high failure rates. Fracture of drive shafts often occurred within months of initial installation; and over a five-year period, approx 5% of all machines for the most prominent manufacturer had failed, even though a 20-year life was contemplated. The present paper provides an historical review of RBC development. A comprehensive design and failure analysis study of an actual drive shaft configuration is performed based on classic fatigue principles (endurance curves) and fatigue/fracture (crack growth) techniques using a finite element approach. Graphs, 17 rcf. Fatigue resistance of a medium carbon steel with a wear resistant thermal spray coating.
Rakitsky, A.A., De Los Rios, E.R. and Miller, K.J. Fatigue Fract. Eng. Mater. Struct. (May 1994) 17 (5), 563-570 The fatigue behaviour of a nickel-chromium-base powder flame-spray coating on a 0.4% C steel is investigated. Fatigue tests were carded out using mild hourglass profile specimens. Cracks were detected and measured using plastic replicas and an image analysis system. Coated specimens showed a slightly lower fatigue endurance than plain specimens under torsion loading, while the opposite was observed for push-pull loading. Microcracks in coated specimens invariably form at pores. Contrary to the usual case of stage I shear growth for a plain 0.4% carbon steel in tension or torsion loading, the coated specimens show initial crack growth from pores along directions perpendicular to the maximum tensile stress. The crucial behaviour of short cracks, and their growth rates, relative to the thickness of the coating, are discussed in some detail. Graphs, 3 ref. Thermomechanical surface treatment of titanium alloys.
Wagner. L. and Gregory, J.K. Mater. Sci. Forum (1994) 163-165, 159-172 Thermomechanical treatments are widely used to optimize the properties of high-strength Ti alloys for a given application. Mechanical surface treatments such as shot peening generally (but not always) result in improved fatigue behaviour of Ti parts. Shot peening changes the surface properties in three respects: surface roughness, residual stresses, and degree of work-hardening are altered. Because these three parameters can independently influence crack nucleation and crack propagation, the overall fatigue life is a complex function of the surface condition. By separating the individual contributions of these parameters to the total fatigue life, it can be determined that an increase in surface roughness leads to early crack nucleation, while an increase in surface strength due to work-harding delays the crack initiation phase. Furthermore, once surface cracks are present, these propagate faster in workhardened material, but their growth is retarded by compressive residual stresses. Based on this analysis, recommendations for surface treatments for service at high temperatures (where the beneficial compressive stresses may anneal out) are presented. Novel methods for improving fatigue performance by combining mechanical surface treatments and thermal treatments are described. Graphs, photomicrographs, 26 ref. Rise in fatigue strength of titanium by extremely mild abrasive processing. Watanabe, S., Amano, J. and Furuichi, H. Wear (May 1994) 174 (1-2), 235-237 It has been reported that mild or to-and-fro grinding raises the fatigue limit of steels. This leads to the idea that mild abrasive processing could raise the fatigue strength of Ti. The main purpose of this paper is to present evidence of the remarkable rise in endurance limit of Ti by extremely mild lapping with SiC papers. Graphs, photomicrographs, 17 ref. Graded thermal barrier coatings: evaluation.
Mendelson, M.I., McKechnie, T.N. and Spiegel, L.B. Ceram. Eng. Sci. Proc. (July-Aug. 1994) 15 (14), 555-562 'Dual-graded' thermal barrier coatings have been developed in order to increase the coating life of conventional plasma ceramic thermal barrier coatings (NiCrAIY bond coat plus zirconia-based top coat). This coating consisted of plasma-sprayed NiCrAIY bond coat, graded interlayer of NiCrAIY/ZrOz-8Y203, and ZrO2-Y203 top coats with a laser-glazed surface layer on Inconel 718 substrate. These coatings had two gradients: an interlayer composition gradient and a density gradient at the surface of the top coat. These coatings were compared with conventional plasma-sprayed coatings after cyclic thermal testing. The thermal fatigue life of the 'dual graded' coatings was significantly greater than that of conventional coatings. Graphs, photomicrographs, 6 ref. Thermomechanical methods for improving the fatigue characteristics of metallic materials.
Mendes, C.A. T. US Patent 5 284 534 (8 Feb. 1994) A thermomechanical method for improving the fatigue characteristics of a metallic material (for example, carbon steel and low-alloy steel) takes advantage of the material's plastic flow characteristics to improve external and internal surface conditions. The material is heated to a temperature in the range of approx. 0.3-0.45 of its homologous temperature, e.g. approx 200 °C to about the Young's modulus transition temperature of the material. While the temperature of the material is in this range, force is applied to the material to produce in at least the region of the material to be treated a tensile stress level greater than the yield point of the material at the temperature, and thereby to produce limited plastic elongation in the region. The material is then cooled under stress, the stress being maintained above the instantaneous yield point of the material during at least part of the
cooling process. As a result of this process, the shape of existing stress raisers (e.g. radii at the base of cracks and flaws) are changed in ways which lower stress concentration factors in the material. Fatigue of welded joints under narrowband non-Ganssian Ioadings.
Sarkani, S., Kihl, D.P. and Beach, J.E. Probab. Eng. Mech. (1994) 9 (3), 179-190 Analytical and experimental results are presented to determine the effect of non-normality of the loading process on the rate of fatigue damage accumulation in welded steel joints. All stochastic loadings used are narrowband, and the parameter kurtosis is used to measure the degree of non-normality of the loading processes. Non-normal loadings are generated by passing Gaussian loadings through non-linear transformation functions. Results of the analytical study as well as results of fatigue experiments on welded cruciform specimens indicate that non-normality can significantly influence the rate of fatigue damage accumulation. Errors in fatigue life prediction can vary from slightly conservative to very unconservative by assuming a normal response when it is not. Results of random load experiments indicate that an endurance limit observed under constant-amplitude tests does not transfer to random load response. Graphs, numerical data, 12 ref. Fatigue strength of welded joints of 6N01 aluminium ahoy extrusions.
Matsuoka, K., Uemura, T., Kamata, K. and Chiaki, S. Kei Kinzoku Yosetsu (J. Light Met. Weld. Constr.) (1994) 32 (5), 1-8 (in Japanese) The fatigue strength of welded joints of 6N01 aluminium alloy extrusion is discussed. Lower copper content (Cu <~ 0.02% ) alloys were chosen, considering corrosion resistance to sea water. Two series of specimens were prepared. One has two longitudinal stiffeners welded on both sides of the main plate (L-type), and the other has the non-load-carrying fillet-welded cruciform joint (T-type). For some specimens, weld toes were ground with a pencil grinder. Using the specimens, residual stress measurements, tensile tests, and fatigue tests were carried out. FEM analyses were carded out both on weld residual stresses and on stress concentrations. Based on both experimental and analytical results, the effects of residual stresses and stress concentrations on the fatigue strength of welded joints are discussed. Graphs, 10 ref. A mathematical model to predict geometry magnification factor for stress intensity factor of butt welded joints.
Nguyen, T.N. and Wahab, M.A. Proc. Conf. Advanced Joining Technologies for New Materials. 11, Cocoa Beach, Florida, United States, 2-4 March 1994, pp. 142-154 An approximate mathematical expression of geometry magnification factor (Mk) for stress intensity factor subjected to various butt weld profile geometry parameters was derived by using linear elastic fracture mechanics (EFM), finite element analysis (FEA) and dimensional analysis techniques (DAT). It has been found that the effect of plate thickness on stress intensity factor and on fatigue strength of butt-welded joints is less significant than that of other weld geometry parameters such as flank angle and weld toe radius. The effect of plate thickness on stress intensity factor is significant only at very small crack length, of the order of 0.025 times plate thickness, while weld toe radius and flank angle affect stress intensity factor at crack lengths of the order of 0.05 and 0.15 times plate thickness respectively. This means that for welded joints with defects like micro-cracks of 0.25 mm at weld toes there is no effect of plate thickness of welded joints with section thickness < 10 mm. The effect of weld toe radius and flank angle dominates the effect of weld geometry on stress intensity factor and fatigue behaviour of welded joints. The strong effect of plate thickness on fatigue strength of butt-welded joints known from literature is revealed as the co-influence effect of plate thickness with other weld geometry parameters such as weld toe radius, flank angle and plate-edge preparation angle. The predicted values of Mk agreed well with that calculated using Bueckner's weight function and superposition principle. The calculated values of fatigue strength of butt-welded joints using the equations developed for Mk agreed well with the available data from literature. Graphs, 19 ref. Corrosion fatigue of silicon carbon whisker or SiC particulate reinforced 6061 aluminum alloy.
Yu, S.-Y., lshii, H. and Tohgo, K. Fatigue Fract. Eng. Mater. Struct. (May 1994) 17 (5), 571-578 Fatigue strengths of SiC whisker or SiC particulate-reinforced 6061-T6 aluminium matrix composite (SiCw/AI or SiCp/AI), fabricated by a highpressure infiltration method, were obtained in laboratory air, ion-exchanged water and a 3% NaCI aqueous solution. A comparison was made with the properties of the matrix aluminium ahoy 6061-T6. The SiC~/AI composite maintained a higher fatigue resistance than the SiCp/AI composite or the monolithic 6061 AI, even in a 3% NaCI solution. Good correlation was observed between a deterioration in the fatigue strength and the value of the corrosive potential and current, when changing the environment from laboratory air or ion-exchanged water to a 3% NaCI aqueous solution. Graphs, photomicrographs, 10 ref. Environmental embrittiement of Fe3AI alloys uuder monotone alld cyclic loading. Stoloff, N.S., Castagna, A., Scott, J. and Duquette, D.J. Proc. Conf. Processing, Properties, and Applications of Iron Aluminides, San Francisco, California, United States, 27 Feb.-3 Mar. 1994, pp. 271-285 The tensile properties and fatigue crack growth behaviour of binary and ternary Fe3AI alloys and a disordered Fe-16 at.% AI alloy, in several gaseous