PSI - Issue 23

Keng Jiang et al. / Procedia Structural Integrity 23 (2019) 451–456 Keng Jiang et al./ Structural Integrity Procedia 00 (2019) 000 – 000

455

5

Fig. 3 plots the damage and failure state over loading cycles, from which the FCG behavior in the microstructure can be described. The simulated count of cycles to failure is 142. At the beginning, weak regions are usually found close to WC-Co phase interfaces, which can be assigned to incompatible strain due to large stiffness gradients. The damage evolution are more common in binder phase than WC phase. It can be interpreted from the fact that accumulation of plastic strain in Co binder increases the damage level simultaneously according to the equation (4). In contrast, the relatively low stress level is not able to split WC grains. These previously formed microcracks tends to form a main crack or multiple subcritical microcracks, and finally the stable growth reaches a limit. To investigate the influence of local thermal residual stress on the fatigue resistance of hardmetals, the simulation of the thermal contraction step before the mechanical loading step is executed. The respective thermal expansion coefficients WC = 5.8 × 10 −6 K −1 and Co = 12 × 10 −6 K −1 are applied (Spiegler et al (1992)). The distribution of residual stresses in the microstructure after cooling from 825 ℃ to the room temperature is given in Fig. 4, accompanied by a histogram plot as a summary. The positive sign of the hydrostatic stress H indicates a tensile state in general. The higher tensile stresses appears in binder regions where crucial microstructural morphologies are found, e.g. narrow sites with tiny binder mean free path. The region near the WC-Co interface commonly hold higher order of magnitude than the interior domain does. On the contrary, WC phase mainly bears compression. (a) (b)

H [MPa]

Fig. 4. The distribution of simulated residual stresses in the microstructure as: (a) a contour plot; (b) a histogram plot.

Simulation results of the damage evolution with residual stress are shown in Fig. 5 and compared with ones from previous residual stress-free simulation. The damage and failure state in the microstructure after same counts of loading cycles for both simulations are displayed. By comparing microcracks at the same time, more damage spots appear in the microstructure from the new simulation. Besides, the simulated lifetime is also reduced by about 40%. A preliminary conclusion can be drawn that the thermal residual stresses decrease the fatigue resistance in hardmetals because the tensile state of stress in binder region accelerates the plastic strain accumulation and damage evolution. (a) (b) (c)

Fig. 5. Simulated damage evolution process with residual stress over loading cycles : (a) = 25 ; (b) = 70 ; (b) = 86 .