Residual stress evaluation in friction stir welding of aluminum plates by means of acoustic emission and ultrasonic waves

© Faculty of Mechanical Engineering, Belgrade. The residual stress assessment in structures is essential for optimization of the structures' design. The attention of this paper is focused on how acoustic emission signals caused by tensile loading of the friction stir welded aluminum plates are expected to vary depending upon the residual stress. To this aim, the distribution of residual stresses in two friction stir welded aluminum specimens was firstly evaluated by ultrasonic stress measurement. AE signals were then produced during tensile tests and captured using AE sensors. The obtained AE signals were analyzed using statistical features including crest factor, cumulative crest factor and sentry function. It was found that the crest factor could be used to identify the presence of the residual stresses and that the trends of sentry function are in good agreement with the results of crest factor and cumulative crest factor

Interlaminar Fracture Toughness Evaluation in Glass/Epoxy Composites Using Acoustic Emission and Finite Element Methods

© 2014, ASM International. Delamination is one of the most common modes of failure in laminated composites and it leads to the loss of structural strength and stiffness. In this paper, mode I, mode II, and mixed of these pure modes were investigated using mechanical data, Finite Element Method (FEM) and Acoustic Emission (AE) signals. Experimental data were obtained from insitu monitoring of glass/epoxy laminated composites with different lay-ups when subjected to different modes of failure. The main objective was to investigate the behavior of delamination propagation and to evaluate the critical value of the strain energy which is required for onset of the delamination (GC). For the identification of interlaminar fracture toughness of the specimens, four methods were used: (a) ASTM standard methods, (b) FEM analysis, (c) AE method, and (d) sentry function method which is a function of mechanical and AE behaviors of the specimens. The results showed that the GC values obtained by the sentry function method and FEM analysis were in a close agreement with the results of nonlinearity methods which is recommended in the ASTM standards. It was also found that the specimens under different loading conditions and various lay-up have different GC values. These differences are related to different stress components distribution in the specimens which induce various damage mechanisms. Accordingly, stress components distribution obtained from FEM analyses were in agreement with SEM observations of the damaged surfaces of the specimens

Prediction of quasi-static delamination onset and growth in laminated composites by acoustic emission

The main objective of this study is to determine the crack tip position during propagation of mode I delamination and also to evaluate interlaminar fracture toughness (G(IC)) in glass/epoxy composite specimens. The crack tip location was identified using two methods: a) localization of the AE signal source and b) the cumulative AE energy. Interlaminar fracture toughness of the specimens was also determined using the ASTM standard methods and the AE-based methods. The AE-based methods results were in a close agreement with the results of ASTM standard. It was found that the novel AE-based methods are more applicable than conventional methods for characterization of the delamination. (C) 2015 Elsevier Ltd. All rights reserved

The effect of mode II fatigue crack growth rate on the fractographic features of CFRP composite laminates: An acoustic emission and scanning electron microscopy analysis

The present study is focused on the characterization of the fatigue damage features in carbon/epoxy laminates under mode-II loading conditions. To this aim, a sinusoidal cyclic load was applied to the End-Notched Flexural (ENF) specimens and the fatigue behavior of specimens was investigated. Scanning Electron Microscope (SEM) was used to identify the damage features on the fracture surface, i.e. fiber imprints, cusps, roller cusps, and striations. It was found that the fatigue damage features, such as cusps and striations, completely depended on the fatigue crack growth rate, da/dN. In addition, a linear relationship between the fatigue striation space and the strain energy release rate range (ΔGs) and the hysteresis loop area was established. The Acoustic Emission (AE) method was also employed to characterize the damage features. The obtained results showed that higher AE energy indicates larger and rougher cusps and striation features.</p

A quantitative assessment of the damage mechanisms of CFRP laminates interleaved by PA66 electrospun nanofibers using acoustic emission

Interleaving composite laminates with nanofibrous mat is one of the most reliable methods for increasing interlaminar fracture toughness. The present study seeks to find out how the damage mechanisms of carbon fiber reinforced polymers (CFRPs), subjected to the mode-I and mode-II fracture tests, are affected while those are modified by interleaved Polyamide 66 (PA66) electrospun layers. For this goal, acoustic emission (AE) and scanning electron microscope (SEM) techniques were used for assessing the damage mechanisms. The mode-I test results showed that adding nanofibers could decrease matrix cracking, fiber breakage, and fiber/matrix debonding by 92%, 27%, and 87%, respectively. The AE demonstrated that no fiber breakage occurred during mode-II loading in both non-modified and nanomodified specimens which was validated by SEM images. On the other hand, the two other damage modes, i.e. matrix cracking and fiber/matrix debonding, decreased about 97% in the nanomodified laminates.</p

Acoustic Emission-Based Analysis of Damage Mechanisms in FilamentWound Fiber Reinforced Composite Tubes

This study investigates the mechanical behavior and damage mechanisms of thin-walled glass/epoxy filament wound tubes under quasi-static lateral loads. The novelty is that the tubes are reinforced in critical areas using strip composite patches to provide a topology-optimized tube, and their damage mechanisms and mechanical performance are compared to that of un-reinforced (reference) tubes. To detect the types of damage mechanisms and their progression, the Acoustic Emission (AE) method is employed, accompanied by data clustering analysis. The loading conditions are simulated using the finite element method, and the results are validated through experimental testing. The findings confirm that the inclusion of reinforcing patches improves the stress distribution, leading to enhanced load carrying capacity, stiffness, and energy absorption. Compared to the reference tubes, the reinforced tubes exhibit a remarkable increase of 23.25% in the load carrying capacity, 33.46% in the tube’s stiffness, and 23.67% in energy absorption. The analysis of the AE results reveals that both the reference and reinforced tubes experience damage mechanisms such as matrix cracking, fiber-matrix debonding, delamination, and fiber fracture. However, after matrix cracking, delamination becomes dominant in the reinforced tubes, while fiber failure prevails in the reference tubes. Moreover, by combining the AE energy and mechanical energy using the Sentry function, it is observed that the reinforced tubes exhibit a lower rate of damage propagation, indicating superior resistance to damage propagation compared to the reference tubes.Materials and Environmen

The fatigue failure study of repaired aluminum plates by composite patches using Acoustic Emission

The aim of this study is to investigate the failure of cracked aluminum plates repaired by one-side composite patches under fatigue loading using Acoustic Emission (AE) and fractography images. Rectangular specimens made of 6061 aluminum alloy with central through thickness pre-cracks were repaired using glass/epoxy laminated patches. The specimens were subjected to the fatigue loading and AE technique was employed to monitor the effect of the repair patch on the damage progression. First, different stages of damage evolution were studied based on the mechanical data and fractography images. Then, the AE energy utilized to characterize failure process of the specimens. To this aim, AE signals of the aluminum cracking and adhesive layer failure were discriminated according to their energy content. The effect of patch thickness and layup on the failure behavior of the specimens were also studied. Finally, it is concluded that AE is a powerful technique to characterize the failure process of a repaired cracked aeronautic structure by composite patches.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Structural Integrity & Composite

A quantitative assessment of the damage mechanisms of CFRP laminates interleaved by PA66 electrospun nanofibers using acoustic emission

Interleaving composite laminates with nanofibrous mat is one of the most reliable methods for increasing interlaminar fracture toughness. The present study seeks to find out how the damage mechanisms of carbon fiber reinforced polymers (CFRPs), subjected to the mode-I and mode-II fracture tests, are affected while those are modified by interleaved Polyamide 66 (PA66) electrospun layers. For this goal, acoustic emission (AE) and scanning electron microscope (SEM) techniques were used for assessing the damage mechanisms. The mode-I test results showed that adding nanofibers could decrease matrix cracking, fiber breakage, and fiber/matrix debonding by 92%, 27%, and 87%, respectively. The AE demonstrated that no fiber breakage occurred during mode-II loading in both non-modified and nanomodified specimens which was validated by SEM images. On the other hand, the two other damage modes, i.e. matrix cracking and fiber/matrix debonding, decreased about 97% in the nanomodified laminates.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Structural Integrity & Composite

The effect of mode II fatigue crack growth rate on the fractographic features of CFRP composite laminates: An acoustic emission and scanning electron microscopy analysis

The present study is focused on the characterization of the fatigue damage features in carbon/epoxy laminates under mode-II loading conditions. To this aim, a sinusoidal cyclic load was applied to the End-Notched Flexural (ENF) specimens and the fatigue behavior of specimens was investigated. Scanning Electron Microscope (SEM) was used to identify the damage features on the fracture surface, i.e. fiber imprints, cusps, roller cusps, and striations. It was found that the fatigue damage features, such as cusps and striations, completely depended on the fatigue crack growth rate, da/dN. In addition, a linear relationship between the fatigue striation space and the strain energy release rate range (ΔGs) and the hysteresis loop area was established. The Acoustic Emission (AE) method was also employed to characterize the damage features. The obtained results showed that higher AE energy indicates larger and rougher cusps and striation features.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Structural Integrity & Composite

Clustering of interlaminar and intralaminar damages in laminated composites under indentation loading using Acoustic Emission

This study focuses on the clustering of the indentation-induced interlaminar and intralaminar damages in carbon/epoxy laminated composites using Acoustic Emission (AE) technique. Two quasi-isotropic specimens with layups of [60/0/-60]4S (is named dispersed specimen) and [604/04/-604]S (is named blocked specimen) were fabricated and subjected to a quasi-static indentation loading. The mechanical data, digital camera and ultrasonic C-scan images of the damaged specimens showed different damage evolution behaviors for the blocked and dispersed specimens. Then, the AE signals of the specimens were clustered for tracking the evolution behavior of different damage mechanisms. In order to select a reliable clustering method, the performance of six different clustering methods consisting of k-Means, Genetic k-Means, Fuzzy C-Means, Self-Organizing Map (SOM), Gaussian Mixture Model (GMM), and hierarchical model were compared. The results illustrated that hierarchical model has the best performance in clustering of AE signals. Finally, the evolution behavior of each damage mechanism was investigated by the clustered AE signals with hierarchical model. The results of this study show that using AE technique with an appropriate clustering method such as hierarchical model could be an applicable tool for structural health monitoring of composite structures.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Structural Integrity & Composite