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Crack growth and crack initiation in cylindrical specimens subject to low cycle strain controlled tension-compression induced fatigue, noting measurement techniques
Interfaces between fatigue, creep and fracture, discussing separation of fatigue process into crack initiation and crack propagation
An investigation of delamination crack growth in a unidirectional composite laminate under static and fatigue loading conditions is presented. An experimental program was conducted to study failure mechanisms and characterize the crack growth rate; analytical modeling based on microscopic observations was carried out using an advanced singular finite element method of the hybrid-stress model. Using a mixed-mode failure criterion, the delamination growth under a monotonically increasing load can be predicted; the rate of delamination crack propagation under fatigue loading is directly related to the amplitudes of cyclic opening and shearing mode stress intensity factors by a power law relationship.
Here, the effect of fatigue on the microstructure of four-point bend specimens of three variations of 304 stainless steels (Commercial 304, 304H, and 304 L) was investigated using synchrotron x-ray tomography and diffraction. X-ray tomography revealed the formation of the fatigue-induced microvoids and crack while the diffraction data was used to quantify the amount of deformation-induced martensite found after fatigue in all samples. Transmission electron microscopy evidenced the role of the precipitates/inclusions on the microvoid formation. It was found to depend on their chemical nature. The shape of the precipitates/inclusions was also found to have an effect on the microvoid shape.
Statistics of distribution of rises and falls in continuous random process, with application to gaussian processes
Fatigue fracture surface macroscopic and microscopic appearance as affected by prevailing stress intensity conditions at moving crack tip
Stress ratio effect on axial load fatigue and crack propagation tests on aluminum alloy sheets
A definition of the envelope of nonstationary random processes is proposed. The establishment of the envelope definition makes it possible to simulate the nonstationary random envelope directly. Envelope statistics, such as the density function, joint density function, moment function, and level crossing rate, which are relevent to analyses of catastrophic failure, fatigue, and crack propagation in structures, are derived. Applications of the envelope statistics to the prediction of structural reliability under random loadings are discussed in detail.
Efficient and practical methods of simulating stationary and nonstationary random envelope processes are presented. The stationary envelope processes are simulated by using the fast Fourier transform while the nonstationary envelope processes are simulated as the square root of the sum of a series of cosine functions and a series of sine functions with random phase angles. Typical applications of the envelope simulation are the simulations of peaks and troughs which play an important role in the analyses of the first excursion probability, fatigue and crack propagation. In particular, applications to the crack propagation under random loadings are demonstrated in detail.
The range of topics covered by the papers includes: rapid quenching (and specifically splat quenching) of metals, spin quenching, levitation melting, rapid quenching from liquid state; amorphous metal phases and metastable phases, metallic glasses, ribbons, splats, films, metal ribbon reinforced resins; crystallization of metallic glasses, and ferromagnetic amorphous alloys. Structural models for amorphous metals, splat-quenching equipment, and measurements of the properties of amorphous and noncrystalline metals and metallic glasses are also discussed, in addition to: fatigue events (crack propagation) in amorphous metals, fracture behavior in rapidly quenched metals, ductile superconducting Cu-Nb-Sn alloys, and semiconducting amorphous V205 alloy. Individual items are announced in this issue.
Studies on the Bell Helicopter 540 Rotor System of the AH-1G helicopter were performed. The stiffness, mass and geometric configurations of the Bell blade were matched to give a dynamically similar prestressed composite blade. A multi-tube, prestressed composite spar blade configuration was designed for superior ballistic survivability at low life cycle cost. The composite spar prestresses, imparted during fabrication, are chosen to maintain compression in the high strength cryogenically stretchformed 304-L stainless steel liner and tension in the overwrapped HTS graphite fibers under operating loads. This prestressing results in greatly improved crack propagation and fatigue resistance as well as enhanced fiber stiffness properties. Advantages projected for the prestressed composite rotor spar concept include increased operational life and improved ballistic survivability at low life cycle cost.
Semiempirical mathematical model of fatigue damage in stressed objects uses experimental histories of strains in those objects to predict fatigue lives. Accounts for initiation and propagation of fatigue cracks on cycle-by-cycle basis. Measured strain history first digitized, then converted to history of turning-point strains for purposes of analysis. Data between turning points not used. When model calibrated against proper test data for each type of object characterized, its predictions of fatigue lives superior to statistical models as one based on root-mean-square strain.
Three-dimensional crack growth simulation was performed on a split-tooth gear design using boundary element modeling and linear elastic fracture mechanics. Initial cracks in the fillet of the teeth produced stress intensity factors of greater magnitude (and thus, greater crack growth rates) than those in the root or groove areas of the teeth. Crack growth simulation was performed on a case study to evaluate crack propagation paths. Tooth fracture was predicted from the crack growth simulation for an initial crack in the tooth fillet region. Tooth loads on the uncracked mesh of the split-tooth design were up to five times greater than those on the cracked mesh if equal deflections of the cracked and uncracked teeth were considered. Predicted crack shapes as well as crack propagation life are presented based on calculated stress intensity factors, mixed-mode crack propagation trajectory theories, and fatigue crack growth theories.
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The theories and research current available on crack propagation under unsteady loadings, especially those of acoustic origin, are reviewed. Since the original theories on fatigue failure did not account for random loading conditions, modified theories which provide statistical methods for evaluating the random loading have emerged. The impact of acoustic fatigue in the aerospace industry, basic principles such as fatigue crack initiation and propagation and load interactions, and testing procedures are discussed. Attention is also given to metal and metal alloy structures, fiber-reinforced composites and nonmetallic structures, short crack growth, and the effects of temperature, moisture, and corrosion on structures. Suggestions for future research in this field are presented, namely, studies on the effect of 'snap-through' response and associated crack growth patterns, studies in microcrack and 'small crack'; propagation under unsteady loading conditions, and the development of an accurate analytical model to predict acceleration and retardation effects in fatigue crack growth under random loading conditions.
Analysis of six service failures in aluminum alloy components which failed in aerospace applications is reported. Identification of fracture surface features from fatigue and overload modes was straightforward, though the specimens were not always in a clean, smear-free condition most suitable for failure analysis. The presence of corrosion products and of chemically attacked or mechanically rubbed areas here hindered precise determination of the cause of crack initiation, which was then indirectly inferred from the scanning electron fractography results. In five failures the crack propagation was by fatigue, though in each case the fatigue crack initiated from a different cause. Some of these causes could be eliminated in future components by better process control. In one failure, the cause was determined to be impact during a crash; the features of impact fracture were distinguished from overload fractures by direct comparisons of the received specimens with laboratory-generated failures.
Crack initiation and propagation formula through fatigue process applied to interface problems, noting creep
The papers contained in this volume focus on various aspects of low cycle fatigue, including cyclic deformation, crack propagation, high-temperature low cycle fatigue, microstructural defects, multiaxial and variable amplitude loading, and life prediction. Papers are presented on the low cycle fatigue of some aluminum alloys, prediction of crack growth under creep-fatigue loading conditions, high-temperature low cycle fatigue behavior and lifetime prediction of a nickel-base ODS alloy, and an integrated approach to creep-fatigue life prediction. Other topics discussed include thermal fatigue testing of coated monocrystalline superalloys, low cycle fatigue of Al-Mg-Si alloys, and the effect of superimposed stresses at high frequency on low cycle fatigue.