Nondestructive evaluation of forced delamination in glass fiber-reinforced composites by terahertz and ultrasonic waves

TLDR: In this paper, the authors used reflective pulsed terahertz imaging to locate and size the forced delamination in polyetherimide resins in 3D dimensions and determined the thicknesses of the delamination and the layers constituting the laminate.

Abstract: Glass fiber-reinforced composite laminates in polyetherimide resin have been studied via terahertz imaging and ultrasonic C-scans. The forced delamination is created by inserting Teflon film between various layers inside the samples prior to consolidating the laminates. Using reflective pulsed terahertz imaging, we find high-resolution, low-artifact terahertz C-scan and B-scan images locating and sizing the delamination in three dimensions. Furthermore, terahertz imaging enables us to determine the thicknesses of the delamination and of the layers constituting the laminate. Ultrasonic C-scan images are also successfully obtained; however, in our samples with small thickness-to-wavelength ratio, detailed ultrasonic B-scan images providing quantitative information in depth cannot be obtained by 5 MHz or 10 MHz focused transducers. Comparative analysis between terahertz imaging and ultrasonic C-scans with regard to spatial resolution is carried out demonstrating that terahertz imaging provides higher spatial resolution for imaging, and can be regarded as an alternative or complementary modality to ultrasonic C-scans for this class of glass fiber-reinforced composites.

Figures

Fig. 1. (a) Typical optical micrograph of the in-plane section of the eight-harness-stain fabric glass fiber-reinforced polyetherimide matrix laminate; (b) Schematic diagram of samples indicating dimensions.

Fig. 5. Three-dimensional THz images for sample 1. [the x position is not indicated]

Fig. 4. (a) and (b) show THz reflection B-scan images through the two yellow delaminations appearing in Fig. 3(a). The vertical direction corresponds to the depth direction in the sample. The THz field is polarized in the vertical direction in all figures.

Fig. 8. Ultrasonic imaging C-scan results for samples 51 and 57. (a) 51 in transmission, (b) 57 in transmission, (c) 51 in reflection, and (d) 57 in reflection modes.

Fig. 9. Ultrasonic time domain waveforms without and with delamination for sample 51 in (a) transmission mode and (b) reflection mode.

Fig. 3. (a) THz reflection C-scan image across 50 mm x 50 mm region of sample 1 using the contrast mechanism associated with the peak-to-valley difference of the reflected THz pulse within the time window 13 ps to 20 ps. (b) THz C-scan image after applying 50 % rule for sizing the delaminations.

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Nondestructive evaluation of forced delamination in
glass ber-reinforced composites by terahertz and
ultrasonic waves
Junliang Dong, Byungchil Kim, A. Locquet, Peter Mckeon, Nico Declercq,
D.S. Citrin
To cite this version:
Junliang Dong, Byungchil Kim, A. Locquet, Peter Mckeon, Nico Declercq, et al.. Nondestructive eval-
uation of forced delamination in glass ber-reinforced composites by terahertz and ultrasonic waves.
Composites Part B: Engineering, Elsevier, 2015, 79, pp.667-675. �10.1016/j.compositesb.2015.05.028�.
�hal-03079427�

1
Nondestructive evaluation of forced delamination in glass fiber-reinforced
composites by terahertz and ultrasonic waves
Junliang Dong,
a,b,*
Byungchil Kim,
a,b
Alexandre Locquet,
a,b
Peter McKeon,
a
Nico Declercq,
a,c
D.S. Citrin
a,b,*
a
Georgia Tech-CNRS UMI2958, Georgia Tech Lorraine, 2 rue Marconi, 57070 Metz, France
b
School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta,
Georgia 30332 USA
c
Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta,
Georgia 30332 USA
Corresponding authors:
Junliang Dong Email: junliang.dong@gatech.edu
D. S. Citrin Email: david.citrin@ece.gatech.edu
Tel: +33(0)38720-1165

2
Abstract:
Glass fiber-reinforced composite laminates in polyetherimide resin have been studied
via terahertz imaging and ultrasonic C-scans. The forced delamination is created by
inserting Teflon film between various layers inside the samples prior to consolidating
the laminates. Using reflective pulsed terahertz imaging, we find high-resolution,
low-artifact terahertz C-scan and B-scan images locating and sizing the delamination in
three dimensions. Furthermore, terahertz imaging enables us to determine the
thicknesses of the delamination and of the layers constituting the laminate. Ultrasonic
C-scan images are also successfully obtained; however, in our samples with small
thickness-to-wavelength ratio, detailed ultrasonic B-scan images providing quantitative
information in depth cannot be obtained by 5 MHz or 10 MHz focused transducers.
Comparative analysis between terahertz imaging and ultrasonic C-scans with regard to
spatial resolution is carried out demonstrating that terahertz imaging provides higher
spatial resolution for imaging, and can be regarded as an alternative or complementary
modality to ultrasonic C-scans for this class of glass fiber-reinforced composites.
Keywords: D. Non-destructive testing
A. Polymer-matrix composites (PMCs)
B. Delamination
D. Ultrasonics
Terahertz imaging

3
1 Introduction
Fiber-reinforced composites provide an alternative to conventional structural
materials such as concrete, steel, aluminum, and wood. Used for structural purposes,
fiber-reinforced composites have the advantage of combining a number of properties
not usually found together in a single material. In particular they combine high strength
and low weight, while at the same time they may be resistant to corrosion and have
thermal and electrical insulation properties. As a result, the wide applicability of
fiber-reinforced composites has created the need for correspondingly advanced
nondestructive evaluation (NDE) techniques for inspection and failure detection during
manufacturing and maintenance.
Various NDE techniques capable of characterizing damage and defects in
fiber-reinforced composites have been developed. Among them, ultrasonic testing is the
most well-known tool to characterize fiber composites, including ultrasonic C-scans
[1-3], ultrasonic polar scans [4-5], nonlinear ultrasonics [6], and guided-wave inspection
[7-8]. However, until now, only the ultrasonic C-scan technique has found widespread
implementation in industry, because of simplicity of analysis and its effectiveness in
geometrically locating damage and defects. In ultrasonic C-scans, the ultrasonic waves
show specific transmission and reflection features depending on the spatial variation of
acoustic impedance within the fiber-reinforced composite. Ultrasonic C-scans can
provide a good trade-off between material penetration and measurement resolution, and
ultrasonic C-scans in pulse-echo mode can also provide qualitative information in depth

4
for thick fiber composite samples [9].
Because of certain limitations associated with ultrasonic C-scans (see below), there
is growing interest in alternative imaging modalities and NDE techniques, such as
shearography, IR thermography, and X-ray radiography, to name a few. Considerable
work has been carried out to compare such approaches with ultrasonic C-scans, which
stands as the reference standard for NDE in these materials [10-15]. These comparisons
highlight some of the difficulties associated with the ultrasonic C-scan technique in
these materials: (1) negligible quantitative information in depth can be obtained in thin
samples with small thickness-to-wavelength ratio due to the relatively large time
duration of ultrasonic pulse signal; (2) because of the attenuation of ultrasonic waves in
fiber-reinforced composites (especially in glass fiber-reinforced composites), the
operating frequency cannot be sufficiently high (usually less than 10 MHz) [16], thus
limiting the resolution; and (3) liquid coupling may be required. Although contactless
ultrasonic techniques using laser [17, 18] and air-coupled transducers [19] have been
proposed, problems (1) and (2) remain. Therefore, alternative nondestructive,
noncontact, and nonionizing (to minimize health risks) techniques with relatively high
resolution are still needed for inspection of fiber-reinforced composites.
As an alternative to ultrasonic waves, we here investigate the use of
terahertz-frequency electromagnetic waves. The terahertz (THz) portion of the
electromagnetic spectrum extends from approximately 100 GHz to 10 THz, and lies
between the microwaves and infrared; the wavelength range in this region is 3 mm

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Nondestructive evaluation of forced delamination in glass fiber-reinforced composites by terahertz and ultrasonic waves" ?

Dong et al. this paper investigated the use of terahertz and ultrasonic waves for inspection and failure detection of fiber-reinforced composites. 

Q2. What are some of the well-known techniques for characterizing fiber composites?

Among them, ultrasonic testing is the most well-known tool to characterize fiber composites, including ultrasonic C-scans [1-3], ultrasonic polar scans [4-5], nonlinear ultrasonics [6], and guided-wave inspection [7-8]. 

Q3. What is the reason for the errors of sizing the delamination in the C-?

Diffraction effects at the edge of the Teflon film may also be responsible for the errors of sizing the delamination in their C-scan images. 

Q4. What is the main merit of THz imaging?

Based on the results, the authors conclude that THz imaging can provide a nondestructive, noncontact, and nonionizing method to evaluate glass fiber-reinforced composites with higher spatial resolution, and can be regarded as an alternative or complimentary to ultrasonic C-scans. 

Q5. How can the authors measure thicknesses of the Teflon film?

For the cases with overlapping echoes, measurement of thicknesses can also be conducted based on the integration of time-domain waveforms mentioned in [39]. 

Q6. What is the important merit of THz imaging?

The authors point out, moreover, the most important merit of THz imaging is the ability of providing quantitative information in depth and three-dimensional imaging of the samples. 

Q7. How can the authors extract the index of refraction of the sample?

Because the authors can measure both the amplitude and the phase of the transmitted THz pulses (not shown), the authors can extract the index of refraction of the sample across the THz band, which varies little in this range and has the value of 2.16 in the frequency range from 0.3 THz to 1.3 THz. 

Q8. Why is the reflected THz signal shifted to a larger optical delay?

Because of this inserted Teflon film, lamina 3 is deformed and the corresponding peak in the time-domain THz signal is shifted to a larger optical delay. 

Q9. Why is there growing interest in alternative imaging modalities and NDE techniques?

Because of certain limitations associated with ultrasonic C-scans (see below), there is growing interest in alternative imaging modalities and NDE techniques, such as shearography, IR thermography, and X-ray radiography, to name a few. 

Q10. What is the largest peak in the THz image?

The largest peak in all cases corresponds to the THz pulse reflected off the surface of the sample 1 on the side from which the THz pulses were incident. 

Q11. Why has the ultrasonic C-scan technique been used in industry?

until now, only the ultrasonic C-scan technique has found widespread implementation in industry, because of simplicity of analysis and its effectiveness in geometrically locating damage and defects. 

Q12. What is the transmission mode of the ultrasonic C-scan images?

In the transmission mode shown in Fig. 9(a), the transmitted waveform in the windowed time slice is chosen to provide sharper contrast for the ultrasonic C-scan images, because this transmitted waveform traverses the samples three times. 

Q13. What are the disadvantages of the ultrasonic C-scan technique?

These comparisons highlight some of the difficulties associated with the ultrasonic C-scan technique in these materials: (1) negligible quantitative information in depth can be obtained in thin samples with small thickness-to-wavelength ratio due to the relatively large time duration of ultrasonic pulse signal; (2) because of the attenuation of ultrasonic waves in fiber-reinforced composites (especially in glass fiber-reinforced composites), the operating frequency cannot be sufficiently high (usually less than 10 MHz) [16], thus limiting the resolution; and (3) liquid coupling may be required. 

Q14. What is the way to evaluate a glass fiber composite?

As a result, the wide applicability of fiber-reinforced composites has created the need for correspondingly advanced nondestructive evaluation (NDE) techniques for inspection and failure detection during manufacturing and maintenance. 

Q15. What is the difference between THz and ultrasonic C-scan?

For these types of samples, THz imaging, which can provide a nondestructive, noncontact, and nonionizing method to evaluate glass fiber-reinforced composites, can be utilized as an alternative or complementary modality to ultrasonic C-scans. 

Q16. What is the pixel with the highest value in the C-scan image?

This method locates the pixel with the highest value in the C-scan image and assigns this a value of 100 %, then colors all pixels red that have a value of at least 50 % of the maximum. 

Q17. What is the difference between terahertz and ultrasonic C-scans?

Comparative analysis between terahertz imaging and ultrasonic C-scans with regard to spatial resolution is carried out demonstrating that terahertz imaging provides higher spatial resolution for imaging, and can be regarded as an alternative or complementary modality to ultrasonic C-scans for this class of glass fiber-reinforced composites.