AD-A262 403 i NAI PGfomApofd ,~~I , ,i,,. ,,I,,(cid:127), EN TATIO N PAGE 48e,, 0(cid:127), 1.AGNNLC U(Ee veblnk " RrOfDATE REPO-RT TYP E AND DATES COVERED ""__January 19932 Final Rer 12/1190-1I130192 4, TITLE AND SUBTITLE 5-- FUNDING NUMBERS Mechanics of Elevated Temperature Fatigue in Fiber-Reinforced Ceramics SDamage 6. AUTHOR(S) 6(cid:127).102F 2306,/BS John W. Holmes 7. PERFORMING ORGANIZATION NAME(S) AND ADORESS(ES) 3. PERFORMING ORGANIZATION REPORT NUMBER University of Michigan - 475 East Jefferson Street SAnn Arbor, MI 48109-1248 9. SPOUSO)IING/TAONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING /MONWTORING AGENCY REPORT NUMBER r c iAFOSR/NE 4Building 410, Boiling AFB DC AFOSR-91-0106 J20332-6448 PRO 11993 11. SUPPLEMENTAflY NOTES 12a. DISTRIBUTION!AVAILABILITY STATEMENT f 12b DISTRIBUTION CODE APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED. <(cid:127) 13. ABSTRACT (Ma):/mum 200 words) The focus of the research conducted under Grant No. 91-0106 (a two year effort) was to -identifying the fundamental mechanisms of fatgue damage that occur in fiber-reinforced ceramics. - Several new findings were made during the research effort: (1) the fatigue life of fiber-reinforced co ___ ceramics decreased markedly during high frequency fatigue loading, (2) fiber-reinforced ceramics - undergo significant internal heating during cyclic loading, (3) because of frictional wear along the =(cid:127) fiber-matrix interface, the frictional shear stress in fiber-reinforced ceramics decreases sharply under 0 cyclic loading. Based upon insight gained from the analytical and experimental parts of the - I (cid:127) investigation, we developed a novel approach to estimate the level of frictional shear stress that exists along the fiber-matrix interface during fatigue. Since this technique allows confirmation of other 0) techniques for estimating frictional shear stress (e.g., fiber pushout technique developed by Marshall at Rnckwell Science Center). Moreover, it is the only approach that allows determination of the in-situ - change in frictional shear stress during cyclic loading (note that the level of frictional shear stress controls many mechanical properties such as strength, toughness and mechanical damping as well as thermophysical properties such as thermal diffusivity). The analysis that was developed to estimate * frictional shear stress can also be used to understand the relationship between composite raicrostrucMre and cyclic energy dissipation in fiber-reinforced ceramics. .14. SUBJECT TERMS$ 15 NUMBSER OFPAE S16. PRICE CODE 4-17 SECURITY CLASSIFICATION 18 SFCURITY CLASSIICATION 1C. SECURITY CLASSIFPCATION 20" LIMITATION OF ABSTRACT OF REPORT WOT HIS PAGE OF ABSTRACT 4 UNCLASSIFIED UNCLASSIFIED UZCLASS! EDL_ _ iJ7S5%40 -0 280 S500 S n ym .' 9 .I 89 MECHANICS OF ELEVATED TEMPERATURE FATIGUE DAMAGE IN FIBER-REINFORCED CERAMICS AFOSR Grant No. 91-0106 Report for the funding period Dec. 90 to Dec. 92 Principal Investigator: John W. Holmes The University of Michigan Ceramics Composites Research Laboratory Department of Mechanical Engineering 2250 G.G. Brown Ann Arbor, MI 48109-2125 t ¶ Table of Contents 1. Overview of Research Results 1I. Dissemination of Research Results III. High Frequency Fatigue Life Degradation of a Ceramic Matrix Coniposite IV. A New Approach for Estimation of Frictional Shear Stress in Fiber-Reinforced Ceramics V. Influence of Fatigue Loading I listor, and Matrix Damage on Frictional I leating in Fiber- Reinforced Ceramics ICQTJALTj-y 7, Acssion PFo r NTIS GPA&i Just r:c._ By .... Dist L I. Overview of Research Results The focus of the research conducted under Grant No. 91-0106 (a two year effon) was to identifying the fundamental mechanisms of fatigue damage that occur in fiber-reinforced ceramics. Several new findings were made during the research effort: ( 1 ) the fatigue life of fiber-reinforced ceramics decreases markedly during high frequency fatigue loading, (2) fiber- reinforced ceramics undergo significant internal heating during cyclic loading, (3) because of frictional wear along the fiber-matrix interface, the frictional shear stress in fiber-reinforced ceramics decreases sharply under cyclic loading. Based upon insight gained from the analytical and experimental parts of the investigation, we developed a novel approach to estimate the level of frictional shear stress that exists along the fiber-matrix interface during fatigue. Since this technique allows confirmation of other techniques for estimating frictional shear stress (e.g., fiber pushout technique developed by Marshall at Rockwell Science Center). Moreover, it is the only approach that allows determination of the in-situ change in frictional shear stress during cyclic loading (note that the level of frictional shear stress controls many mechanical properties such as strength, toughness and mechanical damping as well as thermophysical properties such as thermal diffusivity). The analysis that was developed to estimate frictional shear stress can also be used to understand the relationship between composite microstructure and cyclic energy dissipation in fiber-reinforced ceramics. For convenience, this report is divided into three sections that provide a detailcd account of the research results. The first section discusses the influence of loading frequency on the room temperature fatigue life of Nicalon/CAS composites: these results were obtained after the 1991 progress report. The second and third sections discuss experimental results that show the magnitude and mechanisms of frictional heating in fiber-reinforced ceramics, and the approach that was developed to estimate the frictional shear stress that exists during fatigue loading of composites. 4 II. Dissemination of Research Results A list of the publications, conference presentations and invited talks that were the direct result of this AFOSR project are listed below. Journal Publications 1. C. Cho, J. W. Holmes and J. R. Barber, "Estimation of Interfacial Shear in Ceramic Composites from Frictional Heating Measurements," J. Am. Ceram. Soc., 74 111] 2802-2808 (1991). 2. J. W. Holmes and C. Cho, "Frictional Heating in a Unidirectional Fiber-Reinforced Ceramic Composite," J. Mat. Sci. Lett., 11 (1992) 41-44. 3. C. Cho, J. W. Holmes. and J. R. Barber, "Distribution of Matrix Cracks in a Uniaxial Composite." J. Am. Ceram. Soc., 75 [2] 316-24 (1992). 4 J. W. Holmes and C. Cho, "Experimental Observations of Frictional Heating in a Fiber Reinforcea Ceramic," J. Am. Ceram. Soc., 75 [4] 929-38 (1992). 5. L. Butkus*, J. W. Holmes and T. Nicholas, "Thermomechanical Fatigue of a SiC-Fiber Calcium Aluminosilicate Matrix Composite," J. Am. Ceram. Soc., in press. 6. S. F. Shuler, J. W. Holmes and D. Roach, "Room Temperature Fatigue of a C-Fiber SiC-Matrix Composite," J. Am. Ceram. Soc., accepted. 7. J. W. Holmes, X. Wu, and V. Ramakrishnan, "High-Frequency Fatigue of Fiber-Reinforced Ceramics," submitted to J. Am. Ceram. Soc. Conference Presentations 1. S. F. Shuler, J. W. Holmes and J. Morris, "Influence of Frequency on the Rate of Damage Accumulation in a C-Fiber SiC-Matrix Composite," presented at the 15th Annual Conference on Ceramics and Advanced Composites, Cocoa Beach, FL, (January, 1991). 2. C. Cho and J. W. Holmes, "Frictional Heating in Fiber-Reinforced Ceramic Matrix Composites." presented at the 15th Annual American Ceramic Society Conference, Cincinnati, OH (April 1990). 3. C. Cho and J. W. Holmes, "Influence of Fatigue Loading and Test Frequency on Interfacial Shear Stress in Fiber-Reinforced Composites," 16th Annual Conference and Exposition on Ceramics and Advanced Composites, Cocoa Beach, FL, (January, 1992). 5 4. V. Ramishkrishan, J. W. Holmes and M. Comninou, "High-Frequency Fatigue of Fiber-Reinforced Ceramics," presented at the 16th Annual Conference and Exposition on Ceramics and Advanced Composites, Cocoa Beach, FL, (January, 1992). 5. L. Butlkus*, J. W. Holmes and T Nicholas, "Thermomechanical Fatigue of a SiC-Fiber Calcium Aluminosilicate Matrix Composite," presented at the 16th Annual Conference and Exposition on Ceramics and Advanced Composites, Cocoa Beach, FL, (January, 1992). 6. J. W. Holmes, "Fatigue of Ceramic Matrix Composites Ceramics," 15th Automotive Materials Conference: Tough Ceramics, March 17, 1992, Ann Arbor, MI. Invited Talks 1. J. W. Holmes, "Fatigue of Fiber-Reinforced Ceramics," Detroit TMS meeting, February, 1991, 2. J. W. Holmes, "Role of the Fiber-Matrix Interface in Internal Heating of Fiber-Reinforced Ceramics." TMS Fall Meeting, Cincinnati, OH, October. 1991. 3. J. W. Holmes, "High Frequency Fatigue of Fiber-Reinforced Ceramics," TMS/FEMS Symposium on Fatigue of Advanced Materials, TMS Annual Meeting, Denver, CO, February 22-25, 1993. *Captain, USAF (Capt. Butkus obtained his MS Degree under the supervision of Professor Holmes. The experimental portion of his research was conducted at WPAFB). Dr. Ted Nicholas at WPAFB co- advised the thesis. The results obtained from Butkus' research are being published as a TechnicaL Report at WPAFB and are therefore not elaborated upon in this report. A copy of the TR will be sent to AFOSR). 6 Ill. High Frequency Fatigue Life Degradation of a Fiber- Reinforced Ceramic Matrix Composite III.1 Introduction To-date investigations of fatigue life and fatigue damage accumulation in fiber-reinforced ceramics have been conducted at loading frequencies below 50 Hz.1-8 However, recent studies9 13 have shown that significant internal heating can occur during the fatigue of fiber-reinforced ceramics, which may influence fatigue life. The temperature rise during high frequency fatigue can be very substantial. For example, a temperature rise of approximately 1(X) K has been measured during the fatigue of unidirectional Nicalon/calcium-aluminosilicate composites at a loading trequency of 75 Hz between stress limits of 240 and 10 MPa. 12 Moreover, it has been shown for Nicalon/calcium-aluminosilicate composites, that the frictional shear stress present along the fiber/matrix interface decreases as the fatigue loading frequency is increased (this decrease is caused by differential thermal expansion between the fiber and matrix). 14 It has been previously postulated that the change in frictional shear stress, as well as temperature-induced microstructural damage, could result in a frequency dependence of fatigue life.12 The purpose of the present communication is to report on recent findings that provide clear evidence for a frequency dependence of fatigue life in Nicalon/calcium-aluminosilicate composites. 111.2 Experimental Procedure Unidirectional, 16-ply Nicalori/calcium-aluiiiinosilicate composites (hereafter referred to as [01 -Nicalon/CAS-II) were used in the investigation. The composites, which contained 40 vol.% 16 fibers, were manufactured by hot-pressing at a temperature of approximately 1350'C. Edge-loaded tensile specimens were machined from the hot-pressed billets using diamond tooling. The fibers were parallel :o the specimen loading axis. The specimen gage-length was 33 rmm, with a gage- section width of 6.3 mm and thickness of 3.2 mm (further details of the specimen geometry can be found in Chapter VI). The broad faces of the specimens were not machined. To allow acetate-film replicas of surface cracking to be obtained, the edges (minor faces) of the specimens were polished 7 with diamond paste to a final finish of 0.1 ,tm. An optical micrograph showing the typical fiber distribution in the test specimens is given in Fig. 1. All fatigue experiments were conducted on a high resonant frequency servohydraulic load frame. The load-frame was equipped with a 6.8 kN actuator and edge-loaded grips. Because of the small force capacity of the actuator, monotonic tension tests were conducted on a higher- capacity load-frame equipped with a similar gripping arrangement. An isothermal test chamber, which surrounded the specimen and grips, was used to maintain the ambient temperature surrounding the specimens at 20 ±0.IC. The test chamber had a volume of 0.1 m3. The temperature rise of the specimens was continuously measured throughout the fatigue experiments. For this purpose, optical pyrometers focussed at the center of the gage section were utilized., The pyrometers had a spot size at the specimen surface of 5 mm. Further details concerning the experimental approach can be found in Reference 12. Specimens were subjected to continuous fatigue loading at sinusoidal frequencies of 25, 75, 150 and 350 Hz. All experiments were conducted under load control (i.e., the maximum and minimum load limits, rather than displacement, were controlled). Specimens were fatigued at maximum stresses of 180, 200, 220 and 240 MPa; in all cases the minimum cyclic stress was held constant at 10 MPa. In practice. because of minor control problems, the lower fatigue limit fluctuated between 5 and 12 MPa. The upper fatigue limit was controlled to within 5 MPa. For the stress limits examined, the stress ratio ((Ymin/amax) ranged from approximately 0.055 at 180 MPa to 0.041 at 220 MPa. Fatigue run out was defined as 5 x i06 cycles (this corresponds to approximately 55.5 h of testing at 25 Hz and 4 h of testing at 350 Hz). In addition to providing conventional stress versus cycles to failure data (i.e., an "SN" curve), this series of experiments provided information concerning the dependence of frictional heating on peak fatigue stress and loading frequency. Information concerning the evolution of matrix crack spacing was determined for only tw o," "flEvercst Intcrscience, Fullerton California. Pyrometcr Model No. 5402 (for temperatures< 100'0C); Pyromctcr Modcl No. 5430 (for temperatures > I(WOC). THie two pyromcters were focusscd on opposing sides of tihe specilmen. 8 the fatigue loading histories: 25 and 75 Hl z under a maximum stress of 180%M Pa (since the process of obtaining replicas interferes with the measurement of temperature rise, which was one of the primary goals of the present study, crack spacings were not obtained for higher loading frequencies or stress levels). To avoid time-dependent matrix cracking during the taking of surface replicas, which can significantly influence the subsequent cycle dependence of crack spacing and modulus, all replicas were obtained with specimens loaded to a stress of 10l MPa. For comparison, the crack spacing was also determined for static loading at a maximum stress of 180 MPa (for this loading history the specimen stress was maintained at 180 MPa while obtaining the replicas). The values reported for matrix crack spacing were obtained from measurements made on a minimum of 500 crack-pairs (in many instances the cracks did not traverse across the entire specimen, but rather appeared to start and stop in a random fashion . 111.3 Results and Discussion (1) Monotonic Tensile Behavior The room-temperature monotonic tensile behavior of the composite is shown in Fig. 2. The composite exhibited a linear stress-strain response to a stress of approximately 285 MPa (defined as the proportional limit stress), followed by progressive nonlinear behavior. The failure strength of the composite was approximately 590 MPa. The initial tangent modulus was 131 GPa. (2) Influence of Loading Frequency on Fatigue Life The fatigue life of the composite is shown in Fig. 3 as a function of maximum fatigue stress and loading frequency. At a maximum fatigue stress of 180 MPa, fatigue run out was observed at all loading frequencies. Although there was scatter in the data at 150 and 350 lHz, it is clear for stresses above 180 MPa that the fatigue life decreased as the loading frequency was increased. For example, at 220 MPa the fatigue life was less than 300,000 cycles at frequencies of 150 and 350 Hz; this compares with over 5 x 106 cycles at 25 and 75 Hz. If time dependent (delayed crack growth) were the governing failure mechanism, the failure time would be independent of loading frequency. Plotting the failure time of specimens for the various loading histories, shows that this is clearly not the case: for stress levels above 200 MPa the Laiiure time decreased with increasing 9 loading frequency. It is very important to note that the fatigue experiments were conducted at stress levels ( 180 to 240 MPa) that were significantly below the monotonic proportional limit stress of virgin specimens (= 285 MPa). From the results of numerous investigations 1-7 it has been generally accepted that the proportional limit stress provides a rough approximation of the fatigue limit for many unidirectional fiber-reinforced ceramics fatigue (where the fatigue limit is defined as the stress level below which fatigue failures do not occur). All of these earlier investigations were conducted at loading frequencies of 10 Hz and lower (the 50 Hz fatigue .xperiments conducted by Shuler et a18 were performed with Cf/SiC composites which do not exhibit a proportional limit stress because of processing related microcracking). However, the results obtained from the present investigation clearly suggest that the proportional limit stress cannot be used to predict the fatigue limit of fiber- reinforced ceramics at high loading frequencies. These results do not invalidate the use of the proportional limit stress as a predictor of the fatigue limit at lower loading frequencies (< 10 Hz), since the damage mechanisms at low loading frequencies will be different (i.e., will not be significantly influenced by internal heating). (2) Frictional Heating During Fatigue Loading As discussed in detail elsewhere 10-12, the repeated frictional slip of fibers along debonded interfacial slip-zones causes heat generation during the fatigue loading of fiber-reinforced ceramics. The frequency dependence of temperature rise is plotted in Fig. 4 for a maximum fatigue stress of 220 MPa. Figure 5 shows the influence of peak fatigue stress on temperature rise for a loading frequency of 350 Hz. As both of these figures indicate, the surface temperature rise can be quite substantial. For example, the temperature rise exceeded 160 K during 350 1lz fatigue at a peak stress of 240 MPa. As discussed below, frictional heating is thought to play a significant role in the reduced fatigue life found at high loading frequencies. (3) Mechanisms Responsible for the Frequency Dependence of Fatigue Life There is clear evidence from the present investigation that the rate and extent of nicrostnictural