Materials Week '97: Monday AM 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 morning, September 15.

FATIGUE AND CREEP OF COMPOSITE MATERIALS: Session I

Session Chairs: James M. Larsen, Wright Laboratory Materials Directorate, Wright-Patterson AFB, OH 45433; Paul Bowen, School of Metallurgy and Materials/IRC in Materials for High Performance Applications, The University of Birmingham, UK

MODELLING, CHARACTERISATION AND ASSESSMENT OF SELECTIVELY REINFORCED Ti MMCs: P. Bowen, N. Wang⁺, T.J.A. Doel, A.L. Dore and D.C. Cardona*, School of Metallurgy and Materials/IRC in Materials for High Performance Applications, The University of Birmingham, UK; ⁺British Steel plc., *Rolls-Royce plc

Selectively reinforced (clad) test pieces and model rings can show radically different failure modes compared with uniformly reinforced materials of similar volume fraction. This behaviour has been quantified and modelled as a function of cladding thickness and fibre volume fraction. Extensive transverse damage may often occur for fibre volume fractions of commercial interest at the first reinforced plane ahead of a mode I crack growing in the monolithic cladding layer. This damage may promote premature testpiece failure and prevents crack-tip shielding due to fibre bridging. Such observations have been made both for testpieces in bending and tension. The damage can be quantified by considering the magnitude and position of transverse stresses ahead of the growing crack-tip relative to those stresses that promote such damage in uniformly reinforced composites subjected to transverse loading. To characterise such modes of failure has necessitated extensive experimental and modelling studies. A dimensionless parameter has been developed in analytical form that predicts transverse stresses for a range of loading, testpiece geometry (cladding thickness and cladding: composite thickness ratios) and levels of selective reinforcement. When combined with experimental observations of damage initiation this parameter can define the propensity of a given geometry to transverse damage.

9:00 am INVITED

EFFECT OF IN-SITU MATERIAL PROPERTIES ON FATIGUE DAMAGE MODES IN TITANIUM MATRIX COMPOSITES: David Harmon¹, Kenneth L. Jerina², ¹McDonnell Douglas, St. Louis MO; ²Washington University, St. Louis MO

Titanium matrix composites (TMC) and their behavior under mechanical fatigue loads was the subject of this research. The primary objective was to explain fatigue damage modes in center notched TMC specimens. Two modes of damage have been observed in continuously reinforced, 0 degree unidirectional, SCS-6/Ti-15V-3Cr-3Al-3Sn (SCS-6/Ti-15-3) laminates. The fatigue specimens were destructively analyzed using optical microscopy to determine where cracks originated and how they grew throughout the specimen. A micromechanical model was developed to explain the fatigue crack patterns observed in the interface region surrounding the fibers of the woven and acrylic binder TMC material systems. A two dimensional model of a longitudinal lamina with a center hole was used to obtain a set of displacement boundary conditions for an element near the notch yet within the net section where the spiral crack patterns were observed. These boundary conditions were then used on a 3D unit cell model of the fiber, matrix, and interface.

9:20 am INVITED

ISOTHERMAL AND THERMO-MECHANICAL FATIGUE LIFE PREDICTION OF A TITANIUM MATRIX COMPOSITE, D.C. Slavik¹ and A.H. Rosenberger², ¹General Electric Aircraft Engines, Cincinnati, OH 45150, ²Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433

Higher costs associated with titanium matrix composites (TMCs) must be offset by significant advantages in material weight and/or performance prior to use. Life prediction models for TMCs are typically empirical and require a substantial testing program before the benefits of a new material can be fully assessed. Alternatively, models that rely on only the constituent stresses within the composite and available monolithic life prediction tools can be used for preliminary design without a substantial composite testing program. Fatigue life calculations using the local stresses and the constituent properties are evaluated and compared with composite fatigue data obtained for a continuous fiber SCS-6/Ti-6Al-4V system. Local stresses, calculated from the processing history and applied stress using a concentric cylinder model, are input into life prediction tools that include time-independent fatigue and time-dependent damage modes. These predictions are compared to experimentally measured fatigue lives as a function of test frequency (0.01 Hz to 10 Hz), temperature (23°C to 427°C), and thermo-mechanical temperature history (in-phase and out-of-phase). Life predictions, model limitations, and areas for model improvements are discussed.

9:40 am INVITED

A GOODMAN APPROACH FOR TITANIUM MATRIX COMPOSITES SUBJECTED TO HIGH CYCLE FATIGUE: D.P. Walls¹, T.E. Steyer², F.W. Zok², United Technologies, ¹Pratt & Whitney, West Palm Beach, FL; ²Materials Department, University of California, Santa Barbara, CA

A study has been conducted on high cycle fatigue in fiber reinforced titanium matrix composites. Under this type of loading, the composites generally exhibit the initiation and propagation of distributed matrix cracks and an associated stiffness loss and permanent residual strain. Additionally, the constitutive response of the composite exhibits stress/strain hysteresis. Continued cycling may eventually lead to fracture. Results of such observations for various load amplitudes and stress ratios are presented for a Ti-6Al-4V/SCS-6 system. A methodology for incorporating such information into a conventional Goodman diagram is proposed, as a technique to provide preliminary design guidelines.

10:00 am BREAK

10:20 am INVITED

DEGRADATION IN THE MECHANICAL PROPERTIES OF FIBER-REINFORCED TITANIUM MATRIX COMPOSITES UNDER FATIGUE LOADING, F.W. Zok, T.E. Steyer, S.J. Connell, Materials Department, University of California, Santa Barbara, CA; D.P Walls, United Technologies, Pratt & Whitney, West Palm Beach, FL

Fiber-reinforced titanium matrix composites (TMCs) exhibit a variety of damage processes under cyclic loading, including the formation and propagation of matrix cracks, bridging of the cracks by intact fibers, and debonding and sliding along the fiber-matrix interface. These processes are manifested in several ways, including the development of hysteresis in the stress-strain response, a loss in the elastic modulus, the development of permanent strain, and a reduction in fiber strength associated with fretting of the fiber surfaces. The present paper will address two aspects of this problem. The first involves the growth of a single dominant crack from a sharp notch. Experiments have been performed to directly measure both the interface sliding stress and the fiber strength within the bridged zones and the measurements used to simulate the fatigue crack growth characteristics and the fiber failure threshold. The second deals with the growth of multiple cracks in straight (unnotched) specimens and the effects of the cracks on the subsequent elastic and inelastic deformation characteristics. Micromechanical models for the behavior of bridged cracks of finite length will be presented and compared with the experimental data.

10:40 am INVITED

EFFECTS OF MATRIX MICROSTRUCTURE ON THE FATIGUE CRACK GROWTH RESISTANCE AND THE TOTAL LIFE OF SiC FIBRE REINFORCED Ti MMCS: P. McDonnell, S.V. Sweby, C. Barney, P. Bowen, School of Metallurgy and Materials/IRC in Materials for High Performance Applications, The University of Birmingham, UK

For the composite systems based on Ti-6Al-4V or IMI 834 matrix alloys, and reinforced with Sigma SM1140+ or SM1240 silicon carbide fibres, post processing heat-treatments have been carried out to promote increased matrix fatigue crack growth resistance. In this respect, near and fully matrix microstructures have been shown to improve crack arrest/catastrophic failure limits in the presence of an unbridged defect, and without any decrease in the total life (S-N) performance of such composites. However, because the transus temperature can be increased by approximately 50°C for the matrix present in the composite compared with that expected for the monolithic alloy alone, great care is required if these heat-treatments are not to result in a degradation of fibre strength. If the fibre strength is degraded, then severe reductions in both crack arrest/catastrophic failure limits and total life will result. Of interest, the severity of matrix-fibre interfacial reactions changes markedly for the different fibre-matrix systems considered, and hence different heat-treatment windows can be defined for specific fibre-matrix combinations. These observations will be quantified and discussed in detail and with particular reference to fibre fracture strength distributions and fibre-matrix interfacial strengths.

11:00 am INVITED

FIBER/MATRIX INTERFACIAL DEGRADATION IN SCS-6/TIMETAL^®21S COMPOSITE UNDER HIGH TEMPERATURE FATIGUE: J.L. Moran*, J. M. Larsen, J.R. Jira, M.L. Gambone, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433-7817; *Ball Aerospace & Technologies Corp., Fairborn, OH 45324

Fatigue crack growth in unidirectional titanium matrix composites containing the Textron carbon-coated SiC fiber, SCS-6, has been the subject of numerous investigations. It is now well known that bridging of Mode I matrix fatigue cracks by unbroken fibers has a major influence on crack growth rates at room temperature, particularly at low levels of applied stress. The relatively weak fiber/matrix interface in these materials promotes crack bridging by permitting interfacial sliding. At elevated temperatures, however, degradation of the fiber/matrix interfaces may be extensive, leading to major reductions in the contribution of crack bridging. The phenomenon of degradation of fiber/matrix interfaces in high-temperature fatigue crack growth was examined in tests of SCS-6/Timetal^®21S composite. Acoustic and optical microscopy were used to define the extent of interfacial degradation, and fiber push-out experiments were performed to quantify the effects of the degradation on fiber/matrix interfacial shear strength. Throughout the degraded region, the interfacial shear strength was reduced to near-zero values, while the shear strength beyond the degradation front was equivalent to that of the composite in its as-received condition. These findings are discussed with respect to material performance and life prediction under realistic usage conditions.

11:20 am INVITED

SURFACE-CRACK GROWTH IN A CONTINUOUSLY REINFORCED METAL MATRIX COMPOSITE: J.R. Jira, R. John, J.M. Larsen, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433; *University of Dayton Research Institute, Dayton, OH 45469

Turbine engine and aircraft components fabricated from continuous fiber reinforced titanium-alloy matrix composites (TMC) will experience cyclic loads during service and fatigue crack initiation and growth are expected to control component life. Hence, characterization of the fatigue crack growth behavior of a model TMC was initiated by the USAF under the Metal Matrix Composite Life Prediction Cooperative Program. The model TMC system chosen by the Cooperative was [0]24 SCS-6/Ti-6Al-4V. The results of the experimental investigation of propagation of part-through surface cracks in this material will be presented. Automated fatigue crack growth tests were conducted at a maximum stress level of 600 MPa at 23, 177 and 316°C with stress ratios of 0.1, 0.5 and 0.7. Fully bridged and partially bridged crack growth was observed. Crack extension was monitored using direct current electric potential and optical measurements. During some of the tests, the crack opening displacement profile was measured using the laser interferometric displacement gage system. Some of the tests were stopped prior to failure to determine crack shape. Results will be compared to through-crack behavior and implications for component design will be discussed.

11:40 am INVITED

FATIGUE CRACK GROWTH OF TI-MATRIX COMPOSITES WITH SPATIALLY VARIED INTERFACES: Benji Maruyama, Sunil Warrier*, NIST/Wright Laboratory, 2230 10th ST STE 1, Wright-Patterson Air Force Base, OH 45433; *Universal Energy Systems, 4401 Dayton-Xenia Rd, Dayton, OH 45432

Spatially Varied Interfaces is a design concept for composite synthesis whereby the interface mechanical response is tailored to the composite needs by varying the interface properties in patterns of weak and strong areas. In the SiC_f/Ti-alloy system, longitudinal fatigue crack growth experiments where longitudinal striped areas along the interface are systematically weakened, and the perturbation of the failure process is measured to gain a better understanding of the stress states and interface failure mechanisms.