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The size effects on the mechanical properties and defect evolution process of single crystal aluminum with a center crack under tensile loading have been simulated through molecular dynamic method. With the increase of model size, the yield stress and yield point decreased, while the fracture toughness increased. In the elastic stage, the deformation relies heavily on the point defects and surface defects in materials, while in plastic stage it depends strongly on the dislocation multiplication and its slip. The stress concentration at the crack tip is mainly associated with the phase transformation of the crystal structure near the crack tip due to the stress relaxation after phase transformation.

In this paper, the quasi-static tensile tests and the dynamic compression tests of 20 CrMnTi were carried out using the tensile tester and Split Hopkinson pressure bar（SHPB） device at different strain rate to study the dynamic stress response of 20 CrMnTi steel. The results show that 20 CrMnTi steel has strain rate effect and plasticizing effect. But it is not very sensitive to strain rate change within the scope of dynamic compression experiment. In consideration of the adiabatic temperature rise, the Johnson-Cook constitutive model is modified. The results show that the revised Johnson-Cook constitutive model can well describe the mechanical performance of 20 CrMnTi under the dynamic impact load, which is important to reference value that 20 CrMnTi steel is applied under high speed and overload.

The study on deformation and failure mechanism of materials,and the progressof NEMS/MEMStechnology and biotechnologypromote the development of multi-scale analysis method. However,Multi-scale connectionisthe key of multi-scale model. The finite element method（ FEM） and molecular dynamics simulation（ MD） were combined to simulate the multi-scale models,and the stress intensity factors（ SIF） and displacement mechanics parameters were adopted to connect and convertthecontinuum region and discrete molecular dynamics region,then three multi-scale model of two-dimensional silver crack propagationwasestablished to obtain the crack tip trajectory and the micro-scale crackpropagationprocess. The result is in good agreement with theory and experiment,and proves the rationality of the method.

To obtain the dynamic fracture toughness of 20 Cr Mn Ti,an impact loading test was conducted on three-point bending specimens with the improved Hopkinson pressure bar.Dynamic stress intensity factor was determined by the spring mass model.The onset time of crack initiation is experimentally detected using the strain gauge method.Dynamic fracture toughness of20 Cr Mn Ti with type I crack was obtained through the above experimental procedure.The geometrical dispersion distortion caused by lateral inertia effect is effectively inhibited by the wave shaping technique,to ensure the consistency of stress wave signal in measuring point and loading point is guaranteed.At the same time,the high-speed camera system was used to collect the image and the change of the surface strain field during the loading process was analyzed with GOM Correlate.The results show that samples before crack initiation has reached the dynamic strain balance,ensure the validity of the experiment.

Effect of a type of warm-mix agent, PN2217 on flame-retardant and smoke-suppression properties and pavement performances of aluminum hydroxide (ATH)/organic modified montmorillonite (OMMT) modified asphalt mixture were investigated. ATH/OMMT flame-retardant asphalt and warm-mix flame-retardant asphalt were prepared by means of high-speed shearing and conventional physical properties were tested. Next, cone calorimeter test was adopted to analyze the effect of PN2217 on flame-retardant and smoke-suppression properties of ATH/OMMT asphalt mixture. Thermogravimetric (TG) test was carried out to investigate the mechanism on thermal decomposition characteristic of ATH/OMMT flame-retardant asphalt. The effect of PN2217 on pavement performances of ATH/OMMT asphalt mixtures was analyzed through rutting test and low-temperature bending test. PN2217 has adverse effect on the flame-retardant and smokesuppression properties of ATH/OMMT asphalt mixtures by retarding formation process of char layer. There occurs pores and fissures on the residue surface after combustion and it cannot efficiently prevent heat and gas from being exchanged from outside. PN2217 reduces high-temperature performances and promotes lowtemperature performances by reducing adhesion between asphalt binder and aggregates. [ABSTRACT FROM AUTHOR]