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The microindentation hardness behaviour of carbon filaments, glassy carbons and pyrolytic graphites
al flaw spectrum on the gauge length conclusion. The results demonstrate that, even when failure is due to a single flaw spectrum, the Weibull m parameter is not constant over a gauge length range of 40 times the shortest length. When failure is due to overlapping flaw distributions the possibility of extrapolations assuming constant Weibull parameters is even more remote. The constant gauge length data was used in a modified form of the Poisson distribution proposed by Phoenix to compare the extrapolative ability of the Weibull distribution.
An explanation of the room temperature upper and lower yield points of many amorphous polymers is offered in terms of multiphase composite yielding behaviour. This involves consideration of the lengths and numbers of primary bonds which are the molecular chains and the secondary bonds which are the side chains. It is proposed that the upper yield point corresponds to the statistical relaxation of the maximum number of secondary bonds. The effects of necking and strain hardening are discussed in relation to yielding behaviour.
The influence of oxygen on the chemical reactions during stabilization of PAN as carbon fibre precursor Fitzer, E. and Miiller, D. J. Carbon, Vol 13 (Feb 1975) pp 63-69 DTA measurements were used for studies of the kinetics of the cyclisation and oxidation of PAN during the thermal treatment in air and nitrogen. The cyclisation is a first order reaction with an activation energy of about 30 kcal/mole in nitrogen and about 34 kcal/mole in air. For copolymer pan (5% methacrylate) lower activation energies were found. Oxygen promotes the initiation of the cyclisation but does not catalyse the cyclisation reaction itself. The sequence of cylisation and oxidation reactions is discussed in detail.
On approximations for strength of random fibre composites Hahn, H. T. Journal of Composite Materials, Vol 9 (Oct 1975) pp 316326 It is shown that the combination of the gradual failure model and the rule of mixtures for modulus leads to the rules of mixtures for the strength of a random fibre composite. For analysis purposes, the random composite is replaced by a laminate consisting of unidirectional plies in every direction in the plane of the laminate. When the maximum stress criterion is employed, the strength of the random composite is given in terms of the uniaxial strengths of the unidirectional composite through a simple relation. Other possible modes of failure are also discussed and the corresponding equations for prediction of strengths are given.
The microindentation hardness behaviour of carbon filaments, glassy carbons and pyrolytic graphites Hawthorne, H. M. Carbon, Vol 13 (June 1975) pp 215-223 The microindentation hardness behaviour of carbon filaments has been studied by indenting a Vickers diamond into the sides and end-sections of a range of large, pitch-based filaments. Both side and end-measured hardnesses of filaments heat treated to approx 2700°C decrease considerably with increasing filament anisotropy. Calculations of yield strengths from the measured hardnesses indicate that while the Marsh microplastic concept of fracture may have some validity for carbon filament compression failures, it may not be relevant to tensile fracture behaviour of oriented filaments.
Microscopic composite mechanism on upper and lower yield points of amorphous polymer solids Ito, K., Kasajima, M., Ogawa, Y. and Ojima, T. Fibre Science & Technology, Vol 8, No 3 (July 1975)
202
Prediction of transverse strength and scatter in test data for unidirectional composites Greszczuk, L. B. Conference on Composite Reliability, ASTM STP 580 (American Society for Testing and Materials, 1975) pp 311-326 An approximate theory is presented for predicting the transverse tensile strength of composites from the properties of constituents and composite microstructure. The theory takes into account internal triaxial stress concentrations caused by fibres, triaxial stress concentrations caused by voids, and the interaction of stress concentrations from fibres and voids. probabilistic design of composite structures Maxwell, R. E., Toland, R. H. and Johnson, C.W. Conference on Composite Reliability, ASTM STP 580 (American Society for Testing and Materials, 1975) pp 35-53
The method of statistical trials (Monte Carlo simulation) in combination with a sensitivity analysis by variance separation is presented as an efficient design and analysis system for each specific mode of failure. The method has several advantages: (1)it provides a realistic reliability prediction for each failure mode of a composite structure; (2) it determines the material, geometrical, and manufacturing variables which have a significant role in the structure's reliability with respect to each failure mode; and (3) it determines which material, geometrical, and manufacturing variables should be controlled in order to manufacture the structure at minimum product assurance costs. Application is made to the graphiteepoxy truss tubes of NASA's applications technology satellite.
Reliability of composite zeroexpansion structures for use in orbital environment Berman, L. D. Conference on
Composite Reliability, ASTM STP 580 (American Society for Testing and ' Materials, 1975)pp 288-297 Composite sandwich structures, consisting of graphite/epoxy face sheets on an aluminium honeycomb core, were found to withstand successfully 15 000 thermal cycles between - 157 and 66°C in vacuum. In addition, exposure of the structure to 95% relative humidity resulted in a finite change in dimensions, but had no effect on the coefficient of thermal expansion. Shear property measurements of adhesives in composite material bonded joints Renton, W. J. and Vinson, J. 1L Conference on Composite Reliability, ASTM STP 580 (American Society for Testing and Materials, 19 75) pp 119132 A test specimen was designed, and procedures developed, for determining the effective shear modulus, shear proportional limit and ultimate shear strength for adhesive materials used in bonded lap joints between adherends of either metals or composite materials. The use of adhesives avoided the effects of errors arising from the use of bulk property data. Some approaches to assessing failure probabilities of redundant structures McCarthy, J. F. Jr and Orringer, O. Conference on Composite Reliability, ASTM STP 580 (American Society
COMPOSITES. JULY 1976
An explanation of the room temperature upper and lower yield points of many amorphous polymers is offered in terms of multiphase composite yielding behaviour. This involves consideration of the lengths and numbers of primary bonds which are the molecular chains and the secondary bonds which are the side chains. It is proposed that the upper yield point corresponds to the statistical relaxation of the maximum number of secondary bonds. The effects of necking and strain hardening are discussed in relation to yielding behaviour.
The influence of oxygen on the chemical reactions during stabilization of PAN as carbon fibre precursor Fitzer, E. and Miiller, D. J. Carbon, Vol 13 (Feb 1975) pp 63-69 DTA measurements were used for studies of the kinetics of the cyclisation and oxidation of PAN during the thermal treatment in air and nitrogen. The cyclisation is a first order reaction with an activation energy of about 30 kcal/mole in nitrogen and about 34 kcal/mole in air. For copolymer pan (5% methacrylate) lower activation energies were found. Oxygen promotes the initiation of the cyclisation but does not catalyse the cyclisation reaction itself. The sequence of cylisation and oxidation reactions is discussed in detail.
On approximations for strength of random fibre composites Hahn, H. T. Journal of Composite Materials, Vol 9 (Oct 1975) pp 316326 It is shown that the combination of the gradual failure model and the rule of mixtures for modulus leads to the rules of mixtures for the strength of a random fibre composite. For analysis purposes, the random composite is replaced by a laminate consisting of unidirectional plies in every direction in the plane of the laminate. When the maximum stress criterion is employed, the strength of the random composite is given in terms of the uniaxial strengths of the unidirectional composite through a simple relation. Other possible modes of failure are also discussed and the corresponding equations for prediction of strengths are given.
The microindentation hardness behaviour of carbon filaments, glassy carbons and pyrolytic graphites Hawthorne, H. M. Carbon, Vol 13 (June 1975) pp 215-223 The microindentation hardness behaviour of carbon filaments has been studied by indenting a Vickers diamond into the sides and end-sections of a range of large, pitch-based filaments. Both side and end-measured hardnesses of filaments heat treated to approx 2700°C decrease considerably with increasing filament anisotropy. Calculations of yield strengths from the measured hardnesses indicate that while the Marsh microplastic concept of fracture may have some validity for carbon filament compression failures, it may not be relevant to tensile fracture behaviour of oriented filaments.
Microscopic composite mechanism on upper and lower yield points of amorphous polymer solids Ito, K., Kasajima, M., Ogawa, Y. and Ojima, T. Fibre Science & Technology, Vol 8, No 3 (July 1975)
202
Prediction of transverse strength and scatter in test data for unidirectional composites Greszczuk, L. B. Conference on Composite Reliability, ASTM STP 580 (American Society for Testing and Materials, 1975) pp 311-326 An approximate theory is presented for predicting the transverse tensile strength of composites from the properties of constituents and composite microstructure. The theory takes into account internal triaxial stress concentrations caused by fibres, triaxial stress concentrations caused by voids, and the interaction of stress concentrations from fibres and voids. probabilistic design of composite structures Maxwell, R. E., Toland, R. H. and Johnson, C.W. Conference on Composite Reliability, ASTM STP 580 (American Society for Testing and Materials, 1975) pp 35-53
The method of statistical trials (Monte Carlo simulation) in combination with a sensitivity analysis by variance separation is presented as an efficient design and analysis system for each specific mode of failure. The method has several advantages: (1)it provides a realistic reliability prediction for each failure mode of a composite structure; (2) it determines the material, geometrical, and manufacturing variables which have a significant role in the structure's reliability with respect to each failure mode; and (3) it determines which material, geometrical, and manufacturing variables should be controlled in order to manufacture the structure at minimum product assurance costs. Application is made to the graphiteepoxy truss tubes of NASA's applications technology satellite.
Reliability of composite zeroexpansion structures for use in orbital environment Berman, L. D. Conference on
Composite Reliability, ASTM STP 580 (American Society for Testing and ' Materials, 1975)pp 288-297 Composite sandwich structures, consisting of graphite/epoxy face sheets on an aluminium honeycomb core, were found to withstand successfully 15 000 thermal cycles between - 157 and 66°C in vacuum. In addition, exposure of the structure to 95% relative humidity resulted in a finite change in dimensions, but had no effect on the coefficient of thermal expansion. Shear property measurements of adhesives in composite material bonded joints Renton, W. J. and Vinson, J. 1L Conference on Composite Reliability, ASTM STP 580 (American Society for Testing and Materials, 19 75) pp 119132 A test specimen was designed, and procedures developed, for determining the effective shear modulus, shear proportional limit and ultimate shear strength for adhesive materials used in bonded lap joints between adherends of either metals or composite materials. The use of adhesives avoided the effects of errors arising from the use of bulk property data. Some approaches to assessing failure probabilities of redundant structures McCarthy, J. F. Jr and Orringer, O. Conference on Composite Reliability, ASTM STP 580 (American Society
COMPOSITES. JULY 1976