Initial Damage, The Effect And Mechanism Of The Notch Toughness Of Metallic Materials

In this paper, a series of 4-point normal and reverse bending experiments were carried out in notched specimens of a HSLA steel and LD31 Al alloy, and different levels of micro-cavity damage were introduced and the effect of the notch root radius was eliminated .To separate the effect of damage from residual stress and work hardening in the specimens, the specimens are tempered at high temperature after warm pre-loading. Four- point bending (4PB) tests are conducted subsequently at room temperature and -196℃.By experimental observation, measurement of mechanical parameter, finite element method (FEM)calculation and simulation, the effects of pre-damage on the cleavage fracture toughness and ductile initiation toughness were investigated. The main results obtained are as follows:(1) For CF steel specimens at — 196℃ , experimental results show that in the case of the pre-load ratio P₀/P_gy≤1.05 , there is little damage in the specimens and the notched toughness parameters Pf, Wf and P_f/P_gy and the cleavage fracture stress σ_f do not essentially change. However, in the case of Po/P_gy>1.05, the notched toughness and σ_f decrease rapidly with the increase of the Po/Pgy.(2) The reason for the decrease of the notched toughness with increasing the Po/P_gy is that the micro-cavities, especially the strip defects induced in the pre-loading induce local high stress and strain around the defects. These local high stresses make up the lack of normal stress σ_yy ahead of notch, and promote the cleavage initiation process occurring at low P_f and reduce the values of σ_yy=σ_f.(3) For CF steel specimens at room temperature, when the pre-load ratio Po/Pgy is less than 1.05, the ductile initiation load Pi does not essentially change. But in the case of 1.050/P_gy<1.47, the Pi decreases obviously with increasing the P₀/P_gy. This indicates that pre-damage decreases the ductile initiation toughness. For the LD31 Al specimens, the curve of Pi- Po/P_gy is similar to that of the CF steel specimen, but its toughness begins to decrease at P₀/P_gy=1.0, and decrease quickly for the case of 1.0< P₀/P_gy≤1.25 and the values of Pi arelower than that of the CF steel.(4) The damage volume and the sizes of micro-cavities in specimens increase with increasing the pre-load P₀/P_gy. During the later loading process, the initial cavities grow continually and quickly, and coalesce with the other cavities and the root of the notch at low applied load. At the same time, new cavities nucleate, growth and interact with initial cavities, which makes damage process fast and decreases the toughness of materials.(5) The FEM simulation of damage evaluation for containing pre-damage shows that triaxiality σ_m/σ_e and equivalent strain ε_p promote the cavity growth. The initial cavities with large sizes and the cavities located at special location (present 45° with the S22) grow fast. There exists local high strain ε_p between big cavities, which promotes their coalescence. With the increase of the σ_m/σ_e, the growth rate of micro-cavities and the damage development rate get fast, which decrease the ductile initiation toughness of materials. At the same σ_m/σ_e, different stress states (S11,S22,S33) have effect on the damage evaluation. When the cavity's distributions are not uniform, the damage development rate is faster.