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.

The fatigue ageing of vulcanised rubbers

The cause of the respective rough and smooth fatigue failure surfaces of Neoprene GS : Neoprene W and Neoprene GS : natural rubber vulcanisates is investigated. The contrasting morphology of the vulcanisates is found to be the major factor determining the fatigue behaviour of the blends. Neoprene GS and Neoprene W appear to form homogeneous blends which exhibit physical properties and fatigue failure surfaces intermediate between those of the two horropolymers. Neoprene GS and natural rubber exhibit heterogeneity when blended together. The morphology of these blends is found to influence both the fatigue resistance and failure surface of the vulcanisates. Exceptional uncut and cut initiated fatigue lives are observed for blends having an interconnecting network morphology. The network structure and cross-link density of the elastomers in the blends and the addition of carbon black and antioxidant are all found to influence the fatigue resistance but not the failure mechanism of the vulcanisate.

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.

Fatigue in rubber vulcanisates

A study has been made of the effect of single extensions and continuous fatigue on the structures of various natural rubber networks. The change in network structure of a conventional vulcanisate on a single extension manifests itself as permanent set. The change in network structure has been assessed by the use of the chemical probes, propan-2-thiol/piperidine, hexane-thiol/piperidine and triphenyl phosphine, which determine the polysulphide and disulphide crosslink densities and main chain modification respectively. The permanent set induced on a single extension of a conventional sulphur vulcanisate has been shown to result from the destruction and reformation of polysulphide crosslinks. The magnitude of the effect was dependent upon the degree of extension and showed a maximum at extensions corresponding to the onset of stress-induced crystallisation. The incorporation of a reinforcing filler, HAF-carbon black, magnified the effect. Vulcanisates that possessed only mono and disulphide crosslinks did not show any significant permanent set. The continuous changes in network structure during fatigue have also been determined, and the effects of carbon black and antioxidants on these changes and the fatigue life have been assessed. During fatigue the overall crosslink density increased slightly, which resulted from the destruction of polysulphide crosslinks. and their replacement by principally disulphide crosslinks. Antioxidants reduced the rate of destruction of polysulphide crosslinks and increased the fatigue life of the rubber network. The fatigue life of the network also depended upon the concentration of free chain ends. These chain ends were incorporated into the network by masticating rubber under nitrogen in the presence of bis (diisopropyl)thiophosphoryl disulphide, which improved the fatigue resistance by up to 9%.