R. Craig McClung

A Probabilistically-Based Damage Tolerance Analysis Computer Program for Hard Alpha Anomalies in Titanium Rotors

A probabilistically-based damage tolerance analysis computer program for engine rotors has been developed under Federal Aviation Administration (FAA) funding to augment the traditional safe-life approach. The computer program, in its current form, is designed to quantify the risk of rotor failure due to fatigue cracks initiated at hard alpha anomalies in titanium. The software, DARWIN (Design Assessment of Reliability With Inspection), integrates a graphical user interface, finite element stress analysis results, fracture-mechanics-based life assessment for low-cycle fatigue, material anomaly data, probability of anomaly detection, and inspection schedules to determine the probability-of-fracture of a rotor disk as a function of operating cycles with and without inspections. The program also indicates the relative likelihood of failure of the disk regions. Work is underway to enhance the software to handle anomalies in cast/wrought and powder nickel disks, and manufacturing and maintenance-induced surface anomalies in all disk materials. *Funded under FAA Grant 95-G-04

Application of Probabilistic Fracture Mechanics to Prognosis of Aircraft Engine Components

It is generally accepted that traditional logistics functions including periodic nondestructive inspections and planned maintenance increase the reliability and readiness of turbine engines. Nevertheless, further significant enhancements in reliability and readiness are believed to be possible through the implementation of a prognosis system based on online monitoring and interpretation of critical engine operating parameters and conditions to diagnose potential problems and forecast readiness. An approach is presented for improving probabilistic life prediction estimates through the application of prognosis methods. Actual F-16/F100 usage data from flight data recorders were interfaced with a probabilistic life prediction code to quantify the influence of usage on the probability of fracture of an idealized titanium compressor disk. For the example cases considered, it is shown that usage variability leads to about 6 × × variability in life and from 10 × × to 100 × variability in the probability of fracture. The results suggest that variability in usage could provide a basis for selectively extending the life of aircraft engines.

Fatigue crack growth mechanisms in HSLA-80 steels

Fatigue mechanisms of large and small cracks in Cu-bearing HSLA-80 steels were studied at ambient temperature. Fatigue striations were measured for both large and small fatigue cracks by SEM fractography, while dislocation structures adjacent to the fatigue surfaces were characterized by TEM. The results of the fatigue striation and dislocation structure characterization are compared with crack growth data to assess the fatigue mechanisms in HSLA-80 steels and the cause for the lack of a threshold in the small cracks. Comparisons revealed that both large and small cracks propagated via an intermittent growth mechanism. The growth kinetics of small cracks were consistent with the extrapolation of the power-law regime of the large crack data to stress ranges below the large-crack threshold. The absence of a growth threshold in small cracks was discussed in conjunction with the large-crack threshold.

Efficient Fracture Design for Complex Turbine Engine Components

Many high-energy turbine engine components are fracture critical. However, the complex three-dimensional (3D) geometries and stress fields associated with these components can make accurate fracture analysis impractical. This paper describes a new computational approach to efficient fracture design for complex turbine engine components. The approach employs a powerful 3D graphical user interface (GUI) for manipulation of geometry models and calculated component stresses to formulate simpler 2D fracture models. New weight function stress intensity factor solutions are derived to address stress gradients that vary in all directions on the fracture plane.

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

A Probabilistic Approach to Aircraft Turbine Rotor Material Design

A probabilistic design code is being developed for high energy disks and rotors used in aircraft gas turbine engines. This code is intended to augment, not replace, the current safe-life approach to the design of these critical components. While the code will ultimately be applicable to a range of disk alloys, initial emphasis has been placed on titanium alloys containing hard alpha defects. The approach involves developing an enhanced defect distribution for hard alpha, obtaining crack initiation data for hard alpha and fatigue crack growth data for three titanium alloys, and integrating this information into a software code that is sufficiently efficient that it can be routinely used for design and life prediction.Copyright

A Probabilistic Framework for Risk Prediction of Gas Turbine Engine Components With Inherent or Induced Material Anomalies

Rare anomalies may be introduced during the metallurgical or manufacturing processes that may lead to uncontained failures of aircraft gas turbine engines. The risk of fracture associated with these anomalies can be quantified using a probabilistic fracture mechanics approach. In this paper, a general probabilistic framework is presented for risk assessment of gas turbine engine components subjected to either inherent or induced material anomalies. A summary of efficient computational methods that are applicable to this problem is also provided.Copyright

Integration of Manufacturing Process Simulation with Probabilistic Damage Tolerance Analysis of Aircraft Engine Components

Interfaces between manufacturing process simulation software (DEFORM) and probabilistic damage tolerance analysis software (DARWIN) have been developed for bulk residual stresses and average grain size. These interfaces permit full-field results from manufacturing process simulations to be incorporated in predictions of fracture life and reliability. Approaches were presented for modeling the effects of location-specific bulk residual stress and average grain size on crack growth behavior. The interface and the proposed approaches were implemented in prototype software and used to perform demonstration examples for an idealized engine disk. The exercise demonstrates the practical potential for Integrated Computational Materials Engineering (ICME) that directly addresses component integrity.

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.Copyright

The Impact of Forging Residual Stress on Fatigue in Aluminum

Large aluminum forgings are seeing increased application in aerospace structures, particularly as an enabler for structural unitization. These applications, however, demand an improved understanding of the forging process induced bulk residual stresses and their impact on both design mechanical properties and structural performance. In recent years, significant advances in both computational and experimental methods have led to vastly improved characterization of residual stresses. As a result, new design approaches which require the extraction of residual stress effects from material property data and the formal inclusion of residual stresses in the design analysis, have been enabled. In particular, the impact of residual stresses on durability and damage tolerance can now be assessed, and more importantly, accounted for at the beginning of the design cycle. In an effort to support the development of this next-generation design capability, the AFRL sponsored Metals Affordability Initiative (MAI) consortium 1 has conducted a detailed experimental and analytical study of fatigue crack initiation and fatigue crack growth in aluminum coupons with known, quench induced residual stresses. In this study, coupons were designed and manufactured such that simple ‘design features,’ such as holes and machined pockets, were installed in locations with varying levels of bulk residual stress. The residual stresses at the critical locations in the coupons were measured using multiple techniques and modeled using detailed finite element analysis. Fatigue crack initiation (FCI) and fatigue crack growth (FCG) tests were performed using both constant amplitude and spectrum loading and the results were compared against computed FCI and FCG lives.