EVALUATION OF CRACKING BEHAVIOR OF PIPELINE X70 STEEL FROM DWTT BROKEN SPECIMENS

Fracture surfaces of X70 steel DWTT broken samples are analyzed using statistical methods and fractal concepts. Applying self-affine fracture profile construction method and the first return probability method the Hurst exponent of ductile fracture areas has been found. Increasing percentage of ductile fracture on broken DWTT sample surface the Hurst exponent grows. Modeling of the DWTT fracture surface area by triangle net the statistical distribution of deviations between normal vectors of two neighboring elementary triangles has been discovered. This distribution behaves as Gaussian mixture. The mean value of deviation angles is lower for ductile than for brittle fracture regions whereas scatter of deviation angles are about the same for the both modes of cracking. Calculated fracture surface map of ductile fracture probability highly corresponds to real distribution of ductile and brittle fracture areas on fracture surface of the broken sample. Newly developed statistical methods of the fracture surface treatment of DWTT broken samples is useful tool for objective evaluation of ductile fracture percentage.