Real-time non-destructive monitoring of concrete curing and strength development using hyperspectral remote sensing

This research investigates the use of ultrasonic guided Lamb waves for real-time monitoring of adhesive curing. Two industrial adhesives with different chemical and mechanical were studied: Poly Max and Densolen Primer on an aluminium waveguide. The most sensitive Lamb wave mode-frequency combinations for each adhesive were identified. Two novel amplitude-based indices were developed to track the curing process. These indices showed a strong correlation between changes in adhesive stiffness and Lamb wave amplitude variations. The results demonstrated a decrease in Lamb wave amplitude with adhesive stiffening, enabling effective monitoring of the adhesive phase transitions and the curing process. The adhesives exhibited different overall curing mechanism with minimal room temperature influence: Densolen Primer stiffened rapidly, while Poly Max stiffened gradually. The alignment between the proposed indices and recommended cure times suggests a promising Lamb wave-based technique for adhesive cure monitoring, particularly valuable in limited-access scenarios.

Monitoring the curing process is crucial for guiding and optimizing the curing procedures of composite material repair patches. Traditional embedded online monitoring methods are limited in their ability to track the curing process of these patches. This paper presents a composite material curing monitoring platform designed using dielectric methods. It integrates temperature control, pressure control, dielectric signal acquisition, control and display modules, and is specifically tailored for bag molding curing of repair patches. The platform measures the ionic viscosity of T300 2019B composites, analyzes the curing index, and correlates it with DSC-cured degree tests. The results indicate that the multiple ionic viscosity curves obtained from monitoring exhibit consistent trends, with correlation coefficients between curves exceeding 0.96. The changes in curing index align with the changes in curing degree, demonstrating that the platform can reliably and accurately monitor the ionic viscosity of repair patches. This platform enables effective monitoring of the ionic viscosity during the curing process of composite material repair patches.

Epoxy resins are widely used in the manufacture of composite materials for a wide range of applications. Control of the curing process is an important consideration in ensuring product quality and minimizing production times. The curing of epoxy resin is associated with temperature, strain, and refractive index changes but it is difficult to monitor these quantities individually and hence difficult to achieve accurate control of the curing process. One promising approach for monitoring these quantities is the use of long-period fiber gratings (LPFG). We analyze the spectral response of a LPFG in epoxy resins to temperature, strain, and refractive index. Wavelength shifts and dip amplitudes of cladding mode notches are monitored and are used to decouple temperature, strain, and refractive index for gratings in air, liquid, and hardened resins. The three measurands are found from wavelength shifts and dip amplitudes, employing multiplication by a weighted pseudo-inverse matrix assuming linear dependences between the spectral and external parameters. We propose a new model to describe the influence of fiber parameters and external refractive index, temperature, and strain on the spectral behavior of long-period fiber gratings in epoxy resins during hardening. The results obtained can be utilized for multiparameter cure process monitoring of epoxy resins by using long-period fiber gratings.

Online in-situ monitoring of curing and impact damage of composite structure with embedded CNT sensors

Smart concrete is a structural element that can combine both sensing and structural capabilities. In addition, smart concrete can monitor the curing of concrete, positively impacting design and construction approaches. In concrete, if the curing process is not well developed, the structural element may develop cracks in this early stage due to shrinkage, decreasing structural mechanical strength. In this paper, a system of measurement using fiber Bragg grating (FBG) sensors for monitoring the curing of concrete was developed to evaluate autogenous shrinkage strain, temperature, and relative humidity (RH) in a single system. Furthermore, K-type thermocouples were used as reference temperature sensors. The results presented maximum autogenous shrinkage strains of 213.64 με, 125.44 με, and 173.33 με for FBG4, FBG5, and FBG6, respectively. Regarding humidity, the measured maximum relative humidity was 98.20 %RH, which was reached before 10 h. In this case, the recorded maximum temperature was 63.65 °C and 61.85 °C by FBG2 and the thermocouple, respectively. Subsequently, the concrete specimen with the FBG strain sensor embedded underwent a bend test simulating beam behavior. The measurement system can transform a simple structure like a beam into a smart concrete structure, in which the FBG sensors' signal was maintained by the entire applied load cycles and compared with FBG strain sensors superficially positioned. In this test, the maximum strain measurements were 85.65 με, 123.71 με, and 56.38 με on FBG7, FBG8, and FBG3, respectively, with FBG3 also monitoring autogenous shrinkage strain. Therefore, the results confirm that the proposed system of measurement can monitor the cited parameters throughout the entire process of curing concrete.

Abstract The aerospace industries demand for advanced materials necessitates stringent quality control measures, particularly for composite structures to ensure optimal curing and structural integrity. Traditional methods like differential scanning calorimetry (DSC) and dynamic mechanical analysis, while useful, are limited to laboratory settings. This study explores the use of a printed paper‐based sensor for real‐time cure monitoring of glass fiber‐reinforced phenol‐formaldehyde (PF/GF) prepregs. The sensor, composed of a cellulose paper substrate with screen‐printed electrodes, offers flexibility, ease of integration, and cost‐effectiveness compared to commercial polymer‐based dielectric sensors. Real‐time monitoring with the printed paper‐based sensor was conducted at temperatures ranging from 130 to 180°C, both with and without pressure, and validated against DSC and Fourier‐transform infrared (FTIR) spectroscopy. The results showed a strong correlation between real‐time monitoring and the DSC prediction model, as well as FTIR spectroscopy. Notably, the real‐time monitoring revealed a shorter curing cycle on the production line than the predicted cure kinetic model, which is crucial for large‐scale production, such as fabricating honeycomb panels for aircraft interiors. This research underscores the importance of innovative sensor technologies in improving quality control and manufacturing efficiency in aerospace composites.Highlights Cure Monitoring of PF‐based prepregs was carried out using paper‐based sensor. Sensor offered reproducibility and seamless integration in composites. Sensor data showed a strong correlation with DSC and FTIR spectroscopy results. Paper sensors enable online monitoring of composite in aerospace manufacturing.

Recently, rigid sensors have been commonly applied to online monitoring of the core curing processes of composite materials to prevent both overcuring and under-curing. However, conventional rigid sensors are prone to causing cracks and bubbles in composite materials during the curing process, thereby affecting both the mechanical performance and the overall reliability of the materials. Herein, stretchable interdigital dielectric sensors with flexible substrates and electrodes are designed to conform to complex 3D surfaces, thus enabling embedded nondestructive monitoring of composite curing processes. The sensors obtained can endure 1000 cycles of bending from 0° to 180° and 1000 cycles of stretching at 30% strain while still conforming perfectly to complex 3D surfaces, thus overcoming the inability of traditional curing monitoring sensors to bend. Additionally, sensor integration with an electronic circuit enables real-time data collection and transmission, which makes the device more portable, compact, and lightweight. Moreover, after atmospheric exposure for 5 months, the unit sensitivity of the sensor decreased by only 0.1%, thus demonstrating its excellent reliability and stability. Furthermore, during curing monitoring of the complex three-dimensional surfaces of the Fendouzhe deep-sea submersible, the unit's sensitivity is close to that of conventional planar monitoring equipment, decreasing by only 0.4%. The proposed online nondestructive monitoring technology demonstrates high sensitivity, high monitoring accuracy, and high reliability during surface monitoring, thus enabling long-term curing monitoring under complex nonplanar conditions.

Diffractive optical elements (DOE) offer a wide range of possibilities, from beam shaping to augmented reality and neural networks. Additive manufacturing using photopolymerization curing shows great potential for manufacturing customizable DOEs at low cost. To design and fabricate these, it is essential to monitor both the dynamic evolution of the curing process as well as its spatial distribution during and after curing. To this end, we propose an all-optical monitoring platform comprising a "focused line refractive index microscopy" (FLRIM) technique based on total internal reflection at a prism interface and a large field-of-view interferometric imager, more specifically a "lateral-shearing interferometric microscopy" (LIM) technique. The FLRIM enables dynamic in-situ spatio-temporal quantitative measurements of refractive index (RI) during the curing process, while the LIM technique provides quantitative information of 2D structures by measuring the transmitted phase with high sensitivity via multi-angle illumination. An ultraviolet "digital micromirror device" (DMD) projector is used to photopolymerize pixelated 2D structures. By using the proposed monitoring platform, we investigate the resulting photopolymer structures in-situ during several local curing steps, and, additionally, before and after a global post-curing. Besides showing the capabilities of our technology, we additionally demonstrate that there is potential to fabricate DOEs on a single photopolymer by exploiting tunable local RI changes.

The modulus of elasticity of concrete (Ec) is an essential parameter commonly used in concrete design, concrete curing monitoring, and deterioration evaluation and damage detection of concrete structures. A quick and reliable in situ determination of Ec helps in accurate decision‐making for construction and maintenance. The electromechanical impedance (EMI) technique using surface‐bonded lead zirconate titanate (PZT) patches (referred to as SBP) has become a popular nondestructive method for monitoring concrete structures due to ease of operations. The existing research mainly utilized baseline‐dependent approaches to monitor the changes (e.g., hardening or damage evolution) of concrete structures. However, relevant baselines are greatly influenced by the status of PZT sensors and bonding layers, limiting the practical applications of this technique. This paper presents a baseline‐free EMI resonance method for measuring Ec of concrete without using prior baseline data for the first time. Numerical and experimental studies on standard concrete cubes (100 mm in width) with SBP were conducted to examine the proposed method. A dimensionless physical quantity was first proposed to view the EMI signals. Then, resonance peaks highly related to concrete properties but insensitive to the status of the sensor system were selected, physically described, and correlated to the concrete parameters through numerical analyses. Finally, experimental validations, covering the measurement of Ec, repeatability of the chosen resonance peaks, and temperature effects, were conducted to illustrate the proposed method’s stability, accuracy, and sensitivity.

Passive, Wireless In Situ Millimeterwave Sensor With Mounted Dielectric Channels for Cure Monitoring of Carbon Fiber-Reinforced Polymers

Continuous and accurate monitoring of the degree of curing (DoC) is essential for ensuring the structural integrity of fabricated composites during service. Although machine learning (ML) has shown effectiveness in DoC monitoring, its generalization and extendibility are limited when applied to other curing-related scenarios not included in the previous learning process. To break through this bottleneck, we propose a novel DoC monitoring approach that utilizes transfer learning (TL)-boosted convolutional neural networks alongside Gramian angular field-based imaging processing. The effectiveness of the proposed approach is validated through experiments on metal/polymeric composite co-bonded structures and carbon fiber reinforced polymers using raw sensor data separately collected through the electromechanical impedance and fiber Bragg grating (FBG) measurements. Four indicators, accuracy, precision, recall, and F1-score are introduced to evaluate the performance of generalization and extendibility of the proposed approach. The indicator scores of the proposed approach exceed 0.9900 and outperform other conventional ML algorithms on the FBG dataset of the target domain, demonstrating the effectiveness of the proposed approach in reusing the pre-trained base model on the composite curing monitoring issues.

Application of Ultrasonic Technique for Cure Monitoring of Epoxy/Graphene Oxide–Carbon Nanotubes Composites

Real-time monitoring of concrete curing using fiber Bragg grating sensors: Strain and temperature measurement

Smart Piezoresistive Plaster of Paris for Real-Time Monitoring of Curing and Compressive Stress Changes Quantified Using Vipulanandan Models

Challenges for structural health monitoring of concrete curing using piezoelectric sensor and electromechanical impedance (EMI) technique: A critical review

The ability to measure the degree of cure of epoxy resins is an important prerequisite for making manufacturing processes for fibre-reinforced plastics controllable. Since a number of physical properties change during the curing reaction of epoxy resins, a wide variety of measurement methods exist. In this article, different methods for cure monitoring of epoxy resins are applied to a room-temperature curing epoxy resin and then directly compared. The methods investigated include a structure-borne sound acoustic, a dielectric, an optical and a strain-based observation method, which for the first time are measured simultaneously on one and the same resin sample. In addition, the degree of cure is determined using a kinetic resin model based on temperature measurement data. The comparison shows that the methods have considerable but well-explainable differences in their sensitivity, interference immunity and repeatability. Some measurement methods are only sensitive before and around the gel point, while the strain-based measurement method only reacts to the curing from the gel point onwards. These differences have to be taken into account when implementing a cure monitoring system. For this reason, a multi-sensor node is suitable for component-integrated curing monitoring, measuring several physical properties of the epoxy resin simultaneously.

Field test study on early strain development law of mass concrete in cold weather

A cure kinetics investigation of a high temperature-resistant phenol novolac cyanate ester toughened with polyether sulfone (CE-PES blend) was undertaken using non-isothermal differential scanning calorimetry. Thin ply carbon fiber prepreg, based on the CE-PES formulation, was fabricated, and plates for further in-situ cure monitoring were manufactured using automated fiber placement. Online monitoring of the curing behavior utilizing Optimold sensors and Online Resin State software from Synthesites was carried out. The estimation of the glass transition temperature and degree of cure allowed us to compare real time data with the calculated parameters of the CE-PES formulation. Alongside a good agreement between the observed online data and predicted model, the excellent performance of the developed sensors at temperatures above 260 °C was also demonstrated.

The curing process and thermoresistive response of a single carbon nanotube yarn (CNTY) embedded in a room temperature vulcanizing (RTV) silicone forming a CNTY monofilament composite were investigated toward potential applications in integrated curing monitoring and temperature sensing. Two RTV silicones of different crosslinking mechanisms, SR1 and SR2 (tin- and platinum-cured, respectively), were used to investigate their curing kinetics using the electrical response of the CNTY. It is shown that the relative electrical resistance change of CNTY/SR1 and CNTY/SR2 monofilament composites increased by 3.8% and 3.3%, respectively, after completion of the curing process. The thermoresistive characterization of the CNTY monofilament composites was conducted during heating–cooling ramps ranging from room temperature (RT~25 °C) to 100 °C. The thermoresistive response was nearly linear with a negative temperature coefficient of resistance (TCR) at heating and cooling sections for both CNTY/SR1 and CNTY/SR2 monofilament composites. The average TCR value was −8.36 × 10−4 °C−1 for CNTY/SR1 and −7.26 × 10−4 °C−1 for CNTY/SR2. Both monofilament composites showed a negligible negative residual relative electrical resistance change with average values of ~−0.11% for CNTY/SR1 and ~−0.16% for CNTY/SR2 after each cycle. The hysteresis amounted to ~21.85% in CNTY/SR1 and ~29.80% in CNTY/SR2 after each cycle. In addition, the effect of heating rate on the thermoresistive sensitivity of CNTY monofilament composites was investigated and it was shown that it reduces as the heating rate increases.