COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 61(2001)1561-1570 ww.elsevier. com/locate/com a study of the damage progression from notches in an oxide/oxide ceramic-matrix composite using ultrasonic C-scans Victoria A. Kramb,*, Reji John, David A. Stubbs a University of Dayton Research te, 300 College Park, Dayton, OH 45469-0128, USA b Materials and Manufacturing Directorate(AFRL/MLLN), Air Force Research Laboratory, Wright-Patterson Air Force Base. OH45433-7817,USA Received 10 April 2000: received in revised form 13 March 2001; accepted 19 April 2001 Abstract The damage progression from notches during quasi-monotonic loading was investigated in an oxide/ oxide ceramic-matrix composite using ultrasonic C-scans. Test specimens were monotonically loaded, removed from the test machine, then ultrasonically C-scanned using a through transmission, reflector plate method. The level of ultrasonic attenuation was monitored as a function of applied stress and correlated with the damage observed within the composite. Results of the study showed that the ultrasonic technique successfully monitored the progressive matrix cracking prior to the peak load in specimens tested at 23C. Close to the peak load, fiber breakage ccurred near the notch tip, which was not indicated by the ultrasonic C-scans. At 950C, damage progressed from the notch as a single dominant crack. The extent of enhanced ultrasonic attenuation in the C-scans correlated well with the crack length from the notch. C 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ceramic composites; Damage progression; Non-destructive evaluation; Notched fracture; Oxide/oxide; Ultrasound 1. ntroduction because of the pre-existing porosity and matrix cracking which occurs during processing [4,5 of the Ceramic-matrix composites(CMC) consisting of an us condition of oxide/oxide oxide matrix and oxide fibers with no engineered fiber/ CMCs, previous studies have shown that there was no matrix interphase are currently under consideration for reduction in strength or mechanical behavior of oxide, high temperature aerospace applications due to their oxide CMCs after water exposure. Environmental stabi inherent resistance to oxidation. Oxide/oxide CMC pro- lity in the presence of water makes ultrasonic inspection duced with no fiber/matrix interphase utilize a weak, fri- methods a viable approach for examining damage pro able matrix which offers a low-energy path crack path gression in oxide/oxide CMCs. Damage progression in throughout the matrix [1-3]. In these CMC, nearly all the metal-matrix composites(MMCs) has been monitored load is supported by the fibers. The nearly linear stress- by using a through-transmission, ultrasonic imaging strain behavior exhibited by these composites in the [0/ technique referred to as reflector plate C-scanning 90 ]orientation is typical of fiber-dominated composites [10, 11. In this paper, adaptation of the ultrasonic [3-7]. In contrast, the notched fracture behavior is highly reflector plate C-scan technique to monitor damage non-linear as a consequence of stress redistribution progression from notches in oxide/oxide CMCs will be around the notch during loading [6-9]. Stress redistribu- described. The C-scan results will be correlated with tion and damage around notches have been observed in observed damage mechanisms from monotonically loaded CMC by the use of X-ray [7], thermoelastic emission [6,7] specimens and ultrasonic [8, 9 techniques. Many non-destructive inspection methods typically used for monitoring 2. Expermental procedure damage progression are ineffective in oxide/oxide Cmc The Nextel610/AS CMC used in this investigation - mail address: krambvia(@ flyernet dayton edu(v.A. Kramb) was produced by General Electric Aircraft Engines 0266-3538/01/ S.see front matter C 2001 Elsevier Science Ltd. All rights reserved. PII:S0266-3538(01)00051-3A study of the damage progression from notches in an oxide/oxide ceramic–matrix composite using ultrasonic C-scans Victoria A. Kramba,*, Reji Johnb, David A. Stubbsa a University of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0128, USA bMaterials and Manufacturing Directorate (AFRL/MLLN), Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433-7817, USA Received 10 April 2000; received in revised form 13 March 2001; accepted 19 April 2001 Abstract The damage progression from notches during quasi-monotonic loading was investigated in an oxide/oxide ceramic–matrix composite using ultrasonic C-scans. Test specimens were monotonically loaded, removed from the test machine, then ultrasonically C-scanned using a through transmission, reflector plate method. The level of ultrasonic attenuation was monitored as a function of applied stress and correlated with the damage observed within the composite. Results of the study showed that the ultrasonic technique successfully monitored the progressive matrix cracking prior to the peak load in specimens tested at 23
C. Close to the peak load, fiber breakage occurred near the notch tip, which was not indicated by the ultrasonic C-scans. At 950
C, damage progressed from the notch as a single dominant crack. The extent of enhanced ultrasonic attenuation in the C-scans correlated well with the crack length from the notch. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ceramic composites; Damage progression; Non-destructive evaluation; Notched fracture; Oxide/oxide; Ultrasound 1. Introduction Ceramic–matrix composites (CMC) consisting of an oxide matrix and oxide fibers with no engineered fiber/ matrix interphase are currently under consideration for high temperature aerospace applications due to their inherent resistance to oxidation. Oxide/oxide CMC pro￾duced with no fiber/matrix interphase utilize a weak, fri￾able matrix which offers a low-energy path crack path throughout the matrix [1–3]. In these CMC, nearly all the load is supported by the fibers. The nearly linear stress￾strain behavior exhibited by these composites in the [0
/ 90
] orientation is typical of fiber-dominated composites [3–7]. In contrast, the notched fracture behavior is highly non-linear as a consequence of stress redistribution around the notch during loading [6–9]. Stress redistribu￾tion and damage around notches have been observed in CMC by the use of X-ray [7], thermoelastic emission [6,7] and ultrasonic [8,9] techniques. Many non-destructive inspection methods typically used for monitoring damage progression are ineffective in oxide/oxide CMC because of the pre-existing porosity and matrix cracking which occurs during processing [4,5]. In spite of the highly cracked and porous condition of oxide/oxide CMCs, previous studies have shown that there was no reduction in strength or mechanical behavior of oxide/ oxide CMCs after water exposure. Environmental stabi￾lity in the presence of water makes ultrasonic inspection methods a viable approach for examining damage pro￾gression in oxide/oxide CMCs. Damage progression in metal-matrix composites (MMCs) has been monitored by using a through-transmission, ultrasonic imaging technique referred to as reflector plate C-scanning [10,11]. In this paper, adaptation of the ultrasonic reflector plate C-scan technique to monitor damage progression from notches in oxide/oxide CMCs will be described. The C-scan results will be correlated with observed damage mechanisms from monotonically loaded specimens. 2. Experimental procedure The Nextel610/AS CMC used in this investigation was produced by General Electric Aircraft Engines 0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(01)00051-3 Composites Science and Technology 61 (2001) 1561–1570 www.elsevier.com/locate/compscitech * Corresponding author. E-mail address: krambvia@flyernet.udayton.edu (V.A. Kramb)