Onome Scott-Emuakpor

Development of an Improved High Cycle Fatigue Criterion

An integrated computational-experimental approach for prediction of total fatigue life applied to a uniaxial stress state is developed. The approach consists of the following elements: (1) development of a vibration based fatigue testing procedure to achieve low cost bending fatigue experiments and (2) development of a life prediction and estimation implementation scheme for calculating effective fatigue cycles. A series of fully reversed bending fatigue tests were carried out using a vibration-based testing procedure to investigate the effects of bending stress on fatigue limit. The results indicate that the fatigue limit for 6061-T6 aluminum is approximately 20% higher than the respective limit in fully reversed tension-compression (axial). To validate the experimental observations and further evaluate the possibility of prediction of fatigue life, an improved high cycle fatigue criterion has been developed, which allows one to systematically determine the fatigue life based on the amount of energy loss per fatigue cycle. A comparison between the prediction and the experimental results was conducted and shows that the criterion is capable of providing accurate fatigue life prediction.

Goodman Diagram Via Vibration-Based Fatigue Testing

A new vibration-based fatigue testing methodology for assessing high-cycle turbine engine material fatigue strength at various stress ratios is presented. The idea is to accumulate fatigue energy on a base-excited plate specimen at high frequency resonant modes and to complete a fatigue test in a much more efficient way at very low cost. The methodology consists of (1) a geometrical design procedure, incorporating a finite-element model to characterize the shape of the specimen for ensuring the required stress state/ pattern; (2) a vibration feedback empirical procedure for achieving the high-cycle fatigue experiments with variable-amplitude loading; and finally (3) a pre-strain procedure for achieving various uniaxial stress ratios. The performance of the methodology is demonstrated with experimental results for mild steel, 6061-T6 aluminum, and Ti-6Al-4V plate specimens subjected to a fully reversed bending, uniaxial stress state.

Multi-Axial Fatigue-Life Prediction via a Strain-Energy Method

A strain-energy-based method has been developed to predict the fatigue life of a structure subjected to either shear or biaxial bending loads at various stress ratios. The framework for this method is an advancement of previously conducted research that validates a uniaxial energy-based fatigue-life-prediction approach. The understanding behind the approach states that the total strain energy dissipated during a monotonic fracture and a cyclic process is the same material property, where the experimental strain-energy density of each can be determined by measuring the area underneath the monotonic true stress-strain curve and the area within a hysteresis loop, respectively. The developed framework consists of two elements: a life-prediction method that calculates shear fatigue-life cycles and a multi-axial life-prediction method capable of calculating biaxial fatigue-life cycles. A comparison was made between the two framework elements and experimental results from three different aluminum alloys. The comparison shows encouraging agreement, thus providing credence in the prediction capabilities of the proposed energy-based framework.

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.

An Energy-Based Axial Isothermal- Mechanical Fatigue Lifing Procedure

An energy-based fatigue lifing procedure for the determination of full-life and critical-life of in-service structures subjected to axial isothermal-mechanical fatigue (IMF) has been developed. The foundation of this procedure is the energy-based axial room-temperature fatigue model, which states: the total strain energy density accumulated during both a monotonic fracture event and a fatigue process is the same material property. The energy-based axial IMF lifing framework is composed of the following entities: (1) the development of an axial IMF testing capability; (2) the creation of a testing procedure capable of assessing the strain energy accrued during both a monotonic fracture process and a fatigue process at various elevated temperatures; and (3), the incorporation of the effect of temperature into the axial fatigue lifing model. Both an axial IMF capability and a detailed testing procedure were created. The axial IMF capability was employed in conjunction with the monotonic fracture curve testing procedure to produce fifteen fracture curves at four operating temperatures. The strain energy densities for these fracture curves were compared, leading to the assumption of constant monotonic fracture energy at operating temperatures below the creep activation temperature.

A Promising New Energy-Based Fatigue Life Prediction Framework

An energy-based fatigue life prediction framework has been developed for prediction of axial and bending fatigue life at various stress ratios. The framework for the prediction of fatigue life via energy analysis was developed in accordance with the approach in our previous study which states: the total strain energy dissipated during a monotonic fracture process is a material property that can be determined by measuring the area underneath the monotonic true stress-strain curve. The framework consists of the following two elements: (1) Development of a bending fatigue criterion by observing the total strain energy of the effective volume, which is achieved by computing the total plastic strain energy with consideration of the stress gradient influence through the thickness of a specimen, in the fatigue area, during cyclic loading. A comparison between the prediction and the experimental results from 6061-T6 aluminum specimens was conducted and shows that the new energy-based fatigue criterion is capable of predicting accurate fully reversed bending fatigue life. (2) Development of a new life prediction criterion for axial fatigue at various stress ratios. The criterion was constructed by accounting for both the residual energy dissipated, monotonically, due to the mean stress, and the incorporation of the mean stress effect into the total strain energy density dissipated per cycle. The performance of the criterion was demonstrated by experimental results from 6061-T6 aluminum dog-bone specimens subjected to axial stress at various stress ratios. The comparison shows very good agreement, thus validating the capability of producing accurate fatigue life predictions.Copyright

Material Property Determination of Vibration Fatigued DMLS and Cold-Rolled Nickel Alloys

An experimental procedure for qualifying material properties from cyclically worked parts was investigated in support of aging gas turbine engines and digital twin initiatives. For aging components, remanufacturing or repair efforts are necessary to sustain the life cycles of engines; and for digital twin, the virtual representation of a part requires accurate geometric and component material property measurement. Therefore, having an effective, non-destructive way to assess the material performance of parts is necessary. Since low cycle, low strain, mechanical testing is the ideal experimental approach for non-destructively assessing material properties, investigating the accuracy and trends of tensile properties of fatigue loaded parts was important. The fatigued parts used for this study were specimens tested according to the George Fatigue Method, and the materials observed were cold-rolled Inconel Alloys 625 and 718, and direct metal laser sintering (DMLS) Nickel Alloy 718. The tensile material properties were compared against pristine (non-fatigued) and published data. The comparison for the cold-rolled 625 and 718 results show an increase and a decrease, depending on rolling direction, of tensile strength due to the effects of fatigue cycles; however, the variation of the vibration affected tensile properties are all within one standard deviation of the pristine data. The comparisons of DMLS Nickel Alloys was conducted against two sets of alloys from different suppliers, and the results showed that the tensile properties are sensitive to DMLS manufacturing parameters and post-sintering processes. A digital twin related, nondestructive, material property determination technique is also discussed in this manuscript. The true alloy density was determined with the water displacement method, and elastic modulus is determined with an iterative Ritz method model. The modulus is under-predicted with this method, but suggestions for improving the model are discussed.Copyright

A Hysteretic Energy Method for Determining Damping Properties of Hard Coatings

The following manuscript investigates the use of dissipated and stored energies to extract mechanical damping properties from an axially loaded system. This method is observed for three reasons that will benefit the process for determining the damping capacity of hard coatings: 1) it is capable of determining damping properties at larger strain amplitude levels than conventional methods, 2) small strain ranges at stress ratios greater than -1 can be observed to accurately assess damping properties of strain-dependent materials, and 3) the method for extracting damping properties can be automated by using simplified energy calculations. Despite the pros of the hysteretic energy method, there are some limitations that may make its use impractical. One of the major limitations was explored in this study: inconsistency of the hysteretic energy measurement. The hysteretic energy results were observed for several Aluminum 6061-T6 specimens, where a large sample size was averaged to determine if there was consistency in the mean quality factors (a damping parameter) and the relative standard deviations of the sampled energies. Though the results of this study showed consistency in both observed parameters, the resulting values of the quality factors were too low to assess the storage moduli values seen in typical hard coatings, which are in the range of 20-200GPa. The experimental results of this study brings forth the need to address another limiting factor of the hysteretic energy method. In other words, the mitigation of frictional energy losses at the boundary conditions will be explored in future work.

An Energy Based Critical Fatigue Life Prediction Method

The capability of a critical life, energy-based fat igue prediction method is analyzed in this study. The t heory behind the prediction method states that the strain energy accumulated during monotonic fracture and fatigue are equal. T herefore, a precise understanding of the strain energy density behavior in each failure process is necessary. The initial und erstanding of energy behavior shows that the accumulated strain e nergy density during monotonic fracture is the area under neath the experimental stress-strain curve, whereas the sum o f the constant area within every stress-strain hysteresis loop of the cyclic loading process is the total strain energy d ensity accumulated during fatigue; meaning, fatigue life i s determined by dividing monotonic strain energy density by the strain energy density in one cycle. Further observ ation of the energy trend during fatigue shows that strain energ y density per cycle is not constant throughout the process as initially assumed. This finding led to the incorporation of a critical life effect into the energy-based fatigue prediction met hod. The analysis of the method’s capability was conducted o n Al 6061T6 ASTM standard specimens. The results of the analysis provide further improvement to the characterization of strain energy density for both monotonic fracture and fati gue; thus improving the capability of the energy-based fatigu e life prediction method.