The electro-mechanical response (Electrical Resistance Change method) as a damage index of quasi-isotropic Carbon Fiber Reinforced (CFRPs) laminates under fatigue loading was investigated. The effect ...of dispersed Multi-Wall Carbon Nanotubes (MWCNT) into the epoxy matrix was additionally evaluated and compared with neat epoxy CFRPs. The longitudinal resistance change of the specimens was monitored throughout the fatigue experiment. Three different stress levels were tested. The frequency and the ratio (
R) of the minimum applied load (stress) to the maximum applied load (stress) were kept constant for the different stress levels. The temperature of the specimen was also monitored throughout the process in order to deduce its effect on the electrical resistance of the specimen. The electrical behavior of the quasi-isotropic CFRP deviated from the commonly observed electrical response of unidirectional or cross-ply CFRPs due to the presence of the 45° layers. During initial stages of loading the resistance drops and afterwards it follows a positive slope up to final fracture. This repeatable pattern was observed for both the neat and the CNT-doped specimens, with the latter having smoother electrical recordings. The effect of temperature was calculated to be limited for the specific material and test/measurement configuration. The electro-mechanical response was correlated to stiffness degradation and acoustic emission findings enabling the identification of the specific regions during the fatigue life referring to specific mechanisms of damage accumulation. More specifically the experimental results revealed that the occurrence of the initial drop of the electrical resistance is linked with the occurrence of the Characteristic Damage State (CDS), associated with a specific percentage of stiffness reduction. This finding was used in order to predict the remaining life independently from the applied stress level with a high degree of confidence, assuming a constant stress level throughout the whole lifetime. The remaining life prediction for the CNT-doped specimens had higher coefficient of confidence (
R
2).
Bis-maleimides (BMIs) exhibit healing functionalities on polymer level. In the current investigation, BMI self-healing (SH) resin based on Diels-Alder (DA) reaction mechanism (identification code: ...BMI pp) was integrated into high performance aerospace carbon fiber reinforced plastics (CFRPs). The effect of BMI pp as self-healing agent (SHA) into CFRPs is assessed. More precisely, open-hole reference and BMI pp modified CFRP samples having 45/-45/0/902S stacking sequence, with similar fiber volume fractions have been tested under quasi static tensile and tension-tension fatigue loading conditions. According to quasi static experimental results, it was shown that the incorporation of the BMI pp SHA did not deteriorate the in-plane mechanical properties of the entire composite. In addition, tension-tension fatigue tests revealed that the incorporation of the BMI pp SHA into composites architecture made composites to slightly extend their fatigue life. Finally, the activation of the healing functionality of BMI pp modified composites at certain number of fatigue cycles significantly extends their fatigue life of about 75%. For all fatigue tested material sets a three-stage pattern of stiffness degradation was observed.
Aiming at more efficient fault diagnosis, this research work presents an integrated anomaly detection approach for seeded bearing faults. Vibration signals from normal bearings and bearings with ...three different fault locations, as well as different fault sizes and loading conditions are examined. The Empirical Mode Decomposition and the Hilbert Huang transform are employed for the extraction of a compact feature set. Then, a hybrid ensemble detector is trained using data coming only from the normal bearings and it is successfully applied for the detection of any deviation from the normal condition. The results prove the potential use of the proposed scheme as a first stage of an alarm signalling system for the detection of bearing faults irrespective of their loading condition.
► We employed EMD for feature extraction from vibration signals of bearings. ► The frequency content of the IMFs can provide valuable information for fault detection. ► Simple anomaly detectors combined in a hybrid scheme can perfectly detect anomalies. ► The ensemble scheme compensates for the inefficiencies of the individual detectors.
•Preparation of a future digital-twin-assisted damage diagnosis technique.•Computational models for composite sandwich structure simulation to bird strike.•Validation of computational results against ...experiments.
The constant requirement of aerospace industry to enhance the structural efficiency has driven to the usage of high-performance composite materials, either monolithic or sandwich. However, aerospace composite structures are prone to damage due to high-velocity impact events such as bird strike, hail impact, etc. These impact events can result in extensive damage including structure perforation, which will eventually degrade its post-impact residual strength. Therefore, the early detection of damage in composite structure is imperative to avoid catastrophic failure. This paper develops the computational models which predict the dynamic behavior of a helicopter composite sandwich structure undergoing a bird strike. The models are aimed to be used as virtual tools for a future digital-twin-assisted fault detection technique. Firstly, a high-fidelity (HF) FE/SPH model was developed in LS-DYNA, and it was validated against the soft body impact experiments. Afterwards, a computationally efficient low-fidelity (LF) model was developed and correlated with the high-fidelity model. It was concluded that the high-fidelity model can sufficiently accurately predict the strain history experimentally recorded by the FBG sensors, and that size of the predicted delamination area at the front face of the sandwich structure agrees very well with the experimentally observed delamination area. It was also shown that the LF model can rapidly predict the global dynamic response of sandwich panel under the impact loading, through the good agreement between the numerical strain histories with the FBG measurements. Consequently, the LF model can be used as a quick numerical guide for the identification of the loading condition, whereas the HF model can be used as virtual damage detector and estimator of damage extension before the scheduled inspection.
The goal of the present study was to investigate the influence of multi-wall carbon nanotubes (MWCNTs) on the impact and after impact behaviour of carbon fiber reinforced polymer (CFRP) laminates. ...About 0.5% per weight MWCNTs were dispersed via a high shear device in the epoxy matrix (Bisphenol A) of carbon reinforced quasi-isotropic laminates. Subsequently, the modified CFRPs were subjected to low-energy impact and directly compared with unmodified laminates. In previous studies, the beneficial effect of the MWCNT inclusion to the fracture properties of CFRPs has been demonstrated. In terms of the CFRP impact performance, enhanced performance for the CNT doped specimens was observed for higher energy levels. However, the after-impact properties and more specifically compression after impact were improved for both the effective compression modulus and the compression strength. In addition, compression–compression fatigue after impact performance of the CNT modified laminates was also improved, by extending the fatigue life.
An approach for damage inspection of composite structures utilizing carbon nanotubes (CNT) networks is investigated. CNT are dispersed in an epoxy using a processing technique compatible with ...commonly employed composite manufacturing techniques and subsequently used as matrix for a structural glass fiber reinforced composite. The developed electrical conductivity of the composite system is verified experimentally. The electrically conductive CNT network within the GFRP is exploited through distributed electrical voltage measurements to sense and, ultimately, locate damage in the plane of the composite plate. Damage in the form of cracks or delamination interrupts the continuity of the CNT network separating and isolating regions of the conductive network. Employing electric potential fields these changes can become measurable and can provide information for inversely locating the damage. Electrical Resistance Tomography (ERT) is formulated and experimentally applied to measure changes in the potential fields and deliver electrical conductivity change maps which are used to identify and locate changes in the CNT networks. These changes are correlated to capture the damage in the composite. Different damage modes are studied to assess the capabilities of the technique. The technique shows sensitivity to very small damages; less than 0.1% of the inspected area. The solution of the inverse ERT problem delivers a conductivity change maps which offers an effective localization with nearly 10% error and an inspection area suppression of around 75%. The proposed methodology to create CNT networks enables the application of ERT for Non-Destructive Evaluation of composite materials, previously not possible due to lack of conductivity, thus offering damage sensing and location capabilities even in-situ.
This research paper explores and highlights the effect of aspect ratio (AR) and specific surface area (SSA) of Graphene Nano-Platelets (GNPs) on the interlaminar fracture behavior of carbon ...fiber-reinforced polymer (CFRP) composites. Various types of GNPs having different aspect ratios were dispersed in an epoxy system by using the three roll milling technique at a fixed content of 0.5 wt.%. The blends resulting from this procedure were used for the impregnation of carbon fabrics aiming in obtaining nano-modified epoxy pre-impregnated fabrics (Prepregs). The prepreg material was used for the fabrication of CFRPs by using the autoclave technique. The resistance of composites to interlaminar crack propagation was measured by performing Mode I and II fracture tests utilizing the DCB and ENF configurations. The fracture surfaces of the tested samples were investigated post-mortem using Scanning Electron Microscopy (SEM). We show that the graphene Nano-Platelets with higher AR and SSA combined lead to enhanced interlaminar fracture toughness under mode I and mode II loading as compared to the unmodified composite system.
•This work investigates the hypervelocity impact response of CFRP material and the produced secondary debris using SPH method.•The verification of stiffness and failure of CFRP material to ...quasi-static loading constitutes first-priority target.•SPH code and MAT 59 model provides reasonable results in view of the ballistic limit and the crater diameter of CFRP target.•Model also predicts the downrange ejecta, but it is vulnerable to effective prediction of back-scattered fragments cloud.
The current work investigates the hypervelocity impact response (HVI) of carbon fiber reinforced polymer composites (CFRP) and the produced secondary debris using smoothed-particle hydrodynamics methodology (SPH) in LS-DYNA. The aim of present study is to reproduce numerically the CFRP material response to hypervelocity impact and fragments cloud, to investigate the applicability of SPH modeling technique on composite materials and to determine the suitable numerical solution parameters. A verification procedure for modeling of composite laminate using SPH methodology is proposed. The investigation starts with baseline quasi-static tests, and the results are compared against the available theoretical ones to ensure the stability and efficiency of the SPH kernel function under quasi-static loading. Afterwards, the developed methodology is applied to hypervelocity impact response of CFRPs and the ballistic limit, the crater diameter as well as the secondary debris cloud are numerically calculated and correlated to the published HVI experimental results on CFRP plates.
The implementation and the effectiveness of different types of stepped scarf bonded repairs on Carbon Fiber Reinforced Plastics (CFRPs) are investigated in this study. Two types of bonded repair ...procedures are studied, the soft/wet patch approach and the hard/dry patch approach. Material removal was achieved with both conventional machining as well as laser ablation, to evaluate which technique provides better restored properties of the repaired specimens. Three repair configurations are tested, laser ablation with a wet patch, milling with a wet patch and laser ablation with a dry patch all at a fixed scarf ratio of 1:30. The repaired specimens are tested under tensile loading and pristine specimens are used as a baseline for the tensile strength and stiffness properties recovery assessment. Numerical models are developed and verified against the acquired experimental results towards the development of a prediction tool for the design and implementation of bonded repairs on composite aeronautical structures.
Abstract
Lots of disruptive UAV technologies are rising every day. Especially in the design process, under the prism of the upcoming swarm trend, a novelty touch to new foldable designs has started. ...In combination with electric propulsion systems, those platforms come in reality. Because of their size, the same design can be enlarged and reduced with minor changes at its subsystems. This simplifies the construction process and minimizes the costs. Due to their application range both, civilian and military platforms started to develop their own foldable UAVs, basically for swarm applications. In this study, the basic parameters of such a scalable design will be proposed. A literature survey will be presented, to determine the most common sizes of those platforms and the battery market availability. Afterwards, a mission profile will form the basis of the conceptual process for different dimensional UAVs. Two different methods about the weight estimation of the UAV will be examined. The goal of this multilateral examination is that; enable the designer to precede to an appropriate selection of subsystems of the UAVs, i.e., battery, autopilot and telemetry systems based on the weight estimation.
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