Theodore E. Matikas

Ultrasonic Linear and Nonlinear Behavior of Fatigued Ti–6Al–4V

The change in ultrasonic nonlinear property of a titanium alloy subjected to cyclic loading has been studied, with an objective to develop a new characterization methodology for quantifying the level of damage in the material undergoing fatigue. In order to determine the degree of nonlinearity, the ultrasonic second harmonic generation technique has been used. The second harmonic signal was monitored during the fatigue process, and a substantial increase in the second harmonic amplitude (180% increase in nonlinear factor) was observed. This indicates that the second harmonic signal is very sensitive to the microstructural changes in the material caused by fatigue.

Nanocomposite catalysts producing durable, super-black carbon nanotube systems: Applications in solar thermal harvesting

A novel two-step approach for preparing carbon nanotube (CNT) systems, exhibiting an extraordinary combination of functional properties, is presented. It is based upon nanocomposite films consisting of metal (Me = Ni, Fe, Mo, Sn) nanoparticles embedded into diamond-like carbon (DLC). The main concept behind this approach is that DLC inhibits the growth of Me, resulting in the formation of small nanospheres instead of layers or extended grains. In the second step, DLC:Me substrates were used as catalyst templates for the growth of CNTs by the thermal chemical vapor deposition (T-CVD) process. X-ray photoelectron spectroscopy (XPS) has shown that at the T-CVD temperature of 700 °C DLC is completely graphitized and NiC is formed, making DLC:Ni a very effective catalyst for CNT growth. The catalyst layers and the CNT systems have been characterized with a wide range of analytical techniques such as Auger electron spectroscopy and X-ray photoelectron spectroscopy (AES/XPS), X-ray diffraction, reflectivity and scattering, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and optical and electrical testing. The produced CNTs are of excellent quality, without needing any further purification, durable, firmly attached to the substrate, and of varying morphology depending on the density of catalyst nanoparticles. The produced CNTs exhibit exceptional properties, such as super-hydrophobic surfaces (contact angle up to 165°) and exceptionally low optical reflection (reflectivity <10(-4)) in the entirety of the visible range. The combination of the functional properties makes these CNT systems promising candidates for solar thermal harvesting, as it is demonstrated by solar simulation experiments.

A Lamb wave scanning approach for the mapping of defects in [0/90] titanium matrix composites

In this paper a new scanning technique using leaky Lamb waves is presented. This technique is applied to detect internal defects in a multilayered fiber-reinforced composite plate specimen (SCS-6 fibers in Ti-6Al-4V matrix). Images generated by this new Lamb wave scanning technique (we will refer it as the L-scan technique) are compared with conventional C-scan images. This comparison shows that the L-scan technique is more effective for detecting some internal defects such as missing fibers and fiber breakage type defects in a multilayered specimen than the conventional C-scan technique.

Acoustic emission monitoring of degradation of cross ply laminates

The scope of this study is to relate the acoustic activity of damage in composites to the failure mechanisms associated with these materials. Cross ply fiber reinforced composites were subjected to tensile loading with recording of their acoustic activity. Acoustic emission (AE) parameters were employed to monitor the transition of the damage mechanism from transverse cracking (mode I) to delamination (mode II). Wave propagation measurements in between loading steps revealed an increase in the relative amplitude of the propagated wave, which was attributed to the development of delamination that confined the wave to the top longitudinal plies of the composite.

Ultrasonic reflectivity technique for the characterization of fiber‐matrix interface in metal matrix composites

An ultrasonic plane wave reflected by a cylindrical fiber embedded in a homogeneous isotropic matrix is modeled. The model calculates the ‘‘back‐reflection’’ coefficient by taking in to account the properties of the fiber and the matrix, the ultrasonic wavelength, the angle of incidence, and a coefficient called ‘‘shear stiffness coefficient’’ which characterizes the elastic behavior between the fiber and the matrix. Results obtained from the theoretical analysis for a model metal matrix composite system are shown. The theory developed in this paper and some of the results obtained are equally applicable in ceramic matrix fiber reinforced composites.

Experiments and analysis of fiber fragmentation in single and multiple-fiber SiC/Ti-6Al-4V metal matrix composites

Abstract Single-fiber and multiple-fiber single-ply fragmentation experiments were performed at room temperature on SiC/ Ti-6A1-4V specimens, to understand interface shear failure under fragmentation conditions and to assess load-sharing behavior in longitudinally loaded composites. Tensile specimens were instrumented with two acoustic emission sensors and an extensometer to monitor the strain at which fiber breaks occurred. Following testing, the break locations were determined using a novel ultrasonic shear-wave back reflection (SBR) technique. Data analysis was performed using Curtins exact fiber fragmentation model, wherein the in situ Weibull strength and Weibull modulus of the fiber, and the average shear stress under fragmentation conditions, were determined based on best fit with two essentially independent sets of data from the experiments, i.e. the breaking stress of the fibers, and the fragment length distribution. Results for the SCS-6/Ti-6A1-4V samples are presented in this paper, and they are compared with results from other SiC fibers in the same Ti-alloy matrix. The average shear stress from the fragmentation test was significantly higher than that obtained by push-out tests, and is explained on the basis of high radial clamping stress on the fiber in the immediate vicinity of a fiber break. Experiments were also performed on multi-fiber single-ply specimens. Comparison with the single-fiber results showed evidence of correlated fracture even for the relatively weak interface of the SCS-6 fiber. SBR image and macroscopic slip bands indicate that localized plasticity plays a dominant role in promoting correlated fiber fractures at room temperature, and the mechanism is outlined.

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.

Measurement of the dynamic elastic moduli of porous titanium aluminide compacts

The dynamic elastic moduli of the porous alpha-two titanium aluminide compacts are measured using an ultrasonic technique. Both shear and longitudinal velocities are measured for compacts of different densities, making computation of all the four elastic constants, namely, the Young’s modulus, shear modulus, bulk modulus and Poisson’s ratio. The dependence of these on the relative density are correlated and compared with some earlier models, and some of the uncertainties in the earlier models are discussed.

Microdeformation behaviour of Al–SiC metal matrix composites

The satisfactory performance of metal matrix composites depends critically on their integrity, the heart of which is the quality of the matrix-reinforcement interface. The nature of the interface depends in turn on the processing of the MMC component. At the micro-level, the development of local concentration gradients around the reinforcement can be very different according to the nominal conditions. These concentration gradients are due to the metal matrix attempting to deform during processing. This plays a crucial role in the micro-structural events of segregation and precipitation at the matrix-reinforcement interface. Equilibrium segregation occurs as a result of impurity atoms relaxing in disordered sites found at interfaces, such as grain boundaries, whereas non-equilibrium segregation arises because of imbalances in point defect concentrations set up around interfaces during non-equilibrium heat treatment processing. The amount and width of segregation depend very much on (a) the heat treatment temperature and the cooling rate, (b) the concentration of solute atoms and (c) the binding energy between solute atoms and vacancies. An aluminium–silicon–magnesium alloy matrix reinforced with varying amounts of silicon carbide particles was used in this study. A method of calculation has been applied to predict the interfacial fracture strength of aluminium, in the presence of magnesium segregation at metal matrix interface. Preliminary results show that the model succeeds in predicting the trends in relation to segregation and intergranular fracture strength behaviour in these materials. Microhardness profiles of reinforced and un-reinforced aluminium alloys are reported. The presence of precipitates at alloy-reinforcement interface identified by Nano-SEM.

In situ observation of the single-fiber fragmentation process in metal-matrix composites by ultrasonic imaging

Abstract Single-fiber fragmentation tests with continuous silicon-carbide fibers in a Ti6Al4V alloy matrix have been conducted with in situ ultrasonic imaging to monitor the fragmentation process. Straining proceeded incrementally on a specially designed load frame with acoustic emission detection (AE) performed during each increment, and shear-wave back reflectivity (SBR) ultrasound images were acquired following each increment. Metallographic examination of the fragmented fiber was performed following the straining sequence by electropolishing and scanning electron microscopy. Good agreement was found between the fiber breaks imaged by ultrasound, the number of breaks detected by acoustic emissions, and the breaks observed by metallography.