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Al composite
Fatigue Abstracts Fatigue crack growth and fatigue damage development in flbre.reinfm'ced
¢ompogdes under v a r i n b ~ stresses. Koterazawa, R., Nose, M., Kawai, M., Aramaki, K. and Ho]o, M. J. Soc. Mater. Sci. Jpn (Jan. 1993) 42 (472), 46-51 (in Japanese)
The fatigue crack growth behaviour of continuous fibre-reinforced composite materials was studied under variable-stress conditions. Two kinds of laminate with different ductility were employed: a hybrid laminate of glass-fibrereinforced epoxy and 7075-T6 AI alloy (GLARE1), and a plain-weave roving fabric laminate of glass-fibre-reinforced epoxy (GFRP). Fatigue tests of centre-notched (GLARE1) and double-edge-notched (GFRP) plate specimens were performed for the cases of stress-increase and strcss-decrcase, under repeated tension-tension and alternating tension-compression loading conditions. For GLARE1, acceleration and retardation of crack growth were observed under repeated tension-tension loading conditions, but not under alternating tension-compressiun loading conditions. Fatigue cracks in this material propagated only in the aluminium layer, and their growth rate was not adequately correlated with the stress intensity factor range AK. It was found that the crack growth rate was successfully correlated with an effective stress intensity factor range AKeff defined taking into account both the crackclosure and the fibre-bridging in the crack wake so that AK=~r= AKb~ AKd. The acceleration and retardation were explained using this parameter. For GFRP, on the other hand, no appreciable effect of the stress change on fatigue crack growth was detected. This was attributed to the brittleness of this laminate. The crack growth rate, in this case, was correlated well with the stress intensity factor range. Graphs, 9 ref. Fatigue behaviour of a saflil-reinforced ainminium alloy (AA6061). Levin, M.
and Karlsson, B. Composites (1993) 24 (3), 288-295 Fatigue crack growth properties of squeeze-cast AAt061 alloy reinforced with 20 vol.% of Saffil (alumina) fibres, the squeeze-cast matrix and the matrix ahoy in the form of cold-rolled sheet were studied. Both AKth.,om and AKth.cff are significantly higher in the composite than in the matrix alloys. Conversely, fibre reinforcement impairs the resistance to fatigue crack growth at higher AK where the matrix alloys are superior to the composite. The highest crack closure level was found in the composite. Quantitative fractography showed that the fibres and not the grain size control the crack path in the composite. It is shown, partly quantitatively, that crack deflection and crack branching reduce the local stress intensity factor at the crack tip, an effect that is most pronounced in the composite and in the squeeze-cast matrix. Increased stiffness and cyclic hardening of the composite over the matrix alloys further improve its resistance to near-threshold fatigue crack growth. Photomicrographs, graphs, 17 ref. Fatigue damage development in AI203/AI composite. Liu, X.C. and Bathias, C. Composites (1993) 24 (3), 282-287 Fatigue damage development in two aluminium matrix (AI-7Si-0.6Mg and AI-5Si-3Cu-IMg) composites reinforced with discontinuous A1203 fibres has been monitored by means of acoustic emission (AE). The A E signals (RMS) recorded during the tests clearly exhibit three distinct stages, which correspond to crack initiation, dominant crack formation and stable propagation. Generally speaking, the cracks initiated at a high load level form close together and a dominant crack forms easily. By contrast, at a low load, initiated cracks are widely separated and the formation of a dominant crack is difficult. If there are large defects in the composite, the first stage is absent, even at low load. In the first stage, little change in microstructure and modulus of the composite is observed; in the second, fibre fracture, interface debonding and matrix cracking occur and there are often sinusoidal cracks in the matrix; in the last stage, the principal characteristic is stable propagation of the dominant crack. The degradation of the elastic modulus of the composite in the last two stages is small. Photomicrographs, spectra, graphs, 12 ref. Microstructural influences on fatigue crack initiation in a model particulatereinforced alunflnlum alloy MMC. Lukasak, D.A. and Koss, D.A. Composites (1933) 24 (3), 262-269 Preliminary results are presented for fatigue crack initiation within a particulate-rcinforced aluminium alloy metal-matrix composite. The study examines the response of a model system consisting of Si particles in an AI-Mg-Si matrix age-hardened to the peak strength T6 temper. Experimental observations indicate two sites for crack initiation for the high stress amplitudes (~rm=, ~> 0.95 Oy~) used in this study. In specimens containing smaller Si particles (approx 4 p.m), crack initiation occurs at approx 0.44 Nf within thin ligaments of the matrix, which separate uncracked subsurface particles from the free specimen surface. In contrast, specimens containing larger Si particles (approx 8 p.m) are susceptible to crack initiation from particles which crack within 0.05 Nf. The potential contribution of thermally induced residual stresses is discussed qualitatively for the case of particles located at or near a free surface. Photomicrographs, graphs, 26 ref. Fatigue and fracture toughness of aluminium alloys reinforced with SiC and
alumina particles. Davidson, D.L. Composites (1993) 24 (3), 248-255 The mechanisms and fracture mechanics of fatigue crack initiation and the growth of small and large fatigue cracks and fracture toughness are reviewed. It is concluded that: (1) there are many factors that can affect fatigue crack initiation, some of which are understood; (2) small and large fatigue cracks can be correlated with stress intensity factor if closure is excluded; and (3) fracture toughness is mainly related to matrix plasticity but is strongly
230
Fatigue, 1994, Vol 16, April
influenced by particle characteristics. An AI 2014-T6/SiCo and an AI 6061/ alumina composite are discussed. Graphs, 34 ref. I~ma=~e a~essment and tiling of continuous ilbre-reinforcod metal.matrix
composites, lbbotson, A.R., Beevers, C.J. and Bowen, P. Composites (1993) 24 (3), 241-247. The fatigue response of a continuous silicon carbide (SCS-6) fibre-reinforced Ti-6AI-4V metal-matrix composite in the presence of a sharp precrack has been studied in single-edge-notched test-pieces in bending. Crack growth rates have been measured for different values of span-to-width ratio (s/W) at ambient temperature and at a test temperature of 550 °C in air by the use of a direct current potential difference technique. It was found that in most cases the observed crack growth rates initially decreased with increasing crack length (and hence increasing nominal applied stress intensity range). Effects of frequency on fatigue crack growth rates at 550 °C in air have also been identified. In general, crack growth rates are increased at 550 °C only at low frequencies, relative to the crack growth rates measured at ambient temperature. Based on observations to date it has been shown that fatigue cracks grown at a large span-to-width ratio propagate to failure more rapidly than cracks grown at small span-to-width ratios for equivalent initial nominal stress intensity ranges. Metallographic sections through the composite indicate that the improved fatigue life observed at low values of slW ratio may be attributable to debonding at the fibre-matrix interface, which is deduced to delay the onset of fibre failure. Photomicrographs, graphs, 21 ref.
The bending fatigue behaviour of unidirectionally reinforced SCS6/Ti-I$-3. Greaves, 1., Yates, J.R. and Atkinson, H. V. Composites (1993) 24 (3), 235-240. The fatigue behaviour of a commercial, unidircctionally reinforced metalmatrix composite, Textron SCS6/Ti-15-3 (Ti-15V-3AI-3Cr-3Sn) has been investigated. Tests were conducted on plain unnotched specimens in pure bending at room temperature, and the growth of freely initiating cracks was monitored using acetate replication. The mechanisms of crack growth in these materials are outlined and a simple model is presented to explain the failure mechanics of the composite. This is used to estimate the fatigue limit of the composite material from basic fibre and matrix properties. Photomicrographs, graphs, 8 ref. Fatigue crack propagation in SiC continuous fibre-reinforcod TI-tAI-4V alloy
metal.matrix composites. Barney, C., Beevers, C.J. and Bowen, P. Composites (1993) 24 (3), 229-234 Fatigue crack growth from a through-thickness cut notch has been studied at ambient temperature in a Ti-6A1-4V alloy matrix reinforced with Sigma (SIC) fibres. All tests have been carried out in three-point bending, and localized dominant cracks have been produced in all cases. In these composites such dominant cracks often grow off-axis, and marked effects of stress ratio on crack growth rates have been measured. At low stress ratio, the composites exhibit outstanding crack growth resistance. It has been possible to observe fatigue striations within the matrix alloy and these observations allow local crack growth rates (and hence local effective stress intensity ranges) to be determined. The implications of such studies for defect tolerance and usable stress ranges for these composites have been considered. Photomicrographs, graphs, 17 ref. Fatigue crack growth in a fibre-reinforced titanium MMC at ambient and elevated temperatures. Cotterill, P.J. and Bowen, P. Composites (1993) 24 (3), 214-221 Fatigue crack growth rates in a SCS6 fibre-reinforced Ti-15-3 (Ti-15V-3AI-3Cr-3Sn) based metal-matrix composite have been measured at a number of different temperatures, frequencies and load ratios. In specimens fatigued under a constant load range, crack growth rates decrease with increasing crack length, despite an increase in the nominal applied stress intensity range (AK). At room temperature the crack arrests after growing for < 1 mm when the initial nominal AK is 16 MPa m V2, whereas at elevated temperatures the deceleration of crack growth is interrupted by discrete events, which lead to instantaneous rises in propagation rate of an order of magnitude or greater. These events are associated with the failure of fibres, and crack arrest does not occur at elevated temperatures (200-500 °C). In general, fatigue cracks grow faster at higher temperatures, higher load ratios and lower frequencies. Photomicrographs, graphs, 16 ref. Damage mech=nismms under tensile and fatigue loading of continuous fibre-
reinforced metal-matrix composites. Schulte, K. and Minoshima, K. Composites (1993) 24 (3), 197-208 Metal-matrix composites are gaining increasing attention for structural applications. However, the database relating to their mechanical properties and microstructural characterization remains limited. Aluminium-matrix composites reinforced with SiC fibres, ct-Al203 fibres, and carbon fibres have been investigated. Continuous fibre reinforcements, unidirectional in the 0° and 90° directions, were used. Tensile and compression tests were performed using specially designed test equipment for metal-matrix composites. The best results were achieved when cylindrical hourglass-shaped specimens were used. Fatigue testing of the composites showed that a pronounced improvement in the fatigue behaviour can be achieved upon the addition of fibre reinforcement. AI-2.5Li and commercial-purity AI 1000 are used. Photomicrographs, graphs, 11 ref.
¢ompogdes under v a r i n b ~ stresses. Koterazawa, R., Nose, M., Kawai, M., Aramaki, K. and Ho]o, M. J. Soc. Mater. Sci. Jpn (Jan. 1993) 42 (472), 46-51 (in Japanese)
The fatigue crack growth behaviour of continuous fibre-reinforced composite materials was studied under variable-stress conditions. Two kinds of laminate with different ductility were employed: a hybrid laminate of glass-fibrereinforced epoxy and 7075-T6 AI alloy (GLARE1), and a plain-weave roving fabric laminate of glass-fibre-reinforced epoxy (GFRP). Fatigue tests of centre-notched (GLARE1) and double-edge-notched (GFRP) plate specimens were performed for the cases of stress-increase and strcss-decrcase, under repeated tension-tension and alternating tension-compression loading conditions. For GLARE1, acceleration and retardation of crack growth were observed under repeated tension-tension loading conditions, but not under alternating tension-compressiun loading conditions. Fatigue cracks in this material propagated only in the aluminium layer, and their growth rate was not adequately correlated with the stress intensity factor range AK. It was found that the crack growth rate was successfully correlated with an effective stress intensity factor range AKeff defined taking into account both the crackclosure and the fibre-bridging in the crack wake so that AK=~r= AKb~ AKd. The acceleration and retardation were explained using this parameter. For GFRP, on the other hand, no appreciable effect of the stress change on fatigue crack growth was detected. This was attributed to the brittleness of this laminate. The crack growth rate, in this case, was correlated well with the stress intensity factor range. Graphs, 9 ref. Fatigue behaviour of a saflil-reinforced ainminium alloy (AA6061). Levin, M.
and Karlsson, B. Composites (1993) 24 (3), 288-295 Fatigue crack growth properties of squeeze-cast AAt061 alloy reinforced with 20 vol.% of Saffil (alumina) fibres, the squeeze-cast matrix and the matrix ahoy in the form of cold-rolled sheet were studied. Both AKth.,om and AKth.cff are significantly higher in the composite than in the matrix alloys. Conversely, fibre reinforcement impairs the resistance to fatigue crack growth at higher AK where the matrix alloys are superior to the composite. The highest crack closure level was found in the composite. Quantitative fractography showed that the fibres and not the grain size control the crack path in the composite. It is shown, partly quantitatively, that crack deflection and crack branching reduce the local stress intensity factor at the crack tip, an effect that is most pronounced in the composite and in the squeeze-cast matrix. Increased stiffness and cyclic hardening of the composite over the matrix alloys further improve its resistance to near-threshold fatigue crack growth. Photomicrographs, graphs, 17 ref. Fatigue damage development in AI203/AI composite. Liu, X.C. and Bathias, C. Composites (1993) 24 (3), 282-287 Fatigue damage development in two aluminium matrix (AI-7Si-0.6Mg and AI-5Si-3Cu-IMg) composites reinforced with discontinuous A1203 fibres has been monitored by means of acoustic emission (AE). The A E signals (RMS) recorded during the tests clearly exhibit three distinct stages, which correspond to crack initiation, dominant crack formation and stable propagation. Generally speaking, the cracks initiated at a high load level form close together and a dominant crack forms easily. By contrast, at a low load, initiated cracks are widely separated and the formation of a dominant crack is difficult. If there are large defects in the composite, the first stage is absent, even at low load. In the first stage, little change in microstructure and modulus of the composite is observed; in the second, fibre fracture, interface debonding and matrix cracking occur and there are often sinusoidal cracks in the matrix; in the last stage, the principal characteristic is stable propagation of the dominant crack. The degradation of the elastic modulus of the composite in the last two stages is small. Photomicrographs, spectra, graphs, 12 ref. Microstructural influences on fatigue crack initiation in a model particulatereinforced alunflnlum alloy MMC. Lukasak, D.A. and Koss, D.A. Composites (1933) 24 (3), 262-269 Preliminary results are presented for fatigue crack initiation within a particulate-rcinforced aluminium alloy metal-matrix composite. The study examines the response of a model system consisting of Si particles in an AI-Mg-Si matrix age-hardened to the peak strength T6 temper. Experimental observations indicate two sites for crack initiation for the high stress amplitudes (~rm=, ~> 0.95 Oy~) used in this study. In specimens containing smaller Si particles (approx 4 p.m), crack initiation occurs at approx 0.44 Nf within thin ligaments of the matrix, which separate uncracked subsurface particles from the free specimen surface. In contrast, specimens containing larger Si particles (approx 8 p.m) are susceptible to crack initiation from particles which crack within 0.05 Nf. The potential contribution of thermally induced residual stresses is discussed qualitatively for the case of particles located at or near a free surface. Photomicrographs, graphs, 26 ref. Fatigue and fracture toughness of aluminium alloys reinforced with SiC and
alumina particles. Davidson, D.L. Composites (1993) 24 (3), 248-255 The mechanisms and fracture mechanics of fatigue crack initiation and the growth of small and large fatigue cracks and fracture toughness are reviewed. It is concluded that: (1) there are many factors that can affect fatigue crack initiation, some of which are understood; (2) small and large fatigue cracks can be correlated with stress intensity factor if closure is excluded; and (3) fracture toughness is mainly related to matrix plasticity but is strongly
230
Fatigue, 1994, Vol 16, April
influenced by particle characteristics. An AI 2014-T6/SiCo and an AI 6061/ alumina composite are discussed. Graphs, 34 ref. I~ma=~e a~essment and tiling of continuous ilbre-reinforcod metal.matrix
composites, lbbotson, A.R., Beevers, C.J. and Bowen, P. Composites (1993) 24 (3), 241-247. The fatigue response of a continuous silicon carbide (SCS-6) fibre-reinforced Ti-6AI-4V metal-matrix composite in the presence of a sharp precrack has been studied in single-edge-notched test-pieces in bending. Crack growth rates have been measured for different values of span-to-width ratio (s/W) at ambient temperature and at a test temperature of 550 °C in air by the use of a direct current potential difference technique. It was found that in most cases the observed crack growth rates initially decreased with increasing crack length (and hence increasing nominal applied stress intensity range). Effects of frequency on fatigue crack growth rates at 550 °C in air have also been identified. In general, crack growth rates are increased at 550 °C only at low frequencies, relative to the crack growth rates measured at ambient temperature. Based on observations to date it has been shown that fatigue cracks grown at a large span-to-width ratio propagate to failure more rapidly than cracks grown at small span-to-width ratios for equivalent initial nominal stress intensity ranges. Metallographic sections through the composite indicate that the improved fatigue life observed at low values of slW ratio may be attributable to debonding at the fibre-matrix interface, which is deduced to delay the onset of fibre failure. Photomicrographs, graphs, 21 ref.
The bending fatigue behaviour of unidirectionally reinforced SCS6/Ti-I$-3. Greaves, 1., Yates, J.R. and Atkinson, H. V. Composites (1993) 24 (3), 235-240. The fatigue behaviour of a commercial, unidircctionally reinforced metalmatrix composite, Textron SCS6/Ti-15-3 (Ti-15V-3AI-3Cr-3Sn) has been investigated. Tests were conducted on plain unnotched specimens in pure bending at room temperature, and the growth of freely initiating cracks was monitored using acetate replication. The mechanisms of crack growth in these materials are outlined and a simple model is presented to explain the failure mechanics of the composite. This is used to estimate the fatigue limit of the composite material from basic fibre and matrix properties. Photomicrographs, graphs, 8 ref. Fatigue crack propagation in SiC continuous fibre-reinforcod TI-tAI-4V alloy
metal.matrix composites. Barney, C., Beevers, C.J. and Bowen, P. Composites (1993) 24 (3), 229-234 Fatigue crack growth from a through-thickness cut notch has been studied at ambient temperature in a Ti-6A1-4V alloy matrix reinforced with Sigma (SIC) fibres. All tests have been carried out in three-point bending, and localized dominant cracks have been produced in all cases. In these composites such dominant cracks often grow off-axis, and marked effects of stress ratio on crack growth rates have been measured. At low stress ratio, the composites exhibit outstanding crack growth resistance. It has been possible to observe fatigue striations within the matrix alloy and these observations allow local crack growth rates (and hence local effective stress intensity ranges) to be determined. The implications of such studies for defect tolerance and usable stress ranges for these composites have been considered. Photomicrographs, graphs, 17 ref. Fatigue crack growth in a fibre-reinforced titanium MMC at ambient and elevated temperatures. Cotterill, P.J. and Bowen, P. Composites (1993) 24 (3), 214-221 Fatigue crack growth rates in a SCS6 fibre-reinforced Ti-15-3 (Ti-15V-3AI-3Cr-3Sn) based metal-matrix composite have been measured at a number of different temperatures, frequencies and load ratios. In specimens fatigued under a constant load range, crack growth rates decrease with increasing crack length, despite an increase in the nominal applied stress intensity range (AK). At room temperature the crack arrests after growing for < 1 mm when the initial nominal AK is 16 MPa m V2, whereas at elevated temperatures the deceleration of crack growth is interrupted by discrete events, which lead to instantaneous rises in propagation rate of an order of magnitude or greater. These events are associated with the failure of fibres, and crack arrest does not occur at elevated temperatures (200-500 °C). In general, fatigue cracks grow faster at higher temperatures, higher load ratios and lower frequencies. Photomicrographs, graphs, 16 ref. Damage mech=nismms under tensile and fatigue loading of continuous fibre-
reinforced metal-matrix composites. Schulte, K. and Minoshima, K. Composites (1993) 24 (3), 197-208 Metal-matrix composites are gaining increasing attention for structural applications. However, the database relating to their mechanical properties and microstructural characterization remains limited. Aluminium-matrix composites reinforced with SiC fibres, ct-Al203 fibres, and carbon fibres have been investigated. Continuous fibre reinforcements, unidirectional in the 0° and 90° directions, were used. Tensile and compression tests were performed using specially designed test equipment for metal-matrix composites. The best results were achieved when cylindrical hourglass-shaped specimens were used. Fatigue testing of the composites showed that a pronounced improvement in the fatigue behaviour can be achieved upon the addition of fibre reinforcement. AI-2.5Li and commercial-purity AI 1000 are used. Photomicrographs, graphs, 11 ref.