During a creep crack growth (CCG) test, any change in PD after the initial load-up is attributed to crack growth however, creep strains which accumulate at the crack tip will also influence the PD. This is a possible source of error in the measurement of incubation time subsequent crack growth. A method of differentiating between the influence of creep strains and crack growth is therefore required, particularly for ductile materials where the influence of these strains on PD during may be significant.
It has been predicted using finite element modelling that the relationship between load-line displacement (LLD) and PD is different during incubation and crack growth. A point of inflection on a plot of PD against LLD should therefore identify the onset of crack growth, similar to the approach often employed during J-R curve testing. This paper presents experimental validation of the proposed new method. Three nominally identical CCG tests were performed on C(T) specimens manufactured from ex-service type 316H stainless steel and interrupted after different amounts of crack extension.
The proposed new method of interpreting the PD data can accurately identify the onset of crack growth. The incubation time can be very different to the time for 0.2 mm of crack growth to occur which is the current definition of crack initiation in ASTM E1457 13. This difference in incubation period can also effect the subsequent crack growth rate measurements, particularly for tests where small amounts of crack growth occur
