- Email: [email protected]
Investigation of structural bond lines in wind turbine blades by sub-component tests
International Journal of Adhesion & Adhesives 37 (2012) 129–135
Contents lists available at SciVerse ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Investigation of structural bond lines in wind turbine blades by sub-component tests Florian Sayer n, Alexandros Antoniou, Arnoldus van Wingerde Fraunhofer IWES, D-27572 Bremerhaven, Am Seedeich 45, Germany
a r t i c l e i n f o
abstract
Available online 25 January 2012
In the present study, the structural fatigue performance of an adhesive joint, imitating the connection of the shear web to the spar caps of a wind turbine blade, is investigated. An asymmetric three point bending test has been developed from early design steps to manufacturing and testing. The subcomponent is supposed to fail in the bondline, behavior consistently validated after a comprehensive testing campaign. Similar failure patterns to the full scale wind turbine blades have been observed in the bond line during the fatigue tests. Tensile cracks propagated transverse to the adhesive longitudinal axis, approximately at 10% of the total lifetime. Their growth was hindered from the spar cap and web laminate. After around 60% of the specimen life, cracks changed their direction resulting in disbonds between the adhesive–adherend interface. Shear stress variations and artificial defects seemed to have a relatively low impact on the subcomponent fatigue performance. At the same time it was shown that cover-laminates drastically increase the fatigue performance of the adhesive joint. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Joint design Fatigue Destructive testing Composites Sub-components
1. Introduction Wind turbine blades consist of integrated manufactured composite parts and assembled with adhesive bonds. The mechanical properties of the bond lines are very sensitive to design and material processing technologies. Therefore, the impact of separate parameters on the structural performance of an adhesive joint e.g. the shear stress, different damage geometry and bondline overlaminate should be investigated in representative specimens on the sub-component level. Wind turbine blades are among the highest fatigue loaded technical structures and often made from just a few huge integrally manufactured composite parts. These parts are bonded together, therefore, the adhesives play a key role in the structural integrity. Due to their importance, certification bodies as Germanischer Lloyd (GL) [1] or Det Norske Veritas (DNV) [2] and international standards as IEC 61400 [3], propose tests for the adhesives certification either in coupon or full scale blade test level. The state of the art, certified and widely accepted test methods concerning material properties characterization are performed in coupon level experiments, disregarding geometrical effects e.g. representative thickness or material processing technologies e.g. hand or mechanical mixing and their impact on the structural
n
Corresponding author. Tel.: þ49 471 14290 329; fax: þ 49 471 14290 111. E-mail address: fl[email protected] (F. Sayer).
0143-7496/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2012.01.021
strength. According to the GL Guidelines [4], mechanical tests at two different bond line thicknesses (0.5 mm and 3 mm) are required for the material certification while 10 mm thickness adhesives are common practice in the wind turbine blades industry. In some cases these tests are subjected to intrinsic stress concentrations, resulting in misleading characteristic v as e.g. the single lap shear test [5]. A large number of failures occur during the service life of wind turbine blades; some of these cannot be directly predicted by coupon testing. The reason for that are the small tested volumes, the structural boundary conditions and the stress fields. Therefore the actual research proceeds in two directions. One is heading towards advanced material models, based on mechanics data ([6,7]) aiming to enhance numerical simulation tools. A more direct approach is the structural testing of adhesive joints in the scale of a sub-component. Bond line geometry, adhesive volume and flaw density are considered as intrinsic properties of the structure contributing to the joint mechanical performance. These can be designed and manufactured, imitating the real blade structure. Samborsky et al. [8] addressed stress concentration effects due to the geometry design and porosity in adhesive joints implemented in the wind industry. They studied numerically and experimentally several joints performance with adhesive thickness up to 4 mm. Sayer et al. [9] introduced an asymmetric three point bending test beam in order to study the adhesive performance between the shear web and the spar caps of a wind turbine rotor blade, investigating 10 mm thick bond lines.
130
F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
As a follow up of [9], after finalizing the beam design, a subcomponent test was introduced as a tool in order to determine the structural behavior of an adhesive joint, for a representative adhesive volume and realistic bond line geometry. Different design parameters were addressed and investigated. The proposed test reduces the existing gap between coupon and full scale blade testing, simulating complex stress states in the bond line and reproducing blade like failure patterns.
Manufacturing defects: In a full scale blade test, realistic
2. Sub-component test methodology The commonly accepted test pyramid is ideally filled with a large number of material tests, a lower number of sub-component tests, an even lower number of component tests and at least one full scale test. Fig. 1 shows the test pyramid in the rotor blade industry. As schematically shown, the material and the full scale level are well covered, however, the component and especially the sub-component level is not adequately represented in the current test practice, which might be one of the reasons for the current failure rates. Wind turbine blades are currently certified by material testing and full scale blade testing on a single test blade. The materials specimens and the test blade are often manufactured with special care. However, during the service life of wind turbine blades several failure modes occur, which cannot be detected by coupon testing or in the blade tests. These failures occur mainly due to manufacturing or material defects, handling faults or design faults. The structural approval of wind turbine blades will become more and more challenging as the dimensions of the blades increase for technical and financial reasons, which leads to a drastically higher risk for the designers, manufacturers and investors. Therefore, a reliability based certification methodology is needed. This should bridge the gap between the coupon scale and the full-scale testing — consisting of a mixed approach by destructive testing component and sub-component testing, numerical analysis and manufacturing quality control by non destructive testing methods. With such a reliability based certification methodology the effort of the manufactures to increase knowledge on the structural behavior and the product quality will result in a lower blade weight. It is known that safety factors can be reduced if a larger number of specimens is tested, as the statistical uncertainty is reduced. Sub-component testing offers some very interesting additional information of the full scale blade tests:
defects and defect distributions are detected in the blade. This is obviously not possible in sub-component testing. However, if the defect size and defect distribution is known (e.g. by nondestructive testing), the concepts of sub-component-testing offers the opportunity for systematic investigations on the damage propagation. This can lead to the definition of critical damage sizes and distributions for a wind turbine blade. Realistic loading: Although a large effort is put into realistic loading of wind turbines in the certification tests, there is still a huge potential for improvements. In static blade testing the buckling behavior is influenced and stress concentrations are introduced by the clamping. Furthermore, no realistic loading can be achieved for most load cases. Parametric studies: In full scale blade testing it is not possible or not economically reasonable to do a parameter study. However, it is of strong interest to investigate different materials during the design process under realistic loading. The methodology of sub-component test allows this.
3. Current testing of structural bond lines in the wind industry The proof of reliability over the blade service life is of special interest for the structural bond lines. According to the GL Guidelines [1], mechanical tests at two different bond line thicknesses (0.5 mm and 3 mm) and two different material conditions are required for the material certification (defined lab conditions and water absorption for 1000 h). In the more general GL guidelines [4] for the certification of wind turbines, a glass transition temperature of more than 65 1C is required (tested by a method of thermal analysis, such as Differential Scanning Calometry). To have a broader experimental basis for the material properties, wind turbine manufacturers perform additional static and cyclic tests. Subsequently, a confirmation test on a real blade under static loading is performed. The step from coupon to real blade size implies a risk for the blade manufacturers. Additionally, the bond-lines in modern multi Mega Watt scale turbine blades present a particular design challenge because both, the thickness and length of the adhesive bond are much larger than in other adhesive applications. Shear web to spar cap bond-lines typically run almost the entire length of the blade, up to sixty meters, and a thickness close to a maximum of 10 mm is common.
¨ SN- or Wohler-Lines for materials and designs: In full scale blade testing it is for economic reasons not possible to test a series of blades. By testing sub-components, the fatigue behavior of the material or a structural detail can be investigated with a series of specimens.
4. Adhesive bond line loading in wind turbine blades The GL guidelines [1] provide load limit levels for bond-lines in wind turbine blades. For two-component thermosetting adhesives,
Full scale blade testing
Testing of blade connection (ofte only in full scale blade test)
Large number of static and cyclic tests Fig. 1. Schematic test pyramid in the wind turbine blades.
F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
approved by GL, a static shear strength of 7 MPa can be used without further verification. ‘‘The fatigue verification for joints that feature a steady shear stress curve without discontinuities y is provided for two component thermosetting adhesives approved by the GL if the stress range from the equivalent constant-range spectrum for 107 load cycles is less than 1 MPa’’ [1]. The representative loadings of adhesives used in this investigation were taken from an experimentally validated finite element model using shell elements, developed during the Optimat Blades project [10]. The considered load case was a combined static flap and edge wise loading. The blade structure is shown in Fig. 2 while typical stress states in the adhesive are, for the combined flap and edge wise static loading, illustrated in Table 1. The stresses were calculated with Hooks law from strains given
Spar Cap (SC)
2 1
Web (W) 3
Type of geometry for subcomponent design Fig. 2. Examples of structural Spar Cap to web bond lines.
Table 1 Adhesive loadings in real blade structure for combined flap and edge wise static load case.
s11 s22 s33 s12 s13 s23
Unit
R¼ 5 m
R ¼7 m
R¼ 10 m
MPa MPa MPa MPa MPa MPa
10.70 2.65 0.48 0.43 0.76 Unspecified
12.39 3.84 0.09 0.02 0.85 Unspecified
15.07 1.49 0.02 0.07 1.58 Unspecified
R: rotor blade radius (length position from blade root to tip).
131
for the spar cap and the web using standard material properties for the composite and the adhesive materials as well as an assumed bond line width of 100 mm. The dominating stress is in length direction (1) of the blade. Furthermore, stresses—about 10% to 25% of the longitudinal—in the transverse direction (2) exist. The third significant stress is the shear loading (up to 1.6 MPa). A summary of the calculated stresses from the blade analysis are shown in Table 1. The loadings in the adhesive at radius R¼10 m was chosen to define the stress state for beam development.
5. The Henkel-UpWind-beam The Henkel-UpWind-Beam was developed in a close collaboration between the Henkel AG and the Fraunhofer IWES and the fatigue tests performed and presented were conducted in the framwork of the UpWind [11] project. This sub-component represents the bond line loadings in wind turbine blades. Fig. 3 shows the schematic design of the Henkel-UpWind-Beam. The cross section is mechanically similar to I-beams, commonly known from steel structures. In direct comparison to steel I-beams the web is wider in order to adjust the bond line shear stress to a representative value. In order to have a constant loaded beam section, the spar caps were tapered, resulting in a linear thickness change of the spar cap thickness. Fig. 4 shows the spar cap lay up of the HenkelUpWind-Beams. The ply drops had a fingered design to increase the fatigue strength. With the design information (see Figs. 3 and 4) and material properties (see Table 2) for the composite materials as well as the adhesive, the stresses and strains in the beam structure can be calculated. The used material properties are typical for the materials in the industry; therefore they did not need experimental validation. The adhesive used was MGS Paste 135 G3 from Momentive Speciality Chemicals (Germany) and was processed in a hand mixing and application process. The results of the stress calculation according to beam theory (moments of inertia for discrete cross sections) are shown in Fig. 5. In comparison to the blade loadings in Table 1, an increase in the shear stress by a factor of two can be found. This factor was chosen to compensate for possible higher shear stresses in blades with a different bond line design. The beams were loaded with a hydraulic actuator (see Fig. 6). The load introduction points were realized with height strength steel bolts that were press fitted into the beam. They were 80mm
3.4mm … 10.4mm
26mm
134.6mm … 148.6mm
3.4mm … 10.4mm 32mm Spar Cap
Bond Line
Web (Sandwich material)
Fig. 3. Schematic design of the Henkel-UpWind-Beam (left: side view, right: cross section).
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F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
80mm
80mm 28mm
28mm 100mm
30mm
480 (triangular cut from 450 30mm) 510 (triangular cut from 480 30mm) 540 (triangular cut from 510 30mm) 570 (triangular cut from 540 30mm) 600 (triangular cut from 570 30mm) 630 (triangular cut from 600 30mm) 790 (triangular cut from 690 100mm) 890 (triangular cut from 790 100mm) 690 (triangular cut from 660 30mm) 660 (triangular cut from 630 30mm)
0
250
500
750
1000
1250
1500
Material: Saertex1010g/m² UD; t = 0.7mm Saertex840g/m² +/-45; t = 0.65mm
Fig. 4. Spar Cap design of the Henkel-UpWind-Beam.
Table 2 Selected static material properties for Henkel-UpWind-Beam calculation.
Composite UD Composite 7 451 Adhesive PVC core
Material Type
E11 (MPa)
Material description
Anisotropic Anisotropic Isotropic Isotropic
36000 12000 4800 300
Saertex 1010 g/m2 non crimped fabric, Momentive RIM 135 resin Saertex 840 g/m2 non crimped fabric, Momentive RIM 135 resin Momentive MGS Past 135 G3 Airex C 70
Fig. 5. Axial and shear stress of the adhesive in the Henkel-UpWind-Beam for a loading of 9.7 kN. Fig. 6. Henkel-UpWind-Beam during a cyclic test.
mounted with a rotational degree of freedom. Force, displacement and strain signals were recorded during the tests. The experimental investigation was carried out in two phases. In the first one, the fatigue behavior of the Henkel-UpWind-Beam was determined. In the second part, design, manufacturing and intrinsic flaw parameters were addressed along with their impact on the structural fatigue life of the Henkel-UpWind-Beam. In order to investigate the shear stress influence on the adhesive joint fatigue performance, three additional beam tests were performed. The bond line width was varied in order to change the axial to shear stresses ratio. Therefore, a 22 mm (two beams) and a 42 mm (one beam) width were chosen corresponding to 145% and 76% of the shear value of the reference design.
Examining different techniques to enhance the adhesive joint fatigue performance, a cover laminate was applied on top of the bond line, with two different procedures. Two beams were reinforced with hand laminate and with a pre-manufactured composite bracket, bonded on top of the bond line. In all cases, the reinforcement was one biaxial 745 layer, 840 g/ m2. Small defects i.e. voids and disbonds are common in wind turbine blade bond lines, occurring due to the manufacturing and the application process. Therefore, four beams with artificial flaws were manufactured so as to study their influence on the adhesive joint fatigue life. In two beams, 6 mm diameter cylindrical holes were drilled in the middle of the bond line in the highest loaded area. Furthermore, a notch was made on the hole surface
F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
133
Table 3 Overview of conducted tests.
Henkel-UpWind-Beam Henkel-UpWind-Beam Henkel-UpWind-Beam Henkel-UpWind-Beam Henkel-UpWind-Beam Henkel-UpWind-Beam
Bond line width (mm)
Number of tests
32 22 42 32 32 32
8 2 1 1 3 2
Short description of defects
Artificial void (6 mm diameter drilled hole) Cover laminate (1 premanufactured laminate; 2 hand laminate) Artificial disbond (1 18 mm wide; 1 1 cm2)
Peak to Peak Axial Stress [MPa]
Fig. 7. Schematic illustration of tested beam variants.
enhancing geometric stress concentrations. Additionally, one beam was manufactured with an incipient interface disbond, of 18 mm width, resulting from a Teflon stripe placed over the entire bond line width during the beam manufacturing and one with a 1 cm2 disbonding (resulting from a 1 cm 1 cm big Teflon piece placed in the middle of the spar cap). An overview of the conducted fatigue tests can be found in Table 3. A schematic illustration of the tested beam variants is given in Fig. 7. The midpoint of the artificial defects was located at x¼790 mm from the beam end (see Fig. 3).
60 50 y = 40.92x-1/13 30 20
10 1E+03
1E+04
1E+05 Cycle
1E+06
1E+07
UD/32 MGS Paste 135 G3
6. Experimental results
Fig. 8. Henkel-UpWind-Beam reference SN-line (R-ration: 1).
Spar Cap 1. Axial Cracks
Adhesive Web Spar Cap
2. Start of Interface Failure
Adhesive Web
3. Interface Failure
Spar Cap Adhesive Web
Fig. 9. Failure development in Henkel-UpWind-Beams with 32 mm bond line width.
Fig. 8 shows the SN-line of the Henkel-UpWind-Beams with 32 mm bond line width. The S–N slope parameter m is 13, higher than the standard proposed from the GL for a unidirectional glass laminate. Final failure was considered when the beam could not sustain further loading due to severe damage on the bond line e.g. macroscopic separation between shear web and spar caps. The failure pattern evolution observed in all beam tests is described in Fig. 9. Transverse cracks initiated along the bond line due to the high tensile stresses around 10% of the structure fatigue life. These developed until saturation at around 60% of the component life. Then, interface failure started to develop at the crack tip, between the adhesive and the surrounding web or spar cap laminate. The ultimate failure occurred when horizontal cracks meet each other after propagation. The number of cracks found in the critical beam section was in average 20, resulting in a crack density of 0.5 cracks/cm. However, a very large scatter between 3 and 31 cracks overall was observed. No clear tendency between
F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
Normal Distribution for ε = 0.5%
60 50
y = 40.92x-1/13
30
Rate
Peak to Peak axial Stress [MPa]
134
20 0 100
10 1E+03
1E+04
1E+05 Cycle
1E+06
UD/22 MGS Paste 135 G3
1E+07
1000
Normal distribution -reference 42mm bond line width Delamination -18mm
10000 Cycles Reference +/-45°Cover Laminate Delamination 1cm²
100000
1000000
22mm bond line width Hole
Fig. 13. Henkel-UpWind-Beam—statistic evaluation.
UD/42 MGS Paste 135 G3
Peak to Peak Axial Stress [MPa]
Fig. 10. Henkel-UpWind-Beam SN curve—influence of bond line thickness.
60 50 y = 40.92x-1/13 30 20
10 1E+03
1E+04
1E+05 Cycle
45°premanufactured part MGS Paste 135 G3
1E+06
1E+07
45°hand laminat MGS Paste 135 G3
Peak to Peak Axial Stress [MPa]
Fig. 11. Henkel-UpWind-Beam SN curve—influence of cover laminates.
60
configuration (from Fig. 8) is also given. Fatigue life is significantly boosted with a factor in the range of 30 for high cycle and 50 for low cycle fatigue. However, it should be mentioned that final failure is not directly comparable to the reference beams. Instead of failing in the highest loaded cross section the beam failure was found between the supports. Fig. 12 shows a comparison between the different beam tests with artificial flaws and the reference SN-line. The specific flaws have a negligible impact on the fatigue life. The evaluation of the present results should be based on statistical methods. Therefore, all test results were transformed to a strain of 0.5% (resulting in a stress of 24 MPa) using the slope of the reference SN-line and the corresponding normal distribution for the reference specimens was determined. Fig. 13 shows a logarithmic plot of the ’’events’’ (final failures over number of cycles). Furthermore, the occurrence rate over cycles, using a normal distribution, for the resulting fatigue performance of the reference tests is given. The performance of most of the tested variants performed can be seen within the scatter of the reference tests, however, the test with the 42 mm bond line and the tests with the cover laminates are exceeding typical confidentiality intervals.
50 y = 40.92x-1/13
7. Conclusion
30 20
10 1E+03
1E+04
zylindric holes
1E+05 Cycles 18mm wide disbond
1E+06
1E+07
1cm2 disbond
Fig. 12. Henkel-UpWind-Beam SN curve—influence of defects.
beam life and the number of cracks was found, although the beam with the shortest life showed only 3 cracks. In Fig. 10, it is obvious that all extra experimental data are fitting quite accurately on the reference SN-line (indicated by the best fit curve from Fig. 8), meaning that at least for these complex stress fields, the axial stresses are dominating the structural fatigue life of the component. Shear stresses seem to be of minor importance. Fig. 11 shows the fatigue life of the beam variant with cover laminates. For comparison, the reference the reference
A sub-component test was introduced aiming to replicate critical complex stress states induced in wind turbine rotor blade structures. The specific design was meant to imitate shear web to spar caps adhesive joints, taking under consideration the adhesive representative material volume i.e. bond line width and thickness (10 mm) and geometrical concentration factors i.e. bond line geometry profile. The specimen was used for a parametric study, investigating the influence of design variables i.e. axial to shear stresses ratio, manufacturing techniques i.e. enhancing the bond line with cover laminates along with intrinsic flaws presence on the adhesive joint fatigue life. The dominant stress component defining the adhesive joint failure is found to be the axial one for axial to shear stress ratio from 4 to 8. Single voids with surface stress concentrators seemed also to have negligible effect on the structural fatigue life. A cover laminate on the bond line had a significant influence on the joint life, resulting in an increase of a factor of 50 at low cycles and of 30 at high cycles. References [1] Germanischer Lloyd, Klassifikations- und Bauvorschriften II Werkstoffe und Schweißtechnik – 2 Nichtmetallische Werkstoffe; Edition 2002.
F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
[2] Det Norske Veritas; DNV Standard DNV-DS-J102; Edition October 2010. [3] International Electrical Commission, IEC 61400-1 Wind Turbines – Part 1: Design Requierments, 3rd ed. 2005–08. ¨ die Zertifizierung von Windenergieanla[4] Germanischer Lloyd, Richtlinien fur gen, Edition 2003. [5] Sayer F, Post N, van Wingerde A, Busmann H-G, Kleiner F, Fleischmann W, et al.; Testing of Adhesive Joints in the Wind Industry; European Wind Energy Conference and Exhibition, 16–19 March 2009, Marseille, France. [6] Kickert R, Weerts U, Meister O, Knops M. Fatigue Evaluation of Structural Bondings. DEWEK; 2010. [7] Sorensen BF, Jorgensen K, Jacobsen TK, Ostergaard Rasmus; A general mixed mode fracture mechanics test specimen: The DCB-specimen loaded with
[8]
[9]
[10]
[11]
135
uneven bending moments ; Risø-R-1394(EN); /http://130.226.56.153/ris publ/afm/afmpdf/ris-r-1394.pdfS; November 2011. Samborsky DD, Sears AT, Mandell JF, Kils O Static and Fatigue Testing of Thick Adhesive Joints for Wind Turbine Blades; 2009 ASME Wind Energy Symposium. Sayer F, Kleiner F, Antoniou A, Trusheim M, van Wingerde A. Sub-Component Testing for Adhesive Bond lines for Wind Turbine Blades. Bremen: DEWEK 2010; 2010. Philippidis TP: Quantification of Complex Stress State Effect on Blade Design; Optimat-Blade Report: OB_TG2_R032_ rev. 000; /http://www.kc-wmc.nlS; August 2010. UpWind-project; /http://www.upwind.eu/S; November 2011.
Contents lists available at SciVerse ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Investigation of structural bond lines in wind turbine blades by sub-component tests Florian Sayer n, Alexandros Antoniou, Arnoldus van Wingerde Fraunhofer IWES, D-27572 Bremerhaven, Am Seedeich 45, Germany
a r t i c l e i n f o
abstract
Available online 25 January 2012
In the present study, the structural fatigue performance of an adhesive joint, imitating the connection of the shear web to the spar caps of a wind turbine blade, is investigated. An asymmetric three point bending test has been developed from early design steps to manufacturing and testing. The subcomponent is supposed to fail in the bondline, behavior consistently validated after a comprehensive testing campaign. Similar failure patterns to the full scale wind turbine blades have been observed in the bond line during the fatigue tests. Tensile cracks propagated transverse to the adhesive longitudinal axis, approximately at 10% of the total lifetime. Their growth was hindered from the spar cap and web laminate. After around 60% of the specimen life, cracks changed their direction resulting in disbonds between the adhesive–adherend interface. Shear stress variations and artificial defects seemed to have a relatively low impact on the subcomponent fatigue performance. At the same time it was shown that cover-laminates drastically increase the fatigue performance of the adhesive joint. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Joint design Fatigue Destructive testing Composites Sub-components
1. Introduction Wind turbine blades consist of integrated manufactured composite parts and assembled with adhesive bonds. The mechanical properties of the bond lines are very sensitive to design and material processing technologies. Therefore, the impact of separate parameters on the structural performance of an adhesive joint e.g. the shear stress, different damage geometry and bondline overlaminate should be investigated in representative specimens on the sub-component level. Wind turbine blades are among the highest fatigue loaded technical structures and often made from just a few huge integrally manufactured composite parts. These parts are bonded together, therefore, the adhesives play a key role in the structural integrity. Due to their importance, certification bodies as Germanischer Lloyd (GL) [1] or Det Norske Veritas (DNV) [2] and international standards as IEC 61400 [3], propose tests for the adhesives certification either in coupon or full scale blade test level. The state of the art, certified and widely accepted test methods concerning material properties characterization are performed in coupon level experiments, disregarding geometrical effects e.g. representative thickness or material processing technologies e.g. hand or mechanical mixing and their impact on the structural
n
Corresponding author. Tel.: þ49 471 14290 329; fax: þ 49 471 14290 111. E-mail address: fl[email protected] (F. Sayer).
0143-7496/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2012.01.021
strength. According to the GL Guidelines [4], mechanical tests at two different bond line thicknesses (0.5 mm and 3 mm) are required for the material certification while 10 mm thickness adhesives are common practice in the wind turbine blades industry. In some cases these tests are subjected to intrinsic stress concentrations, resulting in misleading characteristic v as e.g. the single lap shear test [5]. A large number of failures occur during the service life of wind turbine blades; some of these cannot be directly predicted by coupon testing. The reason for that are the small tested volumes, the structural boundary conditions and the stress fields. Therefore the actual research proceeds in two directions. One is heading towards advanced material models, based on mechanics data ([6,7]) aiming to enhance numerical simulation tools. A more direct approach is the structural testing of adhesive joints in the scale of a sub-component. Bond line geometry, adhesive volume and flaw density are considered as intrinsic properties of the structure contributing to the joint mechanical performance. These can be designed and manufactured, imitating the real blade structure. Samborsky et al. [8] addressed stress concentration effects due to the geometry design and porosity in adhesive joints implemented in the wind industry. They studied numerically and experimentally several joints performance with adhesive thickness up to 4 mm. Sayer et al. [9] introduced an asymmetric three point bending test beam in order to study the adhesive performance between the shear web and the spar caps of a wind turbine rotor blade, investigating 10 mm thick bond lines.
130
F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
As a follow up of [9], after finalizing the beam design, a subcomponent test was introduced as a tool in order to determine the structural behavior of an adhesive joint, for a representative adhesive volume and realistic bond line geometry. Different design parameters were addressed and investigated. The proposed test reduces the existing gap between coupon and full scale blade testing, simulating complex stress states in the bond line and reproducing blade like failure patterns.
Manufacturing defects: In a full scale blade test, realistic
2. Sub-component test methodology The commonly accepted test pyramid is ideally filled with a large number of material tests, a lower number of sub-component tests, an even lower number of component tests and at least one full scale test. Fig. 1 shows the test pyramid in the rotor blade industry. As schematically shown, the material and the full scale level are well covered, however, the component and especially the sub-component level is not adequately represented in the current test practice, which might be one of the reasons for the current failure rates. Wind turbine blades are currently certified by material testing and full scale blade testing on a single test blade. The materials specimens and the test blade are often manufactured with special care. However, during the service life of wind turbine blades several failure modes occur, which cannot be detected by coupon testing or in the blade tests. These failures occur mainly due to manufacturing or material defects, handling faults or design faults. The structural approval of wind turbine blades will become more and more challenging as the dimensions of the blades increase for technical and financial reasons, which leads to a drastically higher risk for the designers, manufacturers and investors. Therefore, a reliability based certification methodology is needed. This should bridge the gap between the coupon scale and the full-scale testing — consisting of a mixed approach by destructive testing component and sub-component testing, numerical analysis and manufacturing quality control by non destructive testing methods. With such a reliability based certification methodology the effort of the manufactures to increase knowledge on the structural behavior and the product quality will result in a lower blade weight. It is known that safety factors can be reduced if a larger number of specimens is tested, as the statistical uncertainty is reduced. Sub-component testing offers some very interesting additional information of the full scale blade tests:
defects and defect distributions are detected in the blade. This is obviously not possible in sub-component testing. However, if the defect size and defect distribution is known (e.g. by nondestructive testing), the concepts of sub-component-testing offers the opportunity for systematic investigations on the damage propagation. This can lead to the definition of critical damage sizes and distributions for a wind turbine blade. Realistic loading: Although a large effort is put into realistic loading of wind turbines in the certification tests, there is still a huge potential for improvements. In static blade testing the buckling behavior is influenced and stress concentrations are introduced by the clamping. Furthermore, no realistic loading can be achieved for most load cases. Parametric studies: In full scale blade testing it is not possible or not economically reasonable to do a parameter study. However, it is of strong interest to investigate different materials during the design process under realistic loading. The methodology of sub-component test allows this.
3. Current testing of structural bond lines in the wind industry The proof of reliability over the blade service life is of special interest for the structural bond lines. According to the GL Guidelines [1], mechanical tests at two different bond line thicknesses (0.5 mm and 3 mm) and two different material conditions are required for the material certification (defined lab conditions and water absorption for 1000 h). In the more general GL guidelines [4] for the certification of wind turbines, a glass transition temperature of more than 65 1C is required (tested by a method of thermal analysis, such as Differential Scanning Calometry). To have a broader experimental basis for the material properties, wind turbine manufacturers perform additional static and cyclic tests. Subsequently, a confirmation test on a real blade under static loading is performed. The step from coupon to real blade size implies a risk for the blade manufacturers. Additionally, the bond-lines in modern multi Mega Watt scale turbine blades present a particular design challenge because both, the thickness and length of the adhesive bond are much larger than in other adhesive applications. Shear web to spar cap bond-lines typically run almost the entire length of the blade, up to sixty meters, and a thickness close to a maximum of 10 mm is common.
¨ SN- or Wohler-Lines for materials and designs: In full scale blade testing it is for economic reasons not possible to test a series of blades. By testing sub-components, the fatigue behavior of the material or a structural detail can be investigated with a series of specimens.
4. Adhesive bond line loading in wind turbine blades The GL guidelines [1] provide load limit levels for bond-lines in wind turbine blades. For two-component thermosetting adhesives,
Full scale blade testing
Testing of blade connection (ofte only in full scale blade test)
Large number of static and cyclic tests Fig. 1. Schematic test pyramid in the wind turbine blades.
F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
approved by GL, a static shear strength of 7 MPa can be used without further verification. ‘‘The fatigue verification for joints that feature a steady shear stress curve without discontinuities y is provided for two component thermosetting adhesives approved by the GL if the stress range from the equivalent constant-range spectrum for 107 load cycles is less than 1 MPa’’ [1]. The representative loadings of adhesives used in this investigation were taken from an experimentally validated finite element model using shell elements, developed during the Optimat Blades project [10]. The considered load case was a combined static flap and edge wise loading. The blade structure is shown in Fig. 2 while typical stress states in the adhesive are, for the combined flap and edge wise static loading, illustrated in Table 1. The stresses were calculated with Hooks law from strains given
Spar Cap (SC)
2 1
Web (W) 3
Type of geometry for subcomponent design Fig. 2. Examples of structural Spar Cap to web bond lines.
Table 1 Adhesive loadings in real blade structure for combined flap and edge wise static load case.
s11 s22 s33 s12 s13 s23
Unit
R¼ 5 m
R ¼7 m
R¼ 10 m
MPa MPa MPa MPa MPa MPa
10.70 2.65 0.48 0.43 0.76 Unspecified
12.39 3.84 0.09 0.02 0.85 Unspecified
15.07 1.49 0.02 0.07 1.58 Unspecified
R: rotor blade radius (length position from blade root to tip).
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for the spar cap and the web using standard material properties for the composite and the adhesive materials as well as an assumed bond line width of 100 mm. The dominating stress is in length direction (1) of the blade. Furthermore, stresses—about 10% to 25% of the longitudinal—in the transverse direction (2) exist. The third significant stress is the shear loading (up to 1.6 MPa). A summary of the calculated stresses from the blade analysis are shown in Table 1. The loadings in the adhesive at radius R¼10 m was chosen to define the stress state for beam development.
5. The Henkel-UpWind-beam The Henkel-UpWind-Beam was developed in a close collaboration between the Henkel AG and the Fraunhofer IWES and the fatigue tests performed and presented were conducted in the framwork of the UpWind [11] project. This sub-component represents the bond line loadings in wind turbine blades. Fig. 3 shows the schematic design of the Henkel-UpWind-Beam. The cross section is mechanically similar to I-beams, commonly known from steel structures. In direct comparison to steel I-beams the web is wider in order to adjust the bond line shear stress to a representative value. In order to have a constant loaded beam section, the spar caps were tapered, resulting in a linear thickness change of the spar cap thickness. Fig. 4 shows the spar cap lay up of the HenkelUpWind-Beams. The ply drops had a fingered design to increase the fatigue strength. With the design information (see Figs. 3 and 4) and material properties (see Table 2) for the composite materials as well as the adhesive, the stresses and strains in the beam structure can be calculated. The used material properties are typical for the materials in the industry; therefore they did not need experimental validation. The adhesive used was MGS Paste 135 G3 from Momentive Speciality Chemicals (Germany) and was processed in a hand mixing and application process. The results of the stress calculation according to beam theory (moments of inertia for discrete cross sections) are shown in Fig. 5. In comparison to the blade loadings in Table 1, an increase in the shear stress by a factor of two can be found. This factor was chosen to compensate for possible higher shear stresses in blades with a different bond line design. The beams were loaded with a hydraulic actuator (see Fig. 6). The load introduction points were realized with height strength steel bolts that were press fitted into the beam. They were 80mm
3.4mm … 10.4mm
26mm
134.6mm … 148.6mm
3.4mm … 10.4mm 32mm Spar Cap
Bond Line
Web (Sandwich material)
Fig. 3. Schematic design of the Henkel-UpWind-Beam (left: side view, right: cross section).
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F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
80mm
80mm 28mm
28mm 100mm
30mm
480 (triangular cut from 450 30mm) 510 (triangular cut from 480 30mm) 540 (triangular cut from 510 30mm) 570 (triangular cut from 540 30mm) 600 (triangular cut from 570 30mm) 630 (triangular cut from 600 30mm) 790 (triangular cut from 690 100mm) 890 (triangular cut from 790 100mm) 690 (triangular cut from 660 30mm) 660 (triangular cut from 630 30mm)
0
250
500
750
1000
1250
1500
Material: Saertex1010g/m² UD; t = 0.7mm Saertex840g/m² +/-45; t = 0.65mm
Fig. 4. Spar Cap design of the Henkel-UpWind-Beam.
Table 2 Selected static material properties for Henkel-UpWind-Beam calculation.
Composite UD Composite 7 451 Adhesive PVC core
Material Type
E11 (MPa)
Material description
Anisotropic Anisotropic Isotropic Isotropic
36000 12000 4800 300
Saertex 1010 g/m2 non crimped fabric, Momentive RIM 135 resin Saertex 840 g/m2 non crimped fabric, Momentive RIM 135 resin Momentive MGS Past 135 G3 Airex C 70
Fig. 5. Axial and shear stress of the adhesive in the Henkel-UpWind-Beam for a loading of 9.7 kN. Fig. 6. Henkel-UpWind-Beam during a cyclic test.
mounted with a rotational degree of freedom. Force, displacement and strain signals were recorded during the tests. The experimental investigation was carried out in two phases. In the first one, the fatigue behavior of the Henkel-UpWind-Beam was determined. In the second part, design, manufacturing and intrinsic flaw parameters were addressed along with their impact on the structural fatigue life of the Henkel-UpWind-Beam. In order to investigate the shear stress influence on the adhesive joint fatigue performance, three additional beam tests were performed. The bond line width was varied in order to change the axial to shear stresses ratio. Therefore, a 22 mm (two beams) and a 42 mm (one beam) width were chosen corresponding to 145% and 76% of the shear value of the reference design.
Examining different techniques to enhance the adhesive joint fatigue performance, a cover laminate was applied on top of the bond line, with two different procedures. Two beams were reinforced with hand laminate and with a pre-manufactured composite bracket, bonded on top of the bond line. In all cases, the reinforcement was one biaxial 745 layer, 840 g/ m2. Small defects i.e. voids and disbonds are common in wind turbine blade bond lines, occurring due to the manufacturing and the application process. Therefore, four beams with artificial flaws were manufactured so as to study their influence on the adhesive joint fatigue life. In two beams, 6 mm diameter cylindrical holes were drilled in the middle of the bond line in the highest loaded area. Furthermore, a notch was made on the hole surface
F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
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Table 3 Overview of conducted tests.
Henkel-UpWind-Beam Henkel-UpWind-Beam Henkel-UpWind-Beam Henkel-UpWind-Beam Henkel-UpWind-Beam Henkel-UpWind-Beam
Bond line width (mm)
Number of tests
32 22 42 32 32 32
8 2 1 1 3 2
Short description of defects
Artificial void (6 mm diameter drilled hole) Cover laminate (1 premanufactured laminate; 2 hand laminate) Artificial disbond (1 18 mm wide; 1 1 cm2)
Peak to Peak Axial Stress [MPa]
Fig. 7. Schematic illustration of tested beam variants.
enhancing geometric stress concentrations. Additionally, one beam was manufactured with an incipient interface disbond, of 18 mm width, resulting from a Teflon stripe placed over the entire bond line width during the beam manufacturing and one with a 1 cm2 disbonding (resulting from a 1 cm 1 cm big Teflon piece placed in the middle of the spar cap). An overview of the conducted fatigue tests can be found in Table 3. A schematic illustration of the tested beam variants is given in Fig. 7. The midpoint of the artificial defects was located at x¼790 mm from the beam end (see Fig. 3).
60 50 y = 40.92x-1/13 30 20
10 1E+03
1E+04
1E+05 Cycle
1E+06
1E+07
UD/32 MGS Paste 135 G3
6. Experimental results
Fig. 8. Henkel-UpWind-Beam reference SN-line (R-ration: 1).
Spar Cap 1. Axial Cracks
Adhesive Web Spar Cap
2. Start of Interface Failure
Adhesive Web
3. Interface Failure
Spar Cap Adhesive Web
Fig. 9. Failure development in Henkel-UpWind-Beams with 32 mm bond line width.
Fig. 8 shows the SN-line of the Henkel-UpWind-Beams with 32 mm bond line width. The S–N slope parameter m is 13, higher than the standard proposed from the GL for a unidirectional glass laminate. Final failure was considered when the beam could not sustain further loading due to severe damage on the bond line e.g. macroscopic separation between shear web and spar caps. The failure pattern evolution observed in all beam tests is described in Fig. 9. Transverse cracks initiated along the bond line due to the high tensile stresses around 10% of the structure fatigue life. These developed until saturation at around 60% of the component life. Then, interface failure started to develop at the crack tip, between the adhesive and the surrounding web or spar cap laminate. The ultimate failure occurred when horizontal cracks meet each other after propagation. The number of cracks found in the critical beam section was in average 20, resulting in a crack density of 0.5 cracks/cm. However, a very large scatter between 3 and 31 cracks overall was observed. No clear tendency between
F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
Normal Distribution for ε = 0.5%
60 50
y = 40.92x-1/13
30
Rate
Peak to Peak axial Stress [MPa]
134
20 0 100
10 1E+03
1E+04
1E+05 Cycle
1E+06
UD/22 MGS Paste 135 G3
1E+07
1000
Normal distribution -reference 42mm bond line width Delamination -18mm
10000 Cycles Reference +/-45°Cover Laminate Delamination 1cm²
100000
1000000
22mm bond line width Hole
Fig. 13. Henkel-UpWind-Beam—statistic evaluation.
UD/42 MGS Paste 135 G3
Peak to Peak Axial Stress [MPa]
Fig. 10. Henkel-UpWind-Beam SN curve—influence of bond line thickness.
60 50 y = 40.92x-1/13 30 20
10 1E+03
1E+04
1E+05 Cycle
45°premanufactured part MGS Paste 135 G3
1E+06
1E+07
45°hand laminat MGS Paste 135 G3
Peak to Peak Axial Stress [MPa]
Fig. 11. Henkel-UpWind-Beam SN curve—influence of cover laminates.
60
configuration (from Fig. 8) is also given. Fatigue life is significantly boosted with a factor in the range of 30 for high cycle and 50 for low cycle fatigue. However, it should be mentioned that final failure is not directly comparable to the reference beams. Instead of failing in the highest loaded cross section the beam failure was found between the supports. Fig. 12 shows a comparison between the different beam tests with artificial flaws and the reference SN-line. The specific flaws have a negligible impact on the fatigue life. The evaluation of the present results should be based on statistical methods. Therefore, all test results were transformed to a strain of 0.5% (resulting in a stress of 24 MPa) using the slope of the reference SN-line and the corresponding normal distribution for the reference specimens was determined. Fig. 13 shows a logarithmic plot of the ’’events’’ (final failures over number of cycles). Furthermore, the occurrence rate over cycles, using a normal distribution, for the resulting fatigue performance of the reference tests is given. The performance of most of the tested variants performed can be seen within the scatter of the reference tests, however, the test with the 42 mm bond line and the tests with the cover laminates are exceeding typical confidentiality intervals.
50 y = 40.92x-1/13
7. Conclusion
30 20
10 1E+03
1E+04
zylindric holes
1E+05 Cycles 18mm wide disbond
1E+06
1E+07
1cm2 disbond
Fig. 12. Henkel-UpWind-Beam SN curve—influence of defects.
beam life and the number of cracks was found, although the beam with the shortest life showed only 3 cracks. In Fig. 10, it is obvious that all extra experimental data are fitting quite accurately on the reference SN-line (indicated by the best fit curve from Fig. 8), meaning that at least for these complex stress fields, the axial stresses are dominating the structural fatigue life of the component. Shear stresses seem to be of minor importance. Fig. 11 shows the fatigue life of the beam variant with cover laminates. For comparison, the reference the reference
A sub-component test was introduced aiming to replicate critical complex stress states induced in wind turbine rotor blade structures. The specific design was meant to imitate shear web to spar caps adhesive joints, taking under consideration the adhesive representative material volume i.e. bond line width and thickness (10 mm) and geometrical concentration factors i.e. bond line geometry profile. The specimen was used for a parametric study, investigating the influence of design variables i.e. axial to shear stresses ratio, manufacturing techniques i.e. enhancing the bond line with cover laminates along with intrinsic flaws presence on the adhesive joint fatigue life. The dominant stress component defining the adhesive joint failure is found to be the axial one for axial to shear stress ratio from 4 to 8. Single voids with surface stress concentrators seemed also to have negligible effect on the structural fatigue life. A cover laminate on the bond line had a significant influence on the joint life, resulting in an increase of a factor of 50 at low cycles and of 30 at high cycles. References [1] Germanischer Lloyd, Klassifikations- und Bauvorschriften II Werkstoffe und Schweißtechnik – 2 Nichtmetallische Werkstoffe; Edition 2002.
F. Sayer et al. / International Journal of Adhesion & Adhesives 37 (2012) 129–135
[2] Det Norske Veritas; DNV Standard DNV-DS-J102; Edition October 2010. [3] International Electrical Commission, IEC 61400-1 Wind Turbines – Part 1: Design Requierments, 3rd ed. 2005–08. ¨ die Zertifizierung von Windenergieanla[4] Germanischer Lloyd, Richtlinien fur gen, Edition 2003. [5] Sayer F, Post N, van Wingerde A, Busmann H-G, Kleiner F, Fleischmann W, et al.; Testing of Adhesive Joints in the Wind Industry; European Wind Energy Conference and Exhibition, 16–19 March 2009, Marseille, France. [6] Kickert R, Weerts U, Meister O, Knops M. Fatigue Evaluation of Structural Bondings. DEWEK; 2010. [7] Sorensen BF, Jorgensen K, Jacobsen TK, Ostergaard Rasmus; A general mixed mode fracture mechanics test specimen: The DCB-specimen loaded with
[8]
[9]
[10]
[11]
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uneven bending moments ; Risø-R-1394(EN); /http://130.226.56.153/ris publ/afm/afmpdf/ris-r-1394.pdfS; November 2011. Samborsky DD, Sears AT, Mandell JF, Kils O Static and Fatigue Testing of Thick Adhesive Joints for Wind Turbine Blades; 2009 ASME Wind Energy Symposium. Sayer F, Kleiner F, Antoniou A, Trusheim M, van Wingerde A. Sub-Component Testing for Adhesive Bond lines for Wind Turbine Blades. Bremen: DEWEK 2010; 2010. Philippidis TP: Quantification of Complex Stress State Effect on Blade Design; Optimat-Blade Report: OB_TG2_R032_ rev. 000; /http://www.kc-wmc.nlS; August 2010. UpWind-project; /http://www.upwind.eu/S; November 2011.