This paper attempts to find the possibility of describing the supplementary parameters of the macroscopic crack strength field based on the variation at the microscopic level. The analysis focuses on the development of cracks in the vicinity of the crack tip of a central crack panel made from iron material with a cubic crystal structure under tensile loading. We use the combination of finite element and molecular dynamics simulation methods. Molecular dynamics simulation analysis shows that the motion velocity of molecules near the crack tip region is less than the Rayleigh wave velocity, VR, which is about 0.22 VR. The average velocity is smaller in other directions that are not parallel to the crack. The velocity of the molecular near the crack tip indicates the ductile phenomenon of the crack tip region because it comes from the potential energy, which means that on the crack extension line, the kinetic energy of the molecular motion is greater than that in the other directions but is close to that in the 45◦ direction. It shows that the crack expansion direction is still mainly along the crack extension line under the load.
