Papers by Keyword: Fracture Mechanic

Abstract: The propensity of the magnesium alloys AM30 and AZ91 to environmentally assisted cracking, and in particular to hydrogen embrittlement, was assessed in constant extension rate tensile tests on smooth and pre-cracked specimens which were subjected to monotonic loading in corrosive environment. The experimental findings can be rationalized by model approaches: A meso-scale fibre bundle model was employed to simulate the results obtained in tests on smooth AZ91 tensile specimens, assuming a combination of pitting and subsequent hydrogen embrittlement as the underlying failure mechanism. The experiment data as well as the model results revealed the effect of hydrogen embrittlement on crack growth resistance. The model calculations generated fracture surfaces which were in remarkable correspondence with those observed in the experiments, and stress-strain curves similar to the experimental ones, both reflecting the influence of the applied strain rate on hydrogen induced failure. The effect of hydrogen embrittlement on cracking in AM30 was assessed using a fracture mechanics based approach. A cohesive model which accounts for hydrogen enhanced crack extension and which earlier has been successfully applied to HE of steels is currently readjusted to EAC of magnesium.

Abstract: The stress field around the tip of the blunt crack in rock-type materials under compressive loading is analyzed, and the relation of the stress field is set up. Compared to the stress field of the ideal mathematic crack, the effect on the distribution of the stress field by the thickness or the curvature radius of the blunt crack is considered. Based on the stress field, the two-parameter fracture criterion for the blunt cracks under compressive loading is set up. The two parameters in the criterion are related to the material property, and can be determined by the experiments or theoretical analysis. By the fracture model in the paper, the fracture analysis of the blunt crack under compression are carried out, and the theoretical results are in concordance with the experimental results, which shows the fracture criteria in the paper is available to the fracture analysis of the blunt crack.

Abstract: The relationship between surface crack tip stress intensity factor and calculated parameters of pavement structure is discussed and finite element models based on the fracture mechanics theory is created. From the study in this paper, the following conclusions can be drawn: As the load increases, stress intensity factor will decrease, and it may promote spread of the crack. Surface layer modulus has a great influence on stress intensity factor. In the condition of the same crack depth, stress intensity factor increases while surface layer modulus increases, stress intensity factor decreases gradually while the thickness of the surface layer increases. As base layer modulus increasing, stress intensity factor of surface layer decreases. With the increase of the modulus of subbase layer, stress intensity factor of surface layer decreases gradually. The earthen foundation module has a little big influence. With the increase of the earthen foundation module, stress intensity factor will decrease, so it can delay the appearance of the crack.

Abstract: The fracture energy release rate , which is the important parameter of ice pressure calculation model that is built on energy balance method, is the fracture mechanics performance of ice material. It is related with these factors such as ice material temperature T, which must be measured by experiment. It is the experimental method of fracture mechanics used in this paper. These ice specimens on the different thickness T were tested with fracture mechanics method at low-temperature testing machine, to obtain the fracture flexibility change rate of ice body which contains pre-crack and the values of under different conditions of T, which could determine fracture energy release rate of ice body during the ice pressure generated, which provide the necessary experimental data to establish ice layer pressure calculation model for the application of energy balance method.

Abstract: In order to understand the strain fatigue crack growth rate of pressure vessel steel controlled by displacement or load, we did experiments on strain fatigue of 16MnR steel and describe the fatigue with the energy method. We have obtained delayed cycle curve of strain fatigue controlled by displacement or load and calculated the J-integral at crack tip. In order to compare strain fatigue crack growth rates of 16MnR steel on two conditions，we compute weighted means of the strain fatigue controlled by displacement or load. Comparing two kinds of fatigue growth rate, it is obvious that the crack growth rate of fatigue controlled by displacement is greater than that controlled by load. All experiments show that compress plastic energy is higher, the fatigue growth rate of 16MnR is lower.

Abstract: Taken the Houchuangzi slope debris flow along Kunming- -Songming highway as instance, analyzed the mineral composition of provenance on the slope debris flow adopted the polycrystal X ray diffractometer, which was rich in hydrophilic clay mineralssuch as kaolinite and montmorillnite; analyzed the fornation mechanism of the HouChuangzi slope debris flow by the laboratory model test, compartmentalized the fornation process of the HouChuangzi slope debris flow in to five stages from the aspects of slope debris flow morphology and crack extension mechanism.The results provide certain reference value for further research of the formation mechanical mechanism.

Abstract: Probabilistic analysis aims at providing an assessment of cracked structures and taking relevant uncertainties into account in a rational quantitative manner. The main focus of this research work is on uncertainties aspect which relates with the nature of crack in materials. By using cracked structures modelling, finite element calculation, generation of adaptive mesh, sampling of cracked structure including uncertainties factors and probabilistic analysis using Monte Carlo method, the rigidity of cracked structures is estimated. Assessment of the accuracy in probabilistic structures is essential when limited amount of data is available. The hybrid finite element and probabilistic analysis represents the failure probability of the structures. The probability of failure caused by uncertainties relates to loads and material properties of the structure are estimated using Monte Carlo simulation technique. Numerical examples are presented to show probabilistic analysis based on Monte Carlo simulation provides accurate estimates of failure probability. The comparison shows that the combination between finite element analysis and probabilistic analysis provides a simple and realistic of quantify the failure probability.

Abstract: Delamination is a major problem associated with composite materials that reduce the stiffness of structure used in aerospace, marine and automotive technology. Interlaminar fracture toughness, non dimensional stress intensity factors and delamination crack growth behavior were investigated for carbon fiber (CF)-polyester laminates. All tests were performed with modified version of Arcan specimen. By changing the loading angles in range of 0-90°, mode-I, mode-II and all mixed mode fracture toughness data were obtained. Correction factors were obtained with finite element analysis using Abaqus software. By employing experimentally measured critical loads and the aid of the finite element method, mixed-mode fracture toughness for the composite under consideration determined. The fracture surfaces of the CF-P under different mixed-mode loading conditions were examined by optical and scanning electron microscopy (SEM) to gain insight into the failure responses.

Abstract: The blister test is a promising test method to determine the interface fracture toughness of thin films adhering to rigid fibre reinforced plastics. In this paper nonlinear finite element analysis is used to determine a suitable layout for both the shaft loaded and the pressurised blister test. On the example of a PET film adhering to a quasi-isotropic fibre reinforced plastic, it is shown that energy release rates in a range of 0-1500N/m can be obtained for a 0.5mm thick film if test parameters are carefully selected. The two main causes for deviations of the analytic solution from the FEA results is attributed to infringement of the membrane limit condition and plastic deformation in the film.

Abstract: The second development of ABAQUS is implemented to simulate the initiation, propagation processes of flaws in brittle materials under compressive loading (in the paper ‘flaw’ means ‘the initial crack’, and ‘crack’ means ‘the branch crack’), by which the propagation paths and the corresponding stress intensity factors of the branch crack can be calculated. Further more the experiment is carried out to verify the validity of the above numerical method. By the numerical method, the propagation processes of open flaws and close flaws are simulated, and the comparative analysis of propagation characteristics between the open flaw and the close flaw is carried out. The results show the obvious difference in the propagation characteristics between open flaws and close flaws with the same initial flaw length and angle. Firstly, compared with the close flaw, the branch crack of the open flaw grows along a more obviously curvilinear path, and the propagation path gradually approaches to a line, which passes through the middle point of the open flaw and parallel to the maximum principal stress. Secondly in the early stage of the crack propagation, the stress intensity factors of the branch crack of the open flaw are greater than of the close flaw, but with the further propagation of the branch cracks, the stress intensity factors of the branch crack of the open flaw will be less than of the close flaw. Additionally, according to the close flaw, with the decrease of the friction coefficients, the curve characteristics of the crack propagation paths become more obvious. Therefore, it is noteworthy that the wing crack of the close flaw can be regard as the straight line if the friction coefficient of the flaw surface is very small. The above differences of the propagation characteristics between the open flaws and the close flaw show that the two flaws should be distinguished strictly in the fracture analysis.