To obtain the inner damage mechanism of a new type of thermal stress precision cropping bar. Athermal stress coupling model of the bar with an annular V-shaped notch in the course of cooling was established. The stress field in the inner of the bar was evaluated by means of ANSYS software. The numerical simulation results show that the axial tension stress gets maximum at 0.34 mm from the bottom of the tip of the V-shaped notch on the bar cross-section. As the maximal stress and its nearby stress exceed the ultimate yield strength of the material

an annular damage strip in the bar cross-section of the V-shaped notch and close to V-shaped notch tip appears. At the same cycle cool water velocity

the damage strip increases and then decreases with the increase of width of adiabatic boundary

and reaches to the maximum when the width of adiabatic boundary is 2 mm. With the same width of adiabatic boundary

the damage strip increases with the increase of water velocity

and it changes feebly when the water velocity is greater than 5 m/s. The damage strip disappears when the water velocity is equal to1 m/s. The cropping experimental results show that the cropping time

by heat stress prefabricating an ideal crack at V-shaped notch bottom

is reduced by 15% compared with the simplex variable-frequency vibration cropping method without the action of the heat stress

which validates the thermal stress behaviors.