Authors: Fitzgeorge, D. | Pope, J.A.
Article Type:
Research Article
Abstract:
In these researches, which were carried out on behalf of the British Shipbuilding Research Association, the stresses that occur in combustion-chamber parts are discussed with regard to their nature, magnitude, and likely contribution to failure. The physical and mechanical constants of the material which affect the thermal stresses are given, and the modifying influence of plastic deformation is considered. It is deduced that cracks which commence at the hot side of the walls are due to residual stresses on cooling down, following plastic flow while running. This form of crack is shown to be most likely to occur in
…cast iron (where ductility is low) and the Authors have not so far encountered it in steel components. The danger spots are in the hot face of the cover (particularly where high thermal compressive stresses occur initially as a result of asymmetry) and in the hot face of the pistol crown. The second principal cause of failure is the fatiguing action of the repeated combustion pressures, combined with the tensile stresses occurring as a result of temperature gradients in the walls. The cracks occur at the horizontal junction, of the crown and wall of pistons, and where webs and other scantlings meet the cover plate in cylinder covers. Failures of this type commence at the cooled side of the walls and occur in steel as well as cast-iron components. The effects of growth, additional stresses accompanying load changes, and the fluctuating surface stresses caused by the varying gas temperature are small, but the Authors prefer to regard them as contributing factors causing somewhat earlier failures. Typical failures are discussed. Radial cracks in cylinder covers and circumferential cracks in pistons are selected for detailed study. Direct estimates of the initial thermal compressive stresses in an unsymmetrical cover range from 16 to 28 tons per sq. in. Being higher than previous estimates, these values support the residual-stress theory of failure. A theoretical investigation of the stresses in an idealized piston shows that cracking at the junction of the crown and skirt is caused by fatigue. Examination of four actual failed pistons confirms this finding. Calculated stress curves are drawn showing the variation of piston stresses with changing wall and crown thickness. The stresses due to combustion pressure are independent of the scale of the piston, but dependent on its proportions. This applies to the thermal stresses also, provided that the temperature distribution is the same. Since the piston temperatures increase with increasing size, the thermal stresses in large pistons are greater than in small ones. The stresses produced by the combustion load are shown to be smallest when the crown and wall are thick. For low thermal stresses, a thin crown is necessary; for low thermal stresses in the wall, the wall should be thick, but a thick wall causes high thermal stresses in the crown. An example has been worked out to show graphically the variation in the overall stress conditions at the inner edge of the wall and at the inside face of the crown centre as the crown and wall thicknesses are varied. For the conditions assumed, the wall and crown should be approximately of equal thickness and one-tenth of the piston diameter. These figures include consideration of the compressive stresses in the hot face of the crown. The conclusion is drawn that where circumferential cracking has occurred in the wall, in the absence of cracks elsewhere, design changes should be directed at reducing the stresses at the position of failure, even though the stresses at other vital positions may be increased as a result. This requires the wall to be thickened, which would raise the temperature of the upper ring, but the tendency would be counteracted by the cutting of suitable grooves in the piston wall. Alternative designs are suggested which might be useful in overcoming cracking troubles in the pistons of large or highly rated engines. The qualities required in a material to resist failure by residual stress when used in engine components are: 1. High tensile strength, little affected by continued exposure to the working temperatures. 2. High ductility, little affected by small amounts of repeated plastic strain. 3. Low Young’s modulus in elastic range: low stress-strain ratios in plastic range. 4. Low coefficient of expansion and Poisson’s ratio. 5. High thermal conductivity. 6. High creep resistance. The qualities required to resist fatigue failure, in addition to a high fatigue strength. are identical with (3) to (5) inclusive. A high notched fatigue strength under mildly corrosive conditions is required in the range 50° C. to 250° C. Low creep resistance is beneficial in this case, since creep allows the relaxation of the mean tensile stress. Non-corrosive coolants reduce the danger of fatigue fracture. In Appendix III the result of temperature measurements on oil-cooled and water-cooled pistons are given. The highest temperature recorded for the oil cooled piston was 700° F. (370° C.) while that for the water cooled piston was 730° F. (388° C.).
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DOI: 10.3233/ISP-1955-21403
Citation: International Shipbuilding Progress,
vol. 2, no. 14, pp. 463-499, 1955
Price: EUR 27.50
