Konstantinos G. Dassios

Mechanical properties of alumina Nextel™ 720 fibres at room and elevated temperatures: tensile bundle testing

Abstract Tensile tests on alumina Nextel 720™ fibre bundles were performed and results were analysed using Weibull statistics to calculate the tensile strength and elastic modulus of the fibres as a function of temperature and gauge length. A compliance-correction technique was used to deduce the net fibre strain from the recorded experimental data separately for room and high-temperature tests; the latter tests were performed under inert (argon) gas environments. The Weibull parameters, calculated in terms of strain values, yielded the mean failure strain and the initial compliances yielded the elastic moduli. Both parameters allowed to obtain the mean fibre strength. Statistical analysis was performed to provide a high and low confidence limit for both the Weibull modulus and the strength. Fibre strength was projected to a reference gauge length of 25-mm using the Weibull gauge length correction to allow for comparison with results from single-fibre tests. Room-temperature tests showed that gauge lengths larger than 75 mm are associated with a non-smooth fibre failure regime that can be explained by the larger amount of energy stored in the tested volume at the onset of failure. Results from relevant tests yielded an elastic modulus of 260 GPa and a Weibull modulus of 5.6, 20% less than the value reported by the manufacturer. Fibre strength was substantially less than the value quoted by the manufacturer for both room and high-temperatures; the room-temperature value corresponding to only 53% of the value reported. The observed divergence is a direct effect of the fundamental characteristic of the fibre bundle testing technique which, contrary to the single-fibre test used in the reference studies, is associated with minimal fibre handling and damage and, as such, is believed to fully assess the fibre strength distribution by not eliminating weaker fibres from the population before testing.

Direct in situ measurements of bridging stresses in CFCCs

The macro- and micro-mechanics of large scale bridging in SiC/MAS-L composites were assessed by tensile testing of DEN specimens with in situ LRM measurements. The macromechanical behavior was analyzed in terms of crack growth resistance and bridging laws using an elastic displacement correction approach. A dedicated Raman calibration curve was established for the specific composite fiber, which served in the transformation of wavenumber shifts collected from bridging fibers in the composite to bridging strain, and to stress via the elastic modulus of the fibers. Bridging strain profiles were established along the ligament of the specimen and their shape was discussed in conjunction with local notch effects. Actual bridging strain values up to 0.8% were calculated which compared well with their macromechanical counterparts within the intact-fiber regime. The individual contributions of intact and pulled-out fibers to the total fracture behavior of the composite were distinguished and are discussed in the article.

In-Situ Monitoring of Damage Evolution in Glass Matrix Composites during Cyclic Loading using Nondestructive Techniques

Infrared thermography is a powerful non-destructive testing technique which can be used for the detection of damage in advanced materials such as ceramic matrix composites. The purpose of this study is to apply a non-destructive methodology for analyzing, in real-time, the thermal effects in ceramic matrix composites caused by cyclic loading. Mechanical stresses induced by cyclic loading cause heat release in the composite due to failure of the interface, which results in increasing the material’s temperature. The heat waves, generated by the thermo-mechanical coupling, and the intrinsic energy dissipated during mechanical cyclic loading of the specimen, were detected by an infrared camera. The results were correlated with acoustic emission events that occurred during the damage accumulation process of the material.

Large-scale interfacial damage and residual stresses in a glass–ceramic matrix composite

The current work is concerned with the micro-mechanics of fracture of a SiC-fiber-reinforced barium osumilite (BMAS) ceramic matrix composite tested under both monotonic and cyclic tension. The double-edge notch (DEN) specimen configuration was employed in order to confine material damage within a predefined gage length. The imposition of successive loops of unloading to complete load relaxation and subsequent reloading were found to result in an increase by 20% in material strength as compared to pure tension; the finding is attributed to energy dissipation from large-scale interfacial debonding phenomena that dominated the post-elastic mechanical behavior of the composite. Cyclic loading also helped establish the axial residual stress state of the fibers in the composite, of tensile nature, via a well-defined common intersection point of unloading–reloading cycles. An approach consisting of the application of a translation vector in the stress–strain plane was successfully used to derive the residual stress-free properties of the composite and reconcile the scatter noted in elastic properties of specimens with respect to theoretical expectations.

Damage Accumulation in Cyclically-Loaded Glass-Ceramic Matrix Composites Monitored by Acoustic Emission

Barium osumilite (BMAS) ceramic matrix composites reinforced with SiC-Tyranno fibers are tested in a cyclic loading protocol. Broadband acoustic emission (AE) sensors are used for monitoring the occurrence of different possible damage mechanisms. Improved use of AE indices is proposed by excluding low-severity signals based on waveform parameters, rather than only threshold criteria. The application of such improvements enhances the accuracy of the indices as accumulated damage descriptors. RA-value, duration, and signal energy follow the extension cycles indicating moments of maximum or minimum strain, while the frequency content of the AE signals proves very sensitive to the pull-out mechanism.

Real-time characterization of damage in ceramic matrix composites using IR thermography and acoustic emission

Infrared thermography is one of several non-destructive testing techniques which can be used for detection of damage in materials such as ceramic matrix composites. The purpose of this study is to apply a non-destructive methodology for analyzing the thermal effects in ceramic matrix composites caused by cyclic loading. Mechanical stresses induced by cyclic loading cause heat release in the composite due to failure of the interface, which results in increasing the materials temperature. The heat wave, generated by the thermo-mechanical coupling, and the intrinsic energy dissipated during mechanical cyclic loading of the sample were detected by an infrared camera. The results were correlated with acoustic emission events.

Ni-Ti Shape Memory Alloy Coatings for Structural Applications: Optimization of HVOF Spraying Parameters

This work aims at contributing to the development of a revolutionary technology based on shape memory alloy (SMA) coatings deposited on-site to large-scale metallic structural elements, which operate in extreme environmental conditions, such as steel bridges and buildings. The proposed technology will contribute to improve the integrity of metallic civil structures, to alter and control their mechanical properties by external stimuli, to contribute to the stiffness and rigidity of an elastic metallic structure, to safely withstand the expected loading conditions, and to provide corrosion protection. To prove the feasibility of the concept, investigations were carried out by depositing commercial NiTinol Ni50.8Ti (at.%) powder, onto stainless steel substrates by using high-velocity oxygen-fuel thermal spray technology. While the NiTinol has been known since decades, this intermetallic alloy, as well as no other alloy, was ever used as the SMA-coating material. Due to the influence of dynamics of spraying and the impact energy of the powder particles on the properties of thermally sprayed coatings, the effects of the main spray parameters, namely, spray distance, fuel-to-oxygen feed rate ratio, and coating thickness, on the quality and properties of the coating, in terms of hardness, adhesion, roughness, and microstructure, were investigated.

A novel processing route for carbon nanotube reinforced glass-ceramic matrix composites

The current study reports the establishment of a novel feasible way for processing glass- and ceramic- matrix composites reinforced with carbon nanotubes (CNTs). The technique is based on high shear compaction of glass/ceramic and CNT blends in the presence of polymeric binders for the production of flexible green bodies which are subsequently sintered and densified by spark plasma sintering. The method was successfully applied on a borosilicate glass / multi-wall CNT composite with final density identical to that of the full-dense ceramic. Preliminary non-destructive evaluation of dynamic mechanical properties such as Young’s and shear modulus and Poisson’s ratio by ultrasonics show that property improvement maximizes up to a certain CNT loading; after this threshold is exceeded, properties degrade with further loading increase.

The effect of different surfactants/plastisizers on the electrical behavior of CNT nano-modified cement mortars

Cement-based materials have in general low electrical conductivity. Electrical conductivity is the measure of the ability of the material to resist the passage of electrical current. The addition of a conductive admixture such as Multi-Walled Carbon Nanotubes (MWCNTs) in a cement-based material increases the conductivity of the structure. This research aims to characterize nano-modified cement mortars with MWCNT reinforcements. Such nano-composites would possess smartness and multi-functionality. Multifunctional properties include electrical, thermal and piezo-electric characteristics. One of these properties, the electrical conductivity, was measured using a custom made apparatus that allows application of known D.C. voltage on the nano-composite. In this study, the influence of different surfactants/plasticizers on CNT nano-modified cement mortar specimens with various concentrations of CNTs (0.2% wt. cement CNTs - 0.8% wt. cement CNTs) on the electrical conductivity is assessed.

Nondestructive Damage Evaluation in Ceramic Matrix Composites for Aerospace Applications

Infrared thermography (IRT) and acoustic emission (AE) are the two major nondestructive methodologies for evaluating damage in ceramic matrix composites (CMCs) for aerospace applications. The two techniques are applied herein to assess and monitor damage formation and evolution in a SiC-fiber reinforced CMC loaded under cyclic and fatigue loading. The paper explains how IRT and AE can be used for the assessment of the materials performance under fatigue. IRT and AE parameters are specifically used for the characterization of the complex damage mechanisms that occur during CMC fracture, and they enable the identification of the micromechanical processes that control material failure, mainly crack formation and propagation. Additionally, these nondestructive parameters help in early prediction of the residual life of the material and in establishing the fatigue limit of materials rapidly and accurately.