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A new energy-based life prediction framework for calculation of axial and bending fatigue results at various stress ratios has been developed. The purpose of the life prediction framework is to assess the behavior of materials used in gas turbine engines, such as Titanium 6Al-4V (Ti 6Al-4V) and Aluminum 6061-T6 (Al 6061-T6). The work conducted to develop this energy-based framework consists of the following entities: (1) a new life prediction criterion for axial and bending fatigue at various stress ratios for Al 6061-T6, (2) the use of the previously developed improved uniaxial energy-based method to acquire fatigue life prior to endurance limit region (Scott-Emuakpor et al., 2007, 'Development of an Improved High Cycle Fatigue Criterion,' ASME J. Eng. Gas Turbines Power, 129, pp. 162-169), (3) and the incorporation of a probabilistic energy-based fatigue life calculation scheme to the general uniaxial life criterion (the first entity of the framework), which is capable of constructing prediction intervals based on a specified percent confidence level. The precision of this work was verified by comparison between theoretical approximations and experimental results from recently acquired AI 606-T6 and Ti 6Al-4V data. The comparison shows very good agreement, thus validating the capability of the framework to produce accurate uniaxial fatigue life predictions for commonly used gas turbine engine materials. [DOI: 10.1115/1.2943152]

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. [DOI: 10.111511.2360599]

A new vibration-based multiaxial 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 topological design procedure, incorporating a finite element model, to characterize the shape of the specimens 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 yielding procedure for achieving various uniaxial stress ratios. The performance of the methodology is demonstrated by the experimental results from mild steel, 6061-T6 aluminum, and Ti-6Al-4V plate specimens subjected to fully reversed bending for both uniaxial and biaxial stress states. [DOI: 10.1115/1.1788687]

The repeatability of an additive manufacturing (AM) process that fabricates a unique specimen with inherent damping capability is explored. A laser powder bed fusion (LPBF) technique is used to additively manufacture nickel alloy components with the same geometry/dimensions. Key aspects to the manufacturing process such as scan strategy and build orientation are observed alongside damping performance. The results from forced-response testing show repeatable correlation between damping performance and modal response information. Furthermore, despite different scan strategies and build sequences, the results demonstrate that inherent damping repeatability is driven by the unique component geometry and has minimal sensitivity to manufacturing. [ABSTRACT FROM AUTHOR]

A new vibration-based fatigue testing methodology for assessing high-cycle turbine engine materials fatigue strength at various stress ratios is presented. The performance of the methodology is demonstrated with experimental results for mild steel, 6061-T6 aluminum, Ti-6Al-4V plate specimens subjected to a fully reversed bending, unaxial stress state.

Very high cycle fatigue (VHCF) data and experiments, 107-109 cycles to failure, have traditionally been both a cumbersome and costly task to perform. However, characterizing VHCF behavior of material systems is critical for the design and sustainability of turbine engines as outlined in the turbine engine structural integrity program (ENSIP). With recent advancements, ultrasonic fatigue (UF) test systems have become increasingly available to generate VHCF fatigue data. A primary consideration for ultrasonic fatigue testing is the frequency of loading, the resulting thermal evolution, and its effect on fatigue life. To mitigate the heat generation within the specimen during experiments, cooling air is directed to the specimen and cyclic loading is performed by selecting an appropriate test frequency or defining a duty cycle rather than continuously subjected to fatigue. However, standardization of experimental test procedures remains ongoing and continues to be developed. In this study, a Shimadzu USF-1000A ultrasonic fatigue test system is used to characterize VHCF behavior of Ti-6Al-4V to understand the effect of duty cycle and thermal evolution on fatigue life for ultrasonic fatigue testing. Titanium 6Al-4V test specimens are subjected to fully reversed axial fatigue at 20 kHz exciting resonance in an axial mode to better characterize the experimental process. Three duty cycle-cooling air configurations and their effect on fatigue life due to self-generated heat during the experiment are investigated. Heat generation is monitored in situ via a single-point optical pyrometer, and in situ mechanical and thermal data are collected and compared to standardized servohydraulic fatigue test data performed in this study as well as from data found in the literature. [ABSTRACT FROM AUTHOR]

Centrifugal compressors have a critical impact on the performance of a jet engine and can be made out of aluminum, steel, or titanium alloy. However, if made out of Ti-6Al-4V, it would be more lightweight than if steel were used and stronger than if aluminum were used. Compressors manufactured using traditional techniques can be relatively expensive to manufacture, therefore, fused deposition modeling (FDM) or fused filament fabrication (FFF) can help reduce the cost while maintaining structural integrity. FDM/FFF usually prints polylactic acid (PLA) or acrylonitrile butadiene styrene (ABS) layer-by-layer using an extruder with a spool of material feeding into the extruder. Recently, metal-polymer filaments have become commercially available using this same printing apparatus as common "hobby-class" FDM printers. Using FDM with a metal-polymer matrix allows for a lower cost of production because of the intricate designs it can accomplish with little to no machining involved. Even with the benefits of printing a spool of material using FDM, there has been very little research done of Ti-6Al-4V printed in a FDM format. This document records the feasibility of using Ti-6Al-4V for a centrifugal compressor using FDM by incorporating a three-dimensional structured blue light scanner before heat treatments while also analyzing manufacturing capabilities at the tips of blades as well as the base. This analysis is a first-stage to quantify FDM metal print characteristics and properties. [ABSTRACT FROM AUTHOR]

Vibratory bending fatigue behavior of titanium 6AI-4V plate specimens manufactured via direct metal laser sintering (DMLS), powder bed fusion additive manufacturing (AM), is assessed. Motivation for the work is based on unprecedented performance demands for sixth-generation gas turbine engine technology that requires complex, lightweight components. Due to cost, schedule, and feasibility limitations associated with conventional manufacturing, AM aims to address ubiquitous component concepts. Though AM has promise in the engine community, process controls necessary for consistent material properties remain an enigma. The following manuscript compares variability of DMLS fatigue and strength to cold-rolled data. Results show discrepancies between DMLS and cold-rolled for fatigue and microstructure characteristics. [ABSTRACT FROM AUTHOR]

The fatigue life behavior and internal surface conditions of inherently damped additive manufactured (AM) specimens subjected to vibration bending are under investigation. This study supports research that demonstrated 95% vibration suppression due to damping capability of AM components with 1-3% internal volume of unfused powder; however, the viability of fatigue performance is unknown. The following effort explores whether internal wall damage caused by unfused powder particle motion debits fatigue life. Comparisons between the fatigue behaviors of unfused powder pocket and fully fused nickel based alloy 718 specimens confirms a long suspected powder interaction phenomena critical to damping performance. [ABSTRACT FROM AUTHOR]

The aim of this paper is to provide a novel stochastic life prediction approach capable of predicting the total fatigue life of applied uniaxial stress states from a reduced dataset reliably and efficiently. A previously developed strain energy-based fatigue life prediction method is integrated with the stochastic state space approach for prediction of total cycles to failure. The approach under consideration for this study is the Monte Carlo method (MCM) where input is randomly generated to approximate the output of highly complex systems. The strain energy fatigue life prediction method is used to first approximate SN behavior from a set of two SN data points. This process is repeated with another unique set of SN data points to evaluate and approximate distribution of cycles to failure at a given stress amplitude. Uniform, normal, log-normal, and Weibull distributions are investigated. From the MCM, fatigue data are sampled from the approximated distribution and an SN curve is generated to predict high cycle fatigue (HCF) behavior from low cycle fatigue (LCF) data. [ABSTRACT FROM AUTHOR]

A modified experimental method using digital image correlation (DIC), a noncontact optical method for measuring full-field displacements and strains, is used to interrogate accumulated fatigue damage for low and high cycle fatigue at continuum scales. Previous energy-based fatigue life prediction methods have shown that cyclic strain energy dissipated during fatigue acts as a key damage parameter for accurate determination of total and remaining fatigue life. DIC enables the collection of accurate strain energy measurements or damaging energy of complex geometries that would otherwise be exceedingly difficult and time consuming using traditional strain measurement techniques. Thus, the use of DIC to obtain strain energy measurements of gas turbine engine (GTE) components is highly advantageous for energy-based fatigue life prediction methods. Presented in this study is the experimental characterization of the cyclic strain energy dissipation as a means of predicting fatigue performance and assessment of damage progression of Aluminum 6061 subjected to fully reversed axial fatigue loading utilizing DIC. Validation of total and cyclic strain energy dissipation DIC measurements is accomplished with the simultaneous use of axial extensometery for direct comparison and implementation to strain energy-based life prediction methods. [ABSTRACT FROM AUTHOR]

This paper describes the experimental framework to establish consistent, repeatable results associated with a traveling wave excitation system. The experiment is employed to ascertain the forced response and mistuning patterns for rotors. Several items attributing to existing experimental inconsistencies were identified and their effects were realized during this research. These experimental items were signal input locations, response measurement locations, and rotor alignment position. Accounting for these experimental variables, this paper develops a process which enables consistent rotor forced response results, independent of its orientation on the test stand. [ABSTRACT FROM AUTHOR]

An energy-based fatigue lifting method for the determination of the 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 lifing model, which states: the total strain energy dissipated during both a quasi-static process and a dynamic (fatigue) process is the same material property. The 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 dissipated during both a quasi-static process and a dynamic process at elevated temperatures; and (3) the incorporation of the effect of thermal loading into the axial fatigue lifing model. Both an axial IMF capability and a detailed testing procedure were created. The axial IMF capability was employed to produce full-life and critical-life predictions as functions of temperature, which were shown to have an excellent correlation with experimental fatigue data. For the highest operating temperature, the axial IMF full-life prediction was compared to lifing predictions made by both the universal slopes and the uniform material law prediction and was found to be more accurate at an elevated temperature. [ABSTRACT FROM AUTHOR]

The energy-based firing method is based on the theory that the cumulative energy in all hysteresis loops of a specimens' lifetime is equal to the energy in a monotonic tension test. Based on this theory, fatigue life can be calculated by dividing monotonic strain energy by a hysteresis energy model, which is a function of stress amplitude. Recent studies have focused on developing this method for a sine wave loading pattern--a variable strain rate. In order to remove the effects of a variable strain rate throughout the fatigue cycle, a constant strain rate triangle wave loading pattern was tested. The testing was conducted at various frequencies to evaluate the effects of multiple constant strain rates. Hysteresis loops created with sine wave loading and triangle loading were compared. The effects of variable and constant strain rate loading patterns on hysteresis loops throughout a specimens' fatigue life are examined. [ABSTRACT FROM AUTHOR]