Effect Of Crack Size Research Articles

Fatigue crack propagation thresholds for long and short cracks in René 95 Nickel-base superalloy

A study was made of the near-threshold fatigue crack propagation behaviour of a wrought nickel-base superalloy, René 95, with reference to the effect of crack size on the threshold stress intensity ΔK 0 no detectable crack growth. Measured threshold ΔK 0 values at low load ratios (R = 0.1) for physically short (0.01 – 0.20 mm) cracks were found to be 60% smaller than the corresponding ΔK 0 values for long (about 25 mm) cracks. However, short crack threshold values at R = 0.1 were found to be similar to long crack thresholds at R = 0.8. Such behavior is rationalized in terms of fatigue crack closure, specifically involving the role of fracture surface roughness from crystallographic crack growth in nickel-base alloys. The large difference observed between the threshold values for long and physically short cracks serves to illustrate the potential problems in applying conventional (long crack) fatigue data to defect-tolerant lifetime predictions for structural components containing small flaws.

Discrete dislocation analysis of a tensile crack under fatigue

The discrete dislocation method is employed to determine the effect of pre-existing crack size on the behavior of a plastic tensile crack under static loading. The behavior of a tensile crack under cyclic and reverse loading is also determined by the same method. Three important steps in the fatigue crack growth during each cycle are identified. It is concluded that the details of the fatigue crack growth during each cycle of loading cannot be inferred from the fatigue striations observed on the crack surfaces after fracture. The statistical nature of fatigue is explained by the range of pre-existing crack sizes present in a material which may or may not contain a crack of critical size that can propagate under cyclic or reverse loading.

Effect of crack size on acoustic emission related to moving dislocations

Acoustic emission from a crack vicinity in a uniformly stressed solid is correlated with the crack size based on a continuous theory of dislocations. It is assumed that the total number of acoustic emissions is directly proportional to the total number of nonuniformly distributed moving dislocations in the plastic zone near the crack. The resultant equation of the relationship between acoustic emission and crack size can be expanded into a power series of applied stress. The first-order approximation is consistent with that of a macroscopic fracture mechanics consideration on the emission related to the plastic zone size. The theoretical calculation is verified with experimental results obtained from a flawed carbon steel vessel by a multichannel acoustic emission monitoring system and agrees reasonably well with the experimental data. Further calculations by utilizing the theory and by analyzing available experimental data of acoustic emissions from center-surface-flaw specimens and wedge opening load specimens of various crack sizes also show agreement between the theory and the experimental data.

Effect of crack size on acoustic emission

Acoustic emissions from the vicinities of cracks were monitored by a multichannel acoustic emission system, The experiment was performed on a flawed carbon steel vessel under hydraulic pressure providing a nearly uniform external stress in the vessel wall, However, the stress fields near the cracks were nonuniform. The numbers of acoustic wave packets generated from the individual cracked regions were analyzed by an on-line minicomputer, The correlation between the number of the wave packets and the size of crack at low stress levels was found experimentally and is consistent with a theory of non-uniformly distributed moving dislocations in the vicinity of crack, Further calculations by utilizing the theory and by analyzing available experimental data of acoustic emissions from other types of flawed specimens of various crack sizes also show agreement between the theory and the experimental data.

Effects of Crack Size on the Gross-Strain Crack Tolerance of A-533-B Steel

Gross-strain crack tolerance is a measure of fracture toughness that is useful where yielding precedes fracture yet flaw size must be treated quantitatively. In these tests, rectangular bars containing surface fatigue cracks were pulled in tension at several temperatures. Strains were measured at the edges of the specimen on a gage length spanning the plane of the crack, and also in the gross section above and below the crack. Gross-strain crack tolerance is the strain at maximum load on the gage length designated. It is shown to be a function of crack size, constraint, and temperature.

A laminate composite with a crack normal to the interfaces

The redistribution of stresses in a laminate composite due to the presence of a crack or flaw situated normal to the bond lines is studied. The many-layered composite is idealized to the case of a single layer of dissimilar material containing a crack which is sandwiched between two other layers of infinite height. The elastic properties of the two outer layers are assumed to be averaged properties over a large number of layers. Using the integral transform technique, the problem is formulated in terms of integral equations and solved for the singular stress field near the crack tip. The effects of crack size, layer height and the material properties of the composite on the stress-intensity factor are illustrated graphically. Presumably, this factor can be used to characterize the strength of a composite in the same way as it has been applied successfully to the single phase material within the framework of the linear theory of fracture mechanics.Calculations are also carried out for approximating the stress-intensity factors for a crack inclined at an arbitrary position in the sandwiched layer. This is accomplished by taking the two extreme cases of a crack parallel and normal to the interfaces as the upper and lower bound solutions depending upon the relative stiffness of the layers.