D. A. Exarchos

The shape and the thickness of the anterior cruciate ligament along its length in relation to the posterior cruciate ligament: a cadaveric study.

PURPOSE The purpose of this study was to evaluate the shape of the native anterior cruciate ligament (ACL) along its length in relation to the posterior cruciate ligament (PCL) and compare it with the size of the 3 commonly used autografts (bone-patellar tendon-bone [BPTB], single-bundle hamstring, and double-bundle hamstring). METHODS With the knee in extension, we filled the intercondylar notch with paraffin, fixing the cruciate ligaments in their natural position, in 8 cadaveric specimens. The ACL-PCL tissue specimen, embedded in paraffin, was removed en bloc. Gross sections were prepared in the coronal plane and were evaluated histologically. The width, thickness, and cross-sectional area of both the ACL and PCL were determined. The dimensions of the semitendinosus tendon (ST), gracilis tendon (GT), and BPTB grafts were measured and compared with those of the native ACL. RESULTS The PCL occupies the largest part of the intercondylar area, leaving only a small space for the ACL in knee extension. The ACL midsubstance has a width of 5 mm, resembling a band shape. Only before its tibial insertion does the ACL fan out and take the form of its tibial attachment. The BPTB graft has a thickness of 5.8 mm, whereas the ST and GT grafts have a thickness of 6.25 mm and 4.5 mm, respectively, and are comparable to the midsubstance of the ACL but undersized in the tibial insertion (P = .0016 for BPTB graft, P = .002 for ST graft, and P = .0003 for GT graft). A quadruple-looped ST-GT graft, with a diameter of 8 mm, is oversized in the midsubstance (P = .0002) but fits better in the tibial attachment. CONCLUSIONS The ACL midsubstance has a width of 5 mm, resembling a band shape. Before its tibial insertion, the ACL fans out like a trumpet, taking the form of its wide tibial attachment. CLINICAL RELEVANCE The dimensions of the native ACL have to be considered in graft selection for anatomic ACL reconstruction.

Birefringent phononic structures

Within the framework of elastic anisotropy, caused in a phononic crystal due to low crystallographic symmetry, we adopt a model structure, already introduced in the case of photonic metamaterials, and by analogy, we study the effect of birefringence and acoustical activity in a phononic crystal. In particular, we investigate its low-frequency behavior and comment on the factors which determine chirality by reference to this model.

Self-healing polymers: evaluation of self-healing process via non-destructive techniques

The scope of this study is the real-time monitoring and evaluation of the healing process using novel testing procedures and non-destructive evaluation techniques. In this work, the healing approach that has been followed is that of the intrinsic Diels–Alder (DA) and retro-DA mechanism. Two polymers were used as adhesives in single lap-joint specimens that were subjected to static tensile loading. The healed specimens were subjected to the same testing procedure as the one before the first failure and the healing efficiency was assessed by the correlation of the experimental results prior and after healing. Acoustic emission and infrared thermography were employed with the destructive tests so as to monitor changes in the acoustic and the thermal profile between the virgin and the ‘healed’ specimen.

The effect of CNTs reinforcement on thermal and electrical properties of cement-based materials

This research aims to investigate the influence of the nano-reinforcement on the thermal properties of cement mortar. Nano-modified cement mortar with carbon nanotubes (CNTs) leading to the development of innovative materials possessing multi-functionality and smartness. Such multifunctional properties include enhanced mechanical behavior, electrical and thermal conductivity, and piezo-electric characteristics. The assessment of the thermal behavior was evaluated using IR Thermography. Two different thermographic techniques are used to monitor the influence of the nano-reinforcement. To eliminate any extrinsic effects (e.g. humidity) the specimens were dried in an oven before testing. The electrical resistivity was measured with a contact test method using a custom made apparatus and applying a known D.C. voltage. This study indicate that the CNTs nano-reinforcement enhance the thermal and electrical properties and demonstrate them useful as sensors in a wide variety of applications.

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.

Mechanical damage assessment by means of thermo-electrical lock-in thermography

The present work deals with the nondestructive assessment of the metallic materials’ mechanical damage. An innovative Nondestructive Evaluation (NDE) methodology based on two thermographic approaches was developed in order the state of fatigue damage to be assessed. The first approach allows the detection of heat waves generated by the thermomechanical coupling during the fatigue loading (online method). Specifically, both the thermo-elastic and intrinsic dissipated energy was correlated with the mechanical degradation and the remaining fatigue life. The second approach involves the monitoring of the materials’ thermal behavior using a Peltier device for accurate thermal excitation (offline method). The correlation of the thermal behavior and the state of damage was achieved by the determination of the material’s thermal response. The combination of these two approaches enables the rapid and accurate assessment of the cumulative damage.

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.

Chiral phononic structures

A full elastodynamic multiple scattering approach is employed to investigate the behavior of nonreciprocal phononic structures consisting of periodic helical assemblies of spheres. We report on cases of dense and sparse helical chains, cases with size variation and low frequency behavior.

Towards miniaturization of electronics by developing and characterizing hyperfine solder powders used in printed circuit boards

Today’s electronics industry, due its continues growth and increasing demand for devices such as cell phones, satellite navigation systems, health devices, etc., faces important challenges related to the vast quantity of raw materials needed for sustainability and the quantity of waste generated from electronics equipment. To sustain its growth, the electronics industry needs innovations, such as the miniaturization of printed circuit boards (PCB) for increasing components density. Consequent development of miniaturized electronics design plays, therefore, a key role for the reduction of energy consumption and raw materials sustainable use. A factor, however, that currently limits this endeavor is the availability of hyperfine pitch solder powder pastes. The present work focuses on the development of novel, low cost, type 8 and 9 solder pastes with hyperfine solder particles (with size distribution of 1-10 μm) aiming at printing PCBs with increased component density. The solder joint quality was characterized using nondestructive techniques after manufacturing at different reflow parameters. Infrared thermography and white light interference microscopy provided information on internal defects such as presence of micro-voids, as well as on the topography of geometrical variations of solderbals, solder errors, and warpage of components, which are related to the thermal history of the component during reflow.

Characterization of Ni-P-SiC protective coating nanocomposites produced by electrodeposition and thermal spraying via infrared thermography and scanning acoustic microscopy

This work focuses on the development of novel nano-reinforced composite protective coatings for a wide range of applications, such as aerospace, automotive, energy and cutting tools industries. In the present work, silicon carbide (SiC) nanoparticles of 100nm and purity of 99% were used to form nickel-high phosphorus matrix composite (Ni–P– SiC) coatings on steel plates, which were prepared by direct current electrodeposition with duty cycle values of 50% and 80%, while the frequency of the imposed pulses was varied between 0.1Hz and 100Hz. Nickel sulphate served as the primary Ni source, while nickel chloride was added to improve anode corrosion, solution conductivity, and uniformity of the coating thickness distribution. Phosphorous acid acted as the P source in the solution and H3BO3 was added as buffering agent. Sodium dodecyl sulphate has been used as a wetting agent, and saccharin as a stress reducing additive. XRD characterization showed that the structure of NiP composite coatings as deposited were amorphous, irrespective of the presence of SiC. After heat treatment at 400°C for one hour, the amorphous phase was crystallized at steady phases of Ni and Ni3P. The morphology and structure as well as the elastic property of the coatings with and without the SiC nanoparticles were assessed using infrared thermography and scanning acoustic microscopy.