Key Engineering Materials Vols. 324-325

Abstract: In this study, an experimental method has been studied to measure the residual stresses on a specimen with I-groove weldment. The specimens were extracted from I-groove weldment which was joined by SAW (Submerged Arc Welding) with CO2 shield gas. A FEA (Finite Element Analysis) model was developed for the estimation of the residual stresses for the specimen. Measurements were carried out using ESPI(Electronic Speckle Pattern Interferometry) system which can measure the strain distribution on the surface of specimen. The residual stresses were estimated by the value of strain measured by ESPI system. Strain gages were added to evaluate the accuracy of ESPI system. In addition, a three-dimensional FE model was used to estimate the residual stresses generated by the welding procedure. A thermal elasto-plastic analysis was performed by the FEA. The stresses measured by the experiments were compared with the results of FEA. Also, discussed are the difference and agreement between the stresses obtained by experiments and FEA, respectively.

Abstract: The paper presents the results of a continued study of curved fatigue crack growth in a multiple arbitrarily pre-cracked isotropic sheet under plane stress loading. The predictor-corrector method (PCM) was extended in order to analyse the growth of multiple crack systems in a finite 2D structure. Together with the recently proposed improved modified virtual crack closure integral (MVCCI) method we can obtain accurate SIF values also for interacting cracks, and furthermore we can simulate fatigue crack growth of multiple crack systems in plane sheets under proportional mixed mode loading conditions. As a result, the program PCCS-2D is written to run within ANSYS to simulate interacting curved cracks. In order to check the accuracy and efficiency of the proposed method several example problems are solved. Especially curved cracks emanating from loaded fastener holes in sheets are analysed.

Abstract: This paper presents an experimental study on the anchorage behaviour of long high-strength steel rebars embedded in high-performance concrete (HPC) under both static loading and fatigue loading. The HPC was designed as C60 with its cube compressive strength larger than 60 MPa, and the high-strength steel bar was adopted as HRB500 with its characteristic yield strength equals 500 MPa. Under 3×106 fatigue loading cycles and then followed by a monotonous static loading, the strain and the stress state of the reinforcement bar, and the bond stress between the concrete and the 700 mm-long bar were investigated. Based on the test results and the ANSYS finite element analysis, the bond behaviour between HPC and long high-strength steel bars is discussed.

Abstract: In order to calculate the fracture parameters (Stress intensity factor) in a complicated 3- dimention aircraft model with damage in the aircraft panel, a new two steps global-local hierarchical analysis strategy is used. This paper primarily describes the development and application of advanced computational analysis techniques to determine stress intensity factors for the damaged panels based on the two steps hierarchical analysis strategy from global to 3-D local model, the bulging deformation of crack can be considered in the local model. A fracture parameter calculation programme based on automated global-local procedure to simulate cracked aircraft panel tests is developed for the hierarchical strategy. This programme may create models of two stages, transfer boundary conditions, calculate and obtain fracture parameter automatically. Finally, this paper presents some of the experimental data and the calculated fracture parameters are compared with the experimental results.

Abstract: In engineering reliability analysis, the limit state equation is generally implicit and nonlinear, and large number basic random variables and small failure probability are associated. For this kind of reliability analysis, a novel numerical simulation is presented based on the combination of Markov Chain Simulation and line sampling. In the presented method, Markov Chain Simulation is used to draw samples in the failure domain rapidly, and important direction for the line sampling is determined by these samples. Then the line sampling technique is employed to take samples according to the important direction, and failure probability can be evaluated by line sampling with high efficiency. Comparing to the finite differential method for obtaining the important direction, higher accuracy and higher robustness of the important direction are obtained by the presented method. The application in the reliability analysis of low cycle fatigue life of aeronautical engine turbine disc structure, which is applied by multiple cyclic loads, shows that the presented line sampling combining with the Markov Chain Simulation is efficient and feasible.

Abstract: The effect of undissolved ferrite amount on bending fatigue properties and failure mechanism were studied by using 42CrMo steel with subcritical quenching process The amount of undissolved ferrite were 0%, 10% and 15%, respectively. The hardness of the specimen was treated to medium hardness. The experimental results show that the existence of undissolved ferrite can not only change the microstructure, but also increase the bending fatigue life. The bending fatigue life elongates with increasing of amounts of undissolved ferrite. The grains can be fined by using subcritical quenching process and the area of phase boundaries can also be greatly increased because of undissolved spheroidal carbide. The martensite and carbide can also be changed using subcritical quenching process. Stress relaxation due to the move of dislocations inside the ferrite and the promotion of strength due to occurring of plastic deformation and enwinded dislocations are main reasons of improving the bending fatigue life. The bending fatigue life elongates with the increase of amounts of undissolved ferrite before the amount of undissolved ferrite reaches 10%. Under the experiment conditions, when the amount of undissolved ferrite is 10%, the bending fatigue life will be the longest.

Abstract: In the present paper, singular stress fields at the vertex on the interface in three-dimensional bonded joints are analyzed using BEM. The order of stress singularity is determined solving an eigen equation based on FEM formulation and the stress distribution is expressed using the result of the eigen-value analysis. A simple relationship between the thickness of layer and a value of stress is introduced. Then, stress distributions for various thicknesses of layer are unified using the relationship, and the unified intensity of stress singularity is determined.

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: Most of existing studies on rock damage are focused on investigating the macroscopic stress-strain relationship, and only limited research is available on analyzing the microscopic crack propagation due to measurement difficulty, cumbersome data reduction procedures, and complexity of damage in micro-level. In this study, a powerful image-processing program is developed to extract the microscopic crack distribution from the digital image of microscopic crack status of red sandstone, and the microscopic crack propagation of red sandstone under single-axial pressure is then evaluated by the technique of multifractal. It demonstrates that the microscopic crack distribution possesses the multifractal character, and the capacity dimension of multifractal singular spectrum can be used to quantitatively characterize the microscopic crack propagation of red sandstone. By comparing the stress-strain relationship with the one of stress-capacity dimension, the capacity dimension is capable of acting as a physical quantity to effectively reflect the damage of red sandstone by the means of characterizing its microscopic crack propagation.

Abstract: Temperature field is formed due to heat dissipation when material is subjected to irreversible deformation. In this paper, the heat dissipation in the crack-tip plastic zone was considered. By considering the propagating crack-tip plastic zone as a running heat source and constructing a reasonable heat source density distribution function, the temperature field around a steady running crack was obtained. It is shown that temperature rise is dependent on the crack growth speed and the material parameters. The maximum temperature rise reaches to >50 oC in our example calculations for a steady running crack in PMMA.