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ARALL laminates take wing
This section contains abstracts of selected articles, technical reports, dissertations and patents concerned with fatigue. It is prepared in collaboration with Materials Information, a joint service of The Institute of Metals and ASM International. Readers wishing to obtain the full text of articles abstracted here should contact either: The Institute of Metals, 1 Carlton House Terrace, London S W l Y 5DB, UK, or: ASM International, Metals Park, OH 44073, USA (not International Journal of Fatigue). The fees charged for photocopying articles are £7.00 for the first ten pages and £3.00 per additional ten pages (UK office), or $10.00 for the first ten pages and $4.00 per additional ten pages (US office).
Composites
ARALL ( a r e m i d a l u m i n u m l a m i n a t e ) : a litereture survey. Bogegard, S, Report No Pb90-201542/XAB Feb. 1990 21 pp (in Swedish)
M l c r o m e c h a n i s m s o f f a t i g u e crack propagation in particulste-reinforcod m e t a l m a t r i x composites. (Dissertation). Shang, J. Diss. Abstr. Int. Apr. 1990 50, (10), 189 pp
ARALL is high-strength Kevlar composite bonded between thin sheets of a highstrength AI alloy, it combines the good fatigue and crack growth properties of Kevlar-epoxy composites with the environmental durability of AL The material is 15% less dense than monolithic AI and the tensile properties are claimed to be 15-30% better. The material can be processed like ordinary AI alloys. Commercial exploitation is expected within a few years.
The consequences of the interaction of cracks with SiC particles are examined with the emphasis on micromechanisms influencing fatigue crack propagation in highstrength AI alloy matrix composites. Fatigue crack propagation is found to show three distinct regimes; each accompanied by growth mechanisms reflecting different roles of SiC particles. At near-threshold levels, SiC particles impede fatigue crack growth by deflecting the crack to promote roughness-induced crack closure and by acting as crack traps along the crack front. A two-dimensional crack trapping analysis based on the interaction of a finite crack with an SiC particle indicates that a limiting criterion for fatigue crack growth in SiCp/AI composites can be established, which requires that the maximum plastic-zone size exceeds the effective mean particle size or that the tensile stress in the matrix beyond the particle on the crack front exceeds the yield strength of the material. Implications of crack closure and crack trapping for near-threshold crack growth, including the load-ratio and particle-size dependence of fatigue thresholds, are discussed in terms of contributions from each mechanism. At higher stress intensities, limited fracture of SiC particles ahead of the crack tip leads to the development of uncrecked ligaments along the crack length, resulting in a reduced crack-tip stress intensity from ligament bridging. Micromechanical models are developed for such bridges induced by both overlapping cracks and co-planar ligaments, based on the notion of a limiting crack opening displacement or limiting strain in the ligament. The predicted reduction in crack tip stress intensity is shown to be consistent with experimental observations. Crack path morphology in silicon carbide whisker-reinforced aluminum composite. Lee, E.U. Metal/. Trans. A J u n e 1990 21A, (6), 1783-1785 Metal matrix composites (MMC) of AI alloys reinforced with whisker or particulate silicon carbide have been studied extensively in recent years. These composites have a high specific modulus and strength, and a low coefficient of thermal expansion. In contrast to continuous-filament MMC, they can offer isotropic properties, easier fabricability and formability, and potentially low cost. However, like continuous-filament MMC, their ductility and fracture toughness are tow, they fail in a relatively brittle manner, and their fatigue behaviour is not well understood. The objective is to characterize the fatigue behaviour of a silicon carbide whiskerreinforced AI composite. The specimen material was a silicon carbide whiskerreinforced AI composite, SXA24E/20W-T8E510 (19.8 vol.% SiCw/AI-3.TCu-I.4Mg). Photomicrographs. Effects of interfacial strength on fatigue crack growth in a fiberreinforced t i t a n i u m - a l l o y composite. Chan, K.S. and Davidson, D.L. Metal/. Trans, A J u n e 1990 21A, (6), 1603-1612 The fatigue crack growth behaviour of a Ti-6AI-4V composite with boron fibres was previously studied in the as-received and thermally exposed conditions. The fracture strengths of the composite, fibre and interface were characterized together with the fatigue crack growth rates and failure mechanisms. Utilizing the matrix and fibre properties as input, a recently proposed model was exercised to elucidate the effects of interracial strength on crack growth rates in the composite, Comparison of experimental results with model calculations revealed that a weak fibre/matrix interface combined with a strong, high-modulus fibre led to interface debonding and crack deflection and produced the beneficial effects of increased threshold and reduced transverse crack growth rates. Graphs, photomicrographs. 33 refs. T h e r m a l f a t i g u e o f T i - 2 4 A I - 1 1 N b / S C S - 6 . Russ, S.M. Metal/. Trans. A J u n e 1990 21A, (6), 1595-1602 The effect of thermally cycling in air with no applied load was studied using the Ti-24AI-11Nb (at.%)/SCS-6 composite system. Mechanisms of damage determination were observable cracking and residual tensile propertieS. Either the number of cycles or the temperature range was varied from specimen to specimen. The effects of the number of cycles were investigated using • temperature range of 150-915 °C. Comparisons of temperature range effects were made at a constant cycle count of 500. Matrix cracking was observed at a &T of 450 °C and greater when oxidation was significant. Transverse cracking was not observed in specimens that were cycled to maximum temperatures where oxidation was insignificant, even for values of AT as large as 500 °C. A decrease in tensile properties coincided with the observed transverse matrix cracking. Graphs, photomicrographs. 11 refs.
86
ARALL l a m i n a t e s t a k e w i n g . Gregory, M.A. Adv. Mater. Process. Apr. 1990 137, (4), 115-116 ARALL laminates consist of a prepreg system of unidirectional aremid fibres embedded in a structural epoxy adhesive and sandwiched between layers of aircraft AI alloy sheet (2024, 7075, 7475). The mechanical properties of ARALL-3 laminates are comparable to those of the carbon fibre composite of alloy 7475-T76. The maximum service temperature of ARALL is 160 °C. Its major application is in cargodoor skins, where the circumferential stresses from cabin pressurization can be handled by the orthotropic properties of the laminate. Potential applications involve utilization of adhesive bonding of the laminates for horizontal and vertical stabilizers of the C-17 Airlifter, and its reSistance to fatigue cracking in highly stressed areas of aircraft structures around the emergency-door sills, wings and fuselage skins.
Corrosion The m e c h a n i s m o f corrosion f a t i g u e crack p r o p q a t l o n in structural steels u n d e r v a d a b l o loading. Masuda, H. and Matsuoka, S, Corros. Sci. 1990 30, ( 6 - 7 ) , 6 3 1 - 6 4 2 In order to predict the fatigue and corrosion fatigue life under variable loading conditions, the fatigue crack propagation behaviour of structural alloys was studied both in air and 3% NaCI solution by using a newly developed variable-loading &Kdecreasing test method. The test results show that an acceleration of the crack propagation occurs in air when the overload is applied in the compreSsive direction but not when in the tensile direction. Acceleration of the crack propagation also occurs in 3% NaCI aqueous solution when the overload is applied in the compressive direction. The maximum acceleration of crack propagation in 3% NaCI aqueous solution is similar to that in air under variable loading conditions when &K < 8 MPa ~/m. The mechanism of acceleration of crack propagation under variable loading conditions is discussed with regard to the deformation mode at the crack tip. Graphs. 5 refs. The effect of a p o l y m e r i c coating on t h e corrosion-fatigue o f a suction roll alloy. Hamel, F.G., Blain, J. and Masounave, J. Can. Metal/. Q. J a n . - M a r . 1990 29, (1), 8 1 - 8 6 A urethane coating is proposed as a method to extend the corrosion-fatigue durability of a suction roll KCR 171 duplex stainless-steel alloy in a white water environment. Rotating fatigue specimens with and without coating were tested in air and in white water to produce S-N curves. The results clearly show the improved durability of the coated samples, especially in the high-cycle regime. The improvement is more pronounced at low (6 Hz) than at higher (20 Hz) frequencies because the uncoated samples are more affected by the corrosive environment in low-frequency testing. Other factors such as the surface finish and white water temperature were found to have a minor influence. The concept of a protective polymeric coating for suction rolls seems attractive but additional factors such as wear resistance, permeability and adhesion need further investigation. Graphs. 23 refs. A n a l y s i n g t h e state o f t h e material o f gas pipeline= in connection with t h e i r foiluree. Scherbinin, V.E., Surkov, Y.P., Ryabalko, V.G., Sycheva, T
and Malkova, L.F. J. Cond. Monit. 1990 3, (3), 169-181
An analysis of gas pipeline failures enables one to pinpoint the major factors that have given rise to failure and also to assess the possibility of in-service status monitoring. The aforementioned factors include the state of the structure and the properties of the material used, the possibility of their degradation in the course of sufficiently protracted operation, and also the operational conditions that determine
Int J Fatigue January 1991
Composites
ARALL ( a r e m i d a l u m i n u m l a m i n a t e ) : a litereture survey. Bogegard, S, Report No Pb90-201542/XAB Feb. 1990 21 pp (in Swedish)
M l c r o m e c h a n i s m s o f f a t i g u e crack propagation in particulste-reinforcod m e t a l m a t r i x composites. (Dissertation). Shang, J. Diss. Abstr. Int. Apr. 1990 50, (10), 189 pp
ARALL is high-strength Kevlar composite bonded between thin sheets of a highstrength AI alloy, it combines the good fatigue and crack growth properties of Kevlar-epoxy composites with the environmental durability of AL The material is 15% less dense than monolithic AI and the tensile properties are claimed to be 15-30% better. The material can be processed like ordinary AI alloys. Commercial exploitation is expected within a few years.
The consequences of the interaction of cracks with SiC particles are examined with the emphasis on micromechanisms influencing fatigue crack propagation in highstrength AI alloy matrix composites. Fatigue crack propagation is found to show three distinct regimes; each accompanied by growth mechanisms reflecting different roles of SiC particles. At near-threshold levels, SiC particles impede fatigue crack growth by deflecting the crack to promote roughness-induced crack closure and by acting as crack traps along the crack front. A two-dimensional crack trapping analysis based on the interaction of a finite crack with an SiC particle indicates that a limiting criterion for fatigue crack growth in SiCp/AI composites can be established, which requires that the maximum plastic-zone size exceeds the effective mean particle size or that the tensile stress in the matrix beyond the particle on the crack front exceeds the yield strength of the material. Implications of crack closure and crack trapping for near-threshold crack growth, including the load-ratio and particle-size dependence of fatigue thresholds, are discussed in terms of contributions from each mechanism. At higher stress intensities, limited fracture of SiC particles ahead of the crack tip leads to the development of uncrecked ligaments along the crack length, resulting in a reduced crack-tip stress intensity from ligament bridging. Micromechanical models are developed for such bridges induced by both overlapping cracks and co-planar ligaments, based on the notion of a limiting crack opening displacement or limiting strain in the ligament. The predicted reduction in crack tip stress intensity is shown to be consistent with experimental observations. Crack path morphology in silicon carbide whisker-reinforced aluminum composite. Lee, E.U. Metal/. Trans. A J u n e 1990 21A, (6), 1783-1785 Metal matrix composites (MMC) of AI alloys reinforced with whisker or particulate silicon carbide have been studied extensively in recent years. These composites have a high specific modulus and strength, and a low coefficient of thermal expansion. In contrast to continuous-filament MMC, they can offer isotropic properties, easier fabricability and formability, and potentially low cost. However, like continuous-filament MMC, their ductility and fracture toughness are tow, they fail in a relatively brittle manner, and their fatigue behaviour is not well understood. The objective is to characterize the fatigue behaviour of a silicon carbide whiskerreinforced AI composite. The specimen material was a silicon carbide whiskerreinforced AI composite, SXA24E/20W-T8E510 (19.8 vol.% SiCw/AI-3.TCu-I.4Mg). Photomicrographs. Effects of interfacial strength on fatigue crack growth in a fiberreinforced t i t a n i u m - a l l o y composite. Chan, K.S. and Davidson, D.L. Metal/. Trans, A J u n e 1990 21A, (6), 1603-1612 The fatigue crack growth behaviour of a Ti-6AI-4V composite with boron fibres was previously studied in the as-received and thermally exposed conditions. The fracture strengths of the composite, fibre and interface were characterized together with the fatigue crack growth rates and failure mechanisms. Utilizing the matrix and fibre properties as input, a recently proposed model was exercised to elucidate the effects of interracial strength on crack growth rates in the composite, Comparison of experimental results with model calculations revealed that a weak fibre/matrix interface combined with a strong, high-modulus fibre led to interface debonding and crack deflection and produced the beneficial effects of increased threshold and reduced transverse crack growth rates. Graphs, photomicrographs. 33 refs. T h e r m a l f a t i g u e o f T i - 2 4 A I - 1 1 N b / S C S - 6 . Russ, S.M. Metal/. Trans. A J u n e 1990 21A, (6), 1595-1602 The effect of thermally cycling in air with no applied load was studied using the Ti-24AI-11Nb (at.%)/SCS-6 composite system. Mechanisms of damage determination were observable cracking and residual tensile propertieS. Either the number of cycles or the temperature range was varied from specimen to specimen. The effects of the number of cycles were investigated using • temperature range of 150-915 °C. Comparisons of temperature range effects were made at a constant cycle count of 500. Matrix cracking was observed at a &T of 450 °C and greater when oxidation was significant. Transverse cracking was not observed in specimens that were cycled to maximum temperatures where oxidation was insignificant, even for values of AT as large as 500 °C. A decrease in tensile properties coincided with the observed transverse matrix cracking. Graphs, photomicrographs. 11 refs.
86
ARALL l a m i n a t e s t a k e w i n g . Gregory, M.A. Adv. Mater. Process. Apr. 1990 137, (4), 115-116 ARALL laminates consist of a prepreg system of unidirectional aremid fibres embedded in a structural epoxy adhesive and sandwiched between layers of aircraft AI alloy sheet (2024, 7075, 7475). The mechanical properties of ARALL-3 laminates are comparable to those of the carbon fibre composite of alloy 7475-T76. The maximum service temperature of ARALL is 160 °C. Its major application is in cargodoor skins, where the circumferential stresses from cabin pressurization can be handled by the orthotropic properties of the laminate. Potential applications involve utilization of adhesive bonding of the laminates for horizontal and vertical stabilizers of the C-17 Airlifter, and its reSistance to fatigue cracking in highly stressed areas of aircraft structures around the emergency-door sills, wings and fuselage skins.
Corrosion The m e c h a n i s m o f corrosion f a t i g u e crack p r o p q a t l o n in structural steels u n d e r v a d a b l o loading. Masuda, H. and Matsuoka, S, Corros. Sci. 1990 30, ( 6 - 7 ) , 6 3 1 - 6 4 2 In order to predict the fatigue and corrosion fatigue life under variable loading conditions, the fatigue crack propagation behaviour of structural alloys was studied both in air and 3% NaCI solution by using a newly developed variable-loading &Kdecreasing test method. The test results show that an acceleration of the crack propagation occurs in air when the overload is applied in the compreSsive direction but not when in the tensile direction. Acceleration of the crack propagation also occurs in 3% NaCI aqueous solution when the overload is applied in the compressive direction. The maximum acceleration of crack propagation in 3% NaCI aqueous solution is similar to that in air under variable loading conditions when &K < 8 MPa ~/m. The mechanism of acceleration of crack propagation under variable loading conditions is discussed with regard to the deformation mode at the crack tip. Graphs. 5 refs. The effect of a p o l y m e r i c coating on t h e corrosion-fatigue o f a suction roll alloy. Hamel, F.G., Blain, J. and Masounave, J. Can. Metal/. Q. J a n . - M a r . 1990 29, (1), 8 1 - 8 6 A urethane coating is proposed as a method to extend the corrosion-fatigue durability of a suction roll KCR 171 duplex stainless-steel alloy in a white water environment. Rotating fatigue specimens with and without coating were tested in air and in white water to produce S-N curves. The results clearly show the improved durability of the coated samples, especially in the high-cycle regime. The improvement is more pronounced at low (6 Hz) than at higher (20 Hz) frequencies because the uncoated samples are more affected by the corrosive environment in low-frequency testing. Other factors such as the surface finish and white water temperature were found to have a minor influence. The concept of a protective polymeric coating for suction rolls seems attractive but additional factors such as wear resistance, permeability and adhesion need further investigation. Graphs. 23 refs. A n a l y s i n g t h e state o f t h e material o f gas pipeline= in connection with t h e i r foiluree. Scherbinin, V.E., Surkov, Y.P., Ryabalko, V.G., Sycheva, T
and Malkova, L.F. J. Cond. Monit. 1990 3, (3), 169-181
An analysis of gas pipeline failures enables one to pinpoint the major factors that have given rise to failure and also to assess the possibility of in-service status monitoring. The aforementioned factors include the state of the structure and the properties of the material used, the possibility of their degradation in the course of sufficiently protracted operation, and also the operational conditions that determine
Int J Fatigue January 1991