Mechanics of fracture and fatigue in some common structural configurations

Parker, Anthony Philip (1978) Mechanics of fracture and fatigue in some common structural configurations. University of Southampton, Doctoral Thesis.

Abstract

The main problem investigated is that of the prediction of stress intensity factors and associated fatigue growth rates for radial cracks emanating from single and multiple holes in common structural configurations. A critical review of previous work is conducted. It is opined that the problems experienced by earlier workers on unflawed structures are also important in the flawed structure. Certain unflawed pin load distributions are considered to be physically unacceptable. It is argued that the common practice of employing crack-line stresses obtained Itom an unflawed stress distribution is not valid, neither is the technique of inferring geometrical fracture mechanics parameters for a pin-loaded configuration from those of a geometrically similar remotely loaded configuration.The numerical least squares (overdetermined) boundary collocation technique is reviewed and formulated for configurations having uniaxial and biaxial symmetry. A computer programme, based on this formulation, is described and used to obtain some new stress intensity factor solutions, and modifications to existing ones. Infinite sheet solutions are obtained for various loadings applied to configurations involving a circular hole with two diametrically opposed, unequal length cracks, and for the case of equal length cracks when the hole boundary is loaded by a variable arc of pressure. Stress intensity factors are given for several loadings applied to a finite rectangular panel with equal length cracks emanating from a centrally located, circular hole, including a form of pin loading applied to the panel. Analogous solutions are presented for an array of such cracked holes in a finite height strip, including a form of rivet loading applied to the strip. The results are discussed and explained, in particular a comparison of the pin load results with those of other workers is effected.The results are presented of an experimental programme of fatigue crack growth rate investigations on remotely loaded and pin-loaded, cracked circular holes in aluminium alloy. It is shown that the crack growth rate for the remotely loaded specimens may be predicted from the numerical stress intensity factor solutions. Crack growth rates may also be predicted for the pin-loaded specimens provided the distance from the pin centre to the nearest edge perpendicular to the axis of loading is greater than one half of the strip width. When the distance is smaller than this, deviations are explained by the tendency for crack growth direction to deviate from a direction perpendicular to the loading axis. The Bueckner weight function technique is employed in-the derivation of analytic solutions to a configuration which models an array of cracked, rivet loaded holes. Suggestions are made for future work involving cracked pin and rivet-loaded holes, improving the accuracy of collocation solutions at short crack lengths and in regions of high geometrical eccentricity, and for the application of weight function techniques tonumerical solutions.

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