Grain boundaries have great effects on the mechanical properties and failure behaviors of h-BN sheets.

The adsorption of hydrogen-bonded and stacked nucleobase pairs on the hexagonal boron nitride (h-BN) surface was studied by density functional theory and molecular dynamics methods. Eight types of nucleobase pairs (i.e., GG, AA, TT, CC, UU, AT, GC, and AU) were chosen as the adsorbates. The adsorption configurations, interaction energies, and electronic properties of the nucleobase pair on the h-BN surface were obtained and compared. The density of states analysis result shows that both the hydrogen-bonded and stacked nucleobase pairs were physisorbed on h-BN with minimal charge transfer. The hydrogen-bonded base pairs lying on the h-BN surface are significantly more stable than the stacked forms in both the gas and water phase. The molecular dynamics simulation result indicates that h-BN possessed high sensitivity for the nucleobases and the h-BN surface adsorption could revert the base pair interaction from stacking back to hydrogen bonding in aqueous environment. The h-BN surface could immobilize the nucleobases on its surface, which suggests the use of h-BN has good potential in DNA/RNA detection biosensors and self-assembly nanodevices.

Failure analysis was carried out on a tube-to-tubesheet welded joint of a shell-tube heat exchanger to confirm its failure mechanism. The collected evidence suggests that the failure of the tube-to-tubesheet welded joint was induced by fatigue. Under the morphology analysis, the fracture surface exhibit obvious fatigue crack propagation traces. Fatigue striations were observed clearly in the propagation zone. Detecting results showed that chemical composition and mechanical properties of the tubes was consistent with the standard requirements for the 304 stainless steel. The metallographic test showed that the microstructure of both the tube and tubesheet were normal. However, serious defects were found at the tube-to-tubesheet welded joint. The fatigue crack initiated from the defects at the tube-to-tubesheet welded joint. The periodic load might come from the resonant vibration of the tube or the changes in temperature and pressure of the fluid inside the heat exchanger. Bad welding and unsuitable expansion position of the tubes at the tube-to-tubesheet might give rise to the formation of initial cracks