2009 ASME Wind Energy Symposium Static and Fatigue Testing of Thick Adhesive Joints for Wind Turbine Blades Daniel Samborsky, Aaron Sears, John Mandell, Montana State University and Ole Kils Clipper Windpower Technology, Inc. Technical Monitor Tom Ashwill, Sandia National Laboratories

Outline Background, justification for study Static strength test results Fatigue test results Failure modes Finite element modeling of strain concentration and pore effects Conclusions

Background Reliability issues with adhesive joints in blades Lack of literature data for thick paste adhesives with composite adherends, blade joint geometries –Closest case: general aviation adhesive joint study by Tomblin, et. al., Wichita State, several FAA reports (Ref. 2, 6-8).

Purpose of Study Explore typical blade joint performance Test a large number (>250) of coupons representative of blade joints, four related geometries. Coupons supplied by Clipper. –Normalized static strength statistics (two test rates) –Fatigue lifetime exponents, R = 0.1 and -1 –Failure modes and flaws –FEA based understanding of strain concentration and flaw interactions Test results to be included in DOE/MSU Fatigue Database.

Wedge half angle, θ: Geometry A: 45 o Geometry B: 90 o Geometry C: 45 o Reinforced Geometry D: 90 o Reinforced (Two joints per coupon)

Geometry and location of points of interest and line plot axis. Nominal Dimensions: Width 50 mm Adhesive thickness 4 mm Adherend thickness 4 mm (Adherend ±45 glass/epoxy) Wedge block: ±45 laminate oriented coplanar with adherend

Static Test Results at Two Displacement Rates

Fig. 3. Edge views of typical failed specimens of Geometries A, 45 o (right) and B, 90 o. Cracks initiated in adhesive on left side, propagated toward the right, through the adhesive and into the adherend.

Figure 4(a). Static strength, Geometry A (45 o ). Fig. 4 Static Strength Distributions

Figure 4(b). Static strength, Geometry B (90 o )

Figure 4(c). Static strength, Geometry C (45 o, Reinforced)

Figure 4(d). Static strength, Geometry D (90 o Reinforced).

Static normalized strength data (Normalized by the Geometry A static mean strength,.025 mm/s) Geometry Test Rate (mm/s) Normalized Mean Strength 95/95 Normalized Strength S.D.COV (%) No. Coupons n A (45 o ) A (45 o ) B (90 o ) B (90 o ) C (45 o -R) C (45 o -R) D (45 o -R) D (45 o -R)

Observations, Static Tests Reinforced Geometries C and D are much stronger with lower COV compared with unreinforced Geometries A and B. Geometries A and B show similar average strength, but 95/95 strength 17% lower for Geometry B due to few weak specimens with poorly cured adhesive. No significant effect of test rate.

Fatigue Test Results Geometries A and B: tensile fatigue (R = 0.1) only; reversed loading not tested due to buckling of thin adherends. Geometries C and D : both tensile fatigue and reversed (R = -1) loading.

Tensile Fatigue (R = 0.1) Curve Fit Parameters Average Normalized* Static Strength Normalized* Strength At 10 6 Cycles Fatigue Curve Exponent, B Fatigue Curve Exponent, n Geometry A Geometry B Geometry C Geometry D F/F o = A N B B = -1/n

Figure 5. Tensile fatigue data and curve fits for Geometries A and B, R = 0.1

Figure 6. Tensile (R = 0.1) and reversed (R = -1) load fatigue data for Geometry C

Figure 7. Tensile (R = 0.1) and reversed (R = -1) load fatigue data for Geometry D

Tensile Fatigue (R = 0.1) and Reverse Loading ( R = -1) Curve Fit Parameters, Reinforced Geometries Average Normalized* StaticTensile Strength Normalized* Strength At 10 6 Cycles Fatigue Curve Exponent, B Fatigue Curve Exponent, n Geometry C R = Geometry C R = -1** Geometry D R = Geometry D R = F/F o = A N B B = -1/n *Normalized by Geometry A slow static strength **Shift to interlaminar adherend failure mode, Geometry C, R = -1

Fatigue Observations Fatigue exponents generally show reduced fatigue sensitivity compared to typical fiberglass laminates: n>10 except for adherend failures (Geom. D, R = -1). Geometries A and B have similar fatigue performance. Reinforced Geometries C and D are relatively more fatigue sensitive, but still retain significantly higher strength at 10 6 cycles. Reversed loading causes reduced fatigue strength compared with tensile fatigue, Geometries C and D. Failure in the adherend more likely due to reduced fatigue resistance at R = -1.

Flaws and Failure Modes Flaws which produced reduced strength and fatigue life: –Pores, common in most joints. –Poorly cured adhesive (sticky to the touch on fracture surfaces); a few specimens, Geometries A and B only. (Hand mixed adhesive). –Un-bonded or partially bonded regions at adhesive/adherend interface, Geometries C and D only. –Large pores in adherend surface adjacent to interface, rare.

Failure Modes Geometries A and B: Crack origins cohesive in the adhesive near corner Point A for Geometry A, mostly near point B (away from the corner) at pores for Geometry B. Crack growth across the adhesive, then into adherend, interlaminar (Fig. 3). Fatigue modes similar to static. Few weakest specimens showed poorly cured adhesive, Geometries A and B.

Failure Modes Geometries C and D (Reinforced): –Many weaker specimens failed at poorly bonded areas at reinforcement/adhesive interface, and at adhesive/block interface. –For reversed loading fatigue, extensive interlaminar cracking preceded failure for Geometry C only.

Geometry and location of points of interest and line plot axis.

Fig. 3. Edge views of typical failed specimens of Geometries A, 45 o (right) and B, 90 o. Cracks initiated in adhesive on left side, propagated toward the right, through the adhesive and into the adherend.

Figure 9. Fracture surfaces of Geometry A static specimens, Point A (Fig. 2) at bottom. Left: stronger than average specimen, no major flaws, fails from Point A; Center: weaker specimen, two large pores along edge of adhesive at Point A; Right: weakest specimen, poorly cured adhesive (cohesive mode over entire surface).

Finite Element Results, Geometries A and B Only: Geometric and Pore Interactions

Figure 10. Maximum tensile strain distribution for Geometry A, No Pores.

Max. Tensile Strain vs. Distance from Point A, Four Wedge Block Angles (Geometry A: 45 o ; Geometry B: 90 0 ; 30 o and 60 o not tested)

FEA Results, No Pores Local strain concentration at Point A. Most crack origins near Point A for Geometry A. Strains significantly lower for Geometry B than for Geometry A near Point A. Expect Geometry B to be stronger than Geometry A, but experimental data show similar strength. Crack origins for Geometry B observed to be mostly near Point B, at pores.

Figure 13 Typical pore geometries studied: ellipse, circle, intersecting circle; position varied.

Figure 14. Typical mesh pattern around hole and corner.

Strain Distribution along x from Point A with 12.5 mm dia. hole having different offsets from Point A

FEA Results, Geometry A, Hole offset from Point A Maximum strain still at Point A, increased as pores approach stress concentration point. Consistent with experimental results for crack origins, Geometry A. Strength dominated by geometric strain concentration at Point A, reduced by pores located very close to point A.

FEA, Geometry B with various circular and elliptical pores

Figure 16. Tensile strain distribution at small elliptical hole in Geometry B specimen near Point B in Fig. 2.

Fig. 15. Strain distribution along x from Point A with 2.5 mm dia. pores having different offsets of the pore center from Point A

Figure 17. Maximum tensile strain for elliptical holes, Geometry B, plotted along block interface and near Point B in Fig. 2.

FEA Results, Geometry B, Pore Effects Maximum strains occur at pore boundaries, exceed strains at geometric strain concentration Point A. Experimental results show failure origins at pores distributed around Point B. Strength dominated by pore size and location.

Conclusions Results have been presented for over 250 static and fatigue tests on two basic thick adhesive joint geometries and two reinforced geometries. Basic Joint Geometries A (45 o ) and B 90 o ) have about the same mean static strength, which is rate insensitive. Several poorly cured specimens increased the scatter in B, reducing the 95/95 strength. Most lower strength specimens for both geometries were associated with pores near the stress concentration point for A, but above this location, for B. Fatigue properties were similar for both geometries.

Reinforced Geometries C (45 o ) and D (90 o ) were significantly stronger than the corresponding unreinforced geometries, with lower COV’s. C and D showed moderately increased fatigue sensitivity, but the 10 6 cycle strengths remained significantly higher than for A and B. The primary flaw type was poorly bonded interfacial areas. Reversed loading produced greater fatigue sensitivity for Geometry C, and a shift to interlaminar failure in the adherend. Geometries C and D showed reduced 10 6 cycle fatigue strength in reversed loading compared to tension fatigue.

FEA and specimen fracture surfaces indicate that joint strength for Geometries A and B is a function of the interaction of the geometric stress concentration at the corner of the adhesive, and the location and severity of pores. Failure may occur primarily at pores in the broader high stress region (Geometry B) or at the corner, with the strength decreased by near-by pores (Geometry A). This difference may be due in part to the reduced stress concentration for Geometry B. Sharpness of the adhesive corner is also an issue; FEA assumes a perfectly sharp corner