• Miranda, Roberto M. C.
• Richter-Trummer, Valentin
• De Castro, Paulo
• Albuquerque, Carlos

Abstract

Scanning electron microscopy (SEM) observation of the fracture surface of CT specimens was carried out at CEMUP in the context of a FEUP RD on the contrary, the regions presenting striations seem to have a mainly random orientation. This fact contributes to the absence of correlation between the distance between striations (s) measured using SEM, and the macroscopic fatigue crack growth rate da/dN. Only for substantial values of the crack length/specimen width (a/W) ratio a reasonable agreement between s and da/dN is found. The relationship s/(da/dN) presents values that decrease up to approximately one, as a/W increases. For low values of a/W, s is one or two orders of magnitude greater than da/dN. In the specimens analyzed, fatigue crack growth rates below approximately 2 to 3 x10‑7 m/cycle are associated with approximately constant s values. This border is over the transition between regions I and II of the typical sigmoidal log-log plot of the da/dN vs. DK (I corresponding to the near threshold regime and II to the Paris law regime), and this fact should be accounted for in failure analysis involving SEM of fracture surfaces. Finally, the distance between striations s proved to be insensitive to the presence of important residual stresses in the case of the welded specimens.

Topics

• impedance spectroscopy
• surface
• scanning electron microscopy
• crack
• steel
• fatigue
• random