Ultrasonics inspections and confocal microscopy to evaluate fatigue damage in fiber reinforced...
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Ultrasonics inspections and confocal microscopy to evaluate fatigue damage in fiber reinforced polymer composites V.G. García , J. Sala, L. Crispí, J.M. Cabrera, A. Istúriz, A. Sàez, M. Millán, C. Comes, D. Trias Composites
Ultrasonics inspections and confocal microscopy to evaluate fatigue damage in fiber reinforced polymer composites V. G. García
Ultrasonics inspections and confocal microscopy to evaluate fatigue damage in
fiber reinforced polymer compositesV.G. García, J. Sala, L. Crispí, J.M. Cabrera, A. Istúriz, A. Sàez,
M. Millán, C. Comes, D. Trias
Composites
Moderador
Notas de la presentación
My name is Víctor García and I will present the work titled “Ultrasonics inspections and confocal microscopy to evaluate fatigue damage in fiber reinforced polymer composites ”. This work shows part of the results obtained in the MENDesMaCo project, which was led by the Mapro company in collaboration with, the CTM Technological Centre, the Composites Ate company and AMADE among others members of the project.
3. Results: Wöhler plotUltrasound scans at N=4,000 cyclesUltrasound scans in at N=26,079 cyclesConfocal microscopy in a GFRP at N=26,079 cyclesConfocal microscopy in a CFRP at N=20,000 cycles
4. Conclusions
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Moderador
Notas de la presentación
Complete ultrasound inspections in Carbon Fiber Reinforced Polymers (CFRP) or in Glass Fiber Reinforced Polymers (GFRP) require careful analyses of several composed images (C-scan, B-scans, A-scans, etc.) to assure the integrity of a component. This study set out to improve damage detection, by creating alternative scan images that could summarize internal and superficial damage in one single image, and thereby reducing inspection analysis time. Additionally, when ultrasonics inspection indicated a superficial irregularity, a confocal microscope was used to evaluate, and if possible, quantify the damage. Two different composite materials were subjected to 4-point bending fatigue tests until reaching 20,000 cycles. The fatigue tests were stopped every 2,000 cycles to be inspected by a pulse-echo ultrasound technique. After the 20,000 cycles one GFRP specimen was inspected using the penetrating liquid technique and fatigue was resumed. The objective was to compare the shape of the damage or discontinuity with the image on the ultrasound inspection. This presentation will begin with the experimental procedure, continue with part of the results obtained and will finish the with a few concluding statements.
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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Moderador
Notas de la presentación
The size of the specimens was relatively large to assure enough inspection volume and to increase the possibilities of noticing different types of defects. Each specimen was identified with a code for traceability.
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 0.24mm 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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Moderador
Notas de la presentación
The GFRP was Isovid G-3 manufactured by the Composites Ate company, and consists of layers of 200g/m2 plain weave E-glass, and a modified phenolic resin that enhances flame retardant characteristics. In Fig. 5 the specimens right after manufacture can be observed, and have a yellow color due to the resin. However, after being exposed to sunlight, the surface acquires a brown color, but the interior remains yellow. The surface has a woven appearance, but from a distance this glass fiber/phenolic resin material has a wooden appearance.
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.
The CFRP was manufactured using a Hexcel prepreg at the INTA Materials and Structures Department. This CFRP consisted in 32% of 8552, which is an amine cured, toughened epoxy resin system. The carbon fiber plies consisted in tows of 12,000 individual fibers of IM7 intermediate modulus carbon. In Fig. 8 the finished CFRP specimen bars can be seen, and in Fig. 9 one can appreciate the different surface qualities: smooth on one side and rough on the side that had been in contact with the release film, and also the edges that were high speed milled. In the case of CFRP bars of different thicknesses the sequences were designed to have comparable strengths as can be seen from Table 1 were the percentages of the different orientations are shown. The fourth column shows that predominantly the fibers were oriented in the 0º direction.
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 15dB.-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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Moderador
Notas de la presentación
The ultrasound equipment was designed and built by Mapro, using a Socomate ultrasound acquisition card. Most of the inspections were performed using a 1MHz and 13mm diameter transducer. The gain was set at 15dB. Arbitrarily the acoustic wave propagation was set at 3275m/s for both composite materials. The voltage signals were visualized with a software developed by Mapro using LabView. This software allowed processing the pulse-echo signals to visualize the conventional A-scans, B-scans and C-scan, plus optional scan images that the authors have named as integral scans. In Fig. 10 the general view of the ultrasound equipment is shown. In Fig. 11 an image of the immersion bath can be appreciated. And in Fig. 12 the transducer can be seen while searching the (0,0) origin in the scan and index directions. In Fig. 13 the ripples on the water indicate that an inspection is under progress.
Fig. 14. Screen image of the visualization software, after loading the ultrasound signal data. 7/20
Moderador
Notas de la presentación
This is a screen image of the visualization software developed by Mapro, and shows the view right after loading the inspection data of a GFRP specimen. Below and to the left the typical A-scan plot can be observed. Above there are two different B-scan images. And to the lower right one can see the (x,y) image also known as a C-scan image. The C-scan image is created by first defining a color scale associated to signal intensity values, and then for every (x, y) position a color is assigned in accordance to the highest value observed between the cut-off lines. The B-scan images also show how inclined and/or curved the specimen is, and when analyzing a large specimen this can become a problem. One of the features of this software is that the specimen can be flattened digitally, by clicking on this button.
Fig. 15. Screen image after: digitally aligning the first peak, positioning the cut off lines, and setting a color range. 8/20
Moderador
Notas de la presentación
Here there is an image after digitally flattening the specimen, choosing an adequate color scale, and choosing to only consider the second peak (because the cut-off lines have been placed before and after this second peak).
Fig. 16. In this study the A-scans was passed through different algorithms to create alternative scan images. 9/20
Moderador
Notas de la presentación
On the A-scan label, a bigger A-scan signal can be seen. A typical A-scan (of signal intensity versus material depth) when using a low frequency transducer consists in an initial and larger multi-peak echo that is due to the interface crossing of the acoustic wave, here designated as I echo and later in depth another smaller multi-peak echo due to the signal bouncing from the back-wall of the material, and here this back-wall echo is designated as B echo. In this work, besides calculating a C-scan image, the A-scan curve was integrated and different scan images were created depending whether or not the echoes were considered. The value of the integral, in each case, was the datum used to assign a color to each (x, y) site.
Fig. 17. A menu in the C-scan label allows processing the A-scan to create different ∫-scans. 10/20
Moderador
Notas de la presentación
It is on the C-scan label of the software where a menu allows selecting one of these alternative integral scan images. As can be seen on the menu one can select a C-scan, an integral scan including both interface and back-wall echoes, an integral scan but without the I&B echoes, an integral scan with the I&B echoes minus the mean and an integral scan minus the I&B echoes minus the mean. For every case a constant color scale was chosen to allow comparing scans of different inspections.
The maximum strength σ0 for the GFRP was 391MPa, and for the CFRP was 1135MPa.
L
L4
L4
S F2
F2
F2
F2
Fig. 18. Loading diagram according to ASTM D6272-
02 (2008).
Fig. 19. GFRP specimen tested until fracture to determine σ0.
Fig. 20. 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 σmax / σ0 where σmaxis the maximum flexural fatigue
stress in the outer layers.
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Moderador
Notas de la presentación
The four-point bending tests were performed using an MTS 322 Test Frame, and the flexural stress in the outer layer was calculated using this formula in accordance to the loading diagram suggested by ASTM standard D6272. First one specimen of each the composite materials was tested until fracture to determine the maximum strength, sigma sub zero. Figures 19 and 20 show the GFRP and CFRP specimens respectively at the moment of fracture. And the maximum strength for the GFRP was 391MPa and 1135MPa for the CFRP. The normalized fatigue stresses for the GFRP were set between 0.2 and 0.9 and for the CFRP specimens between 0.36 and 0.60. The normalized fatigue stress is defined as sigma max over sigma sub zero, where sigma max is the maximum flexural fatigue stress in the outer layers.
The two plots above show the actual maximum stress at which each of the specimens was tested for every stage of 2,000 cycles, until reaching 20,000 cycles. Ideally each specimen should have been tested at the same conditions every 2,000 cycles, but some specimens were not, especially the CFRP specimens. However, these inconsistencies did not prevent from extracting useful information. The GFRP specimens, which were mostly tested at the programmed conditions, allowed creating a maximum stress versus cycles to failure plot (or a partial S-N plot). This partial S-N plot in Figure 23 shows that when flexural fatigue is considered, the GFRP Isovid G-3 should not be stressed above 235MPa, because at that stress a specimen failed at 13,344 cycles. Of course, the plot could and should be extended. As for the CFRP specimens, none of them broke.
3. Results: Ultrasound scans a GFRP at N=4,000 cycles
C-scan of the B echo
∫-scan with I & B
∫-scan minus I & B
∫-scan with I & B minus mean
∫-scan minus I & B minus mean
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 24. In (a) specimen GFRP-P031-40-710-16_5/6 that broke at N=4001 cycles is shown. Images (b) to (f) show different types of
scans performed at N=4000.
X (scan)
Y (In
dex)
X (scan)Y (In
dex)
Moderador
Notas de la presentación
Above in Figure 24a, there is a specimen that broke at 4,001 cycles, right after having been inspected at 4,000 cycles. The main fracture appears to have originated at the line of contact with one of the upper load noses. In Figure 24b appears a conventional C-scan of the B echo, and shows two blue lines of low intensity ultrasound signal. These blue lines coincided with the lines of contact with the upper load noses. Also in Fig. 24b, red spots of high ultrasound intensity signal appeared, but these spots could not be associated to any type of damage. In Figures 24c, d, e, and f appear the mentioned integral scans, and in general show different contrast spots than those in the conventional C-scan in Fig. 24b. The authors preliminarily prefer Fig. 24d, the integral scan minus the I&B echoes, because it was the scan image that showed the greatest contrast at the areas that were known to have been damaged. However, more tests are required that actually compare the shape to the damage with the shape of the spots observed in the integral scans.
3. Results: Ultrasound scans in a GFRP at N=26,079 cycles
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C-scan of the B echo
∫-scan with I & B
∫-scan minus I & B
∫-scan with I & B minus mean
∫-scan minus I & B minus mean
Fig. 25. In (a) specimen GFRP-P031-40-710-16_3/6 that broke at N=26,504 cycles is shown. Images (b) to (f) show different types of
scans performed at N=26,079.
(a)
(b)
(c)
(d)
(e)
(f)
X (scan)Y
(Inde
x) Y
(Ind
ex)
N=26,504
Moderador
Notas de la presentación
In this case, Fig. 25a shows the specimen that was examined at 20,000 cycles using the penetrating liquid technique, which should have dyed the delaminations in color red and the undamaged material should have remained color yellow. This specimen finally broke at 26,504 cycles, and the last inspection was carried out at 26,079 cycles. The C-scan image in Fig. 25b also shows the dark blue lines of low intensity ultrasound signal due to the contact line with the upper load noses. However this specimen broke at the middle length, at the maximum strain point, and the C-scan also showed a dark blue spot on that site. Below appear the integral scans, and again the authors prefer the integral scan without the I&B echoes, because it was the only scan image that showed red contrast spots in the middle region where the penetrating liquid had entered. Unfortunately no scan image was able to reproduce the shape left by the red dye.
3. Results: Ultrasound scans in a GFRP at N=26,079 cycles
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C-scan of the B echo
∫-scan with I & B
∫-scan minus I & B
∫-scan with I & B minus mean
∫-scan minus I & B minus mean
Fig. 26. In (a) specimen GFRP-P031-40-710-16_3/6 was cracked open to reveal the red color of the penetrating liquid. Images (b) to (f) show
different types of scans performed at N=26,079.
(a)
(b)
(c)
(d)
(e)
(f)
X (scan)
Y (In
dex)
N=26,504
Moderador
Notas de la presentación
As can be compared from the photo in Fig. 26a.
3. Results: Confocal microscopy in a GFRP at N=26,079 cycles
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Fig. 27. Image in (a) shows specimen GFRP-P031-40-710-16_3/6 being inspected by a Sensofar Plμ 2300 confocal microscope, in (b) and (c) two contiguous surface areas show a
crack on the site of maximum bending stress.
(a)(b)
(c)
Moderador
Notas de la presentación
The same GFRP specimen that was treated with the penetrating liquid was additionally examined using a confocal microscope, at those areas where the C-scan and B-scans indicated a superficial irregularity. In a confocal image the color scale is used to represent micron scale topography. The confocal microscope used was a Sensofar Plu 2300, as shown in Fig. 27a. In retrospect, if only the confocal images of the middle length area had been considered, then the insipient crack that was found should have been recognized as the place of imminent failure, but damage was also found at the lines of contact with the upper loading noses. Nevertheless the confocal microscope was capable of revealing a crack that was not visible a simple sight. The confocal images in Figs. 27b and c are contiguous areas that show how the surface to the left of the crack bulged, probably due to the absorption of penetrating liquid.
3. Results: Confocal microscopy in a CFRP at N=20,000 cycles
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Fig. 28. Images in (a) and (b) shows specimen CFRP-P021-40-1040-27_1/2, in (c) the C-scan image, in (d) a photo of the damaged surface, and in (e) and (f) two contiguous confocal
images show a normal surface and a worn surface, respectively.
X (scan)
Y (In
dex)
X (scan)
Y (In
dex)
(a)
(b)
(c)
(d)
(e) (f)
Moderador
Notas de la presentación
In the case of the CFRP specimens the confocal microscope was capable of quantifying the superficial damage that was already visible at simple sight. Figures in 28a and b show both sides of a CFRP specimen that had shown superficial damage in the C-scan, as pointed in Fig. 28c and then shown up-close in a photo in Fig. 28d. The confocal image in Fig. 28e shows a left half of normal appearance, but on the right half the wear has eliminated much of the resin. In Fig. 28f the damage has arrived at least to the first ply of carbon fiber.
3. Results: Confocal microscopy in a CFRP at N=20,000 cycles
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Fig. 28. Images in (a) and (b) shows specimen CFRP-P021-40-1040-27_1/2, in (c) the C-scan image, in (d) a photo of the damaged surface, and in (e) and (f) two contiguous confocal
images show a normal surface and a worn surface, respectively.
(e) (f)
3. Results: Confocal microscopy in a CFRP at N=20,000 cycles
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Fig. 29. Image in (f) is again the worn surface of specimen CFRP-P021-40-1040-27_1/2. A closer observation in shown in (g) and the damage can be quantified in image (h).
(f)
(g) (h)
Moderador
Notas de la presentación
A closer at the confocal image in (f) also shows that damage, at the edge, reached the second ply. A large rupture in the carbon fibers was observed further in the center, and in average the depth was of 90 microns, which represents 70% of the thickness that first ply. A closer observation of the middle of the confocal image shows that many fibers have been broken and lost. An even closer observation in 3D shows that the wear reaches 20 microns, which in this case is 15% of the thickness of that carbon fiber ply.
4. Conclusions
-The additional information (i. e. newer spots) provided by the ∫-scans still requirefurther corroboration with the actual internal damage.
-However, the observations of this study seem to point to the ∫-scan without the I &B echoes, as the image that can combine the most information both from internaland superficial damage.
-Ultrasound inspections in the GFRP specimens of this study helped locatepossible surface damage, and confocal microscopy was capable of noticing crackson that surface.
-In CFRP specimens the damage was visually evident, but confocal images helpedquantify the depth and type of damage.
