A fractographic study of crack propagation under creep-fatigue conditionso Koterazawa, R., Nosho, T. andMukai, Y. J. Soc. Mater. ScL, Jpn. J u l y 1989 38, (430), 8 0 2 - 8 0 8 (in Japanese) Crack propagation behaviour under creep-fatigue conditions at elevated temperatures was studied with a SUS 304 stainless steel, a quenched 21/4Cr-1Mo steel and a Hastelloy X, and the results were discussed in the light of fractography. The crack propagation rates of the 304 steel and the Hastelloy were correlated linearly with cyclic J-integrel range ~J. In the region of low propagation rate, the crack propagation rates of the 304 steel, the Cr-Mo steel and the Hastelloy were about the same. In the region of high propagation rates, on the other hand, those of the Cr-Mo steel were higher and those of the Hastelloy were slightly lower than those of the 304 steel. This difference in crack propagation rate is probably associated with the difference in creep ductility of these materials. In the case of C-P-type loading, in all materials, a transition of fracture morphology from the transgranular fatigue type to the intergrenular creep type occurred when the ratio of creep component to fatigue component ~ exceeded a critical value of approximately one. In the C-C-type loading, transgranular fatigue fracture appeared also in the case of a ~-ratio greater than one, where the fracture morphology in C-P-type loading was intergrenular. It is suggested that reversible slip along grain boundaries caused by compressive creep possibly recovers forward slip along the boundaries caused by tensile creep. No appreciable difference was found between crack propagation rates of the 304 steel in air and vacuum, and it was consistent with a small difference observed on the fracture surfaces in these environments. Graphs, Photomicrographs. 10 refs. Mechanical properties o f sintered stainless steel. Kuroda, Y., Izumi, H.,
Sunada, H. and Fukaura, K. J. Soc. Mater, ScL, Jpn. J u l y 1989 38, (430), 7 7 1 - 7 7 6 (in Japanese)
injection moulding for siotered products has recently attracted a great deal of interest because it has a relative freedom to shape intricate geometries. The purpose of the investigation was to clarify the effects of the Mn/Si ratio and the sintering temperature on the mechanical properties of sintered stainless steel. The materials used were two kinds of stainless-steel powder having an approximately equal particle size and a similar chemical composition except for Mn content. The results obtained are as follows: (1) The tensile strength of the sintered stainless steels was 540 MPa regardless of the Mn/Si ratio and was almost equivalent to that of the conventional steel SUS304. In addition, it was found that the logarithm of the tensile strength was proportional to the volume fraction of the pores. (2) The fatigue limit of the specimen having a relatively low Mn/Si ratio was 328 MPa, while the specimen having a high Mn/Si ratio exhibited a significantly lower fatigue limit, that is 265 MPa. (3) It became clear from these results that the fatigue properties varied considerably with the Mn/Si ratio. The ratio affected the volume fraction, the composition and the shape of the oxides. The high Mn/Si ratio seemed to produce sharp pores from which the fatigue cracks initiated easily. Graphs, Photomicrographs. 12 refs.
Moire interferometric fringe patterns about crack tips: experimental observations and numerical simulation. Nicoletto, G. Opt. Lasers Eng. 1990 12, ( 2 - 3 ) , 135-150 The local deformation about fatigue cracks has been investigated by the coherent optics method of Moir~ interferometry. The complexity of the interference fringe patterns motivated the parallel development of companion numerical models for simulating a Moird interferometric response for known boundary conditions. Experimental patterns on aluminium alloy are examined in the light of computergenerated fringe systems for a stationary crack in elastic as well as elastic-plastic solids. Various effects arising from material plasticity, specimen-dependent nonsingular stress, and mixed-mode loading are identified in the fringe patterns. Diffraction patterns. 22 refs.
Fracture toughness and fatigue strength at room temperature and 423 K in melt-forged aluminlum casting alloys. Egashira, H., Hirota, I., Kobayashi, T. and Sakai, S. J. Jpn. Inst. Light Met. Dec. 1989 39, (12), 8 8 6 - 8 9 2 (in Japanese) Fracture toughness, S-N curve and fatigue crack propagation behaviour in meltforged aluminium casting alloys such as AC4C-T6 and AC8A-T6 were investigated at room temperature and 423 K, and were compared with those in a commerciaF 6061-T6 wrought alloy. The static or dynamic fracture toughness, Jtc or Jd, for AC4CT6 and AC8A-T6 alloys was lower than that for a 6061-T6 alloy. The S-N curves for AC4C-T6 and AC8A-T6 alloys were situated lower than that for a 6061-T6 alloy. This was because of a pre-existing surface flaw effect of coarse silicon particles in the casting alloys. The fatigue crack propagation in AC4C-T6 and AC8A-T6 was unstable in a high stress intensity range, because a large number of fracture-initiation sites such as silicon and compounds were distributed in these alloys compared with a 6061-T6 alloy. On the contrary, the value of AKthincreased in a lower stress intensity range because silicon particles and compounds acted as obstacles against crack propagation. Graphs, Photomicrographs. 17 refs. Effects o f metallurgical factors on t h e fatigue crack propa~lation characteristics in h i g h - p u r i t y A I - S i casting alloys. Egashira, H., Nimomi,
M., Kobayashi, T. and Kohmura, S. J. Jpn. Inst. Light Met. Dec. 1989 39, (12), 8 7 8 - 8 8 5 (in Japanese)
The effects of Si particle size, strength of the matrix and addition of Mn and Fe on the fatigue crack propagation were investigated for high-purity AI-Si casting alloys. The fine distribution of Si particles resulted in the retardation of the crack propagation rate (in the da/dN-~K curves) in an Al-12Si alloy. In AI-7Si and AI-6.3Si-0.32Mg alloys the retardation of the crack propagation rate was pronounced in a high stress intensity range when the strength of the matrix was high. At the threshold region, however, the crack propagation rate was retarded in the decreased matrix strength due to the crack closure effect. The fatigue crack propagation rate (da/dN) in a high stress intensity range was retarded by the addition of Mn and Fe in an AI-Si-Mg alloy. Graph, Photomicrographs. 15 refs. Structural reliability assessment a n d large scale testing. Lucia, A. C. Comm. Eur. Communities Report No 12274 1989 pp 43"/-453 The main results and plans are presented of a project on the assessment of structural reliability and lifetime of pressure components. The project is based on two test rigs: room temperature fatigue on 1/5 scaled pressure vessels and pressurized
thermal shocks on the safe end and nozzle corner. Materials were SA508, SA533 and 347SS. The relevant theoretical development is on damage process modelling, treatment of uncertainties, fracture mechanics analysis and application of artificial intelligence tools. Graphs. 25 refs. T h e r m o p l e s t i c i t y and fracture mechanics, Bum, H. D. and Inglebert, G. Mec. Mater. Electr. Sept. 1989 431, 8 - 1 4 (in French) Thermoplasticity and fracture are discussed and plastic deformation and irreversible deformation are emphasized. Cyclic loading in an area free of initial defects eventually produces an unstable back and forth response, which leads to fracture. Bree tubes, used in cyclic tests, have a thin wall which is subject to a constant internal pressure (primary load) with thermal cycling (secondary load). The complete work covers fatigue, and the relaxation of fatigue stresses during test. The joining of thermoplastics and fracture mechanics involves a study of the crack from its propagation by heating until it becomes macroscopic. It covers the effects of localized stresses, interfaces between two materials, the effects of temperature differences and the resulting deformation. Several diagrams and curves are included. The material mentioned was steel. Graphs, Photomicrographs, Phase diagrams. 18 refs.
Nonlinear high-temperature fracture mechanics basis for strain-range
partitioning. Kitamura, T. and Ha/ford, G. R. NASA Lewis Research Center Report No N90-14642/4/XAB Oct. 1989 17 pp A direct link was established between strain-range partitioning (SFP) and hightemperature fracture mechanics by deriving the general SRP inelastic strain range against cyclic life relationships from high-temperature, nonlinear, fracture mechanics considerations. The derived SRP-life relationships are in reasonable agreement based on the experience of the SRP behaviour of many high-temperature alloys. In addition, fracture mechanics has sewed as a basis for derivation of the ductilitynormalized SRP-tife equations, as well as for examination of SRP relations that are applicable to thermal fatigue life prediction. Areas of additional links between nonlinear fracture mechanics and SRP were identified for future exploration. These include effects of multiaxiality as wetl as low-strain, nominally elastic, long-life creep fatigue interaction.
Mechanical properties o f a l u m i n i u m alloys at v e r y l o w temperatures.
Sail, S. and Hori, S. J. Jpn. Inst. Light Met. Aug. 1989 39, (8), 5 7 4 - 5 8 3 (in Japanese)
Technical information on the properties of aluminium alloys at low temperatures is vital for the development of a magnetic levitation railroad, magnetic-confinementtype fusion reactors, MHD generation, and many other applications. However, data found in the literature are very limited. The yield strength, tensile strength, fracture elongation, and reduction ratio of cross sections of 1099, 2014, 2199, 2024, 2090, 3003, 5083, 5454, 5456, 6061, 7005, 7079, and 8090 alloys at 4.2, 20.4, 77.4 K and RT are tabulated. Special characteristics of tensile, elastic and fatigue properties at low temperatures are summarized for selected alloys. Graphs. 48 refs.
Fatigue d a m a g e and crack nucleation mechanisms at intermediate strain amplitudes. Lim, L.C., Tay, Y.K. and Fong, H.S. Acta MetalL Mater. Apr. 1990 38, (4), 595-601 Polycrystalline copper was fatigued in rotary bending at constant intermediate surface strain amplitudes at 26 Hz under ambient conditions. The specimens were interrupted at various life fractions, their surfaces prepared metallogrephically and scrutinized to ascertain the types of fatigue damage, namely, short cracks that are confined to individual grains or isolated grain boundary facets, and their role in fatal crack formation. The results show that, at intermediate strain amplitudes, slip band and twin boundary crack damage predominate during the early stages of cycling, while grain boundary crack damage remains relatively insignificant even at the stage when fatal cracks have developed. However, depending on the strain amplitude level, the transgranular crack damage may or may not be instrumental in fatal-crack formation. At the lower amplitude end of the transition region, fatal cracks are formed by interlinkage of the slip band and twin boundary damage. At the higher amplitude end, even though grain boundary damage is negligible initially, the specimens degenerate rapidly on further cycling and eventually fatal cracks evolve. The present findings show that some 0.05% plastic strain amplitude is required to propagate intergranular cracks. Once the above condition is met, cracks would propagate rapidly along the interface and the crack nucleation mode would change from transgranular to intergranulsr. Graphs, Photomicrographs. 20 refs. Fatigue life i m p r o v e m e n t o f an autofrettaged t h i c k - w a l l e d pressure vessel w i t h an external groove. (Dismwtation). Koh, S.K. Dies. Abatr. Int. Mar. 1990 50, (9), 290 pp An investigation into fatigue life improvement of an autofrettaged thick-walled pressure vessel with an external groove subjected to pulsating internal pressure along with mean strain and mean stress effects on strain-controlled low-cycle fatigue behaviour was carried out. Linear elastic stress analysis of an autofrettaged thickwalled pressure vessel with an external groove is done using a finite-element method. Autofrettage loading is performed using a thermal loading analogy. A change of external groove geometry is made using a quasioptimization technique and finite-element method to achieve longer fatigue life by relieving the stress concentration at the groove root. Surface treatment using shot peening is employed to produce compressive residual stresses at the vulnerable surface of the groove root to counteract the high tensile stresses. An evaluation of the fatigue life of an autofrattaged thick-walled pressure vessel with an external groove was done through a series of simulation fatigue tests using C-shaped specimens taken from the thickwalled pressure vessel. The effect of mean strain and mean stress in the highstrength pressure vessel steel of ASTM A723, Grade 1, Class 4, is reported from the strain-controlled low-cycle fatigue tests using smooth cylindrical axial specimens. Three mean stress parameters from Morrow, Smith et al and Lorenzo-Laird are used to reduce the low-cycle fatigue test data including a wide range of strain ratios into a single line on the strain-life plot. Life estimation of an autofrettaged thickwalled pressure vessel is made by incorporating the linear and modified Neuber's rule with results from linear elastic finite-element stress analysis and mean stress parameters obtained in low-cycle fatigue testing.
Int J Fatigue November 1990