Advanced Materials Research Vols. 690-693

Abstract: Experiments of fatigue crack growth rate were conducted for standard compact tension specimen of Q345E steel; the resulting experimental data were processed by the use of incremental polynomial method for mathematical statistical analysis. The crack growth rate formula can be obtained under different survival rates by the use of Pairs Formula.

Abstract: Railway track steel, U71Mn and U75V were fatigued in this study, with the help of ultrasonic fatigue test system, to investigate the high cycle fatigue life behaviors. The results showed that the fatigue damage still occurs when the fatigue life exceeds 107, and the evolution of S-N curve showed a ladder type. This test showed that the traditional view of fatigue design and life prediction method were unable to meet the requirements of machinery and equipment working in gigacycle fatigue range, very high cycle fatigue behavior of fatigue has become a major challenge for researchers. The scanning electron microscope analysis of crack initiation was performed to clarify the mechanism of very high cycle fatigue failure.

Abstract: The high temperature creep test of P92 materials were carried out under different stresses at 600 °C and 650 °C, the microstructure of creep fracture appearance of samples was examined, the stress index n was fitted by Power Law Equation, and damage tolerance factor λ was calculated out. The results show that the ratio of creep fracture time vs. starting time of accelerating creep stage is range 1.4 to 1.8; the stress index n is 17 and 15, respectively; and damage tolerance factor λ is 3.1 and 3.26, respectively. According to stress index n and damage tolerance factor λ , the movement of dislocation leads to accelerating creep and final fracture. The results also show that P92 steel has good high temperature plasticity; micro fracture shows obvious dimple fracture characteristics and the second phase. Precipitation hardening effect from Laves-phase has the most influence on creep stability. At high temperature, dislocation density is decreased by rapid formation of Laves-phase, which lowers the fracture strengthening and accelerates creep fracture.

Abstract: The fatigue endurance behaviors of SiC nanoparticle reinforced 6061 Al alloy composites (T6-treated SiC_np/6061Al) were simulated by finite element (FE) method with the help of recently developed cyclic viscoplastic constitutive model and its implementation into a FE code Solidworks simulation. The Rice-Tracey damage parameter was used for the crack path calculation did the prediction agree completely with the observed behavior. FE simulation of the materials showed that the composite samples exhibit regular crack propagation behavior. At early stages of loading the hydrostatic stress was low and the damage evolution was affected by the plastic flow. The damage grew in outer layers where the maximum equivalent plastic strain occurs. By increasing the number of fatigue failure cycles at 3×10⁵ cycles, the hydrostatic stress increased and its effect became dominant. In the 200 MPa tests the slope of the tensile portion of the hysteresis loops decreased with fatigue cycling, indicating progressive decrease in the tensile modulus. Stress concentration, due to the presence of nanoparticle reinforcements, produces controlled crack growth and higher stresses, which are related to regular energy release by the material during fracture. The need for higher stresses for a crack to propagate reveals the materials microstructural strength.

Abstract: Study on the fractural mechanism of thin sheet metals focuses on how to efficiently fracture and recycle the scrapped vehicles and electrical equipments. By using the experimental fracture mechanics, the failure mode was studied for 10F rimmed steel sheets to be crushed and recycled. In-plane mode I, out-plane mixed mode I /III and mode III fracture tests were conducted under different loading angles. The effects and contributions of mixed mode crack extensions for 10F rimmed steel sheets were analyzed and some sensitive fractural factors were studied. The experimental results show that pure mode III is most viable to fracture the thin sheet 10F rimmed steels.

Abstract: Rupture reason of boiler drainage pipe socket was analyzed by means of chemical composition analysis, macro-observation, metallurgical and SEM microstructure examination. Results showed that the rupture of the drainage pipe socket was a result of creep cracking initiated in intercritical zone of HAZ due to long period operation under high temperature. During creep process, carbides precipitated on fine grain boundaries of intercritical zone and grew up gradually into new coarse carbides as favorite nucleation site for creep void. With the growth, coalesce and connection of creep voids carries on, micro-cracks formed on original austenite boundaries and finally led to fracture of weld joint.

Abstract: This study examined the microstructures, mechanical and fatigue properties of the recently developed Eco7075 alloy. Eco7075 is made using Eco-Mg (Mg-Al₂Ca) in place of the element Mg during the manufacture of alloy 7075, having economically advantageous and superior properties. In the microstructure observation, average grain size was measured to be 5.2 μm. It consisted of Al matrix containing minute amounts of Al₂CuMg, MgZn₂, and Ca phases and showed microstructures with reduced amounts of Fe-based phases or oxides. Tensile tests exhibited that this alloy had yield strength of 492 MPa, tensile strength of 548 MPa, and elongation of 12.8%, which showed higher strengths than the conventional 7075 alloy but the similar elongation. Fatigue properties improved significantly compared to those of conventional 7075 alloys (Eco7075: fatigue limit of 330MPa). The superior tensile and fatigue properties of Eco7075-T73 alloy were mainly attributed to grain size refinement, homogeneous distribution of main strengthening phases, and reduced harmful phases of Fe-based intermetallic and oxide.

Abstract: The study is focused on the investigations into applying the grey model with rolling check to the prediction of fatigue crack growth. Fatigue crack growth data of compact tension specimens made of 2024-T351 aluminum-alloy plate tested under constant-amplitude loads were carried out for verifications. The optimal values of parameter affecting the accuracy of prediction were found by variational analysis. Using four experimental crack lengths as the source series and the optimal value of parameter for modelling with rolling check, it was found almost entire fatigue crack growth curve of the specimen can be predicted accurately. Besides, the analyzed results including number of rolling check performed, loading cycle corresponding to the maximum predicted crack length, fractured cycle of specimen, cycle ratio of loading cycle and fractured cycle, and percentage of error between maximum predicted and experimental crack length for all specimens were tabulated.

Abstract: In this paper, the effect of pre-stress of ball screws on bending stiffness is studied. By using ANSYS APDL parameterized method and adding stress stiffness effect, the relationship between the maximum deflection of ball screws under gravity load and pre-stress is given. The research result shows that with the increasing of pre-stress, the maximum deflection of the ball screw under gravity load gets smaller nonlinearly. The effects of pre-stretching of the ball screw on bending stiffness vary with different cross sections.

Abstract: One of the key issues is in crystal growth technologies is control of crystal morphology. This paper attends to understand film growth in its full generality is to propose a growth law, and it is capable of describing all types of growth mechanisms and determining the growth morphology at different stages of growth. We refer to an axiomatic approach to identify the important scaling in growth processes. One way to determine which processes dominate the competition is to form dimensionless parameters which embody the competitions between pairs of kinetic processes.