Materials Week '97: Tuesday 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 Tuesday morning, September 16.

FATIGUE AND CREEP OF COMPOSITE MATERIALS: Session III

Session Chairs: Peter K. Liaw, Dept. of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200; D.B. Miracle, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433

CALCULATION OF THE SHEAR DECOHESION STRENGTH OF TI-MMC INTERFACE: D. Osborne, H. Ghonem, Mechanics of Materials Laboratory, Department of Mechanical Engineering, University of Rhode Island, Kingston, RI 02881

An experimental / numerical procedure has been carried out in order to calculate the shear decohesion strength of the fiber/matrix interface in a continuous fiber reinforced titanium metal matrix composite. Transverse loading experiments are used to determine the externally applied stress at which the interface decohesion under Mode I loading conditions occurs. The applied stress necessary to cause initiation of the Mode I interface decohesion was verified by measuring the displacement of the material near the interface during the loading process through a high magnification long range microscope. Using Finite Element Analysis, the corresponding local stress is calculated. This local stress is used to determine the minimum valid thickness for a pushout sample. Fiber pushout tests on samples which exceed this minimal thickness are then carried out in order to determine the applied load required to cause debonding under shear loading conditions. The stress state in a sample of this thickness is numerically calculated in order to identify the shear stress in the interface at the point when the interface begins to debond. This shear stress is taken in this study to be the shear decohesion strength of the fiber/matrix interface. This shear strength was found to decrease with increasing temperature due to the relaxation of residual stresses with increasing temperature. Results of this study were compared to the interface shear strength calculated from pushout tests based on the assumption of constant shear stress along the thickness of the sample.

9:00 am INVITED

ISOTHERMAL & THERMOMECHANICAL FATIGUE OF TITANIUM MATRIX COMPOSITE SUBJECT TO TRANSVERSE LOADING: A.H. Rosenberger, R. John*, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433-7817, *University of Dayton Research Institute, Dayton, OH 45469-0128

Unidirectional titanium matrix composites (TMCs) have a significant strength to weight benefit over monolithic materials when loaded in the fiber direction. The benefit of TMCs, however, is severely diminished when off-axis loading becomes appreciable such as the centrifugal loading in hoop wound rotating ring structures. This study examined the isothermal and thermomechanical fatigue (TMF) behavior of a transversely loaded unidirectional SCS-6/Ti-6Al-4V composite. Isothermal fatigue experiments were conducted at temperatures of 23, 371°C, and 427°C. Generally the isothermal fatigue life was short at applied stresses above the fiber debond stress and considerably longer at lower stresses. In-phase and out-of-phase TMF were conducted with a temperature range of 23 to 427°C. A simple model was used to conduct life predictions of these composites using fatigue properties of the matrix, taking into consideration the geometry of the transverse specimens, and temperature phasing of the TMF. Experimental results, life prediction modeling, and areas for model improvements are discussed.

9:20 am INVITED

LOW-CYCLE FATIGUE BEHAVIOR OF TRIMARC-1/Ti-6Al-2Sn-4Zr-2Mo: D.J. Buchanan^*, R. John^*, J.M. Larsen, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433-7817; ^*University of Dayton Research Institute, Dayton, OH 45469-0128

Continuous fiber reinforced titanium matrix composites (TMC) are planned as replacement materials for conventional steel components in elevated temperature aerospace applications. The results of an experimental and analytical investigation of isothermal low-cycle fatigue behavior of Ti-6Al-2Sn-4Zr-2Mo monolithic and [0]₁₀ metal matrix composite reinforced with silicon-carbide (Trimarc-1) fibers are presented. The fatigue tests were conducted at temperatures of 23, 163 and 371°C. The fatigue tests were load-controlled with stress ratios -1.3 and 0.1 for longitudinal tests and 0.1 for transverse tests. Fractographic studies are being conducted to document the influence of fiber spacing and fiber touches on the fatigue lives. This paper will also describe the application of a life fraction model based on micromechanics for longitudinal and transverse fatigue loading.

9:40 am INVITED

A COMPARISON OF DEFORMATION AND FATIGUE BEHAVIORS OF HIPPED FOIL AND SHEET TITANIUM ALLOYS: M.G. Castelli^*, B.A. Lerch, *NYMA, Inc., Brook Park, OH 44142, *NASA-LeRC, Cleveland, OH 44135

Micromechanics modeling of fiber reinforced MMCs requires a good and accurate knowledge of constituent properties. Much emphasis has been placed upon fabricating unreinforced matrix materials which have processing histories, and ideally mechanical properties, which are similar to the matrix materials in the foil/fiber/foil composites (in-situ matrix). This has required the processing of matrix plates which have been fabricated via foil consolidation rather than by more conventional wrought methods. However, the fabrication of such plates is complicated by the fact that the matrix material must be rolled into foils, a process which is not always easily accomplished. In addition, a consolidated foil plate, while giving comparable properties to those of the matrix in the composite (ideally), is expensive when compared to the standard wrought product. To alleviate these problems and yet maintain appropriately representative properties, a plate can be consolidated out of matrix sheets. These sheets do not require the extensive working required to produce the foils, and fewer sheets are needed to obtain the desired plate thickness. The question is, does the consolidated sheet material exhibit the same properties as that of the consolidated foils? This study investigates the mechanical properties of HIPped foil vs. HIPped sheet of Ti-matrix materials commonly employed in SiC reinforced Ti-composites. The Ti-materials investigated are: Ti-15-3, TIMETAL 21S, and Ti-6-4. Tensile, LCF, creep, and stress relaxation properties were examined and compared for both HIPped foil and sheet materials. Microstructural examinations were conducted to provide input to interpreting the macroscopic behaviors.

10:00 am BREAK

10:20 am INVITED

DEFORMATION AND RUPTURE MODEL OF [0] METAL MATRIX COMPOSITE UNDER SUSTAINED LOAD: N.E. Ashbaugh*, J. Metzcar*, A.H. Rosenberger, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433-7817; *University of Dayton Research Institute, Dayton, OH 45469-0128

A one-dimensional model for the deformation and rupture of a [0] titanium alloy matrix composite (TMC) has been developed for creep (sustained load) conditions. The model was based on the statistical strength distribution of the fibers and time dependent characterization of the matrix. Acoustic emissions from broken fibers have been monitored to assess the progression of fiber damage. The evaluation of the number and location of fiber breaks in interrupted test specimens has provided relevant information for assessment of the fiber/matrix load interactions and for correlation with acoustic emission events. Deformation and rupture predictions will be compared to experimental data from continuous fiber reinforced [0]8 SCS-6/Ti-6Al-4V tested at 427, 482 and 538°C and at various stress levels. Application of the model to forward predict the test results or to infer characteristic fiber strength distributions will be discussed.

10:40 am INVITED

CREEP OF SiC/Ti-1100 COMPOSITE IN AN AIR AND A VACUUM ENVIRONMENT: M.L. Gambone, A.H. Rosenberger, Wright Laboratory Materials Directorate, Wright-Patterson Air Force Base, OH 45433-7817

The titanium-alloy matrix composites in turbine engine components are required to withstand prolonged exposures at elevated stress and temperature; thus, understanding and predicting creep behavior is important. The near-alpha titanium alloy, Ti-1100, was developed for greater strength and creep resistance at temperatures as high as 593°C and has been proposed as a composite matrix to exploit these improved properties. This study examines the creep behavior of a unidirectionally-reinforced SiC fiber/Ti-1100 composite at 540°C in both laboratory air and vacuum. The dramatically reduced creep life of the composite tested in air compared to vacuum is attributed to environmentally assisted intergranular cracking in the Ti-1100 matrix. Modification of the matrix microstructure through heat treatments of the composite is also shown to increase creep life. The McLean/Curtin creep model is demonstrated to describe creep behavior of the composite in vacuum where matrix cracking does not occur.

11:00 am INVITED

ESTIMATIONS OF MMC TRANSVERSE RUPTURE AND RESIDUAL STRENGTH AFTER CREEP - A MICROMECHANICS APPROACH: E. Wung, Allison Engine Company, Rolls-Royce Aerospace Group, Indianapolis, IN 46206-0420

The high temperature creep behavior of continuous fiber reinforced SCS-6/Ti64 materials subjected to loading in the transverse direction is investigated with a micromechanics approach. The objective is to identify a suitable damage parameter which can be used to predict the rupture time or the residual strength if the material does not rupture during the hold time. Due to the weakly bonded fiber/matrix interface and the ductile nature of the Ti-6-4 material at elevated temperature, a large deformation theory approach is utilized in the stress analysis procedure which allows for the calculation of the true stresses and the logarithmic strains at the macro level as well as in the constituents. Owing to the evaluation of the true stress and strain quantities, the model prediction can accurately reproduce even the earlier stages of the traditional tertiary creep regime. A damage parameter based on the reduction of the cross-sectional area of the matrix material is proposed. This parameter is shown to correlate well with creep rupture and residual strength data. The incorporation of this damage parameter into an elaborated FEM computational scheme for the creep life prediction and the residual strength estimation of gas turbine engine components will be discussed.

11:20 am INVITED

TRANSVERSE CREEP OF SiC/Ti-6Al-4V FIBER-REINFORCED MMC's: D.B. Miracle¹, B.S. Majumdar², and S.G. Warrier²; ¹AF Wright Laboratory/ Materials Directorate, Dayton, OH; ²UES, Inc., 4401 Dayton-Xenia Rd., Dayton, OH 45432

The creep response of an 8-ply SiC (SCS-6)/Ti-6Al-4V composite has been measured at 427°C at several levels of stress. In addition to a conventional transverse creep sample geometry, two additional sample geometries were considered, with the intent of assessing the influence of the stress concentration at the sample free edge. These two geometries include a cruciform sample and a straight-sided sample with a layer of matrix material diffusion-bonded over the cut fiber ends. The transverse creep response of each sample geometry will be presented and discussed. Microstructural observations will be reported to describe the mechanisms of creep damage and failure. The contribution of the interfacial region to the transverse properties will be discussed.

11:40 am INVITED

DEGRADATION OF RESIDUAL STRENGTH IN SCS-6/Ti-15-3 DUE TO FULLY REVERSED FATIGUE: J.R. Calcaterra, S. Mall, Department of the Air Force, AFIT/ENY Building 640, 2950 P Street, Wright-Patterson AFB, OH 45433

Little attention has been given to residual strength degradation in Titanium Matrix Composites (TMCs) after cycling. To address this concern, fatigue tests on SCS-6/Ti-15-3 have recently been carried out at the Air Force Institute of Technology (AFIT). These tests were conducted in strain control at 427°C with R=-1. The main goal of the experiments was to determine their effect on the residual strength behavior of TMCs with fiber volume fractions (Vf) of 15, 25 and 42%. After test completion, fracture surfaces of each specimen were examined in a scanning electron microscope. Results indicate that fiber volume fraction seems to have an effect on both strain-controlled fatigue life and residual strength degradation. Lower fiber percentages result in material where the properties of the matrix, such as hardening or cracking, play a much larger role in the composite response. Despite these distinctions, all specimens tested retained the majority of their strength prior to failure.