M.L. Antti et al. Journal of the European Ceramic Society 24(2004)565-578 aluminosilicate(AS) matrix. The results have been pre- However, in a study of edge-notched specimens of a 0/ sented in general terms in an earlier study. 4 The focus of 90 Nextel 610/AS composite it was found that the fail this study is the thermal instability of the composite ure mode changed from multiple matrix fracture at microstructure when exposed to elevated temperatures room temperature to self-similar crack growth at 950C. and in particular how this influences notch strength and This change led to a significantly increased notch sensi- notch sensitivity.Both090°and±45° orientations tivit were investigated Fibre degradation as a possible source of composite In common with polymer matrix composites, degradation should also be considered. Nextel 720 fibres matrix composites and several non-oxide CF( consist of alumina grains with an approximate diameter porous matrix oxide/oxide composites exhibit me of 0. 1 um distributed among larger (0.5 um) mullite notch sensitivity. -12 For example, centre hole plate grains consisting of many smaller subg samples with 0/90 orientation at room temperature gations of the response of the fibres to thermal exposure exhibit a moderate decrease in net section strength with are somewhat conflicting. Deleglise et al. 5 observed increasing hole diameter. The strength falls towards a significant strength degradation only above 1400C for limiting value for large hole diameters that generally lies 5 h exposure times while Milz et al. 6 observed severe between 60 and 80% of the unnotched strength. The degradation after 2 h at 1300oC. Petry and Mah" elatively good notch tolerance is associated with the report a small loss in strength after exposure for 2 h at development of an extensive zone of damage developing 1100C. The causes of degradation are also not clear; from the hole with increasing load which effectively surface grooving, structural coarsening 7 and local reduces the stress intensity at the hole. In dense matrix impurity enrichment have been suggested. 6 The con composites the damage consists of multiple tensile flicting observations could as well be due to differences cracking of the matrix combined with fibre/matrix in the fibre batches used as to experimental differences. debonding In porous matrix composites, shear damage of the matrix is thought to make a significant contribu- tion. Composites with +45 orientation are notch 2. The notch sensitivity test insensitive. 2 12 Their fracture is dominated by shear failure of the matrix. During the composite fracture As already indicated the main objective of the present process the fibre tows can separate without significant study was to investigate the degradation at high tem- fibre fracture. A function of the fibres is to aid the peratures of oxide/oxide, porous matrix composites and extension of matrix damage thereby reducing the stress in particular through its effect on the notch sensitivity intensity. The nature of the centre hole notch test is measured in terms of the tensile properties of centre discussed in more detail in Section 2 hole panels. This form of notch sensitivity test, though Reports on the thermal degradation of porous matrix, of practical relevance, is somewhat complex in inter all-oxide CFCCs are scarce. Levi et al. report, for an pretation. In an isotropic, elastically deforming solid alumina fibre(Nextel 610) reinforced mullite/alumina the maximum stress intensification at the edge of a cir composite, little change in unnotched tensile behaviour cular notch in an infinitely wide plate loaded in uniaxial after up to 100 h in air at 1200 oC. However, a 2 h tension is a factor of three regardless of hole size. 8 Thus exposure at 1300C led to reduced fracture stress and the strength of an ideally, fully notch sensitive material reduced strain to failure. The authors attributed this to will be reduced by a factor of three when such a notch is fibre degradation rather than to changes in the matrix introduced. A fully notch insensitive material will not be ind suggested that the use of the more stable Nextel 720 weakened since the stress intensification will be dis fibre should lead to improved thermal stability. Jurf and sipated, for example, by plastic deformation. In reality Butner3observed a decrease to about 70% unnotched most materials, including long-fibre reinforced compo- strength of the 0/90 material studied here after 1000 h at sites, exhibit intermediate behaviour leading to a mod- 100oC. It was suggested that the primary explanation erate loss of strength that is generally dependent on hole for the loss in strength is matrix densification rather size. The dependence on hole size arises because the than degradation of fibre strength. 3 Long-term ageing circular notch affects and interacts with the material in has also been performed by Siemens-Westinghouse. 4 its immediate vicinity, thereby altering the distribution At 1100C the unnotched strength after 100 h was of the stress intensification; the volume of affected reduced by over 25% and after 2600 h by over 55%.A material scales with the hole size. 8 An added compli heat-treatment of 3000 h at 1000C led to a decrease in cation in experimental situations is that the stress strength of about 23%. The suggested explanation of intensification of a circular hole is affected by the width the degradation was embrittlement due to densification of the tested plate, decreasing from a factor of three to a of the matrix and possibly a phase change. 4 The pre- factor of two as the ratio of hole size to plate width(a/w) sent authors are not aware of any earlier reports on the approaches 1, thus necessitating a finite width correc notch strength or notch sensitivity of such composites. tion. It is also to be noted that the stress intensificationaluminosilicate (AS) matrix. The results have been pre￾sented in general terms in an earlier study.4 The focus of this study is the thermal instability of the composite microstructure when exposed to elevated temperatures and in particular how this influences notch strength and notch sensitivity. Both 0
/90
and 45
orientations were investigated. In common with polymer matrix composites, carbon matrix composites and several non-oxide CFCCs,58 porous matrix oxide/oxide composites exhibit moderate notch sensitivity.912 For example, centre hole plate samples with 0/90 orientation at room temperature exhibit a moderate decrease in net section strength with increasing hole diameter. The strength falls towards a limiting value for large hole diameters that generally lies between 60 and 80% of the unnotched strength. The relatively good notch tolerance is associated with the development of an extensive zone of damage developing from the hole with increasing load which effectively reduces the stress intensity at the hole. In dense matrix composites the damage consists of multiple tensile cracking of the matrix combined with fibre/matrix debonding. In porous matrix composites, shear damage of the matrix is thought to make a significant contribu￾tion.2,12 Composites with 45 orientation are notch insensitive.2,12 Their fracture is dominated by shear failure of the matrix. During the composite fracture process the fibre tows can separate without significant fibre fracture. A function of the fibres is to aid the extension of matrix damage thereby reducing the stress intensity. The nature of the centre hole notch test is discussed in more detail in Section 2. Reports on the thermal degradation of porous matrix, all-oxide CFCCs are scarce. Levi et al.2 report, for an alumina fibre (Nextel 610) reinforced mullite/alumina composite, little change in unnotched tensile behaviour after up to 100 h in air at 1200
C. However, a 2 h exposure at 1300
C led to reduced fracture stress and reduced strain to failure. The authors attributed this to fibre degradation rather than to changes in the matrix and suggested that the use of the more stable Nextel 720 fibre should lead to improved thermal stability. Jurf and Butner13 observed a decrease to about 70% unnotched strength of the 0/90 material studied here after 1000 h at 1100
C. It was suggested that the primary explanation for the loss in strength is matrix densification rather than degradation of fibre strength.13 Long-term ageing has also been performed by Siemens-Westinghouse.14 At 1100
C the unnotched strength after 100 h was reduced by over 25% and after 2600 h by over 55%. A heat-treatment of 3000 h at 1000
C led to a decrease in strength of about 23%. The suggested explanation of the degradation was embrittlement due to densification of the matrix and possibly a phase change.14 The pre￾sent authors are not aware of any earlier reports on the notch strength or notch sensitivity of such composites. However, in a study of edge-notched specimens of a 0/ 90 Nextel 610/AS composite it was found that the fail￾ure mode changed from multiple matrix fracture at room temperature to self-similar crack growth at 950
C. This change led to a significantly increased notch sensi￾tivity.11 Fibre degradation as a possible source of composite degradation should also be considered. Nextel 720 fibres consist of alumina grains with an approximate diameter of 0.1 mm distributed among larger (0.5 mm) mullite grains consisting of many smaller subgrains.15 Investi￾gations of the response of the fibres to thermal exposure are somewhat conflicting. Dele´glise et al.15 observed significant strength degradation only above 1400
C for 5 h exposure times while Milz et al.16 observed severe degradation after 2 h at 1300
C. Petry and Mah17 report a small loss in strength after exposure for 2 h at 1100
C. The causes of degradation are also not clear; surface grooving, structural coarsening17 and local impurity enrichment have been suggested.16 The con- flicting observations could as well be due to differences in the fibre batches used as to experimental differences. 2. The notch sensitivity test As already indicated the main objective of the present study was to investigate the degradation at high tem￾peratures of oxide/oxide, porous matrix composites and in particular through its effect on the notch sensitivity measured in terms of the tensile properties of centre hole panels. This form of notch sensitivity test, though of practical relevance, is somewhat complex in inter￾pretation. In an isotropic, elastically deforming solid, the maximum stress intensification at the edge of a cir￾cular notch in an infinitely wide plate loaded in uniaxial tension is a factor of three regardless of hole size.18 Thus the strength of an ideally, fully notch sensitive material will be reduced by a factor of three when such a notch is introduced. A fully notch insensitive material will not be weakened since the stress intensification will be dis￾sipated, for example, by plastic deformation. In reality most materials, including long-fibre reinforced compo￾sites, exhibit intermediate behaviour leading to a mod￾erate loss of strength that is generally dependent on hole size. The dependence on hole size arises because the circular notch affects and interacts with the material in its immediate vicinity, thereby altering the distribution of the stress intensification; the volume of affected material scales with the hole size.18 An added compli￾cation in experimental situations is that the stress intensification of a circular hole is affected by the width of the tested plate, decreasing from a factor of three to a factor of two as the ratio of hole size to plate width (a/w) approaches 1,19 thus necessitating a finite width correc￾tion. It is also to be noted that the stress intensification 566 M.-L. Antti et al. / Journal of the European Ceramic Society 24 (2004) 565–578