Andrew H. Rosenberger

Effect of vacuum level on the embrittlement of timetal®21S

The objective of this research was to experimentally determine the influence of the vacuum level or P(O{sub 2}) on the embrittlement of Timetal{reg_sign}21S. Tests were conducted at 650 C and 760 C in a variable total pressure vacuum chamber and gettered, recirculated helium environment which is amenable to thermal mechanical fatigue testing. Qualitative measurements of oxygen diffusion depth were obtained through optical and scanning electron fractography, Knoop hardness profiles, and {beta}-Ti to {alpha}-Ti phase transformations.

Understanding Materials Uncertainty for Prognosis of Advanced Turbine Engine Materials

Abstract : Materials damage prognosis offers the opportunity to revolutionize life management of advanced materials and structures through a combination of improved state awareness, physically based predictive models of damage and failure, and autonomic reasoning. Historically, lifetime and reliability limits for advanced fracture-critical turbine engine materials have been based on expected worst-case total life under fatigue. Recent findings in a variety of advanced propulsion alloys indicate that the life-limiting mechanisms are typically dominated by the growth of damage that begins at the scale of key microstructural features. Such behavior provides new avenues for management and reduction of uncertainty in prognosis capability under conditions that depend on damage tolerance. To examine a range of sources of uncertainty in behavior and models of such behavior, this paper explores the following topics: (1) Duality in Fatigue, (2) Relaxation of Surface Residual Stresses in Laboratory Specimens, (3) Relaxation of Bulk Residual Stresses in Components, (4) Nonlinear Acoustic Parameter for the Detection of Precursor Fatigue Damage, (5) Elevated Temperature Fretting Fatigue, (6) Crack Growth under Spin Pit Environments, and (7) Crack Growth Under Variable Amplitude High Cycle Fatigue (HCF) Loading. Based on the findings, we outline avenues for further technology development, maturation, validation, and transition of mechanistically based models that have the potential to reduce predictive uncertainty for current and future materials.

Emerging Methods for Matching Simulation and Experimental Scales

Structures are on the scale of meters, yet the material deforms on the scale of the microstructure– micrometers or smaller. This chapter examines the experimental methods that are emerging at the different length scales that are important tools in building models for location-specific design. Current design practice is discussed to provide a baseline understanding of today’s design methodology. Many of the design decisions are based on the stress or constitutive response of the structure to loading. This has been sufficient, but the idea behind location-specific design is that the material at each location can be tailored to the design requirements. Location-specific design will request a variation of material at each location, wherein each material will have a distinct constitutive response and specific damage accumulation. While special experimental techniques are needed to probe the material at finer scales to assess the local behaviors, testing methods at all scales are discussed to demonstrate the breadth of experimental capability available at each scale of the material. The chapter concludes with the thoughts on the experimental capabilities and material understanding that are still elusive and need to be developed.