The effect of CO2 saturation on mechanical properties of Australian black coal using acoustic emission

Acoustic emission (AE) methods are now widely used for damage evaluation. For a better understanding of the damage mechanics of materials such as rocks, AE has been used to monitor stresses which induce crack closure, crack initiation and crack damage. In the present study, an AE system was used to study the damage behaviour of some Australian black coal samples subjected to uniaxial compression. Several samples were left in a container filled with 100% carbon dioxide (CO₂) at a certain pressure for 72 h prior to testing. The results were compared with samples which had only been exposed to the atmosphere to see if CO₂ had any adverse effect on the strength of coal. Strain gauges were installed on the samples and the measured axial and volumetric strains were studied in conjunction with the AE counts.The AE method was successfully used for detecting the onset of crack initiation and the crack damage stress threshold of the black coal samples. Of the coal samples examined, crack initiation and crack closure of the samples subjected to saturation with CO₂ occurred at stress corresponding to a higher percentage of the peak strength when compared to the samples which had only been exposed to atmospheric conditions. However, crack damage occurred at a higher percentage of peak strength and the average peak strength showed a higher value for samples in atmospheric condition when compared to CO₂ saturated samples. The results show that sorption of CO₂ can cause a reduction in strength of the black coal samples when tested under uniaxial compression. As the coal samples were highly inhomogeneous more tests are required in order to be able to confirm whether the adsorption of CO₂ will cause strength reduction in coal and to identify the actual underlying mechanisms.     

Hot deformation behaviour of 7075 alloy

The hot deformation behaviour of Stir cast 7075 alloy was studied using processing map technique. The map has been interpreted in terms of the microstructural processes occurring in situ with deformation, based on the values of a dimensionless parameter η which is an efficiency index of energy dissipation through microstructural processes. An instability criterion has also been applied to demarcate the flow instability regions in the processing map using another parameter (ξ). Both the parameters (η and ξ) were computed from the experimental data generated by compression tests conducted at various temperatures and strain rate combinations over the hot working range (300-500 °C and 0.001-1.0 s⁻¹) of the present material. The processing map exhibits one distinct η domains without any unstable flow conditions under the investigated temperature and strain rate conditions. The dynamic recrystallization zone and instable zones, i.e. adiabatic shear band formation, interface crack, and wedge cracking, were identified in the processing map. Microstructural examination was performed for validation. The processing maps can be used to select optimum strain rates and temperatures for effective hot deformation of 7075 alloy.     

Experimental and theoretical investigation on microwave melting of metals

Experiments were conducted for microwave heating and melting of lead, tin, aluminium and copper with the aid of susceptors and the detailed results were presented for various microwave power levels and sample loading. Aluminium and copper samples were heated in presence of inert gas to minimize oxidation. Compared to conventional melting, microwave melting was twice as fast and more energy efficient. Lumped parameter model of the heating process showed that the conversion of microwave to thermal energy was enhanced at higher temperatures, justifying this a favourable process for metal melting applications.     

Evaluation of the corrosion resistance of anodized aluminum 6061 using electrochemical impedance spectroscopy (EIS)

The corrosion resistance of anodized Al 6061 produced by two different anodizing and sealing processes was evaluated using electrochemical impedance spectroscopy (EIS). The scanning electron microscope (SEM) was employed to determine the surface structure and the thickness of the anodized layers. The EIS data revealed that there was very little change of the properties of the anodized layers for samples that were hard anodized in a mixed acid solution and sealed in hot water over a 365 day exposure period in a 3.5 wt% NaCl solution. The specific admittance A_s and the breakpoint frequency f_b remained constant with exposure time confirming that the hard anodizing process used in this study was very effective in providing excellent corrosion resistance of anodized Al 6061 over extended exposure periods. Some minor degradation of the protective properties of the anodized layers was observed for samples that were hard anodized in H₂SO₄ and exposed to the NaCl solution for 14 days.     

Experimental exploration of damage propagation in rocks using acoustic emission

Bulletin of Engineering Geology and the Environment - The formation of geological disasters, such as earthquakes and rock bursts, is a process similar to avalanches. These sudden disasters are...     

Aqueous slurry processing of monolithic films for SOFC - YSZ, LSM and YSZ-NiO systems

The powder precursor for yttria stabilized zirconia (YSZ) was prepared by reverse strike co-precipitation method while those for lanthanum strontium manganite (LSM) and nickel oxide (NiO) were prepared by gel combustion method. The thermal decomposition and phase evolution behavior of these powder precursors was carried out by thermogravimetry and XRD. Phase pure cubic YSZ formed around 430 °C from the dried amorphous gel and exhibited a mass loss of 23%. Even though the as formed LSM and NiO precursors exhibited nano-crystallinity, they contained some amount of volatiles (up to 8%), elimination of which required a post heat treatment. The optimum calcination temperature for these powders to obtain sintered bodies with desired densities (viz. >95% T.D. for YSZ and 65-75% T.D. for LSM, YSZ-NiO) at the desired sintering temperatures (1350, 1400 and 1500 °C respectively), was found to vary in the range of 900 1350 °C. Fine YSZ powder with size (D₅₀) 0.7-1.2 μm was used in formation of the electrolyte film while YSZ, LSM and NiO powder with size (D₅₀) 3-5 μm along with carbon pore former (15 wt% in LSM) were used for formation of electrode films. The conditions for slurry formation for film casting were evaluated through surface charge and rheological studies. The study of the effect of pH of aqueous suspension on zeta potential showed that YSZ and NiO were charged to sufficient extent (>20 mV) in both acidic and alkaline media while LSM and pore former exhibited sufficient surface charging only in alkaline medium. The slurries for tape casting were formulated using a polyvinyl binder solution and the composition was optimized through rheological studies. Compositions were fixed to form slurries with desired amount of pseudo-plasticity that could exhibit controlled flow to form flexible films with desirable thickness. The process conditions were optimized to form flat sintered electrolyte films possessing about 95% T.D. and electrode films possessing 65-75% T.D. Sintered bodies of the electrolytes exhibited fine-grained microstructure while the electrodes exhibited composite structure of grains and inter-connected pores.     

Effect of Hydrogen and Magnetic Field on the Mechanical Behavior of High Strength AISI 4340 Steel

Presence of hydrogen in materials is known to affect their mechanical properties due to hydrogen embrittlement problem. Steels used in various applications are prone to be exposed to aqueous electrochemical environments, which may introduce hydrogen into the alloy. These alloys are also prone to be simultaneously exposed to magnetic field, which may affect the hydrogen embrittlement susceptibility of these alloys. Therefore, it is important to examine the effect of hydrogen and magnetic field on the mechanical behavior of iron-based alloys. In this work, the effect of hydrogen and magnetic field on the fracture behavior of high strength AISI 4340 steel was examined. Three-point bend test was used to study the fracture behavior. In all the cases, the samples tested with hydrogen charging show a drastic reduction in ductility and fracture stress values. The effect of magnetic field was seen to be negligible. The hydrogen embrittlement was characterized by a change in the fracture surface from a ductile-type fracture to a brittle cleavage-type fracture. Acoustic emission signals collected during the test corresponds to the fracture behavior.