A novel method to determine critical CTOA directly by load-displacement curve

The critical Crack Tip Opening Angle (CTOAC) has been proved to be a promising parameter to characterize the crack arrest toughness of natural gas pipeline. However, the accuracy and consistency of its experimental measuring results are seriously affected by the uncertainties in locating the crack tips and the points on both crack surfaces at a certain distance behind the crack tips. Load-displacement curve is a comprehensive reflection of geometry, mechanical property and fracture behaviour of a loaded specimen. It would be highly advantageous to deduce CTOAC from load-displacement curve directly. This is exactly what this paper has done. Firstly, in order to combine the load, displacement and deformation of single edge notched bend (SENB) specimen together organically, the Martinelli-Venzi ductile fracture model is used to analyze its dynamic fracture process together with energy equilibrium theory and a correlation between CTOA and the slope of the energy-load curve is established. Secondly, based on the fact that load decreases almost linearly with the increment of displacement during steady-state crack growth, attempts have been made to regress the load-displacement curve linearly and express the slope of the energy-load curve by the linear regression equation. Thirdly, the correlation between the load-displacement curve and instantaneous CTOA is found by the bridge of energy load gradient. Finally, CTOAC can be determined by averaging the stable state CTOA values. The experiments and corresponding finite element simulations of SENB specimens with X80 pipeline steel are conducted. CTOAC results obtained by both methods are in good agreements with the theoretical ones, indicating that the method proposed in this paper is satisfactory. The CTOAC derived from the published Drop Weight Tear Test (DWTT) load-displacement curves is also consistent with the experimental results, which further proves its wide applicability in different steel grades and loading rates. The method presented in this paper can significantly improve the analysis efficiency and has great potentialities for industry application.