composes Part A: applied science and manufacturing ELSEVIER Composites: Part A 33(2002)1209-1218 www.elsevier.com/locate/composites fracture behaviour of cross-ply Nicalon/CAS-II glass-ceramic matrix composite laminate at room and elevated temperatures A.Yasmin"P. bowen igh Performance Applications, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 23 January 2002; revised 23 May 2002: accepted 26 June 2002 Abstract The fracture behaviour of cross-ply Nicalon/CAS-nI glass ceramic matrix co mposite laminate has been investigated as a function of temperature, loading rate and environment. Tests were carried out at 20, 600, 800, 1000C in air and also at 1000.C in vacuum. Increased flexural strength was obtained with increased loading rate but it decreased with increasing test temperature. Although the change in flexural rength was not very significant for the loading rates(10-and 10-mm/min)employed in this study except at 600.Cin air, the influence of environment was dramatic. The flexural strength at 1000C in vacuum was comparable to the value obtained at room temperature in air, however, the flexural strength at 1000 C in air reduced significantly from its room temperature value. Two major damage modes have been identified: fibrous at room temperature and non-fibrous at high temperature. c 2002 Elsevier Science Ltd. All rights reserved Keywords: A Ceramic-matrix composites(CMCs); B. Fracture; D. Mechanical testing: D. Electron microscopy 1. Introduction advance [4-7]. For example, by employing continuous fibres the fracture toughness of CMCs or G-CMCs can be When combined with high specific strength and stiffness, increased to =30 MPavm [8] compared to monolithic the ability of ceramic materials to withstand severe ceramics that show only about 3 MPam[4]. In addition environments including heat, abrasion and high oxidation they retain load carrying capacity after matrix cracks are compared to conventional materials have made these initiated whereas monolithic ceramics show catastrophic materials particularly suitable for the development of high failure temperature structural components such as aero-engine, gas After achieving excellent properties from unidirectional turbine and nuclear reaction furnaces [1, 2]. Consequently, composites, the next interest goes to cross-ply or angle-ply they have the potential to produce damage tolerant and laminates due to their potential to be applied under tough materials that can be used at temperatures as high as multiaxial stress conditions. An extensive research on the 1000C for several 100 h. However. the main drawback of damage mechanism of cross-ply G-CMCs under uniaxial these materials lies in their reduced ductility and fracture nsile loading condition is available in the literature toughness, which makes ceramic components potentially [9-11]. It shows that the presence of off-axis plies changes prone to catastrophic failure 3]. Therefore, in the last two the failure mechanisms. There are two modes of decades, considerable attention has been given to the cracking in the cross-ply laminates: one is transverse development of continuous silicon carbide fibre reinforced cracking in the 90 plies and the other is matrix cracking in ceramic( CMCs)and glass-ceramic matrix composites he 0 plies. Therefore, the failure processes start at lower stresses than that for the unidirectional composite (G-CMCs)to provide excellent toughness by providing However, the presence of transverse cracks in the cross- various energy dissipating processes (i.e. matrix micro- cracking, fibre/matrix debonding, fibre pullout) via load ply laminate has only a limited effect on the evolution of matrix cracks and hence, the composite strength is transfer and crack deflection mechanisms during crack controlled by the o ply proportional limit point where orresponding autho address: Centre for Intelligent matrix cracking starts, and not by the 90 ply failure point Processing of Composite #330, Northwestern University However, the room temperature behaviour of cross-ply Evanston. IL 60208 USA. 491-7961;fax:+1-847-491-5227 laminates may deteriorate at high temperatur E-mail address: a-yasmi western.edu(A. Yasmin) the early cracking of 90 plies, which may penetrate past 1359-835X/02/S-see front matter e 2002 Elsevier Science Ltd. All rights reserved. PI:S1359-835X(02)000799
Fracture behaviour of cross-ply Nicalon/CAS-II glass–ceramic matrix composite laminate at room and elevated temperatures A. Yasmin*, P. Bowen School of Metallurgy and Materials/IRC in Materials for High Performance Applications, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 23 January 2002; revised 23 May 2002; accepted 26 June 2002 Abstract The fracture behaviour of cross-ply Nicalon/CAS-II glass–ceramic matrix composite laminate has been investigated as a function of temperature, loading rate and environment. Tests were carried out at 20, 600, 800, 1000 8C in air and also at 1000 8C in vacuum. Increased flexural strength was obtained with increased loading rate but it decreased with increasing test temperature. Although the change in flexural strength was not very significant for the loading rates (1023 and 1021 mm/min) employed in this study except at 600 8C in air, the influence of environment was dramatic. The flexural strength at 1000 8C in vacuum was comparable to the value obtained at room temperature in air, however, the flexural strength at 1000 8C in air reduced significantly from its room temperature value. Two major damage modes have been identified: fibrous at room temperature and non-fibrous at high temperature. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Ceramic–matrix composites (CMCs); B. Fracture; D. Mechanical testing; D. Electron microscopy 1. Introduction When combined with high specific strength and stiffness, the ability of ceramic materials to withstand severe environments including heat, abrasion and high oxidation compared to conventional materials have made these materials particularly suitable for the development of high temperature structural components such as aero-engine, gas turbine and nuclear reaction furnaces [1,2]. Consequently, they have the potential to produce damage tolerant and tough materials that can be used at temperatures as high as 1000 8C for several 100 h. However, the main drawback of these materials lies in their reduced ductility and fracture toughness, which makes ceramic components potentially prone to catastrophic failure [3]. Therefore, in the last two decades, considerable attention has been given to the development of continuous silicon carbide fibre reinforced ceramic (CMCs) and glass–ceramic matrix composites (G–CMCs) to provide excellent toughness by providing various energy dissipating processes (i.e. matrix microcracking, fibre/matrix debonding, fibre pullout) via load transfer and crack deflection mechanisms during crack advance [4–7]. For example, by employing continuous fibres the fracture toughness of CMCs or G–CMCs can be increased to $30 MPapm [8] compared to monolithic ceramics that show only about 3 MPapm [4]. In addition, they retain load carrying capacity after matrix cracks are initiated whereas monolithic ceramics show catastrophic failure. After achieving excellent properties from unidirectional composites, the next interest goes to cross-ply or angle-ply laminates due to their potential to be applied under multiaxial stress conditions. An extensive research on the damage mechanism of cross-ply G–CMCs under uniaxial tensile loading condition is available in the literature [9–11]. It shows that the presence of off-axis plies changes the failure mechanisms. There are two major modes of cracking in the cross-ply laminates: one is transverse cracking in the 908 plies and the other is matrix cracking in the 08 plies. Therefore, the failure processes start at lower stresses than that for the unidirectional composite. However, the presence of transverse cracks in the crossply laminate has only a limited effect on the evolution of matrix cracks and hence, the composite strength is controlled by the 08 ply proportional limit point where matrix cracking starts, and not by the 908 ply failure point. However, the room temperature behaviour of cross-ply laminates may deteriorate at high temperature due to the early cracking of 908 plies, which may penetrate past 1359-835X/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S1 35 9 -8 35 X( 02 )0 0 07 9 -9 Composites: Part A 33 (2002) 1209–1218 www.elsevier.com/locate/compositesa * Corresponding author. Present address: Centre for Intelligent Processing of Composites, Room #330, Northwestern University, Evanston, IL 60208, USA. Tel.: þ1-847-491-7961; fax: þ1-847-491-5227. E-mail address: a-yasmin@northwestern.edu (A. Yasmin)
A. Yasmin, P. Bowen/Composites: Part A 33(2002)1209-1218 the fibres of the 0 plies and expose them to environmental An optical micrograph of the cross-section of cross-ply attack Nicalon/CAS is shown in Fig. 1. Nicalon/CAS consists of The high strength and high toughness values obtained at aIcium-alumino-silicate (CA amic matrIx room temperature are believed to be due to the presence of a reinforced with 35% continuous silicon carbide fibre weakly bonded carbon-rich interfacial layer developed(Nicalon) by volume. Specimens with dimensions of about during hot pressing [12-14]. However, at higher tempera- 50 mm long, 5 mm wide and 3. 7 mm thick were cut from the ture the carbon interface layer is removed by oxidation composite plate at O to the plate axis using diamond tooling reaction upon matrix cracking and a very strong silica bond fracture tests were performed on as-received and plane is developed [15-17]. This silica bond increases interfacial sided specimens under three-point bending with a total shear strength [13, 18-20] and causes failure of the fibres loading span of 40 mm. The specimens were edge-loaded along with the matrix instead of any type of crack deflecting and the schematic of this loading configuration is given in mechanism. The degradation of strength of fibres by thermal Fig. 2. The three-point bend technique allows simple decomposition may be another reason of embrittlement; specimen geometry to be used under a relatively simple however, it is reported by others [21] that it is less crucial for fixture and also utilization of small quantity of plate material Nicalon/CAS glass-ceramic matrix composite system. The very efficiently. Tests were carried out at room temperature embrittlement process of cross-ply Nicalon/CAS (9 is 600, 800, 1000C in air and also at 1000C in vacuum found to be similar to unidirectional Nicalon/CAS except Crosshead displacement rates of 10 and 10 mm/min 90 plies crack prior to 0 ply matrix cracking; however, were used in this study. For all high temperature tests, embrittlement does not occur until the matrix in the 0 plies specimens were heated at a rate of 10 C/min and held at the cracks. Therefore, if these are the materials to be used at required temperature for I h under a low load of 10n to high temperature, a detailed investigation on their behaviour allow the system to reach equilibrium. An Instron 8501 under loading is important servohydraulic machine equipped with a 5 kN load cell was The present work illustrates the effects of temperature sed for fracture tests in air. whereas vacuum tests were and loading rate on the failure mechanism of cross-ply carried out in a further Instron 8501 servohydraulic machine Nicalon/CAS glass-ceramic matrix composite laminate equipped with a vacuum chamber and a load cell of 10 kN under three-point bending subjected to monotonic loading During the test, a load versus crosshead displacement to failure. the environmental effect on the fracture record was taken for all tests and the nominal maximum behaviour has also been investigated. In addition, stress or flexural strength was calculated using the following comparison has been drawn between the fracture behaviour formula of unidirectional and cross-ply Nicalon/CAS glass-ceramic 1) cre P is the maximum load, L is the half of the loading 2. Experimental B is the specimen thickness and w is the specimen width. This is called nominal maximum stress. since the The material used in this study is a cross-ply [0/90J4s stresses in the O and 90 plies are not the same and also shift icalon/CAS-Il glass-ceramic matrix composite laminates in proportion as damage takes place manufactured by Corning Industries, USA. The composite The fracture surfaces of the specimens were investigated was supplied as a plate of dimensions 150 X 150 x 3.7 mm. using a Hitachi $4000 Field Emission Gun(FEG)scanning electron microscope(SEM) with an accelerating voltage of 4 kv, which did not cause any charging of the specimen and 0°fibr 4 250 Fig. 1. Optical micrograph of the n of as-received cross-ply s all dimensions are in mm Nicalon/CAS-ll glass-ceramic matrix composite. Fig. 2. A schematic of three-point bend loading configurati
the fibres of the 08 plies and expose them to environmental attack. The high strength and high toughness values obtained at room temperature are believed to be due to the presence of a weakly bonded carbon-rich interfacial layer developed during hot pressing [12–14]. However, at higher temperature the carbon interface layer is removed by oxidation reaction upon matrix cracking and a very strong silica bond is developed [15–17]. This silica bond increases interfacial shear strength [13,18–20] and causes failure of the fibres along with the matrix instead of any type of crack deflecting mechanism. The degradation of strength of fibres by thermal decomposition may be another reason of embrittlement; however, it is reported by others [21] that it is less crucial for Nicalon/CAS glass–ceramic matrix composite system. The embrittlement process of cross-ply Nicalon/CAS [9] is found to be similar to unidirectional Nicalon/CAS except 908 plies crack prior to 08 ply matrix cracking; however, embrittlement does not occur until the matrix in the 08 plies cracks. Therefore, if these are the materials to be used at high temperature, a detailed investigation on their behaviour under loading is important. The present work illustrates the effects of temperature and loading rate on the failure mechanism of cross-ply Nicalon/CAS glass–ceramic matrix composite laminate under three-point bending subjected to monotonic loading to failure. The environmental effect on the fracture behaviour has also been investigated. In addition, a comparison has been drawn between the fracture behaviour of unidirectional and cross-ply Nicalon/CAS glass–ceramic matrix composite laminates. 2. Experimental The material used in this study is a cross-ply [0/90]4s Nicalon/CAS-II glass–ceramic matrix composite laminates manufactured by Corning Industries, USA. The composite was supplied as a plate of dimensions 150 £ 150 £ 3.7 mm3 . An optical micrograph of the cross-section of cross-ply Nicalon/CAS is shown in Fig. 1. Nicalon/CAS consists of calcium–alumino-silicate (CAS) glass–ceramic matrix reinforced with 35% continuous silicon carbide fibre (Nicalon) by volume. Specimens with dimensions of about 50 mm long, 5 mm wide and 3.7 mm thick were cut from the composite plate at 08 to the plate axis using diamond tooling. Fracture tests were performed on as-received and plane sided specimens under three-point bending with a total loading span of 40 mm. The specimens were edge-loaded and the schematic of this loading configuration is given in Fig. 2. The three-point bend technique allows simple specimen geometry to be used under a relatively simple fixture and also utilization of small quantity of plate material very efficiently. Tests were carried out at room temperature, 600, 800, 1000 8C in air and also at 1000 8C in vacuum. Crosshead displacement rates of 1023 and 1021 mm/min were used in this study. For all high temperature tests, specimens were heated at a rate of 10 8C/min and held at the required temperature for 1 h under a low load of 10 N to allow the system to reach equilibrium. An Instron 8501 servohydraulic machine equipped with a 5 kN load cell was used for fracture tests in air, whereas vacuum tests were carried out in a further Instron 8501 servohydraulic machine equipped with a vacuum chamber and a load cell of 10 kN. During the test, a load versus crosshead displacement record was taken for all tests and the nominal maximum stress or flexural strength was calculated using the following formula s ¼ 3PL BW2 ð1Þ where P is the maximum load, L is the half of the loading span, B is the specimen thickness and W is the specimen width. This is called nominal maximum stress, since the stresses in the 0 and 908 plies are not the same and also shift in proportion as damage takes place. The fracture surfaces of the specimens were investigated using a Hitachi S4000 Field Emission Gun (FEG) scanning electron microscope (SEM) with an accelerating voltage of 4 kV, which did not cause any charging of the specimen and Fig. 1. Optical micrograph of the cross-section of as-received cross-ply Nicalon/CAS-II glass–ceramic matrix composite. Fig. 2. A schematic of three-point bend loading configuration. 1210 A. Yasmin, P. Bowen / Composites: Part A 33 (2002) 1209–1218
A. Yasmin, P. Bowen /Composites: Part A 33(2002)1209-1218 thereby permitted scanning without gold coating 图 Ramp rate,0001 mm/min Occasionally, the pull-out length of fibres was measured B Ramp rate, 0.1 mm/min from SEM FEG micrographs taken at a tilt angle of 4. Fig 3 shows the variation of flexural strength of cross-ply 8"900 Nicalon/CAs glass-ceramic matrix composite with tem- perature, (in air)at the two different loading rates of 10-3 300 and 10 mm/min. From this figure, it is apparent that there 2 200 increased from room temperature to 1000C in air, and 800 is a steady decrease of flexural strength as the temperature is approximately 41, 32 and 26% of the room temperature value is obtained at 600, 800 and 1000C, respectively. lus, the temperature effect is highly significant and this gnificant decrease in flexural strength with temperature is 20°C,air1000°c,air1000°c,vac. true for both loading rates (10 and 10mm/min) Fig. 4. Effects of environment and temperature on the flexural streng employed in this study. However, the influence of loading cross-ply Nicalon/CAS glass-ceramic matrix composite rate for a given test temperature was not found to be pronounced except at 600C where a large degree of variation of flexural strength was obtained at the higher The load versus displacement curves for all test loading rate temperatures at both loading rates (10-and In Fig. 4, the flexural strength of cross-ply Nicalon/CAS 10 mm/min) are summarised in Fig. 5(a)and(b), at 1000C in vacuum has been compared with the values respectively. It is found that as the temperature increases obtained at both room temperature and 1000C in air. The oth peak load and the corresponding displacement flexural strength values at 1000C in vacuum are found to decreases and two types of failure modes can be identified be 7-14% higher than the room temperature values for the from these curves loading rates employed in this study. However, the flexural trength values at 1000 C in air are found to be significantl 1. Non-catastrophic or fibrous: tests at 20C in air and lower, just 22-26% of the room temperature and 20-23% 1000C in vacuum show non-linear behaviour until the of the 1000c in vacuum values. this observation maximum load is reached, which is typical of damage therefore, suggests a strong influence of environment on tolerant materials. After reaching the peak value the load the flexural strength of Nicalon/CAS at high temperatures drops gradually but in a discontinuous manner. Finally the specimens fail by gross collapse Catastrophic or non-fibrous: tests at 600, 800 and 1000C in air show linear behaviour until the maximum Ramp rate, 0.001 mm/mim load is reached and then fail catastrophically Since the area under the load-deflection curve is proportional to the ability of the material to absorb energy ng testing, Fig. 5(a)and(b) also energy involved in non-catastrophic failure specimens is 5300 higher than the catastrophic failure specimens ig. 6(a) illustrates an SEM micrograph of the side face of a specimen, loaded monotonically to failure at a ading rate of 10 mm/min at room temperature in air There are no dominant mode -I cracks and cracking is diffuse. Damage consists of multiple matrix microcrack ing, debonding and fibre pullout. The corresponding load- displacement curve in Fig. 5(a) shows non-linearity to a 0 200 400 600 800 1000 1200 displacement value of 0.6 mm to reach the peak load (780N). The load then drops to a residual value(240 N, The variation of flexural strength of cross-ply Nicalon/CAS glass 0.67 mm), which is about 30%o of the peak load and the ic matrix composite in air with temperature at two different loadin long tail afterwards is attributed to extensive pullout of he individual fibres. The load-displacement curve
thereby permitted scanning without gold coating. Occasionally, the pull-out length of fibres was measured from SEM FEG micrographs taken at a tilt angle of 458. 3. Results Fig. 3 shows the variation of flexural strength of cross-ply Nicalon/CAS glass–ceramic matrix composite with temperature (in air) at the two different loading rates of 1023 and 1021 mm/min. From this figure, it is apparent that there is a steady decrease of flexural strength as the temperature is increased from room temperature to 1000 8C in air, and approximately 41, 32 and 26% of the room temperature value is obtained at 600, 800 and 1000 8C, respectively. Thus, the temperature effect is highly significant and this significant decrease in flexural strength with temperature is true for both loading rates (1023 and 1021 mm/min) employed in this study. However, the influence of loading rate for a given test temperature was not found to be pronounced except at 600 8C where a large degree of variation of flexural strength was obtained at the higher loading rate. In Fig. 4, the flexural strength of cross-ply Nicalon/CAS at 1000 8C in vacuum has been compared with the values obtained at both room temperature and 1000 8C in air. The flexural strength values at 1000 8C in vacuum are found to be 7–14% higher than the room temperature values for the loading rates employed in this study. However, the flexural strength values at 1000 8C in air are found to be significantly lower, just 22–26% of the room temperature and 20–23% of the 1000 8C in vacuum values. This observation, therefore, suggests a strong influence of environment on the flexural strength of Nicalon/CAS at high temperatures. The load versus displacement curves for all test temperatures at both loading rates (1023 and 1021 mm/min) are summarised in Fig. 5(a) and (b), respectively. It is found that as the temperature increases both peak load and the corresponding displacement decreases and two types of failure modes can be identified from these curves: 1. Non-catastrophic or fibrous: tests at 20 8C in air and 1000 8C in vacuum show non-linear behaviour until the maximum load is reached, which is typical of damage tolerant materials. After reaching the peak value the load drops gradually but in a discontinuous manner. Finally, the specimens fail by gross collapse. 2. Catastrophic or non-fibrous: tests at 600, 800 and 1000 8C in air show linear behaviour until the maximum load is reached and then fail catastrophically. Since the area under the load–deflection curve is proportional to the ability of the material to absorb energy during testing, Fig. 5(a) and (b) also clearly show that the energy involved in non-catastrophic failure specimens is higher than the catastrophic failure specimens. Fig. 6(a) illustrates an SEM micrograph of the side face of a specimen, loaded monotonically to failure at a loading rate of 1023 mm/min at room temperature in air. There are no dominant mode-I cracks and cracking is diffuse. Damage consists of multiple matrix microcracking, debonding and fibre pullout. The corresponding load– displacement curve in Fig. 5(a) shows non-linearity to a displacement value of 0.6 mm to reach the peak load (780 N). The load then drops to a residual value (240 N, 0.67 mm), which is about 30% of the peak load and the long tail afterwards is attributed to extensive pullout of the individual fibres. The load–displacement curve, Fig. 4. Effects of environment and temperature on the flexural strength of cross-ply Nicalon/CAS glass–ceramic matrix composite. Fig. 3. The variation of flexural strength of cross-ply Nicalon/CAS glass– ceramic matrix composite in air with temperature at two different loading rates. A. Yasmin, P. Bowen / Composites: Part A 33 (2002) 1209–1218 1211
A. Yasmin, P. Bowen/ Composites: Part A 33(2002)1209-1218 Tensile side 1000 000°C,vac 600°C,air 750pm 100C, air Tensile side 0.00.20.40.60.81.0121.41.6 1000 0°,vac 400三 750pu Non-Fibrous 600°C,ai ig. 6. SEM micrographs of the side face of specimens subjected to 800°C,air monotonic failure at room temperature Loading rate (a)10 mm/min and 00c (b)10 mm/min 10 mm/min(Fig 3). Fig. 7(b) shows the fracture surface 0.00.20.4060.81.01.21.41.6 of the mid-section of the specimen that showed the lowest Displacement, (mm) peak load(332.5 N) among all specimens tested at the higher loading rate(10 mm/min). The fractograph shows Fig. 5. Load-displacement curves for all temperatures employed in this significant pullout compared to the specimen tested at the udy Loading rate(a)10-mm/min and(b)10-mm/min lower loading rate, and the average pullout length is approximately 50 um. The corresponding load-displace therefore, indicates a gross collapse and this type of ment curve in Fig. 5(b) also shows that the peak load occurs failure mechanism is usually referred to as ' fibrous'in at a higher displacement (0.27 mm) compared to the nature specimen tested at the lower loading rate, 10 mm/min However,a dominant crack and fibre bridging are After that the load drops stepwise with a higher displace found as the loading rate is increased to 10 mm/min as ment, perhaps an indication of fibre failure as bundles shown in Fig. 6(b). The corresponding load-displacement Fig. 7(e) illustrates the side face of the specimen that curve in Fig. 5(b) also shows similar behaviour to th showed the highest peak load(636.2 N) among all speci load-displacement curve at the lower loading rate mens tested at the higher loading rate of 10 mm/min 10-mm/min), however, exhibits a residual load which (Note that, the load-displacement curve for this test is not is about 30% of the peak load at a higher displacement available due to a problem with the chart recorder). In this (275N, 0.9 mm) than that observed at the lower loading case, the specimen shows a tortuous mode-I crack with random fibre failure and the fibre pullout length is about ig. 7(a) shows the fracture surface of the mid-section of 135 um. Therefore, this type of fracture behaviour can be a specimen tested at 600C in air with a loading rate of characterised as'mixed and a transition from non-fibrous to 10 mm/min. The 0 fibre layers of the fracture surface are mixed fracture occurs as the loading rate is increased at found nearly flat with limited fibre pullout. The correspond- 600C in air g load-displacement curve in Fig. 5(a) also shows Fig &(a)and(b) show SEM micrographs of the side faces catastrophic behaviour. The specimen loses its load carrying of the specimens tested at a loading rate of 10 mm/min at capacity(95%)instantly after the peak load(293N) is 800 and 1000C in air, respectively. Damage consists of reached. However, the specimens show more scatter in mode-I cracking in both conditions. However, specimens flexural strength values as the loading rate is increased to tested at 800 C in air show occasional presence of a little o
therefore, indicates a gross collapse and this type of failure mechanism is usually referred to as ‘fibrous’ in nature. However, a dominant crack and fibre bridging are found as the loading rate is increased to 1021 mm/min as shown in Fig. 6(b). The corresponding load–displacement curve in Fig. 5(b) also shows similar behaviour to the load–displacement curve at the lower loading rate (1023 mm/min), however, exhibits a residual load which is about 30% of the peak load at a higher displacement (275 N, 0.9 mm) than that observed at the lower loading rate, 1023 mm/min. Fig. 7(a) shows the fracture surface of the mid-section of a specimen tested at 600 8C in air with a loading rate of 1023 mm/min. The 08 fibre layers of the fracture surface are found nearly flat with limited fibre pullout. The corresponding load–displacement curve in Fig. 5(a) also shows catastrophic behaviour. The specimen loses its load carrying capacity (,95%) instantly after the peak load (293 N) is reached. However, the specimens show more scatter in flexural strength values as the loading rate is increased to 1021 mm/min (Fig. 3). Fig. 7(b) shows the fracture surface of the mid-section of the specimen that showed the lowest peak load (332.5 N) among all specimens tested at the higher loading rate (1021 mm/min). The fractograph shows significant pullout compared to the specimen tested at the lower loading rate, and the average pullout length is approximately 50 mm. The corresponding load–displacement curve in Fig. 5(b) also shows that the peak load occurs at a higher displacement (0.27 mm) compared to the specimen tested at the lower loading rate, 1023 mm/min. After that the load drops stepwise with a higher displacement, perhaps an indication of fibre failure as bundles. Fig. 7(c) illustrates the side face of the specimen that showed the highest peak load (636.2 N) among all specimens tested at the higher loading rate of 1021 mm/min. (Note that, the load–displacement curve for this test is not available due to a problem with the chart recorder). In this case, the specimen shows a tortuous mode-I crack with random fibre failure and the fibre pullout length is about 135 mm. Therefore, this type of fracture behaviour can be characterised as ‘mixed’ and a transition from non-fibrous to mixed fracture occurs as the loading rate is increased at 600 8C in air. Fig. 8(a) and (b) show SEM micrographs of the side faces of the specimens tested at a loading rate of 1021 mm/min at 800 and 1000 8C in air, respectively. Damage consists of mode-I cracking in both conditions. However, specimens tested at 800 8C in air show occasional presence of a little 08 Fig. 6. SEM micrographs of the side face of specimens subjected to monotonic failure at room temperature. Loading rate (a) 1023 mm/min and (b) 1021 mm/min. Fig. 5. Load–displacement curves for all temperatures employed in this study. Loading rate (a) 1023 mm/min and (b) 1021 mm/min. 1212 A. Yasmin, P. Bowen / Composites: Part A 33 (2002) 1209–1218
A. Yasmin, P. Bowen /Composites: Part A 33(2002)1209-1218 1213 Tensile side 90 ply 0°ply 231um Tensile side 90°py 273pm Tensile side Fig. 8. SEM micrographs of the side face of specimens subjected to monotonic failure at a loading rate of 10 mm/min in air(a)800C and(b) 1000°C. load and therefore, the specimen can maintain a high residual strength for a higher displacement value (218 N, 1.1mm) Table 1 summarises the flexural strength values and failure modes for all test conditions applied in this study pecimens tested at room temperature and 1000C in vacuum show fibrous failure, whereas specimens tested at Fig. 7. SEM f the specimens subjected to monotonic failure at ate:10-3mm/min;(b)loadin 800 and 1000C in air show non-fibrous failure at both 600°Cin 0.1 mm/min showed the lowest flexural strength; (c) loading loading rates employed in this study. A transition from non- min; specimen showed the highest flexural strength. fibrous to mixed failure is observed at 600C when the loading rate is increased from 10 to 10 mm/min. the above observation, therefore, signify that the flexural fibre pullout but specimens tested at 1000C in air show strength of cross-ply Nicalon/CAS is strongly dependent absolutely no fibre pullout. In general, in both cases the on temperature and environment, and the loading rate can fracture surfaces are flat and the corresponding loa also be important. displacement curves in Fig. 5(b) show catastrophic failure at a very low peak load and displacement values. However,a higher flexural strength is obtained at 800C than at 1000C. Tensile side This type of failure is usually referred to as 'non-fibrous However a fibrous failure can be obtained at 1000C uum as shown in Fig. 9. The failure is characterised by multiple transverse matrix microcracking, fibre debonding, fibre bridging and fibre pullout. This type of failure mechanism is similar to that observed in roor temperature tests and consequently a high flexural strengt is obtained which is even higher than the room temperature values. The correspo ponding load-displacement curve Fig. 9. SEM micrograph of the side face of a specimen subjected to Fig 5(a) shows that the load drops gradually after the peak monotonic failure at 1000C in vacuum at a loading rate of 10-mm/min
fibre pullout but specimens tested at 1000 8C in air show absolutely no fibre pullout. In general, in both cases the fracture surfaces are flat and the corresponding load– displacement curves in Fig. 5(b) show catastrophic failure at a very low peak load and displacement values. However, a higher flexural strength is obtained at 800 8C than at 1000 8C. This type of failure is usually referred to as ‘non-fibrous’ failure. However, a fibrous failure can be obtained at 1000 8C, but in vacuum as shown in Fig. 9. The failure is characterised by multiple transverse matrix microcracking, fibre debonding, fibre bridging and fibre pullout. This type of failure mechanism is similar to that observed in room temperature tests and consequently a high flexural strength is obtained which is even higher than the room temperature values. The corresponding load–displacement curve in Fig. 5(a) shows that the load drops gradually after the peak load and therefore, the specimen can maintain a high residual strength for a higher displacement value (218 N, 1.1 mm). Table 1 summarises the flexural strength values and failure modes for all test conditions applied in this study. Specimens tested at room temperature and 1000 8C in vacuum show fibrous failure, whereas specimens tested at 800 and 1000 8C in air show non-fibrous failure at both loading rates employed in this study. A transition from non- fibrous to mixed failure is observed at 600 8C when the loading rate is increased from 1023 to 1021 mm/min. The above observation, therefore, signify that the flexural strength of cross-ply Nicalon/CAS is strongly dependent on temperature and environment, and the loading rate can also be important. Fig. 8. SEM micrographs of the side face of specimens subjected to monotonic failure at a loading rate of 1021 mm/min in air (a) 800 8C and (b) 1000 8C. Fig. 7. SEM micrographs of the specimens subjected to monotonic failure at 600 8C in air. (a) Loading rate: 1023 mm/min; (b) loading rate: 0.1 mm/min, specimen showed the lowest flexural strength; (c) loading rate, 1021 mm/min; specimen showed the highest flexural strength. Fig. 9. SEM micrograph of the side face of a specimen subjected to monotonic failure at 1000 8C in vacuum at a loading rate of 1023 mm/min. A. Yasmin, P. Bowen / Composites: Part A 33 (2002) 1209–1218 1213