G. Gkikas

Corrosion and environmental degradation of bonded composite repair

Purpose – Bonded composite patches are ideal for aircraft structural repair as they offer enhanced specific properties, case‐tailored performance and excellent corrosion resistance. Bonding minimizes induced stress concentrations unlike mechanical fastening, whilst it seals the interface between the substrate and the patch and reduces the risk of fretting fatigue that could occur in the contact zone. The purpose of this paper is to assess the electrochemical corrosion performance and the environmentally induced mechanical degradation of aerospace epoxy adhesives when carbon nanotubes (CNTs) are used as an additive to the neat epoxy adhesive.Design/methodology/approach – The galvanic effect between aluminium substrates and either plain or CNT enhanced carbon fibre composites, was measured using a standard galvanic cell. Also, rest potential measurements and cyclic polarizations were carried out for each of the studied systems. The effect of the CNT introduction to a carbon fiber reinforced plastic (CFRP) o...

Monitoring strain and damage in multi-phase composite materials using electrical resistance methods

The variation of the electrical properties of fiber reinforced polymers when subjected to load offer the ability of strain and damage monitoring. This is performed via electrical resistance and electrical potential measurements. On the other hand Carbon Nanotubes (CNTs) have proved to be an efficient additive to polymers and matrices of composites with respect to structural enhancement and improvement of the electrical properties. The induction of CNTs increases the conductivity of the matrix, transforming it to an antistatic or a conducting phase. The key issue of the structural and electrical properties optimization is the dispersion quality of the nano-scale in the polymer phase. Well dispersed CNTs provide an electrical network within the insulating matrix. If the fibers are conductive, the CNT network mediates the electrical anisotropy and reduces the critical flaw size that is detectable by the change in conductivity. Thus, the network performs as an inherent sensor in the composite structure, since every invisible crack or delamination is manifested as an increase in the electrical resistance. The scope of this work is to further exploit the information provided by the electrical properties with a view to identify strain variation and global damage via bulk resistance measurements. The aforementioned techniques were employed to monitor, strain and damage in fiber reinforced composite laminates both with and without conductive nanofillers.

Effect of carbon nanotube enhanced adhesives on degradation of bonded joints in corrosive environments

Abstract The aim of this work is to study the lap shear strength as well as the response to galvanic corrosion and environmental degradation of Multi Wall Carbon Nanotube (MWCNT) enhanced adhesive bonding on metallic substrates so as to establish whether the response of MWCNT enhanced adhesive patches to environmental degradation is percolation or material system dependent. The incorporation of MWCNTs into an epoxy patch may mediate the galvanic effect between substrate/patch, but also enhances localised degradation phenomena into the polymer matrix. The introduction of 0·1 wt-%MWCNTs positively affects both the galvanic and environmental durability of the adhesive, whereas higher contents adversely affect its behaviour suggesting a percolation threshold between 0·1 and 0·5 wt-%CNT, which, however, is system dependent.

Simultaneous acoustic and dielectric real time curing monitoring of epoxy systems

The attainment of structural integrity of the reinforcing matrix in composite materials is of primary importance for the final properties of the composite structure. The detailed monitoring of the curing process on the other hand is paramount (i) in defining the optimal conditions for the impregnation of the reinforcement by the matrix (ii) in limiting the effects of the exotherm produced by the polymerization reaction which create unwanted thermal stresses and (iii) in securing optimal behavior in matrix controlled properties, such as off axis or shear properties and in general the durability of the composite. Dielectric curing monitoring is a well known technique for distinguishing between the different stages of the polymerization of a typical epoxy system. The technique successfully predicts the gelation and the vitrification of the epoxy and has been extended for the monitoring of prepregs. Recent work has shown that distinct changes in the properties of the propagated sound in the epoxy which undergoes polymerization is as well directly related to the gelation and vitrification of the resin, as well as to the attainment of the final properties of the resin system. In this work, a typical epoxy is simultaneously monitored using acoustic and dielectric methods. The system is isothermally cured in an oven to avoid effects from the polymerization exotherm. Typical broadband sensors are employed for the acoustic monitoring, while flat interdigital sensors are employed for the dielectric scans. All stages of the polymerization process were successfully monitored and the validity of both methods was cross checked and verified.

On the use of dielectric spectroscopy for the real time assessment of the dispersion of carbon nanotubes in epoxy

In this work, we report the on line monitoring of the dispersion of carbon nanotubes in a typical aerospace epoxy system. The dispersion quality is well known to dominate all the properties of the nanocomposite. Process monitoring was performed using a dielectric spectroscopy system to measure the evolution of the impedance of the mixture during ultrasonication. Interdigital sensors were employed as flat capacitors immersed in the mixture. The evolution of the dispersion process was accurately monitored via the changes in the real and the imaginary parts of the impedance. Furthermore, the distinct stages of the dispersion process were modelled using an equivalent circuit which was found to accurately represent the involved phases and their interaction during dispersion process. Overall, this work demonstrates how dispersion control may become possible through real-time in situ measurement of the dielectric response from the system, analysis of its main components and identification of the significant features.

Dispersion monitoring of carbon nanotube modified epoxy systems

The remarkable mechanical and electrical properties exhibited by carbon nanotubes (CNTs) have encouraged efforts to develop mass production techniques. As a result, CNTs are becoming increasingly available, and more attention from both the academic world and industry has focused on the applications of CNTs in bulk quantities. These opportunities include the use of CNTs as conductive filler in insulating polymer matrices and as reinforcement in structural materials. The use of composites made from an insulating matrix and highly conductive fillers is becoming more and more important due to their ability to electromagnetically shield and prevent electrostatic charging of electronic devices. In recent years, different models have been proposed to explain the formation of the conductive filler network. Moreover, intrinsic difficulties and unresolved issues related to the incorporation of carbon nanotubes as conductive fillers in an epoxy matrix and the interpretation of the processing behavior have not yet been resolved. In this sense, a further challenge is becoming more and more important in composite processing: cure monitoring and optimization. This paper considers the potential for real-time control of cure cycle and dispersion of a modified epoxy resin system commonly utilized in aerospace composite parts. It shows how cure cycle and dispersion control may become possible through realtime in-situ acquisition of dielectric signal from the curing resin, analysis of its main components and identification of the significant features.

Interlaminar shear strength and thermo-mechanical properties of nano-enhanced composite materials under thermal shock

The introduction of nanoscaled reinforcement in otherwise conventional fiber reinforced composite materials has opened an exciting new area in composites research. The unique properties of these materials combined with the design versatility of fibrous composites may offer both enhanced mechanical properties and multiple functionalities which has been a focus area of the aerospace technology on the last decades. Due to unique properties of carbon nanofillers such as huge aspect ratio, extremely large specific surface area as well as high electrical and thermal conductivity, Carbon Nanotubes have benn investigated as multifunvtional materials for electrical, thermal and mechanical applications. In this study, MWCNTs were incorporated in a typical epoxy system using a sonicator. The volume of the nanoreinforcement was 0.5 % by weight. Two different levels of sonication amplitude were used, 50% and 100% respectively. After the sonication, the hardener was introduced in the epoxy, and the system was cured according to the recommended cycle. For comparison purposes, specimens from neat epoxy system were prepared. The thermomechanical properties of the materials manufactured were investigated using a Dynamic Mechanical Analyser. The exposed specimens were subjected to thermal shock. Thermal cycles from +30 °C to -30 °C were carried out and each cycle lasted 24 hours. The thermomechanical properties were studied after 30 cycles . Furthermore, the epoxy systems prepared during the first stage of the study were used for the manufacturing of 16 plies quasi isotropic laminates CFRPs. The modified CFRPs were subjected to thermal shock. For comparison reasons unmodified CFRPs were manufactured and subjected to the same conditions. In addition, the interlaminar shear strength of the specimens was studied using 3-point bending tests before and after the thermal shock. The effect of the nanoreinforcement on the environmental degradation is critically assessed.

Structural health monitoring of aerospace materials used in industry using electrical potential mapping methods

The increasing use of composite materials in aerostructures has prompted the development of an effective structural health monitoring system. A safe and economical way of inspection is needed in order for composite materials to be used more extensively. Critical defects may be induced during the scheduled repair which may degrade severely the mechanical properties of the structure. Low velocity impact LVI damage is one of the most dangerous and very difficult to detect types of structural deterioration as delaminations and flaws are generated and propagated during the life of the structure. In that sense large areas need to be scanned rapidly and efficiently without removal of the particular components. For that purpose, an electrical potential mapping was employed for the identification of damage and the structural degradation of aerospace materials. Electric current was internally injected and the potential difference was measured in order to identify induced damage in Carbon Fiber Reinforced Polymer (CFRP) structures. The experimental results of the method were compared with conventional C-scan imaging and evaluated.