Terahertz Inspection Research Articles

Evaluation of static bending caused damage of glass-fiber composite structure using terahertz inspection

Abstract Composite materials find increasing applications in modern industry and transport. However, they may lose desirable mechanical properties due to external mechanical excitations, ultraviolet radiation, moisture penetration, or other factors. Therefore, an effective way to assess the condition of materials is necessary. In this article, a nondestructive evaluation of glass fiber-reinforced composite subjected to five-stage static bending is presented. For this reason, pulsed excitation terahertz imaging was utilized, and a data processing/exploration scheme was proposed. The proposed, novel approach consists of an efficient data registration algorithm based on surface approximation (for surface roughness and unevenness elimination) and a parametrization scheme applied for the signals gained from the time response of the glass fiber-reinforced polymer layer. Obtained parameters enable the global description of the evaluated material state and prediction of failure effectively, even in the early stages of destruction.

Rapid sensing of hidden objects and defects using a single-pixel diffractive terahertz sensor

Terahertz waves offer advantages for nondestructive detection of hidden objects/defects in materials, as they can penetrate most optically-opaque materials. However, existing terahertz inspection systems face throughput and accuracy restrictions due to their limited imaging speed and resolution. Furthermore, machine-vision-based systems using large-pixel-count imaging encounter bottlenecks due to their data storage, transmission and processing requirements. Here, we report a diffractive sensor that rapidly detects hidden defects/objects within a 3D sample using a single-pixel terahertz detector, eliminating sample scanning or image formation/processing. Leveraging deep-learning-optimized diffractive layers, this diffractive sensor can all-optically probe the 3D structural information of samples by outputting a spectrum, directly indicating the presence/absence of hidden structures or defects. We experimentally validated this framework using a single-pixel terahertz time-domain spectroscopy set-up and 3D-printed diffractive layers, successfully detecting unknown hidden defects inside silicon samples. This technique is valuable for applications including security screening, biomedical sensing and industrial quality control.

THz-Based Monitoring of the Deformation of the Inner Woven Structure of Glass-Fiber Reinforced Polymers

This work exploits terahertz (THz) imaging in the real-space and the spatial-frequency domain to determine nondestructively the evolution of the main characteristics of a woven glass-fiber reinforced polymer (GFRP) laminate in three-dimensions in-situ during a tensile test. These characteristics include tow orientation and the weave period. The surface and the inner plies can be investigated with the proposed method. The results show how the tows tend to realign with the load direction. In addition, we detect a decrease in the weave period, manifesting a distortion of the surface and inner layers as the stress is increased. Contrary to conventional optical inspection, THz inspection can provide quantitative measurements in all material layers and not only on the surface. This technique could be helpful to monitor distortion and damage due to mechanical stress or defects incorporated during the manufacturing process and to validate parameters and results of draping process modeling, especially for the inner plies.

Terahertz Barcodes Enabled by All‐Silicon Metasurfaces for Process Control and Monitoring Applications

AbstractProcess control and monitoring (PCM) is crucial for the quality control and yield improvement of semiconductor products. Terahertz inspection of semiconductor devices has been of interest since the progress of terahertz spectroscopic techniques. However, due to the diffraction limit, high‐resolution measurement of geometric sizes and multiple parameter monitoring are challenging for terahertz PCM techniques. In this article, XX present terahertz all‐dielectric metasurfaces as the interface on a silicon substrate for monitoring material properties and fabrication errors in the silicon deep reactive ion etching (DRIE) process. It is shown that four different metasurfaces can encode multiple information simultaneously, including the carrier density of silicon substrates, etching depth and linewidth error, forming a terahertz barcode for PCM. By correlating the measured reflection spectra of the barcode with those in the simulation database, the three parameters are retrieved with a mean square error (MSE) of ≈0.5. The results indicate that this proposed technique using emerging artificial photonic materials as PCM structures paves a pathway toward high‐efficiency semiconductor quality control and other sensing applications.

Real-time terahertz characterization for composite delamination using a lightweight CPU adaptive network

Delamination defects, as common damages occurring in fiber reinforced composite laminates, are often difficult to be detected due to their invisibility and concealment. Terahertz (THz) technique, as a promising alternative to conventional nondestructive testing (NDT) approaches, has emerged great potentials in quantitative characterization of delamination defects in composites. However, since THz signals are susceptible to confounding interference during THz inspection, such as the noise, interlayer reflection, dispersion and overlap, the accuracy and speed of defect localization and identification are usually contradictory. Generally, to improve the accuracy, many complex manual signal processing methods can be applied, which however will inevitably decrease the inference speed for delamination inspection and cannot fulfill the automatic and real-time analysis requirements in practical THz applications. Here, an intelligent THz 3D characterization system based on the lightweight deep learning (DL) model is proposed to realize the automatic and real-time characterization of hidden delamination defects in composites with high accuracy and low CPU latency. The core aims to specifically design a lightweight and end-to-end CPU adaptive network to obtain the optimal balance between the accuracy and speed. A series of experiments are implemented to validate the superior comprehensive performance of the system in terms of classification accuracy and inference speed on low-cost CPU-based platform. Compared with other methods, the proposed system can be not only able to obtain the superior accuracy, but also to obtain low CPU latency. Therefore, our work provides a novel and general paradigm for DL-based THz intelligent 3D characterization, which will promote the deployment of the system for the automatic and real-time THz characterization in industrial applications.

Industrial Applications of THz Imaging Based on Resonant Slit-Type Probe

In this study, the possibility of the industrial application of terahertz (THz) imaging technology was verified. It was applied to the inspection of voids in multistack semiconductors that require safe inspection and to the high-resolution detection and inspection of foreign substances in tablets in the pharmaceutical field. To acquire a high-resolution THz image, a resonant slit probe operating in the THz region was designed, and a high-speed scanning system was established. For the inspection of a multistack semiconductor, a lateral scan method was proposed, and voids with a diameter of 0.5 mm in the multistack semiconductor were detected. In addition, the proposed probe even enables the distinguishment of the positions of voids in the multistack semiconductor. For pharmaceutical inspection, we investigated the application of THz imaging to detect mixed foreign objects frequently occurring in the tablet manufacturing process. For metals, plastics, and rubber, which are the most frequently mixed materials in the tablet manufacturing process, the foreign objects were identified in tablets using a transmission THz system. The measured THz image was compared with the conventional X-ray test result to confirm the potential of THz inspection. In the X-ray image, only metal and some polymer foreign objects were detected. In contrast, in the THz image, although the materials could not be distinguished, most foreign substances were detected. Consequently, the THz imaging test was verified as a possible new tool in fields where X-ray or existing tests are not possible.

Terahertz Inspection of Buildings and Architectural Art

We present a broad literature review on the uses of terahertz radiation for the inspection architectural art, as well as building structures. As an example of the uses of terahertz in this field, we also include original results on a non-destructive assessment of a conservation intervention on murals of the Konstantinbasilika in Trier, Germany while using terahertz time-of-flight spectroscopy.

Terahertz imaging and vibro-thermography for impact response in carbon fiber reinforced plastics

In this paper, terahertz (THz) imaging and vibro-thermography (VT) are implemented for non-destructive evaluation (NDE) of CFRP subjected to impact damage. THz inspection is well-known for its outstanding detection depth in dielectric materials but unreliable in conductive materials like carbon fiber reinforced plastics (CFRP): an unwanted electro-magnetic wave will be excited under the irradiation of THz waves in conductive molecules and interrupted the reflective waves leading to a blur in the detection results. However, our experimental results showed THz imaging is comparable when it’s used to detect surface defects and shallow ones caused by impact loads. We conducted a set of VT tests as a contrast test, and the VT data are processed by Fourier transform and eigenvector decomposition algorithms, i.e. independent component analysis (ICA). Compared with VT, THz imaging show more details on surface defects and shallow-buried ones due to the absence of thermal diffusion effects. THz imaging could perform non-invasive, non-ionizing and non-contact inspection on the impact damage based on the differences in reflective index. In the end, signal-to-noise (SNR) evaluation is used as an effective means to quantitatively compare the results of these two techniques.

Effective surface emissivity and heat dissipation among integrated bamboo-like super-black vertical carbon nanotube array electrodes in silicon via holes

Recently, carbon nanotube (CNT)-based micro-devices have attracted extensive attention at the frontiers of intelligent electronics, but the practical application of these devices are seriously limited by the low CNT package rates, poor heat dissipation, and intricate in-situ growth processes. Here, microelectrodes with low impedance based on in-situ-grown super-black vertical bamboo-like multi-walled CNT arrays (∼70 μm) were fabricated inside silicon via holes using face-down catalytic plasma enhanced chemical vapor deposition. Interestingly, the unique CNT array exhibited uniformly high absorption efficiency with a broad range of wavelength (exceeding 99.65% in the UV-NIR band and 99% in the mid-infrared region). Moreover, the excellent cooling performance of CNT array microelectrodes based on in-situ-grown bamboo-like CNT arrays within silicon via holes was demonstrated. Our study highlights the benefits of CNT-based micro functional devices, which indicates the integrated in-situ-grown bamboo-like CNT arrays will have potential applications in advanced microelectrodes, CNT-based field-effect transistors, and portable terahertz inspection devices.

熔石英激光诱导损伤的太赫兹检测

熔石英激光诱导损伤的太赫兹检测

Non-Contact Terahertz Inspection of Water Content in Concrete of Infrastructure Buildings

Concrete is the most widely used for construction materials in the world. Water content of concrete is an important parameter in terms durability of concrete structures. Terahertz (THz) waves, for which concrete is a porous and absorbable material, have been studied in order to establish a new non-contact inspection technology for maintenance of concrete structures. In this study, THz transmittance and reflectance of concrete in drying process were measured with a 60 GHz GUNN diode and absorption coefficient is analyzed for concrete with various water contents. It is shown that quantitative detection below 10% is possible for the water content at surface area of concrete.

Measuring low-level porosity structures by using a non-destructive terahertz inspection system

It is demanded a non-contact, non-destructive and reliable system and method of porosity measurement because conventional techniques are contact and cubersome. We have developed a method by using THz inspection system, which allows measuring the porosity rapidly and non-invasively, by introducing an external perturbation. The embodiments of the external perturbation can be mechanical or pulsed laser.

Automatic defect recognition for pulsed terahertz inspection of basalt fiber reinforced composites

Purpose – The purpose of this paper is to present a system for automatic recognition of defects detected in non-conductive polymer composites using pulsed terahertz imaging. Design/methodology/approach – On the beginning, non-destructive evaluation of composites using electromagnetic waves in terahertz frequency is shortly introduced. Next automatic defects recognition (ADR) algorithm is proposed, focussing on new features calculation. Dimensionality of features space is reduced by using principal component analysis. Finally, results of basalt fiber reinforced composite materials inspection and identification using artificial neural networks is presented and discussed. Findings – It is possible to develop ADR system for non-destructive evaluation of dielectric materials using pulsed terahertz technique. New set of features in time and frequency domains is proposed and verified. Originality/value – ADR in non-destructive testing is utilized in case of digital radiography and ultrasonic testing. Terahertz inspection with pulsed excitation is reported as a source of many useful information about the internal structure of the dielectric material. Up to now ADR based on terahertz non-destructive evaluation systems was not utilized.

Terahertz detection and identification of defects in layered polymer composites and composite coatings

Modern industry makes more and more extensive use of various composite materials. In this paper, for the purposes of various composite materials evaluation, the terahertz imaging method is presented. Basalt fibre-reinforced composites and polymeric anticorrosion coatings are considered. Basalt fibre composites are the innovative materials that are being increasingly used in modern industry. The paper also briefly introduces a specific type of complex coating of steel applied in the industry (e.g. oil or chemical). Two methods of defects detection in the mentioned structures are presented. The first method is based on a system identification, whereas the second one is on the estimation of time-domain signal parameters. Finally, the results achieved during terahertz inspection of coatings are compared with those obtained using active thermography.

Energy and Entropy Based Signal Processing for Terahertz Inspection of Homogeneous and Composite Materials Adhesive Joints

Modern industry makes extensive use of various composite materials In this paper terahertz imaging method is proposed for evaluation of various composite materials adhesive joints. The results of inspection (A-scans and C-scans) as well as 2-D distributions of proposed parameters are presented for method validation.