| Summary: | It is known that cracks grow in materials subject to both mechanical and thermomechanical fatigue. Work has been undertaken to investigate the shapes of cracks in two materials in low cycle fatigue (LCF) and thermomechanical fatigue (TMF) conditions utilising novel Alternating Current Potential Difference (ACPD) and beach marking methods to measure the crack length. This has been added to by the development of a novel beach marking technique which enables the user to define the position and size of marks.
Crack growth measurement work has been undertaken on two materials, 316 Stainless Steel (SS316) and Nickel Alloy RR1000 (RR1000), using a fixed frequency ACPD system. In this work a range of crack shapes and directions have been recorded but due to the fixed frequency of the Alternating Current Potential Difference (ACPD) system this could not be measured during the tests. In this both quarter-circular and non-uniform crack shapes have been present, depending on the loading and temperature exerted during the test.
An advanced ACPD technique has also been developed and tested using a range of specimen and feature geometry. In experiments where the cross-sectional area has remained the same the technique has been shown sensitive to the cross-sectional shape. The change in the response (impedance magnitude and phase) with frequency has been shown to systematically vary for the different cross-section shapes, suggesting the method is capable of discerning different cross sectional geometries. A range of crack like features have also been tested, in this the direct current (DC) response has been shown to vary as is expected due to the area of the feature. In addition to this as the frequency increases and hence the skin depth is reduced the response changes to reflect not just the feature area but the shape.
A novel fracture mechanics beach marking technique has also been developed, which enables the user to program the position and size of beach marks on a fracture surface without needing additional instrumentation. This technique has been shown to be able to produce beach marks down to 25μm, the technique gives crack position predictions to about 2% of the experimental value.
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