Availableonlineatwww.sciencedirect.com BCIENCEODIRE。T Engineering Fracture Mechanics ELSEVIER Engineering Fracture Mechanics 71(2004)2589-2605 www.elsevier.com/locate/engfracm fracture behaviour of 2D-weaved. silica-silica continuous fibre-reinforced ceramic-matrix composites(CFCS) N. Eswara Prasad", Sweety Kumari, S.v. Kamat, M. Vijayakumar, G. Malakondaiah Defence Metallurgical Research laboratory, PO Kanchanbagh, Hyderabad 500058, India Received 18 February 2003: received in revised form 29 January 2004: accepted 24 February 2004 Abstract Significantly improved fracture resistance (in terms of fracture toughness and fracture energy) can be imparted to monolithic ceramics by adopting composite design methodology based on fibre reinforcement technology. The present paper describes the fracture behaviour of one such fibre- reinforced material, namely the silica-silica based continuous fibre-reinforced, ceramic-matrix composite(CFCC)in two orthogonal notch orientations of crack divider and crack arrester orientations. Different fracture resistance parameters have been evaluated to provide a quantitative treatment of the observed fracture behaviour. From this study, it has been concluded that the overall fracture resistance of the CFCC is best reflected by total fracture energy release rate Je), which parameter encompasses most of the fracture events/processes. The Je values of the composite are found to be more than an order of magnitude higher than the energy values corresponding to the plane strain fracture toughness (Ko, derived from Kle, the plane strain fracture toughness)and >200% higher than elastic-plastic fracture toughness (ie). Apart from this, the composite is found to exhibit high degree of anisotropy in the fracture resistance and also, a significant variation in the relative degree of shear component with crack extension. g 2004 Elsevier Ltd. All rights reserved Keywords: Continuous fibre 2D silica-silica composites: Fracture behaviour and modes of failure; Fracture resistance; Total fracture energy release rate; R-curve behaviour 1. Introduction Ceramic materials have assumed significant technological importance as structural materials because the newer design and development methodologies, adopting fibre reinforcements, have resulted in enhancement of the fracture resistance of monolithic ceramics by several fold [1-6]. Among various ceramic materi- als, amorphous silica uniquely combines different properties to suit several select technological applications Corresponding author.Tel:+91-40-24340051;fax:+91-40-24340683/4341439 E-mailaddresses:nep(@dmrl.ernet.in,neswarap@rediffmail.com(N.EswaraPrasad) 0013-7944/S. see front matter 2004 Elsevier Ltd. All rights reserved doi: 10. 1016/j-engfracmech 2004.02.005
Fracture behaviour of 2D-weaved, silica–silica continuous fibre-reinforced, ceramic–matrix composites (CFCCs) N.Eswara Prasad *, Sweety Kumari, S.V. Kamat, M.Vijayakumar, G.Malakondaiah Defence Metallurgical Research Laboratory, PO Kanchanbagh, Hyderabad 500058, India Received 18 February 2003; received in revised form 29 January 2004; accepted 24 February 2004 Abstract Significantly improved fracture resistance (in terms of fracture toughness and fracture energy) can be imparted to monolithic ceramics by adopting composite design methodology based on fibre reinforcement technology.The present paper describes the fracture behaviour of one such fibre-reinforced material, namely the silica–silica based continuous fibre-reinforced, ceramic–matrix composite (CFCC) in two orthogonal notch orientations of crack divider and crack arrester orientations.Different fracture resistance parameters have been evaluated to provide a quantitative treatment of the observed fracture behaviour.From this study, it has been concluded that the overall fracture resistance of the CFCC is best reflected by total fracture energy release rate (Jc), which parameter encompasses most of the fracture events/processes.The Jc values of the composite are found to be more than an order of magnitude higher than the energy values corresponding to the plane strain fracture toughness (JKQ, derived from KIc, the plane strain fracture toughness) and >200% higher than elastic–plastic fracture toughness (JIc).Apart from this, the composite is found to exhibit high degree of anisotropy in the fracture resistance and also, a significant variation in the relative degree of shear component with crack extension. 2004 Elsevier Ltd.All rights reserved. Keywords: Continuous fibre 2D silica–silica composites; Fracture behaviour and modes of failure; Fracture resistance; Total fracture energy release rate; R-curve behaviour 1. Introduction Ceramic materials have assumed significant technological importance as structural materials because the newer design and development methodologies, adopting fibre reinforcements, have resulted in enhancement of the fracture resistance of monolithic ceramics by several fold [1–6].Among various ceramic materials, amorphous silica uniquely combines different properties to suit several select technological applications * Corresponding author.Tel.: +91-40-24340051; fax: +91-40-24340683/4341439. E-mail addresses: nep@dmrl.ernet.in, neswarap@rediffmail.com (N. Eswara Prasad). 0013-7944/$ - see front matter 2004 Elsevier Ltd.All rights reserved. doi:10.1016/j.engfracmech.2004.02.005 Engineering Fracture Mechanics 71 (2004) 2589–2605 www.elsevier.com/locate/engfracmech
N. Eswara Prasad et al. Engineering Fracture Mechanics 71(2004) 2589-2605 [7-10). These properties include, high melting point combined with high thermal shock resistance and xcellent thermal as well as electrical insulating properties [8, 10]. However, the mechanical properties of lica material in the monolithic form are far from acceptable levels. Silica, in its bulk form, has low strength (both tensile and flexural) and extremely low fracture toughness as compared to several structural ceramic materials [8]; thus, needing significant improvements so that it can be accepted for any structural application. One of the means of achieving improved mechanical properties is by using either two-or three- dimensional(designated commonly as 2D- and 3D, respectively) networks of continuous fibres as rein- forcements to the ceramic-matrix material leading to newer structural materials, known as"continuous fibre-reinforced, ceramic-matrix composites(CFCCs)". Numerous studies have been conducted in the last two decades on the fibre/whisker toughening of this class of ceramics. These studies have been compre- hensively reviewed by Evans [2] as well as by Becher [4] and later, by Faber [6]. However, to the best of our knowledge, there are no fracture toughness/energy studies reported so far for the silica-silica CFCCs During the fracture process of a CFCC, various events/developments take place in the three regions of the fracture, namely the wake of the crack, at the crack tip and finally in the region of process zone ahead of the crack tip These influence the net enhancements in the fracture resistance of a CFCC. They include some or most of the following [2, 3, 5 1. Local increase in the stress level with the application of external loading 2. relative displacement of matrix/interface elements 3. matrix microcracking, leading to matrix failure(with or without significant crack path meandering, i.e rack deflection and/or branching) 4. debonding of matrix/fibre interface(with or without significant frictional forces), fibre pull-out and fibre breakage in the crack tip process zone, 6. frictional sliding of the fibres along the matrix/fibre interfaces, 7. loss of residual strain energy These processes/stages, schematically shown in Fig. 1, result in significant energy dissipation through frictional events in the wake and process zones, acoustic emission and fibre debonding, pull-out and breakage. Contributions from these stages of crack tip and fibre reinforcements interactions, with or ULL-oUT FRIC TIONAL DISSIPATION ENERGY DISSIPATED ACOUSTIC WAVES MATRIX CRACK RESIDUA SURFACES STRESS-FREE LOSS OF RESIDUAL Fig. l. Schematic showing various events and processes of crack bridging mechanism in fibre-reinforced composites(from Ref. 2). Note that the crack extension process essentially involves matrix microcracking, fibre/matrix debonding, fibre fracture and fibre pull
[7–10].These properties include, high melting point combined with high thermal shock resistance and excellent thermal as well as electrical insulating properties [8,10].However, the mechanical properties of silica material in the monolithic form are far from acceptable levels.Silica, in its bulk form, has low strength (both tensile and flexural) and extremely low fracture toughness as compared to several structural ceramic materials [8]; thus, needing significant improvements so that it can be accepted for any structural application.One of the means of achieving improved mechanical properties is by using either two- or threedimensional (designated commonly as 2D- and 3D-, respectively) networks of continuous fibres as reinforcements to the ceramic–matrix material leading to newer structural materials, known as ‘‘continuous fibre-reinforced, ceramic–matrix composites (CFCCs)’’.Numerous studies have been conducted in the last two decades on the fibre/whisker toughening of this class of ceramics.These studies have been comprehensively reviewed by Evans [2] as well as by Becher [4] and later, by Faber [6].However, to the best of our knowledge, there are no fracture toughness/energy studies reported so far for the silica–silica CFCCs. During the fracture process of a CFCC, various events/developments take place in the three regions of the fracture, namely the wake of the crack, at the crack tip and finally in the region of process zone ahead of the crack tip.These influence the net enhancements in the fracture resistance of a CFCC.They include some or most of the following [2,3,5]: 1.Local increase in the stress level with the application of external loading, 2.relative displacement of matrix/interface elements, 3. matrix microcracking, leading to matrix failure (with or without significant crack path meandering, i.e., crack deflection and/or branching), 4.debonding of matrix/fibre interface (with or without significant frictional forces), 5.fibre pull-out and fibre breakage in the crack tip process zone, 6.frictional sliding of the fibres along the matrix/fibre interfaces, 7.loss of residual strain energy. These processes/stages, schematically shown in Fig.1, result in significant energy dissipation through frictional events in the wake and process zones, acoustic emission and fibre debonding, pull-out and breakage.Contributions from these stages of crack tip and fibre reinforcements interactions, with or Fig.1.Schematic showing various events and processes of crack bridging mechanism in fibre-reinforced composites (from Ref.[2]). Note that the crack extension process essentially involves matrix microcracking, fibre/matrix debonding, fibre fracture and fibre pullout. 2590 N. Eswara Prasad et al. / Engineering Fracture Mechanics 71 (2004) 2589–2605
N. Eswara Prasad et al. Engineering Fracture Mechanics 71(2004)2589-2605 without the contributions from matrix fracture events. have led to unified models for the fracture resistance in materials that exhibit crack bridging [2-6]. The toughening in these cases of crack bridging is essentially due to ductile or brittle reinforcements. In the case of present CFCCs, it is the later that makes contri butions to the toughening. In the present paper, the fracture behaviour of a two-dimensional (2D) silica fibre-reinforced, silica matrix composite is presented and discussed. Various parameters of fracture resistance have been used to quantify the fracture resistance of the material. These include, the plane strain fracture toughness(Kl) elastic-plastic fracture toughness (ic) and total fracture energy release rate (e). Also reported and dis- cussed are the effects of notch orientation and notch depth on the fracture resistance in these composites. 2. Experimental details he two-dimensionally weaved silica fibre preforms are vacuum impregnated using colloidal silica Ition precursor to provide the matrix for the silica-silica continuous fibre-reinforced, ceramic-matrix composites (referred to as"silica-silica CFCC"or simply"CFCC"). The interconnected network of capillaries in the preforms facilitates solution impregnation, thus providing uniform matrix for the CFCC After infiltration, the CFCC is dried and during this drying process, water content of the matrix gel solution is gradually removed. These dried CFCCs are then sintered to impart interparticle bonding and in turn, this Facilitates load transfer from the matrix to the fibre and vice-versa There are no standard test procedures for the evaluation of fracture toughness/energy of ceramic materials, especially for the advanced ceramic composites such as CFCCs. However, several studies have been reported in the recent past which describe in detail the procedures adopted for and the fracture behaviour observed of the monolithic ceramics and ceramic-matrix composites, including CFCCs(see Refs. [11-18] for details and a summary of these details in Ref [19D. Since ceramic materials exhibit brittle fracture, the AsTM Standard E-399, describing the standard practice for the evaluation of plane strain fracture toughness of metallic materials [20], can conveniently be adopted to determine the fracture oughness of these materials. However, since the CfCCs also exhibit limited extent of non-linear fracture, the J-integral technique once again developed for metallic materials(fundamentals and standard practices described in Refs. [21-23] and [24], respectively) also applies equally Single edge notch beam(SENB)specimens of 8 mm thickness, 10 mm width and a span length of 40 mm were used. The fracture toughness/energy was evaluated in two notch orientations, namely (i)crack divider orientation, in which the notch is along the orientation of the plies in the thickness direction and (ii)crack arrester orientation, in which the notch is perpendicular to the orientation of the plies in the thickness direction(the third orientation of crack delamination could not be studied because of specimen size limi tations). In both cases, notch is perpendicular to the longitudinal plies Notches of varied length were introduced using 0.3 mm thick diamond wafer blades, mounted on a standard Isomet cutting machine. A specially designed jig was used to obtain straight notches by moving the job across the cutting plane. The notches thus introduced were found to have a finite root radius, p, typically of the order of 160 um. The p values were determined by Delta TM 35 x-y profile projector. The notch root radii. either in the crack divider or crack arrester orientation were found to be similar. The crack lengths were maintained in the range of 0.35 to 0. 7 times the specimen width. Among these, specimens with crack lengths in the range specified by the ASTM standard E-399[20](0.45-0.55 times the specimen width) were only considered for the determination of Klc values. The other specimens with large lengths were employed essentially to determine the work of fracture [25], which results will be reported separately. The fracture energy determined from the load-displacement data were used to determine the elastic-plastic fracture toughness, JIe and the total fracture energy release rate, Jc. The later two fracture resistance parameters are based on J-integral [21]
without the contributions from matrix fracture events, have led to unified models for the fracture resistance in materials that exhibit crack bridging [2–6].The toughening in these cases of crack bridging is essentially due to ductile or brittle reinforcements.In the case of present CFCCs, it is the later that makes contributions to the toughening. In the present paper, the fracture behaviour of a two-dimensional (2D) silica fibre-reinforced, silica– matrix composite is presented and discussed.Various parameters of fracture resistance have been used to quantify the fracture resistance of the material.These include, the plane strain fracture toughness (KIc), elastic–plastic fracture toughness (JIc) and total fracture energy release rate (Jc).Also reported and discussed are the effects of notch orientation and notch depth on the fracture resistance in these composites. 2. Experimental details The two-dimensionally weaved silica fibre preforms are vacuum impregnated using colloidal silica solution precursor to provide the matrix for the silica–silica continuous fibre-reinforced, ceramic–matrix composites (referred to as ‘‘silica–silica CFCC’’ or simply ‘‘CFCC’’).The interconnected network of capillaries in the preforms facilitates solution impregnation, thus providing uniform matrix for the CFCC. After infiltration, the CFCC is dried and during this drying process, water content of the matrix gel solution is gradually removed.These dried CFCCs are then sintered to impart interparticle bonding and in turn, this facilitates load transfer from the matrix to the fibre and vice-versa. There are no standard test procedures for the evaluation of fracture toughness/energy of ceramic materials, especially for the advanced ceramic composites such as CFCCs.However, several studies have been reported in the recent past which describe in detail the procedures adopted for and the fracture behaviour observed of the monolithic ceramics and ceramic–matrix composites, including CFCCs (see Refs.[11–18] for details and a summary of these details in Ref.[19]).Since ceramic materials exhibit brittle fracture, the ASTM Standard E-399, describing the standard practice for the evaluation of plane strain fracture toughness of metallic materials [20], can conveniently be adopted to determine the fracture toughness of these materials.However, since the CFCCs also exhibit limited extent of non-linear fracture, the J-integral technique once again developed for metallic materials (fundamentals and standard practices described in Refs.[21–23] and [24], respectively) also applies equally. Single edge notch beam (SENB) specimens of 8 mm thickness, 10 mm width and a span length of 40 mm were used.The fracture toughness/energy was evaluated in two notch orientations, namely (i) crack divider orientation, in which the notch is along the orientation of the plies in the thickness direction and (ii) crack arrester orientation, in which the notch is perpendicular to the orientation of the plies in the thickness direction (the third orientation of crack delamination could not be studied because of specimen size limitations).In both cases, notch is perpendicular to the longitudinal plies. Notches of varied length were introduced using 0.3 mm thick diamond wafer blades, mounted on a standard Isomet cutting machine.A specially designed jig was used to obtain straight notches by moving the job across the cutting plane.The notches thus introduced were found to have a finite root radius, q, typically of the order of 160 lm.The q values were determined by Delta TM 35 x–y profile projector.The notch root radii, either in the crack divider or crack arrester orientation, were found to be similar.The crack lengths were maintained in the range of 0.35 to 0.7 times the specimen width. Among these, specimens with crack lengths in the range specified by the ASTM standard E-399 [20] (0.45–0.55 times the specimen width) were only considered for the determination of KIc values.The other specimens with larger crack lengths were employed essentially to determine the work of fracture [25], which results will be reported separately.The fracture energy determined from the load–displacement data were used to determine the elastic–plastic fracture toughness, JIc and the total fracture energy release rate, Jc.The later two fracture resistance parameters are based on J-integral [21]. N. Eswara Prasad et al. / Engineering Fracture Mechanics 71 (2004) 2589–2605 2591
2592 N. Eswara Prasad et al. Engineering Fracture Mechanics 71(2004)2589-2605 All the fracture toughness tests were conducted on a computer controlled, servohydraulic Instron 8801 est system using a self-articulating 3-point bend fixtures of MTS 880 test system. The tests were conducted at ambient temperature(23C)and in laboratory air atmosphere. The notched specimens were loaded in ramp control at a constant ramp rate of 0.5 mm/min. The load-displacement curves thus obtained were analysed to obtain various measures of fracture resistance, and the results are presented and discussed in the following sections 3. Results and discussion 3. 1. Load-displacement data and crack path observations The load-displacement data obtained for crack divider and crack arrester orientations are shown in Figs 2 and 3, respectively. Crack lengths are given as normalised values(crack length'a, normalised with the specimen width, ' W). Though six tests with different a/w values were conducted in the crack arrester direction, for the sake of clarity, only three load-displacement plots are included in Fig 3. On the other hand, all the four load-displacement plots obtained are included in Fig. 2 for the crack divider direct CRACK DIVIDER DIRECTION 0.32a 0.44 DISPLACEMENT (6) Fig. 2. Load-displacement data obtained during the evaluation of fracture resistance using specimens with varied crack length(given in terms of the normalised crack length with specimen width) in case of the material in the"crack divider"orientation
All the fracture toughness tests were conducted on a computer controlled, servohydraulic Instron 8801 test system using a self-articulating 3-point bend fixtures of MTS 880 test system.The tests were conducted at ambient temperature (23 C) and in laboratory air atmosphere.The notched specimens were loaded in ramp control at a constant ramp rate of 0.5 mm/min. The load–displacement curves thus obtained were analysed to obtain various measures of fracture resistance, and the results are presented and discussed in the following sections. 3. Results and discussion 3.1. Load–displacement data and crack path observations The load–displacement data obtained for crack divider and crack arrester orientations are shown in Figs.2 and 3, respectively.Crack lengths are given as normalised values (crack length ‘a’, normalised with the specimen width, ‘W ’).Though six tests with different a=W values were conducted in the crack arrester direction, for the sake of clarity, only three load–displacement plots are included in Fig.3.On the other hand, all the four load–displacement plots obtained are included in Fig.2 for the crack divider direction. Fig.2.Load–displacement data obtained during the evaluation of fracture resistance using specimens with varied crack length (given in terms of the normalised crack length with specimen width) in case of the material in the ‘‘crack divider’’ orientation. 2592 N. Eswara Prasad et al. / Engineering Fracture Mechanics 71 (2004) 2589–2605
N. Eswara Prasad et al. Engineering Fracture Mechanics 71(2004)2589-2605 DISPLACEMENT (6) g. 3. of hd nosa isee ct ak etah wit d sng me ewa dta in otsr at the rmsisteanae u ihespecckeas ter an e tation length (grven It can be seen from these curves that increase in crack length decreases maximum load attained prior to the commencement of crack extension. The load initially increases linearly with the displacement in all the cases.This corresponds to the stage in which the specimen largely experiences elastic stresses. Followed by his stage, the crack extension takes place. This is reflected in nonlinear increase in the load with dis- placement followed by noticeable drop in the load with further increase in the displacement. The load- displacement curves show distinctly different characteristics at this stage of crack extension in the two notch orientations The material in the crack divider orientation shows a steep, but continuous fall in the load with increase in the displacement(Fig. 2). Such a behaviour is seen in specimens with lower crack lengths(a) and(b)in Fig. 2 with a/W=0.32 and 0.44, respectively). This clearly indicates gradual extension of the crack front However, higher crack length specimens in this notch orientation((c)and(d)in Fig. 2 with a/W=0.56 and 0.65, respectively) do not show such steep load drop. Instead, these specimens show near-saturation in the variation of load with displacement, up to a displacement of 800 um. In this case, the fibre bundles undergo significant bending without breakage and crack extension essentially occurs along fibre/matrix interface. On the other hand, the specimens in the crack arrester orientation show distinct and sudden load drops with crack extension(shown as A1 B1, A2B2 etc, in the curves(a),()and(c)of Fig 3). Again, at large crack lengths(a/W=0.64, case(c)in Fig 3)the specimen shows gradual load drop with displacement Such a behaviour is attributable to the change in the nature of crack extension. As shown schematically in ne specimens in the crack arrester orientation show complete mode I(tensile) fracture dominant fibre bundle breakage leading to relatively insignificant crack extension along the fibre/matrix interface when the crack lengths are smaller(a/w<0.45). As crack length increases, the extent of crack extension along the fibre/matrix interface also increases, leading to significant extent of'H'and'Tcracking (Fig 4b). This results in mode I crack extension in the initial stages and a mixed mode fracture in the later stages, comprising mode I and mode II (in-plane shear or sliding) components. This occurs in case of specimens with a/W values in the range 0.55-0.6. At still higher crack lengths(cases(c)and(d)in Fig. 2 and (c)in Figs. 3 and 4), the crack extension occurs essentially in mode II with interply shearing being the
It can be seen from these curves that increase in crack length decreases maximum load attained prior to the commencement of crack extension.The load initially increases linearly with the displacement in all the cases.This corresponds to the stage in which the specimen largely experiences elastic stresses.Followed by this stage, the crack extension takes place.This is reflected in nonlinear increase in the load with displacement followed by noticeable drop in the load with further increase in the displacement.The load– displacement curves show distinctly different characteristics at this stage of crack extension in the two notch orientations. The material in the crack divider orientation shows a steep, but continuous fall in the load with increase in the displacement (Fig.2).Such a behaviour is seen in specimens with lower crack lengths ((a) and (b) in Fig.2 with a=W ¼ 0:32 and 0.44, respectively). This clearly indicates gradual extension of the crack front. However, higher crack length specimens in this notch orientation ((c) and (d) in Fig.2 with a=W ¼ 0:56 and 0.65, respectively) do not show such steep load drop. Instead, these specimens show near-saturation in the variation of load with displacement, up to a displacement of 800 lm.In this case, the fibre bundles undergo significant bending without breakage and crack extension essentially occurs along fibre/matrix interface.On the other hand, the specimens in the crack arrester orientation show distinct and sudden load drops with crack extension (shown as A1B1, A2B2 etc., in the curves (a), (b) and (c) of Fig. 3). Again, at large crack lengths (a=W ¼ 0:64, case (c) in Fig.3) the specimen shows gradual load drop with displacement. Such a behaviour is attributable to the change in the nature of crack extension.As shown schematically in Fig.4a, the specimens in the crack arrester orientation show complete mode I (tensile) fracture with predominant fibre bundle breakage leading to relatively insignificant crack extension along the fibre/matrix interface when the crack lengths are smaller (a=W < 0:45).As crack length increases, the extent of crack extension along the fibre/matrix interface also increases, leading to significant extent of ‘H’ and ‘T ’ cracking (Fig.4b).This results in mode I crack extension in the initial stages and a mixed mode fracture in the later stages, comprising mode I and mode II (in-plane shear or sliding) components.This occurs in case of specimens with a=W values in the range 0.55–0.6. At still higher crack lengths (cases (c) and (d) in Fig. 2 and (c) in Figs.3 and 4), the crack extension occurs essentially in mode II with interply shearing being the Fig.3.Load–displacement data obtained during the evaluation of fracture resistance using specimens with varied crack length (given in terms of the normalised crack length with specimen width) in case of the material in the ‘‘crack arrester’’ orientation. N. Eswara Prasad et al. / Engineering Fracture Mechanics 71 (2004) 2589–2605 2593