A method for evaluation of fatigue process zone (FPZ) dimensions near a notch root in metal and alloy specimens by using a two-step phase shifting interferometry (TS PSI) technique is proposed. In comparison with other destructive and nondestructive methods evaluating the FPZ dimensions, it possesses higher accuracy and performance. The method uses a criterion for the FPZ dimensions definition based on an assumption that the surface roughness of notched specimens after cyclic loading reaches its maximum values at the FPZ boundary. To realize this method, first, a phase map (PM) of a total surface relief near a notch root is retrieved; second, roughness and waviness PMs are extracted from the retrieved total surface relief PM by using the TS PSI; and finally, a surface roughness parameter Ra spatial distribution is calculated according to the offered criterion and the FPZ size d* is defined. The FPZ size was measured for specimens made of low-carbon steel and aluminum alloys 2024-Т6 and 7075-T3. Obtained experimental results have shown that the proposed criterion allows defining the FPZ size for notched specimens made of metals and alloys possessing high, moderate, and low plasticity.
A new two-step phase shifting interferometry technique for evaluation of a fatigue process zone (FPZ) in notched metal
and alloy specimens is proposed. In comparison with well-known destructive and nondestructive methods evaluating
FPZ, this technique possesses higher accuracy and performance and allows defining the FPZ size for notched specimens
made of metals and alloys with low, moderate or high plasticity. The technique is fulfilled by retrieval of a total surface
relief of a studied notched specimen, extraction of surface roughness and waviness phase maps from the retrieved surface
relief, calculation of a surface roughness parameter Ra spatial distribution and definition of the FPZ size by using an
extracted surface roughness phase map. Obtained experimental results have confirmed assumption that the surface
roughness of notched specimens after cyclic loading reaches its maximum values at the FPZ boundary. This boundary is
produced as the narrow strip containing pixels possessing the maximum values on the spatial distribution of the
roughness parameter Ra near a notch root. The basic distances d* defining the FPZ sizes were measured for notched
specimens made of a low-carbon steel and aluminum alloys 2024–Т6 and 7075–T3. Results of the distances d*
measurement are very close to respective results obtained with the help of other methods for the FPZ evaluation.
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