This paper addresses the quasi-static and dynamic response of multi-layered steel beams using finite element analysis. The steel beam has four corrugated cores in between the top and bottom substrates. The cores are arranged with corrugated sheets of different thickness. Quasi-static test is first performed to select a suitable material, either thin steel substrate made of steel 1008 or thick steel substrate made of steel 1018, for the top substrate plate, and is then validated with finite element analysis. Upon quasi-static tests and finite element validations, the top plate is selected and dynamic responses of various core layups are further evaluated. The stress-strain relations of carbon steel 1018 at high strain rates are measured using the Split-Hopkinson pressure bar. A finite element modeling procedure for dynamic analysis is validated with a set of shock tube experiments. Corrugated multi-layered steel core arrangements are taken into account for design parameters in order to maximize mitigation of blast load effects onto the main structure.

Thermoelectric power generation is based on the Seebeck effect and occurs with a temperature gradient. Recently, we prepared a single crystalline Ba₈Au_xSi_46-x clathrate with Au composition gradient and reported a heat-electric power conversion with narrow band gap. However, the mechanism was not clearly elucidated. In this study, we investigated carrier-generation behavior using the thermal-probe method during the heating. A single crystal of Ba₈Au_xSi_46-x clathrate was grown by the Czochralski method. Wavelength- dispersive X-ray spectroscopy showed that this single crystal has an Au-content gradient. The Seebeck coefficient of the sample was evaluated by the thermal probe method at room temperature and 300 °C. Results for 300 °C show that heating promotes a decrease of the Seebeck coefficient near the n/p interface. This decrease of the Seebeck coefficient indicated the electrons were excited by heating near the n/p interface.

Coating of Al-Cu alloy with Ti-N graded film was examined by reactive DC sputtering, in order not only to improve the surface hardness and the corrosion resistance or the biocompatibility of the aluminum alloy but also to relax the stress concentrated at the interface between the coated film and the alloy substrate. The deposition of the Ti-N graded coatings were realized by varying continuously nitrogen content in Ar-N₂ sputter gas during the reactive sputtering. Deposited films appeared to be uniform and adhesive, and their color tone was bronze with a red tinge. Their surface hardness reached over Hv1400. According to GDOES (Glow Discharge Optical Emission Spectroscopy) in-depth profiles, the nitrogen concentration in the film gradually decreased in depth direction from the surface toward the alloy substrate, confirming that Ti-N graded film had formed on the alloy substrate. On the basis of XRD, it was assumed that not only a-titanium phase but also titanium nitride phases such as TiN and Ti₂N were formed in the graded film. Furthermore it was confirmed that the corrosion protective property of the AlCu alloy could be improved by coating it with the graded coating.

Halloysite nanotube (HNT) is a natural clay mineral with a hollow and tubular structure, which can be a potential reinforcement for biodegradable polymers such as poly(vinyl alcohol) (PVA). This study investigates the improvement of interfacial adhesion between HNT and PVA by means of controlled acid etching on HNT surfaces. Such an improvement is attributed to a porous structure with rough HNT surfaces in that acid favors the dissolution of AlO₆. The strength of PVA/HNT nanocomposites increased up to 10% when HNT were subjected to 1 h acid treatment. However, the reinforcement effect could have been more pronounced if HNT aspect ratio were maintained during the composite fabrication process.

Quantitative microstructural measurements were made on the oriented and graded structures of the liquid-phase sintered and subsequently carburized Fe-Cu-C compacts. The possible mechanism of the formation of the microstructure has been proposed, in which a concurrent effect of chemically driven liquid film migration, and the decrease in the grain boundary energy, both originated from carbon dissolution, as well as of the surface flattening by capillarity, is the main cause for the formation of oriented and elongated grains. The energy minimization in the graded mirostructure was discussed in terms of the total interface energy in liquid phase sintered alloys. The constancy of the total interface energy throughout the graded compact, the linear relationship between the liquid volume fraction and dihedral angles, as well as the liquid flow towards carbon-diffusion front, strongly suggest that the graded compact is energetically in equilibrium.

Changes of the Young's modulus and hardness of thermal barrier coatings (TBCs) isothermally held at high temperatures or subjected to heat cycles were experimentally studied using a nano-indenter. 8mass% yittria-stabilized zirconia (YSZ) was plasma-sprayed on a Ni-based superalloy with a CoNiCrAlY bond coat layer. The specimen was isothermally held or heatcycled at temperatures of 1323 K, 1373 K, and 1473 K, and the Young's modulus and hardness were then measured using the nano-indenter. Both the Young's modulus and hardness results for the isothermally held specimens showed almost constant values at a maximum indentation load of 100 mN, and monotonically increased with holding time at a maximum indentation load of 1000 mN before reaching constant values. When the specimens were heatcycled at maximum temperatures of 1323 K, 1373 K, and 1473 K, both the Young's modulus and hardness, measured at a maximum indentation load of 1000 mN, decreased with increasing heating time after reaching maximum temperature. The heating times corresponding to the maximum indentation values decreased as the heating temperatures increased. Cross-sectional observation showed that sintering of the intersplat followed by cracking occurred in the top coat. Equations to predict the changes of the Young's modulus and hardness were semi-theoretically derived and agreed with the experimental result.

Journal of Fuctionally Graded Materials Vol.33, 23-27(2019)

Formation of Oriented and Graded Grain Structure in Fe-Cu-C Alloy by Liquid Phase Sintering and Carburization

We have fabricated various porous nickel aluminides with bi-modal pore size distribution by combustion synthesis reaction between Ni and Al with NaCl space holder powder. Effects of powder blending ratio in Ni+Al+NaCl powder mixture on porosity and size of small pores derived from the reaction. NaCl powder, which is a former of large pores, affects not only the large pores but also the small pores derived from the reaction between Ni and Al. Tendencies of change in porosity and size of the small pores with the amount of NaCl were different depending on Ni:Al molar ratio. The reason was discussed in shrinkage of volume of metallic cell wall due to the formation of nickel aluminides from Ni and Al mixture.

In this paper, the transient thermal fracture problem of a functionally graded thermal barrier coating (FGTBC) containing a normal crack bonded to a homogeneous substrate is considered. The problem is solved for the laminate that is suddenly heated from the upper surface of the FGTBC. The bottom surface of the homogeneous substrate is maintained at the initial temperature. The crack faces are supposed to be completely insulated. Material properties are assumed to be exponentially dependent on the distance from the interface. By using both the Laplace and Fourier transforms, the thermo-mechanical fracture problem is reduced to a singular integral equation, which is solved numerically. The stress intensity factors of the embedded crack and the edge crack are computed and presented as a function of the normalized time for the various values of the nonhomogeneous and geometric parameters.

Coal ash generated in thermal power plants is kneaded, extruded through a pipe by a screw, and formed into a pellet shape in extruder machines. As the screw rubs with high-hardness ash at high pressure, the edge becomes most susceptible to wear. By welding a wear-resistant material to the edge of the screw, it is possible to increase the continuous use time of such machines. Therefore, a weldable functionally graded material (FGMs), cemented carbide, was developed using the spark plasma sintering (SPS) method. This FGMs consists of three layers: wear-resistant, intermediate, and weldable layers. As these three layers have different sintering temperatures, sintering is carried out via the SPS method using an appropriate mold. The application of this material to the screw surface of an extruder results in a lifetime of approximately 3000 h, which is about three times longer than that of a conventional product coated by thermal spraying.

In this paper, the fracture problem of a penny-shaped crack in a functionally graded thermal barrier coating (FGTBC) bonded to a homogeneous substrate is considered. The problem is solved for the laminate that is suddenly heated from the upper surface of the FGTBC. The bottom surface of the homogeneous substrate is maintained at the initial temperature. The crack faces are supposed to be completely insulated. Material properties are assumed to be exponentially dependent on the distance from the interface. By using both the Laplace and Hankel transforms, the thermo-mechanical fracture problem is reduced to a singular integral equation and a system of singular integral equations which are solved numerically. The stress intensity factors of the crack are computed and presented as a function of the normalized time for the various values of the nonhomogeneous and geometric parameters.