Ultrasound inspections were performed on glass fiber/phenolic resin and carbon fiber/epoxy resin composites during flexural fatigue testing. Specimens were scanned using ultrasound equipment after 10,000 and 12,000 cycles, revealing initial damage in the glass fiber composite. A glass fiber specimen failed at 13,344 cycles, showing a fracture. The study characterized the fatigue behavior of the materials and showed ultrasound can detect early stage damage during flexural fatigue testing of fiber reinforced polymer composites.

1. Ultrasound inspections on glass fiber/phenolic
resin and on carbon fiber/epoxy resin composites
during flexural fatigue
V.G. García, J. Sala, L. Crispí, J.M. Cabrera, A. Istúriz, A. Sàez, M.
Millán, C. Comes, D. Trias
Composites

2. 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
2/18

3. 2. Experimental Procedure
Fig. 1. Dimensions of the glass fiber/phenolic resin bars
Fig. 3. Ultrasound inspections
every 2,000 cycles.
Until
20,000
Fig. 2. Dimensions carbon fiber/epoxy resin bars cycles.
Fig. 4. 4-Point bending flexural fatigue. 3/18

4. 2. Experimental Procedure: Materials inspected and tested
The Glass Fiber Reinforced Polymer (GFRP) was Isovid G-3
manufactured by Composites Ate.
-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. 6. GFRP sample after Fig. 7. Woven appearance.
exposure to sun light.
4/18

5. 2. Experimental Procedure: Materials inspected and tested
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.
Fig. 8. CFRP samples. -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.
Table 1. Stacking sequences and percentages of layers oriented 90º, 0º or ±45º.
% % % % #
Specimens Stacking sequence
90º 0º 45º -45º layers
[(0/±45/02/±45/02/90) / (
CFRP-P011 8.8 54.4 18.4 18.4 283
02/±45/02/±45/02/90)12]S
[(0/90) / (
Fig. 9. Smooth, rough, and CFRP-P021 9.5 54.7 17.9 17.9 201
02/±45/02/±45/02/90)9]S
machined surfaces of CFRP [(0/90/0/±45/02/90) / (
samples. CFRP-P041 11.9 54.2 16.9 16.9 59
02/±45/02/±45/02/90)2]S
5/18

6. 2. Experimental Procedure: Ultrasound equipment
-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,
Fig. 10. Ultrasound equipment built by Mapro B-scans, C-scans, plus optional ∫-scans.
using a Socomate USPC3100LA card.
Y (Index)
X (scan)
Fig. 11. A pulse-echo inspection in an Fig. 12. Before an inspection the Fig. 13. Inspection of
immersion bath. transducer is positioned at the (0,0) a GFRP specimen.
origin.
6/18

7. 2. Experimental Procedure: Ultrasound visualization software
Fig. 14. Screen image of the visualization software, after loading the ultrasound signal data. 7/18

8. 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

9. 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

10. 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

11. 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

12. 2. Experimental Procedure: Ultrasound visualization software
Fig. 19. Filtered A-scan has less peaks. 12/18

13. 2. Experimental Procedure: 4-Point bending fatigue tests
Flexural Stress
The maximum strength σ0
σ = (3FL)/(4Wh2)
for the GFRP was
F→ force, 391MPa, and for the
L → support length, CFRP was 1135MPa.
W → specimen width,
h → specimen thickness. Normalized fatigue stresses
L for GFRP specimens was
F F set at 0.2 to 0.9 and for the
Fig. 21. GFRP specimen tested until
2 2 CFRP specimens at 0.36 to
fracture to determine σ0.
0.60.
F L S L F Normalized fatigue stress is
2 4 4 2 defined as σ0 / σmax where
L
Fig. 20. Loading diagram σmax is the maximum flexural
according to ASTM fatigue stress in the outer
D6272-02 (2008). layers.
L
Fig. 22. CFRP specimen tested until
fracture to determine σ0. 13/18

14. 3. Results: Wöhler plot
600 400
Maximum stress, σmax (MPa)
GFRP-P011-40-1000-38_1/3 GFRP-P031-40-710-16_2/6
CFRP-P011-40-1440-38_1/2
Maximum stress, σmax (MPa)
400 CFRP-P011-40-1440-38_2/2 300 GFRP-P011-40-1000-38_2/3
GFRP-P011-40-1000-38_3/3
GFRP-P031-40-710-16_3/6
GFRP-P031-40-710-16_4/6
CFRP-PO21-40-1040-27_1/2 GFRP-P021-40-1000-27_1/3 GFRP-P031-40-710-16_5/6
200 GFRP-P021-40-1000-27_2/3 GFRP-P031-40-710-16_6/6
200 f = 0,8 Hz CFRP-P021-40-1040-27_2/2 GFRP-P021-40-1000-27_3/3 GFRP-P041-40-307-8_1/2
R=0,1 CFRP-P041-40-307-8_2/2 100 GFRP-P031-40-710-16_1/6 GFRP-P041-40-307-8_2/2
0
0 2 4 6 8 10 12 14 16 18 20 0 0 2 4 6 8 10 12 14 16 18 20
Number of cycles (x1000)
-200 Number of cycles (x1000)
-100
4-point bending tests
-400 -200
-600
-300 4-point bending tests
f = 0,8 Hz
-400 R = 0,1
-800
Fig. 23. Maximum stresses during fatigue Fig. 24. Maximum stresses during fatigue
every 2,000 cycles for the CFRP specimens. every 2,000 cycles for the GFRP specimens.
400
Maximum stress, σmax (MPa)
350
300 4-point
bending
250
fatigue tests, 235 MPa
200 R=0,1
150 f=0,8Hz 13,344 cycles
100 Isovid G-3:
50
Plain weave 200g/m2 fiber glass
with flame retardant phenolic resin
0
101 102 103 100104 105
Number of cycles to failure, N
Fig. 25. Maximum stresses versus cycles to failure of Isovid G-3 GFRP. 14/18

15. 3. Results: Ultrasound scans at N=10,000 cycles
Fig. 26. Specimen GFRP-P031-40-710-16_4/6 inspected only
between the white lines.
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
Fig. 27. At N=10,000 cycles. Fig. 28. At N=10,000 cycles with mean
signal filter.
Without filtering the A-scan. After filtering the A-scan.
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16. 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.
C-scan minus I & B
∫-scan with I & B
∫-scan minus I & B
∫-scan with I & B minus mean
∫-scan minus I & B minus
Fig. 30. At N=12,000 cycles. Fig. 31. At N=12,000 cycles with mean
signal filter.
Without filtering the A-scan. After filtering the A-scan.
16/18

17. 3. Results: A failure at N=13,344 cycles
(a)
(b)
(c)
(d)
Fig. 32. Specimen GFRP-P031-40-710-16_4/6 after fracture at
N=13,344 cycles.
17/18

18. 4. Conclusions
-The additional information (i. e. newer spots) provided by the ∫-scans still
require further corroboration with the actual internal damage.
-However, these algorithms allow revealing both superficial and internal
damage in one single scan, and ultimately may improve damage
detection of composite materials.
-The fatigue behavior of a glass fiber/phenolic resin composite was
characterized by means of a partial Wöhler plot.
-CFRP specimens only showed superficial damage after the fatigue tests
of this study.
18/18

19. Thank you for your attention.