Effect of thermal residual stresses on fatigue crack opening and propagation behavior in an Al/SiCp metal matrix composite

The effects of a thermal residual stress field on fatigue crack growth in a silicon carbide particle-reinforced aluminum alloy have been measured. Stress fields were introduced into plates of material by means of a quench from a solution heat-treatment temperature. Measurements using neutron diffraction have shown that this introduces an approximately parabolic stress field into the plates, varying from compressive at the surfaces to tensile in the center. Long fatigue cracks were grown in specimens cut from as-quenched plates and in specimens which were given a stress-relieving overaging heat treatment prior to testing. Crack closure levels for these cracks were determined as a function of the position of the crack tip in the residual stress field, and these are shown to differ between as-quenched and stress-relieved samples. By monitoring the compliance of the specimens during fatigue cycling, the degree to which the residual stresses close the crack has been evaluated. © 1995 The Minerals, Metals & Material Society.

Role of reinforcement/matrix interfacial strength in fatigue crack propagation in particulate SiC reinforced aluminum alloy 8090

A study has been made of the influence of the reinforcement/matrix interfacial strength on fatigue crack propagation in a powder metallurgy aluminum alloy 8090-SiC particulate composite. The interfacial region has been altered by two separate routes, the first involving aging of the 8090 matrix, with the subsequent formation of precipitate free zones at the boundaries, and the second consisting of oxidizing the surface of the SiC particles before their incorporation into the composite. In the naturally aged condition, oxidation of the SiC leads to a reduction in fatigue crack growth resistance at higher values of stress intensity range ΔK. This is due to a proportion of the crack growth occurring through voids formed in association with many of the weak SiC interfaces which have retained a layer of thick surface oxide after processing. On overaging no difference in crack growth rate is discernible between the oxidized and unoxidized SiC composites. It is proposed that this is due to similar levels of interfacial weakening having occurred in both composites, indicating that this is an important factor in the reduction of the high ΔK crack growth resistance of the unoxidized SiC composite on aging.

Fatigue crack propagation in nickel-base superalloys:effects of microstructure, load ratio, and temperature

The current state of knowledge and understanding of the long fatigue crack propagation behavior of nickel-base superalloys are reviewed, with particular emphasis on turbine disk materials. The data are presented in the form of crack growth rate versus stress intensity factor range curves, and the effects of such variables as microstructure, load ratio, and temperature in the near-threshold and Paris regimes of the curves, are discussed.

Crack closure and residual stress effects in fatigue of a particle-reinforced metal matrix composite

A study of the influence of macroscopic quenching stresses on long fatigue crack growth in an aluminium alloy-SiC composite has been made. Direct comparison between quenched plate, where high residual stresses are present, and quenched and stretched plate, where they have been eliminated, has highlighted their rôle in crack closure. Despite similar strength levels and identical crack growth mechanisms, the stretched composite displays faster crack growth rates over the complete range of ΔK, measured at R = 0.1, with threshold being displaced to a lower nominal ΔK value. Closure levels are dependent upon crack length, but are greater in the unstretched composite, due to the effect of surface compressive stresses acting to close the crack tip. These result in lower values of ΔKeff in the unstretched material, explaining the slower crack growth rates. Effective ΔKth values are measured at 1.7 MPa√m, confirmed by constant Kmax testing. In the absence of residual stress, closure levels of approximately 2.5 MPa√m are measured and this is attributed to a roughness mechanism.

Temperature effects on the mechanism of time independent hydrogen assisted fatigue crack propagation in steels

The effects of temperature on hydrogen assisted fatigue crack propagation are investigated in three steels in the low-to-medium strength range; a low alloy structural steel, a super duplex stainless steel, and a super ferritic stainless steel. Significant enhancement of crack growth rates is observed in hydrogen gas at atmospheric pressure in all three materials. Failure occurs via a mechanism of time independent, transgranular, cyclic cleavage over a frequency range of 0.1-5 Hz. Increasing the temperature in hydrogen up to 80°C markedly reduces the degree of embrittlement in the structural and super ferritic steels. No such effect is observed in the duplex stainless steel until the temperature exceeds 120°C. The temperature response may be understood by considering the interaction between absorbed hydrogen and micro-structural traps, which are generated in the zone of intense plastic deformation ahead of the fatigue crack tip. © 1992.

The role of primary carbides in fatigue crack propagation in aeroengine bearing steels

Fatigue crack propagation, tensile and fracture toughness data for four aeroengine bearing steels are reported. The steels involved are the through-hardened tool steels 18-4-1 (T1) and M50, and two similar carburized steels, RBD and Volvic. Crack growth data have been obtained at 20 °C and 280 °C to cover the range of oil temperatures experienced in aeroengine bearing operations. At 20 °C threshold ΔK values (ΔKth) ranged between 3.5 and 4.5 MPa √m with Paris exponents (m) of between 2.0 and 3.5. The lowest m-values were seen in the carburizing steels, which also exhibited lower Paris regime crack growth rates than M50 and 18-4-1. For all the steels, growth rates were higher at 280 °C,than 20 °C, although there was a slight tendency for ΔKth to increase, probably associated with oxide-induced closure at 280 °C. The effects of primary carbides, strength and toughness on fatigue crack growth behaviour are discussed, in relation to the importance of static-mode cracking. © 1990.

Effects of crack length and shape on the fracture toughness of a high strength steel 300m

The results of fracture toughness tests on a high strength steel 300m are presented. These results show (i) that in the presence of through-thickness cracks the toughness remains constant down to (a/W)-ratios as low as 0.01 and failure loads up to 0.85σy, and (ii) that the material is more resistant to crack growth when the cracks are semi-elliptical in shape, giving a toughness value which is almost 25 per cent higher than the through-thickness one. Three independent stress analyses are used to obtain stress intensity values for the semi-elliptical cracks and additional confirmation of the increase in toughness comes from stretch zone measurements.

Growth of short fatigue cracks in titanium alloys

The initiation and early propagation of short fatigue cracks has been studied in detail in two alpha / beta titanium alloys as a function of microstructure. Detailed metallography is presented relating short crack growth rates to the microstructural features present. The work shows the significant differences in short crack propagation rates which can be achieved by microstructural changes within a single alloy.

Relevance of microstructural influences in the short crack regime to overall fatigue resistance

The behaviour of short fatigue cracks is shown to be relevant only to a limited number of engineering situations. Within these situations, further restrictions on the extent to which metallurgical control can be exerted to improve fatigue crack growth behaviour are identified. The degree of control remaining is discussed in terms of two separate regimes which are described as intrinsic and extrinsic crack growth resistance. These separate effects are highlighted by comparisons both within and between a wide range of alloy systems. The implications of such an analysis are discussed in terms of aerospace applications.

Effects of microstructure, temperature and R-ratio on fatigue crack propagation and threshold behaviour in two Ni-base alloys

Fatigue crack propagation and threshold data for two Ni-base alloys, Astroloy and Nimonic 901, are reported. At room temperature the effect which altering the load ratio (R-ratio) has on fatigue behaviour is strongly dependent on grain size. In the coarse grained microstructures crack growth rates increase and threshold values decrease markedly as R rises from 0. 1 to 0. 8, whereas only small changes in behaviour occur in fine grained material. In Astroloy, when strength level and gamma grain size are kept constant, there is very little effect of processing route and gamma prime distribution on room temperature threshold and crack propagation results. The dominant microstructural effect on this type of fatigue behaviour is the matrix ( gamma ) grain size itself.

Classical and technological convergence:beyond the Solow-Swann model

Recent investigations into cross-country convergence follow Mankiw, Romer, and Weil (1992) in using a log-linear approximation to the Swan-Solow growth model to specify regressions. These studies tend to assume a common and exogenous technology. In contrast, the technology catch-up literature endogenises the growth of technology. The use of capital stock data renders the approximations and over-identification of the Mankiw model unnecessary and enables us, using dynamic panel estimation, to estimate the separate contributions of diminishing returns and technology transfer to the rate of conditional convergence. We find that both effects are important.

The properties of advanced alumimium alloy systems

2XXX and 7XXX series aluminium alloys have been the accepted materials for airframe construction for many decades. However, only minor improvements in properties have been possible by the development of these alloys since the early 1970's. The constant need to reduce weight in aircraft has therefore led to a resurgence in the research for higher performance aluminium alloys. The reason for this investigation was to evaluate possible alternatives for the existing conventional aluminium alloy 2014 for aircraft wheel applications. Three new technologies in alloy development were considered: a metal matrix composite, an aluminium-lithium alloy and a powder metallurgical alloy. The basic mechanical properties of these advanced materials have already been established to an extent, but their fatigue behaviour has yet to be fully understood. The purpose of this work was to investigate the fatigue properties of the materials concerned, in both air and an aerated 3.5% NaCl solution, and compare these properties to 2014-T6. As well as the basic mechanical properties, fatigue crack propagation data is presented for all of the materials concerned. Additionally, fatigue crack initiation data is presented for the aluminium-lithium alloy and 2014. The D.C. electrical potential method was used to monitor crack growth. Of the materials investigated, the most promising was the aluminium-lithium alloy. However, short transverse properties need to be increased and the commercial cost of the material needs to be decreased before it can be considered as a direct replacement for 2014 for aircraft structural applications. It was considered that the cost of the powder metallurgical alloy would limit its further use. The metal matrix composite material proved to be unsuitable for most ambient temperature applications

Noisy random Boolean formulae:a statistical physics perspective

Properties of computing Boolean circuits composed of noisy logical gates are studied using the statistical physics methodology. A formula-growth model that gives rise to random Boolean functions is mapped onto a spin system, which facilitates the study of their typical behavior in the presence of noise. Bounds on their performance, derived in the information theory literature for specific gates, are straightforwardly retrieved, generalized and identified as the corresponding macroscopic phase transitions. The framework is employed for deriving results on error-rates at various function-depths and function sensitivity, and their dependence on the gate-type and noise model used. These are difficult to obtain via the traditional methods used in this field.

Creep and fatigue of a single crystal nickel base superalloy

The introduction of single crystal casting techniques has led to the development of existing nickel-base superalloys to produce materials with optimum mechanical properties in the single crystal condition. As single crystals are known to be anisotropic, a study is needed to determine the general mechanical properties of these materials, and determine the effects of crystal orientation upon them. A study has been carried out to identify the effect of orientation and temperature on the creep and fatigue properties of a development single crystal superalloy, SRR 99. Creep testing and crystal rotation experiments have been made on SRR 99 and an earlier development alloy, SRR 9. Fatigue experiments at elevated temperatures have been carried out on both notched and un-notched specimens of alloy SRR 99. To aid in this analysis, several analytical techniques have been employed including Laue x-ray orientation analysis, measurement of strain by photographic methods and microstructural examination. Crystal rotation experiments have indicated that shear of 1 precipitates by lbrace111rbrace< 112> slip systems is operative during primary creep deformation at temperatures of 750oC and 850oC. The effect of orientation variation obtained by standard casting practices was not found to be significant. Creep rupture was found to be associated with multiple crack initiation from micropores. Fatigue crack initiation in un-notched specimens was found to be related to microporosity and microstructural defects. Failure was predominantly by crystallographic crack growth on lbrace111rbrace planes. The use of linear elastic fracture mechanics to describe fatigue crack propagation in alloy SRR 99 was found to be acceptable at temperatures up to 850oC. Variation of temperature, frequency and crystal orientation was found to have only moderate effect upon crack propagation rates.

Fatigue of commercial aluminium alloys

Fatigue crack propagation has been observed for a number of commercial aluminium alloys. Comparable data was obtained for a variety of crack and specimen geometries over a range of crack lengths for a given alloy. Where crack propagation only was of interest the initiation event has been excluded by pre-cracking the specimen using a fin of material adjacent to the crack face. By this method a controlled defect size is introduced in to the specimen. By modification of the D.C. potential drop method it has been shown possible to measure the growth of cracking from 0.12mm by this method. Crack growth from defects greater than 0.6mm have been shown to give conventional crack propagation deduced by principle of similitude. Fatigue fracture surface analysis has been conducted for cracking from both free surfaces and from blunt notches. A `quasi cleavage' feature has been identified and is shown to be prominent when the fatigue stress intensity range is below 10 MNm-3/2.