ACOUSTO-ULTRASONIC COMPOSITE TRANSDUCERS INTEGRATION INTO THERMOPLASTIC COMPOSITE STRUCTURES VIA ULTRASONIC WELDING

Acousto-ultrasonic composite transducers (AUCT), which are made of piezoceramic materials embedded in a reinforced polymeric matrix, are promising for the health monitoring of composite structures. However, when they are integrated into highly loaded thermoplastic composite structures, ensuring proper joining properties is a challenge. The conventional approach of attaching the AUCT using adhesive may not be sufficiently reliable in aeronautic applications for low surface energy materials such as polyaryletherketone composites, where surface treatments are needed for adhesion. Welding techniques can be used to create a joint in which the interface material interfuses with the AUCT embedment and the structure matrix, resulting in a homogeneous interface with properties comparable to the host structure matrix throughout its service life. With this in mind, the main objective of the present work is to investigate the viability of attaching AUCT to low-melting polyaryletherketone carbon fiber reinforced thermoplastic composite structures using the ultrasonic welding (UW) procedure and characterize the joint performance. The ultrasonic welded joint using an external energy director in the interface is investigated by comparing the findings to those of a reference AUCT system integrated into the structure with autoclave co-consolidation. Infrared thermography is employed to monitor the process, and a parameter study of the UW process is carried out. The AUCT survivability during the UW process is determined by measuring the capacitance, and C-scan is used to assess joint quality. The results show the challenges of attaching AUCT to thermoplastic composite structures using UW and surviving the procedure

Assessing stiffness degradation of stiffened composite panels in post-buckling compression-compression fatigue using guided waves

The application of structural health monitoring (SHM) in composite airframe structural elements under long- term realistic fatigue loading needs to consider the structural behavior on the global level, which is an intri- cate task. The overall structural stiffness is a key design parameter for composite structures and the stiffness degradation under fatigue loading is closely related to the damage accumulation and failure mechanism which can be used as an indicator for the structural degradation. Therefore, this paper investigates the use of guided waves in axial stiffness degradation estimation for stiffened carbon fiber reinforced polymer (CFRP) composite panels under post-buckling compression-compression (C-C) fatigue loads. Impacted or artificially debonded stiffened composite panels are tested under fatigue until failure and guided waves are acquired using a network of piezoelectric (PZT) sensors at fixed cycle intervals. The guided wave phase velocity along the loading direction is extracted to estimate the axial stiffness degradation with the consideration of mode conversion and failure of PZT sensors. The estimated stiffness of five stiffened composite panels matches well with the stiffness calculated from the load–displacement curves. The estimated stiffness is also assessed using prognostic performance metrics and shows good potential for being used as a health indicator for prognostic purposes.Projet européen H2020 REMA

Fusion of SHM techniques for synergetic degradation monitoring of composite aircraft wing structure under compression fatigue

In the pursue of smart structures for cyber-physical health management of lightweight engineering structures, a number of structural health monitoring methods have been developed. Each SHM technique has different coverage and sensitivity to certain types of damage. In particular, the structural degradation of composites under fatigue loading is multi-causal and intricate, and none of the techniques alone is able to fully capture the fatigue degradation phenomenon. This paper presents the considerations and results in a fusing strategy of two different SHM techniques, distributed optical strain sensing and guided wave, on the in-situ monitoring of an aircraft wing structure under compression-compression fatigue loading. The emergence and growth of localized damage (disbond of stiffener foot) and the degradation of mechanical performance (stiffness degradation) caused by the accumulation of distributed fatigue damage have been monitored in the synergy of the two different SHM techniques. The result shows that the fusion strategy unveils the fatigue degradation phenomenon in a more extensive manner than using two techniques separately.H2020 REMA

Health Monitoring of Aerospace Structures Utilizing Novel Health Indicators Extracted from Complex Strain and Acoustic Emission Data

The development of health indicators (HI) of diagnostic and prognostic potential from generally uninformative raw sensor data is both a challenge and an essential feature for data-driven diagnostics and prognostics of composite structures. In this study, new damage-sensitive features, developed from strains acquired with Fiber Bragg Grating (FBG) and acoustic emission (AE) data, were investigated for their suitability as HIs. Two original fatigue test campaigns (constant and variable amplitude) were conducted on single-stringer composite panels using appropriate sensors. After an initial damage introduction in the form of either impact damage or artificial disbond, the panels were subjected to constant and variable amplitude compression–compression fatigue tests. Strain sensing using FBGs and AE was employed to monitor the damage growth, which was further verified by phased array ultrasound. Several FBGs were incorporated in special SMARTapesTM, which were bonded along the stiffener’s feet to measure the strain field, whereas the AE sensors were strategically placed on the panels’ skin to record the acoustic emission activity. HIs were developed from FBG and AE raw data with promising behaviors for health monitoring of composite structures during service. A correlation with actual damage was attempted by leveraging the measurements from a phased array camera at several time instances throughout the experiments. The developed HIs displayed highly monotonic behaviors while damage accumulated on the composite panel, with moderate prognosability

Investigation of fatigue crack growth in a single cycle by means of acoustic emission

Results of experiments using acoustic emission to investigate crack growth in an epoxy adhesive bonded joint

Data underlying the article: Early fatigue damage accumulation of CFRP Cross-Ply laminates considering size and stress level effects

Ply-block size and stress level effect on accumulation of transverse cracks and delamination are investigated during early fatigue life of CFRP laminates. Tension-tension fatigue tests under different stress levels were performed for two cross-ply configurations. Edge observation with digital cameras, digital image correlation and acoustic emission were employed for in-situ damage monitoring. Transverse cracks were dominant for [0/902]s laminates with almost non-existent delamination, while different interactive levels between both damage mechanisms occurred for [02/904]s laminates. Poisson’s ratio identifies whether early fatigue damage is dominant by transverse cracks or involves delamination. Cumulative AE energy is a helpful indicator of crack density.</p

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

Composite specimens under fatigue and impact loading conditions

Open-hole specimens were subjected to constant amplitude fatigue loading up to failure while in-situ impact and manufacturing imperfections were used so as to demonstrate unexpected phenomena. Acoustic emission and digital image correlation techniques were employed in order to collect condition monitoring data which were used for the training and testing processes.Eight specimens were used for the training process, and they were subjected only to fatigue loading. Four specimens were used for testing the proposed adaptive model. Three of them were subjected to fatigue and in-situ impact, and created a left, a right outlier and an inlier performer respectively to the training specimens. The last one was subjected just to fatigue loading but created one more left outlier case since it had a manufacturing imperfection

Damage Diagnostics on Post-buckled Stiffened Panels Utilizing the Digital-Twin Concept

A digital twin representative of a typical composite stiffened panel is utilized to monitor skin-to-stringer disbonds. A validated finite element model of the composite panel estimates the longitudinal strains of the pristine state, at the exact location where integrated fiber Bragg grating sensors are permanently installed. Experimental strains are acquired and compared to those provided by the digital twin in order to reveal the presence of disbonds. The integrated sensor grid is used in a manner that some sensors identify the load acting on the panel, leveraging on the digital twin baseline, whilst the remaining ones are dedicated for diagnostic purposes. Two damaged single-stringer panels are tested under compression-compression fatigue conditions. Static strains are received during quasi-static test intervals among the fatigue cycles. The historical strain data are analyzed in a near real-time manner to detect and localize the induced damage throughout the test span.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