scripta MATERIALIA pta mater.44(2001)531-537 PERGAMON www.elsevier.com/locate/scriptamat RELIABILITY OF THE CHEVRON NOTCH TECHNIQUE FOR FRACTURE TOUGHNESS DETERMINATION IN GLASS COMPOSITES REINFORCED BY CONTINUOUS FIBRES I. Dlouhy' and A.R. boccaccini* I Institute of Physics of Materials, Cz-61662 Brno, Czech Republic 2 Institute of Materials Technology, Ilmenau University of Technology, D-98684 Ilmenau, Germany (Received July 18, 2000) (Accepted in revised form August 25, 2000) Keywords: Composites; Glass composite; Fracture; Fracture toughness, Chevron notch 1. Introduction In ceramics and glasses reinforced by ceramic fibres, elastic fibre bridging and fibre pull-out mecha- nisms mainly cause the toughening. Both these mechanisms increase in some extent behind the crack tip along the process zone wake(1-3). Thus, the crack growth resistance rises as the crack propagates and leaves the wake. In these materials, it is difficult to define the intrinsic fracture toughness(Klc)as a material parameter due to the increasing crack growth resistance curve(2). Nevertheless, an exact method of fracture behaviour quantification is needed if a further development of fibre reinforced brittle matrix composites is desired, including the assessment of their possible structural degradation in service The chevron notched specimen technique is a well-established method used to determine the fracture toughness and the work of fracture of brittle materials (2, 4-6), including particle reinforced glass matrix composites(7). As indicated elsewhere(8), however, the technique has not received wide application to measure the fracture toughness of fibre reinforced brittle matrix composites. Other specimen configurations, for example straight notched specimens (9-11), have been more extensively used in these materials. Apart from our own previous research (8, 11), few reports are available on the applicability of the chevron notched specimen to fibre reinforced ceramics and glasses (12, 13) Mecholsky has pointed out, however, that as long as the notch is large enough to cover many fibres at the critical crack length, the chevron notch test should represent a useful method to measure the fracture toughness and the work of fracture in fibre reinforced ceramic composites (10). In the work of Ha and Chawla(12), it was shown that using the chevron notched specimen technique the different mechanical behaviour of mullite matrix composites containing mullite fibres with different coatings could be detected. In our previous investigation( 8, 11), the differences in fracture behaviour of glass matrix composites after different thermal ageing conditions in non-oxidising atmosphere were detected by this technique and reliable fracture toughness data were obtained. On the basis of such previous xperiences, the research was extended to samples thermally loaded under different oxidising condi- tions, including thermal shock and thermal cycling, as reported in a companion paper(14). It was s Present address: Department of Materials, Imperial College, London SW7 2BP, England 1359-6462/01/S-see front matter. c 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PI:S1359-6462(0000601-1
RELIABILITY OF THE CHEVRON NOTCH TECHNIQUE FOR FRACTURE TOUGHNESS DETERMINATION IN GLASS COMPOSITES REINFORCED BY CONTINUOUS FIBRES I. Dlouhy´ 1 and A.R. Boccaccini2 * 1 Institute of Physics of Materials, CZ-61662 Brno, Czech Republic 2 Institute of Materials Technology, Ilmenau University of Technology, D-98684 Ilmenau, Germany (Received July 18, 2000) (Accepted in revised form August 25, 2000) Keywords: Composites; Glass composite; Fracture; Fracture toughness; Chevron notch 1. Introduction In ceramics and glasses reinforced by ceramic fibres, elastic fibre bridging and fibre pull-out mechanisms mainly cause the toughening. Both these mechanisms increase in some extent behind the crack tip along the process zone wake (1–3). Thus, the crack growth resistance rises as the crack propagates and leaves the wake. In these materials, it is difficult to define the intrinsic fracture toughness (KIc) as a material parameter due to the increasing crack growth resistance curve (2). Nevertheless, an exact method of fracture behaviour quantification is needed if a further development of fibre reinforced brittle matrix composites is desired, including the assessment of their possible structural degradation in service. The chevron notched specimen technique is a well-established method used to determine the fracture toughness and the work of fracture of brittle materials (2,4–6), including particle reinforced glass matrix composites (7). As indicated elsewhere (8), however, the technique has not received wide application to measure the fracture toughness of fibre reinforced brittle matrix composites. Other specimen configurations, for example straight notched specimens (9–11), have been more extensively used in these materials. Apart from our own previous research (8,11), few reports are available on the applicability of the chevron notched specimen to fibre reinforced ceramics and glasses (12,13). Mecholsky has pointed out, however, that as long as the notch is large enough to cover many fibres at the critical crack length, the chevron notch test should represent a useful method to measure the fracture toughness and the work of fracture in fibre reinforced ceramic composites (10). In the work of Ha and Chawla (12), it was shown that using the chevron notched specimen technique the different mechanical behaviour of mullite matrix composites containing mullite fibres with different coatings could be detected. In our previous investigation (8,11), the differences in fracture behaviour of glass matrix composites after different thermal ageing conditions in non-oxidising atmosphere were detected by this technique and reliable fracture toughness data were obtained. On the basis of such previous experiences, the research was extended to samples thermally loaded under different oxidising conditions, including thermal shock and thermal cycling, as reported in a companion paper (14). It was * Present address: Department of Materials, Imperial College, London SW7 2BP, England. Scripta mater. 44 (2001) 531–537 www.elsevier.com/locate/scriptamat 1359-6462/01/$–see front matter. © 2001 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00601-1
CHEVRON NOTCH TECHNIQUE Vol 44. No. 3 TABLE 1 Properties of the Glass Matrix, Fibres and Composite(8, 11, 15) Density Youngs modulus Poi Thermal exp coeff. Tensile strength Ig/cml MatrIx DURANs Glas 3.25*10-6 Fibre sic nicalon 3.0*10-6 Composite 0.21 3.1*10-6 600=700 possible to correlate changes of Kc data, as measured by the chevron notched specimen technique, with the microstructural damage that occurred after thermal loading(14) The purpose of the present contribution is to analyse the relative suitability of the chevron notch technique for the assessment of the fracture behaviour of fibre reinforced glass matrix composites in comparison with the more frequently used straight notch technique 2. Material and Experimental Methods The material investigated was a commercially available unidirectional SiC Nicalon(NL202) fibre reinforced borosilicate(DURAN) glass matrix composite fabricated by Schott Glaswerke(Mainz, Germany ). Information on the composite constituents is given in Table 1. The composite was prepared by the sol-gel-slurry method (15). The samples were received in the form of rectangular test bars of nominal dimensions 4.5 3.8X 100 mm. The density of the composites was 2.4 g/em and their fibre volume fraction 0.4. Fairly regular fibre distribution and the absence of porosity were found by microstructural investigations, as shown elsewhere(11) The chevron-notched (CN) specimen technique was employed for fracture toughness determination (Figure la). Chevron notches with angles of 90 were cut using a thin diamond wheel. A three point bending(with span of 16 mm)at a constant cross-head speed of 0. 1 mm/min was employed. Graphs of load versus deflection were recorded and the maximum force was determined from each trace. The fracture toughness value was calculated from the maximum load(Fmax)and the corresp minimum value of geometrical compliance function(Y min). The calculation of the function Y chevron notch bend bars was based on the use of Bluhm's slice model. The procedure used for the purposes of this investigation has been described elsewhere(6). The chevron-notch depth ao was measured from optical micrographs of fractured specimens. Additionally, the straight-notched(SN)specimen technique( Figure 1b), which is recommended as a standard procedure(16), was employed for comparison. Straight notches having width of about 150 mm and depth of about 1.5 mm were cut by using a thin diamond wheel. Then the tip was sharpened by hand by polishing with a conventional razor blade and diamond paste of size of 0. I umm. Other Figure 1. Schematic diagram showing the loading geometry and fracture plane for (a) chevron notch and(b) straight notch
possible to correlate changes of KIC data, as measured by the chevron notched specimen technique, with the microstructural damage that occurred after thermal loading (14). The purpose of the present contribution is to analyse the relative suitability of the chevron notch technique for the assessment of the fracture behaviour of fibre reinforced glass matrix composites in comparison with the more frequently used straight notch technique. 2. Material and Experimental Methods The material investigated was a commercially available unidirectional SiC Nicalon (NL202) fibre reinforced borosilicate (DURAN®) glass matrix composite fabricated by Schott Glaswerke (Mainz, Germany). Information on the composite constituents is given in Table 1. The composite was prepared by the sol-gel-slurry method (15). The samples were received in the form of rectangular test bars of nominal dimensions 4.5 3 3.83 100 mm3 . The density of the composites was 2.4 g/cm3 and their fibre volume fraction 0.4. Fairly regular fibre distribution and the absence of porosity were found by microstructural investigations, as shown elsewhere (11). The chevron-notched (CN) specimen technique was employed for fracture toughness determination (Figure 1a). Chevron notches with angles of 90° were cut using a thin diamond wheel. A three point bending (with span of 16 mm) at a constant cross-head speed of 0.1 mm/min was employed. Graphs of load versus deflection were recorded and the maximum force was determined from each trace. The fracture toughness value was calculated from the maximum load (Fmax) and the corresponding minimum value of geometrical compliance function (Y* min). The calculation of the function Y* min for chevron notch bend bars was based on the use of Bluhm’s slice model. The procedure used for the purposes of this investigation has been described elsewhere (6). The chevron-notch depth a0 was measured from optical micrographs of fractured specimens. Additionally, the straight-notched (SN) specimen technique (Figure 1b), which is recommended as a standard procedure (16), was employed for comparison. Straight notches having width of about 150 mmm and depth of about 1.5 mm were cut by using a thin diamond wheel. Then the tip was sharpened by hand by polishing with a conventional razor blade and diamond paste of size of 0.1 mmm. Other TABLE 1 Properties of the Glass Matrix, Fibres and Composite (8, 11, 15) Density [g/cm3 ] Young’s modulus [GPa] Poisson’s ratio Thermal exp. coeff. [K21 ] Tensile strength [MPa] Matrix DURANt Glass 2.23 63 0.22 3.25 p 1026 60 Fibre SiC Nicalont 2.55 198 0.20 3.0 p 1026 2750 Composite 2.40 118 0.21 3.1 p1026 600–700 Figure 1. Schematic diagram showing the loading geometry and fracture plane for (a) chevron notch and (b) straight notch technique. 532 CHEVRON NOTCH TECHNIQUE Vol. 44, No. 3
Vol 44. No. 3 CHEVRON NOTCH TECHNIQUE Figure 2. Fracture surface of a straight-notched specimen showing both pull-out and partial delamination. stress intensity factor value, KIC, as used for standard pre-cracked specimens, was appliedof critical testing conditions were the same as in case of CN technique. The equation for calculation The acoustic emission technique(AE) was used during the test. Traces of cumulative number of counts(AE events)were obtained in the same time scale as the load vs. time plots. This technique allows for an accurate detection of the microcrack initiation at the chevron notch. which occurs when a sharp increase in the number of AE events is observed. Valid measurements for computing Kic are those in which this increase of AE events coincides with the end of the linear part of the force versus time trace, as explained below As important supporting technique, scanning electron microscopy(SEM) was used to investigate acture surfaces Results and discussion The load-deflection records from specimens with straight notch are typical by pop-in effects that can be observed at maximum load. The pop-ins have been assigned to the stopping of major crack propagation at fibre/matrix interfaces followed by the interface debonding and delamination. Fracto- graphic observations support this explanation, as Figure 2 shows. Delamination is observed in fracture surfaces very near the notch root and in almost all cases also on specimen side surfaces. For the chevron-notched specimen, the crack develops at about 1/2 to 2/3 of maximum load and it propagates in controlled manner up to maximum load after which unstable fracture occurs. A typical fracture surface of a chevron notched sample is shown in Figure 3. Besides the extensive fibre pull-out, typical of this kind of composites( 8), the crack propagation from the chevron notch tip is observed. As compared to Figure 2, the fracture surface of the chevron-notched specimen is uniform in the sense that the matrix fracture has taken place mainly in one plane (mode I crack) as discussed below Acoustic emission(AE), a technique that has gained application for detecting the onset of microc- racking when testing ceramic composite materials(17), was used to assess whether or not valid conditions for obtaining fracture toughness data from the Cn test were met. Typical load-deflection curves obtained from 3-point bending test of both the chevron notched and the straight-notched specimens are shown in Figure 4, together with the AE traces When a mechanical stress is applied to a composite, several fracture phenomena can occur in the material, including matrix microcracking, fibre-matrix debonding, delamination and fibre failure. For the chevron notched specimens, the acoustic emission technique showed that individual fracture events started at about 1/2 to 2/3 of the maximum load(a smooth increase in the cumulative number of AE
testing conditions were the same as in case of CN technique. The equation for calculation of critical stress intensity factor value, KIC, as used for standard pre-cracked specimens, was applied. The acoustic emission technique (AE) was used during the test. Traces of cumulative number of counts (AE events) were obtained in the same time scale as the load vs. time plots. This technique allows for an accurate detection of the microcrack initiation at the chevron notch, which occurs when a sharp increase in the number of AE events is observed. Valid measurements for computing KIC are those in which this increase of AE events coincides with the end of the linear part of the force versus time trace, as explained below. As important supporting technique, scanning electron microscopy (SEM) was used to investigate fracture surfaces. Results and Discussion The load-deflection records from specimens with straight notch are typical by pop-in effects that can be observed at maximum load. The pop-ins have been assigned to the stopping of major crack propagation at fibre/matrix interfaces followed by the interface debonding and delamination. Fractographic observations support this explanation, as Figure 2 shows. Delamination is observed in fracture surfaces very near the notch root and in almost all cases also on specimen side surfaces. For the chevron-notched specimen, the crack develops at about 1/2 to 2/3 of maximum load and it propagates in controlled manner up to maximum load after which unstable fracture occurs. A typical fracture surface of a chevron notched sample is shown in Figure 3. Besides the extensive fibre pull-out, typical of this kind of composites (8), the crack propagation from the chevron notch tip is observed. As compared to Figure 2, the fracture surface of the chevron-notched specimen is uniform in the sense that the matrix fracture has taken place mainly in one plane (mode I crack) as discussed below. Acoustic emission (AE), a technique that has gained application for detecting the onset of microcracking when testing ceramic composite materials (17), was used to assess whether or not valid conditions for obtaining fracture toughness data from the CN test were met. Typical load-deflection curves obtained from 3-point bending test of both the chevron notched and the straight-notched specimens are shown in Figure 4, together with the AE traces. When a mechanical stress is applied to a composite, several fracture phenomena can occur in the material, including matrix microcracking, fibre-matrix debonding, delamination and fibre failure. For the chevron notched specimens, the acoustic emission technique showed that individual fracture events started at about 1/2 to 2/3 of the maximum load (a smooth increase in the cumulative number of AE Figure 2. Fracture surface of a straight-notched specimen showing both pull-out and partial delamination. Vol. 44, No. 3 CHEVRON NOTCH TECHNIQUE 533
CHEVRON NOTCH TECHNIQUE Vol 44. No. 3 347071000PA igure 3. Fracture surface of chevron-notched specimen showing extensive and uniform fibre pull-out(no delamination). events as labelled by arrow I in Figure 4). Microstructural changes corresponding to this acoustic emission should be matrix microcracking and partial local interfacial decohesion. The first non-linearity observed in all samples occurred at loads close to the maximum load, and it was associated with a strong increase in the cumulative number of acoustic emission events. This increase is so high that crack propagation through the glass matrix and fibre debonding and fracture only could be responsible for this significant effect. An increase in the number of Ae events observed at the end of the linear part of the load-deflection trace(arrow 2-Figure 4), indicates that the actual crack is developed at the chevron- notch tip. In these cases, the measurements of Kic can be taken as valid, since the unstable fracture(at maximum force)occurs from a propagating crack perpendicular to the fibre axis. For straight-notched specimens, the number of AE events is higher due to the larger process volume at the notch root. The increase of the number of AE events at the stage corresponding to the first pop-in(arrow 3 in Fig. 4) is extremely high, followed by a region of very low activity. Interfacial debonding connected with delamination in larger extent could be responsible for these effects. According to fractographic observations, mixed mode crack propagation could be assigned to this behaviour(see Figure 2). After certain increase of load, the failure process(re-initiation) takes place again(arrow 4 in Fig. 4), this process is repeated in almost all cases for several times Differences have been found when comparing the fracture toughness values determined by using the straight notched specimens with those obtained from chevron notched specimens. The summary of 乙 8400 6E+004 2E+004 0E+000 deflection [ 0. 1 mm] Figure 4. Load deflection curves obtained from 3-point bending tests of chevron notched(CN)and straight notched specimens (SN), showing also the cumulative traces of AE events. The arrows indicate different fracture stages, as discussed in the text
events as labelled by arrow 1 in Figure 4). Microstructural changes corresponding to this acoustic emission should be matrix microcracking and partial local interfacial decohesion. The first non-linearity observed in all samples occurred at loads close to the maximum load, and it was associated with a strong increase in the cumulative number of acoustic emission events. This increase is so high that crack propagation through the glass matrix and fibre debonding and fracture only could be responsible for this significant effect. An increase in the number of AE events observed at the end of the linear part of the load-deflection trace (arrow 2—Figure 4), indicates that the actual crack is developed at the chevronnotch tip. In these cases, the measurements of KIC can be taken as valid, since the unstable fracture (at maximum force) occurs from a propagating crack perpendicular to the fibre axis. For straight-notched specimens, the number of AE events is higher due to the larger process volume at the notch root. The increase of the number of AE events at the stage corresponding to the first pop-in (arrow 3 in Fig. 4) is extremely high, followed by a region of very low activity. Interfacial debonding connected with delamination in larger extent could be responsible for these effects. According to fractographic observations, mixed mode crack propagation could be assigned to this behaviour (see Figure 2). After certain increase of load, the failure process (re-initiation) takes place again (arrow 4 in Fig. 4), this process is repeated in almost all cases for several times. Differences have been found when comparing the fracture toughness values determined by using the straight notched specimens with those obtained from chevron notched specimens. The summary of Figure 3. Fracture surface of chevron-notched specimen showing extensive and uniform fibre pull-out (no delamination). Figure 4. Load deflection curves obtained from 3-point bending tests of chevron notched (CN) and straight notched specimens (SN), showing also the cumulative traces of AE events. The arrows indicate different fracture stages, as discussed in the text. 534 CHEVRON NOTCH TECHNIQUE Vol. 44, No. 3
Vol 44. No. 3 CHEVRON NOTCH TECHNIQUE 555 TABLE 2 Primary Data of Fracture Toughness Determination in Fibre Reinforced Glass Matrix Composites by Straight Notch and Chevron Notch Techniques Straight notch ture load Notch depth Fract. toughness Sam- Fracture load Notch depth Fract. toughness ple KIc [MPam ple Fmax n mm 2,117 25,2 295,3 770 24,3 563,5 2.082 77 2,058 303,4 3770 24,8 285,5 25,2 6 6 283,4 6782 2,069 7 1,760 primary data is given in Table 2. In a Weibull plot the differences are evident from the shift along the Kic axis(Figure 5)showing the overestimation of toughness incurred for specimens with straight notch (KIc value of 26 19 MPamagainst 24.59 MPam" for CN technique). Not only the average KI values but also the scatter characteristic differ in remarkable extent. Only the sn data obtained from load-deflection traces having no more than one pop-in are on a level comparable with data from CN specimens The higher Kic values obtained from specimens with straight notch can be explained by the premature localisation of fracture events related to fibre-matrix delamination in a larger process zone It is a known phenomenon in long fibre reinforced brittle matrix composites that delamination may occur also in the case the crack is already running perpendicularly to the fibres. Thus, the standard matrix cracking preceding the unstable crack formation is more or less suppressed and/or it is strongly affected by delamination. The pop-ins observed during SN tests correspond well to this phenomenon, which is supported by the simultaneous strong increase of AE events(arrow 3 in Figure 4). After delamination, some time is needed for further macrocrack development. The microcracking localisation is connected with larger energy dissipation, causing the shifting of unstable(macro)crack initiation to later stages of loading. Because of the impossibility of crack length measurement during the tests, an uncertainty arises on how to obtain the real crack length values corresponding exactly to the critical conditions of load, both quantities being necessary for Kc values calculation SiC fibre/glass+° + 3.1323334 In(Kc) Figure 5. Weibull plot of Kic data from chevron and straight notch test
primary data is given in Table 2. In a Weibull plot the differences are evident from the shift along the KIC axis (Figure 5) showing the overestimation of toughness incurred for specimens with straight notch (KIC value of 26.19 MPam1/2 against 24.59 MPam1/2 for CN technique). Not only the average KIC values but also the scatter characteristic differ in remarkable extent. Only the SN data obtained from load-deflection traces having no more than one pop-in are on a level comparable with data from CN specimens. The higher KIC values obtained from specimens with straight notch can be explained by the premature localisation of fracture events related to fibre-matrix delamination in a larger process zone. It is a known phenomenon in long fibre reinforced brittle matrix composites that delamination may occur also in the case the crack is already running perpendicularly to the fibres. Thus, the standard matrix cracking preceding the unstable crack formation is more or less suppressed and/or it is strongly affected by delamination. The pop-ins observed during SN tests correspond well to this phenomenon, which is supported by the simultaneous strong increase of AE events (arrow 3 in Figure 4). After delamination, some time is needed for further macrocrack development. The microcracking localisation is connected with larger energy dissipation, causing the shifting of unstable (macro)crack initiation to later stages of loading. Because of the impossibility of crack length measurement during the tests, an uncertainty arises on how to obtain the real crack length values corresponding exactly to the critical conditions of load, both quantities being necessary for KIC values calculation. TABLE 2 Primary Data of Fracture Toughness Determination in Fibre Reinforced Glass Matrix Composites by Straight Notch and Chevron Notch Techniques Straight Notch Chrevron Notch Sample Fracture load Fmax [N] Notch depth [mm] Fract. toughness KIC [MPam1/2] Sample Fracture load Fmax [N] Notch depth [mm] Fract. toughness KIC [MPam1/2] 1 593,1 2,117 25,2 1 295,3 1,770 24,3 2 563,5 2,082 23,6 2 296,4 1,770 24,6 3 642,5 2,148 28,0 3 279,9 1,790 24,1 4 656,1 2,058 26,9 4 303,4 1,770 26,1 5 663,2 1,912 24,8 5 285,5 1,800 25,2 6 653,9 2,056 26,8 6 283,4 1,800 24,2 7 678,2 2,069 28,0 7 297,2 1,760 23,6 Figure 5. Weibull plot of KIC data from chevron and straight notch test. Vol. 44, No. 3 CHEVRON NOTCH TECHNIQUE 535
CHEVRON NOTCH TECHNIQUE Vol 44. No. 3 initial notch defaming secondary mixed m crack Figure 6. Schematics of composite failure micromechanisms for straight (left)and chevron notch technique(right) Based on fractographic observations and other results presented above, schematics showing the differences in failure micromechanisms in both specimen geometries are shown in Fig. 6. For the specimens with straight notch a mixed fracture mode was observed. Just below the notch root the propagation of minor cracks along the low energy interfaces was evident. Fracture was not entirely confined to the propagation of single crack(see Figure 2). Mixed mode crack was connected with higher deformation energy dissipation and thus the values of fracture toughness are higher when comparing to the specimens with chevron notch(Table 2) For chevron notched specimens, the failure initiates in a relative small volume near the chevron otch tip and the condition for crack propagation, without premature localisation near fibre interfaces, is more advantageous. For these specimens the crack trajectory is controlled by the notch geometry keeping its plane, at least at the stage when data for fracture toughness detern are sampled. The crack tip driving force is higher than in case of straight notch test. It is an important feature of the chevron notch that the conditions for unstable crack propagation are formed from the running crack. This is confirmed by the load-deflection curves, since the traces exhibit no sharp maximum. Thus, for Kic determination, the initial notch depth is taken for SN specimens that the major crack tip will meet a weak matrix-fibre interface and cha l The fracture initiation in CN specimens is thus connected with a comparably lower probability than trajectory along the interface. In SN specimens, on the contrary, due to a larger"process zone, some microcracks perpendicular to the major crack, i. e along fibre-matrix interfaces, may be initiated. Thus, he total deformation energy will be higher and an overestimation of fracture toughness results. The differences observed and the evidently more advantageous condition for accurate Kic measurement should lead to the preferable use of chevron notch technique for fracture toughness determination in long fibre reinforced brittle matrix composites Conclusion The work has demonstrated that the chevron-notch technique can be a reliable method to assess fracture behaviour in brittle matrix composites reinforced by continuous fibres The fracture toughness(KIc) values determined using the chevron notch technique on the SiC-fibre reinforced glass matrix composite investigated(having the mean value of 2459 MPam")are compa- rable to data in the literature obtained in similar materials. In comparison with the straight notched specimen technique, chevron notch technique offers higher accuracy for Kic determination because more reproducible conditions for crack initiation are assured. For specimens with straight notch, separate effects that may take place due to a larger"process zone, i.e. microcracking along the
Based on fractographic observations and other results presented above, schematics showing the differences in failure micromechanisms in both specimen geometries are shown in Fig. 6. For the specimens with straight notch a mixed fracture mode was observed. Just below the notch root the propagation of minor cracks along the low energy interfaces was evident. Fracture was not entirely confined to the propagation of single crack (see Figure 2). Mixed mode crack was connected with higher deformation energy dissipation and thus the values of fracture toughness are higher when comparing to the specimens with chevron notch (Table 2). For chevron notched specimens, the failure initiates in a relative small volume near the chevron notch tip and the condition for crack propagation, without premature localisation near fibre/matrix interfaces, is more advantageous. For these specimens the crack trajectory is controlled by the chevron notch geometry keeping its plane, at least at the stage when data for fracture toughness determination are sampled. The crack tip driving force is higher than in case of straight notch test. It is an important feature of the chevron notch that the conditions for unstable crack propagation are formed from the running crack. This is confirmed by the load-deflection curves, since the traces exhibit no sharp maximum. Thus, for KIC determination, the initial notch depth is taken. The fracture initiation in CN specimens is thus connected with a comparably lower probability than for SN specimens that the major crack tip will meet a weak matrix-fibre interface and change the trajectory along the interface. In SN specimens, on the contrary, due to a larger “process zone,” some microcracks perpendicular to the major crack, i. e. along fibre-matrix interfaces, may be initiated. Thus, the total deformation energy will be higher and an overestimation of fracture toughness results. The differences observed and the evidently more advantageous condition for accurate KIC measurement should lead to the preferable use of chevron notch technique for fracture toughness determination in long fibre reinforced brittle matrix composites. Conclusion The work has demonstrated that the chevron-notch technique can be a reliable method to assess fracture behaviour in brittle matrix composites reinforced by continuous fibres. The fracture toughness (KIC) values determined using the chevron notch technique on the SiC-fibre reinforced glass matrix composite investigated (having the mean value of 24.59 MPam1/2) are comparable to data in the literature obtained in similar materials. In comparison with the straight notched specimen technique, chevron notch technique offers higher accuracy for KIC determination because more reproducible conditions for crack initiation are assured. For specimens with straight notch, separate effects that may take place due to a larger “process zone,” i.e. microcracking along the Figure 6. Schematics of composite failure micromechanisms for straight (left) and chevron notch technique (right). 536 CHEVRON NOTCH TECHNIQUE Vol. 44, No. 3
Vol 44. No. 3 CHEVRON NOTCH TECHNIQUE fibre-matrix interface (mixed mode crack propagation), may affect the maximum load or overall deformation energy for fracture, leading to an overestimation of Kic values Acknowledgment The research was financially supported by grant Nr. A2041003 of the Grant Agency of the Academy of Sciences and partially by the Project of binational Czech-German collaboration Nr. TSR-007-98 supported by Ministry of Education( Czech Republic)and International Buro des bmbF(Germany) The authors gratefully acknowledge Prof. w. Beier and Mr. R. Liebald of Schott Glaswerke, Mainz, Germany, for supplying the composite samples References 1. KK. Chawla, Ceramic Matrix Composites, Chapman and Hall, London(1993) 2. T. Akatsu, E. Yasuda, and M. Sakai, Fract. Mechan Ceram. 11, 245(1996) 3. M. D. Thouless and A. G. Evans, Acta Metall, p. 517(1988) 4. P. A Withey, R. L. Brett, and P, Bowen, Mater. Sci. Technol, 8, 805(1992) 5. A Ghosh, M. G. Jenkins, and K. w. White, Ceramic Materials and Components for Engines, p 592(1989) 6. 1. Dlouhy, M. Holzmann, J. Man, and L. Valka, Metall. Mater. 32, 3(1994) 7. 1. Dlouhy, M. Reinisch, A.R. Boccaccini, and J. F. Knott, Fatigue Fract. Eng. Mater. Struct. 20, 1235(1997) 8. A.R. Boccacini, J. Janczak-Rusch, and I. Dlouhy, Mater. Chem. Phys. 53, 155(1998) 9. J.J. Brennan and K. M. Prewo, J Mater. Sci. 17, 2371(1982) 10. J. J. Mecholsky, Ceram. Bull. 65, 336(1986) 11. I Dlouhy, M. Reinisch, and A R. Boccaccini, in Fracture Mechanics of Ceramics, Moscow, in press 12. J.S. Ha and KK. Chawla, Mater. Sci. Eng. A203, 1271(1995) 13. R. Venkatesh, Mater. Sci. Eng A268, 47(1999) 14. A.R. Boccaccini, H. Kern, and I, Dlouhy, Mater. Sci Eng. A submitted 15. W. Pannhorst, Ceram. Eng. Sci. Proc. 11, 947(1990) 6. R J. Primas and R. Gstrein, Fatigue Fract. Eng. Mater. Struct. 20, 513(1997) 17. P. J. Lamicq, G. A Bernhart, M. M. Dauchier, and J G. Mace, Ceram. Bull. 65, 336(1986)
fibre-matrix interface (mixed mode crack propagation), may affect the maximum load or overall deformation energy for fracture, leading to an overestimation of KIC values. Acknowledgments The research was financially supported by grant Nr. A2041003 of the Grant Agency of the Academy of Sciences and partially by the Project of binational Czech-German collaboration Nr. TSR-007-98 supported by Ministry of Education (Czech Republic) and International Bu¨ro des BMBF (Germany). The authors gratefully acknowledge Prof. W. Beier and Mr. R. Liebald of Schott Glaswerke, Mainz, Germany, for supplying the composite samples. References 1. K. K. Chawla, Ceramic Matrix Composites, Chapman and Hall, London (1993). 2. T. Akatsu, E. Yasuda, and M. Sakai, Fract. Mechan. Ceram. 11, 245 (1996). 3. M. D. Thouless and A. G. Evans, Acta Metall. p. 517 (1988). 4. P. A. Withey, R. L. Brett, and P. Bowen, Mater. Sci. Technol, 8, 805 (1992). 5. A. Ghosh, M. G. Jenkins, and K. W. White, Ceramic Materials and Components for Engines, p. 592 (1989). 6. I. Dlouhy´, M. Holzmann, J. Man, and L. Va´lka, Metall. Mater. 32, 3 (1994). 7. I. Dlouhy´, M. Reinisch, A. R. Boccaccini, and J. F. Knott, Fatigue Fract. Eng. Mater. Struct. 20, 1235 (1997). 8. A. R. Boccacini, J. Janczak-Rusch, and I. Dlouhy´, Mater. Chem. Phys. 53, 155 (1998). 9. J. J. Brennan and K. M. Prewo, J. Mater. Sci. 17, 2371 (1982). 10. J. J. Mecholsky, Ceram. Bull. 65, 336 (1986). 11. I. Dlouhy´, M. Reinisch, and A. R. Boccaccini, in Fracture Mechanics of Ceramics, Moscow, in press. 12. J. S. Ha and K. K. Chawla, Mater. Sci. Eng. A203, 1271 (1995). 13. R. Venkatesh, Mater. Sci. Eng. A268, 47 (1999). 14. A. R. Boccaccini, H. Kern, and I. Dlouhy´, Mater. Sci. Eng. A. submitted. 15. W. Pannhorst, Ceram. Eng. Sci. Proc. 11, 947 (1990). 16. R. J. Primas and R. Gstrein, Fatigue Fract. Eng. Mater. Struct. 20, 513 (1997). 17. P. J. Lamicq, G. A. Bernhart, M. M. Dauchier, and J. G. Mace, Ceram. Bull. 65, 336 (1986). Vol. 44, No. 3 CHEVRON NOTCH TECHNIQUE 537