Mode I / Ii Mixed-load Material Fracture Behavior

In this paper, a series of asymmetry four-point bending(AS4P) experiments are carried out on the HSLA steel and LD31 specimens. By the mechanical tests, fracture surface and metallographical observations of unloaded specimens of HSLA steel and LD31,combing with the detailed FEM calculations, the deformation, the deactivation shape and the ductile damage and fracture behaviors of the notch tip of the HSLA steel and LD31 under mixed Mode Ⅰ/Ⅱ, and the cleavage fracture behavior of HSLA steel are investigated. The main results are as follows:(1) When the specimens are only under Mode Ⅰ loading, the deformation and the plastic zones ahead of the notch are symmetry. One side ahead of the notch is blunted, another is sharpened, and the plastic zone is rotated by an angle θ. With the increasing of the ratio of Mode Ⅱ, the degree of the deformation of blunting and sharpening and the angle θ areincreasing, but the σθθ, εp and σm /(?) decrease. The distribution figure of the equalvalue of σm /(?) shows that σm /(?) is symmetry on the center of the notch when the specimens are loaded under Mode Ⅰ, and the value of σm /(?) is maximum at the center of the notch. The value of σm /(?) in the blunted side of the notch is high, but the value of σm/ (?) in the sharpening side of the notch is low, even a negative value. With the increasing of the ratio of Mode n, the value of σm/(?) in the blunted side decreases, but the value of σm/(?) in the sharpening side increases.(2) In this study, It is shown that the mechanisms and the locations of the deformation, damage, and fracture which are ahead of a notch change with the increasing of the ratio of the Mode Ⅱ when the specimens are loaded under mixed Mode Ⅰ/Ⅱ .When the ratio of Mode Ⅱ is low, the ductile damage and fracture are controlled by the growth and the coalescence of the micro-voids located at the blunted side of the notch. Then with the increasing of the ratio of Mode Ⅱ, the mechanism gradually becomes the competition between the micro-void damage of the blunted side and the shear damage of the sharpening side of the notch. It is controlled by the shear damage and fracture mechanisms of the sharpening side of the notch when the specimens are loaded under high ratio of Mode Ⅱ. The analysis and explanation of the transition of the damage and fracture behavior are made by the results of FEM calculations.(3) It is shown that the macro maximum tangential stress criterion (MTS) can be satisfied to the cleavage fracture of the notched specimens which are under mixed Mode I/UU loading by the investigation of the effects of mixed Mode Ⅰ/Ⅱ loading on the initiation behavior of cleavage fracture of HSLA steel. The mechanisms of the initiation and propagation ofcleavage fracture and the local cleavage fracture stress σf remain nearly constant with the change of the ratio of Mode Ⅰ/Ⅱ. The cleavage fracture of the notched specimens which are under mixed Mode Ⅰ/Ⅱ loading also obey the criterion (εp≥εPC and σyy ≥ σf ).