Issue 51

K. Bahram et alii, Frattura ed Integrità Strutturale, 51 (2020) 467-476; DOI: 10.3221/IGF-ESIS.51.35 469 0 1 i R pic max pic a a K K K R              (5) where eq R is expressed by the following equation: 2 1 max R eq y K K R                   (6)  is state of stress in the direction of propagation of the crack (plane stress or plane strain) , and  define the level of residual stress induced by application of overload, it is given by the following equation: 1 SOR   1 THO K K            (7) where 0 TH K  represents the threshold stress intensity factor for a load ratio R = 0. SOR: Shut-off Ratio - Ratio of overload maximum stress to the subsequent maximum stress required to arrest crack growth The value of K pic is updated whenever the maximum load exceeds the previous maximum value, or when the current plasticized area extends beyond the area created by the overload. The exact value of the SOR is varied to adjust the life prediction to match test results. Ideally, the SOR should be a material parameter that is insensitive to spectrum or stress level. However, this does not always work out. The following is a list of common SOR values for some materials: As an indication, 2 SOR  for the steels, 2.7  SOR  for the titanium alloys and 3  SOR  for the aluminum alloys. It is therefore assumed that the delay disappears as soon as the current plasticized zone enters a zone of marerial not affected by the overload. Figure 1: Willenborg retardation model