M.-H. H. Shen

Hysteresis-loop representation for strain energy calculation and fatigue assessment

Improvements have been made to the cyclic strain energy density expression used in a fatigue life prediction method. The theory behind the prediction method is based on the understanding that the same amount of strain energy is dissipated during a monotonic fracture and a cyclic fatigue process. From this understanding, the failure cycle for a fatigue process can be determined by dividing monotonic strain energy by the average strain energy per cycle. Though this technique has been shown to be acceptable, it needs to be improved to account from the experimentally observed increase in the strain energy per cycle as the loading cycles approach fatigue. In order to improve the fatigue life prediction technique, experimental strain energy density per cycle is observed during the fatigue process of Aluminium 6061-T6 (Al 6061-T6) specimens. The results show exponential change in the strain energy density through the first 20 per cent and the last 30 per cent of the total failure cycles. The results lead to a new representation of strain energy density per cycle, which leads to an improved fatigue life prediction method. A comparison is made between the improved prediction method and experimental fatigue results. The comparison result validates the precision of the new hysteresis-loop representation.

Extension of an energy-based life prediction method to low and combined cycle fatigue regimes

An energy-based fatigue life prediction method has been developed to accurately predict lifetimes of coupon specimens in excess of 105 cycles. The method has been shown to agree with empirically determined room temperature high-cycle fatigue data for both Al 6061-T6 and Ti-6Al-4V in uniaxial, bending, and shear at various stress ratios (R). As with any life prediction method, using a testing scheme to accurately predict fatigue performance from a reduced data set greatly reduces test time and material costs. For gas turbine engine components, this can account for a large portion of development costs, making the use of reduced order models very attractive. The stress state of these components can be difficult to characterize and simulate, as they are subjected to both low-cycle fatigue and high-cycle fatigue from both mechanical and vibrational loading. Mechanical loading is generally within the low-cycle fatigue regime and attributed to throttle excursions of various flight maneuvers or engine start-up/shut-down cycles over the course of a component’s lifetime, typically less than 105 cycles. Vibrational loading causes high-cycle fatigue, sometimes of a multiaxial stress state, and is attributed to various forced and free vibration sources manifested as high-order bending or torsion modes. Understanding the interaction of these two fatigue regimes, as combined cycle fatigue, is necessary to develop robust design techniques for gas turbine engine and turbomachinery in general. This study focuses on extending a previously developed energy-based fatigue life prediction method to account for both low-cycle fatigue and combined cycle fatigue of Al 6061-T6511 cylindrical test specimens subjected to various stress ratios, mean stresses, and high-cycle fatigue–low-cycle fatigue interaction.

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.