R Boccaccini et al./ Composites Science and Technology 65(2005 )325-333 energy dissipation before fracture [13]. Several mullite 2. Experimental matrix composites with oxide and non-oxide fibre rein- forcement have been developed in the last 20 years 2. 1. Material 4, 5, 8,9, 12, 14-19]. Most previous investigations have concentrated on the study of the fracture strength of The material investigated was a composite formed by the composites at room and elevated temperatures, with a mullite matrix containing homogeneously distributed less emphasis given to other properties such as thermal ultra-fine (70-350 nm) porosity reinforced with hock resistance or fracture behaviour under dynamic NdPOa-coated mullite woven fibre mats(Nextel 720) (e.g. ballistic impact) loads. Moreover, there has been The details of the material fabrication, which was done only limited work aimed at measuring the fracture by a combination of electrophoretic deposition, pressure toughness of fibre reinforced-mullite matrix composites filtration and pressureless sintering, have been published [5,19 elsewhere [26]. The typical microstructure of the com- In brittle matrices reinforced by ceramic fibres posite is shown in Fig. 1 fibre bridging and fibre pull-out mechanisms cause the toughening [13]. Both these mechanisms in- 2. 2. Fracture toughness determination crease, to some extent, behind the crack tip in the proc. ess zone wake [20, 21]. Thus, the crack growth resistance Samples in the form of rectangular test bars of nom- rises as the crack propagates and leaves the wake In inal dimensions 4.9x 1.9 x 45 mm were used for frac- these materials, it is difficult to define the intrinsic frac- ture toughness determination by means of the CN ture toughness(Klc)as a material parameter due to a ris- specimen technique. Chevron notches with angles of ing crack growth resistance curve [20]. It is not possible 90 were cut using a thin diamond wheel. A 3-point to characterise this group of materials by known meth- bending test(with span of 16 mm) at a constant cross- ods of R-curve determination either, because of major head speed of 0.01 mm/min was employed. Graphs of crack deflection along the fibre matrix interface and load versus time were recorded and the maximum force the resultant delamination instead of single was determined from each trace. The fracture toughness crack propagation. Nevertheless, an exact method of value was calculated from the maximum load (Fmax)and fracture behaviour quantification is needed, mainly for the corresponding minimum value of the geometrical comparison purposes, if a further development of fibre calibration function(Ymin). The calculation of the func reinforced-brittle matrix composites is desired, includ tion y foran bend bars was based on the use of ing the assessment of their possible structural degrada- Bluhm's slice model [27]. Details of the procedure used tion in service. In the work of Ha and Chawla [19] it for calculating Kl have been given elsewhere [28]. The was shown that using the chevron notched (CN) speci- Cn depth ao. necessary for calculating the calibration men technique the fracture toughness of mullite matrix function Ymin, was measured from optical micrograph composites containing mullite fibres with different coat- of fractured specimens. The acoustic emission technique ings could be measured. In our previous investigations, (AE) was used during the test. Traces of cumulative the CN technique has been successfully used to measure number of counts(AE events) were obtained in the same fracture toughness in SiC fibre reinforced-glass matrix time scale as the load vs time plots. This technique al- composites which have been subjected to thermal shock lows for an accurate detection of the microcrack initia and thermal ageing [22-25] tion at the Cn, which occurs when a sharp increase in la he purpose of the present work was to analyse the the number of AE events is observed. Valid measure- acture behaviour of a novel mullite fibre/mullite ma- ments for computing Klc are those in which this increase trix composite, which has been recently developed [26]. of AE events coincides with the maximum load follow- Since some preliminary characterisation of the mechan- ing the linear part of the force versus time trace, as ical properties of this composite already exists, i.e. explained below. The crack propagation in different high-temperature fracture strength and thermal cycling samples was observed by low magnification optical resistance [26]. the focus of the present study was on microscopy crack propagation behaviour under quasi-static condi- tions and on the composite macroscopic response under 2.3. Ballistic impact test and evaluation ballistic impact loading. The suitability of the CN tech nique for determination of the fracture toughness of this Ballistic impact tests were carried out by impacting fibre reinforced-mullite matrix composite was also composite tiles of dimensions 75 mm x 75 mm x 2.65 assessed. The materials response under impact loads mm with projectiles of varying velocities using a labo- was studied for low-velocity ballistic conditions using ratory gas gun. The projectiles were glass balls meas a gas gun. The remanent load carrying capability of uring 10.15 mm in diameter and weighing 1. 4 g. The composite samples after the ballistic test was measured projectile velocity range during this investigation was to quantify ballistic impact induced damage 77.6-207.5 m/s. The samples were mechanicallyenergy dissipation before fracture [13]. Several mullite matrix composites with oxide and non-oxide fibre rein￾forcement have been developed in the last 20 years [4,5,8,9,12,14–19]. Most previous investigations have concentrated on the study of the fracture strength of the composites at room and elevated temperatures, with less emphasis given to other properties such as thermal shock resistance or fracture behaviour under dynamic (e.g. ballistic impact) loads. Moreover, there has been only limited work aimed at measuring the fracture toughness of fibre reinforced–mullite matrix composites [15,19]. In brittle matrices reinforced by ceramic fibres, elastic fibre bridging and fibre pull-out mechanisms mainly cause the toughening [13]. Both these mechanisms in￾crease, to some extent, behind the crack tip in the proc￾ess zone wake [20,21]. Thus, the crack growth resistance rises as the crack propagates and leaves the wake. In these materials, it is difficult to define the intrinsic frac￾ture toughness (KIc) as a material parameter due to a ris￾ing crack growth resistance curve [20]. It is not possible to characterise this group of materials by known meth￾ods of R-curve determination either, because of major crack deflection along the fibre matrix interface and the resultant delamination process instead of single crack propagation. Nevertheless, an exact method of fracture behaviour quantification is needed, mainly for comparison purposes, if a further development of fibre reinforced–brittle matrix composites is desired, includ￾ing the assessment of their possible structural degrada￾tion in service. In the work of Ha and Chawla [19], it was shown that using the chevron notched (CN) speci￾men technique the fracture toughness of mullite matrix composites containing mullite fibres with different coat￾ings could be measured. In our previous investigations, the CN technique has been successfully used to measure fracture toughness in SiC fibre reinforced–glass matrix composites which have been subjected to thermal shock and thermal ageing [22–25]. The purpose of the present work was to analyse the fracture behaviour of a novel mullite fibre/mullite ma￾trix composite, which has been recently developed [26]. Since some preliminary characterisation of the mechan￾ical properties of this composite already exists, i.e. high-temperature fracture strength and thermal cycling resistance [26], the focus of the present study was on crack propagation behaviour under quasi-static condi￾tions and on the composite macroscopic response under ballistic impact loading. The suitability of the CN tech￾nique for determination of the fracture toughness of this fibre reinforced–mullite matrix composite was also assessed. The materials response under impact loads was studied for low-velocity ballistic conditions using a gas gun. The remanent load carrying capability of composite samples after the ballistic test was measured to quantify ballistic impact induced damage. 2. Experimental 2.1. Material The material investigated was a composite formed by a mullite matrix containing homogeneously distributed ultra-fine (70–350 nm) porosity reinforced with NdPO4-coated mullite woven fibre mats (NextelTM 720). The details of the material fabrication, which was done by a combination of electrophoretic deposition, pressure filtration and pressureless sintering, have been published elsewhere [26]. The typical microstructure of the com￾posite is shown in Fig. 1. 2.2. Fracture toughness determination Samples in the form of rectangular test bars of nom￾inal dimensions 4.9 · 1.9 · 45 mm3 were used for frac￾ture toughness determination by means of the CN specimen technique. Chevron notches with angles of 90 were cut using a thin diamond wheel. A 3-point bending test (with span of 16 mm) at a constant cross￾head speed of 0.01 mm/min was employed. Graphs of load versus time 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 the geometrical calibration function ðY
minÞ. The calculation of the func￾tion Y
min for CN bend bars was based on the use of Bluhm
s slice model [27]. Details of the procedure used for calculating KIc have been given elsewhere [28]. The CN depth a0, necessary for calculating the calibration function Y
min, was measured from optical micrographs of fractured specimens. 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 al￾lows for an accurate detection of the microcrack initia￾tion at the CN, which occurs when a sharp increase in the number of AE events is observed. Valid measure￾ments for computing KIc are those in which this increase of AE events coincides with the maximum load follow￾ing the linear part of the force versus time trace, as explained below. The crack propagation in different samples was observed by low magnification optical microscopy. 2.3. Ballistic impact test and evaluation Ballistic impact tests were carried out by impacting composite tiles of dimensions 75 mm · 75 mm · 2.65 mm with projectiles of varying velocities using a labo￾ratory gas gun. The projectiles were glass balls meas￾uring 10.15 mm in diameter and weighing 1.4 g. The projectile velocity range during this investigation was 77.6–207.5 m/s. The samples were mechanically 326 A.R. Boccaccini et al. / Composites Science and Technology 65 (2005) 325–333