Experimental and Theoretical Investigations of Fatigue Crack Growth in D16 Alloy

In this work the arising and development of fatigue crack in aluminium alloy D16, taking into consideration the influence of notch in the form of hole with incisions which were done on its sides were tested. The process of working out of tests' programme was preceded by numeric analysis of stresses pattern and strains distribution in the zone of notch’s influence which was focused on determination of stresses' face values. To examine stress and strain distributions, the specialised FEM software (MSC.Patran and MSC.Nastran) was applied. Findings have been presented in the form of a statement of σ_max and ε_max values, and functions of the following factors: a_σ, a_ε, and a_k, computed on the grounds of these values for both different distances from the bottom of the notch and assumed levels of loading the specimens. Theoretical analysis has been supplemented with experimental investigation into the microstructure of fatigue-fracture surfaces in the area of crack initiation and that of fatigue of a propagating crack. The paper has been intended to present a model of the probabilistic estimation of fatigue life of structural members. The model has been based on the deterministic description of the cracking. Analyzed were components with notches in the form of centrally located holes with side cuts. In the method of probabilistically approaching the crack propagation, some dependences have been used that take account of the presence of areas showing plastic strains in front of crack tips. It has been assumed that the cracking can be modeled on the grounds of some general-purpose quantity used to describe the energy state in the area of the crack tip, i.e. the Rice's integral (J). The formulated computational model has been used to estimate fatigue life of model components made of D16 alloy. Experimental work was carried out using some flat specimens with centrally positioned holes. They were exposed to flat bending at R = 0. Analytical and experimental results have shown pretty good conformity.