January 2005 Matrix Cracking in 3D Orthogonal Mell-Infiltrated Composites X Table I. Properties for Three Types of 3D Architectures Y- and y-tow ends direction per cm Thickness ZMIT 2050.1740.1520.032 Syl-iBN 7.9 1950.2030.1600.001 (Atlanta GA) 1000 fibers per tow, average fiber diameter= 7 um-Property Data Sheet supplied by manufacturer. ICF Industries(New York, NY), 400 fibers fiber diameter= 12 um-Data supplied by Albany International Techniweave, Inc.NASA-modified Sylramie fiber-by heat treatment of the Sylramic fiber to produce a thin, -100 nm, in layer on the surface of the fiber. Dow Corning(Midland, MI) 800 fibers per tow, average diameter 10 m-Property Data Sheet supplied by manufacturer. For all three architectures, 3.95 epcm of a double tow was woven in the x-direction. As will be discussed, a key aspect of the Z-fiber types is their tow size in the final as-processed composite, which was largest for the ZMI fibers and smallest for the rayon fibers due to significant decomposition during composite fabrication. For convenience he three different types of 3D orthogonal composites are referred <PXPLY- X to in this paper by their particular Z-fiber type. T zes all the key properties of the 3D architectures Tensile tests were performed on specimens 12-mm wide by 150-mm long with a contoured gage section(dog-bone)of width 10 mm using a universal testing machine(Instron Model 8562, Instron Ltd, Canton, MA) with an electromechanical actuator. The 3D composites were tested with the y-direction oriented in the loading direction. Glass-fiber-reinforced epoxy tabs were ounted on both sides of the specimen in the grip regions and the specimens were gripped with rigidly mounted hydraulical actuated wedge grips. A clip on strain gage, with a range of 2.5% strain over 25 4-mm gage length was used to measure the deformation of the gage section Modal ae was monitored during the tensile tests with two ig. 1. Schematic representation of (a)3D orthogonal composite and ide-band, 50 kHz to 2.0 MHz, high-fidelity sensors placed just b)regions aligned in the Y-direction. Only nine layers are represented in outside the tapered region of the dog-bone specil b). the composites actually contained 15 layers(see Fig. I(a)) uum grease was used as a couplant and mechanical clips were used to mount the sensors to the specimen. The aE waveforms were recorded and digitized using a 4-channel, fracture wa detector(FWD) produced by Digital Wave Corporation(Engle- treated at NASa to convert the Sylramic fibers to the higher wood, CO). The load and strain were also recorded with the performance Sylramic-iBN SiC fiber. 4 The Sylramic-iBN or FWD. Location of the ae events was determined from the dif- Sylramic fibers in the preforms were then coated by CVI with ference in times of arrival of the first peak, the measured stress- BN interphase coatings and Sic matrices. Porosity between the dependent speed of sound, and the distance between the two CVI-coated"SiC/SiC mini-composite"tows was then filled by sensors as in Morscher. 12,13 However, the threshold technique slurry infiltration of Sic particles, followed by molten infiltra typically used for determination of the time of arrival was not tion of Si, commonly known as melt-infiltration, or MI.The used. Instead, the time of arrival of the first peak of the exten- composite panels were very dense, usually with less than 5% sional wave for each event waveform on the two sensors was porosity, most of which was in the form of inter-tow porosity determined by manual inspection in order to get a location ac- The 3D orthogonal panels(see Fig. I)had seven layers in the curacy of less than +0.25 mm. In this way, the AE activit Y-direction and eight layers in the Y-direction and were conse- within the UNI and XPLY regions of the 3D composites or quently not balanced in terms of fiber content. In addition, two ented in the y-direction( Fig. 1(b)could be distinguished. Sections from the the gage section of the tested tensile spe woven together in the X-direction at 3.95 tow-ends-per-cm nens at least 10 mm in length were polished and then plasma (epcm) for a fiber volume fraction of 15-18%. For the Y-direc-(CF4)etched at 500 W for 30 min. Etching was required to ob- tion, single tows were woven at either 7.I or 7.9 epcm for fiber serve transverse matrix cracks: however. the etchant reacts with ractions of 17-23%(see Table D). For the out-of-plane rein- the free Si in the matrix, removing much of it, making it im- forcement, very low fiber volume fractions(<3%)were used possible to observe the extension of matrix cracks through the MI part of the matrix. Matrix cracks can only be observed in the fiber types: II um ZMI SiC fibers from(800 fibers/tow ) 7 dense CVI SiC layer between the BN and the MI matrix T300 carbon fibers (1000 fibers/tow), and 12 um polymer- derived rayon fibers from ICF Industries(400 fiberstow ). The X- Y fibers of the rayon composite were not converted to Sylramic iBN fibers because the rayon fiber is subject to decomposition at The accuracy was dependent on the resolut the separation dis- the elevated temperatures required for Sylramic-iBN treatment ured during the test from AE that occurred outside the i.e., in the grips Interphase and fabrication was performed by General Electric Power Systems tapered section of the dog-bone. As the speed of sound decreased, the accuracy of event location increasedtreated at NASA to convert the Sylramic fibers to the higher￾performance Sylramic-iBN SiC fiber.14 The Sylramic-iBN or Sylramic fibers in the preforms were then coated by CVI with BN interphase coatings and SiC matrices. Porosity between the CVI-coated ‘‘SiC/SiC mini-composite’’ tows was then filled by slurry infiltration of SiC particles, followed by molten infiltra￾tion of Si, commonly known as melt-infiltration, or MI.z15 The composite panels were very dense, usually with less than 5% porosity, most of which was in the form of inter-tow porosity. The 3D orthogonal panels (see Fig. 1) had seven layers in the X-direction and eight layers in the Y-direction and were conse￾quently not balanced in terms of fiber content. In addition, two Sylramic SiC fiber tows (800 fibers per tow) were combined and woven together in the X-direction at 3.95 tow-ends-per-cm (epcm) for a fiber volume fraction of 15–18%. For the Y-direc￾tion, single tows were woven at either 7.1 or 7.9 epcm for fiber fractions of 17–23% (see Table I). For the out-of-plane rein￾forcement, very low fiber volume fractions (o3%) were used based on the single-tow weaving of three different Z-direction fiber types: 11 mm ZMI SiC fibers from (800 fibers/tow), 7 mm T300 carbon fibers (1000 fibers/tow), and 12 mm polymer￾derived rayon fibers from ICF Industries (400 fibers/tow). The X– Y fibers of the rayon composite were not converted to Sylramic￾iBN fibers because the rayon fiber is subject to decomposition at the elevated temperatures required for Sylramic-iBN treatment. As will be discussed, a key aspect of the Z-fiber types is their tow size in the final as-processed composite, which was largest for the ZMI fibers and smallest for the rayon fibers due to significant decomposition during composite fabrication. For convenience, the three different types of 3D orthogonal composites are referred to in this paper by their particular Z-fiber type. Table I summa￾rizes all the key properties of the 3D architectures. Tensile tests were performed on specimens 12-mm wide by 150-mm long with a contoured gage section (dog-bone) of width 10 mm using a universal testing machine (Instron Model 8562, Instron, Ltd., Canton, MA) with an electromechanical actuator. The 3D composites were tested with the Y-direction oriented in the loading direction. Glass-fiber-reinforced epoxy tabs were mounted on both sides of the specimen in the grip regions and the specimens were gripped with rigidly mounted hydraulically actuated wedge grips. A clip on strain gage, with a range of 2.5% strain over 25.4-mm gage length was used to measure the deformation of the gage section. Modal AE was monitored during the tensile tests with two wide-band, 50 kHz to 2.0 MHz, high-fidelity sensors placed just outside the tapered region of the dog-bone specimen.12,13 Vac￾uum grease was used as a couplant and mechanical clips were used to mount the sensors to the specimen. The AE waveforms were recorded and digitized using a 4-channel, fracture wave detector (FWD) produced by Digital Wave Corporation (Engle￾wood, CO). The load and strain were also recorded with the FWD. Location of the AE events was determined from the dif￾ference in times of arrival of the first peak, the measured stress￾dependent speed of sound, and the distance between the two sensors as in Morscher.12,13 However, the threshold technique typically used for determination of the time of arrival was not used. Instead, the time of arrival of the first peak of the exten￾sional wave for each event waveform on the two sensors was determined by manual inspection in order to get a location ac￾curacy of less than 70.25 mm.J In this way, the AE activity within the UNI and XPLY regions of the 3D composites ori￾ented in the Y-direction (Fig. 1(b)) could be distinguished. Sections from the the gage section of the tested tensile spec￾imens at least 10 mm in length were polished and then plasma (CF4) etched at 500 W for 30 min. Etching was required to ob￾serve transverse matrix cracks; however, the etchant reacts with the free Si in the matrix, removing much of it, making it im￾possible to observe the extension of matrix cracks through the MI part of the matrix. Matrix cracks can only be observed in the dense CVI SiC layer between the BN and the MI matrix. Fig. 1. Schematic representation of (a) 3D orthogonal composite and (b) regions aligned in the Y-direction. Only nine layers are represented in (b), the composites actually contained 15 layers (see Fig. 1(a)). Table I. Properties for Three Types of 3D Architectures Z-Fiber type X- and Y-direction fiber type Y-tow ends per cm (epcm)ww Thickness (mm) fy fx fz ZMIw Syl-iBNz 7.1 2.05 0.174 0.152 0.032 T300z Syl-iBN 7.9 1.75 0.226 0.178 0.014 Rayony SylramicJ 7.9 1.95 0.203 0.160 0.001 w Ube Industries (Tokyo, Japan), 800 fibers per tow, average fiber diameter 5 11 mm—Property Data Sheet supplied by manufacturer. z Amoco Performance Prod￾ucts, (Atlanta, GA), 1000 fibers per tow, average fiber diameter 5 7 mm—Property Data Sheet supplied by manufacturer. y ICF Industries (New York, NY), 400 fibers per tow, average fiber diameter 5 12 mm—Data supplied by Albany International Techniweave, Inc. z NASA-modified Sylramic fiber—by heat treatment of the Sylramic fiber to produce a thin, B100 nm, in situ BN layer on the surface of the fiber. J Dow Corning (Midland, MI); 800 fibers per tow, average diameter B10 mm—Property Data Sheet supplied by manufacturer. wwFor all three architectures, 3.95 epcm of a double tow was woven in the X-direction. z Interphase and matrix fabrication was performed by General Electric Power Systems Composites (Newark, DE). J The accuracy was dependent on the resolution in the time domain, the separation dis￾tance of the sensors, and the speed of sound across the material. For this study, those pa￾rameters were 0.1 microseconds, 50 mm, and B9000 m/s for a pristine composite, respectively. As matrix cracking occurs, the speed of sound decreases. The speed of sound was measured during the test from AE that occurred outside the sensors,12 i.e., in the grips or tapered section of the dog-bone. As the speed of sound decreased, the accuracy of event location increased. January 2005 Matrix Cracking in 3D Orthogonal Melt-Infiltrated Composites 147