Sajedur R. Akanda

A modified closed form energy-based framework for fatigue life assessment for aluminum 6061-T6: Strain range approach

It has been shown in our previous effort in the development of an energy-based fatigue life prediction framework that, in order to obtain the fatigue toughness of a nonviscoplastic material at room temperature from the areas of hysteresis loops, a fatigue test need to be conducted at the ideal frequency to minimize the effect of nondamaging energy. For aluminum 6061-T6, the ideal frequency is 0.05 Hz. A fatigue test with such a low frequency requires extensive time to complete. Therefore, the present investigation focuses on modifying the energy-based framework in order to obtain the fatigue toughness from fatigue tests conducted at an arbitrary frequency higher than the ideal frequency. The fatigue toughness is calculated from the average strain range developed in a material during a fatigue test and the Ramberg–Osgood cyclic parameters. The measurement of average strain range by an extensometer associated with a nonviscoplastic test specimen at room temperature is independent of the test frequency. The cyclic parameters are obtained from the fatigue lives at two different stress ranges which are also independent of the test frequency at room temperature. Similar to the case of the previous framework, the modified framework is found to predict the room temperature fatigue life of aluminum 6061-T6 with a promising accuracy.

Fracture toughness of welded joints of a steam turbine low pressure rotor

In order to ensure safety and reliability of a steam turbine welded rotor, the present investigation focused on evaluation of the crack resistance parameter of the base metal (BM), weld metal (WM), and heat affected zone (HAZ) of a turbine rotor welded joint constituent at both room temperature and 300℃. The property of crack resistance was evaluated in terms of fracture energy JIc or plane strain fracture toughness KJIc. Three-point bend experiments were performed to obtain a J-integral based crack resistance curve (J–R curve) from a single specimen in accordance with ASTM E1820, from which the fracture toughness was determined. To construct a J–R curve, the corresponding crack extension was calculated by incorporating the crack opening displacement measured by a clip-gage, in a compliance method. From the experimental results, the WM was found to have the lowest KJIc values whereas the HAZ was found to have the highest KJIc values at both room and high temperature although the HAZ had little or no crack extension during the experiments. It was also observed that the HAZ samples fractured with crack jumped toward the fusion line. Microstructural root-cause analysis of the abnormal HAZ behavior was performed and discussed.

Fatigue Crack Growth Threshold Determination for Welded Joint Constituents of a Steam Turbine LP Rotor

In the present investigation, fatigue crack growth threshold of the base metal, heat affected zone (HAZ), and weld metal of a steam turbine rotor’s welded joint constituents were determined by performing fatigue crack growth rate (FCGR) tests. Two types of test specimens were considered: (1) single-edge notch bend (SENB) and (2) compact tension (CT). Although a CT specimen is more appropriate for an FCGR test according to ASTM E647, in the present investigation the SENB and CT specimens were found to yield an equivalent fatigue crack growth threshold of the welded joint constituent. But, the SENB specimen provided some advantages over the CT specimen in performing the FCGR tests on the welded joints. It was also found that the fatigue crack growth threshold is a function of loading ratio rather than a single material parameter.

Assessment of Fracture Toughness and Fatigue Crack Growth Threshold of a Welded Joint Constituent of a Steam Turbine LP Rotor

In order to ensure safety and reliability of steam turbine welded rotors, the present investigation focuses on evaluation of crack initiation, growth, and resistance parameters of base metal (BM), weld metal (WM) and heat affected zone (HAZ) of a steam turbine rotor welded joint constituent. The experimental part consists of three-point bending conducted on single edge notch bend specimens to induce stable crack propagation. The crack size was calculated by incorporating the crack opening displacement measured by a clip-gage, in a compliance method. The fatigue crack threshold was obtained from a crack growth rate curve according to ASTM E647 and the fracture toughness was determined from a J-based resistance curve according to ASTM E1820. From the experimental results the fatigue crack threshold is found to be a function of loading ratio rather than a single material parameter. From the fracture toughness results, the WM and the BM are found to have similar KIc values whereas HAZ is found to have slightly better KIc values although HAZ had little crack extension during the experiments.Copyright

Investigation of Manganese Cobalt Oxide (MCO) Coatings on Fuel Cell Interconnects

Planar Solid Oxide Fuel Cells (SOFCs) are composed of repeating cathode-electrolyte-anode units separated by electrically conductive interconnects. With the reduction in fuel cell operating temperature to approximately 800°C, it has become possible to use chromium-based, ferritic stainless steel or Crofer for interconnects. These interconnects must survive the high temperature oxidizing and reducing environments while maintaining electrical conductivity. Unfortunately the formation of chromium oxide scale poisons the cell by significantly reducing cathodic activity. Chromium scale formation can be inhibited by applying an electrically conductive manganese cobalt oxide (MCO) spinel coating to the interconnect prior to its installation in the fuel cell. The most costeffective way to apply protective coatings to interconnects involves spray coating. To investigate the quality of the coatings and the coating adhesion, four-point bend experiments were undertaken at room temperature. Tensile cracking patterns on the convex surfaces of the bend specimens were used to determine the interfacial shear strengths of the coatings. SEM images of the cracked coating surfaces were processed to analyze the interface failure mechanisms, the crack spacing, and areas that spalled at higher strains. These investigations were able to show distinct differences between coatings formed with different processes parameters.