Microstructural examination of fatigue crack tip in high strength steel

Fatigue Abstracts Effect of single overload on fatigue crack propagation behaviour of aluminium 7039 alloy. Kim, J.-S., Byon, U.S. and Kim, S.H. Korean Inst. Met. Mater. (Apr. 1993) 31 (4), 407-416 (in Korean) The characteristics of fatigue crack propagation behaviour under constant cyclic load and cyclic load including tensile overload have been compared in the thermomechanically treated aluminium 7039 alloy and the peak aged alloy. The crack closure mechanism due to surface roughness is found to be related to the higher fatigue resistance in the peak aged condition under constant load. During overload, crack branching, in addition to residual stress-induced crack closure, was observed to be another important mechanism of the retardation of fatigue crack growth rate following overload. More significant crack branching observed in the thermomechanically treated condition is closely related to the higher retardation of fatigue crack growth. The primary mechanism of fatigue crack growth retardation due to overload varies with the loading condition and microstructures. Consequently the retardation of fatigue crack growth becomes more obvious in the lower stress intensity factor range and for higher overload ratio. Graphs, photomicrographs, 22 reI. Near threshold fatigue testing Freeman, D.C. and Strum, M.J. Lawrence Livermore National Laboratory Report DE93009069/XAB (1993) 14pp Measurement of the near-threshold fatigue crack growth rate (FCGR) behaviour provides a basis for the design and evaluation of components subjected to high-cycle fatigue. Typcally, the near-threshold fatigue regime describes crack growth rates of less than approx 10 s mm cycle ~ (4 × 10 -7 in cycle i). One such evaluation was performed for the binary alloy U-6Nb. The procedures developed for this evaluation are described in detail to provide a general test method for near-threshold FCGR testing. In particular, techniques for high-resolution measurements of crack length performed in situ through a direct-current potential-drop (DCPD) apparatus, and a method that eliminates crack-closure effects through the use of loading cycles with constant maximum stress intensity, are described. NDE of PWA 1480 single crystal turbine blade material. Klima, S.J., Orange, 7".W. and Dreshfield, R.L. NASA Lewis Research Center, NASA TM-106140 (1993) 11 pp Cantilever bending fatigue specimens were examined by fluorescent liquid penetrant and radioactive gas penetrant (krypton) NDE methods and tested. Specimens with cast, ground, or polished surfaces were evaluated to study the effect of surface condition on NDE and fatigue crack initiation. Fractographic and metallurgical analyses were performed to determine the nature of crack precursors. Preliminary results show that fatigue strength was lower for specimens with cast surfaces than for specimens with machined surfaces. The liquid penetrant and gas penetrant techniques both provided indications of a large population of defects on the cast surfaces. On ground or polished specimen surfaces, the gas penetrant appeared to estimate the actual number of voids more accurately than the liquid penetrant. Graphs, photomicrographs, 5 rcf. New knowledge about 'white spots' in superalloys. Jackman, L.A., Maurer, G.E. and Widge, S. Adv. Mater. Proc. (May 1993) 143 (5), 18-25 Findings of workshops sponsored by the Gas Turbine Superalloy Committee of the Aerospace Division of ASM International are reported. The workshops aimed to improve upon definitions and to expand knowledge about segregation in superalloys and white spots in solute-lean areas. In respect of the three types of white spot (discrete, dendritic and solidification white spots), characteristics are described and illustrated for each, including niobium ranges in Alloy 718. Observations are listed as to how white spots affect properties and fatigue. Formation mechanisms of the three types, and what can be achieved by thermomechanical processing, are described. Etchants and etching methods suitable for detecting white spots are listed. Photomicrographs, graphs, 6 ref. Fatigue and creep-fatigue damage of austenitic steels under multiaxial loading. Weiss, J. and Pinau, A. Metall. Trans. A (Oct. 1993) 24A (10), 2247-2261 The objectives are to observe and model physical damage induced by cyclic multiaxial (tension-torsion) loading of 316L stainless steel both at room temperature and at elevated temperature (600 °C). Four types of experiment were carricd out on thin tubular specimens: continuous pure fatigue (PF) tests; PF sequential tests with different sequences of push-pun and torsional loading; creep-fatiguc (CF) tests with superimposed hold time at maximum tensile strain; and sequential tests involving sequences of PF and CF loadings. Optical microscopy and scanning electron microscopy (SEM) were used to study the damage quantitatively: in particular, to determine the orientation of cracks and to measure the kinetics of crack nucleation and crack growth. It is shown that in pure fatigue at 600 °C, the classical crack initiation stage I is bypassed owing to a strong interaction between cyclic plasticity, oxidation, and cracking. Intense slip bands act as diffusional short-circuits, leading to the formation of external (FE203) and internal ((FeCr)3 04) oxide scales. The orientation of the microcracks during the initiation and propagation stages, which is strongly affected by oxidation effects, explains qualitatively the significant deviations observed in the sequential tests from Miner's linear damage cumulative rule. It is also shown that creep-fatigue damage, which involves intergranular damage, is a complex process rather than a simple superposition of fatigue and creep damage. A stochastic model based on a Monte Carlo simulation is developed. This model, which accounts very well

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for the situations in which crack initiation and crack propagation are coplanar, includes damage equations based on quantitative metallographical observations. Damage is modelled as the continuous nucleation of the population of growing cracks, which eventually coalesce to lead to final fracture. It is shown that this simulation is able to reproduce with good accuracy the fatigue lives measured under multiaxial continuous and sequential tests. Graphs, photomicrographs, 30 ref. A theoretical evaluation of crack closure. Louat, N., Duesbery, M., Vasudevan, A.K. and Sadananda, K. Metall. Trans. A (Oct. 1993) 24A (10), 2225-2232 Premature crack closure has been considered an important factor affecting the applied driving force under cyclic load. Among several factors that induce crack closure, plasticity and oxidation or corrosion have been recognized as the most significant. An analytical estimation of both is made using dislocation theory. The analysis indicates that plasticity originating from the crack tip does not induce crack closure; and closure arising from asperity ridges due to oxides, corrosion products or surface roughness is small and insignificant unless the crack is completely packed with asperities. Graphs, 16 ref. Microstructural examination of fatigue crack tip in high strength steel. Fukuoka, C., Yoshizawa, H,, Nakagawa, Y.G. and Lapides, M.E. Metall. Trans. A (Oct. 1993) 24A (10), 2209-2216 Fatigue tests were performed to examine how microstructural conditioning influences crack initiation and propagation in SA508 class 3 low-carbon steel. A 3 mm long crack was introduced in compact tension (CT) fatigue test specimens under four different loads to obtain crack tip plastic zones at different stress intensity factor ranges, AK = 18, 36, 54 and 72 MPa m l/: The microstructures of the plastic zones around the crack tip were examined by transmission electron microscopy (TEM) and selected area electron diffraction (SAD). Micro-orientation of the dislocation cells in the plastic zones of all of the CT samples increased to 4° from the level of an asreceived sample. Four-point bending fatigue tests were performed for plate shape samples with a large cyclic strain range. The SAD value of the bending samples was also 4° in the damaged area where cracks already initiated at an early stage of the fatigue process. These test results indicate that the microstructural conditioning is a prerequisite for fatigue crack initiation and propagation in SA508. These observations may lead to better understanding of how fatigue initiation processes transit to cracks. Graphs, photomicrographs, 14 ref. Microstructure and mechanical properties of 18 mass% chromium cast stainless steel for marine propeller. Ono, S., Saito, M. and Matsuo, S. J. Jpn Inst. Met. (July 1993) 57 (7), 761-766 The relationship between microstructure and mechanical properties in air of various high-chromium cast stainless steels were investigated to develop a new marine propeller material 1.5 times (265 MPa) greater in cor~'osion fatigue strength than nickel aluminium bronze. The results are summarized as follows. (1) To achieve the target for tensile properties, it is necessary that the microstructure of the new steel consists of martensitic, austenitic and ferritic phases. (2) Concerning the volume of the metallurgical phases, it is desirable to ensure 5-30 vol.% of the austenitic phase and < 25 vol.% of the ferritic phase. (3) Judging from the above-mentioned metallurgical phases, the 18% Cr cast stainless steel containing ~< 0.04% carbon, 16-18% chromium, 5-7% nickel and 1% molybdenum is nearly suitable for a new marine propeller material that satisfies the intended tensile properties. The characteristic of this 18%Cr cast stainless steel is that the 0.2% proof stress of this steel is half of that of 12% Cr cast martensitic stainless steel, although this steel has almost the same tensile strength as that of 12% Cr cast martensitic stainless steel. Graphs, photomicrographs, 14 ref. Fatigue behaviour of a commercial aluminium ahoy in sea water at different temperatures. Bayoumi, M.R. Eng. Fract. Mech. (June 1993) 45 (3), 297-307 Fatigue tests of a commercial aluminium alloy (A1-0.85Si-0.72Mg0.48Mn-0.25Fe-0.05Cu) are conducted in sea water at different temperatures using a specially designed experimental environmental chamber installed on a standard rotating bending fatigue testing machine. The tests are carried out in air at 20 °C to establish the tr-N curve for this alloy as a reference curve, while tests in sea water are at 20, 30, 50 and 80 °C to investigate the role of temperature on the fatigue behaviour. Changing the medium from air to sea water during fatigue results in a significant decrease in the fatigue endurance limit. Increasing the temperature from 20 to 80 °C reduces the endurance limit from 0.58 to 0.3cry. In a generalized empirical estimation formula, the present study suggests an incorporation of an environmental factor (CE), which depends on both the working medium and the temperature. Fracture surface examination of the scanning electron microscope indicates a great dependence of the pitting and/or intergranular corrosion fatigue cracks on the testing temperature and the applied cyclic stress levels. Photomicrographs, graphs, 28 ref. Low cycle fatigue behavior of polycrystalline NIAI at 300 and 1000 K. Lerch, B.A. and Noebe, R.D. NASA Lewis Research Center NASA TM-105987 (1993) 48 pp The low-cycle fatigue behaviour of polycrystalline NiAI was determined at 300 and 1000 K - temperatures below and above the brittle-to-ductile transition temperature (BD'I'T). Fully reversed plastic strain-controlled fatigue tests were conducted on two differently fabricated alloy samples: hot isostatically pressed (HIPed) prealloyed powder and hot extruded castings. HIPed powder