Crack growth under general load spectrum: numerical simulation and experimental tests

摘要

Experimental observations of three- and two-dimensional fatigue crack growth are compared to numerical predictions obtained by the computer code BEASY, which is based on Dual Boundary Element Method (DBEM). General modelling capabilities are allowed by this approach, with the allowance for general crack front shape and a fully automatic propagation process. A part-through corner crack is initiated on a pre-notched specimen undergoing a traction fatigue load, as defined by a general load spectrum. Experimental tests on a fatigue machine are carried out in order to validate the numerical simulation and to provide the necessary material fatigue data for the aluminium plates. By means of a nonlinear regression analysis, applied on in house obtained experimental data, the material parameters for the NASGRO 2.0 crack propagation law are defined, capable to effectively keep into account the threshold effect, the unstable final propagation and the stress ratio influence. Moreover the crack growth retardation, after an overload, is simulated by the Generalised Willenborg model; to this aim the shutoff ratio is obtained by fitting experimental data coming from tests on specimens undergoing a general load spectrum. A satisfactory agreement between numerical and experimental crack growth rates are displayed, both for three-dimensional (part-through cracks) and two dimensional (through the thickness cracks) simulation.