V. Kostopoulos

Enhanced Fracture Properties of Carbon Reinforced Composites by the Addition of Multi-Wall Carbon Nanotubes

In the present study, the fracture energy of hybrid carbon fiber reinforced polymers was investigated. The composites were modified by the addition of multi-walled carbon nanotubes into the matrix material. The interlaminar fracture properties under Mode I and Mode II remote loading were studied as a function of the carbon nanotube content in the matrix. With the addition of carbon nanotubes in the epoxy matrix, a significant increase in the load bearing ability as well as in the fracture energy was observed, for both Mode I and Mode II tests. It is speculated that carbon nanotubes due to their large aspect ratio have a significant toughening effect since extra energy is needed in order to pull them out from the matrix and start the crack propagation following a kinking out pattern at nanoscale.

On the identification of the failure mechanisms in oxide/oxide composites using acoustic emission

Abstract In the present work, a new class of oxide/oxide composites made of Nextel™ 720 fibre reinforcement and a mullite-based matrix, fabricated by using liquid polymer infiltration process, were studied. A fibre coating was applied via sol–gel in order to achieve improved damage tolerant behaviour. Mechanical properties were investigated at ambient temperature under quasi-static loading in the presence of continuous Acoustic Emission (AE) monitoring. Statistical pattern recognition analysis is the proposed tool for the classification of the monitored AE events. Lacking an a priori knowledge of different signal classes, unsupervised pattern recognition algorithms were used. A complete methodology including descriptor selection methods, procedures for numerical verification and cluster validity criteria is followed. Cluster analysis of AE data was achieved and the resulted clusters were correlated to the damage mechanisms of the material under investigation. This process was assisted by systematic microscopic examination. Furthermore, the initiation and evolution of each mechanism is described by plotting the cumulative hits of each class as a function of the applied load.

Finite element analysis of impact damage response of composite motorcycle safety helmets

Abstract The energy absorption during impact provided by a motorcycle safety helmet is always of critical importance in order to protect the rider against head injury during an accident. In the present study, a parametric analysis has been performed in order to investigate the effect of the composite shell stiffness and the damage development during impact, on the dynamic response of a composite motorcycle safety helmet. This kind of parametric analysis may be used as a tool during helmet design for minimising testing needs. The LS-DYNA3D explicit hydrodynamic finite element code was used to analyse a detailed model of the helmet-headform system (composite shell/foam liner/metallic headform) and to simulate its dynamic response during impact. A significant part of the work was focused on the modelling of the mechanical behaviour of the composite materials, including damage and delamination development. The dynamic response of the different helmet-headform systems was judged in terms of the maximum acceleration monitored at the centre of gravity of the headform and the maximum value of head injury criterion. It was shown that composite shell systems exhibiting lower shear performance provide additional energy absorbing mechanisms and result to better crashworthiness helmet behaviour.

Failure mechanisms analysis of 2D carbon/carbon using acoustic emission monitoring

The present work aims toward the application of an innovative methodology for the analysis of acoustic emission (AE) activity monitored during the quasistatic tensile loading of centre-hole carbon/carbon. An in-house developed algorithm is proposed, which utilises the results of unsupervised pattern recognition classification of AE data. Correlation between clusters and specific material failure modes was achieved, using algorithm results and cluster activation parameters. During the analysis a dependence of AE activity on the hole diameter was observed.

Damage Monitoring of Carbon Fiber Reinforced Laminates Using Resistance Measurements. Improving Sensitivity Using Carbon Nanotube Doped Epoxy Matrix System

In this study, carbon nanotubes (CNTs) were used as additives in the epoxy matrix of unidirectional (UD) carbon fiber reinforced laminates. CNTs were employed not only for improving the mechanical performance of composite, but also for providing an innovative way for monitoring the damage accumulation during the loading of the laminate via the monitoring of the changes in the conductivity of the material. The CNT inclusion improved the transverse conductivity rendering the UD composite more electrically isotropic. Both plain and modified laminates were subjected to monotonic and cyclic tensile loading with simultaneous monitoring of the longitudinal resistance. The resistance change due to mechanical loading was more pronounced for the laminates with the modified matrices. As it was expected, the presence of the electrically conductive CNT network acted as a direct sensor of matrix related damage phenomena, which was complementary to changes related to failure of the reinforcing phase. This was not the case for the reference composite laminates where the monitored changes of the electrical conductivity mirrored the damage exclusively related to the reinforcing phase i.e., the carbon fibers.

Experimental investigation of the tension band in fractures of the patella

We investigated the strain pattern developed in the anterior and posterior part of the fixed patella during knee motion. Eight fresh cadaver knees were used but two were excluded because of non reliable measurements due to misplacement of gauges. Two strain gauges were bonded in the midline of the anterior and two in the posterior surface of the patella. Threaded steel rods were cemented into the intramedullary femoral and tibial canals. The knee was placed on a special device. The quadriceps tendon was gripped and a 4.5 kg weight was attached to the tibial rod 16.5 cm distal to the joint line. Ten flexion/extension cycles were performed before testing. Initially the intact patella was tested. A transverse osteotomy was performed before being stabilized by the AO recommended tension band technique. The knee was retested again as above. Finally an additional circular wire was passed around the patella and the knee was tested again under the same loading configuration. The intact patella showed weak tensile strain on the anterior and compressive strain on the posterior surface through the range of knee motion. Tension band fixation produced weak tensile strains in the first few degrees of flexion and then weak compressive strains in the posterior surface. The presence of the additional circular wire significantly increased the compressive strain. The classical tension band is highly effective for the fixation of the fractured patella but is improved by an additional circular wire.

Intelligent health monitoring of aerospace composite structures based on dynamic strain measurements

This work presents a study on an intelligent system for structural health monitoring of aerospace structures based on dynamic strain measurements, in order to identify in an exhaustive way the structural state condition. Four fiber Bragg grating (FBG) optical sensors were used for collecting strain data, representing the dynamic response of the structure and the expert system that was developed was based on the collected response data. Multi-sensor data fusion in a feature-level approach was followed. Advanced signal processing and pattern recognition techniques such as discrete wavelet transform (DWT) and support vector machines (SVM) were used in the system. For the current analysis, independent component analysis (ICA) was additionally used for the reduction of feature space. The results showed that SVMs using non-linear kernel is a powerful and promising pattern recognition scheme for damage diagnosis. The system was developed and experimentally validated on a flat stiffened composite panel, representing a section of a typical aeronautical structure. Within the frame of the present work the flat stiffened panel was manufactured using carbon fiber pre-pregs. Damage was simulated by slightly varying the mass of the panel in different zones of the structure by adding lumped masses. The analysis of operational dynamic responses was employed to identify both the damage and its position. Numerical simulation with finite element analysis (FEA) was also used as a support tool.

Damage identification in carbon fiber reinforced polymer plates using electrical resistance tomography mapping

This study addresses the issue of structural damage identification and location in carbon fiber reinforced polymer plates using electrical measurements. Electrical resistance tomography is presented as a method for structural damage localization in composite parts. A set of electrodes is fixed on the edges of the part and combinations of DC current injections and voltage measurements are applied to the system. The change of voltage between different times in the part’s service life (e.g. start and degraded) are monitored. These sets of measurements are used as input to inversely calculate conductivity maps for the complete composite part and thus indirectly assess its structural health. Such processes are inherently ill-posed. Data post-processing approaches are proposed here to diminish this uncertainty and to conclude to an optimally converge solution of the inverse problem. To assist the process, a material-originating mathematical constraint is introduced. The method is applied on carbon fiber reinforced polymer plates for different damage modes. Experimental recordings show that the analysis of electrical fields allows detecting the presence of damage. Discontinuities as small as 0.1% of the inspected area can be sensed. The proposed data post-processing techniques were applied and conductivity maps were calculated. The results show that using these techniques locating damage is possible with less than 10% error. Material-based constraints greatly enhance the prediction of the data post-processing techniques. It is believed that by overcoming certain implementation issues, electrical resistance tomography could evolve in the direction of a non-destructive evaluation or a structural health monitoring technique for composite structures.

Toughness characterization and acoustic emission monitoring of a 2-D carbon/carbon composite

Abstract The fracture behavior of carbon/carbon composites is mainly governed by the activation of multiple matrix cracking, shear band formation, fiber debonding and fiber pull out, promoting the stress redistribution and offering a kind of “stress shielding effect” against crack propagation. In order to understand these effects and quantify the evolution of different damage mechanisms in time, experiments have been conducted on compact tension (CT) test coupons, accompanied by continuous acoustic emission (AE) monitoring. Then, fracture mechanics analysis was applied on the experimental results and a signal pattern recognition classification process was developed for the AE data, supported by extensive microscopical examination and systematic ultrasonic inspection. Correlation between clusters, resulting from the classification algorithm of AE data, and damage mechanisms activated by load increase during the CT experiment was accomplished, using classification algorithm parameters. A relation between the felicity ratio and the effective crack length was introduced and the multiple matrix cracking resulting by the shear failure of carbon matrix was found to act as the dominant stress redistribution mechanism.

A new method for the determination of viscoelastic properties of composite laminates: a mixed analytical–experimental approach

Abstract The present work deals with the determination of the viscoelastic properties of composite laminates based on the assumption of viscoelastic behaviour of the single lamina. A viscoelastic lamination theory is developed assuming the lamina stiffness matrix fully complex and frequency dependent. For the measurement of the complete set of frequency dependent viscoelastic properties of the single lamina (i.e. complex moduli and Poissons ratio) a new methodology has been introduced, based on the free and forced vibration of unidirectional and/or 45 −45,0 ns composite laminates. All the measurements have been performed in air. Since the vibration amplitude was very small, in order to fulfil linearity requirements, the results do not have any significant dependence on air damping. The effects of experimental errors on the evaluation of the viscoelastic properties of the single lamina have also been discussed analytically.