Key Engineering Materials Vols. 353-358

Abstract: This paper is focused on the nonlinear dynamics of a spur gear pair with slight wear fault, where the backlash, time-varying stiffness and wear fault are all included. The Incremental Harmonic Balance Method (IHBM) is used in studying the periodic solution of this system. Based on the Kronecker’s notation, step function and sign function, the general forms of the periodic solutions are founded, which is useful to obtain the periodic solutions with arbitrary precision. Finally the typical frequency-response diagrams are obtained to illustrate the properties of gear system with fault.

Abstract: In this paper, failure behavior of basalt specimen containing 3D-flaw with different angles is investigated under triaxial compression. The process of crack growth was observed by computerized comography (CT). Based on the experimental results, the following conclusions were obtained: Under 3D loading condition, the angle of pre-existing flaw has a dominating influence on the zone of stress concentration. When the flaw angle is parallel to the direction of vertical loading, the failure peak strength is greater than that of sample containing other flaw angle, and the final failure mode is irregular. When the pre-existing flaw angle is about 60 degree with horizontal loading direction, stress concentration is easy to form at the tip of pre-existing flaw, and lead many small radicalized cracks to propagate. Under 3D loading condition, the cracking surface generally is distorted and lead crack to propagate in irregular direction, and then form irregular space failure with mixed mode. The experimental results observed here has shown that the experimental system and method in this study is efficient and robust.

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: In fire resistant design traditional method based on experiment is being replaced by method based on calculation.in this paper Damage mechanics is applied to calculate and analyze the process of damage and failure of structures at high temperature through solving jointly the closed equations composed of the thermal damage evolving equation based on the residual strength, heat conduction equations, equilibrium equations, geometry equations and physics equations. An example of a truss is given to illustrate the calculation of thermal damages and stresses at high temperature. The results suggest, the bearing capacity at high temperature is influenced by these three main factors: thermal damage resulted from the elevated temperature, the thermal stresses and the original load. This provides a new research method for the modern fire-resistant design based on calculation and can be developed and applied in the numerical simulation of the process of damage and failure of the integrated structures.

Abstract: Structural operating conditions may significantly differ from those applied during laboratory tests where the structure is well known, well installed and properly excited. For structures under their natural loading conditions, or excited by random forces, excitations cannot be measured and are usually non stationary. Hence, an improvement operational modal analysis is a useful complement to the traditional modal analysis approach. The aim of this paper is to present the application of a new identification procedure, named wavelet-based identification technique of structural modal parameters. Wavelet-based identification that works in time-frequency domain is used to identify the dynamic characteristics of the structural system in terms of natural frequencies, damping coefficients and mode shapes. The paper has shown how the amplitude and the phase of the wavelet transform of operational vibration measurements are related to eigenfrequencies and damping coefficients, and the wavelet-based spectrum analysis is used to identify the mode shapes of the structure. Those modal parameters can be used to detect damage of structures. A simulation example has demonstrated that current identified results are comparable with those previously obtained from the peak pick method in frequency domain and stochastic subspace identification in time domain.

Abstract: A new continuum damage theory (CDT) has been proposed by Lee et al. (1997) based on the SEEP. The CDT has the apparent advantage over the other related theories because the complete constitutive law can be readily derived by simply replacing the virgin elastic stiffness with the effective orthotropic elastic stiffness obtained by using the proposed continuum damage theory. In this paper, the CDT is evaluated by comparing the mode shapes and natural frequencies of a square plate containing a small line-through crack with those of the same plate with a damaged site replaced with the effective orthotropic elastic stiffness computed by using the CDT.

Abstract: The fracture mechanisms of SiCp/AZ91 composites were investigated by scanning electron microscopy (SEM). For the as-cast composites, the decohesion at SiCp/matrix interface is the main fracture mechanism because of the high stress concentration resulting from the segregation of particles in grain boundaries formed during solidification process. But for the extruded composites, the main fracture mechanism is the particle crack or ductile rupture of the matrix between the particles. So the fracture mechanism of SiCp/AZ91 composites is altered by extrusion because the segregation of particles and defects in the grain boundaries are largely eliminated by extrusion.

Abstract: Fatigue crack bending and propagation behaviors were studied under mixed-mode conditions using annealed and fatigue slant precracks. The bent fatigue crack initiated from the fatigue slant precrack propagated under mixed-mode conditions with mode II stress intensity factor evaluated from the crack sliding displacement measured along the crack. On the other hand, bent fatigue cracks propagated under the mode I condition for an annealed slant precrack specimen. The forces which suppress the crack opening/sliding were calculated along the slant precrack and the bent crack by FEM (Finite Element Method) analysis. As a result, the crack opening suppress forces were generated by the compressive residual stress around the fatigue slant precrack, while the forces which promote the crack sliding were caused by the residual stress field in front of the fatigue slant precrack.

Abstract: Fiber reinforced metal matrix composites (MMCs) are recently used in automobile, ship, aerospace and manufacturing industry because they have high stiffness and strength. The effective utilization of the strength and stiffness of the fiber reinforced MMCs depends on efficient load transfers from the matrix to fibers through the interfacial region. However, during the fabrication and afterward utilization of composites, so many numbers of micro crack may extend, especially at the interface, even before any load has been applied. Thus, in this study, the interfacial stress state and behavior of the interfacial perpendicular crack for transversely loaded unidirectional fiber reinforced MMCs investigated by using the elastic-plastic finite element analysis.

Abstract: Fatigue tests were performed in laboratory air using extruded Ti-6Al-4V to determine the effect of microstructure on S-N curve, and the crack initiation and crack propagation behavior of the alloy. A modified linear elastic fracture mechanics approach was used in the analysis of the short crack propagation behavior to predict the S-N curve and the crack propagation curve. The predictions agreed well with the experimental results.