Mechanical behaviour of thick structural adhesives in wind turbine blades under multi-axial loading

Figures & data

Figure 1. A cross section of a WTB with the bonded joints highlighted.

Figure 2. The curing process of the tubes.

Figure 3. The adhesive tubular specimens.

Figure 4. The experimental set-up.

Table 1. The tensile elastic properties of the adhesive tube.[18]

	Et (MPa)	Strength (MPa)	Failure ε (μstrain)	vt
Mean	5412	48.2	10,228	0.398
st.dev.	141.8	5.4	1151	0.018
c.o.v %	2.6	11.2	11.3	4.4

Table 2. The compressive elastic properties of the adhesive tube.[18]

	Ec (MPa)	Strength (MPa)	Failure ε (μstrain)	vc
Mean	5638	–97.7	–27,784	0.397
st.dev.	255.5	3.7	3283	0.009
c.o.v. %	4.5	–3.8	11.8	2.2

Table 3. The shear elastic properties of the adhesive tube.[18]

	G (MPa)	Strength (Mpa)	Failure γ (μstrain)
Mean	1510	37.9	24,501
st.dev.	64.6	3.7	8295
c.o.v. %	4.3	9.8	33.9

Table 4. The biaxial failure loads and stress.

Specimen identification	Ratio axial to shear stress	Load (kN)	Torque (Nm)	Axial stress (MPa)	Shear stress (MPa)
T01_020	5/1	12.3	33	48.6	9.7
T01_021	5/1	9.2	22	53.3	11.2
T01_022	5/1	12.6	33	56.4	11.8
T01_008	1/1	5.2	67	24.7	24.4
T01_023	1/1	6.4	82	29.4	29.8
T01_025	1/1	7.6	98	35.7	36.1
T01_027	–1/1	–10.0	120	–43.6	43.8
T01_028	–1/1	–9.3	119	–44.0	44.4
T01_030	–1/1	–9.5	121	–43.6	44.5
T01_024	–2.5/1	–16.5	85	–74.3	30.1
T01_026	–2.5/1	–16.6	86	–77.9	31.7
T01_029	–2.5/1	–16.1	83	–75.2	30.4

Figure 5. The tension stress–strain response for the biaxial ratios.

Figure 6. The compression stress–strain response for the biaxial ratios.

Figure 7. The shear stress–strain response for the biaxial ratios.

Table 5. The model parameters for different biaxial stress ratios.

Biaxial ratio	Et0	Ec0	G0	σ0	τ0	nt	nc	ns
1/0	5412	–	–	80.0	–	2.50	–	–
5/1	5412	–	1540	70.0	18.0	2.45	–	2.00
1/1	5412	–	1540	42.0	32.0	2.35	–	2.80
0/1	–	–	1540	–	43.0	–	–	2.85
–1/1	–	5638	1540	–55.0	48.0	–	3.00	2.83
–2.5/1	–	5638	1540	–79.0	34.0	–	3.22	2.80
–1/0	–	5638	–	–108.0	–	–	3.40	–

Figure 8. Compression modulus degradation as a function of the stress level for the biaxial ratios.

Figure 9. Shear modulus degradation as a function of the stress level for the biaxial ratios.

Table 6. Degraded elastic properties of the adhesive.

Failure of the adhesive elements D≥1	Material property degradation
Elastic modulus	E ≈ 0
Shear modulus	G ≈ 0
Poisson’s ration	V ≈ 0

Table 7. Failure experimental and numerical stresses.

Ratio axial to shear stress	Experiments		FEM		Difference
	Failure axial stress (MPa)	Failure shear stress (MPa)	Failure axial stress (MPa)	Failure shear stress (MPa)
1/0	46.5	0	48.2	0	3.5%
1/0	56.2	0	48.2	0	14.2%
1/0	48.8	0	48.2	0	1.2%
1/0	41.2	0	48.2	0	14.5%
5/1	48.6	9.7	48.6	9.7	0%
5/1	53.3	11.2	48.6	9.7	8.8% / 13.4%
5/1	56.4	11.8	48.6	9.7	13.8% / 17.8%
1/1	24.7	24.4	32.6	32.6	24.2% / 25.2%
1/1	29.4	29.8	32.6	32.6	9.8% / 8.6%
1/1	35.7	36.1	32.6	32.6	8.7% / 9.7%
0/1	0	38.4	0	37	3.6%
0/1	0	31.8	0	37	14.1%
0/1	0	41.1	0	37	9.9%
0/1	0	40.4	0	37	8.4%
−1/1	−43.6	43.8	−39	39	10.6% / 11%
−1/1	−44.0	44.4	−39	39	11.3% / 12.2%
−1/1	−43.6	44.5	−39	39	10.6% / 12.4%
−2.5/1	−74.3	30.1	−69.5	27.8	6.4% / 7.6%
−2.5/1	−77.9	31.7	−69.5	27.8	10.8% / 12.3%
−2.5/1	−75.2	30.4	−69.5	27.8	7.6% / 8.5%
−1/0	−99.4	0	−97.8	0	1.6%
−1/0	−100.0	0	−97.8	0	2.2%
−1/0	−99.0	0	−97.8	0	1.2%
−1/0	−99.7	0	−97.8	0	1.9%
−1/0	−90.4	0	−97.8	0	7.6%
−1/0	−97.7	0	−97.8	0	0.1%

Figure 10. The flowchart of the progressive damage scenario.

Figure 12. Comparison between the experimental observations and numerical results for the 1/0 loading ratio (uniaxial tension).

Figure 13. Comparison between the experimental observations and numerical results for the 5/1 loading ratio (biaxial tension–torsion).

Figure 14. The biaxial failure envelope.

Zarouchas DS, Nijssen RPL, van Delft DRV. Failure analysis of structural adhesives in wind turbine blades under multiaxial loading. Paper presented at DURACOSYS Conference. 2012 Sep 17–19; Brussels, Belgium.

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