Terahertz (THz) imaging is an attractive alternate to ultrasonic based Non-destructive Evaluation (NDE) especially for Fiber Reinforced Polymers (FRPs) such as Glass FRP (GFRP) composites as the latter demands proximity and additional coupling medium for the best performance. Typically, THz imaging system uses a single emitter-detector configuration employing raster scan method for image acquisition. The image acquisition speed is greatly limited by the speed of the mechanical stages and hence its usage in real-time industrial NDT applications such as in-line quality control has been limited. Alternatively, having an array of detectors will significantly increase the system cost. As an optimal compromise for speed and cost, line scanners are highly desirable. In this work, rapid imaging performance of a THz line scanner has been studied by imaging closely spaced defects in GFRP composites using a 100 GHz source. The total acquisition time for imaging the GFRP sample of dimensions 55× 35 mm2 is 10 s, which is >100 times faster compared to a conventional raster scanning technique. In addition, image deconvolution techniques such as Lucy Richardson and Weiner deconvolution have been adopted to improve the quality of the acquired THz images. The results show that the THz line scanners can successfully be employed for rapid defect detection in GFRP composites.
Nanosilica incorporation in cement has been of great interest for its accelerating effect on the hydration process as well as providing higher compressive strength and durability. During hydration, cement constituents, such as tricalcium silicate (C3S) and dicalcium silicate (β-C2S) react with water to form key hydration products, such as calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)2, CH). In this work, Mid-infrared and Terahertz spectroscopy has been employed to study the effect of nanosilica incorporation in cement hydration. The acceleration due to the presence of nanosilica has been demonstrated by the reduction in peak intensity of the resonances related to Si-O stretching (925 cm-1) and Si-O bending modes (520 cm-1) which confirms faster consumption of the cement constituents. Furthermore, the formation of the hydration products C-S-H and CH is vital since C-S-H contributes to the early stage strength development in concrete and CH is an undesirable hydration product. CH content in the cement matrix can be minimized by nanosilica incorporation resulting in pozzolanic reactions as CH reacts with nanosilica to produce more C-S-H. Formation of C-S-H has been demonstrated by the prominence of the resonances related to deformations of SiO4 chains around 455 cm-1 and 1100 cm-1. The type of C-S-H can also be predicted by tracking the shift of resonances to higher/lower wavenumbers, denoting polymerization which is more prominent for the nanosilica incorporated sample. Formation of the other key hydration product is observed as the resonance related to CH around 314 cm-1 is seen to get sharper with hydration. This study has also been able to show a reduced carbonation effect in the nanosilica incorporated sample as evident from the less prominent carbonate peaks around 1425 cm-1 after 28 days of hydration.
Nafion membranes, known for their excellent proton conductivity, are widely used in Proton Exchange Membrane Fuel Cells (PEMFC), a promising clean energy source. Proton conductivity is highly dependent on membrane hydration. Water has strong absorption in the Terahertz (THz) frequency range and hence THz spectroscopy can be potentially used to understand the water dynamics in fuel cell membranes. In this study, Terahertz spectroscopy has been employed to track the water retention in a hydrated Nafion membrane by measuring the transmitted THz time domain pulses through the hydrated membrane for every minute up to 25 minutes. In addition, the complex permittivity has been extracted for each measurement by considering multiple reflections in the sample and is fitted with the double-Debye model. From these calculations, the dielectric constant and relaxation timescale have been extracted which provides more information on the population of bulk and ‘free’ water in the hydrated membranes. Results show 45 % decrease from the initial bulk-like water population by the end of 25 minutes which confirms reasonable water retention of Nafion for the measurement duration.
Terahertz (THz) technology is a competent non-destructive evaluation (NDE) technique, particularly for advanced materials such as Fibre Reinforced Polymer (FRP) composites due to its ability to penetrate most non-metallic and nonpolar substances. Typically, THz NDE studies are carried out using expensive and broadband pulsed THz systems limiting their widespread use in practical applications. In contrast, Continuous wave (CW) THz systems can potentially be a narrowband, cost-effective and scalable solution for NDE applications. However, conventional CW THz systems employ a coherent detection scheme which results in large acquisition time per pixel thus limiting their real-time applicability. In this paper, a CW THz system with incoherent detection scheme using Schottky receiver along with spatial adaptive sampling technique is employed to achieve rapid THz imaging of Glass Fibre Reinforced Polymer (GFRP) composite with artificial defects. Here, an initial coarse scan of 2 mm step size has been done, and gradient based thresholding criterion is used for identifying the regions of interest to progressively scan the sample with finer resolution down to a step size of 0.5 mm. Results demonstrate a total reduction in the image acquisition time by a factor of 50 compared to the coherent CW THz imaging. Further, the THz image acquired through adaptive sampling shows excellent correlation with that of the traditional uniformly sampled THz image with 0.5 mm step size.
Engineering of novel structures with high strength to weight ratio for applications in aerospace, renewable energy and naval industries has resulted in an increased popularity of sandwich structured composites. A sandwich-structured composite is fabricated by bonding a thick lightweight core between two stiff, thin skins such as Glass Fiber Reinforced Plastic (GFRP). Balsawood is a type of homogeneous core which is widely used for renewable energy structures, such as wind turbine blades. In this paper, a GFRP-balsawood sandwich structure is evaluated non-destructively for internal defects such as holes, using a CW Terahertz system in transmission mode. Internal defects will give rise to differential THz transmission and hence can be identified using THz imaging. The imaging studies are carried out with a central frequency of 0.35 THz and the sample is raster scanned using 2-D translational stages controlled by high precision stepper motors in x-y directions to obtain the THz image. The image acquired using CW THz system clearly identifies the defects in the GFRP-balsawood composite structure with good contrast demonstrating the potential of THz imaging for non-destructive testing of sandwich composite structures.
Ordinary Portland Cement (OPC) primarily constitutes Tricalcium Silicate (C3S) and Dicalcium Silicate (C2S) making up 60–70 % and 20–30 % of the cement matrix respectively. During cement hydration, C3S starts to react faster contributing to early stage strength in comparison to C2S, which reacts slowly and is responsible for long term strength development of concrete. C2S is manufactured at lower temperatures compared to C3S, resulting in lesser emission of carbon dioxide as compared to C3S. Moreover, C2S produces less Ca(OH)2 than C3S, which is an undesirable hydration product. Thus, incorporation of greater percentages of C2S in cement matrix will be highly beneficial, provided it’s early stage reactivity can be increased. One of the key methods to increase reactivity of C2S is incorporating nanosilica which accelerates the hydration along with the formation of greater amount of calcium silicate hydrate (C-S-H) which is responsible for the strength development of concrete. Hence, understanding the acceleration in hydration dynamics of the nanosilica incorporated β-C2S can help in optimizing the percentages of C3S and C2S in cement. In this study, Terahertz spectroscopy has been employed to track the acceleration of hydration of C2S due to the addition of nanosilica. Results show early stage reduction in peak height of the resonance around 520 cm-1 in nanosilica incorporated sample which indicates faster hydration of C2S during hydration. Furthermore, early stage formation of a prominent resonance around 453 cm-1 for the nanosilica incorporated C2S sample implies formation of C-S-H like structures confirming the accelerated hydration rate.
Proton Exchange Membrane (PEM) fuel cells are increasingly gaining importance as a clean energy source. PEMs need to possess high proton conductivity and should be chemically and mechanically stable in the fuel cell environment. Proton conductivity of PEM in fuel cells is directly proportional to water content in the membrane. Among the various PEMs available, Nafion has high proton conductivity even with low water content compared to SPEEK (Sulfonated Poly(ether ether ketone)) but is also expensive. SPEEK membranes and it’s composites have better mechanical properties and have comparatively higher thermal stability. Operating the fuel cell at higher temperatures and at the same time maintaining the water content of the membrane is always a great challenge. In this paper, to increase water retention capacity, Nafion, SPEEK and it’s composite (SPEEK PSSA-CNT) membranes are exposed to Ultra-Violet (UV) radiation for varied times. Terahertz Spectroscopy, in both pulsed and CW mode has been used as an efficient tool to quantify the water retention of the membrane. Results using Terahertz spectroscopy show that even though the initial water absorption capacity of Nafion membranes is more, SPEEK membranes and it’s composites show considerable improvement in the water retention capacity upon high intensity UV irradiation.
THz rays have higher penetration depth compared to infrared rays and hence can be effectively used to measure tablet coating thickness. In addition, THz wavelength (1 mm - 0.1 mm) provides an optimal depth resolution for the thickness measurement. This method can be non-invasive and hence ideal for inline quality monitoring. Tablet coating thickness is one of the major parameters of interest in Process Analytical Technology (PAT). In this paper, a reflection mode Continuous Wave (CW) Terahertz (THz) system has been employed to measure the tablet coating thickness. A frequency scan of the sample has been carried out from 0.1 THz to 1.1 THz and the reflection coefficient of the sample is inverse fourier transformed to obtain the tablet thickness. The calculated thickness has also been validated using the optical microscope. Results show that the thickness can be measured with considerable accuracy.
Cement is mixed with water in an optimum ratio to form concrete with desirable mechanical strength and durability. The ability to track the consumption of major cement constituents, viz., Tri- and Dicalcium Silicates (C3S, C2S) reacting with water along with the formation of key hydration products, viz., Calcium-Silicate-Hydrate (C-S-H) which gives the overall strength to the concrete and Calcium Hydroxide (Ca(OH)2), a hydration product which reduces the strength and durability, using an efficient technique is highly desirable. Optimizing the amount of water to be mixed with cement is one of the main parameters which determine the strength of concrete. In this work, THz spectroscopy has been employed to track the variation in hydration kinetics for concrete samples with different water-cement ratios, viz., 0.3, 0.4, 0.5 and 0.6. Results show that for the sample with water-cement ratio of 0.3, significant amount of the C3S and C2S remain unreacted even after the initial hydration period of 28 days while for the cement with water-cement ratio of 0.6, most of the constituents get consumed during this stage. Analysis of the formation of Ca(OH)2 has been done which shows that the concrete sample with water-cement ratio of 0.6 produces the highest amount of Ca(OH)2 due to higher consumption of C3S/C2S in presence of excess water which is not desirable. Samples with water-cement ratio of 0.4 and 0.5 show more controlled reaction during the hydration which can imply formation of an optimized level of desired hydration products resulting in a more mechanically strong and durable concrete.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.