Ultrasound inspections on glass fiber/phenolicresin and on carbon fiber/epoxy resin composites

during flexural fatigueV.G. García, J. Sala, L. Crispí, J.M. Cabrera, A. Istúriz, A. Sàez, M.

Millán, C. Comes, D. Trias

Composites

1. Introduction: Outline

2. Experimental Procedure:

Materials inspected and tested

Ultrasound equipment

Ultrasound visualization software

4-Point bending fatigue tests

3. Results:

Wöhler plot

Ultrasound scans at N=10,000 cycles

Ultrasound scans at N=12,000 cycles

A fracture at N=13,344 cycles

4. Conclusions

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2. Experimental Procedure

Fig. 1. Dimensions of the glass fiber/phenolic resin bars

Fig. 2. Dimensions carbon fiber/epoxy resin bars

Fig. 3. Ultrasound inspections every 2,000 cycles.

Fig. 4. 4-Point bending flexural fatigue.

Until 20,000 cycles.

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2. Experimental Procedure: Materials inspected and tested

The Glass Fiber Reinforced Polymer (GFRP) was Isovid G-3 manufactured by Composites Ate.

Fig. 6. GFRP sample after exposure to sun light.

-Isovid G-3 consists of 200g/m2 plain weave E-glass, and a modified phenolic resin that enhances flame retardant characteristics.-Layers are 0.10 to 0.11mm thick.-All plies were stacked to match 0º and 90º.-Composite was high speed milled to reach dimensions.

Fig. 5. GFRP samples.

Fig. 7. Woven appearance.

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Table 1. Stacking sequences and percentages of layers oriented 90º, 0º or ±45º.

2. Experimental Procedure: Materials inspected and tested

Fig. 9. Smooth, rough, and machined surfaces of CFRP

samples.

Fig. 8. CFRP samples.

The Carbon Fiber Reinforced Polymer (CFRP) was manufactured using Hexcel 8552/32%/134/IM7(12K) at the INTA Materials and Structures Department.

-The 8552/32%/134/IM7(12K) pre-impregnated carbon fiber plies consisted of tows of 12,000 individual fibers of IM7 intermediate modulus carbon.-The pre-preg contained 32% of 8552, an amine cured, toughened epoxy resin system.-The nominal ply thickness was 134µm.-Measured ply thickness was 129µm to 134µm.

Specimens Stacking sequence %90º

%0º

%45º

%-45º

# layers

CFRP-P011 [(0/±45/02/±45/02/90) / ( 02/±45/02/±45/02/ )12]S

8.8 54.4 18.4 18.4 283

CFRP-P021 [(0/90) / ( 02/±45/02/±45/02/ )9]S

9.5 54.7 17.9 17.9 201

CFRP-P041 [(0/90/0/±45/02/90) / ( 02/±45/02/±45/02/ )2]S

11.9 54.2 16.9 16.9 59

90

90

90

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2. Experimental Procedure: Ultrasound equipment

Fig. 10. Ultrasound equipment built by Mapro using a Socomate USPC3100LA card.

Fig. 11. A pulse-echo inspection in an immersion bath.

-A single crystal longitudinal wave transducer of 1MHz and 13mm diameter was used for most inspections.-The gain was set at 5dB.-Specimens CFRP-P041 were inspected using a 5MHz (10mm diameter) transducer at 5dB.-Acoustic wave propagation was set at 3275m/s.-Mapro developed a software, based on LabView, to process the pulse-echo signals and visualize A-scans, B-scans, C-scans, plus optional ∫-scans.

X (scan)

Y (In

dex)

Fig. 12. Before an inspection the transducer is positioned at the (0,0)

origin.

Fig. 13. Inspection of a GFRP specimen.

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2. Experimental Procedure: Ultrasound visualization software

Fig. 14. Screen image of the visualization software, after loading the ultrasound signal data. 7/18

2. Experimental Procedure: Ultrasound visualization software

Fig. 15. Screen image after: digitally aligning the first peak, positioning the cut off lines, and setting a color range.8/18

2. Experimental Procedure: Ultrasound visualization software

Fig. 16. In this study the scale of the signal intensity (potence %) and cut off lines were set as shown above. 9/18

2. Experimental Procedure: Ultrasound visualization software

Interface (I) echo

Back-wall (B) echo

Fig. 17. In this study the A-scans was passed through different algorithms to create alternative scan images. 10/18

2. Experimental Procedure: Ultrasound visualization software

C-scan minus I & B

∫-scan with I & B

∫-scan minus I & B

∫-scan with I & B minus mean

∫-scan minus I & B minus mean

Fig. 18. A menu in the C-scan label allows processing the A-scan to create different ∫-scans. 11/18

2. Experimental Procedure: Ultrasound visualization software

Fig. 19. Filtered A-scan has less peaks. 12/18

2. Experimental Procedure: 4-Point bending fatigue tests

Flexural Stressσ = (3FL)/(4Wh2)F→ force,L → support length,W → specimen width,h → specimen thickness.

L

L

The maximum strength σ0 for the GFRP was

391MPa, and for the CFRP was 1135MPa.

L

L4

L4

S F2

F2

F2

F2

Fig. 20. Loading diagram according to ASTM D6272-02 (2008).

Fig. 21. GFRP specimen tested until fracture to determine σ0.

Fig. 22. CFRP specimen tested until fracture to determine σ0.

Normalized fatigue stresses for GFRP specimens was

set at 0.2 to 0.9 and for the CFRP specimens at 0.36 to

0.60.

Normalized fatigue stress is defined as σ0 / σmax where

σmax is the maximum flexural fatigue stress in the outer

layers.

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3. Results: Wöhler plot

0 2 4 6 8 10 12 14 16 18 20

-800

-600

-400

-200

0

200

400

600

4-point bending tests

f = 0,8 HzR=0,1

CFRP-P011-40-1440-38_1/2 CFRP-P011-40-1440-38_2/2 CFRP-PO21-40-1040-27_1/2 CFRP-P021-40-1040-27_2/2 CFRP-P041-40-307-8_2/2

Max

imum

stre

ss, σ

max

(MPa

)

Number of cycles (x1000)

0 2 4 6 8 10 12 14 16 18 20

-400

-300

-200

-100

0

100

200

300

400

4-point bending tests

GFRP-P031-40-710-16_2/6GFRP-P031-40-710-16_3/6GFRP-P031-40-710-16_4/6GFRP-P031-40-710-16_5/6GFRP-P031-40-710-16_6/6GFRP-P041-40-307-8_1/2GFRP-P041-40-307-8_2/2

f = 0,8 HzR = 0,1

GFRP-P011-40-1000-38_1/3GFRP-P011-40-1000-38_2/3GFRP-P011-40-1000-38_3/3GFRP-P021-40-1000-27_1/3GFRP-P021-40-1000-27_2/3GFRP-P021-40-1000-27_3/3GFRP-P031-40-710-16_1/6

M

axim

um s

tress

, σm

ax (M

Pa)

Number of cycles (x1000)

Fig. 23. Maximum stresses during fatigue every 2,000 cycles for the CFRP specimens.

Fig. 24. Maximum stresses during fatigue every 2,000 cycles for the GFRP specimens.

Fig. 25. Maximum stresses versus cycles to failure of Isovid G-3 GFRP.

235 MPa

100 101 102 103 104 1050

50

100

150

200

250

300

350

400

Max

imum

stre

ss, σ

max

(MPa

)

Isovid G-3:Plain weave 200g/m2 fiber glasswith flame retardant phenolic resin

4-pointbendingfatigue tests,R=0,1f=0,8Hz

Number of cycles to failure, N

13,344 cycles

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3. Results: Ultrasound scans at N=10,000 cycles

Fig. 27. At N=10,000 cycles. Fig. 28. At N=10,000 cycles with signal filter.

Without filtering the A-scan. After filtering the A-scan.

A-scan with the interface (I) and back-wall (B) echoes.

C-scan minus I & B

∫-scan with I & B

∫-scan minus I & B

∫-scan with I & B minus mean

∫-scan minus I & B minus mean

Fig. 26. Specimen GFRP-P031-40-710-16_4/6 inspected only between the white lines.

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3. Results: Ultrasound scans at N=12,000 cycles

Fig. 29. The triangle in specimen GFRP-P031-40-710-16_4/6 points to the damaged area found in the scans.

Fig. 30. At N=12,000 cycles. Fig. 31. At N=12,000 cycles with signal filter.

C-scan minus I & B

∫-scan with I & B

∫-scan minus I & B

∫-scan with I & B minus mean

∫-scan minus I & B minus mean

Without filtering the A-scan. After filtering the A-scan. 16/18

3. Results: A failure at N=13,344 cycles

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Fig. 32. Specimen GFRP-P031-40-710-16_4/6 after fracture at N=13,344 cycles.

(d)

(c)

(b)

(a)

4. Conclusions

-The additional information (i. e. newer spots) provided by the ∫-scans stillrequire further corroboration with the actual internal damage.

-However, these algorithms allow revealing both superficial and internaldamage in one single scan, and ultimately may improve damagedetection of composite materials.

-The fatigue behavior of a glass fiber/phenolic resin composite wascharacterized by means of a partial Wöhler plot.

-CFRP specimens only showed superficial damage after the fatigue testsof this study.

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Thank you for your attention.