Key Engineering Materials Vols. 324-325

Abstract: Impact tests were carried out by use of a 74-mm-diameter split Hopkinson pressure bar to investigate the impact damage behaviors of plain concrete (PC), steel fiber reinforced concrete (SFRC) and steel fiber reinforced and polymer modified concrete (SFRPMC). The results show that all three kinds of materials appear strain rate strengthening effects, and SFRPMC appears a better impact resistance and energy absorbing ability than PC and SFRC. Based on the analysis of experimental results, a rate-dependent damage model is suggested to depict the dynamic behaviors of SFRC and SFRPMC, which derives the impact damage evolution of the materials. It shows that the damages in SFRPMC develop more slowly than that in SFRC.

Abstract: Under the action of compressive load, the growth and coalescence containing flaws in brittle materials (rock and rocklike materials e.g.) will result in the local buckling and global fracture of rockmass. But, the mechanisms on propagation and coalescence of 3-dimensional internal flaws are not clear till now. We examine brittle fractures of manmade specimens using frozen casting resin and rocklike material to observe 3D internal flaws growth process at about -30° C. A team of specimens containing three internal flaws is measured; flaws are made of three parallel oblong aluminum films. The propagation and coalescence pattern of three internal flaws is observed under compressive stress. An interesting phenomenon is that the crack initiated from the second flaw quickly turns to the one induced from the third flaw and forms a bigger fracture plane, then splits the specimen. It shows that the flaw distribution pattern will greatly affect the flaws growth and coalescence process. The mechanisms that lead to the wing and anti-wing crack initiation and coalescence are described.

Abstract: Flutter characteristics of composite curved wing are investigated in this study. The efficient and robust computational system for the flutter optimization has been developed using the coupled computational method based on the micro genetic algorithms. The present results show that the micro genetic algorithm is very efficient in order to find optimized lay-ups for a composite curved wing model. It is found that the flutter stability of curved wing model can be significantly increased using composite materials with proper optimum lamination design when compared to the case of isotropic wing model under the same weight condition.

Abstract: Two multiaxial damage parameters are proposed in this paper. The proposed fatigue damage parameters do not include any weight constants, which can be used under either multiaxial proportional loading or non-proportional loading. On the basis of the research on the critical plane approach for the tension-torsion thin tubular multiaxial fatigue specimens, two multiaxial fatigue damage models are proposed by combining the maximum shear strain and the normal strain excursion between adjacent turning points of the maximum shear strain on the critical plane. The proposed multiaxial fatigue damage models are used to predict the fatigue lives of the tension-torsion thin tube, and the results show that a good agreement is demonstrated with experimental data.

Abstract: In order to study the dynamics property and damage evolution of frozen aeolian soil in the west of Liaoning Province, laboratory test have been done by using the DDS-70 dynamic triaxial test machine. The damage evolution formula of frozen soil is established, and the dynamic evolution curve between damage variable and strain is sketched. The research results show that the process on damage of frozen soil can be divided into three stages, and the increase of damage variable is non-linear. The speed of damage growth is fast in plasticity damage stage, but the damage growth is relatively flat and smooth in micro-crack damage stage. Aiming at initial stages of damage in frozen soil, its microstructure is observed by using scanning electronic microscope (SEM). The occurrence of interior micro-cracks begins with the middle of sample. From the middle to ends in the sample, the micro-cracks are in circular growth. Through transfixion and collection of cracks, the inclined cracks come into being and enlarge step by step.

Abstract: Based on the Hamiltonian theory and method of elasticity, a ring and a circular hyper-analytical-elements are constructed and formulated. The hyper-analytical-elements give a precise description of the displacement and stress fields in the vicinity of crack tip for the bilinear cohesive crack model. The new analytical element can be implemented into finite element method program systems to solve crack propagation problems for plane structures with arbitrary shapes and loads. Numerical results for typical problems show that the method is simple, efficient and accurate.

Abstract: The furthest dangerous cracking type of asphalt pavement is usually considered of the simplex
type crack(namely shear crack) under traffic load, but the so-called
type crack should be the compound cracking at the concurrence of I type crack (namely splay crack) and
type crack. In order to study the compound type crack propagation behavior of asphalt concrete, a three point bending beam with compound type notched has been designed. Asymmetry-gap small girder specimens of asphalt concrete are adopted in the test, and the central testing equipment is MTS. A series of compound type three point bending beam tests have been performed to simulate compound type crack initiation and growth. The propagation of compound type crack is studied by a newly developed numerical code, Rock Failure Process Analysis (RFPA2D).it is shown that the crack initiation angle and the peak load increased as the distance of the preexisting crack from the midpoint of the beam increased, the crack propagation path follows some regularity in general and the main force of crack propagation is still tensile stress. Through this work, the understanding of the mechanism about damage and early destroy of asphalt pavement can be advanced and it can provide guidance for asphalt pavement design and maintenance.

Abstract: The cell model with twenty-five random dispersion voids was employed to analyze the damage and fracture mechanism of the nodular cast iron. The results show that the growth velocity of the voids has obvious difference with each other due to the random dispersion of voids. In the early stage of the deformation, the growth of the voids is mainly determined by the distance of the voids since the triaxiality stress parameter of the matrix around the voids is approximately equal. With the increase of the triaxiality stress parameter of the matrix materials around the void, the evolution velocity of the voids increases quickly. At the same time, this will influence the neighboring voids to grow quickly. The chain reaction of the rapidly increase of voids lead to the final material failure. The results can explain the fracture appearance of the smooth bar specimens under uniaxial tensile load really.

Abstract: The dynamic properties of the model of vibrating screen are obtained using the software Pro/ MECHANICA. The natural frequencies and mode shapes of the screen are calculated, and the distribution law of dynamic stress is found out. It shows that the structure design of the screen is reasonable. Under the working conditions designed, its dynamic strength is satisfied. This paper provides engineering estimates of non-linear fracture mechanics parameters for vibrating screen arthropleure cracks, subject to alternate stress and Vibrating. Solutions are given in the form of two different approaches.

Abstract: Traditional laminate strength analysis only considers face failure under in-plane loads. In fact, owing to the mismatch of the mechanical properties of the adjacent layers, a three-dimensional interlaminar singular stress fields develop in a small boundary region in the vicinity of the free edges of the laminate under mechanical load, which may lead to interlaminar delamination failure. Neglecting this interlaminar failure mode, the failure strength of laminate will be overestimated. In this paper, face failure and interlaminar failure are both considered. So for a lamina, three major failure modes are considered: matrix failure, fiber breakage and delamination. Finite element method is used to obtain the stresses in a laminate under mechanical loads. Stress-based criterions are adopted to predict the failure mode of laminas. When a lamina is failed, the lamina stiffness is reduced according to the corresponding failure mode, and the stresses of the laminate are re-analyzed. This procedure is repeatedly performed until the whole laminate fails and thus the ultimate strength is determined. The predicted ultimate strengths are in good agreement with experiment results in the open literature.