Papers by Author: Bo Lin He

Abstract: In recent years, the core engineering components of high-speed train, automobiles and aircrafts are required to endure fatigue loads up from 10⁸ to 10¹⁰ cycles. The present study results show that in the very high cycle fatigue (VHCF) regimes of more than 10⁷ cycles, the fatigue failure of high strength steel materials can occur below the traditional fatigue limit, hence the VHCF investigations of high strength steels not only help to further understand the fatigue essence and mechanism, but also do research on the fatigue design and life assessment method. This paper summarizes works of VHCF researches for high strength steels in recent years, such as the characteristics of S-N curve, the observations on fish-eye, which is one of the typical characteristics of fracture surface, crack initiation, crack propagation, etc. The present work also analyzes the fatigue mechanisms and briefly discusses several factors that affect VHCF properties, such as hydrogen effect, inclusion effect, frequency effect. Some possible and prospective aspects of future researches are also proposed.

Abstract: In recent years, very high cycle fatigue has become a major concern in design and durability of engineering components and structures such as railway wheels, rails, offshore structures, bridges, load bearings, etc. There are some factors which have been assumed to influence the fatigue behavior for metallic materials in VHCF regime. But most factors influencing the VHCF behavior have not been studied thoroughly. In order to deeply understand the essence of material fatigue, the experimental and theoretical aspects of the factors influencing on very high cycle fatigue for metallic materials should be studied further more. This paper deals with an overview on the effect of factors on the property in very high cycle fatigue regime. Research trends and some conclusions in this field are briefly discussed and obtained.

Abstract: With the increase of design fatigue life of many critical mechanical components and engineering structures, research on very high cycle fatigue (VHCF) has become a new topic for engineering components failure. This paper summarizes works of VHCF of high strength steel, such as the observations on fish-eye, which is one of the typical characteristics in VHCF regime; Characteristics of crack initiation and crack propagation are analyzed based on fracture surface; The present work also analyzes the fatigue mechanism and related models. Loading frequency effect on the VHCF behavior is also discussed. Some prospective aspects of future researches are proposed.

Abstract: In this paper, the effect of welding processes such as friction stir welding (FSW), laser beam welding (LBW) and pulsed current gas tungsten arc welding (PCGTAW) on comprehensive mechanical properties of AZ31B magnesium alloy was reviewed. Among the three different welding processes, the LBW joints showed superior comprehensive properties to FSW and PCGTAW joints. The formation of very fine grains in weld region, higher fusion zone hardness, uniformly distributed finer precipitates were the main reasons for superior comprehensive mechanical performance of LBW joints compared to PCGTAW and FSW joints.

Abstract: Surface treatment was carried out on the butt joint weldment of P355NL1 steel by using the HJ-II-type ultrasonic impact machine. The ultrasonic impact current is 1.5A, the impact amplitude is 20 microns and ultrasonic impacting time is 5min. Tensile test was carried out for both treated specimen and un-treated specimen. The fracture observed with the scanning electron microscope of 6360LA type. The experimental results show that although the compressive residual stress can be obtained in the surface of weld toe area, and the grain size in the welded joint can be refined, but the mechanical property of the butt joint of P355NL1 steel can not be improved through the ultrasonic impact treatment. The main reason is that the ultrasonic impact layer is only 120um, it is to thin to compared to the thickness of the specimen.

Abstract: Geometrical parameters of welded joint affect the stress concentration coefficient seriously. In order to increase the fatigue property of welded structure, it has great significance to reduce stress concentration coefficient of welded structures by researching and improving the geometry of welded joints. In this paper, the effects of weld toe inclination angle θ and weld edge transition arc radius r on the stress concentration coefficient of welded butt joints of magnesium alloy were analyzed by using ABAQUS finite element program, and the change rule of stress concentration coefficient with the variation of the two parameters was also researched. The calculation results indicate that reducing weld toe inclination angle or increasing transition arc radius can effectively decrease the stress concentration coefficient of welded butt joints, so as to improve the fatigue property of welded structures. For the safe use of welded structures, the true weld edge transition arc radius r should be greater than 3mm, and weld toe inclination angle θ should be smaller than 30°.

Abstract: The weld toe surface and its nearby area of welded cruciform joints were treated by ultrasonic impact. Under the same stress concentration and after heat treatment to eliminate residual stress, the effect of residual stress on the fatigue life of joint was researched. The fatigue tests are performed on the joints of 16MnR both for the un-treated and treated joints by using EHF-EM200K2-070-1A type fatigue tester when the load ratio is 0.1, frequency is 10Hz. The experimental results indicate that the severe plastic deformation in the vicinity of weld toe surface was formed by impact treating for 2 minutes, the thickness of the plastic deformation layer is about 60μm. Residual tensile stress in the weld toe surface can be changed to residual compressive stress by impact treatment. The fatigue life of welded joint is 0.260×10⁶ cycle, and the fatigue life of treated joint is 0.499×10⁶ cycle. Compared to the un-treated joint, the fatigue life of treated joint has been increased by 91.92%. The residual stress contributed to fatigue life is about 16%. Residual stress has great effect on the fatigue life of welded cruciform joint.

Abstract: The weld seam and weld toe surface were treated by ultrasonic impact method. The contrast corrosion tests were performed in 3.5% NaCl aqueous solution both for the un-treated and treated joint. The experimental results indicate that the severe plastic deformation on the surface of weld seam and weld toe were formed by ultrasonic impact for different time, the maximum depth of the plastic deformation layer is about 300μm. Residual tensile stress in the surface of weld seam and weld toe can be changed to residual compressive stress by impact treatment, and the grain in the surface of welded joint could be refined. Compared to the un-treated joint, when the impact current is 1.2A and 1.5A, respectively, the corrosion rate of treated joint was reduced by 22.67%, 54.59%, 43.99% and 33.92%, 56.54%, 49.29%, respectively for 10, 20 and 30 min. treating. The corrosion resistance of welded joint has a certain relationship with the residual stress on the surface of welded joint. The ultrasonic impact treatment has distinct effect on the corrosion resistance of 16MnR welded joint.

Abstract: It is important for the design engineer to realize that scatter and variability of materials properties are inevitable and must be dealt with appropriately. On occasion, data must be subjected to statistical treatments and probabilities determined. The fracture toughness of pipe steel has great effect on the safety of welded pressure pipe. The fracture toughness test of circumferential welded joint was carried out in this paper for X65 pressure pipe steel. The fracture toughness experimental results were analyzed by using computer program developed by ourselves. The distribution of weld fracture toughness of X65 pressure pipe steel which may accord with normal distribution, lognormal distribution or weibull distribution can be determined by calculating the results only one time. The program, which plays an important role in structure reliability analysis, can be used to determine the probability statistic distribution of the mechanical properties. The calculated results indicate that the weld fracture toughness of the X65 pressure pipe steel can either accord with normal distribution, lognormal distribution or weibull distribution, but the optimum fitting distribution of the fracture toughness is weibull distribution.

Abstract: The composite of carbon fiber reinforced polyethylene resin was prepared by using a twin-screw extruder. The effect of carbon fiber oxidation treating on the mechanical properties of carbon fiber reinforced polyethylene resin composite was researched. The tensile fracture failure mechanism of composite was analyzed for both untreated and air oxidation treated specimen. The experimental results indicate that when carbon fiber content is equal, the tensile strength and the elastic modulus of air oxidation-treated carbon fiber-reinforced polyethylene composite are improved than that of the untreated. When the fraction of adding carbon fiber is 3.99%, compared with the pure polyethylene resin matrix, the tensile strength, elastic modulus is increased by 13.12% and 172.91%, respectively. Compared to the untreated carbon fiber reinforced polyethylene resin composite, the tensile strength and tensile modulus is increased by 4.71% and 13.14%, respectively.