Materials Week '97: Tuesday PM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Tuesday afternoon, September 16.

HIGH CYCLE OF FATIGUE OF STRUCTURAL MATERIALS: Session II: High Cycle Fatigue I

Session Chairs: Prof. Robert Ritchie, Univ. of California-Berkeley, Dept. of Materials Science & Eng., 282 Hearst Mining Bldg., Berkeley, CA 94720; Dr. Ted Nicholas, USAF Wright Laboratory, Wright-Patterson AFB, Dayton, OH 45433

ISSUES IN HCF OF MATERIALS IN GAS TURBINE ENGINES: Ted Nicholas, USAF Wright Laboratory, Wright-Patterson AFB, Dayton, OH 45433

HCF failures in materials used in rotating components of gas turbine engines have often been found to be attributable to fatigue loading on materials which have sustained damage from other sources. Damage can be present from initial material or manufacturing defects, or can develop during service operation. IN-service damage, while not catastrophic by itself, can degrade the HCF resistance of the material so that the fatigue runout stress, plotted on a Goodman diagram, is reduced. In studying failures in military and civil engines, three major sources of in-service damage have been identified which can alter the HCF resistance individually or in conjunction with one another: low cycle fatigue (LCF), foreign object damage and fretting. Experiments are being conducted to evaluate the type and intensity of damage which causes a reduction in the HCF behavior of a common compressor blade titanium alloy, Ti-6Al-4V. Some recent results on the effects of LCF on the Goodman diagram are presented.

1:25 pm INVITED

A 2.5 kHz LOADING STAGE FOR THE SEM AND DESCRIPTION OF EXPERIMENTS USING IT TO STUDY HCF PHENOMENA: D.L. Davidson, Southwest Research Institute, P.O. Box 28510, San Antonio, TX 78228-0510

A loading stage for the SEM that cycles at a resonant frequency of about 2.5 kHz and that is capable of high levels of meanload will be described. This system was constructed to allow the surfaces of specimens to be observed under high resolution conditions during simulated operating conditions of gas turbine blades at ambient temperature in the compressor section. Fatigue cracks have been started from notches machined by section. Fatigue cracks have been started from notches machined by focused ion beams and under fretting fatigue conditions in Ti-6Al-4V. The initiation and growth of cracks will be described and some results of measurements of crack tip micromechanics parameters are given.

1:50 pm INVITED

ENSURING DAMAGE TOLERANCE OF TITANIUM ALLOYS UNDER LOADING SPECTRA CONTAINING HIGH CYCLE FATIGUE: J.M. Larsen, B.D. Worth, J.R. Jira, D.C. Maxwell, Materials Directorate, Wright Laboratory (WL/MLLN), Wright-Patterson AFB, OH 45433; The University of Dayton Research Institute, Dayton, OH 45419-0128

Damage tolerance methods are widely used to assure reliability of fracture critical components in advanced turbine engines. Although low cycle fatigue often controls component lifetimes, high cycle fatigue of airfoils and airfoil attachment regions is a growing concern. An overview is presented of the role of damage tolerance methods available for life management of titanium alloy turbine engine components that experience loading spectra containing low- and high-cycle fatigue. While crack growth rate behavior may be a dominant factor affecting component lifetime under low cycle fatigue, the threshold crack growth condition is crucial under high cycle fatigue conditions. Key factors controlling fatigue crack growth thresholds in titanium alloys used in turbine engines are discussed, including effects of alloy composition, microstructure, realistic loading spectra and crack size. In addition, the influence of realistic statistical variation in properties of current materials is examined in light of damage tolerance requirements.

2:15 pm INVITED

MICROMECHANISMS OF FATIGUE IN STRUCTURAL AEROENGINE ALLOYS: S. Dubey, V. Sinha, M. Foster, C. Mercer, D. DeLuca*, W.O. Soboyejo, Dept. of Materials Science and Engineering, The Ohio State University, 2041 College Rd., Columbus, OH 43210-1179; *Pratt & Whitney, government Engines and Space Propulsion, P.O. Box 109600, West Palm Beach, FL 33410-9600

The micromechanisms of fatigue crack initiation and crack growth in nickel and titanium base alloys are discussed in this paper. Following an initial review of the existing applications of aerospace alloys, the effects of microstructure, thickness and stress ratio on the fatigue crack growth behavior of Ti-6Al-4V are discussed. The micromechanisms of fatigue crack nucleation and growth will be identified in Ti-6Al-4V with equiaxed and Widmanstatten microstructural morphologies. The trends in the fatigue crack growth rate data will be rationalized using crack closure concepts. The micromechanisms of cyclic crack-tip deformation in single crystal and polycrystalline ingot metallurgy nickel base superalloys will then be discussed within the context of recent evidence of crack-tip deformation obtained from crack-tip TEM analysis. The implications of the results will be discussed for the future modeling of fatigue crack initiation and fatigue crack growth.

2:40 pm

EFFECTS OF MICROSTRUCTURE ON THE SUBSURFACE CRACK INITIATION OF Ti-6Al-4V ALLOYS: O. Umezawa, K. Nagai, National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305, Japan; H. Yokoyama, T. Suzuki, Dept. of Mechanical Engineering, Kogakuin University, Shinjyuku, Tokyo, Japan

The materials having various microstructures such as acicular- or equiaxed-type were obtained by heat-treatment or hot-working for Ti-6Al-4V alloys. The high-cycle fatigue test with an R ratio equal to 0.01 was done at 77 K. Subsurface crack initiation was detected in longer-life range such as over 10⁵ cycles for each material. The subsurface crack initiation site reveals a facet or facets which was identified as the phase. The size of subsurface crack initiation site was evaluated. The dependence of initiation site size on the maximum stress can be explained by a threshold condition assumption.

3:00 pm BREAK

3:15 pm INVITED

OVERLOAD EFFECTS IN FATIGUE CRACK PROPAGATION-A REVIEW: Arthur McEvily, University of Connecticut, 97 N. Eagleville Rd., Storrs, CT 06269

Paul Paris has made a number of major contributions to the field of fatigue crack growth. Among these are the use of the stress intensity factor as a correlating parameter for determining the rate of fatigue crack growth, the introduction of the Paris-Erdogan law, the study of the threshold behavior of fatigue cracks, the early recognition of the importance of Elber's discovery of crack closure, and the discovery (with Hermann) that two opening events could be detected following an overload. The present paper is chiefly concerned with this last topic, but in a sense each of the other topics also plays a role. A brief review of past work on overload effects will be given, and the present state of our understanding of the physical factors involved as well as quantitative analysis of these overload effects will be discussed.

3:40 pm INVITED

FATIGUE THRESHOLD MAPS OF PWA 1480 SUPERALLY SINGLE CRYSTAL IN AIR AND VACUUM AT ROOM TEMPERATURE: R.L. Holtz, Geo-Centers, Inc., 10903 Indian Head Highway, Suite 502, Ft. Washington, MD 20744; M.A. Imam, K. Sadananda, Materials Science & Technology Div., Naval Research Laboratory, Washington, DC 20375

The fatigue crack growth thresholds of PWA 1480 superalloy single crystal were measured in air and in vacuum at room temperature for stress ratios 0.1, 0.5 and 0.7. The DK versus K_max maps of the fatigue crack growth thresholds are correlated with microstructure features and dislocation densities near the crack-tip. The relative importance of crack closure is examined.

4:05 pm INVITED

THRESHOLD BEHAVIOR OF A NICKEL-BASE SUPERALLOY AT VERY HIGH FREQUENCY AND HIGH R-RATIO: W.W. Milligan, S.A. Padula, Dept. Of Metallurgical and Materials Engineering, Michigan Technological Univ., Houghton, MI 49931

Initial experiments on fatigue crack propagation and threshold behavior of a nickel-base turbine disk alloy at frequencies up to 1,000 Hz will be presented. Effects of R-ratio and frequency, as well as grain size, will be discussed. Experimental issues related to closed-loop servohydraulic testing of metals at 1,000 Hz, including the challenge of measuring crack length, will be included. We thank the Air Force of Scientific Research for sponsoring the work.

4:30 pm

HIGH-CYCLE FATIGUE CRACK INITIATION OF ULTIMET ALLOY: E.Y. Shen, P.K. Liaw, C.R. Brooks, Dept. Of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996; D.L. Klarstrom, Haynes International, Inc., Kokomo, IN 46904; J.Y. Huang, Institute of Nuclear Energy Research, Chlaan Village, Lungtan, Taiwan

ULTIMET alloy is a cobalt-base (approximately 55 wt. % Co, 26 wt. % Cr and 9 wt. % Ni) superalloy which has excellent wear and corrosion resistance. IN this paper, preliminary results of fatigue testing will be presented. Specifically, fatigue crack initiation in high-cycle, tension-tension loading at 25°C has been studied. By using different techniques (scanning electron microscopy, atomic force microscopy, scanning tunneling microscopy, laser extensometer, krak gage), the crack initiation behavior has been examined. The effect of microstructure on crack initiation is discussed.

4:50 pm

THE CRYSTALLOGRAPHY OF FATIGUE CRACK INITIATION IN TWO AUSTENITIC Fe-Ni SUPERALLOYS: C.R. Krenn, J.W. Morris, Jr., Center for Advanced Materials, Lawrence Berkeley National Laboratory and Dept. of Materials Science and Mineral Engineering, Univ. of California, Berkeley, CA 94720; Z. Mei, Hewlett Packard Company, 1501 Page Mill Road, Palo Alto, CA 94303

Fatigue crack initiation in the austenitic Fe-Ni superalloys Incoloy-908 and A-286 is examined using local crystallographic orientation measurements. Preliminary results are consistent with sharp transgranular initiation and propagation occurring almost exclusively on {111} planes in Incoloy-908 but on a variety of low index planes in A-286; ongoing research will be presented. Initiation in each alloy occurred both intergranularly and transgranularly and was often associated with blocky surface oxide and carbide inclusions. Taylor factor and resolved shear stress and strain crack initiation hypotheses were found to convincingly describe preferred crack initiation sites in either alloy. Subsurface inclusions are thought to play a significant role in crack initiation. This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.

5:15 pm INVITED

FATIGUE STRENGTH OF TiN AND TiCN CERAMIC COATED 1Cr-1Mo-0.25V STEEL: C.M. Suh, K.Y. Kim, S.G. Do, Department of Mechanical Engr., Kyungpook National University, Teagu City, South Korea

In order to clarify the effect of ceramic coating films on the fatigue strength, and crack propagation properties of material, fatigue tests (R=0.1, R=-1) were carried out in room air, using the round plain specimens and compact tension (CT) specimens of 1Cr-1Mo-0.25V steel coated with TiN and TiCN by physical vapor deposition (PVD). It was observed that the scatter band of fatigue life at low fatigue strengths was wider than that of fatigue life at high fatigue strengths. The obvious improvement of fatigue life was confirmed in TiCN coated specimens for the region of low fatigue strengths, as compared with uncoated and TiN coated specimens. It was explained that the increase of fatigue life in the TiCN coated material was attributed to the retardation of crack initiation due to the restriction of surface plastic deformation in the substrate with hard coating layer. Also, the fatigue strength of 107 cycles of ceramic coated material was increased about 15 - 21% higher than that of base material.

5:40 pm

ON A PHENOMENOLOGICAL MODEL OF FRETTING FATIGUE DESTRUCTION: J.Z. Shtilerman & V.I. Iogansen, Scientific Research Institute, St. Petersburg, Russia

We considered an element (sample or detail) subjected to cyclic deforming with presence of fretting. Our model is based on two main hypotheses: 1. The work in fixed size of the element during the cycle of deforming is a criterion of fretting fatigue destruction, but the work of normal and tangent surface forces must be taken into account together with the work of stress in material (i.e. elastic energy). 2.The fretting influence area depth depends essentially on the element's stress amplitude. The simple expressions of "equivalent" (with fretting) stress and fretting fatigue factor (which is similar to the reduced factor of stress concentration) have been obtained. The model constants' values of high-strength rotor steel have been estimated. Calculations based of this model have confirmed directly well-known experimental fretting fatigue limit dependencies of various parameters that characterize the element's material, the contact conditions, the absolute dimensions etc. Therefore, the adequacy of proposed model has been proven.