Abstract
Fatigue surface crack growth and the in-plane and out-of-plane constraint effects are studied through experiments and computations for aluminium alloy D16T. Subjects for studies are cruciform specimens under different biaxial loading and bending central notched specimens with external semi-elliptical surface crack. Both the optical microscope measurements and the crack opening displacement (COD) method are used to monitor and calculate both crack depth and crack length during the tests. The variation of crack growth rate and surface crack paths behaviour is studied under cyclic pure bending and biaxial tension-compression fatigue loading. This work is centered on the relations between crack size on the free surface of specimen considered configurations, COD and aspect ratio under different fatigue loading conditions. For the experimental surface crack paths in tested specimens the T-stress, the local triaxiality parameter h, the out-of-plane TZ factor and the governing parameter for the 3D-fields of the stresses and strains at the crack tip in the form of In-integral were calculated as a function of aspect ratio by finite element analysis to characterization of the constraint effects along semi-elliptical crack front. The plastic stress intensity factor approach is applied to the fatigue crack growth on the free surface of the tested bending and cruciform specimens as well as the deepest point of the semi-elliptical surface crack front. As result fatigue surface crack paths or crack front positions as a function of accumulated number of cycle of loading are obtained.
Highlights
I n order to provide operation in a safe condition, it is necessary to perform fracture mechanics assessment of a structural component under cyclic loading
Firstly experimental results of fatigue crack growth for a crack starting from a semi-elliptical notch in an cruciform specimens under biaxial loading and bending plate are given
Different crack growth rate is observed in the direction of the deepest point of the crack front with respect to the free surface of the bending specimen
Summary
I n order to provide operation in a safe condition, it is necessary to perform fracture mechanics assessment of a structural component under cyclic loading. The fatigue failure of structural elements subjected to biaxial stress system maybe develops from surface flaws, and only several analyses have been carried out to determine the stress intensity factors along the front of an edge defects and crack growth rate study on this base [1,2,3,4]. Firstly experimental results of fatigue crack growth for a crack starting from a semi-elliptical notch in an cruciform specimens under biaxial loading and bending plate are given. The computational 3D fracture analyses deliver a governing parameter of elastic-plastic stress field distributions along the crack front On this base crack growth interpretation is performed using the traditional elastic and new plastic stress intensity factors [5,6,7]. The test material is aluminum alloy D16T which main mechanical properties are listed in Tab. 1 where E is the Young’s modulus, b is the nominal ultimate tensile strength, 0 is the monotonic tensile yield strength, u is the true ultimate tensile strength, is the elongation, is the reduction of area, n is the strain hardening exponent and α is the strain hardening coefficient
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