Examining the Efficacy of using IRT as an NDE method for CFRP and ZTCFRP Composites

A film adhesive is commonly used to form the bond-line between composite parts. The bond-line\u27s quality and performance can be affected by defects such as voids, impurities, and agglomerations. Identifying these defects is possible with non-destructive evaluation testing (NDE). In this thesis, the joule-heating effect (JH) through carbon nanofiber (CNF) and carbon nanotube (CNT) modified film adhesive bonded to control carbon fiber reinforced polymer (CCFRP) was used along with infrared thermography testing (IRT) for bond-line defect inspection as a preliminary study. Due to the difference in the electrical conductivity between the modified epoxy and the defect, JH can cause a difference in temperature that can be viewed by IRT. The percentage of carbon nanofiller in a film adhesive is related to its electrical conductivity. A three-dimensional (3D) printed acrylonitrile butadiene (ABS) equilateral triangle defect was used. A more in-depth investigation was conducted to determine how both the number of plies and z-threaded carbon fiber reinforced polymer (ZTCFRP) can influence the defect image. Flash thermography (F-IRT) was used to provide a comparison against this NDE method

Development of Non-Destructive Testing by Eddy Currents for Highly Demanding Engineering Applications

Defect detection with Non-Destructive Testing (NDT) is essential in accidents prevention, requiring R&TD to generate new scientific and procedural knowledge for new products with high safety requirements. A current challenge lies in the detection of surface and sub-surface micro defects with NDT by Eddy Currents (EC). The main objective of this work was the development of applied research, technological innovation and experimental validation of EC customized systems for three highly demanding inspection scenarios: micro defects in tubular geometries; brazed joints for the automotive industry; and high-speed moving composite materials. This objective implied starting from the scientific fundamentals of NDT by EC to design and simulate EC probes and the prototypes developed were tested in industrial environment, reaching a TRL ≈ 5. Another objective, of a more scientific and disruptive nature, was to test a new technique for the creation of EC in the materials to be inspect, named Magnetic Permeability Pattern Substrate (MPPS). This technique consists on the development of substrates/films with patterns of different magnetic permeabilities rather than the use of excitation bobbin coils or filaments of complex geometry. The experimental results demonstrated that the prototypes developed for the three industrial applications studied outperformed the state of the art, allowing the detection of target defects with a very good signal-to-noise ratio: in tubular geometries defects with depth of 0.5 mm and thickness of 0.2 mm in any scanning position; in the laser brazed weld beads pores with 0.13 mm diameter and internal artificial defects 1 mm from the weld surface; in composite materials defects under 1 mm at speeds up to 4 m/s and 3 mm lift-off. The numerical simulations assisted the probe design, allowing to describe and characterize electrical and magnetic phenomena. The new MPPS concept for the introduction of EC was validated numerically and experimentally

Nondestructive Evaluation and Structural Health Monitoring of Manufacturing Flaws and Operational Damage in Composite Structures

Advanced composite materials have begun to be used extensively in the manufacturing of aerospace structures. Composite aerospace structures can develop complex types of damages during the manufacturing stages and operational lifetime. This creates an indispensable demand for appropriate nondestructive evaluation (NDE) and structural health monitoring (SHM) methods that are tailored to specific types of damage. This dissertation addresses innovative methods for NDE and SHM of manufacturing flaws and operational damage in composite structures. For the NDE of manufacturing flaws in carbon fiber reinforced polymer (CFRP) composites, an eddy current testing (ECT) NDE method has been developed by conducting multiphysics modeling and simulation of ECT detection of different types of manufacturing flaws. In addition, extensive experiments have also been conducted on in-house manufactured composite specimens with embedded manufacturing flaws, and specimens obtained from Boeing which had realistic manufacturing flaws. To validate NDE and SHM methods, controlled impact testing experiments have been conducted on quasi-isotropic CFRP composites of increasing thicknesses (2-mm, 4-mm and 6-mm) to create approximately 1 impact damage diameter. The impact testing experiments were conducted with increasing energy and it was observed that it was easier to experimentally obtain desired impact damage size in thin composites compared to thicker composites. Each impact damage was characterized using ultrasonic NDE, X-ray micro computed tomography (CT) and contact profilometry methods. A pure mode guided wave excitation method using a variable angle beam transducer (ABT) as the excitation, and a phased array transducer (PAT) as the receiver, has been developed. This method has been used for exciting pure SH0 mode guided wave in quasi-isotropic composites for the detection of impact damage. Experiments have demonstrated that the presence of impact damage in thin composites leads to amplitude drop in the received signal. On the other hand, in thicker composites, in addition to amplitude drop we can also observe mode conversion. An in-situ acoustic emission recording method for impact damage ascertainment has been implemented. This method utilizes a quasi-isotropic composite coupon which has been instrumented with four piezoelectric wafer active sensors (PWAS) to record real-time acoustic emission signals during an impact event. The impact event is created by conducting drop weight impact tests. Through this method, it is possible to ascertain if an impact event has indeed caused damage to the composite or not. The dissertation finishes with concluding remarks which include summary, conclusions and suggestions for future work

Terfenol-D Carbon Fiber Reinforced Polymer (CFRP) Embedded Sensing for Early Damage Detection

Carbon Fiber Reinforced Polymers (CFRPs) have become an essential part of designing and engineering lightweight rigid bodies; predominantly in the aerospace and automotive industries. Typical epoxy-based CFRPs exhibit virtually no plasticity with minimal strain to failure. Although CFRPs have high specific strengths and elastic moduli, the brittle fracture mechanism presents unique challenges in failure detection for the Army\u27s vertical lift vehicles since failure occurs catastrophically. The current state of the art structural health monitoring (SHM) for aerospace structures are intrusive to the surface of the part and/or requires electrical connectivity. Army uses a safe-life interval-based service methodology where components are replaced with regards to a usage spectrum rather than the component\u27s actual state of structural health. This paper explores a method for solving this problem by investigating the possibility of embedding Terfenol-D (~100 microns in diameter), a magnetostrictive material, into the CFRPs for embedded non-contact structural health monitoring. For baseline results, the change in localized (32 mm2 field of view) magnetic flux was only 0.02% for an applied load of 0-100% of the material\u27s ultimate tensile strength (UTS). For quasi-static testing procedure of specimen 5714 (15 wt.% Terfenol-D embedded CFRP) on a 0-40% loading interval of the material\u27s UTS, there was an observed localized (32 mm2 field of view) magnetic flux gradient of more than 5 mT (4%) with a reversible flux of 100%. For quasi-static testing procedure of specimen 5714 (15 wt.% Terfenol-D embedded CFRP) on a 0-70% loading interval of the material\u27s UTS, there was an observed localized (32 mm2 field of view) magnetic flux gradient of more than 3 mT (2%) with a reversible flux of only 25%. Terfenol-D embedded CRFPs have shown promising results for detecting instantaneous levels of degradation. Acoustic emission (AE), X-ray computed tomography (CT) scanning, Finite Element Analysis (FEA) and analytical modeling were used to validate the observed results

Dielectric Characteristics of Microstructural Changes and Property Evolution in Engineered Materials

Heterogeneous materials are increasingly used in a wide range of applications such as aerospace, civil infrastructure, fuel cells and many others. The ability to take properties from two or more materials to create a material with properties engineered to specific needs is always very attractive. Hence engineered materials are evolving into more complex formulations or heterogeneities in multiple disciplines. Design of microstructure at multiple scales controls the global functional properties of these materials and their structures. However, local microstructural changes do not directly cause a proportional change to the global properties (such as strength and stiffness). Instead, local changes follow a latent evolution process including significant interactions for the most of the life and only shows significant bulk property change prior to failure. Therefore, in order to understand property evolution of engineered materials and predict potential catastrophic failure, microstructural changes need to be effectively captured. Characterizing these changes and representing them by material variables will enable us to further improve our material level understanding. In this work, we will demonstrate how microstructural features of heterogeneous materials can be described quantitatively using broadband dielectric spectroscopy (BbDS). The frequency dependent dielectric properties can capture the change in material microstructure and represent these changes in terms of material variables, such as complex permittivity. These changes in terms of material properties can then be linked to a number of different conditions, such as increasing damage due to impact or fatigue. Two different broadband dielectric spectroscopy modes are presented: i) standard BbDS for measurements of bulk properties, and ii) continuous scanning mode (Scanning BbDS) to measure dielectric property change as a function of position across the specimen. In this study, we will focus on both ceramic materials and fiber reinforced polymer matrix composites as test bed material systems. In the first part of the thesis, we will present how different micro-structural design of porous ceramic materials can be captured quantitatively using BbDS. These materials are typically used in solid oxide fuel cells (SOFC) as anode materials. Results show significant effect of microstructural design on material properties at multiple temperatures (up to 800 C). In the later part of the thesis, we will focus on microstructural changes of fiber reinforced composite materials due to impact and static loading. The changes in dielectric response can then be related to the bulk mechanical properties of the material and various damage modes. Observing trends in dielectric response enables us to further determine local mechanisms and distribution of properties throughout the damaged specimens. A 3D X-ray microscope and a digital microscope have been used to visualize these changes in material microstructure and validate experimental observations. The increase in damage observed in the material microstructure has been captured by the changes in BbDS characteristics. Results show that BbDS is an extremely useful tool for identifying microstructural changes within a heterogeneous material and particularly useful in relating remaining properties. For example, the remaining property (such as modulus) after low velocity impact shows significantly higher sensitivity to dielectric properties and hence provided a more accurate representation of material state change. This sensitivity holds for both predominantly dielectric glass fiber to mixed conductor carbon fiber reinforced composite. The great advantage of using material variables is that these variable can be defined at multiple scales, and hence used directly in property degradation laws to help develop a framework for future predictive modeling methodologies

Surface and inter-phase analysis of Composite Materials using Electromagnetic Techniques based on SQUID Sensors

In this thesis an electromagnetic characterization and a non-destructive evaluation of new advanced composite materials, Carbon Fiber Reinforced Polymers (CFRP) and Fiber-Glass Aluminium (FGA) laminates, using an eddy-current technique based on HTS dc-SQUID (Superconductive QUantum Interference Device) magnetometer is proposed. The main goal of this thesis is to propose a prototype based on a superconducting sensor, such as SQUID, to guarantee a more accuracy in the quality control at research level of the composite materials employed in the aeronautical applications. A briefly introduction about the superconductivity, a complete description of the SQUID properties and its basic working principles have been reported. Moreover, an overview of the most widely used non destructive technique employed in several industrial and research fields have been described. Particular attention is given to the eddy current testing and the technical improvement obtained using SQUID in NDE. The attention has been focused on two particular application, that are the main topics of this thesis. The first concerns with the investigation of the damage due to impact loading on the composites materials, and the second is the study of the corrosion process on the metallic surface. The electrical and mechanical properties of the tested advanced composite materials, such as Carbon Fiber Reinforced Polymers (CFRPs) and Fiber-glass Aluminium (FGA) laminates are investigated. The experimental results concern the non-destructive evaluation of impact loading on the CFRPs and FGA composites, by means of the electromagnetic techniques; the investigation of the electromechanical effect in the CFRPs using the SQUID based prototype and the AFM analyses; and the study of corrosion activity of the metallic surface using magnetic field measurement

Wireless Sensors and Actuators for Structural Health Monitoring of Fiber Composite Materials

This work evaluates and investigates the wireless generation and detection of Lamb-waves on fiber-reinforced materials using surface applied or embedded piezo elements. The general target is to achieve wireless systems or sensor networks for Structural Health Monitoring (SHM), a type of Non-Destructive-Evaluation (NDE). In this sense, a fully wireless measurement system that achieves power transmission implementing inductive coils is reported. This system allows a reduction of total system weight as well as better integration in the structure. A great concern is the characteristics of the material, in which the system is integrated, because the properties can have a direct impact on the strength of the magnetic field. Carbon-Fiber-Reinforced-Polymer (CFRP) is known to behave as an electrical conductor, shielding radio waves with increasing worse effects at higher frequencies. Due to the need of high power and voltage, interest is raised to evaluate the operation of piezo as actuators at the lower frequency ranges. To this end, actuating occurs at the International Scientific and Medical (ISM) band of 125 kHz or low-frequency (LF) range. The feasibility of such system is evaluated extensively in this work. Direct excitation, is done by combining the actuator bonded to the surface or embedded in the material with an inductive LF coil and setting the circuit in resonance. A more controlled possibility, also explored, is the use of electronics to generate a Hanning-windowed-sine to excite the PWAS in a narrow spectrum. In this case, only wireless power is transmitted to the actuator node, and this lastly implements a Piezo-driver to independently excite Lamb-waves. Sensing and data transfer, on the other hand, is done using the high-frequency (HF) 13.56 MHz. The HF range covers the requirements of faster sampling rate and lower energy content. A re-tuning of the antenna coils is performed to obtain better transmission qualities when the system is implemented in CFRP. Several quasi-isotropic (QI) CFRP plates with sensor and actuator nodes were made to measure the quality of transmission and the necessary energy to stimulate the actuator-sensor system. In order to produce baselines, measurements are prepared from a healthy plate under specific temperature and humidity conditions. The signals are evaluated to verify the functionality in the presence of defects. The measurements demonstrate that it is possible to wirelessly generate Lamb-waves while early results show the feasibility to determine the presence of structural failure. For instance, progress has been achieved detecting the presence of a failure in the form of drilled holes introduced to the structure. This work shows a complete set of experimental results of different sensor/-actuator nodes

Microwave NDT&E using open-ended waveguide probe for multilayered structures

Ph. D. Thesis.Microwave NDT&E has been proved to be suitable for inspecting of dielectric structures due to low attenuation in dielectric materials and free-space. However, the microwave responses from multilayered structures are complex as an interrogation of scattering electromagnetic waves among the layers and defects. In many practical applications, electromagnetic analysis based on analytic- and forward structural models cannot be generalised since the defect shape and properties are usually unknown and hidden beneath the surface layer. This research proposes the design and implementation of microwave NDT&E system for inspection of multilayered structures. Standard microwave open-ended rectangular waveguides in X, Ku and K bands (frequency range between 8-26.5 GHz) and vector network analyser (VNA) generating sweep frequency of wideband monochromatic waves have been used to obtain reflection coefficient responses over three types of challenging multilayered samples: (1) corrosion progression under coating, (2) woven carbon fibre reinforced polymer (CFRP) with impact damages, and (3) thermal coated glass fibre reinforced polymer (GFRP) pipe with inner flat-bottom holes. The obtained data are analysed by the selected feature extraction method extracting informative features and verify with the sample parameters (defect parameters). In addition, visualisation methods are utilised to improve the presentation of the defects and material structures resulting in a better interpretation for quantitative evaluation. The contributions of this project are summarised as follows: (1) implementation of microwave NDT&E scanning system using open-ended waveguide with the highest resolution of 0.1mm x 0.1 mm, based on the NDT applications for the three aforementioned samples; (2) corrosion stages of steel corrosion under coating have been successfully characterised by the principal component analysis (PCA) method; (3) A frequency selective based PCA feature has been used to visualise the impact damage at different impact energies with elimination of woven texture influences; (4) PCA and SAR (synthetic aperture radar) tomography together with time-offlight extraction, have been used for detection and quantitative evaluation of flat-bottom hole defects (i.e., location, size and depth). The results conclude that the proposed microwave NDT&E system can be used for detection and evaluation of multilayered structures, which its major contributions are follows. (1) The early stages (0-12month) of steel corrosion undercoating has been successfully characterised by mean of spectral responses from microwave opened rectangular waveguide probe and PCA. (2) The detection of low energy impact damages on CFRP as low as 4 Joules has been archived with microwave opened rectangular waveguide probe raster scan together with SAR imaging and PCA for feature extraction methods. (3) The inner flat-bottom holes beneath the thermal coated GFRP up to 11.5 mm depth has been successfully quantitative evaluated by open-ended waveguide raster scan using PCA and 3-D reconstruction based on SAR tomography techniques. The evaluation includes location, sizing and depth. Nevertheless, the major downside of feature quantities extracted from statistically based methods such as PCA, is it intensely relies on the correlation of the input dataset, and thus hardly link them with the physical parameters of the test sample, in particular, the complex composite architectures. Therefore, there are still challenges of feature extraction and quantitative evaluation to accurately determine the essential parameters from the samples. This can be achieved by a future investigation of multiple features fusion and complementary features.Ministry of Science and Technology of Royal Thai Government and Office of Educational Affairs, the Royal Thai Embass