating/tape application The influences of the blade production method and manufacturing tolerances on the blade leading edge area should be considered during testing. For these considerations, all materials and layers at the leading edge shall be considered. For generic specimens, appropriate assumptions for the items listed above shall be made Layer thicknesses All materials at the leading edge shall be applied with the minimum thicknesses compared to the real blade production. The thicknesses of all layers shall be specified and measured. The thickness of the dried leading edge protection coating shall be measured in micrometres by one of the procedures specified in ISO Accelerated ageing The reference baseline testing should be performed on virgin specimens. If climate influences are part of the test campaign, accelerated ageing of the test specimens shall be carried out in the same manner, to the highest possible extent, as the conditions the blades are subjected to. The following climatic parameters should be considered: Table 4-3 Parameters for accelerated ageing Parameter for accelerated ageing Nominal condition extreme temperatures UV exposure humidity salt spray to be documented to be documented to be documented to be documented Recommended practice DNVGL-RP Edition February 2018 Page 19

20 To reduce the list of climatic influences for the tests, it shall be shown that the neglected climatic parameter has no influence on the test result, or that the parameter is not relevant for the application purpose. It must be emphasized that there is currently no approach available to reliably relate accelerated ageing of test specimens to wind turbine site conditions. 4.5 Tapes as erosion protection The transition area of the tape edges with the blade surface shall be investigated during testing. In addition to that, tape edges, start and end positions, overlaps, as well as transitions between two tapes shall be subject to erosion testing. It shall be ensured that the differences in failure modes are appropriately covered. Tape peeling is a critical failure mode. Since the failure modes for coatings and tapes may be different, any comparison of test results for coatings and tapes shall be performed very carefully. Recommended practice DNVGL-RP Edition February 2018 Page 20

21 SECTION 5 TEST PARAMETERS 5.1 Test condition parameters Overview of test condition parameter The test condition parameters that shall be specified and or monitored during test are listed in Table 5-1. Table 5-1 Test condition parameters Test parameter Unit Nominal condition duration of test [min] to be specified normal impact velocity at centre of specimen, v s,c [m/s] to be calculated water temperature [ C] to be monitored water quality [μs/cm] to be documented test specimen temperature [ C] to be monitored during test or alternatively during inspection test chamber temperature [ C] to be monitored during test test chamber pressure [Pa] to be documented if room pressure or vacuum is present mean droplet size, diameter, d [mm] ~2.0 droplet size standard deviation [mm] to be monitored prior to test Duration of test The test duration shall be defined depending on the individual incubation period and breakthrough time of the protection system. Since this is not known at the onset of testing new protection systems, careful monitoring of the first samples is needed to establish the test durations for subsequent test samples to establish a baseline. The test is completed when the required level of information on the erosion progress is reached Water temperature The influence of water temperature on erosion is not clearly understood. As a result, it is important to measure the water temperature as close to the needle as possible for each test performed. A possible effect of water temperature should then be evaluated during post processing of the results Water quality The selected water quality shall be documented by measuring the conductivity or composition. Deionized water, de-mineralized water, tap water, chloride-containing water or artificial sea water may be selected. Recommended practice DNVGL-RP Edition February 2018 Page 21

22 5.1.5 Test chamber pressure The chamber pressure should be monitored. The erosion test should be performed at normal atmospheric pressure Mean droplet size The droplet size has a direct impact on the erosion damage. Therefore, test results should only be compared for similar sized droplets. The mean droplet size and standard deviation shall be determined and reported with a reasonable accuracy. Furthermore, the droplet size distribution should be determined to give a better understanding of the impact on the specimen. Droplet sizes are dependent on many parameters and have a large influence on the erosion behaviour. The droplet size shall be regularly measured using a laser disdrometer or appropriate methods. 5.2 Derived test parameters Overview of derived test parameters The following test parameters shall be derived from the test condition parameters specified in [5.1]. Table 5-2 Derived test parameters Derived test parameter Unit Nominal condition rain intensity, I [m/s] to be measured or computed from rig design (which needs to be defined) max impact velocity, v s,max [m/s] to be computed min impact velocity, v s,min [m/s] to be computed droplet velocity when entering rotor plane, v drop,rp [m/s] to be computed specific impact frequency per unit time at centre of gauge zone, Ṅ c (based on mean drop diameter, d) [Impacts /(m 2 *s)] to be computed The impact frequency should be selected in a way that the sample surface is able to recover after each impact, and no water film is generated on the sample surface. It is believed that, on an operating wind turbine, the impact frequency is not high enough for any point of the protection system to simultaneously experience stresses resulting from separate impacts. Further details on the calculation of these parameters are provided in App.B. Recommended practice DNVGL-RP Edition February 2018 Page 22

23 SECTION 6 CALIBRATION 6.1 General To ensure the accuracy of the test equipment and to quantify the variation between different tests or test rigs a standardised calibration scheme is required. Further information on calibration test results and possible reference bands are provided in [C.2]. 6.2 Calibration intervals A calibration is mandatory when a new test rig is set up. In addition to that, calibrations shall be performed as a minimum every second month and after any change of the test parameters. Table 6-1 Parameters calibration intervals Parameter Unit Nominal condition date and time of calibration [YYYYMMDD] to be documented Any modification of test parameters during a test campaign shall be explicitly listed. 6.3 Calibration specimens Geometry For the calibration specimens, the geometry, which is specified in App.A, may be applied Aluminium calibration specimens For calibration specimens, the material defined in Table 6-2 and Table 6-3 may be used. Table 6-2 Parameters for aluminium calibration material Parameter Unit Nominal condition specimen composition [-] EN-AW-3003, aluminium alloy temper code [-] H112 average hardness ISO :2005 [HV 2] 33 density [kg/m 3 ] ~2700 Young s modulus [GPa] ~70 Table 6-3 Parameters related to manufacture and preparation of aluminium calibration specimen Parameter Unit Nominal condition manufacturing process [-] extruded from blocks and polished annealing [-] none Recommended practice DNVGL-RP Edition February 2018 Page 23

24 Parameter Unit Nominal condition surface roughness, Ra [μm] <1 6.4 Evaluation of calibration results The evaluation of the calibration results is based on the end of the incubation period which is determined from visual inspections. Alternatively, the calibration with aluminium specimens may be based on mass loss Repeatability From the results of the round robin tests, it is assumed that the repeatability of the calibration tests using aluminium specimens, as specified in [6.3], should lead to a coefficient of variation COV of less than 20%. with s = standard deviation for specific number of impacts N = mean value for specific number of impacts N v s,ref = reference impact speed of droplet with the sample N fit,j = specific number of impacts N following the best fit reference line for the data points of sample j This limit for the coefficient of variation should be used for calibration tests on one test rig. As a minimum, one set of calibration specimens, in this case 3 specimens, shall be used for the regular standard calibrations Reproducibility For the reproducibility of the calibration test results between different test rigs, it shall be shown that the results are similar. The test results of calibration testing should be compared with a reference curve, preferably in a similar way as shown in Figure C-3 through Figure C-6. The reproducibility of the calibration test results may be shown using the calibration reference curve described in [C.2]. It is recommended that a comparison between aluminium reference specimens test results and the reference curve is included in each test report. Recommended practice DNVGL-RP Edition February 2018 Page 24

25 SECTION 7 INSPECTION PARAMETERS Currently, no unambiguous evaluation method of the rain erosion test results exists. The specimens are commonly inspected visually and subsequently compared to other test results in terms of damage severity versus test execution time. 7.1 Overview of inspection parameters The relevant inspection parameters are listed in Table 7-1: Table 7-1 Parameters related to inspections Inspection parameter Unit Nominal condition inspection interval [min] to be specified cleaning method before inspection [-] to be specified time of inspection in relation to exposure time [min] to be documented picture at every inspection [-] high resolution pictures including a scale to be documented 7.2 Inspection interval and time Before the erosion test is started, an initial visual inspection of the test specimens shall be performed and documented with pictures. As additional information, the specimen mass may be recorded. The inspection interval is individually determined. It is recommended to adjust the inspection intervals and the test time to cover both stages of erosion progress, the end of the incubation period and breakthrough, with sufficient accuracy. The time of inspection in relation to the execution time shall be recorded for every inspection. The inspection interval has an influence on the accuracy of the test result. For calibration purposes, the inspection interval should therefore be kept constant for each run. Loss of gloss is not used to evaluate the performance of the erosion protection system. 7.3 Cleaning method The cleaning method, which is used before each inspection, shall be specified. The cleaning is mainly used for drying the specimens to avoid an influence of the water on the result of the visual inspection or mass measurement. 7.4 Inspection method The test specimens shall be inspected visually. The results shall be documented with high resolution pictures, including a reference scale, for every inspection. It shall be ensured that the quality of the pictures is good enough to derive the end of the incubation period. More elaborate inspection methods using microscopes may be applied. Each location on the specimen shall be correlated with an impact speed. Depending on the rig configuration, the impact frequency might change along the length of the sample. Recommended practice DNVGL-RP Edition February 2018 Page 25

26 SECTION 8 RESULT PARAMETERS 8.1 Overview of result parameters The relevant result parameters are listed in Table 8-1: Table 8-1 Result parameters Result parameter Unit Nominal condition mass loss [g] optional failure modes [-] optional stages of erosion progress [-] reference point in time: end of incubation period and breakthrough end of incubation period [min] document time of initial surface damage for each location breakthrough [min] document time of breakthrough for each location 8.2 Mass loss The mass loss of the calibration specimens may be measured and monitored at the inspections. The mass loss is used to monitor the erosion damage development on the complete specimen and independent of the eroded layers. 8.3 Failure modes The different failure modes of the leading edge protection systems, such as cracking, peeling, and abrasion, may be documented as additional information. This information might be important to evaluate the reasons for varying performance levels of the leading edge protection systems. It shall be ensured that the failure modes, which are triggered during testing, are comparable to the failure modes seen on the turbines. 8.4 Stages of erosion progress The stages of erosion progress are reference points in time which may be used to assess the remaining protective efficacy of the erosion protection system and to compare the performance of different systems with each other. In context of this recommended practice, the end of the incubation period and breakthrough are specified as they are the most commonly used stages of erosion progress. 8.5 End of incubation period The incubation period is defined as the exposure time until the first damage is visually detectable on the outer surface of the test specimen. The incubation period depends on the impact speed and thus, for rotating arm test rigs, on the position on the specimen. An illustration of an initial surface damage for a protected specimen is shown in Figure 8-1: Recommended practice DNVGL-RP Edition February 2018 Page 26

27 Figure 8-1 Illustration of initial surface damage at the end of the incubation period The first visual surface damage can be caused by different failure modes (e.g. cracking, peeling, abrasion). In some cases, the initial damage is not located on the outer surface, but in the underlying layers, and is thus not visible until a piece of the material is suddenly removed. Methods to measure damage below an undamaged surface are not yet common in rain erosion testing. Loss of gloss is not considered to be damage. For determining the end of the incubation period on rotating arm test rigs, as described in [3.1], measuring mass loss is not an appropriate parameter, since it is providing information about the status of the complete specimen, independent of the rotational speed and the affected layer. Thus, for visualization purposes only, Figure 8-2 uses mass loss to describe the meaning of incubation period. Figure 8-2 Visualization of the incubation period based on mass loss for one specific section, e.g. section A-A of Figure 8-1 Recommended practice DNVGL-RP Edition February 2018 Page 27

28 Generally, it shall be clearly specified how the end of the incubation period is defined, how it is detected and which approximate resolution the detection method has. Especially for tapes, a detailed definition of the end of the incubation period and the differentiation to breakthrough (see [8.6]) is important. 8.6 Breakthrough Breakthrough is defined as the point in time when the erosion breaks through the protective layer to the underlying substrate. It shall be clearly defined which layers belong to the substrate (e.g. laminate, topcoat etc.). The time of breakthrough depends on the impact velocity and thus, for rotating arm test rigs (as described in [3.1]), it also depends on the location on the specimen. The end of the incubation period and breakthrough may be equal in some cases. This might apply when the initial damage is caused on the underlying layers and develops without visible damage on the surface, until a piece of protection layer is suddenly removed. An illustration of the breakthrough erosion stage is shown in Figure 8-3. Figure 8-3 Illustration of breakthrough erosion stage in B-B compared to the initial surface damage at the end of the incubation period in A-A As described in [8.5], loss of mass is not an appropriate parameter to define erosion progress stages for the chosen test rig configuration (see [3.1]). However, for visualization purposes only, Figure 8-4 uses mass loss to provide further information on the breakthrough erosion stage. Recommended practice DNVGL-RP Edition February 2018 Page 28

29 Figure 8-4 Visualization of breakthrough based on mass loss for homogeneous specimen material Looking at the theoretical graph of mass loss vs number of droplet impacts, the point of breakthrough will be shown as a change of slope, since the underlying substrate has different erosion properties than the protective layer. Breakthrough times should be determined conservatively, by using the time-step before breakthrough is detected on the pictures. Recommended practice DNVGL-RP Edition February 2018 Page 29

30 SECTION 9 DISPLAYING RESULTS 9.1 Displaying results An incubation curve may be expressed in terms of the recorded ends of incubation periods at different impact velocities and specific numbers of impacts, see Figure 9-1. Figure 9-1 Result data for end incubation period shown as droplet impact velocity vs specific number of droplet impacts (axes on a logarithmic scale) The v s versus N diagrams, as shown in Figure 9-1 may be developed assuming that the data cloud is described by a power law: As often used for traditional fatigue S/N curves, the equation may be specified with N as the dependent parameter: As a next step, the data is transformed into a log-log scale and the parameters k and m are determined using a least square fit: If the end of the incubation period is plotted for droplet impact velocity versus specific number of impacts, the diagram resembles traditional fatigue S/N curves where the induced stresses are displayed versus the number of cycles. Since, for the test rig configuration specified in [3.1], the impact speed increases with the radial position on the specimen, one test specimen provides information for several impact velocities. However, the establishment of an incubation curve requires visual detection of initial damages at the individual specimen cross-sections. Recommended practice DNVGL-RP Edition February 2018 Page 30

31 Breakthrough data may be expressed in the same way, as shown in Figure 9-2: Figure 9-2 Breakthrough data shown as droplet impact velocity versus specific number of droplet impacts (axes on a logarithmic scale) Recommended practice DNVGL-RP Edition February 2018 Page 31

32 SECTION 10 TEST REPORT 10.1 Test report The test report should comply with ASTM G Further a test parameter overview, as shown in Table 10-1, shall be summarized by collecting the parameters specified in Table 3-1 to Table 8-1 in this document. Table 10-1 Summary of parameters to be documented in the test report. Test parameter Unit Nominal condition Deviations from nominal condition specimen carrier arm [-] aerofoil shaped with an integrated specimen number of specimen carrier arms [-] max. 3 Test rig radius position of centre of specimen attachment, [m] min. 1.0 r c distance from origin of droplet to centre of specimen in rotor plane, x [m] min. 0.2 angle of incidence [ ] 90 Specimen geometry cross-sectional shape of specimen gauge zone length of specimen, l gz (zone where erosion is evaluated) [-] [m] min. 0.2 U-shaped and integrated in the aerofoil design of the carrier arm. Leading edge curvature to be measured. exposure zone [m] larger than gauge zone Recommended practice DNVGL-RP Edition February 2018 Page 32

33 Test parameter Unit Nominal condition Deviations from nominal condition identification number of specimen [-] to be documented materials (fibres, resins, filler, coating etc.) and material suppliers [-] as in blade production to be documented Specimen preparation layup [-] surface preparation [-] curing cycles, temperatures and duration for base laminate and coating all layer thicknesses (filler, primer, coating etc.) [-] [μm] as in blade production to be documented as in blade production to be documented as in blade production to be documented minimum values from blade production to be documented coating/tape application method [-] as in blade production to be documented coating application quality [-] as in blade production to be documented extreme temperatures [-] to be documented UV exposure [-] to be documented Accelerated ageing humidity [-] to be documented salt spray [-] to be documented duration of test [min] to be specified Test conditions normal impact velocity at centre of specimen, v s,c [m/s] to be calculated water temperature [ C] to be monitored water quality [μs/cm] to be documented test specimen temperature [ C] to be monitored during test test chamber temperature [ C] to be monitored during test Recommended practice DNVGL-RP Edition February 2018 Page 33

34 Test parameter Unit Nominal condition Deviations from nominal condition test chamber pressure [Pa] to be monitored during test mean droplet size, diameter [mm] ~2.0 droplet size standard deviation rain intensity in exposure zone (exposure zone needs to be defined) [mm] [m/s] to be monitored prior to test to be measured or computed from rig design (which needs to be defined) Derived test parameters max impact velocity, v s,max [m/s] to be computed min impact velocity, v s,min [m/s] to be computed droplet velocity when entering rotor plane, [m/s] to be computed v drop,rp specific impact frequency per unit time in exposure zone, Ṅ c (based on mean drop diameter) [Impacts / (m 2 *s)] to be estimated Calibration date and time of calibration [YYYYMMDD] to be documented specimen composition [-] EN-AW-3003 temper code [-] H112 Calibration material average hardness, ISO :2005 [HV 2] 33 density [kg/m 3 ] ~2700 Young s modulus [GPa] ~70 Recommended practice DNVGL-RP Edition February 2018 Page 34

35 Test parameter Unit Nominal condition Deviations from nominal condition manufacturing process [-] extruded from blocks and polished Calibration process annealing [-] none surface roughness, Ra [µm] <1 inspection interval [min] to be specified cleaning method before inspection [-] to be specified Inspections time of inspection in relation to exposure time picture at every inspection [min] [-] to be documented high resolution pictures including a scale to be documented mass loss [g] optional failure modes [-] optional Results end of incubation period breakthrough [min] [min] document time of initial surface damage for each location document time of breakthrough for each location stage of erosion progress [-] define if data point is representing end of incubation period or breakthrough Recommended practice DNVGL-RP Edition February 2018 Page 35

36 SECTION 11 SUMMARY 11.1 Summary In this recommended practice, the influencing parameters for erosion tests on rotating arm test rigs were specified and in some cases nominal values were recommended. Furthermore, an aluminium calibration specimen is introduced. The performed round robin tests show comparable erosion performances for the three rotating arm test rigs. The round robin test results are listed in App.C. Recommended practice DNVGL-RP Edition February 2018 Page 36

37 APPENDIX A SPECIMEN GEOMETRY A.1 Specimen geometry Figure A-1 Gauge length explanation Figure A-2 Specimen cross-section based on NACA Recommended practice DNVGL-RP Edition February 2018 Page 37

38 APPENDIX B DERIVED TEST PARAMETERS B.1 Droplet velocity If the initial speed of the droplet at the needle is zero, the velocity of the drop is a function of how far the drop falls from the needle to the sample, x, and the droplet diameter, d. Figure B-1 Illustration of specimen impact velocity and droplet falling velocity v drop,rp x = droplet falling velocity when reaching the centre of specimen in the rotor plane = distance from origin of droplet (needle) to centre of specimen in the rotor plane For droplet sizes of 0.1 mm to 3 mm, the terminal velocity of the droplet may be defined by using the following empirical relation in ASTM G73-10: where d is the mean droplet diameter in [mm]. If the travel distance is not high enough for the droplets to reach terminal velocity when reaching the rotor plane (v drop,rp = v drop,max ), the droplet velocity may be derived from the two following relations: using x(t = 0) = 0 Recommended practice DNVGL-RP Edition February 2018 Page 38

39 The velocity as function of time is established from the equilibrium equation in terms of an object falling freely in air. Using this equation, the velocity of the droplet when reaching the rotor plane, = v drop,rp, may be calculated. Alternatively, the droplet velocity may be determined experimentally by using a laser disdrometer. B.2 Rain intensity In line with ASTM G73-10, the rain intensity, I, may be directly derived from the water flow, P, and the area the water is distributed over, A. Thus, the rain intensity is calculated as: The area is dependent on how the rain field is generated. In cases where the rain is only generated directly over the specimen gauge zone, the area may be computed as: where: r o = r i = φ = In this case, θ is the angle coverage where the rain is generated, and φ the distribution ratio. B.3 Droplet impact velocity The droplet falling velocity is very small compared to the sample travelling velocity. Thus, the resulting impact velocity is assumed to be equal to the sample speed. For the test rig configuration specified in this document, the impact velocity distribution, v s (r), across the gauge zone is linearly related to the radial position on the specimen carrier arm and the angular velocity: B.4 Specific impact frequency The number of specific impacts, in terms of number of droplet impacting on a unit area during the exposure time, t is computed from the droplet concentration, q, and the impact speed, v s : Thus, the specific impact frequency per unit time at the gauge zone centre is quantified as: Recommended practice DNVGL-RP Edition February 2018 Page 39

40 The droplet concentration, q, is the number of droplets per cubic meter. It is estimated from the water volume concentration, ψ, and the volume of a single droplet, V drop, which is based on the mean droplet diameter: The water volume concentration, ψ, may either be experimentally characterized or estimated from the rain intensity, I, and the droplet falling velocity, v drop,rp, when entering the rotor plane: The leads to the following equation for the droplet concentration, q: The specific impact frequency, Ṅ c shall be reported to give an indication of the impact rate of the test setup. The number of specific impacts N should be used for expressing results. On some machine setups, in order to have a constant specific impact frequency along the sample's length, the rain intensity is intentionally inhomogeneous. The evaluation of the specific impact frequency for one example of such a rain intensity repartition is shown below. In order to keep the specific impact frequency, Ṅ, constant along the samples being tested, one solution is to generate the rain field through dispensers organized radially, in a spider web shape. Recommended practice DNVGL-RP Edition February 2018 Page 40

41 Figure B-2 Dispensers set following a spider web pattern For most machines, the above drawing is not exactly accurate as dispensers would, by design, not have exactly the same radial position from one row to the other, in order to homogenize the rain flow along the sample. It is also common, as shown in the drawing, to have a certain angular section without dispensers, required for example, to fit the needs of automatic inspection equipment. The complementary angle is called the angle of coverage θ. With such a distribution of dispensers, if we consider an area delimited by 2 circles of arbitrary radius r and r + Δr, we can see that the number of dispensers do not depend on r (provided that the discreet distribution of dispensers is approximated by an equivalent continuous distribution). We can therefore write that: with θ being the angle of coverage in radians. In order to keep the specific impact frequency, Ṅ c constant, it has to be independent of the radius r and the rain intensity I(r) has to be proportional to l/r: We find the constant K through the total flow of water P which us poured over the covered area: Recommended practice DNVGL-RP Edition February 2018 Page 41

42 Replacing constant K, the rain intensity in the covered area can then be expressed as: As shown earlier, q can be expressed through the water volume concentration and the volume of a drop: Replacing q and v s, the specific impact frequency in the covered area, can be expressed as: And by replacing the rain intensity I : Deducing from this, we see that the specific impact frequency is independent of the radial position, hence constant along the sample. By multiplying that impact frequency with the time the sample spent in the rain covered area, we can get the specific number of drop impacts after a certain test time t: with the rotational speed:. Thus, the specific number of impacts is independent of the angle of coverage: Recommended practice DNVGL-RP Edition February 2018 Page 42

43 APPENDIX C RESULTS FROM ROUND ROBIN TESTS C.1 Parameter overview The test parameters for the three rotating arm test rigs, which were used for the calibration tests on aluminium specimens, are listed in the tables below. C.1.1 Parameters constant for all tests Table C-1 Test rig parameters for round robin tests Test parameter Unit Round robin value specimen carrier arm [-] aerofoil shaped with an integrated specimen. NACA number of specimen carrier arms [-] 3 radial position of centre of specimen attachment, r c [m] 1.0 Table C-2 Specimen design parameters for round robin tests Test parameter Unit Round robin value Specific value for test rig A Specific value for test rig B Specific value for test rig C cross-sectional shape of specimen [-] U-shaped and integrated in the aerofoil design of the carrier arm as specified in App.A as specified in App.A as specified in App.A gauge zone length of specimen, l gz [m] distance from origin of droplet to centre of specimen in rotor plane, x [m] ~ Table C-3 Test condition parameters for round robin tests Test parameter Unit Round robin value Specific value for test rig A Specific value for test rig B Specific value for test rig C water quality [μs/cm] / test specimen temperature test chamber pressure mean droplet size, diameter [ C] NA [Pa] NA [mm] ~ Recommended practice DNVGL-RP Edition February 2018 Page 43