Browsing by Author "Hockenhull, B. S."

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    Corrosion Fatigue Crack Propagation Behaviour of a High Strength Low Alloy Steel in a Synthetic Sea Water Environment.
    (Cranfield University, 1986-12) Millar, Peter G.; Hockenhull, B. S.
    The corrosion fatigue crack propagation behaviour of a high strength low alloy steel, N-A-XTRA 70, in a synthetic sea water solution was tested using S. E. N. specimens subjected to a loading frequency of 0.1 Hz and a load ratio of 0.6. In order to simulate the conditions encountered by a thumbnail type crack several specimens from each of the microstructural types tested, namely parent plate, heat affected zone and heat treated material, had their crack sides covered by transparent plastic covers. Severe overprotection and slight underprotection conditions were produced using cathodic protection potentials of -1400, -1300, -1200 and -700 mV (S. C. E. ). The Paris relationship da/dN = CLKm was found to be a useful tool in describing the crack propagation rate data. Results obtained, presented in the form of plots of log da/dN against log AK, show that for parent plate, H. A. Z. and heat treated material, covering the crack sides of specimens produces enhanced corrosion fatigue crack propagation rates, at cathodic protection potentials of -1400 and -1300 mV (S. C. E. ), when compared to non covered specimens. This trend was also true for H. A. Z. specimens at a potential of -700 mV (S. C. E. ). For parent plate specimens, however, covering the crack sides at a potential of -700 mV (S. C. E. ) produced reduced crack propagation rates over non covered specimens. It is believed restriced oxygen access may account for these results. Plots of the Paris exponent m and constant C for the three microstructures tested produced three lines of the form m= alnC +b where a and b were found to be dependent upon material parameters. Comparison of results with BS 4360: 50D revealed that N-A-XTRA 70 exhibited superior fatigue performance when tested in air but behaved worse under conditions of free corrosion.
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    The effects of exfoliation corrosion on the fatigue and fracture behaviour of thin aluminium alloys
    (Cranfield University, 1982-12) Morad, T. A.; Chubb, J.; Hockenhull, B. S.
    The present study is concerned with the interaction of stress and corrosion in high strength aluminium a11oys in sheet form used in the aircraft industry. Many of these alloys are susceptible to exfoliation corrosion under service conditions. This type of corrosion was reproduced in the susceptible a11oys using the Exco test method. The fracture toughness of an A1 - Zn - Mg - Cu a11oy (7178 - T6) was assessed using R - curves. The interaction between exfoliation corrosion and the thinning effect of exfoliation on the fracture toughness are discussed. Fatigue crack growth behaviour in two naturally aged A1 - Cu ~ Mg alloys, 2024 - T351 and 3L7O (2014 - T4) was evaluated under conditions of low and high humidity using constant amplitude loading. A similar assessment of the effect of humidity on the propagation rates in the higher strength 7178 - T6 was carried out. Additionally, the effect of the presence of exfoliation corrosion on crack growth in 2024 - T351 and 7178 -- T6 was examined. Both humidity and exfoliation corrosion enhanced fatigue crack growth rates in the alloys, though to differing extents, with 7178 - T6 being affected more. The fatigue resistance of 7178 - T6 diminished even further in the presence of humidity and corrosia together. Finally, a mechanism to explain the way by which exfoliation corrosion affects the fatigue and fracture behaviour of thin aluminium sheets is proposed.
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    An investigation into the fatigue and corrosion fatigue properties of two high strength low alloy steels and their HAZ’s.
    (1992-11) Drury, J. A.; Billingham, J.; Spurrier, J.; Hockenhull, B. S.
    Testing was performed on three point bend specimens machined from parent plate and welded samples machined such that the crack propagated down the heat affected zone. Specimens were subjected to tension-tension loading using a stress ratio of 0.6 and a testing frequency of 0.5 Hz. Tests were performed in air and in a synthetic sea water solution. Three levels of corrosion protection were investigated, freely corroding, protected at -800 mV and over protected at -1100 mV. The resulting crack growth rate data were plotted on a log scale against the log of the stress intensity range. This made comparisons between different conditions and materials easier. Both high strength steels and the heat affected zones associated with their weldments compared favourably with BS4360:50D structural steel when tested in air and with a cathodic potential of -800 mV. Using over protection conditions the materials showed a susceptibility to hydrogen concentration. A particularly sensitive technique was used to investigate the effects of heat affected zone microstructures on the crack propagation rate. This was achieved using a continuous monitoring gauge mounted onto the side of the specimen. The gauge was used to obtain the crack length and a computer was coupled to the fatigue machine to control the applied loads. The resulting data showed that there was a variation in growth rates between different weldment regions which was in turn influenced by the weld heat input used. The other finding was that when low heat inputs were used for welding there was rapid increase in crack growth rate associated with the coarse grained region, which was attributed to the aligned carbides of this region which act to stop crack blunting. This effect was only seen in the 1.5 kJ/mm weld, higher heat inputs cause a break up of these carbides and hence the crack is able to blunt.
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    Phenomenological investigation of the influence of Cathodic protection on corrosion fatigue crack propagation behaviour, in a BS 4360 50D type structural steel and associated weldment microstructures, in a marine environment.
    (Cranfield University, 1984-06) Thompson, J. W. C.; Hockenhull, B. S.
    The influence of Cathodic Protection potential upon corrosion fatigue crack propagation rates in a medium7strength ferritic-pearlitic structural steel (ES 4360 grade 50D) and associated weldment microstructures in simulated sea-water was studied and the results were presented in bi-modal da/dN vs AK curves. Above transition propagation rate data was satisfactorily described by the Paris relationship da/dN = C. AKm and a relationship of the formin = Aln C+D between Paris exponent m and constant C was confirmed. In all microstructures the influence of cathodic protection on crack propagation rate was found to be dependent upon the level of cathodic potential applied and crack depth. A. critical crack depth of approximately 5.0mm was identified. Optical crack monitoring combined with graphical data processing, was found to be particularly effective for the characterisation of corrosion fatigue crack propagation behaviour and resulted in minimal data scatter. The phenomena of transition which was evident in the bi-modal da/dN vs LK curves was found to be associated with secondary or branched-crack activity. Back-extrapolation of below transition data was used to estimate Arm values and showed that the weldment beat-affected zone microstructure was a major potential source of fatigue or corrosion fatigue fracture. The effect of a single cycle of simulated overload, to stormload levels, on both fatigue and corrosion fatigue crack propagation behaviour in parent plate material was studied and the phenomenon of retardation was confirmed.
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    Some further experiments in fatigue testing at 20 kH2
    (College of Aeronautics, 1968-10) Hockenhull, B. S.
    Considerations are made of the suitability of acoustic transformers applied to fatigue testing at 20 kH2. A suitable system has been constructed which allows the use of plain cylindrical specimens. The results of fatigue tests on an aluminium alloy are given and discussed in relation to structure and environment.
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    Some microstructural features of fatigue in an aluminium alloy
    (College of Aeronautics, 1969-01) Hockenhull, B. S.; Panakal, J. D.; Hacking, R. G.
    The microstructures produced by the heat treatment of a commercial age hardening At-4.4 Cu alloy have been examined by thin foil electron microscopy. Whilst there is similarity of the microstructures in the commercial alloy to those which have been reported for simple binary At-Cu alloys, there is a strong association of dislocation structures and incoherent precipitates with undissolved Mn bearing intermetallic particles. Fatigue tests made on the alloy at both 50 Hz and 20 kHz have shown that there are changes in microstructure during fatigue; more markedly at 20 kHz in which tests the heating effect has some importance. There is some evidence of both accelerated ageing and also the by-passing or resolution of coherent phases during fatigue, particular at 20 kHz.
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    A study of fatigue crack propagation in quenched and tempered and controlled rolled HSLA steels.
    (Cranfield University, 1987-09) Callister, D. R.; Billingham, J.; Hockenhull, B. S.
    A range of HSLA steels reflecting the two major processing routes, quench and tempering and controlled rolling, have been tested in fatigue to assess their potential wider application in the offshore Industry. The six steels chosen have a wide range of yield strenghts (470 to 690Nmm-2), fracture toughness (31 to 260J at -40°C) and carbon equivalent values (0.19 to 0.33). Fatigue testing has in general been carried out at low frequency (0.5Hz) and high load ratio (0.6) however some tests have been conducted at very low frequency (0.1Hz) and low load ratio (0.1). An in-air study was first used to assess the fatigue performance of all six parent plates. Five steels were welded by the Submerged Arc Welding process at high heat input (1.5kJmm -1) to evaluate the Heat Affected Zone (HAZ) fatigue performance. A new test was devised to grow a fatigue crack through a single pass, bead on plate-, HAZ whilst maintaining a constant stress intensity range. The surface crack length was continuously monitored and recorded to an accuracy of 0.01mm. A corrosion fatigue study evaluated the performance of one controlled rolled and one quenched and tempered steel at three levels of impressed current cathodic protection. Extensive metallographic examination was made to study the influence of microstructural features and types on fatigue crack propagation. Techniques used include optical microscopy, scanning electron microscopy, fatigue crack and surface replication and crack profile digitising. These techniques give an assessment of crack path deviation and branching, the influence of precipitates and inclusions, and an indication of the mode of fatigue crack propagation. The wide range of microalloyed HSLA steels tested have shown a significant improvement in fatigue crack propagation resistance over structural steels conforming to BS 4360 grade 50D. Typically an improvement by a factor of two has been observed. Whilst the observed increase in fatigue life was slightly reduced by high heat input welding the slope of the Paris curves remained unaffected thus indicating a similar stress intensity range sensitivity in the HAZ to that shown by the parent plate. The newly developed crack monitoring system coupled to a computer controlled fatigue testing machine has shown a wide variation in fatigue crack propagation rates through a heat affected zone microstructural gradient. Growth rates have increased by a factor of ten in localised coarse grained microstructural regions compared to the adjacent weld metal and outer heat affected zone. The corrosion fatigue study has also indicated that in general HSLA steels retain their superiour fatigue resistance compared to structural steels and in particular respond more favourably to cathodic protection. Both in-air and corrosion fatigue studies have indicated that the controlled rolled steel microstructures developed mainly for line pipe application has the greatest potential for increased use offshore.