Molecular dynamics study on the nano-void growth and coalescence at grain boundary

Molecular dynamics simulations using embedded atom method (EAM) potential were performed to study nano-void growth and coalescence at grain boundary in face-centered cubic bicrystal copper. Thin-plate specimens subjected to uniaxial tension strain with one-void and two-void at the centered grain boundary were employed to analyze the effect of specimen size, temperature and applied strain rate on the stress-strain response, incipient yield strength and macroscopic effective Young's modulus. The evolutions of dislocations, twin bands and void shapes under different specimen sizes were also presented. The obtained results show that, regardless of the void numbers, the specimen sizes, temperature, the applied strain rate had significant influence on the void shape evolution, stress-strain curve and incipient yield strength, while negligible effects on the macroscopic effective Young's modulus except for the temperature. Moreover, the voids growth rate along the grain boundary was also found to be associated with the specimen sizes.

Comparison of the very high cycle fatigue behaviors of INCONEL 718 with different loading frequencies

In order to clarify the differences of very high cycle fatigue(VHCF) behavior of nickel based superalloy IN718 with different loading frequencies,stress-controlled fatigue tests were carried out by using ultrasonic testing method(20 KHz) and rotary bending testing method(52.5 Hz),both at room temperatures,to establish stress versus cycles to failure(S-N) relationships.Results disclosed that cycles to failure at a given stress level increased with an increase of the applied frequency,i.e.,the higher frequency produced an upper shift of the S-N curves.Fractographic analysis suggested that crack initiation and propagation behaviors had large differences:cracks in low-frequency tests preferentially initiated from multiple sources on the specimen surface,while in high-frequency tests,cracks mostly originated from a unique source of subsurface inclusions.Subsequently,frequency-involved modeling was proposed,based on the damage accumulation theory,which could well illustrate qualitatively those comparisons due to different loading frequencies.

Determination of mixed-mode interfacial fracture toughness for thermal barrier coatings

In this work, a test method was developed to determine the interfacial fracture toughness of the air plasma sprayed （APS） thermal barrier coatings （TBCs） over a wide range of mode mixities. For this mixed-mode test method, the analytical expres- sions for the energy release rate and stress intensity factors were derived based on the energy theory and the concept of ＂equi- valence＂. The fidelity of these expressions was affirmed by selected finite element analysis. The experimental results showed that the critical energy release rate increased with the increase of the positive mode mixity, which was mainly due to the increase in contact/friction effect and plastic work dissipation with increasing shear mode loading. Furthermore, an elliptical interfacial failure criterion in terms of the stress intensity factors was proposed. The agreement between the experimental results in the literature and those in our work indicated that our test method and the corresponding analytical solutions can well determine the interfaeial fracture toughness of the TBCs over a wide range of mode mixities.