Loading Process Simulation In Tensile Test，charpy Impact Test, COD Test And Critical Parameters’determination

This paper investigates the mechanism of cleavage fracture in precracked specimen andnotch specimen made of modified C-Mn steel with three different grain sizes and carbideparticles sizes (fine grain and fine carbide-particle microstructure、fine grain and coarsecarbide particle microstructure、coarse grain and coarse carbide particle microstructure)atlower shelf region of toughness-transition temperatures(-60℃~-100℃). Moreover, Tensileproperties of NiCrMoV low-carbon bainitic high strength steel of two weldedjoints(DM4-1,DM4-5) in different welding variables at various testing temperature(roomtemperature,-50℃,-110℃,-196℃)，and charpy properties of base metal and five welded jointsin different welding variables at-196℃are also calculated and analyzed.Three point bending COD crack model, Charpy-V notch model of modified C-Mn steelsand and tensile model of NiCrMoV low-carbon bainitic high strength steel welding jointswere established by using the commercial finite element software ABAQUS and the differentstages of the stress and strain distribution were simulated.Combined with the experiment dataCOD, fracture distance Xfand the finite element calculation of stress and strain distributioncurves,the local fracture stress σf, fracture strain εpcand the critical stress triaxiality Tcweremeasured when cleavage fracture was happened for specimens with three different grain sizesand carbide particles sizes in different temperatures.By adding the G-T-N damage model totensile simulating of welded joints at different temperatures, tensile curves were fitted wellwith experiments, and the normal stress σyy, equivalent plastic strain εpq, and stress triaxialityT in the cross section along radial direction were calculated at fracture load, the followingmain conclusions are drawn from these study：(1) XFEM can simulate brittle crackpropagation well, and is not depend on the mesh, but it is still not perfect for plastic crackpropagation. DEBOND standard is also need, plastic crack propagation can be realized bysetting master and slave surfaces and cracking statements.(2)For the precrackedspecimen,carbide size has large influence on tougheness brittle transition temperature andfracture toughness in three point bending precracked test, however, effect of grain size issmall.(3) For the notch specimens in the same carbide sizes, impact toughness of finegrain-coarse carbide specimens is higher than that of coarse grain-coarse carbide specimens,so carbide size has large influence on tougheness brittle transition temperature and fracturetoughness in notch specimens.(4) material properties and G-T-N damage parameters ofNiCrMoV low-carbon bainitic high strength steel of two welded joints(DM4-1,DM4-5) areobtained.(5)To study impact properties of the base metal and five kinds of welding joint at -196℃, orderof the cleavage fracture stress σfis listed: MC> DM4-1> T100> M100. Thesurface energy is calculated by two cleavage criterion. The main reasons of various charpytoughness for different welding joints in different welding variables are attributed to thedifference of cleavage critical cleavage fracture stress and plastic surface energy.