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Assessing the damage evolution in carbon-fiber-reinforced polymer (CFRP) composites is an intricate task due to their complex mechanical responses. The acoustic emission technique (AE) is a non-destructive evaluation tool that is based on the recording of sound waves generated inside the material as a consequence of the presence of active defects. Proper analysis of the recorded waves can be used for monitoring the damage evolution in many materials, including composites. The acoustic track associated with the entire loading history of the sample or the structures is usually followed by using some descriptors, such as the amplitude of the sound waves and the number of counts. In this study, the acoustic emission in CFRP single-lap shear joints was monitored by using a multiparameter approach based on the contemporary analysis of multiple features, such as the absolute signal level (ASL), initiation frequency, and reverberation frequency, to understand whether a proper combination of them can be adopted for a more robust description of the damage propagation in CFRP structures. For selecting the best features, principal component analysis (PCA) was used. The selected features were classified into different clusters using fuzzy c-means (FCM) data clustering for analyzing the damage modes.

An innovative way of using the Acoustic Emission (AE) technique is introduced in this research work. The ratio of recorded acoustic energy and the counts recorded for each acoustic event were used for characterizing Carbon Fiber Reinforced Plastic (CFRP) laminates adhesively bonded with and without mechanical fasteners. The cumulative counts and cumulative energy of the recorded acoustic events were used for identifying the critical points of failure under loading of these hybrid joint specimens. The peak amplitude distribution was used for identifying the different damage modes such as delamination, matrix cracking and fiber breakage, albeit, ineffectively. The new parameter energy per count was introduced in this work, which can successfully identify the different damage modes under loading. To differentiate the damage modes using the energy per count, they were clustered using k-means++ pattern recognition technique. The method introduced in this work can estimate the damage modes of the CFRP specimens.

The acousto-ultrasonic approach is experimented on CFRP laminates. The test was organized in two modes: before the impact event and after the impact event; along longitudinal and transverse directions. Finally, the Compression after impact (CAI) test is performed on the impacted specimens to characterize its residual compressive strength . The AE descriptors loss along the longitudinal and transverse directions of the specimen are utilized to characterize the interlaminar strength of the specimen. With the help of Continuous Wavelet Transform (CWT) analysis, a hypothesis is created to discriminate the loss variation of AE descriptors to characterize the interlaminar strength of the material and the presence of damage or flaw in the material. The hypothesis holds true that the direction along which the relative loss of AE descriptors is low, has the lowest interlaminar strength. The mechanical results support the hypothesis created. With the proper channelling of acoustic signals and discrimination of data, the acousto-ultrasonic approach can be extended to characterize the interlaminar strength of the mechanical structures.

Two different configurations of adhesive-bonded carbon fiber-reinforced plastic (CFRP) specimens, joggled lap-joint specimens and single lap-joint specimens, are mechanically tested. The mechanical tests show that the joggled lap specimens have lower strength than the single lap specimens. The damage modes in both the specimens are analysed by the Acoustic Emission descriptors recorded during the mechanical tests. The acoustic data as cumulative counts and cumulative energy show the critical points of failure in both the specimen groups under loading. Moreover, they also show that the damage modes in both the specimens are dissimilar. Finally, the data provided by acoustic emission descriptors are verified by fractographic analysis on the failed surface.

Three different groups of AlSi10Mg specimens are built using Selective Laser Melting (SLM) technique. Each of these groups represents the orientation of the specimens with respect to the building plane. The mechanical characteristics of the specimens are evaluated under uniaxial tensile loading. The Acoustic Emission (AE) technique is used for characterizing the failure modes of the specimens under the uniaxial loading. An innovative parameter, the slope coefficient bAE, is formulated for directly relating the different failure modes in the tested specimens with the acoustic results. The slope coefficient bAE proves to be a powerful tool for understanding the damage characteristics of the specimens. In addition to that, the AE signals are recorded during the tests and are analysed in the Hilbert Spectrum. The signals are first decomposed using Empirical Mode Decomposition (EMD) and then reconstructed after removing the noise. The instantaneous frequencies obtained from the Hilbert Spectrum of the reconstructed signals are used for analysing the damage modes. From the results obtained, it is safe to say that the AE technique is powerful for characterizing the failure modes in materials during their entire loading history.

The integrity of the CFRP specimens is tested using acousto-ultrasonic testing method. To validate the acousto-ultrasonic test mode, the specimens are tested before and after a Barely Visible Impact Damage induced by an impactor. A special model is created to use both Wavelet Packet Transform and Empirical Mode Decomposition, for decomposing the recorded waveforms. This mode also enables the reconstruction of the decomposed waveforms, discarding the residual signal in the parent waveform, and calculates the energy associated with each frequency band of the reconstructed signal. By using the percentage of energy recovered by the receiver compared to the signal sent through the specimen, the integrity of the specimens is identified. Moreover, the properties of each specimen and the extent of its damage, albeit qualitatively along the longitudinal and transverse directions can also be assessed by using this technique.

Carbon Fiber Reinforced Plastic (CFRP) laminates are tested using Acousto-Ultrasonic approach. The materials are subjected to out of plane impact and the Acousto-Ultrasonic approach is used to test the materials both before and after the impact event. The Waveform of the recorded signal before and after the impact is used to characterize the structural integrity of the material. Wavelet Packet Transform (WPT) analysis was used to decompose the Wavelets. The WPT analysis is preferred over the conventional Discrete Wavelet Analysis (DWT) owing to the possibility of analyzing both the approximation and details of the decomposed Wavelet. From the WPT results, the spectral energy in the time domain associated with each of the recorded wavelets are analyzed. The spectral energy is successfully used to characterize the integrity of the CFRP material before and after the impact event. The magnitude of the spectral energy in both the longitudinal and transverse direction of the specimen is observed to be lowered after the impact event. Using this technique, it is possible to characterize the integrity of the material even before it is subjected to loading.

In this paper, the full field experimental analysis, and numerical and morphological investigation on two groups of adhesive bonded CFRP laminates are presented. The Digital Image Stereo Correlation (Stereo DIC) technique is used to characterize the out of plane deformation of the specimens under loading. Despite the simplicity of the adopted approach, the analysis and the comparison between the load-displacement curve and the stereo DIC results show that a significant damage in the specimens can be found during the early stages of the test. This first significant failure is the result of out of plane deformation due to the adhesive opening, which plays a crucial role in the ultimate failure of the specimen. The stereo DIC and mechanical results prove the phenomena; while the localization of the first significant failure has also been verified using a numerical model. The morphological studies of the adhesive coated specimen after failure shows the nature of the fracture and the load carrying behaviour of the adhesive. It has been quantitatively proved that the thickness of the adherend has a very less substantial role in the failure of specimens in comparison with the thickness of the adhesive.

The thermal degradation of thermally cured vinyl ester resin systems is studied for different heating rates. The kinetic triplets, the activation energy, pre-exponential factor and the reaction model f(α) for the different reaction extent of conversions (α) are estimated using advanced isoconversional methods. Although the thermal degradation curves show the degradation occurs as a single stage, the kinetic parameters suggest the otherwise. The activation energy remains constant for α = 0.3–0.575 but varies during the initial and final stages of conversion. Similarly, the pre-exponential factor shows considerable variation between the lower and higher reaction extent (α) values. This shows the complexity in the reaction. The probable reaction mechanism that the degradation follows has been explained. The complexity of the thermal degradation and the changes in reaction model f(α) over different reaction extent has been related. The appropriate working temperature for different thermal lifetime of the cured vinyl ester resin system for the failure of conversion α = 0.2 has been predicted under the nitrogen atmosphere.

Damage propagation in DCB specimens is characterized by Acoustic Emission (AE) Technique. Signal-based AE data is used to relate the Mechanical Property of the Material with the Acoustic Activity; Sentry function is used to relate the Strain Energy and Acoustic Energy. The cumulative acoustic energy provides information about the critical points before failure in each specimen. Nonetheless, it was observed that the Sentry function reveals concealed information about the damage propagation and material deterioration which are concealed in the Load-Displacement curve. Sentry function can be used to characterize the damage propagation in the material even before its critical point of failure. The signal-based data, the wavelets, are decomposed into different levels using the Wavelet Packet Transform (WPT). These WPT results discriminate the different frequency band associated with the different types of damage propagation. The false signals from both the mechanical results and acoustic results can be identified using the WPT results.

The present work describes the mechanical characterization combined with the thermal degradation kinetics of Carbon Fiber Reinforced Polymers (CFRP). The thermal degradation kinetics of CFRP have never been studied in the past. In that regard, the present work focuses on studying the thermal degradation kinetics of CFRP tested mechanically at different environmental conditions. Tensile tests were performed on the specimens with different lay-ups at room temperature, elevated temperature (71 &deg, C), and cryogenic conditions (&minus, 54 &deg, C), and the same specimens were used for thermal degradation kinetic studies. Mechanical tests show different responses respect to the different environmental conditions and different fibers orientation. On the other hand, the thermogravimetric results, mass loss, and derivative mass loss, show no significant difference in the degradation of CFRP tested at different temperatures. However, the thermal degradation kinetics shows more insight into the degradation pattern of the materials. The activation energy of degradation shows that the degradation of materials subjected to elevated conditions increases rapidly in the later stages of degradation, suggesting the formation of high char yield. The varying activation energy has been related to different degradation mechanisms. Lastly, the morphology of the materials was studied under SEM to understand the structural change in the material after tested in different weather conditions.