Papers by Author: Xi Shu Wang

Abstract: In the present study, a unified crack growth law based on the small fatigue crack growth law was investigated using specimens with different configurations and loading types for the annealed 0.42% carbon steel. Then, a convenient prediction method of fatigue life was proposed. This small fatigue crack growth rate was uniquely determined by the modified small crack growth law, , despite of changing for specimen configurations and loading types. The constants of can be estimated by an empirical equation of without stress/strain gradient. Considered the effect of stress gradient for different specimen configurations and loading types, the stresses under bending loadings and specimen configurations was calculated from that under push-pull loading. Therefore, the fatigue life of the specimens with different configuration and under different loading types can be simply estimated through the fatigue crack growth law based on the only tensile strength of carbon steels. The availability of proposed method was confirmed experimentally by the results of several carbon steels.

Abstract: In this work, the microstructure of the dragonfly wing vein was investigated by the finite element method (FEM). It is a bionic view to simulate the microstructure of the wing vein, which could be used to construct the micro air vehicles (MAVs). From the FEM results, the sandwich structure of the dragonfly wing vein was proved, which could supply more torsional deformation and reduce the weight of dragonfly. And the protein layer in the sandwich structure almost not bear the bending loadings, which could protect the protein not to be destroyed. It could assist us to utilize such design for the new micro air vehicle (MAV), especially ornithopter.

Abstract: To study the fatigue microcrack initiation and propagation behaviors of cast magnesium alloys, the small fatigue crack propagation tests were carried out using the in-situ observation with scanning electron microscope (SEM). All initiations and propagations of fatigue small cracks focused on effects of the interaction of artificial two small holes, which there are the different distances and alignments of two small holes. The results indicate that the fatigue small cracks of cast magnesium alloys occurred mainly at the defects or root of notch but the early stage crack propagations were influenced on the distance and alignment between two small holes. For cast AM50 and AM60B alloys, the fatigue small cracking prior to occurred at the weak dendrite boundary and had some concomitances such as the plastic deformation on surface of α-Mg phase. For AZ91 alloy, the fatigue cracking characterization depended mainly on the brittle properties of β-Mg17Al12 phase, which the multi cracks occurred at the boundaries of β-Mg17Al12 phase. The effect of notch on the fatigue cracking behavior becomes weaker when the radius of notch is over 3-4 times than that of average α-Mg grain size. The fatigue crack propagation behaviors varied with the different arrangements of two small holes. The effects of distance and alignment of two small holes on the fatigue crack propagation behaviors are also obvious.

Abstract: We studied the effects of the Ca and Sr additions on the microstructure and the cracking process of magnesium alloys with in-situ scanning electron microscope (SEM) images. We found that the Ca addition is not only able to refine the Mg17Al12 phase but also form a new Al2Ca phase, which lead to improved ductile of cast magnesium alloys at elevated temperature. The MgAlCaSr phase can be improved the hot resistance. However, the number of cracks per unit area increases with increasing Ca addition.

Abstract: Specially designed SEM in-situ tensile and fatigue tests have been conducted to trace the entire process of crack initiation and propagation till fracture in an ultra-high strength steel MA250. TiN is a typical inclusion and its average size is in the range of 8~10μm in MA250 steel. The micro-mechanism of the effect of TiN inclusion on crack initiation and propagation at tensile and fatigue tests both have been studied in detail. Experimental results show the harmful effect of TiN on tensile and fatigue properties both. This work is helpful to establish the practical life prediction model for the characteristic inclusion parameters in ultra-high strength steel components. It also enlightens us to eliminate TiN in the further development of ultra-high strength steels.

Abstract: Some differences between the growth behaviors of small fatigue crack of cast AM50 magnesium alloy at different elevated temperature and at open and closed states were investigated based on in-situ observations with scanning electron microscope (SEM). These results indicate that the growth rates of small fatigue cracks depend on not only the stress levels but also the elevated temperature and crack states. The fatigue crack growth rates were estimated based on novel and conventional methods.

Abstract: This work focuses on the damage mechanisms and the resulting failure behavior of structures made of anodized coatings on magnesium alloy substrates. The failure of anodized coatings of about 30µm thickness on AZ91D substrates was investigated under three-points bending loading with online scanning electron microscope (SEM) observations. The obtained SEM images show that void nucleation and crack initiation occurs mainly at sites near the coating-substrate interface, and the evolutionary microcracking damage diffuses from the interface to the coating surface and also to the bulk substrate with the increasing in loading.

Abstract: It is of interest to understand damage and failure mechanisms of microcracks and their evolution as a function of loading history, especially in the case of complex loading. Owing to their superior mechanical and physical properties, carbon nanotubes (CNTs) seem to hold a great promise as an ideal reinforcing material for composites of high-strength and low-density. HOWEVER, In most of the experimental results, only modest improvements in the strength and stiffness have been achieved by incorporating carbon nanotubes in polymers. There are many factors that influence the overall mechanical property of CNT-reinforced composites, e.g. the weak bonding between CNTs and matrix, the waviness and agglomeration of CNTs. In the present paper, we use the Mori-Tanaka method to evaluate the effect of these factors on the moduli of CNTs-CNT-reinforced composites. It is established that the waviness and agglomeration may significantly reduce the stiffening effect of CNTs, while the interface between the matrix and CNTs influence the moduli of CNTs-reinforced composites little.In this paper, the frictional sliding of microcracks under complex triaxial loading is analyzed, and the obtained results are incorporated into the constitutive relation of microcrack-weakened brittle materials.