Papers by Keyword: Dynamic Fracture

Abstract: Static and dynamic loading fracture experiments were conducted on magnesium alloy under equal and unequal biaxial stress. Cross shape specimens with a crack on their center were used for tests. Fracture behavior was observed by two methods; the caustic and the one point gauge method. From the observation, the stress intensity factors and the fracture toughness values were calculated. It was found that the one point gauge method was not suitable for calculating the stress intensity factor with a strain gauge widely available. The fracture toughness values obtained under equal biaxial stress were 1.2~2.3 times greater than those of magnesium alloy naturally inherited. Moreover, plate thickness was negatively related to the fracture toughness value.

Abstract: Static and dynamic ductile crack propagation tests were carried out using thin single edge notched tension (SENT) specimens of carbon-manganese steel, each of which had a fatigue pre-crack or a sharp V-notch as a crack initiator. The crack tip opening angle (CTOA) was measured using digital images on the surface of the SENT specimens, and the critical values of CTOA for crack propagation decreased with increasing crack length while initial crack growth was still small. After the initial crack growth up to the distance of the specimen thickness, the critical CTOA remained almost constant. These tendencies were common in static and dynamic crack propagation specimens as well as fatigue pre-cracked and sharp V-notched specimens. There was no particular difference in the static crack propagation characteristics of both fatigue pre-cracked and sharp V-notched specimens. On average, it was observed that higher crack speeds affected the constant values of the critical CTOA by slightly reducing them. The constant CTOA tends to decrease with an increasing global constraint factor, and this suggests that the factor is insensitive to a crack starter, fatigue pre-crack or a sharp V-notch, but relatively sensitive to crack speed.

Abstract: The dynamic stress intensity factor was studied about the Griffith crack between the visco-elastic layer and elastic body under shear loading. Adopting integral transform and singular integral equation method, the time response of dynamic stress intensity factor was calculated. Considering the influence of materials parameter, it was found that the dynamic stress intensity factor increases with crack length increasing, and it posses distinct size effect when the thickness of visco-elastic layer reaching the character size of materials.

Abstract: In this paper, the optical method of caustics is developed to apply in dynamic fracture problems of orthotropic materials. The initial curve and the caustics formulas about mode-I crack are derived first from the distribution of stress field to determine the dynamic stress intensity factor (SIF). Meanwhile, dynamic caustics experiments are performed to study the dynamic fracture process of orthotropic materials, during which the dynamic fracture toughness and the velocity of crack propagation are calculated directly from the series of caustics patterns. Moreover, typical mesoanalysis of fracture section are done by a scanning electron microscope (SEM), revealing the fracture mechanism of fiber reinforced material.

Abstract: The dynamic fracture properties of polypropylene/nylon-6 (PP/PA6) blends, with different weight fractions and different compatibilizars, are investigated by reflected dynamic caustics experiments. According to the shadow spot patterns around a moving crack tip, which are recorded during the dynamic fracture process, dynamic fracture toughness and crack growth velocity can be estimated to evaluate the fracture properties of polymer blends. Moreover, combined with microscopic observation, the damage mechanism of the polymer blends is discussed.

Abstract: The paper considers a bi-material with a crack propagating rapidly, penetrating or deflecting at the interface under dynamic loading. The hybrid time-domain boundary element method, together with the multi-region technique, is applied to simulate the dynamic process of propagation and penetration or deflection of the crack. Moving of the crack tip in the matrix, penetration and deflection of the crack at the interface, and propagation of the crack along the interface are controlled by criteria developed from the quasi-static ones. A bi-material rectangular plate with an edged crack under impact loading is computed and compared with the photoelastic experiments. Good agreement between numerical and experimental results implies that the present boundary element numerical method can provide an excellent simulation for the dynamic crack propagation in a bi-material involving interfaces.

Abstract: A crack propagation perpendicular to gradient in a large scale functionally gradient materials, which has (1) a linear variation of Young’s modulus with a constant mass density and Poisson’s ratio, and (2) a exponential variation of Young’s modulus with a constant mass density and Poisson’s ratio, is modelled by finite element methods. Based on the experimental result of large scale functionally gradient materials, the dynamic propagation process of the FGMs is modelled and the dynamic parameters, like the energy release rate and crack tip opening angle, are calculated through a generation phase.

Abstract: Manifold Method provides a unified framework for solving problems with both continuous and discontinuous media. In this paper, by introducing a criterion of crack initiation and propagation, the second order manifold method is used to simulate two-hole blasting and Hopkinson spalling of rock. In the study of two-hole blasting, two different initiation conditions are considered. One is simultaneous initiation, the other is 0.1 ms delay initiation. The whole blasting process including crack initiation, crack growth and fragment formation is simulated. In the study of Hopkinson spalling, the propagation and interaction of stress waves and the spalling process caused by a reflected tensile stress wave are reproduced. The simulation results including the thickness of the formed scab and its velocity are in good agreement with theoretical values. Manifold method proves to be an efficient method in the study of dynamic fracture of rock.

Abstract: In this paper, a new algorithm based on Meshless Local Petrov-Galerking (MLPG) method is presented for analyzing the crack dynamic propagation. A new modified Moving Least Squares approximation is proposed to simplify the treatment of essential boundary conditions. Explicit central difference with lumped mass matrix is adopted for the time integral. Visibility criterion with crack edge node adding technique and the maximum hoop stress criterion are used to describe the crack propagation and forecast the crack propagation direction. Based on this algorithm, three-point bend specimen for impact fracture test is investigated. Comparing the results with those obtained by the laser caustic method and high-speed photographs, the accuracy of the present algorithm is proved.

Abstract: In this study, the dynamic behaviors of cracks under dynamic biaxial stress are investigated. We conduct dynamic loading fracture experiments on the aluminum (2024-T3) and the magnesium alloy (AZ31B-O) under equitable biaxial stress with a hydraulic high-speed biaxial experimental machine. The processed specimens are cruciform with a crack. Different kinds of cracks are defined by their crack angles. We analyze the results by the caustic method. We obtained the stress intensity factor and the fracture toughness value in the neighborhood of the crack tip under dynamic biaxial stress. We analyzed the obtained data, and then, we compared results.