Several samples of the common ferritic tool steel AISI 1045 were fatigued in cyclic load tests. The local distribution of the Von-Mieses stress VM was simulated using the finite elements method (FEM). In the regions of interest, where VM reaches maximum values, the defect distribution was measured spatially resolved by Doppler-spectroscopy (DBAR) employing the Bonn Positron Microprobe (BPM). The lateral distribution of the S-parameter, which could be described by a simple model derived from linear fracture mechanics, corresponds well with the simulated Von-Mieses stress
The concentration of lattice defects in plastically deformed metals can be measured by positron annihilation spectroscopy (PAS) with an outstanding sensitivity. The positron acts as a highly mobile atomic probe sensitive to all defects forming an open volume in the lattice. Using a positron microbeam, like the Bonn positron microprobe (BPM), the lateral distribution of these defects in the sub-surface layer can be mapped with a resolution down to one micrometer. In this work the changes in the defect concentration were determined during tension tests on the aluminium alloys AA2024, AA6013 and AA6082. The results show that these changes depend on the configuration and the heat treatment of the alloys. Moreover, alternating load fatigue tests were performed on AA6082. The defect distribution was measured laterally resolved employing the BPM in several early stages of fatigue. Using those results the number of cycles to fatigue failure was extrapolated. The trueness of the prediction was tested by further fatiguing the sample until failure occurs.
We describe an alternative approach for a reliable lifetime prediction employing the local concentration of lattice defects as a precursor for fatigue failure. We present positron annihilation spectroscopy (PAS) as a non-destructive technique sensitive for defect concentrations in the range relevant to plasticity in metals.The Bonn Positron Microprobe (BPM), a currently unique device, provides a fine focused positron beam with a selectable beam diameter from 5 to 200 μm assisted by an inbuilt fully functional scanning electron microscope (SEM). Using the BPM, plasticity and fatigue can be measured with a lateral resolution from some microns up to the range of millimeters.Employing laterally resolved PAS and the empirical supposition of a linear relation between the defect concentration and the logarithm of the number of fatigue cycles, the point of failure was successfully predicted on the common carbon steel AISI 1045. For a generalization of the precursor method, a minimal model of fatigue based on a cellular automaton was developed. First results from a one-dimensional implementation are presented.
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