Materials Week '97: Monday 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 Monday afternoon, September 15.

FATIGUE AND CREEP OF COMPOSITE MATERIALS: Session II

Session Chairs: David P. Walls, United Technologies, Pratt & Whitney, West Palm Beach, FL A.H. Rosenberger, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433

STRESS RATIO AND TEMPERATURE EFFECTS ON FATIGUE CRACK GROWTH BEHAVIOR OF SCS-6/Ti-6Al-4V: R. John^*, J.R. Jira, J.M. Larsen, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433-7817; ^*University of Dayton Research Institute, Dayton, OH 45469-0128

Titanium alloy matrix composites (TMC) are targeted for use in many aerospace applications. Critical turbine engine and aircraft components fabricated from continuous fiber reinforced TMC will experience cyclic loads during service, and many of these components typically contain crack initiators. Hence, characterization of the fatigue crack growth behavior of [0]₈ SCS-6/Ti-6Al-4V was initiated by the USAF Wright Laboratory under the MMC Life Prediction Cooperative Program. The results of the experimental and analytical investigation of fatigue crack propagation in SCS-6/Ti-6Al-4V material will be presented. Automated fatigue crack growth tests were conducted using middle tension, M(T) specimens at 23, 177 and 316C with stress ratios of 0.1, 0.5 and 0.7. During some of the tests, the crack opening displacement profile was measured to verify the stress distributions predicted by the fiber bridging models. This presentation will also discuss the capability of the shear lag models to predict the crack growth life of SCS-6/Ti-6Al-4V.

2:30 pm INVITED

ELASTIC SHIELDING AT BI-MATERIAL INTERFACES DURING FATIGUE CRACK GROWTH OF TITANIUM MATRIX COMPOSITES: S.G. Warrier¹, B.S. Majumdar¹, D.B. Miracle, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433-7817; ¹UES, Inc., Dayton, OH 45432

A "weak" fiber-matrix interface can act as a debonding layer and facilitate fiber bridging, whereas a "strong" interface can assist in the transfer of load to the fibers and retard the crack as the crack approaches a fiber. The latter phenomenon, elastic shielding which is caused by a mismatch in the modulus of the two materials, has not received much attention in the past. In this study, controlled experiments were carried out using single-ply composites with several carefully selected interfaces possessing different normal and shear strengths. Results show that elastic shielding is dependent on the mechanical properties of the interface; the strongest interface offering the largest extent of crack retardation. The crack shielding mechanism was further examined using finite element analysis. Results of numerical simulation and experimental results provide a detailed understanding of the influence of interface properties on elastic shielding and crack growth rates in titanium matrix composites.

2:50 pm INVITED

EFFECTS OF SiC FIBRE VOLUME FRACTION ON FATIGUE CRACK GROWTH RESISTANCE IN UNIFORMLY REINFORCED Ti MMCS: A.L. Dore, H. Whitlow, X. Wu, P. Bowen, School of Metallurgy and Materials/IRC in Materials for High Performance Applications, The University of Birmingham, UK

Effects of fibre volume fraction on the fatigue crack growth resistance of Ti MMCs have been assessed for volume fractions in the range from 10 to 40%. Mode I bridged cracks result from unbridged defects growing perpendicular to the fibre axis, and the influence of fibre volume fraction on crack growth resistance can be quantified and predicted. Acoustic emission techniques and in-situ observations have been used to establish the critical role of local fibre fracture in controlling the crack growth resistance of such composites. Attention will also be given to the resistance to crack growth parallel to the fibre axis as a function of fibre volume fraction. For a fibre volume fraction of 10% the role of the partial debonding of individual fibre-matrix interfaces can be distinguished, and cracks deviate towards such regions of debonding. At a fibre volume fraction of 35% several areas of fibre matrix debonding are observed to occur simultaneously ahead of a growing fatigue crack and the rapid linkage of such damaged regions produces a marked acceleration of crack growth. Clearly, as the fibre volume fraction increases, for such transverse growth the resistance of the composite to cyclic loading now decreases sharply. Within the overall paper, the compromise required to obtain adequate crack growth resistance as a function of fibre volume fraction in biaxial stress fields will be outlined.

3:10 pm INVITED

FIBER DAMAGE MECHANISMS IN TITANIUM METAL MATRIX COMPOSITES: M.N. Tamin, H. Ghonem, Mechanics of Materials Laboratory, Department of Mechanical Engineering, University of Rhode Island, Kingston, RI 02881

This work examines the fatigue damage mechanisms of SiC fibers at different temperatures. Results show that static and fatigue strengths of SCS-6 fibers are unaffected by the test temperature of 650°C and below. In addition, the onset of fiber fracture is instantaneous. Temperature influences the fracture process of these fibers through the density of cracks in the outermost carbon-rich coating layer. The composite damage in terms of crack distribution along the fiber is examined using the finite element method. In support of these calculations, Ti-MMC specimens were fatigued to the same number of cycles as employed in the fiber residual strength tests at the respective temperature. Results indicate that the mechanism of crack initiation in the coating/interphase region of a composite and in the carbon-rich coating of a SCS-6 fiber specimen are similar. In addition, the interphase zone does not influence the strength of the fiber in the temperature range of up to 650°C.

3:30 pm BREAK

3:50 pm INVITED

EVALUATION OF THE MMC LIFE 3.0 CODE IN PREDICTING CRACK GROWTH IN TITANIUM ALUMINIDE COMPOSITES: David Harmon¹, Alonso Peralta², James A. Hall², James M. Larsen³, ¹McDonnell Douglas, St. Louis MO, ²Allied Signal Engines, Phoenix, AZ; ³Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433

Crack growth and fatigue life predictions made with the MMCLIFE 3.0 code are compared to test data for unidirectional continuously reinforced SCS-6/Ti-24Al-11Nb (SCS-6/Ti-24-11) laminates. The MMCLIFE 3.0 analysis package is a design tool capable of predicting strength and fatigue in metal matrix composite laminates. The code uses a combination of micromechanic lamina and macromechanic laminate analyses to predict stresses and uses linear elastic fracture mechanics for crack growth. The crack growth analysis now includes a fiber bridging model to predict matrix flaws in 0 degree laminates and is capable of predicting the effects of interfacial shear stress and thermal residual stresses. The code has also been modified to include edge notch flaws in addition to center notch flaws. The model was correlated with constant amplitude, isothermal data from crack growth tests conducted on 0 degree and 90 degree SCS-6/Ti-24-11 laminates. Verification tests were conducted which included dwell times and frequency effects. Strengths and areas for improvement for the analysis are discussed.

4:10 pm INVITED

PREDICTIONS OF TOTAL LIFE IN SiC FIBRE REINFORCED Ti MMCS: J.G. Pursell, J. Liu, W. Ding, D.C. Cardona* and P. Bowen, School of Metallurgy and Materials/IRC in Materials for High Performance Applications, The University of Birmingham, UK; *Rolls-Royce plc.

In fibre reinforced regions of envisaged components the possibility of minor defects and damage occurring cannot be ignored in any lifting assessment. Such damage could plausibly arise from a local fibre failure and/or defects at the ends of fibres which may be impossible to eliminate fully in some envisaged components. This paper will address primarily the growth of such defects under cyclic loading in a local environment where crack bridging can occur. The crack bridging analyses and models have been extended to simulate and incorporate the influence of fibre failure and fibre strength distribution changes on the integrated crack life of such composites. Model predictions of integrated fatigue life will be compared with S-N curves obtained experimentally: for testpieces containing damaged surface fibres; for testpieces containing totally embedded fibres; and for testpieces containing fibre ends. Typical values of initial defect sizes required to match model predictions with S-N curves obtained experimentally will be shown to be in good agreement with the sizes of defects that are expected to occur for composites reinforced with such large diameter (100-140 mm) fibres. The paper will thus attempt to summarize the progress made to date in specifying integrated lives for such composites and will consider if further dramatic improvements in integrated lives are likely to occur in the near future for the composites currently under development.

4:30 pm INVITED

FIBER-MATRIX INTERFACE SLIDING IN CONTINUOUS FIBER SCS-6/Ti MMC: E.A. DeBartolo, B.M. Hillberry, Purdue University, West Lafayette, IN; G.T. Ward, Allison Engine Company

In modeling the fatigue and fracture of continuous fiber metal matrix composites, the behavior of the metal matrix interface has been elusive. Debonding at the interface plays an important role in the degree of fiber bridging that affects the crack growth rate. The interface shear stress in the debond region has been assumed to be constant, vary along the length or used as a fitting parameter in fatigue crack growth models. In this study the debond region is investigated using a fatigue loading stage inside the chamber of an Environmental Scanning Electron Microscope (ESEM). The SCS6/Timetal®21S unidirectional test specimens are masked and a 4 to 7 mm section of the matrix dissolved through the thickness leaving the four rows of fibers exposed and intact. The area in the dissolved region where the fiber enters the matrix is clearly visible in the ESEM and the fiber pull-out can be observed and readily measured. In the paper, load and distance that the fiber slips on the first cycle and periodically during fatigue cycling will be presented. The change in fiber pull-out during cycling, fiber surface roughness, and presence of wear particles will provide insight to the fiber-matrix behavior.

4:50 pm

CREEP BEHAVIOR OF POWDER METALLURGY SiC-Al COMPOSITES AND THEIR Al MATRICES: Farghalli A. Mohamed, Materials Science and Engineering, Dept. of Chemical and Biochemical Engineering, University of California, Irvine, CA 92697

The effect of stress and temperature on the creep behavior of silicon carbide particulate reinforced Al alloys, produced by powder metallurgy, has been studied over several orders of magnitude of strain rate. The experimental data of the composites are examined in reference to those of their Al matrices that were creep tested under similar experimental conditions.