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The influence of fibre discontinuities on the stress-strain behaviour of composites
Abstracts Of journals Prepared by P. Curtis, K. Phillips, P. Manders and 7-. Feest
CARBON MA TRICES
Biomedical applications of carbon fibre reinforced carbon in implanted prostheses Jenkins, G.M. and De Carvalho, F.X. Carbon Vol 15 1977pp 33-37 Unidirectional carbon fibre-reinforced carbon rods have been fabricated by a method involving successive impregnations of carbon fibres with a phenolic resin, each impregnation being followed by a heat treatment. Two applications of these rods are described; (i) as pins for bone stretching where the rapid tissue in growth and good tissue adhesion without inflamation or rejection are an advantage over similar metal components, and (ii) as retaining pins for an all-carbon heart valve disc which exploits the low probability of carbon inducing blood clotting (thromboembolism).
monomer, which was then polymerised thermally using a free radical initiation. The results of compressive and flexural tests on these composites are presented and the fracture behaviour is examined by scanning electron microscopy.
effective in demonstrating a concentric three zone skin/sheath/core structure. Both transverse and longitudinal sections of fibres subjected to different heat treatment temperatures and oxidation times are examined, and a mechanism is proposed to explain the etching phenomenon.
CERAMIC MA TRICES
Evaluation of relative wettability of carbon fibres Aggarwal, R.K. Carbon Vo115 (1977) pp 291-293 A theory is deveolped which relates wicking rate of a liquid into a fibre bundle to contact angle, and is then applied to experimental data for two pan-based carbon fibres of different origin. Relative contact angles are obtained for the furfuryl alcohol impregnant on the two fibre types.
Effect of SiC content and orientation on the properties of Si/SiC ceramic composites Mehan, R.L. Journal of Materials Science Vol 13No 2 1978 pp 358-366 Silicon/silicon carbide ceramic composites were made by infiltrating carbonaceous material with liquid silicon to form SiC crystallites dispersed in a silicon matrix. The variation of density, elastic modulus, strength and fracture toughness is reported as a function of the volume fraction and distribution of the SiC.
CEMENT MA TRICES
GENERA L
Explosive testing of fibre reinforced concrete Robins, P.J. and Calderwood, R.W. Concrete Vo112 No 1 January 1978 pp 26-28 Concrete slabs reinforced with steel and polypropylene fibres were subjected to explosive tests using a high speed cine camera to record the damage. The degree of spalling is significantly reduced by the presence of fibres and the polypropylene fibres are particularly effective in retaining the concrete fragments.
Anisotropic photoelasticity with application to composites Hahn, H.T. and Morris, D.H. Fibre Science and Technology Vo111 No 2 (March 19 78) pp 113-125 A formulation of photoelasticity equations for composite materials using the refraction tensor is presented. A distinction is made between optical behaviour and elastic behaviour in the derivation of the equations. The paper also discusses the additive property of the refraction tensor, analysis of residual fringes, derivation of stressproportioning models and symmetry of stress/optic and strain/optic relations for composites.
Fracture morphology and mechanical properties of thermocatalytically polymerised MMA-impregnated mortar Munoz-Escalona, A. and Ramos, C. Journal o f Materials Science Vol 13 No 2 1978 pp 301-310 Mortar mixes with different water/ cement ratios and consistencies were impregnated with methyl-methacrylate
COMPOSITES. OCTOBER 1978
Carbon fibre structure by electrolytic etching Manders, P.W. Nature Vo1271 (12th January 1978)pp 142-143 An electrolytic oxidation etching technique employing a 2 M sulphuric acid bath is described and shown to be
High temperature compatability of carbon fibres with nickel Warren, R., Anderson, C.H. and Carlsson, M. Journal of Materials Science Fo113 No 1 (1978) pp 178-188 The elevated temperature degradation of nickel coated carbon fibres is shown to be independent of the nickel coating process. Resistance to recrystallisation is however related to the fibre type and increases in the order HTS-panbased, HM-pan-based and HM-rayonbased. Recrystallisation of HTS fibres can be explained by a simple diffusion controlled dissolution and reprecipitation model. The HM fibres behaviour suggests a stable surface layer between 0.1 and 0.5/am thick, but it is sensitive to the presence of air during heat-treatment. The influence of fibre discontinuities on the stress-strain behaviour of composites Schultrich, B., Pompe, W. and Weiss, H.-J. Fibre Science and Technology Vo111 No 1 (January 1978) pp 1-18 By considering a short fibre-reinforced composite in terms of regular arrays of plates in a ductile matrix, attempts are made to calculate the stress/strain curve for the material. Other
283
parameters, such as stress variation along the fibre, Young's modulus and yield stress are also calculated as functions of the material and structural parameters of the composites. It is suggested that overlap of the fibres may significantly affect the properties. An orthotropic sandwich plate containing a part-through crack under mixed mode deformation Ko, W.L Engineering Fracture Mechanics Vol 10 p 15 (1978) A laminated structure consisting of two dissimilar orthotropic bonded plates, one containing a part through crack, is analysed for the case of mixed mode deformation. Basic equations are deduced for calculating the stress intensity factors and the boundary of the debonded zone which develops from the cracked plate. Non-Hookean behaviour in the fibre direction of carbon fibre composites and the influence of fibre waviness on the tensile properties Van Dreumel, W.H.M. and Kamp, J.L.M. Journal of Composite Materials 11 (October 1977) pp 461-469 Stress/strain curves of unidirectional carbon fibre composites are found to show an increasing modulus with stress, contrary to results normally quoted in the literature, and an empirical relationship is developed to describe the behaviour. No significant influence of fibre misalignment on modulus or uts is detected, but Poisson's ratio is observed to vary from 0.28 to 0.55 over the range of fibre waviness investigated. A preliminary investigation of pan based carbon fibre surfaces by flow microcalorimetry Rand, B. and Robinson, R. Carbon Vo115 (1977) pp 311-315 The adsorption of n-butylamine, n-butyric acid, n-butanol, and 1.2 epoxy propane from n-heptane solutions onto pan-based carbon fibres has been investigated by flow microcalorimetry. The n-butylamine irreversible exotherm is suggested as a semiquantitative estimate of surface acidity, and shows surface acidity to be increased by the manufacturers' surface treatment, and more so by oxidation in HNO3. The proportion of edge, or active, surface occupied by acid groups is greater on HNO3-treated than manufacturer-treated fibres.
284
Shear stress correction factors in hybrid composite material beams Miller, A.K., Adams, D.F. and Mentock, W.A. Materials Science and Engineering Vol 33No 1 (April 1978) pp 81-90 A mechanics of materials approach previously developed elsewhere is used to calculate values of the static shear stress correction factors (K1) for hybrid, fibre-reinforced composite materials. The values of K1 thus obtained are then used to derive values of deflections and strain energies for the proposed hybrid materials. These latter data do not agree with previously obtained experimental values for deflection and strain energies; it is suggested that the discrepancies may be due to stress concentrations at the loading and support points of the hybrid beam originally tested.
The surface composition and energetics of type 'A' graphite fibers Drzal, L.T. Carbon Vo115 (1977) pp 129-138 A variety of techniques, (thermal desoption, Krypton adsorption, thermodynamics, ion scattering spectroscopy, and positive secondary ion mass spectroscopy) were used to characterise the surfaces of both treated and untreated pan-based carbon fibres. Comparison of the two fibre types shows the manufacturer's surface treatment to enhance surface energetics over 30% of the surface area, and to increase the surface adsorbed material by a factor of eight. This surface adsorbed material may be removed below 200°C and may contribute to voiding in a composite. Both fibre types are shown to possess significant quantities of sodium on the surface.
Stresses in adhesive joints due to moisture and temperature Weitsman, Y. Journal of Composite Materials Vol 11 (October 19 77) p378 An aluminium/epoxy resin joint is analysed using an appropriate variational approach and tensile and interlaminar shear stresses evaluated across the joint as a function of exposure time and joint thickness. Moisture sorption is described by Fick's diffusion law and it is assumed that the only effect of both moisture and temperature is to induce dilation strains in the adhesive layers. All the stresses are found to be essentially confined to regions near the free edge and in some cases are quite significant. The peak tensile normal stress is found to be slightly inside the joint edge.
Transverse diffusivity of dual phase composites Ko, W.L. Fibre Science and Technology Vol 11 No 2 (March 1978J pp 157-162 Exact and approximate theories developed for predicting the transverse diffusivity of dual phase composite systems (containing a rectangular lattice of uniform parallel circular cylindrical inclusions) are compared. The two most commonly-used approximate theories used in estimating diffusivity (or conductivity) - parallel/series and series/parallel are also described in detail.
Surface characteristics of carbon fibres from pan Rand, B. and Robinson, R. Carbon 1Iol 15 (1977) pp 257-263 High modulus pan-based carbon fibre surfaces have been investigated using N2 adsorption isotherms to elucidate physical characteristics, and 02 chemisorption to determine the active surface area. Untreated fibres, fibres treated by the manufacturer, and fibres oxidised in HNO3 are compared, and mechanical properties resulting from the treatments presented. The surface treatments increase active surface areas by factors of 3-30, but have little effect on surface area. Both treatments increase the proportion of edge, or active, sites.
METALLIC MA TRICES
The effects of microstructure on the fatigue of Co, Cr, Ni-TaC eutectic alloys Koburger, C.W., Duquette, D.J. and Stoloff, N.S. Acta Metallurgica Vo126 (1978) pp 81-91 The effect of various treatments on the fatigue life of this directionally solidified alloy are investigated. At low stresses random single fibre fracture is observed, but at high stresses a sharp drop in fatigue life resulted from gross fibre fracture. Rapid growth rates produced a finer fibrous structure and a significant increase in fatigue resistance, apparently by limiting dislocation pile ups between fibres and reducing the number of fibre fractures. Precipitation of matrix carbides increased fatigue life by inhibiting crack propagation and
COMPOSITES . OCTOBER
1978
CARBON MA TRICES
Biomedical applications of carbon fibre reinforced carbon in implanted prostheses Jenkins, G.M. and De Carvalho, F.X. Carbon Vol 15 1977pp 33-37 Unidirectional carbon fibre-reinforced carbon rods have been fabricated by a method involving successive impregnations of carbon fibres with a phenolic resin, each impregnation being followed by a heat treatment. Two applications of these rods are described; (i) as pins for bone stretching where the rapid tissue in growth and good tissue adhesion without inflamation or rejection are an advantage over similar metal components, and (ii) as retaining pins for an all-carbon heart valve disc which exploits the low probability of carbon inducing blood clotting (thromboembolism).
monomer, which was then polymerised thermally using a free radical initiation. The results of compressive and flexural tests on these composites are presented and the fracture behaviour is examined by scanning electron microscopy.
effective in demonstrating a concentric three zone skin/sheath/core structure. Both transverse and longitudinal sections of fibres subjected to different heat treatment temperatures and oxidation times are examined, and a mechanism is proposed to explain the etching phenomenon.
CERAMIC MA TRICES
Evaluation of relative wettability of carbon fibres Aggarwal, R.K. Carbon Vo115 (1977) pp 291-293 A theory is deveolped which relates wicking rate of a liquid into a fibre bundle to contact angle, and is then applied to experimental data for two pan-based carbon fibres of different origin. Relative contact angles are obtained for the furfuryl alcohol impregnant on the two fibre types.
Effect of SiC content and orientation on the properties of Si/SiC ceramic composites Mehan, R.L. Journal of Materials Science Vol 13No 2 1978 pp 358-366 Silicon/silicon carbide ceramic composites were made by infiltrating carbonaceous material with liquid silicon to form SiC crystallites dispersed in a silicon matrix. The variation of density, elastic modulus, strength and fracture toughness is reported as a function of the volume fraction and distribution of the SiC.
CEMENT MA TRICES
GENERA L
Explosive testing of fibre reinforced concrete Robins, P.J. and Calderwood, R.W. Concrete Vo112 No 1 January 1978 pp 26-28 Concrete slabs reinforced with steel and polypropylene fibres were subjected to explosive tests using a high speed cine camera to record the damage. The degree of spalling is significantly reduced by the presence of fibres and the polypropylene fibres are particularly effective in retaining the concrete fragments.
Anisotropic photoelasticity with application to composites Hahn, H.T. and Morris, D.H. Fibre Science and Technology Vo111 No 2 (March 19 78) pp 113-125 A formulation of photoelasticity equations for composite materials using the refraction tensor is presented. A distinction is made between optical behaviour and elastic behaviour in the derivation of the equations. The paper also discusses the additive property of the refraction tensor, analysis of residual fringes, derivation of stressproportioning models and symmetry of stress/optic and strain/optic relations for composites.
Fracture morphology and mechanical properties of thermocatalytically polymerised MMA-impregnated mortar Munoz-Escalona, A. and Ramos, C. Journal o f Materials Science Vol 13 No 2 1978 pp 301-310 Mortar mixes with different water/ cement ratios and consistencies were impregnated with methyl-methacrylate
COMPOSITES. OCTOBER 1978
Carbon fibre structure by electrolytic etching Manders, P.W. Nature Vo1271 (12th January 1978)pp 142-143 An electrolytic oxidation etching technique employing a 2 M sulphuric acid bath is described and shown to be
High temperature compatability of carbon fibres with nickel Warren, R., Anderson, C.H. and Carlsson, M. Journal of Materials Science Fo113 No 1 (1978) pp 178-188 The elevated temperature degradation of nickel coated carbon fibres is shown to be independent of the nickel coating process. Resistance to recrystallisation is however related to the fibre type and increases in the order HTS-panbased, HM-pan-based and HM-rayonbased. Recrystallisation of HTS fibres can be explained by a simple diffusion controlled dissolution and reprecipitation model. The HM fibres behaviour suggests a stable surface layer between 0.1 and 0.5/am thick, but it is sensitive to the presence of air during heat-treatment. The influence of fibre discontinuities on the stress-strain behaviour of composites Schultrich, B., Pompe, W. and Weiss, H.-J. Fibre Science and Technology Vo111 No 1 (January 1978) pp 1-18 By considering a short fibre-reinforced composite in terms of regular arrays of plates in a ductile matrix, attempts are made to calculate the stress/strain curve for the material. Other
283
parameters, such as stress variation along the fibre, Young's modulus and yield stress are also calculated as functions of the material and structural parameters of the composites. It is suggested that overlap of the fibres may significantly affect the properties. An orthotropic sandwich plate containing a part-through crack under mixed mode deformation Ko, W.L Engineering Fracture Mechanics Vol 10 p 15 (1978) A laminated structure consisting of two dissimilar orthotropic bonded plates, one containing a part through crack, is analysed for the case of mixed mode deformation. Basic equations are deduced for calculating the stress intensity factors and the boundary of the debonded zone which develops from the cracked plate. Non-Hookean behaviour in the fibre direction of carbon fibre composites and the influence of fibre waviness on the tensile properties Van Dreumel, W.H.M. and Kamp, J.L.M. Journal of Composite Materials 11 (October 1977) pp 461-469 Stress/strain curves of unidirectional carbon fibre composites are found to show an increasing modulus with stress, contrary to results normally quoted in the literature, and an empirical relationship is developed to describe the behaviour. No significant influence of fibre misalignment on modulus or uts is detected, but Poisson's ratio is observed to vary from 0.28 to 0.55 over the range of fibre waviness investigated. A preliminary investigation of pan based carbon fibre surfaces by flow microcalorimetry Rand, B. and Robinson, R. Carbon Vo115 (1977) pp 311-315 The adsorption of n-butylamine, n-butyric acid, n-butanol, and 1.2 epoxy propane from n-heptane solutions onto pan-based carbon fibres has been investigated by flow microcalorimetry. The n-butylamine irreversible exotherm is suggested as a semiquantitative estimate of surface acidity, and shows surface acidity to be increased by the manufacturers' surface treatment, and more so by oxidation in HNO3. The proportion of edge, or active, surface occupied by acid groups is greater on HNO3-treated than manufacturer-treated fibres.
284
Shear stress correction factors in hybrid composite material beams Miller, A.K., Adams, D.F. and Mentock, W.A. Materials Science and Engineering Vol 33No 1 (April 1978) pp 81-90 A mechanics of materials approach previously developed elsewhere is used to calculate values of the static shear stress correction factors (K1) for hybrid, fibre-reinforced composite materials. The values of K1 thus obtained are then used to derive values of deflections and strain energies for the proposed hybrid materials. These latter data do not agree with previously obtained experimental values for deflection and strain energies; it is suggested that the discrepancies may be due to stress concentrations at the loading and support points of the hybrid beam originally tested.
The surface composition and energetics of type 'A' graphite fibers Drzal, L.T. Carbon Vo115 (1977) pp 129-138 A variety of techniques, (thermal desoption, Krypton adsorption, thermodynamics, ion scattering spectroscopy, and positive secondary ion mass spectroscopy) were used to characterise the surfaces of both treated and untreated pan-based carbon fibres. Comparison of the two fibre types shows the manufacturer's surface treatment to enhance surface energetics over 30% of the surface area, and to increase the surface adsorbed material by a factor of eight. This surface adsorbed material may be removed below 200°C and may contribute to voiding in a composite. Both fibre types are shown to possess significant quantities of sodium on the surface.
Stresses in adhesive joints due to moisture and temperature Weitsman, Y. Journal of Composite Materials Vol 11 (October 19 77) p378 An aluminium/epoxy resin joint is analysed using an appropriate variational approach and tensile and interlaminar shear stresses evaluated across the joint as a function of exposure time and joint thickness. Moisture sorption is described by Fick's diffusion law and it is assumed that the only effect of both moisture and temperature is to induce dilation strains in the adhesive layers. All the stresses are found to be essentially confined to regions near the free edge and in some cases are quite significant. The peak tensile normal stress is found to be slightly inside the joint edge.
Transverse diffusivity of dual phase composites Ko, W.L. Fibre Science and Technology Vol 11 No 2 (March 1978J pp 157-162 Exact and approximate theories developed for predicting the transverse diffusivity of dual phase composite systems (containing a rectangular lattice of uniform parallel circular cylindrical inclusions) are compared. The two most commonly-used approximate theories used in estimating diffusivity (or conductivity) - parallel/series and series/parallel are also described in detail.
Surface characteristics of carbon fibres from pan Rand, B. and Robinson, R. Carbon 1Iol 15 (1977) pp 257-263 High modulus pan-based carbon fibre surfaces have been investigated using N2 adsorption isotherms to elucidate physical characteristics, and 02 chemisorption to determine the active surface area. Untreated fibres, fibres treated by the manufacturer, and fibres oxidised in HNO3 are compared, and mechanical properties resulting from the treatments presented. The surface treatments increase active surface areas by factors of 3-30, but have little effect on surface area. Both treatments increase the proportion of edge, or active, sites.
METALLIC MA TRICES
The effects of microstructure on the fatigue of Co, Cr, Ni-TaC eutectic alloys Koburger, C.W., Duquette, D.J. and Stoloff, N.S. Acta Metallurgica Vo126 (1978) pp 81-91 The effect of various treatments on the fatigue life of this directionally solidified alloy are investigated. At low stresses random single fibre fracture is observed, but at high stresses a sharp drop in fatigue life resulted from gross fibre fracture. Rapid growth rates produced a finer fibrous structure and a significant increase in fatigue resistance, apparently by limiting dislocation pile ups between fibres and reducing the number of fibre fractures. Precipitation of matrix carbides increased fatigue life by inhibiting crack propagation and
COMPOSITES . OCTOBER
1978