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Influence of predetermined delaminations on buckling and postbuckling behaviour of composite sandwich beams
with silicon carbide monofilaments. The addition of whiskers was found to increase the strength of monolithic zircon and also to increase the first matrix cracking strength of the composite. The strength of the composites remained unchanged, but the work of fracture was increased from 16 kJ m-2 to 35 kJ m-2. The increase in work of fracture is related to the decrease in interfacial shear strength measured by an indentation technique.
Microstructure property relationships in Pyrex glass composites reinforced with Nicalon fibres Bleay, S.M. and Scott, V.D. Journal o f Materials Science Vol 26 No 8 (1991) pp 2229-2239 Composites, formed by hot pressing Pyrex glass reinforced with Nicalon silicon carbide fibres, have been extensively characterized by optical, scanning electron and analytical electron microscopes. Tensile tests and measurements of the fibre/matrix bond strength have also been carried out. Large amounts of cristobalite were observed in the matrix in some cases; these lead to large differential contractions on cooling, resulting in extensive microcracking which is detrimental to the mechanical properties. The importance of the fibre/matrix interface was also observed, the presence of graphite here being related to the fibre/matrix bond strength and the mechanical properties of the composites. NMR imaging of slipcast SiC whiskerreinforced alumina Karunanithy. S. Journal o f Materials Science Vo126 No 8 (1991) pp 2169-2172 The application of NMR imaging to the detection of flaws and the observation of whisker distribution in the green state of slip-cast alumina reinforced with SiC whiskers, is described. SEM was used to examine sintered samples for comparison. High grey levels in the NMR image are shown to be related to clusters of whiskers. The detection of large open pores was also possible. Flaws smaller than the detection limit of the NMR were also observed by SEM, but these should still contribute to darker regions in the NMR image. Pressureless sintering of AIzOa--SiC whisker composites Kim, Y.-W. and Lee, J.-G. Journal o f Materials Science Vo126 No 5 (1991) pp 1316-1320 High green densities are achieved in the processing of AI2OJSiC composites by pressure casting and subsequent impregnation with aluminium nitride. Samples were sintered without pressure to a relative density of 95%. The further application of hot isostatic pressing allowed densities of >99% to be achieved. Typical flexural strengths achieved were 680 MPa, with a fracture toughness of 4.70 MPa m 1:--. Tensile creep behaviour of a fibre-reinforced SiC--SijjN4 composite Holmes, J.W. Journal o f Materials Science Vol26 No 7 (1991) pp 1808-1814 Tensile creep tests have been carried out on Si3N4 reinforced with SiC monofilaments. The tests were carried out at 1350°C in air, with stress levels from 70 to 190 MPa. Up to 150 MPa steady-state creep was observed; the rates being 2.5 × I(Y m s t at 70 MPa and 5.6 x 10-8 s ~at 150 MPa. Only tertiary creep
was observed at 190 MPa. Over the stress range examined no conclusive evidence for the acceleration of the creep rate was observed. The results obtained are compared with the published data on monolithic Si3N4.
Thermal stability of refractory materials for high temperature composite applications Wiedemeier, H. and Singh, M. Journal of Materials Science Vol 26 No 9 (1991) pp 2421-2430 Thermodynamic theory is applied to calculate the stability of Group IV, V and VI transition metal borides, carbides, nitrides and oxides at high temperatures and under different ambient conditions. The study shows the thermodynamic stability of Group IV, V and VI compounds increases with atomic number within a group and decreases with atomic number within a period. Using the computations V3B4, HfC, HfN and TiO5 are found to be the most stable in each class. Overall, refractory oxides are found to be more stable than borides, carbides and nitrides.
GENERAL Analysis of test specimens for interlaminar mode 11 fracture toughness, Part 1. Elastic laminates Chatterjee. S.N. Journal of Composite Materials Vo125 No 5 (1991) pp 460-493 Exact and approximate stress analyses were produced for two different Mode II interlaminar fracture toughness specimens: (1) thick beam specimens with implanted delaminations; and (2) end-notch flexure test specimens. Data analysis procedures for fitting to compliance curves were discussed and were found to be necessary when the delamination thickness was small compared with the laminate thickness and/or when the thickness shear modulus was small compared with the axial modulus. Analysis of test specimens for interlaminar mode 11 fracture toughness, Part 2. Effects of adhesive layers and material nonlinearities Chatterjee, S.N. Journal o f Composite Materials Vo125 No 5 (1991) pp 494-511 Using the approximate elastic stress analysis obtained in Part I for end-notch flexure test specimens, expressions were obtained for determining energy release rates from compliance curves. The effect of subcritical microcracking damage or elastoplastic interlaminar shear response was studied. A bound on the error in classical theory of laminated plates Van Kculen, F. International Journal o f Engineering Science Vol 29 No 7 (1991) pp 869882 Full 3-D solutions for laminated plates were obtained by constructing 3-D kinematically admissible displacement fields and 3-D statically admissible stress distributions. These were used to derive a bound on the error produced by using classical laminated plate theory. Examples were used to demonstrate the range of applicability of classical laminate theory. A generalized Navier's approach for solution of clamped moderately-thick cross-ply plates Kabir, H.R.H. and Chaudhuri, R.A. Corn-
posite Structures Vol 17 No 4 (1991) pp 351366 The authors have generalized Navier's approach to obtain a unique boundarycontinuous analytical solution for the problem of bending of a moderately thick, rectangular, cross-ply clamped plate. The solution is compared with classical laminate theory and the characteristics of the plate considered. Heat conduction in laminated anisotropic composites with a debonding Choi, H.J. and Thangjitham, S. International Journal o f Engineering Science Vol 29 No 7 (1991) pp 819~29 The problem of two-dimensional heat conduction in a laminated isotropic slab containing interracial debonding was solved using an extension of the fexibility matrix approach originally intended for perfectly bonded media. The solution was applied to temperature distributions in a laminated slab with a uniform temperature rise on the boundary planes and a completely insulated debonded surface. Heterogeneous medium, is an equivalent macroscopic description possible Auriault, J.L. International Journal o f Engineering Science Vol 29 No 7 (1991) pp 785796 The behaviour of a heterogeneous medium subject to an excitation can be modelled by replacing it with an equivalent homogeneous medium exhibiting the average behaviour of the medium. Most homogenizing or averaging methods produce a result whether or not the medium and/or excitation are homogenizable; i.e., there exists an equivalent macroscopic description. The author presents a method to avoid similar problems based upon a double scale method using asymptotic developments. A methodology is developed and applied to the dynamics of elastic composite materials. Influence of predetermined delaminations on buckling and postbuckling behaviour of composite sandwich beams Somers, M., Weller, T. and Abramovich, H. Composite Structures Vol 17 No 4 (1991) pp 292-329 An analytical model was developed to predict the buckling and post-buckling behaviour of composite sandwich beams with disbonding between the face plate and the less rigid core. The stability of the delamination was predicted using a Griffith fracture energy release rate model. The beams were shown to be sensitive to delaminations, buckling occurring at loads considerably less than those required for undamaged beams. If composite face plates were used, the lay-up sequence affected the load-carrying capacity of the beams containing delaminations. A plasticity-based, constitutive model for fibre-reinforced composite laminates Vaziri, R., Olson, M.D. and Anderson, D.L. Journal o f Composite Materials Vol 25 No 5 (1991) pp 512-535 A constitutive model which took into account progressive cracking and inelastic deformation was developed for single fibre-reinforced composite layers (either uni or bidirectional). Using a continuum mechanics approach these "homogeneous' layers are superimposed using classical laminate theory. Good agree-
COMPOSITES. NOVEMBER 1991
473
Microstructure property relationships in Pyrex glass composites reinforced with Nicalon fibres Bleay, S.M. and Scott, V.D. Journal o f Materials Science Vol 26 No 8 (1991) pp 2229-2239 Composites, formed by hot pressing Pyrex glass reinforced with Nicalon silicon carbide fibres, have been extensively characterized by optical, scanning electron and analytical electron microscopes. Tensile tests and measurements of the fibre/matrix bond strength have also been carried out. Large amounts of cristobalite were observed in the matrix in some cases; these lead to large differential contractions on cooling, resulting in extensive microcracking which is detrimental to the mechanical properties. The importance of the fibre/matrix interface was also observed, the presence of graphite here being related to the fibre/matrix bond strength and the mechanical properties of the composites. NMR imaging of slipcast SiC whiskerreinforced alumina Karunanithy. S. Journal o f Materials Science Vo126 No 8 (1991) pp 2169-2172 The application of NMR imaging to the detection of flaws and the observation of whisker distribution in the green state of slip-cast alumina reinforced with SiC whiskers, is described. SEM was used to examine sintered samples for comparison. High grey levels in the NMR image are shown to be related to clusters of whiskers. The detection of large open pores was also possible. Flaws smaller than the detection limit of the NMR were also observed by SEM, but these should still contribute to darker regions in the NMR image. Pressureless sintering of AIzOa--SiC whisker composites Kim, Y.-W. and Lee, J.-G. Journal o f Materials Science Vo126 No 5 (1991) pp 1316-1320 High green densities are achieved in the processing of AI2OJSiC composites by pressure casting and subsequent impregnation with aluminium nitride. Samples were sintered without pressure to a relative density of 95%. The further application of hot isostatic pressing allowed densities of >99% to be achieved. Typical flexural strengths achieved were 680 MPa, with a fracture toughness of 4.70 MPa m 1:--. Tensile creep behaviour of a fibre-reinforced SiC--SijjN4 composite Holmes, J.W. Journal o f Materials Science Vol26 No 7 (1991) pp 1808-1814 Tensile creep tests have been carried out on Si3N4 reinforced with SiC monofilaments. The tests were carried out at 1350°C in air, with stress levels from 70 to 190 MPa. Up to 150 MPa steady-state creep was observed; the rates being 2.5 × I(Y m s t at 70 MPa and 5.6 x 10-8 s ~at 150 MPa. Only tertiary creep
was observed at 190 MPa. Over the stress range examined no conclusive evidence for the acceleration of the creep rate was observed. The results obtained are compared with the published data on monolithic Si3N4.
Thermal stability of refractory materials for high temperature composite applications Wiedemeier, H. and Singh, M. Journal of Materials Science Vol 26 No 9 (1991) pp 2421-2430 Thermodynamic theory is applied to calculate the stability of Group IV, V and VI transition metal borides, carbides, nitrides and oxides at high temperatures and under different ambient conditions. The study shows the thermodynamic stability of Group IV, V and VI compounds increases with atomic number within a group and decreases with atomic number within a period. Using the computations V3B4, HfC, HfN and TiO5 are found to be the most stable in each class. Overall, refractory oxides are found to be more stable than borides, carbides and nitrides.
GENERAL Analysis of test specimens for interlaminar mode 11 fracture toughness, Part 1. Elastic laminates Chatterjee. S.N. Journal of Composite Materials Vo125 No 5 (1991) pp 460-493 Exact and approximate stress analyses were produced for two different Mode II interlaminar fracture toughness specimens: (1) thick beam specimens with implanted delaminations; and (2) end-notch flexure test specimens. Data analysis procedures for fitting to compliance curves were discussed and were found to be necessary when the delamination thickness was small compared with the laminate thickness and/or when the thickness shear modulus was small compared with the axial modulus. Analysis of test specimens for interlaminar mode 11 fracture toughness, Part 2. Effects of adhesive layers and material nonlinearities Chatterjee, S.N. Journal o f Composite Materials Vo125 No 5 (1991) pp 494-511 Using the approximate elastic stress analysis obtained in Part I for end-notch flexure test specimens, expressions were obtained for determining energy release rates from compliance curves. The effect of subcritical microcracking damage or elastoplastic interlaminar shear response was studied. A bound on the error in classical theory of laminated plates Van Kculen, F. International Journal o f Engineering Science Vol 29 No 7 (1991) pp 869882 Full 3-D solutions for laminated plates were obtained by constructing 3-D kinematically admissible displacement fields and 3-D statically admissible stress distributions. These were used to derive a bound on the error produced by using classical laminated plate theory. Examples were used to demonstrate the range of applicability of classical laminate theory. A generalized Navier's approach for solution of clamped moderately-thick cross-ply plates Kabir, H.R.H. and Chaudhuri, R.A. Corn-
posite Structures Vol 17 No 4 (1991) pp 351366 The authors have generalized Navier's approach to obtain a unique boundarycontinuous analytical solution for the problem of bending of a moderately thick, rectangular, cross-ply clamped plate. The solution is compared with classical laminate theory and the characteristics of the plate considered. Heat conduction in laminated anisotropic composites with a debonding Choi, H.J. and Thangjitham, S. International Journal o f Engineering Science Vol 29 No 7 (1991) pp 819~29 The problem of two-dimensional heat conduction in a laminated isotropic slab containing interracial debonding was solved using an extension of the fexibility matrix approach originally intended for perfectly bonded media. The solution was applied to temperature distributions in a laminated slab with a uniform temperature rise on the boundary planes and a completely insulated debonded surface. Heterogeneous medium, is an equivalent macroscopic description possible Auriault, J.L. International Journal o f Engineering Science Vol 29 No 7 (1991) pp 785796 The behaviour of a heterogeneous medium subject to an excitation can be modelled by replacing it with an equivalent homogeneous medium exhibiting the average behaviour of the medium. Most homogenizing or averaging methods produce a result whether or not the medium and/or excitation are homogenizable; i.e., there exists an equivalent macroscopic description. The author presents a method to avoid similar problems based upon a double scale method using asymptotic developments. A methodology is developed and applied to the dynamics of elastic composite materials. Influence of predetermined delaminations on buckling and postbuckling behaviour of composite sandwich beams Somers, M., Weller, T. and Abramovich, H. Composite Structures Vol 17 No 4 (1991) pp 292-329 An analytical model was developed to predict the buckling and post-buckling behaviour of composite sandwich beams with disbonding between the face plate and the less rigid core. The stability of the delamination was predicted using a Griffith fracture energy release rate model. The beams were shown to be sensitive to delaminations, buckling occurring at loads considerably less than those required for undamaged beams. If composite face plates were used, the lay-up sequence affected the load-carrying capacity of the beams containing delaminations. A plasticity-based, constitutive model for fibre-reinforced composite laminates Vaziri, R., Olson, M.D. and Anderson, D.L. Journal o f Composite Materials Vol 25 No 5 (1991) pp 512-535 A constitutive model which took into account progressive cracking and inelastic deformation was developed for single fibre-reinforced composite layers (either uni or bidirectional). Using a continuum mechanics approach these "homogeneous' layers are superimposed using classical laminate theory. Good agree-
COMPOSITES. NOVEMBER 1991
473