numero25

D. Nowell et alii, Frattura ed Integrità Strutturale, 25 (2013) 1-6; DOI: 10.3221/IGF-ESIS.25.01 3 4 ( ) 2 I i K r u r E    (1) so that a plot of log(relative displacement) against log(r) may be used to estimate the stress intensity factor at any particular time during the crack loading history. Figure 2 : Typical image showing the crack tip, together with the five pairs of measurement points. R ESULTS hilst the main experimental programme concerned the investigation of behaviour under constant amplitude loading, a small number of specimens were subjected to a single overload in an otherwise constant amplitude load history. Analysis of this situation represents a useful first step towards a full theory which can be applied in any general loading case. Fig. 3a shows the schematic loading history, whereas Fig. 3b shows crack growth rate data (plotted against nominal  K) for a number of 3mm thick specimens, including two (CTF6 and CTF8) which were subjected to an overload. The resulting retardation in crack growth rate can clearly be seen. Figure 3 : (a) Schematic load history for single overload specimen; (b) Variation of crack growth rate with nominal elastic  K for 3mm thick specimens, including two with single overloads. W Load Time (a) (b)