High performance quasi-isotropic thin-ply carbon/glass hybrid composites with pseudo-ductile behaviour loaded off-axis

The aim of this work was to investigate the effect of loading angle variation on the pseudo-ductility of quasi-isotropic (QI) hybrid composite laminates. Previously, hybrids of thin-ply carbon fibres and standard glass fibres were found to have an excellent pseudo-ductile behaviour both in unidirectional (UD) and QI configurations when subjected to axial tension in the fibres’ orientations. In this work, the QI laminates, with 60° intervals, have been subjected to a quasi-static tensile load at various off-axis orientations – i.e. 5°, 10° and 20°. The QI hybrid composites were made by sandwiching a QI T300-carbon laminate between the two halves of a QI S-glass laminate. The results showed a pseudo-ductile behaviour with a linear elastic part and a desirable plateau for all the loading directions, however the pseudo-ductile strain decreases when increasing the off-axis angle. Comparing the 20° off-axis with the other cases, there was more active matrix cracking damage before fragmentation in the 20° off-axis plies and it failed earlier than the other samples. Acoustic emission (AE) results confirmed this, with more matrix cracking related AE signals in the 20° off-axis case compared to the other configurations

The thickness effect of rubbery nanofibrous mat on modes I–II fracture mechanism of composite laminates

This study investigates the effect of the interleaving nanofibers, made of NBR/PCL blend, on the interlaminar fracture toughness of carbon/epoxy laminates. Different nanomat thicknesses, ranging from 20 μm to 120 μm, were tested at Mode-I and mode-II and results were compared to the non-modified laminates. Acoustic Emission (AE) technique was used to assess the influence of interleaving nanofibers on dominant damage modes of the specimens, i.e., matrix cracking, fiber/matrix debonding, and fiber breakage. Moreover, the damage mechanism and the nanofiber toughening contribution were investigated by means of crack path and surface analysis. Results indicated that the optimum nanomat thickness for mode-I is 40 μm (GI,R = + 333 %), while for mode-II is 20 μm (GII,R = + 43 %). The study also confirmed by AE the significant impact of nanofibers on various damage modes, especially during mode-I loading

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

Classification of damage mechanisms during delamination growth in sandwich composites by acoustic emission

Sandwich composites are widely used in structural applications because of their appropriate mechanical properties and low strength/weight ratio. Delamination is common failure mode in these structures that lead to a reduction in strength and stiffness of composite. In this paper, using acoustic emission, initiation and propagation of delamination in sandwich composite specimens was investigated. The specimens were loaded under mode I loading. Then the characteristics of the signals related to different damage mechanisms were specified. The acoustic emission signals were classified based on their frequency ranges. Then the acoustic emission signals were recorded during the test specimens were processed using wavelet transform. Thus the percentage of energy in each components of the acoustic emission signal was specified. Each of these components has a certain frequency range corresponding to a damage mechanism. Thus the percentages of different damage mechanisms in each specimen were specified. The Scanning Electron Microscopy (SEM) was also employed to verify the results which were obtained from acoustic emission and wavelet transform method. The results showed acoustic emission is efficient tool for identification and separation of different damage mechanisms in sandwich structures

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

Determination of fracture toughness of heat treated AISI D2 steel using Finite Element and Acoustic Emission methods

The AISI D2 steel is a high-chromium and high-carbon tool steel which has good mechanical properties such as high compressive strength and good through-hardening. Despite these advantages, fracture toughness of this steel is moderate. In this study, fracture toughness of AISI D2 steel was determined using Finite Element and Acoustic Emission methods. Selected steel (AISI D2 cold-work tool steel) was heat treated and tempered at different conditions. Then Compact testing specimens were prepared according to ASTM E399 standard and fracture toughness of the specimens was specified according to the standard method. The specimens were modeled in the commercial FE software (ABAQUS) and fracture toughness of the specimens was determined using FEM. Determination of fracture toughness using AE technique was carried out according to three methods: Acoustic Emission Energy Rate (AEER), Acoustic Emission Count Rate (AECR) and integral of sentry function. The results obtained from ASTM E399, Finite Element and Acoustic Emission methods were compared with each other. It was found that fracture toughness values which were obtained using AECR and integral of sentry function techniques are lower bound and the results obtained from FEM are upper bound values of the fracture toughness. Furthermore, fracture toughness values obtained using AEER were the most consistent with the results obtained from ASTM E399 standard method. Finally, it could be concluded that Acoustic Emission method can be used as a useful method for determination of fracture toughness of engineering materials

The study of buckling and post-buckling behavior of laminated composites consisting multiple delaminations using acoustic emission

© 2018 Elsevier Ltd This study introduces a comprehensive set of designed and tested glass/epoxy composites, AE monitoring and signal processing techniques; (i) to investigate the effect of multiple delaminations on buckling and post-buckling behaviors of laminated composites and (ii) to evaluate Acoustic Emission (AE) technique ability to monitor the buckling delamination growth and to classify the occurred damage mechanisms. The pre-delaminations were made by inserting a Teflon film at the plies interfaces during fabrication. Three different types of specimens were fabricated and subjected to compression loading to study the effects of the location, the number of delaminations, and the thickness of the Teflon film on buckling and post-buckling behaviors of the specimens. The mechanical results showed that the number of delaminations has a major effect on the critical and maximum loads and the location of delamination and the thickness of the Teflon film have minor effects on the critical and maximum loads. The AE signals of the specimens were then classified using Gaussian Mixture Model (GMM) and the evolution of different damage mechanisms was investigated. The AE results showed that AE is a robust technique to classify damage mechanisms in buckling of laminated composites and could identify delamination propagation earlier and with a lower standard deviation, compared with the conventional methods

Correlation of acoustic emission with finite element predicted damages in open-hole tensile laminated composites

This paper focuses on quantification of damage mechanisms in Standard Open-Hole Tensile (OHT) laminated composites using Acoustic Emission (AE) and Finite Element Method (FEM). To this aim, OHT tests were carried out in unidirectional glass/epoxy composite materials. AE accompanied with a wavelet and fuzzy C-means clustering methods were used to distinguish damage mechanisms of the specimen. These damages were consisted of three main mechanisms, including matrix cracking, fiber/matrix debonding and fiber breakage. FE analysis was utilized to validate the AE results. Scanning Electron Microscope (SEM) images were also used to investigate damage mechanisms in the specimen. The comparison of the applied methods shows that the difference in results of FE analysis and wavelet transform methods are less than 15 percent but there is a more difference (around 42 percent) between results of fuzzy C-means and FEM

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

© 2015 Elsevier Ltd. All rights reserved. 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 (GIC) 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