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 Wednesday morning, September 17.
Program Organizer: Prof. Wole Sobojeyo, The Ohio State University, Dept. of Materials Science and Engineering, Columbus, OH 43210
Room: 206
Session Chairs: Dr. George Yoder, Office of Naval Research, Washington, D.C.; Dr. Jim Larsen, Wright Laboratories, WPAFB, OH
ON THE GROWTH OF LARGE AND SMALL FATIGUE CRACKS IN DUCTILE AND BRITTLE MATERIALS: R.O. Ritchie, C.J. Gilbert, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720
The mechanisms of fatigue-crack propagation are examined with particular emphasis on the similarities and differences between cyclic crack growth in ductile materials, such as metals, and corresponding behavior in brittle materials, such as intermetallics and ceramics. This is achieved by considering the process of fatigue-crack growth as a mutual competition between intrinsic mechanisms of crack advance ahead of the crack tip (e.g., alternating crack tip blunting and resharpening), which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip (e.g., crack closure and bridging), which impede it. The widely differing nature of these mechanisms in ductile and brittle materials and their specific dependence upon the alternating and maximum driving forces (e.g., DK and Kmax) provide a useful distinction of the process of fatigue-crack propagation in different classes of materials; moreover, it provides a rationalization for the effect of such factors as crack size, load ratio and variable-amplitude loading.
8:25 am INVITED
EFFECT OF ADHESIVE LAYER-THICKNESS ON FATIGUE CRACK GROWTH ALONG POLYMER/METAL INTERFACE: J.K. Shang, Z. Xing, University of Illinois, Urbana, IL 61801
The effect of adhesive layer-thickness on fatigue crack growth along polymer-metal interface was examined by conducting fatigue experiments on the flexural peel specimens and by performing elastoplastic finite element analysis of the interfacial crack tip field. The crack growth rate along the polymer/metal interface was correlated with the total strain energy release rate, G, and obeyed the Paris law at the intermediate growth rates. The fatigue crack growth threshold was found to increase with adhesive thickness for small thicknesses but to decrease gradually with increasing adhesive thickness for large thicknesses. Elastoplastic analysis of the crack tip field and microscopic examination of the failure mechanism indicated that the thickness dependence of interfacial fatigue threshold could be explained by the variations in the crack tip loading condition and in the fracture mechanism with adhesive thickness.
8:50 am INVITED
EFFECT OF FATIGUE LOADING ON THE ADHESION AND PROGRESSIVE DELAMINATION OF POLYMER/METAL INTERFACES: R.H. Dauskardt, Dept. of Materials Science and Engr., Stanford University, Stanford, CA 94305
Bonding of metals using polymers has significantly increased in a wide range of modern applications including aerospace structures, microelectronic packages and bio-prosthetic components. The reliability of these structures are profoundly influenced by the interfacial fracture resistance (adhesion) and resistance to progressive debonding of resulting polymer/metal interfaces. In this study we examine such interfacial fracture properties of representative metal/polymer interfaces commonly found in microelectronic and biomedical applications. Specifically, interface fracture mechanics techniques are described to characterize adhesion and progressive debonding behavior under cyclic fatigue loading. Cyclic fatigue debond-growth rates were measured from ~ 107 to 10-4 mm/cycle and found to display a power-law dependence on the applied strain energy release rate range, DG. Fracture toughness test results show that the interfaces typically exhibit resistance-curve behavior, with a plateau interface fracture resistance, Gss, strongly dependent on the interface morphology and the thickness of the polymer layer. Micromechanisms controlling interfacial adhesion and progressive debonding are discussed in terms of the prevailing deformation mechanisms and related to interface structure and morphology.
9:15 am INVITED
PREDICTIVE METHODOLOGIES FOR ELEVATED TEMPERATURE HIGH CYCLE FATIGUE OF STRUCTURAL COMPOSITE MATERIALS: K. Reifsnider, Virginia Polytechnic Institute and State University, Blackburng, VA 24061
Structural composite materials are heterogeneous, generally anisotropic, and often brittle, even at elevated temperatures. However, they have the remarkable characteristic under high cycle fatigue loading of changing their stiffness and strength by significant fractions (one half is not uncommon) before fracture. This creates special challenges for the analyst, as well as for the designer of components made from such materials. Over the last fifteen years or so, several concepts have evolved that are now being widely used to approach problems of this type. The present paper will discuss the combination of those concepts that are embodied in the code series MRLife, used by a number of major industries to predict remaining strength and life of composite components under combined mechanical, thermal, and chemical long-term applied conditions. Some of those concepts include the "critical element" method, damage evolution integrals, and damage accumulation analysis. Comparisons of predicted remaining strength and life under combined fatigue, creep, and stress rupture conditions will be made with measured results for some of the applications to industrial components that have been completed to date.
9:40 am INVITED
HIGH CYCLE FATIGUE AND CRACK GROWTH OF INTERMETALLICS: N.S. Stoloff, Rensselaer Polytechnic Inst., Troy, NY 12180
The great interest in the mechanical behavior of ordered intermetallic compounds over the past three decades has focused upon monotonic strength and fracture properties. Consequently, the literature on high cycle fatigue of these compounds has been scattered and difficult to interpret. Recently, increasing attention has been paid to cyclic properties, especially under stress controlled crack growth conditions. In view of the close association between fatigue lives and growth rates under stress control conditions, this review will address both phenomena. Emphasis will be on recent experimental results for nickel, titanium, niobium and iron aluminides as well as on MoSi2 alloys. Particular attention will be directed towards microstructural and environmental effects on stress-controlled lives.
10:05 am BREAK
10:20 am INVITED
FATIGUE OF GAMMA TiAl BASED ALUMINIDES: P. Bowen, School of Metallurgy and Materials / IRC in Materials for High Performance Application, The Univ. of Birmingham, Edgbaston, Birmingham B15 2TT, UK
Where specific stiffness improvements outweigh their increased cost Gamma (TiAl) based titanium aluminides are poised for industrial applications in selected components for both automotive and aerospace sectors. Engineering concerns relate to their low ductility, modest fracture toughness and limited fatigue crack growth resistance in the presence of a sharp defect. The topics of total life and fatigue crack growth resistance will be addressed in detail in this presentation. The influence of test temperature, microstructure, lamellar colony size, and lamellar colony orientation on fatigue crack growth resistance and total life will all be highlighted. In addition, effects of environment (air versus vacuum) will be considered for a range of test temperatures and cyclic frequencies. Particular emphasis throughout the presentation will be placed on the fully lamellar and/or near (fully) lamellar microstructures. The importance of both aligned lamellar colonies and randomized lamellar colonies on fatigue crack growth resistance and total life will be addressed. In particular, the problem of premature interlamellar failure under cyclic loading, and its engineering significance, will be explored. The concept of stress-sampling volume correlation's will be considered to rationalize the total life of gamma based aluminides under cyclic loading observed for both plane sided and notched test pieces. Such correlation's will be essential and must be quantified if the performance of large scale components is to be predicted (and approved) on the basis of tests carried out on relatively small scale test pieces. The general challenges of designing against fatigue failure in such brittle materials will also be outlined.
10:45 am
FOREIGN OBJECT DAMAGE AND FATIGUE BEHAVIOR OF GAMMA TiAl: T. Harding, J.W. Jones, Univ. of Michigan, Ann Arbor, MI 48109; T.M. Pollock and P. Steif, Carnegie Mellon University, Pittsburgh, PA 15213
A study is underway to examine the relationship between foreign object damage (FOD) and the fatigue behavior of gamma titanium aluminide. Axial fatigue specimens fabricated from cast Ti-47.9 Al-2Cr-2Nb alloy were impacted under controlled conditions with various indentor shapes to simulate FOD. The damage was quantified and related to impact parameters. A measure of the ambient temperature fatigue strength in the damaged specimens was obtained by standard fatigue testing employing a step-loading approach. Fractographic studies were performed to differentiate impact damage from subsequent fatigue crack growth and to elucidate the mechanisms responsible for the dependence of fatigue strength on FOD. A threshold-based fracture mechanics analysis of crack advance from damage zones, and its use in fatigue life prediction, will be described.
11:05 am
HIGH-CYCLE FATIGUE CRACK GROWTH IN THE PARIS AND THRESHOLD REGIME AT ULTRASONIC FREQUENCIES: Stefanie Stanzl-Tschegg, University of Agriculture, Türkenschanzstraße
Measurement of fatigue limit, fatigue crack growth thresholds and life times under constant or varying amplitudes needs high numbers of cycles and thus long testing times. This means that time and costs can be saved, if the testing frequency is increased up to ultrasonic frequencies. Ultrasound fatigue testing equipments have achieved high technical standard during the last years, so that, besides scientific investigations, reliable results of such measurements for industrial application can be obtained. Improvements of the ultrasound fatigue technique have been obtained mainly in the control accuracy and the development of several new testing possibilities. Testing, for example is possible now at various positive and negative axial mean loads. Similarly, low frequency loads or static mode II or mode III loads may be superimposed to the axial ultrasonic fatigue load. Recently, also torsional loading at ultrasonic frequency has been developed. In addition, not only constant amplitude, but also multi-step or random loading sequences can be performed. The techniques of these developments, the results on material testing with the ultrasound technique as well as comparison with results obtained at conventional frequencies are reviewed in this paper.
11:25 am INVITED
FATIGUE BEHAVIOR OF CONTINUOUS FIBER-REINFORCED CERAMIC-MATRIX COMPOSITES (CFCCS) AT AMBIENT AND ELEVATED TEMPERATURES: P.K. Liaw, M. Miriyala, C.J. McHargue, Dept. of Materials Science and Engineering, The Univ. of Tennessee, Knoxville, TN 37996; L.L. Snead, Oak Ridge National Laboratory, Oak Ridge, TN 37831
Fatigue tests were performed at room temperature in air and at 1000C in an argon environment, on two continuous fiber-reinforced ceramic-matrix composites. Both composites were reinforced with Nicalon fiber-fabrics, with alumina and silicon carbide being the respective matrix materials. Using four-point bend specimens, loads were applied either parallel or normal to the laminate plies to study the effects of fabric orientation on the mechanical behavior. The fatigue behavior of the Nicalon/Alumina composite was significantly affected by the laminate orientation at room and elevated temperatures, while the effects were insignificant in the Nicalon/SiC composite. Creep was observed in both composites at The damage mechanisms responsible for the differences in the fatigue behavior of the two composites, with regard to fabric orientation and test temperature, will be elucidated. Some results of finite element analysis (FEA) also will be presented to explain the effects of fabric orientation on the flexural behavior of laminate composites. Research supported by DOE under a subcontract from Lockheed Martin Energy Research Corporation (No. 11X-SV483V), and by the National Science Foundation, under Contract No. EEC-9527527 with Mrs. Mary Poats as a contract monitor.
11:50 am INVITED
INFLUENCE OF DUCTILE PHASE REINFORCEMENT ON THE CYCLIC FATIGUE BEHAVIOR OF AN OXIDE DISPERSION STRENGTHENED COPPER ALLOY: T.S. Srivatsan, Dept. of Mechanical Engr., The Univ. of Akron, Akron, OH 44325; J.D. Troxell, OMG Americas (formerly SCM Metal Products Inc., Research Triangle Park, Raleigh, NC 27709)
A study has been made to understand the cyclic stress response characteristics, fatigue properties and fracture behavior of an oxide dispersion strengthened copper-niobium composite. The composite specimens were cyclically deformed over a range of stress amplitudes at both ambient and elevated temperatures. Under strain-amplitude control the composite specimens displayed combinations of hardening and softening to failure. The cyclic stress-strain characteristics, fatigue properties and fracture behavior of the composite will be compared with the unreinforced alloy and observed differences rationalized in light of the competing and mutually interactive influences of cyclic strain amplitude and resultant response stress, cyclic stress amplitude, intrinsic microstructural effects, matrix deformation characteristics and macroscopic aspects of fracture.
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