Lightweight Composite Materials Research Articles

Aqueous Photoiniferter Polymerization of Acrylonitrile.

Polyacrylonitrile (PAN) is a key industrial polymer for the production of carbon fiber for high-strength, lightweight composite material applications, with an estimated 90% of the carbon fiber market relying on PAN-based polymers. Traditionally, PAN synthesis is achieved by conventional radical polymerization, resulting in broad molecular weight distributions and the use of toxic organic solvents or surfactants during the synthesis. Additionally, attempts to improve polymer and processing properties by controlled radical polymerization methods suffer from low monomer conversions and struggle to achieve molecular weights suitable for producing high-performance carbon fiber. In this study, we present an aqueous photoiniferter (aqPI) polymerization of acrylonitrile, achieving high monomer conversion and high PAN molecular weights with significantly faster kinetics and dispersity control when compared to traditional methods. This approach allows for the unprecedented control of polymer properties that are integral for downstream processing for enhanced carbon fiber production.

Experimental Evaluation of Geometric and Environmental Effects on Mechanically Fastened Non-Crimp Fabric Composites.

Corresponding to marine environmental regulations is important in shipbuilding and marine industries. The application of lightweight composite materials on ships is an effective approach to reducing the emission of greenhouse gases. The mechanical fastening method is a good candidate to assemble composites and conventional metals. The joint geometric and environmental effects are two important factors in mechanically fastened ship and marine structures. In this study, we evaluated the W/D (hole diameter to width ratio) and environmental effects on the bearing strength and failure mode of a mechanically fastened non-crimp fabric (NCF) composite material. To consider the effect of joint geometry, wherein hole diameters of 5, 6, 8, and 10 mm were machined. Further, by selecting three environmental conditions (UV, saltwater and low temperature), we evaluated environmental effects on bearing strength and failure modes of NCF composite specimens. The bearing strength increased as W/D decreased, and the bearing strength of the specimen exposed to low temperature and UV environments increased, while that of the specimen exposed to saltwater remained the same. From the failure mode analysis, the specimen that was exposed to salt fog showed the same failure mode as the unaged specimen. It was observed that the changes in the transition section and new failure mode in the xenon arc and low-temperature specimens.

Structure-Mechanical Property Relationships in Carbon Nanotube Yarns

Carbon nanotube (CNT) is an innovative material with significant potential for a wide range of applications, including but not limited to the development of lightweight composite materials or superconductors. A single CNT demonstrates an exceptional degree of tensile strength. CNTs are commonly employed in a structure of yarn, wherein several CNT strands are arranged and aligned together. CNT yarns, on the other hand, have a lower tensile strength than individual CNTs due to the different parameters of the yarn. This study aimed to investigate the effect of different structural parameters on the mechanical properties of CNT yarn. Sixty CNT yarn models with different structures were simulated with the molecular dynamic (MD) simulation. The varied parameters are the chirality of the CNTs, CNTs’ inner diameter, number of walls, crosslink density, and yarn twist angle. Tensile strength results from the simulations were compared concerning the varied parameters, and their influence on the nominal tensile strength of the CNT yarn was studied. It was found that the parameters for the CNT yarn that yields a higher tensile strength are the armchair type CNT with a small diameter, a large number of walls, crosslink density higher than approximately 1%, and a low twist angle.

A wave-transparent electrothermal sandwich composite for high-efficient anti-icing/de-icing

Light-weight composite materials with multifunction are widely used on aircrafts, antennas and wind turbines. Its anti-icing/de-icing is still a great challenge due to its poor heat transmission and low heat resistance. To address these issues, a novel electrothermal sandwich composite (NESC) was prepared by hot pressing an electrothermal film into load-bearing composite materials. By optimizing the ingredient of boron nitride nanoparticle, the heat diffusion rate was improved by 86 %, saving ∼14 % of energy consumption for complete anti-icing in dynamic icing conditions compared to samples without thermal conductivity enhancement. Furthermore, high electromagnetic transmittance up to 80 % of NESC was ensured through electromagnetic design. Besides the high-efficient and electromagnetic compatible anti-icing performance, NESC also performs high resistance to acid/alkali, abrasion and impact, good heating uniformity, stable photothermal effect, and potential of large-area manufacture. This paper presents a promising solution for durable, multi-functional, compatible, and energy-saving composites for anti-icing and de-icing.

Advances in Adhesive Bonded Joints for Composite Materials: A Comprehensive Review

Adhesive bonded joints have become increasingly significant across various high-tech industries, including aerospace, automotive, marine, and construction. This growing importance is largely due to their efficiency in joining lightweight composite materials, which are integral to modern engineering applications. In this review paper, we offer a comprehensive update on the latest advancements in adhesive bonded joints specifically for composite materials. The paper delves into several critical aspects of adhesive bonding, beginning with surface preparation. Proper surface preparation is crucial for achieving optimal bond strength and durability, and this paper explores the latest techniques and best practices in this area. It also examines current testing methods used to evaluate the performance of adhesive joints, ensuring that they meet stringent industry standards. Durability is another key focus of the review. We discuss various considerations related to the long-term performance of adhesive bonded joints, including environmental factors, load-bearing capacity, and resistance to aging and degradation. The paper also highlights recent innovations in adhesive technologies. These advancements include new formulations and types of adhesives that enhance bonding capabilities and overall joint performance. Additionally, we cover the latest developments in surface treatments, which play a critical role in improving adhesion by modifying the surface properties of composite materials. In summary, this review offers a thorough and up-to-date examination of adhesive bonded joints for composite materials, highlighting both recent advancements and ongoing challenges, while providing valuable insights into future research directions

Development of fishbone powder and carbon-jute fiber reinforced unsaturated polyester based novel green composites: investigation of tensile, impact strength, and morphology microstructure

In the last decade, hybrid composites have been the subject of significant advancements and study. This fast expansion necessitated the development of lightweight composite material with enhanced mechanical strength. In the past few years, the utilization of organic wastes from the food industry, like powder from cow bone, pig bone, and fishbone, has substantially improved the mechanical characteristics of composite materials. Owing to the increasing growth of the meat-based fisheries industry, fish bones are readily available. These bones are often thrown, causing environmental damage. Hence, a sustainable and unique hybrid composite was created employing fish bone powder (FBP) as filler in carbon/jute fibers reinforced polyester matrix. The hybrid composite was compression-molded. FTIR, SEM, and EDS studies were used to characterize the composite. The ASTM standard was used as a reference to explore the mechanical strength (tensile and impact). Additionally explored are the effects of alkali treatment of jute fiber on the above-mentioned processes. It was revealed that the inclusion of FBP and alkali treatment enhanced the mechanical characteristics of the composite. Fishbone powder with a particle size range of 200–250 μm and a weight percentage of 10 to 15% had significant fiber attachment which resulted in improved strength. The produced green composites have proven to be a viable choice for bio-waste treatment in the food sector.

Fabrication of low-cost metal polishing machine for preparation of microstructure samples of lightweight alloy and composite material

A metallographic specimen should be performed to determine the microstructure of a material. One of the processes of preparation of metallographic specimen is polishing process. The designing and manufacturing a polishing machine will be very helpful in the process of polishing the metallographic specimen. This apparatus includes a motor, aluminum circular disc, switch and speed controller, and base plate. The polishing machine is constructed from these individual parts. This paper is proposed to design and fabrication a metal polishing machine for laboratory scale. In this paper, several stages of the process are performed, such as designing the model, preparing tools and materials used, making components, assembling and testing the polishing machine. The results obtained are polishing machine that works well and can be used for the metallographic test for students and researchers. It can be concluded that this polishing machine can produce the level of specimen preparation for a metallographic test.

Common incidents and prevention of urban water supply pipeline accidents

With rapid urbanization, urban water supply pipeline accidents have become increasingly prevalent. Understanding their common occurrences and prevention strategies is crucial for ensuring water safety. This study aims to explore the underlying causes of such incidents, ranging from ageing infrastructure to design flaws and construction issues. By analyzing existing preventive measures and international best practices, this research seeks to contribute to the development of effective strategies for mitigating urban water supply pipeline accidents. This thesis focuses on urban water supply pipeline accidents, addressing both common incidents and preventive measures. It begins by highlighting the unique challenges of fires in high-rise buildings, including their rapid spread and the difficulties in evacuating occupants. The paper then emphasizes the importance of innovative fire-resistant materials, classifying them into various types such as phenolic, sprayed, calcium silicate, and lightweight composite materials. Each type is explored in detail, discussing their properties and potential applications. Finally, the paper outlines the practical use of these materials in fire engineering, emphasizing their role in enhancing the safety and resilience of urban infrastructure. Overall, this thesis contributes valuable insights to understanding and mitigating the risks associated with urban water supply pipeline accidents, promoting disaster prevention and management efforts.

On low velocity impact response of Sandwich composite with jute/epoxy facesheet and cenosphere reinforced functionally graded Core: Experimental and finite element approach

AbstractThe assessment of energy absorption behavior has a vital role in determining suitability of lightweight composites for impact protection applications. This study focuses on evaluating the energy absorption characteristics of cenosphere reinforced epoxy syntactic foam core and jute epoxy facesheet sandwich composite subjected to low velocity impact. The syntactic foam is composed of lightweight cenospheres embedded within an epoxy matrix, resulting in a high‐strength and lightweight composite material. Low velocity impact tests were conducted using a drop tower apparatus to simulate realistic impact conditions. The impact force, energy absorption and damage mitigation of the syntactic foam specimens with varied weight percentage of Cenosphere (0%, 10%, 20% and 30%) were measured and analyzed to assess the energy absorption behavior. The gradation revealed that the weight of top layer when compared to bottom layer is 50.49%, 70.23% and 95.74% less in syntactic foam core with 10, 20 and 30 wt% Cenosphere respectively, indicating gradation. The study demonstrates that the addition of Cenosphere up to 20 wt% enhances the energy absorption capacity of the sandwich composites by 59.37%–93.44% compared to sandwich without Cenosphere reinforced core. However, beyond this threshold, a decline in energy absorption is observed when subjected to low‐velocity impact (LVI). Fractography analysis demonstrates that the incorporation of Cenosphere induces a rough fracture surface with deep river‐like topography, indicative of greater energy absorption during impact loading. This substantiates the conclusion that Cenosphere‐reinforced syntactic foams exhibit enhanced impact resistance, making them promising materials for structures subjected to low‐velocity impact events.Highlights Development of functionally graded core sandwich composite for low velocity impact applications. Assessing the performance of developed composites under low velocity impact loading. Determining the optimal weight percentage of cenospshere in functionally graded core. Studying the fractography of developed sandwich composites. Corelation between experimental and finite element studies.

Research on compressive strength and thermal conductivity of lightweight phosphogypsum-based composite cementitious materials

Investigation of the mechanical and thermal properties of lightweight phosphogypsum-based cementitious materials can enhance the utilization rate of phosphogypsum-based composites and reduce building energy consumption. In this study, various amounts of hollow glass microspheres (HGMs) are incorporated into phosphogypsum-based composite cementitious materials to investigate their effects on mechanical and thermal insulation properties. Subsequently, comparisons are made between lightweight phosphogypsum-based composite cementitious materials produced by foaming and those with added HGMs. The research findings indicate that HGMs significantly mitigate the loss of compressive strength and notably reduce thermal conductivity, aligning with the development needs of lightweight and high-strength construction materials. Specifically, at a 20 % HGM dosage, the specimen exhibits a thermal conductivity of 0.2512 W·m−1·K−1, marking a 46.0 % decrease compared to specimens without HGMs. Additionally, the compressive strength measures at 25.45 MPa, reflecting a 36.4 % decline relative to specimens without HGMs. However, because of the uncontrolled pore structure and size in lightweight phosphogypsum-based composite cementitious materials produced by direct foaming, the decrease in compressive strength of foamed materials is greater than that of materials with added HGMs when the reduction in the coefficient of thermal conductivity is the same. Moreover, the compressive strength of phosphogypsum-based composite cementitious materials made with HGMs decreases more than that made with the addition of HGMs. This can be attributed to the uniform particle size of HGMs, contributing to a singular and complete pore structure within the phosphogypsum-based cementitious materials, thereby optimizing their performance.

Shear Behavior Investigation Between Lightweight Metals and Composite Materials

Recently, composites have attracted many researchers, which have advanced favored properties compared to metallic materials, such as high strength with lightweight as well as corrosion and erosion resistance and others. However, these resources are exposed to yield during the working life. For these new products and their employing in different applications, and for the mentioned importance, this study has investigated the mechanical characteristics of these structures to avoid potential failure. An investigation of Arcan (shear [Formula: see text]-notch test) procedure has been conducted for epoxy resin which is used for reimpregnations with Eglass and compared with lightweight metal of Aluminum as a benchmark to show the shear behavior for the composite one under shear load experimentally and numerically. The butterfly-shaped sample was modeled and tested accordingly with Arcan (shear [Formula: see text]-notch test) numerically in order to produce pure shear force by tensile subjection condition using ANSYS software. The test verified that a state of pure shear was present during the testing on each ± 45∘ inclined from the axis of subjection although the changes recorded as G magnitude of shear modulus between Aluminum and the thermoplastic of Epoxy resin is nearby 94%, which may be because of the effect of material nature (i.e. mechanical properties) for the selected materials. The mentioned novel constituent shear behaviors have been investigated and compared expediently. These novel constituents can be adopted in different applications of shear loads.

Composite repair method for internally damaged cylinders subjected to external pressure

In deep ocean environment, it is feasible to effectively repair damaged cylinders by externally wrapping them with lightweight composite materials. An analytical method was developed to determine the optimal thickness and length for composite repairs of internally damaged cylinders under uniform external pressure. The repair layer thickness was evaluated based on the assumption of full thickness reduction, and the repair length was determined by axial shearing theory. Repairs were designed for three groups of damaged cylinders, and the repaired and damaged cylinders were compared with pristine cylinders. The finite-element method was used to validate the analytical method and investigate the failure modes of the cylinders. Furthermore, the analytical and numerical results were validated through experiments. The results indicate that the load-bearing capacity of damaged cylinders can be completely restored using the designed repair method. Moreover, at collapse, the instability locations of the repaired cylinders exist near an end instead of the middle region. The analytical, numerical, and experimental results are in favorable agreement. The research contributes to offering a robust solution to repairing internally damaged cylindrical structures, promising substantial advantages in terms of maintenance and extending the lifespan of these structures.

Sustainable high-strength and dimensionally stable composites through in situ regulation and reconstitution of bamboo-derived lignin and hemicellulose contents

The development of modern construction and transportation industries demands increasingly high requirements for thin, lightweight, high-strength, and highly tough composite materials, such as metal carbides and concrete. Bamboo is a green, low-carbon, fast-growing, renewable, and biodegradable material with high strength and toughness. However, the density of its inner layer is low due to the functional gradient (the volume fraction of vascular bundles decreases from the outer layer to the inner layer), resulting in low performance, high compressibility, and significant amounts of bamboo waste. We utilized chemical and mechanical treatments of bamboo's low-density, low-strength inner layers to create lightweight, ultra-thin, high-strength, and high-toughness composites. The treatment included the partial removal of lignin and hemicellulose to alter the chemical components, followed by mechanical drying and hot pressing. The treated bamboo had 100.8 % higher tensile strength (150.35 MPa), 47.7 % higher flexural strength (97.67 MPa), and 132.0 % higher water resistance and was approximately 68.9 % thinner than the natural bamboo. The excellent physical and mechanical properties of the treated bamboo are attributed to the contraction of parenchyma cells during delignification, the interlocking due to the collapse of parenchyma cells during mechanical drying, and an increase in the density of hydrogen bonds between cellulose molecular chains during hot pressing. Our research provides a new strategy for obtaining sustainable, ultra-thin, lightweight, high-strength, and high-toughness composite materials from bamboo for construction and transportation applications.

Development of a Novel Lightweight Utility Pole Using a New Hybrid Reinforced Composite—Part 1: Fabrication and Experimental Investigation

This paper is the first part of a two-part paper that discusses the development of a novel lightweight and cost-effective hybrid 3D composite material and its and utilization for constructing utility poles. The main objective was to generate a material/pole with a comparable performance to the commercially available poles made of 2D fiber-reinforced polymer (FRP) and examine its feasibility. The novel hybrid composite was configured using a recently developed and marketed 3D E-glass fabric–epoxy composite reinforced with wood dowels, referred to as 3D dowel-reinforced FRPs (3D-drFRPs) hereafter. Firstly, the compressive and flexural properties of the 3D-drFRPs are evaluated. Then, the development of the 3D pole is discussed followed by the fabrication details of two 3D-drFRPs using the standard test method, and their responses are compared. For the second part, robust finite element (FE) models were developed in an LS-DYNA environment and calibrated based on the experimental results. A sophisticated nonlinear FE model was used to simulate the performances of ASTM standard-size compression and three-point bending specimens and tapered 2D and prismatic 3D poles. Moreover, the responses of equivalent 2D and 3D poles were simulated numerically, as the task could not be accommodated experimentally due to our laboratory’s deficiencies. The integrity of the numerical simulation results was validated against experimental results, confirming the accuracy of the developed model. As an example, the stiffness values for the 3-pt bending specimens and the 3D poles obtained through the simulations were very close to the experimentally obtained results, with small margins of errors of 3.2% and 0.89%, respectively. Finally, a simplified analytical calculation method was developed so practicing engineers can determine the stiffnesses of 3D-DrFRP poles very accurately and quickly.

Investigation of Thermomechanical Analysis of Carbon/Epoxy Composite for Spacecraft Structure Material

When building spacecraft structures, it is crucial to use lightweight and high-strength composite materials with the necessary characteristics. Aerospace applications benefit significantly from the exceptional properties of carbon/epoxy composite materials. As part of a study on composite materials, this work focuses on exploring the thermo-mechanical properties of carbon fiber. The matrix used in this research is LY-5052 epoxy, applied through a vacuum infusion technique. To achieve optimal composite properties, various tests are conducted to evaluate its thermo-mechanical behavior. These tests may include measuring Thermal Conductivity and performing thermogravimetric analysis (TGA). Most importantly, the composite is subjected to tensile testing at room temperature to 200 °C. This is done because most tensile tests on carbon/LY5052 composites are carried out at room temperature. The results obtained from the measurement of the thermal conductivity of the carbon/LY5052 composite were 0.419 W/mK; from the Thermogravimetric Analysis (TGA), the carbon/LY5052 composite began to decompose at a temperature of 365.63 °C and the tensile test was carried out simultaneously with variations in temperature from room temperature, 50 °C, 100 °C, 150 °C and 200 °C have tensile strengths of 553, 507, 340, 266, and 242 MPa, respectively. This trend confirms that strength decreases with higher temperature loads. Several image observations are also presented in this report to understand composite materials' failure behavior at these various temperatures.

Development and study of lightweight recycled composite materials based on linear low-density polyethylene and W for radiation application

Composite materials based on a polymer matrix of linear low-density polyethylene (LLDPE) and W were produced by thermal pressing. The content of W in the samples varied from 0 to 70 %. The recycling properties of LLDPE are demonstrated in this study, which significantly helped to reduce the defects. The microstructure of the composites consists of well-defined W grains covered by elastic LLDPE fibers. A homogeneous distribution of tungsten in the polymer matrix was observed for sample W70. The EDX analysis showed the presence of tungsten and carbon (from the polymer component). The XRD analysis confirms the increase in W content in the samples. The FTIR spectra of the composites showed an increase in the content of terminal methyl groups, a decrease in the molecular weight, and a decrease in the degree of crystallinity of the polyethylene matrix with an increase in the W content in the composite. The sample with 70 % W content has the highest effective density (2.61 g/cm3). The sample relative density ranges from 93.3 to 97.7 %. The porosity of LLDPE-W composites does not exceed 7 %. Gamma radiation shielding efficiency parameters such as LAC, HVL, and MFP were calculated using Phy-X/PSD. The radiation source was Co60, with an emission range of 0.8–2.5 MeV. As the gamma energy increases, it is observed that the values of all the parameters deteriorate. However, the sample with a maximum W content of 70 % has the best values of LAC, HVL, and MFP among the other samples.

A novel co-continuous Si–Zr hybrid phenolic aerogel composite with excellent antioxidant ablation enabled by sea-island-like ceramic structure at high temperature

Lightweight phenolic matrix composites are highly potential in ablative thermal protective materials for future aerospace vehicles, and hybrid modification is considered as one of effective ways to improve the oxidation and ablation resistance of phenolic matrix. Herein, the co-continuous silica zirconium hybrid phenolic resin (SZRx) was synthesized, which can be used to prepare lightweight ablation resistant aerogel composites by the sol-gel method. The SZRx aerogel showed stable porous structure and excellent hydrophobicity, and the silicon zirconium hybrid greatly improved the high temperature carbon residue and oxidation resistance of phenolic resin. The SZRx aerogel composites also possessed excellent integrated characteristics of heat protection and insulation, with a linear ablation rate as low as 0.032 mm/s and a back temperature below 200 °C. The synergistic effect of silicon and zirconium formed the multiphase ceramic layer (C, SiO2, SiC and ZrO2) with the sea-island-like structure on the surface of the composite material, which can effectively resist high-temperature ablation and aerodynamic exfoliation. This can indicate that silicon zirconium dual element hybrid phenolic resin is the highly competitive matrix for lightweight and ablation resistant composite materials.

Development of a flexible composite based on vulcanized silicon casting with bismuth oxide and characterization of its radiation shielding effectiveness in diagnostic X-ray energy range and medium gamma-ray energies

The study aims to develop a novel, lead-free, flexible and lightweight composite shielding material against ionizing radiation. For this, it was used bismuth oxide (Bi2O3) in RTV-2 silicon matrix. The shielding tests were carried out in both diagnostic X-ray energies and intermediate gamma-ray energy range of up to 662 keV to determine the radiation attenuation properties of this material in terms of attenuation ratio, half value layer, tenth value layer, mean free path and lead equivalency of samples in weight of 30%, 40%, 50% in Bi2O3. In the diagnostic X-ray energy range, half value layer, tenth value layer and lead equivalency (in mm Pb) of the produced samples were measured at 80 and 100 kVp narrow beam conditions according to the requirements of EN IEC 61331-1 standard. The results show that lead equivalent values of the produced novel sheets was measured to be 0.16 mm Pb, corresponding to a 6 mm thickness of the flexible sample when it contains 30% wt. Bi2O3 in RTV matrix. The experimental findings for durability and flexibility also indicated that this new RTV-based flexible, lead -free shielding composite can be used safely for especially for manufacturing aprons, garments and thyroid guards used in mammography, radiology, nuclear medicine and dental applications in practice.

The multifunctional flexible conductive viscose fabric prepared by thiol modification followed by copper plating

Based on the current rapid development of electronic products, the development of light-weight, processable, environmentally friendly, long-life, durable, less corrosive, and tunable conductive composite materials with multiple applications may be the development direction of next-generation electronic devices. In this work, for the first time, we employed 3-Mercaptopropyltrimethoxysilane (3-MT) to modify viscose nonwovens and enhance the copper plating process. The prepared samples were characterized by Fourier transform infrared, Wide-angle X-ray diffraction (WAXD), scanning electron microscope + energy dispersive X-ray spectroscopy (SEM + EDS), thermogravimetric analysis (TGA), electrical resistivity, anti-corrosion, Joule heating, and electromagnetic interference (EMI) shielding. Results showed that 3-MT was covalently bound to the viscose surface through hydrolysis and condensation reactions and introduced SH groups. WAXD confirmed that the thiol modification did not change the internal crystal structure of viscose and copper ions. TGA and surface morphology analysis confirmed that the modified viscose promoted the deposition of metal particles in the copper plating process due to the affinity of thiol to metal so that copper particles almost completely wrapped the viscose fibers. In addition, 3MT@Cu@Viscose exhibits extremely low surface and volume resistivity (346.6 and 333.2 mΩ·m), improved corrosion resistance (corrosion rate reduced by 58% compared to the unmodified sample), fast Joule heating response (within 10 s) in low voltage (1 V) and excellent EMI shielding effectiveness (EMI SE > 50 dB). It showed great potential in future multi-functional electronic products such as electric heating sensors, smart clothing, and EMI shielding barrier.

Quantitative Detection of Defects in Multi-Layer Lightweight Composite Structures Using THz-TDS Based on a U-Net-BiLSTM Network.

Multi-layer lightweight composite structures are widely used in the field of aviation and aerospace during the processes of manufacturing and use, and, as such, they inevitably produce defects, damage, and other quality problems, creating the need for timely non-destructive testing procedures and the convenient repair or replacement of quality problems related to the material. When using terahertz non-destructive testing technology to detect defects in multi-layer lightweight composite materials, due to the complexity of their structure and defect types, there are many signal characteristics of terahertz waves propagating in the structures, and there is no obvious rule behind them, resulting in a large gap between the recognition results and the actual ones. In this study, we introduced a U-Net-BiLSTM network that combines the strengths of the U-Net and BiLSTM networks. The U-Net network extracts the spatial features of THz signals, while the BiLSTM network captures their temporal features. By optimizing the network structure and various parameters, we obtained a model tailored to THz spectroscopy data. This model was subsequently employed for the identification and quantitative analysis of defects in multi-layer lightweight composite structures using THz non-destructive testing. The proposed U-Net-BiLSTM network achieved an accuracy of 99.45% in typical defect identification, with a comprehensive F1 score of 99.43%, outperforming the CNN, ResNet, U-Net, and BiLSTM networks. By leveraging defect classification and thickness recognition, this study successfully reconstructed three-dimensional THz defect images, thereby realizing quantitative defect detection.