Sai V. Raj

Power-law and exponential creep in class M materials: discrepancies in experimental observations and implications for creep modeling

Abstract This paper discusses our current understanding of the processes thought to be dominant in the exponential creep regime as well as the implications for creep modeling relating to both power-law and exponential creep regions. The significance and implications of creep controlled by vacancy diffusion along dislocation cores are discussed. It is pointed out that creep substructures, other than subgrains, have been reported in the literature, and a bifurcation diagram is presented to demonstrate how this evolution can occur from an initially homogeneous dislocation substructure. The use of nonlinear dislocation dynamics in creep modeling is advocated to rationalize the observed diversity in the creep substructures. It is demonstrated that the dislocation substructure evolution models can be coupled with a viscoplastic model through the volume fractions of the ‘hard’ and ‘soft’ phases. This coupling is shown to lead to the stress-subgrain size relationship in a simple and a natural way.

A Phenomenological Description of Primary Creep in Class M Materials

Observations of deformation microstructures in the primary creep region in class M materials show a remarkable similarity with those formed in the exponential creep regime. As a result, it is proposed that the constitutive creep law for normal primary creep is similar to that for the exponential creep regime. A phenomenological description is discussed to rationalize these microstructural observations in terms of a normalized strain rate versus stress plot. The implications of this plot in describing different testing procedures, steady-state flow, and on the observed deviations from the universal creep law are discussed. The plot is also extended to explain the observed similarities in the transient creep behavior in pre-strained materials and in stress change experiments.

Analysis of Stainless Steel Sandwich Panels with a Metal Foam Core for Lightweight Fan Blade Design

The quest for cheap, low density and high performance materials in the design of aircraft and rotorcraft engine fan and propeller blades poses immense challenges to the materials and structural design engineers. Traditionally, these components have been fabricated using expensive materials such as light weight titanium alloys, polymeric composite materials and carbon-carbon composites. The present study investigates the use of a sandwich foam fan blade made up of solid face sheets and a metal foam core. The face sheets and the metal foam core material were an aerospace grade precipitation hardened 17-4 PH stainless steel with high strength and high toughness. The stiffness of the sandwich structure is increased by separating the two face sheets by a foam core. The resulting structure possesses a high stiffness while being lighter than a similar solid construction. Since the face sheets carry the applied bending loads, the sandwich architecture is a viable engineering concept. The material properties of 17-4 PH metal foam are reviewed briefly to describe the characteristics of the sandwich structure for a fan blade application. A vibration analysis for natural frequencies and a detailed stress analysis on the 17-4 PH sandwich foam blade design for different combinations of skin thickness and core volume are presented with a comparison to a solid titanium blade.

Evaluation of Ti-48Al-2Cr-2Nb Under Fretting Conditions

Kazuhisa Miyoshi, Bradley A. Lerch, Susan L. Draper, and Sai V. RajNational Aeronautics and Space AdministrationGlenn Research CenterCleveland, Ohio 44135SUMMARYAn investigation was conducted to examine the fretting behavior of 7-TiAI (Ti-48AI-2Cr-2Nb) in contact with anickel-base superalloy (Inconel 718) in air at temperatures from 23 to 550 °C. Fretting wear experiments were con-ducted with 9.4-mm-diameter hemispherical Inconel (IN) 718 pins in contact with Ti-48AI-2Cr-2Nb fiats (and thereverse) at loads from 1 to 40 N and fretting frequencies from 50 to 160 Hz with slip amplitudes from 50 to 200 gmfor 1 to 20 million fretting cycles. The results were similar for both combinations of pin and fiat. Reference frettingwear experiments were also conducted with 9.4-ram-diameter hemispherical Ti-6AI-4V pins in contact withIN718 flats.The interfacial adhesive bonds between Ti-48AI-2Cr-2Nb and IN718 in contact were generally stronger than thecohesive bonds in the cohesively weaker Ti-48AI-2Cr-2Nb. The failed Ti-48AI-2Cr-2Nb subsequently transferred tothe IN718 surface at any fretting condition. The wear scars produced on Ti-48AI-2Cr-2Nb contained metallic andoxide wear debris, scratches, plastically deformed asperities, cracks, and fracture pits. Oxide layers readily formedon the Ti-48AI-2Cr-2Nb surface at 550 °C, but cracks easily occurred in the oxide layers. Factors including frettingfrequency, temperature, slip amplitude, and load influenced the fretting behavior of Ti-48AI-2Cr-2Nb in contactwith IN718. The wear volume loss of Ti-48AI-2Cr-2Nb generally decreased with increasing fretting frequency. Theincreasing rate of oxidation at elevated temperatures up to 200 °C led to a drop in wear volume loss at 200 °C.However, the fretting wear increased as the temperature was increased from 200 to 550 °C. The highest tempera-tures of 450 and 550 °C resulted in oxide film disruption with generation of cracks, loose wear debris, and pits onthe Ti-48AI-2Cr-2Nb wear surface. The wear volume loss generally increased as the slip amplitude increased. Thewear volume loss also generally increased as the load increased. Increasing slip amplitude and increasing load bothtended to produce more metallic wear debris, causing severe abrasive wear in the contacting metals.1.0 INTRODUCTIONAdhesion, a manifestation of mechanical strength over an appreciable area, has many causes, including chemi-cal bonding, deformation, and the fracture processes involved in interface failure. A clean metal in contact with aclean metal will fail either in tension or in shear because some of the interfacial bonds are generally stronger thanthe cohesive bonds in the cohesively weaker metal (ref. 1). The failed metal subsequently transfers to the other con-tacting metal. Adhesion undoubtedly depends on the surface cleanliness, the area of real contact, the chemical,physical, and mechanical properties of the interface, and the modes of junction rupture. The environment influencesthe adhesion, deformation, and fracture behaviors of contacting materials in relative motion.Clean surfaces can be created by repeated sliding, making direct contact of the fresh, clean surfaces unavoidablein practical cases (ref. 2). This situation applies in some degree to contact sliding in air, where fresh surfaces arecontinuously produced on interacting surfaces in relative motion. Microscopically small surface-parallel relativemotion, which can be vibratory (in common fretting or false brinnelling) or creeping (in common fretting), producesfresh, clean interacting surfaces and causes junction (contact area) growth in the contact zone (refs. 3 to 5).Fretting wear produced between contacting elements is adhesive wear taking place in a nominally static contactunder normal load and repeated microscopic vibratory motion (refs. 6 to 10). The most damaging effect of frettingis the possibly significant reduction in fatigue capability of the fretted component even though the wear producedby fretting appears to be quite mild (ref. 10). It was reported that the reduction in fatigue strength by fretting ofTi-47AI-2Nb-2Mn with 0.8 vol.% TiB 2 was approximately 20 percent.Fretting fatigue is a complex problem of significant interest to aircraft engine manufacturers (refs. 11 to 14).Fretting failure can occur to a variety of engine components. Numerous approaches, depending on the componentand the operating conditions, have been taken to address the fretting problem. The components of interest in thisinvestigation were the fan and compressor blades. Many existing fan and compressor components have titaniumNASA/TM--2001-210902 i

Tensile creep fracture of polycrystalline near-stoichiometric NiAl

Abstract Tensile creep fracture behavior of polycrystalline near-stoichiometric NiAl has been studied between 700 and 1200 K under initial applied stresses varying between 10 and 200 MPa. The stress exponents for fracture varied between 5.0 and 10.7 while the activation energy for fracture was 250 ± 22 kJ mol –1 . The fracture life was inversely proportional to the secondary creep rate in accordance with the Monkman-Grant relation although there was extensive scatter in the data. This observation suggests that the fracture life for near-stoichiometric NiAl was influenced by creep under these stress and temperature conditions. Several different fracture morphologies were observed. Transgranular ductile cleavage fracture occurs at 700 K and at the higher stresses at 800 K. The fracture mode tr ansitions to transgranular creep fracture at 900 and 1000 K and at lower stresses at 800 K, while plastic rupture and grain boundary cavitation occur at 1100 and 1200 K. An experimental fracture mechanism map is constructed for near-stoichiometric NiAl.

A combined NDE/FEA approach to evaluate the structural response of a metal foam

Metal foams are expected to find use in structural applications where weight is of particular concern, such as space vehicles, rotorcraft blades, car bodies or portable electronic devices. The obvious structural application of metal foam is for light weight sandwich panels, made up of thin solid face sheets and a metallic foam core. The stiffness of the sandwich structure is increased by separating the two face sheets by a light weight metal foam core. The resulting high-stiffness structure is lighter than that constructed only out of the solid metal material. Since the face sheets carry the applied in-plane and bending loads, the sandwich architecture is a viable engineering concept. However, the metal foam core must resist transverse shear loads and compressive loads while remaining integral with the face sheets. Challenges relating to the fabrication and testing of these metal foam panels remain due to some mechanical properties falling short of their theoretical potential. Theoretical mechanical properties are based on an idealized foam microstructure and assumed cell geometry. But the actual testing is performed on as fabricated foam microstructure. Hence in this study, a detailed three dimensional foam structure is generated using series of 2D Computer Tomography (CT) scans. The series of the 2D images are assembled to construct a high precision solid model capturing all the fine details within the metal foam as detected by the CT scanning technique. Moreover, a finite element analysis is then performed on as fabricated metal foam microstructures, to calculate the foam mechanical properties with the idealized theory. The metal foam material is an aerospace grade precipitation hardened 17-4 PH stainless steel with high strength and high toughness. Tensile and compressive mechanical properties are deduced from the FEA model and compared with the theoretical values for three different foam densities. The combined NDE/FEA provided insight in the variability of the mechanical properties compared to idealized theory.