An Introduction To Terahertz Technology For Non-Destructive Testing- Terametrix

Applications that benefit from Terahertz technology include aircrafts and fiber-reinforced composites. Shutterstock/Frank_peters

Sponsored by Ether NDE Ltd.
https://www.azom.com/article.aspx?ArticleID=18439

What is Terahertz technology?

In the electromagnetic spectrum, Terahertz (THz) radiation sits between the infrared and microwave regions with frequencies between 300GHz and 3000GHz (0.3-3THz), and corresponding wavelengths from 1mm to 0.1mm. Historically, difficulties in generating and detecting THz radiation limited research into its interactions with matter [1]. However, recent advances now mean that it is possible to generate and detect 0.1-3THz frequencies.

THz radiation can penetrate a wide variety of non-conducting materials (such as cardboard, plastic, ceramics, clothing, wood, masonry, and paper.)). Penetration depth is relatively large (although usually less than that of microwave radiation), making THz-radiation suitable for probing the inner structure of samples in search of defects and inclusions [2]. Many chemicals have defining fingerprints at THz-frequencies so THz spectroscopy can be used to identify and characterize substances such as drugs and explosives.

Advantages of THz radiation

THz radiation penetrates many materials (more than infrared, for example) and, as an imaging technique, also exhibits good spatial resolution (better than microwaves, for example) [2]. This higher resolution means it is well-suited for analyzing complex systems with a low contrast range where established methods (like an ultrasound) may fail.

As THz pulses carry very low energies (compared to X-ray or UV radiation), they do not ionize and damage the material under study. This makes them particularly suitable for applications such as non-invasive imaging and non-destructive quality control. THz technology can be used where ultrasonic and other inspections cannot, such as when physical contact is not possible or when sensitive materials require examination (such as lightweight aircraft composites) or where materials do not conduct ultrasound [1].

Low energies mean this type of radiation is also safe for people to use; there are no associated health or safety risks.

Terahertz technology provides highly accurate, reliable and repeatable inspection data.

Applications

Pulsed terahertz imaging can provide a range of information about a sample. It can measure the thickness of a coating and study how well it is bonded, and give highly precise multi-layer thickness measurements in one go [3].   It can also detect the presence of internal voids or a specific gas, and detail the density distribution in a layer of foam [4]. Its major advantage is that can be used with the most sensitive of materials and testing environments.

Applications include NDT in aircraft and fiber-reinforced composites; ceramic coating thickness measurements; inspection and repair of pipelines; evaluation of seams; detection of voids; inspection of radomes (enclosures that protects radar antennae) and automotive fuel tanks; security applications such as checking for explosives in airports; and imaging of internal structures [1].

TeraMetrix Products

T-Ray 5000 series [1]

TeraMetrix Inc has designed robust compact pulsed terahertz measurement systems for use in an industrial environment. Sensor heads are coupled by an optical fibre to a rack mount package containing the electro-optical acquisition hardware. This gives complete flexibility as sensor heads can be replaced and configured to a user’s specific requirements. Cutting edge research can be performed quickly and easily as the T-Ray 5000 systems can be adapted to handle a large number of experiments thanks to their modular construction.

Sensor Heads

T-Ray 5000 series Control Units can be combined with a handheld gauge or  line scanner for nondestructive testing [4]. The Single Point Gauge (SPG) determines the thickness of several coating layers with unparalleled precision, and the Line Scan Gauge (LSG) produces images of structures under the surface. Measurements with the SPG can be taken either straight-on or at right-angle depending on which tip is selected. While essentially non-contact, a measurement tip helps the user to position the object at the focus of the THz beam when using a hand-held tool. Measurement configuration and mode of operation are selected using the touchscreen display. The system is capable of working on metallic or composite substrates.

Applications areas include aerospace where the system is used to measure coating thickness and panel alignment and identify defects; marine (coating thickness and corrosion under paint); building products (seam inspection and void detection); and petrochemical (steel pipe coatings and pipe repair inspection).

TeraMetrix Online Sensor Head - C1D1 [5]

The sensor head allows the  

T-Ray 5000 systems to work in flammable atmospheres, such as paint spray booths or coating facilities, as the transmitter and receiver are housed within sealed stainless steel and the lens is coated with Teflon to withstand solvents. It can also be mounted on a robot. While its working distance is set to 115 mm, this can be changed if needed.

Applications of TeraMetrix products

Aircraft Non-Destructive Testing [6]

A good example is the F-35 Joint Strike Fighter aircraft. Its exterior is coated with specialty chemicals that are particularly challenging to measure. However, THz pulses have proved useful in measuring coating thickness soon after application once the coatings have dried. A handheld THz scanner can then provide the information necessary to assemble the airframe ensuring that parts are properly aligned.

Once in use, the aircraft needs to be inspected and serviced regularly. THz systems can be used to evaluate external coatings, as well as helping with paint removal when coatings need removing. In this case, they can provide information to control laser power and cutting depth.

Spacecraft Non-Destructive Testing [7]

NASA uses terahertz systems to examine the sprayed-on-foam insulation on the exterior of spacecraft fuel tanks. The Previous model T-Ray 4000 was also used to inspect the tiles of the shuttle heat shield as corrosion can occur underneath. By measuring the layers which attached the tiles to the orbiter, NASA could decide if tiles need replacing.

NASA is also investigating terahertz inspection of heat shields, thermal blankets and ultra-high pressure tanks as it develops the new Ares launch platform; as well as for checking ultra-high pressure gas tanks used for attitude adjustment rockets. Terahertz imaging can monitor the tanks’ Kevlar fiber coatings to check for broken fibers. Any rupture would result in the loss of the orbiter.

Examination of Packaged Goods [8]

THz technology can be used to detect contamination in packaged products such as powdered antibiotics while the product remains inside its packaging. It can also test the weight of tablets in blister packs and powdered antibiotics can be non-destructively examined inside their packages.

Pipeline Coating and Repair Inspection [9]

Pipelines need regular monitoring for corrosion and leaks and THz technology can be used to measure the thickness of multi-layer coatings and monitor pipe repairs. Usually, fiberglass composite wraps are used as patches and THz imaging can check these patches for degradation. The advantage of THz technology is that it can probe below the patch to the surface of the pipe and check for problems with the pipe surface as well as the patch.

References:

[1] http://bw-nde.com/partner-products/terametrix

[2] https://www.recendt.at/en/THz.html

[3] https://lunainc.com/high-speed-waveform-acquisition-key-terahertz-imaging/

[4] http://bw-nde.com/images/tetrametrix/TeraMetrix_NDT_App_Sheet_v1_060518.pdf

[5] https://lunainc.com/product/online-sensor-head-class-division-1-cid1/

[6] https://lunainc.com/market/aircraft-non-destructive-testing/

[7] https://lunainc.com/market/spacecraft-non-destructive-testing/

[8] https://lunainc.com/market/examination-package-goods/

[9] https://lunainc.com/market/pipeline-coating-repair-inspection/

Radiophysicists will double the quality of terahertz devices

http://en.tsu.ru/news/radiophysicists-will-double-the-quality-of-terahertz-devices/

Radiophysicists from Tomsk and Novosibirsk are planning to double the resolution of the terahertz range. In particular, this will help to more thoroughly examine works of art - old books, manuscripts, and paintings - without touching them. The experiment will begin in mid-August.

- Terahertz radiation is used for various purposes, including the study of works of art, because unlike optical illumination, it penetrates deeply, but unlike X-ray, does not shine through. It is possible for example to view text or drawings in old books layer by layer at different frequencies without touching them and therefore without damaging them, - said Valentin Suslyaev, manager of the project, associate professor at TSU, head of the Laboratory of Terahertz Research. - The same radiation is used to diagnose and treat skin diseases, for example, various types of skin cancer.

In both cases, the terahertz beam must be as focused as possible, that is, its resolution must be high. The finer this beam, the greater the resolution it has, and therefore the more carefully it can identify the features of the object that needs to be studied.

- There is a physical law that limits the resolution to half the wavelength, but it can be overcome using lenses. Our colleagues from Novosibirsk, Oleg Minins and Igor Minins, showed that it is possible to use simple manufactured lenses and simple geometric shapes - cuboids and ellipsoids. Making a cube is easier than a lens, and using it, you can double the resolution, - explained Valentin Suslyaev.

Oleg and Igor Minins used mathematical modeling to prove the possibility of increasing the resolution of terahertz radiation using cuboids, and Tomsk radiophysicists will conduct an experimental study. TSU scientists have already developed an experimental design in free space and in an open resonator to prove the Minins’ theoretical results.

- One of the problems that we are solving is the influence of the tool on the object of study. We created a probe that when it is moved, makes it possible to determine the size of the area of focused terahertz radiation, but the probe affects the size of the focused radiation. Now in Minsk, our employees are developing solid-state detectors based on graphene to consider this area with less impact on the object, -added Valentin Suslyaev.

A physical experiment will take place in mid-August. According to the forecasts of TSU radiophysicists, they will get the first results by the beginning of September.

Backscanning from out of the box with THz technology

http://idm.net.au/article/0012514-backscanning-out-box-thz-technology

The laborious and expensive process of backscanning boxes of paper archives could become a point and click operation in the future, via new Terahertz imaging technology that reads pages through closed books with invisible radiation.

Terahertz technology, pioneered by Barmak Heshmat, a researcher at the MIT Media Lab, is capable of time-gated spectroscopic imaging that allows for content extraction through layered structures.

Today, that means this technology can read through 9-20 pages of a closed book using Terahertz waves. But for the future, it unlocks the possibility of reading entire boxes of paper without removing the lid.

US startup Ripcord has just appointed Heshmat to its advisory board. It plans to integrate Heshmat’s groundbreaking work in Terahertz imaging technology with Ripcord’s existing OCR (optical character recognition) capabilities and AI powered entity extraction.

Heshmat joins an already stellar roster on the Ripcord advisory board, which includes Apple co-founder, Steve Wozniak, former NASA CTO, Chris Kemp and Oracle co-founder, Bruce Scott.

Currently, Heshmat serves as the founder and CEO of BRELYON, a stealth startup. His past work has disrupted the world of imaging all the way from fundamentals in design of optics to introduction to new applications such as batch scanning.   

 “I’m honoured to join Ripcord’s advisory board to help leverage lessons learned in THz imaging and time-of-flight imaging that can help Ripcord radically accelerate data capture from paper documents,” said Barmak Heshmat, founder and CEO of BRELYON.

“Ripcord is an innovative and forward-thinking company that combines data that we can uncover through cutting-edge non-optical imaging with machine learning and artificial intelligence to produce insights that previously could not have been imagined. It’s exciting to think about the possibilities this creates as we transition from physical to digital.”

“Barmak’s help in commercializing this approach will make Ripcord’s mission of taking the entire world paperless a reality,” said Alex Fielding, founder and iCEO, Ripcord.

“It’s not every day that you meet a person as brilliant and curious as Barmak, much less have the opportunity to learn from him and work alongside him to commercialize and scale a technology approach that can help us create a paperless future. This advancement in imaging is the technological equivalent of going from an X-Ray to a high-resolution MRI.”  

Co-founded by former Apple and NASA employees, Ripcord enables companies to digitize paper records and connect those records to existing enterprise systems. The company says it provides an alternative to outdated records management systems that are overly complex, require expensive customization and take years to effectively implement.

OT-LUNA Blog-Self-Driving Vehicles: Evaluating Damage and Wear with Smart Parts

Fiber optic sensors have unique characteristics such as their low profile, flexibility and resistance to chemicals and corrosion that make them uniquely suited for long-term and embedded applications. This creates an opportunity for a design to incorporate “smart parts” that can easily communicate their health to an inspector. Instead of relying on visual confirmation or disassembly of the vehicle to check on hard-to-get-to structural elements, an inspector could connect to fiber optic sensor leads tucked away in the vehicle and scan components for damage quickly and reliably.

Consider advanced self-driving vehicles, where no human needs to be present or attentive to operate. These are already on the roads in some areas. These machines can’t be viewed as only personal vehicles because they are ideal for fleets or shuttles that provide transportation services in populated areas. So there could quickly be large quantities of such vehicles without an individual owner who would be able to keep track of performance history.

On top of general wear and tear, no matter how smart the system, there will always be unpredictable and unavoidable impacts with varying levels of severity over the lifetime of a vehicle. A sudden large pothole, a deer that runs into the road, a human driver who bumps the vehicle … depending on the conditions, these events could be serious or trivial to the vehicle’s structural health. What if no one was present to witness the event? Can the condition of key components in the vehicle be quickly determined in that situation?

Fiber optic sensors are small and lightweight enough to embed within composite structures, thermoplastics and even metal components during manufacturing. Using embedded sensors to create “smart parts,” which can provide critical data from inaccessible areas, can be beneficial in countless applications providing access to valuable health monitoring throughout the entire life cycle of the part.

If your part could talk, what would it say about the vehicle’s structural health?

Download our White Paper Now! Measurement and Validation of Strain in New Materials and Complex Structures

Abstract-Terahertz Analysis of Phthalocyanine Pigments

A. D. Squires, R. A. Lewis,

https://link.springer.com/article/10.1007/s10762-019-00599-9

In situ, non-invasive and non-destructive analysis of important artworks and cultural pieces is largely important in art conservation science. Terahertz time-domain spectroscopy and imaging delivers these requirements but lacks a materials database with fundamental understanding of salient pigments, binders and substrates. In this study, the most important synthetic pigments, the copper phthalocyanines, are investigated through terahertz time-domain spectroscopy. The terahertz spectrum reveals a series of characteristic modes in a 0.1–3-THz range across 14 pigment samples. Identification and distinction of copper phthalocyanines’ α, β and 𝜖 crystal polymorphs is demonstrated. This uniqueness within the terahertz regime is extended to two halogenated variants and a metal-free form. This invites the use of THz spectroscopy in investigation of contemporary artworks, post 1935, containing these pigments and promotes applications such as identifying fraudulent works of art.

LUNA Blog-Using Terahertz Technology to Measure Total Thickness of Manufacturing Materials

https://lunainc.com/terahertz-technology-measure-total-thickness-manufacturing-materials/

Following a successful trip to Tire Tech Expo last month here’s a closer look at the ability of Terahertz measurement to perform an on-demand on-production material property check to ensure the THz measurement remains accurate. Especially through material batch changes, shift changes, production stops / restarts and new product starts.

Terahertz radiation is part of the electromagnetic spectrum lying between microwaves and the far-IR. This region has frequencies ranging from 0.1 – 10 THz and wavelengths from 3 mm to 0.03 mm. This spectral region is often referred to as the “Terahertz gap” as these frequencies fall between electronic (measurement of field with antennas) and optical (measurement of power with optical detectors) means of generation. Historically, little study of the interactions between these wavelengths and matter has been undertaken. The reason for this was the difficulty in generating and detecting terahertz frequencies. Recent advances in combining optical and electronic methods have allowed for generation and detection of very high signal-to-noise ratio and high data acquisition rates of 0.1 – 3 THz.

Most materials, especially polymers and other manufactured materials, can be relied on to be very consistent and thus have the same material properties. In tire manufacturing, gum rubber is a relatively unusual material in that it is derived from many sources, both natural rubber sources and recycled material sources. Thus, consistency in the exact formulation of the rubber product is difficult to guarantee. Tire manufacturers undertake extensive multiple processing steps to ensure the material is uniform, but small changes in the mixed material properties are possible.

For thickness measurement, the most important property is the Refractive Index (RI) of the material. While this term sounds complicated, it is simply the ratio of the Speed-of-Light (in vacuum) to the velocity-of-light of the THz pulse (through a material).  Electromagnetic energy (i.e., light) always slows down when passing through matter. For a consistent material (e.g., polymer), determination of the RI value takes less than 2 minutes and only needs to be done once ever.

However, there was a concern that this property value could vary for a natural product. Variation in this value would lead to accuracy errors. Thus, this 2-minute check needs to be performed in real-time on-line on the exact material being produced. For THz, there is a simple method to do so.

By adding a window between the sample and the sheet and a metal reflector behind the sheet, it is possible to make two THz measurements (one of the empty structure, and one when the sheet is present) to determine the sheet total thickness without needing any calibration. From these two measurements it is straightforward to find the instantaneous value for RI for the product being manufactured.

The Empty structure needs to be measured to calculate the “thickness” of the air layer between the window bottom and rear reflector surface (TotAir). This value needs to be stored.

When the structure moves on sheet, then two air layers are created; one between the window and sheet (TopAir) and one between the sheet and rear reflector (BotAir).

The thickness of the sample is then:

Sample Thickness  =  TotAir  –  TopAir  –  BotAir

Thus, the thickness of the sample is now known without any knowledge the sample material properties. From this thickness value, the Refractive Index can be immediately calculated. With the RI value, guaranteed accurate thickness measurements are obtained. This structure is set up on the scanning frame and can be run on demand or on schedule as set by the operators.

Other topics discussed considered the measurement of cord spacing with the ply sheet, especially the distance from the last cord to the gum sheet edge. This would be a new application for THz inspection and is being considered now.

For more information on how TeraMetrix is using terahertz technology in tire manufacturing please download our application note on calendering.

Abstract-THz spectroscopy application for analyzes of internal structure damage due to moisture influence

Magdalena Mieloszyk,  Katarzyna Majewska,  Wieslaw Ostachowicz

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10972/109720Z/THz-spectroscopy-application-for-analyzes-of-internal-structure-damage-due/10.1117/12.2513265.short?SSO=1

Glass composite structures are recently very popular in many branches of industry, such as marine (e.g. ship hull), civil engineering (e.g. composite bridge deck) or energy (e.g. wind turbine blades). Due to high safety requirements related to the objects structural health monitoring systems based on fiber optics techniques are recently widely applied. One of problems that can influence on material durability is moisture introduced into element structure during its manufacturing or exploitation processes. Moisture changes material characteristics, affect element durability and can be a damage origin especially during exposure on negative temperature influence. One of the non-destructive techniques that can be applied for evaluation of internal structure of non- conductive materials (like glass fiber reinforced polymers) is THz spectroscopy. This method can be used for identification of material structural disintegrations that results in changes of absorption coefficient, refractive index or scattering of THz waves propagating throughout analyzed material. The paper presents an application of THz spectroscopy for inspection of glass composite samples internal structure. The method was used for evaluation of internal material structure as well as detection, localization and determination of size of internal damage due to influence of moisture and exposure on negative temperature. During analysis the limitations of proposed method will be determined.

© (2019) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Broadband stepped-frequency modulated continuous terahertz wave tomography for non-destructive inspection of polymer materials

Xiaoxuan Zhang, Qijia Guo, Tianying Chang, Hong-Liang Cui,


https://www.sciencedirect.com/science/article/pii/S0142941818320683

An all-solid-state electronic three-dimensional terahertz tomography system designed specifically for non-destructive inspection of polymer materials is demonstrated, which is capable of determining the positions and shapes of hidden defects accurately. The imaging radar system, based on stepped-frequency modulated continuous wave (SFMCW), with center frequency at 180 GHz, bandwidth of 60 GHz, and average power 0.5 mW, is tested against thick Teflon (polytetrafluoroethylene, PTFE) plates with internal voids as samples. The locations and shapes of the hidden holes are obtained in both electromagnetic simulation and experimental measurements with a three-dimensional image reconstruction algorithm, which features advantages of accurate reconstructed image details and fast computation speed, demonstrating that the terahertz imaging radar system combined with the algorithm developed is capable of detecting internal defects of thick polymers. Key resolution parameters are established experimentally in detail, demonstrating 2.5 mm and 1.7 mm range resolutions in free space and in Teflon separately, and lateral spot diameters ranging from 3.4 mm to 8 mm at imaging distances from 5 mm to 60 mm.

Abstract-Synthesis of novel rambutan-like graphene@aluminum composite spheres and non-destructive terahertz characterization

Zhongbo Yang, Shuanglong Feng, Wei Yao, Jiaguang Hanc, Huabin Wang

https://pubs.rsc.org/en/Content/ArticleLanding/2019/RA/C8RA09129C#!divAbstract

Graphene reinforced Al (graphene@Al) spheres were synthesized using microwave plasma chemical vapor deposition technique in which H2, CH4, and Ar were used as the reduced gas, carbon source, and plasma enhancement gas, respectively. The obtained graphene@Al spheres presented a rambutan-like structure and had a graphene shell wrapped on the sphere surface, which was proved by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The thickness of the graphene shell on the Al sphere is difficult to be characterized by conventional techniques. However, it was successfully measured with a sophisticated terahertz (THz) time-domain spectroscopic technique. To the best of our knowledge, neither have graphene@Al spheres been synthesized before nor has a THz-based technique been exploited to characterize the thickness of a shell structure. Therefore, the present work sheds useful insights on both the rational synthesis and non-destructive characterization of graphene reinforced functional structures.

Abstract-THz imaging techniques for nondestructive inspections

Kodo Kanwkse,  Takayuki Shibuya, Shin'ichiro Hayashi, Koji Suizu

https://www.sciencedirect.com/science/article/pii/S1631070510000423

We have suggested a wide range of real-life applications using novel terahertz imaging techniques. A high-resolution terahertz tomography has been demonstrated by ultra short terahertz pulses using optical fiber and a nonlinear organic crystal. We also describe a nondestructive inspection system that can monitor the soot distribution in a ceramic filter using millimeter-to-terahertz wave computed tomography. Further, we report on the thickness measurement of very thin films using high-sensitivity metal mesh filter. These techniques are directly applicable to the nondestructive testing in industries.

OT LUNA -Fiber-optic sensors shed light on the toughest composite design challenges

Designers of composites are learning that if they are to be able to guarantee component and system performance, repeatability and reliability, it is critical that they have access to more sensor and test data.

Fig 1:  High-definition fiber optic sensors embedded into this composite cantilever beam provide continuous strain data with resolution less than 1 mm in all 3 dimensions and interior to the part.

BY DAVID POTTER, DIRECTOR OF MARKETING, LUNA INNOVATIONS

https://www.compositesworld.com/articles/fiber-optic-sensors-shed-light-on-the-toughest-composite-design-challenges

Designers of composites are learning that if they are to be able to guarantee component and system performance, repeatability and reliability, it is critical that they have access to more sensor and test data. Whether they are striving to certify a complex structural component, validate the structural integrity of a part over its entire lifecycle or simply assess the performance of an adhesive bond, designers now have a new measurement tool that provides unprecedented visibility into the structural behavior of composite components. 

For decades, the go-to device for materials engineers and stress analysts was the metal foil strain gage. Invented in the 1950s, the foil strain gage quickly became the standard tool for spot measurement of strain levels in metals. However, due to the cost and complexity of applying foil gages (gage size, attachment and bonding, wiring, etc.), stress analysts were forced to focus on the most critical locations and select only a handful of key stress points to instrument with gages. In an era dominated by standardized isotropic materials, such as metal, with well-understood properties and knowledge of how stress propagates through the material, this approach of using relatively sparse test data was an efficient and effective way to test materials and validate the accuracy of high-fidelity models.

The current state of composite materials and systems, however, is another story. In addition to their orthotropic properties, composites encompass a vast and growing range of material types and manufacturing. Although the models and simulation software available for composite materials are advancing at an impressive rate, the number of variables and complexity inherent in composite materials and systems remains a significant challenge for the designer looking to guarantee the performance and reliability of a new design. As carbon fiber composites are more frequently used in structural applications, requiring higher levels of verification and certification, spot checking with strain gages leads to problematic blind spots.

Fortunately, the gap between reality and simulation can be greatly reduced with fiber-optic sensors. Pioneered in the 1990s for monitoring civil structures, fiber-optic sensing has developed, evolved and matured to a level of capability and robustness useful in the composites industry, enabling much more complete and accurate testing and verification. In fact, the technology promises to move beyond the test lab because it is proving useful in building smart parts that incorporate thousands of integrated sensors.

High-definition fiber-optic sensing

These developments are a direct result of the advent of high-definition fiber-optic sensing (HD-FOS). HD-FOS can use a standard optical fiber as a very sensitive, continuous sensor of strain or temperature. By exploiting the minute effects that strain and temperature have on how light travels through that optical fiber, HD-FOS systems are able to detect and measure more than 1,000 discrete points of strain or temperature per meter of the fiber length, with accuracy and sensitivity on par with legacy sensors, including strain gages.

Moreover, optical fibers are physically compatible with composite materials. Extremely lightweight, small and flexible, optical fiber sensors can easily be installed on composite parts regardless of geometry. They even can be embedded within a part and measure internal or interlaminar strain — loads that are invisible to strain-gage or vision-based systems. Multiple studies have shown that embedding fiber-optic sensors into a composite structure has no detrimental effect on the structure’s performance. Additionally, unlike metal-foil strain gages, fiber-optic sensors are passive, immune to electrical fields, chemically inert, and exhibit excellent fatigue performance.

For example, Fig. 1 illustrates a composite cantilever beam with three-dimensional carbon fiber reinforcement. A 125-µm optical fiber was integrated into the preform fabric along the x (warp), y (fill) and z axes. These HD-FOS sensors are, therefore, able to capture thousands of discrete strain locations in all three dimensions. The figure includes a sample of the measured strain data along the y-axis, which is transverse to the applied strain. However, the labeled points where the fiber makes a turn (A, B, C, etc.) experienced tensile strain that increased with load; those data are also mapped onto the beam for easier visualization.

Multi-material joints and adhesive bonds

One particularly challenging area where this type of high-density measurement data can make a difference is the use of adhesively bonded interfaces, particularly for mixed materials. Although adhesive bonds have many significant advantages, mechanical fasteners remain a requirement in many applications because it is a challenge to certify adhesive bonds due to their unpredictable performance and durability.  

Conventional nondestructive evaluation or test (NDE/NDT) methods can be used to inspect bonds, and to some extent, quantify damage or disbonding. But HD-FOS is proving to be a valuable tool to measure residual strain directly, along or within the bond lines, to verify the integrity of the bond and to provide detailed insight into disbond growth. Fig. 2 shows measurement data from an HD-FOS bonded to both sides of a Center Cut Plies (CCP) test specimen, which is about 30 cm long. After a fatigue test, the CCP was ultrasonically scanned, and the C-scan was compared to the HD-FOS data. The HD-FOS data are able to more precisely pinpoint the locations of the disbond tips, allowing the calculation of disbond growth rate. Continuous strain along identified sections 1, 2 and 3, at a single moment in time, is displayed along with the C-scan inspection.

Smart parts for damage detection

Although gaining insight into these complex material systems and building more accurate damage and fatigue models are important, the holy grail may be the ability to monitor the performance of the material, bond or system during operation. Currently, a variety of NDE/NDT methods are typically used to detect flaws or damage. However, these methods are time-consuming and require significant downtime and labor costs. The ability to use onboard sensors to provide in-situ health monitoring or instant detection of structural issues is the promise of structural health monitoring (SHM) and damage detection technologies.   

Consider a composite structure that has lightweight and unobtrusive fiber-optic sensors embedded throughout its geometry during fabrication. At any time, a snapshot of the stress and strains throughout the structure can be taken to provide an assessment of residual strain, which research has shown to be a reliable indicator of fatigue damage, impact damage, crack growth, disbonding, delamination or general damage.

Fig. 3, for example, shows a composite overwrapped pressure vessel (COPV) with multiple meters of fiber-optic sensors woven into its overwrapping. A quick interrogation of the embedded HD-FOS sensors generates thousands of residual strain measurements over the entire surface, which is mapped onto the 3D model of the COPV. This vessel had previously been drop tested and the 3D data map easily identifies areas of sustained internal damage, indicated by the red areas. 

The ability to embed these HD-FOS sensors into materials, components, bonds and systems can potentially change the way composite parts and structures are designed and managed through their lifecycles. HD-FOS sensors are starting to enable truly smart parts and smart structures that are enhanced with a “nervous system” of sensors able to detect and analyze internal stresses, deformation and movement while in operation. In the meantime, the same sensor technology is already providing unprecedented visibility into the performance of new materials, processes and designs.    

About the Author

 

David Potter is director of marketing for Luna Innovations (Blacksburg, VA, US), a manufacturer of fiber-optic sensing systems. He holds a BSEE from Vanderbilt University and an MSEE from M.I.T. He has more than 25 years of experience in the testing and measurement industry, developing and managing products and solutions for data acquisition, industrial measurements and control. He is responsible for product management and strategic marketing at Luna.

Abstract-Damage Assessment in Composite Beam Using Infrared Thermography, Optical Sensors, and Terahertz Technique

Rohan N. Soman, Katarzyna Majewska, Magdalena Mieloszyk and Wieslaw Ostachowicz

http://nondestructive.asmedigitalcollection.asme.org/article.aspx?articleid=2673224


Composite materials find wide range of applications due to their high strength-to-weight ratio. Due to this increasing dependence on composite materials, there is a need to study their mechanical behavior in case of damage. There are several extended nondestructive testing (ENDT) and structural health-monitoring (SHM) methods for the assessment of the mechanical properties each with their set of advantages and disadvantages. This paper presents a comparative study of three distinct damage detection methods (infrared thermography (IRT), neutral axis (NA) method based on optical strain sensor measurements, and terahertz spectroscopy) for the detection of delamination and temperature-induced damage in a simple glass fiber reinforced polymer (GFRP) beamlike structure. The terahertz spectroscopy is a specialized technique suitable for detecting deterioration inside the structure but has limited application for in-service performance monitoring. Similarly, the IRT technique in the active domain may be used for in situ monitoring but not in in-service assessment. Both methods allow the visualization of the internal structure and hence allow identification of the type and the extent of damage. Fiber optic sensors (especially fiber Bragg grating (FBG)) due to their small diameter and no need of calibration can be permanently integrated within the sample and applied for continuous dynamic strain measurements. The measured strain is treated as an input for neutral axis (NA) method, which as a damage-sensitive feature may be used for in-service monitoring but gives absolutely no information about the type and extent of damage. The results for damage detection based on proposed comparative studies give a complete description of the analyzed structure.