G. R. Leverant

Probabilistic methods for Design Assessment of Reliability with Inspection (DARWIN)

Conventional rotor life prediction methodologies are based on nominal conditions that do not account for material and manufacturing anomalies that can degrade the structural integrity of high energy rotors. In order to account for these anomalies, an industry committee recommended adoption of a probabilistic damage tolerance approach to supplement the current safe life methodology. The DARWIN computer program computes the probability-of-fracture as a function of flight cycles, considering random defect occurrence and location, random inspection schedules, and several other random variables. This study focuses on the probabilistic fatigue analysis methodology developed and implemented in DARWIN Version 3.2. The computational efficiency and accuracy of this computer program is illustrated for several realistic rotor models provided by aircraft engine manufacturers. It is shown that the life approximation function (LAF) and importance sampling (IS) methods significantly reduce computation time (nearly two orders of magnitude) compared to the Monte Carlo method.

A New Tool for Design and Certification of Aircraft Turbine Rotors

This paper summarizes recent enhancements to a probabilistic damage tolerance software code, DARWIN that can be used for design certification of aircraft jet engine titanium disks/rotors that may contain melt-related anomalies. Evaluations of DARWIN by engine manufacturers are also discussed, including comparisons with existing codes for accuracy and time efficiency In addition, relevant test results, including various fatigue tests on material containing melt-related anomalies, are summarized.

Fatigue crack growth in MAR-M200 single crystals

The effects of crystallographic orientation on the fatigue crack growth behavior of MAR-M200* single crystals were examined. Using compact-tension specimens tested at 20 Hz, fatigue crack growth rates were determined at ambient temperature at minimum stress to maximum stress ratios,R, of 0.1 and 0.5. In most cases, subcritical crack growth occurred either along a single {111} slip plane or a combination of {111} planes. The mode of cracking was generally mixed and contained mode I, II, and III components. Considerable crack deflection and branching were also observed. Some fracture surfaces were found to contain a significant amount of asperities and, in some specimens, black debris. Based on Auger spectroscopic analyses and the fracture surface appearance, it appears that the black debris represented oxides formed due to rubbing of the fracture surfaces. Using stress intensity solutions obtained based on the Boundary-Integral-Equation technique, an effective ΔK was successfully used for correlating the crack growth rate data. The results indicate that the effect of crystallographic orientation on crack growth rate can be explained on the basis of crack deflection, branching, and roughness-induced crack closure.

Fatigue crack propagation in Ni-base superalloy single crystals under multiaxial cyclic loads

The effects of crystallographic orientation and stress state on the multiaxial fatigue behavior of MAR-M200* single crystals were examined. Using notched tubular specimens subjected to combined tension/torsion cyclic loads, crack growth rates were determined at ambient temperature as functions of stress intensity range, the shear stress range-to-normal stress range ratio, and crystallographic orientation. Comparison of crack growth data at the same effective ΔK reveals a weak dependence of the crack growth rate on both the tube axis and the notch orientation. For a given set of tube axis and notch orientation, the crack growth rate might or might not vary with the applied stress state, depending on whether roughness-induced crack closure is present. In most cases, subcritical cracking occurs either along a single 111 slip plane or on ridges formed with two 111 slip planes. Neither fracture mode is altered by a change in the applied stress state. This complex crack growth behavior will be discussed in terms of the crack-tip stress field, slip morphology, and crack closure.

Elevated-temperature fatigue crack growth

The fatigue crack growth behavior of MAR-M200 single crystals was examined at 982 °C. Using tubular specimens, fatigue crack growth rates were determined as functions of crystallographic orientation and the stress state by varying the applied shear stress range-to-normal stress range ratio. Neither crystallographic orientation nor stress state was found to have a significant effect on crack growth rate when correlated with an effective ΔK which accounted for mixed-mode loading and elastic anisotropy. For both uniaxial and multiaxial fatigue, crack growth generally occurred normal to the principal stress direction and in a direction along which ΔKII vanished. Consequently, the effective ΔK was reduced to ΔKI and the rate of propagation was controlled by ΔKI only. The through-thickness fatigue cracks were generally noncrystallographic with fracture surfaces exhibiting striations in the [010], [011], and [111] crystals, but striation-covered ridges in the [211] specimen. These fracture modes are contrasted to crystallographic cracking along slip bands observed at ambient temperature. The difference in cracking behavior at 25 and 982 °C is explained on the basis of the propensity for homogeneous, multiple slip at the crack tip at 982 °C. The overall fracture mechanism is discussed in conjunction with Koss and Chan’s coplanar slip model.

Tensile and Fatigue Behavior of Aluminum Oxide Fiber Reinforced Magnesium Composites: Part I. Fiber Fraction and Orientation

The mechanical behavior of commercially pure magnesium reinforced with FP aluminum oxide fibers has been studied as a function of fiber fraction and orientation. Test specimens included material of two different volume fractions of fiber and four different fiber orientations. Axial properties were dependent on the fiber content and generally followed the rule of mixtures. Of the off-axis properties, only the elastic modulus exhibited a significant dependence on fiber content. Off-axis loading resulted in large reductions in both the tensile and fatigue properties. The reductions coincided with a change in fracture morphology from fracture across fibers during axial loading to fracture along the fiber direction for off-axis loading. A weak fiber/matrix interface was found to be responsible for the drop in tensile properties, and a combination of a weak matrix and a weak fiber/matrix interface were responsible for the reduced fatigue resistance.

Tensile and Fatigue Behavior of Aluminum Oxide Fiber Reinforced Magnesium Composites: Part II. Alloying Effects

The effect of matrix alloy additions on mechanical properties was examined by comparing the tensile and fatigue properties of commercially pure magnesium and ZE41A (Mg-4.25 Zn-0.5 Zr-1.25 RE) which were both reinforced with FP aluminum oxide fibers. The alloy additions were found to improve the off-axis properties but decrease the axial properties. This was brought about by an increase in the matrix and interfacial strengths and a decrease in the fiber strength. It was also determined that the reaction zone in both materials was MgO and that strengthening of the interface was due to an increased particle size and/or a thicker reaction zone and not to any segregation of alloying elements to the interface.

A Software Framework for Probabilistic Fatigue Life Assessment of Gas Turbine Engine Rotors

An enhanced life management process based on probabilistic damage tolerance methods has been developed to address material anomalies in titanium rotating components of gas turbine engines. Related methods are being used as tools to investigate the impact of engine monitoring and usage variability on prognosis for field readiness and life management. This paper begins with an overview of the process of probabilistic damage tolerant design, using the DARWIN® computer program to illustrate the interplay between various random variables and the conventional elements of structural design and life prediction. Special attention is then given to the initial distribution of material anomalies, scatter in fatigue crack growth data, and variability in complex mission histories. The significance of each source of variability for different applications is discussed.

Probabilistic Surface Damage Tolerance Assessment of Aircraft Turbine Rotors

This paper describes some of the new surface damage capabilities in DARWIN™, a probabilistic fracture mechanics software code developed to evaluate the risk of fracture associated with aircraft jet engine titanium rotors/disks. An initial framework is presented in which a graphical user interface (GUI) is used to explicitly define the stresses and temperatures at the crack location for several crack geometries. A summary of the approach used to develop new stress intensity factor solutions for these geometries is also presented, including selected validation results.Copyright

Coating Life Prediction for Combustion Turbine Blades

A life prediction method for combustion turbine blade coatings has been developed by modeling coating degradation mechanisms including oxidation, spallation, and aluminum loss due to inward diffusion. Using this model, the influence of cycle time on coating life is predicted for GTD-11 1 coated with an MCrAlY, PtAl, or aluminide coating. The results are used to construct a coating life diagram that depicts failure and safe regions for the coating in a log-log plot of number of startup cycles versus cycle time. The regime where failure by oxidation, spallation, and inward diffusion dominates is identified and delineated from that dominated by oxidation and inward diffusion only. A procedure for predicting the remaining life of a coating is developed. The utility of the coating life diagram for predicting the failure and useful life of MCrAlY, aluminide, or PtAl coatings on the GTD-11 substrate is illustrated and compared against experimental data.