Fracture Mechanical Analysis of Thin-Walled Cylindrical Shells with Cracks

The fracture mechanical behaviour of thin-walled structures with cracks is highly significant for structural strength design, safety and reliability analysis, and defect evaluation. In this study, the effects of various factors on the fracture parameters, crack initiation angles and plastic zones of thin-walled cylindrical shells with cracks are investigated. First, based on the J-integral and displacement extrapolation methods, the stress intensity factors of thin-walled cylindrical shells with circumferential cracks and compound cracks are studied using linear elastic fracture mechanics, respectively. Second, based on the theory of maximum circumferential tensile stress of compound cracks, the number of singular elements at a crack tip is varied to determine the node of the element corresponding to the maximum circumferential tensile stress, and the initiation angle for a compound crack is predicted. Third, based on the J-integral theory, the size of the plastic zone and J-integral of a thin-walled cylindrical shell with a circumferential crack are analysed, using elastic-plastic fracture mechanics. The results show that the stress in front of a crack tip does not increase after reaching the yield strength and enters the stage of plastic development, and the predicted initiation angle of an oblique crack mainly depends on its original inclination angle. The conclusions have theoretical and engineering significance for the selection of the fracture criteria and determination of the failure modes of thin-walled structures with cracks.

Study on Formation Mechanism of Zonal Disintegration of Surrounding Rock in Deep Tunnel

With the construction of underground rock engineering, the surrounding rock in deep tunnels appears zonal disintegration of fracture and intact zones alternate distribution, which is a special engineering geological phenomenon. This study establishes a partitioned fracture model under the coupling of high in-situ stress and osmotic pressure, and identify the key influencing factors of the fracture model. Furthermore, a stress intensity factor (SIF) of initial cracks on surrounding rock elastoplastic boundary is derived using the transformation of complex functions. Considering the high seepage pressure of the surrounding rocks, a zonal fracture initiation criterion is established combined with the analysis of redistributed stress fields. Finally, the obtained criterion is embedded into an extended finite element method (XFEM) platform for numerical simulation. Taking the maximum circumferential tensile stress as a cracking criterion, the propagation trajectory of rock cracks is traced by contour methods. Calculation results have realized the modelling of a whole process of crack initiation, propagation, and formation. And the established criterion has be verified.

Experimental and Analytical Study on Mechanical Properties of High Rock Temperature Diversion Tunnel

The high temperature of rock used in different working conditions has a significant effect on the deformation characteristics and the mechanics of tunnel lining support structure. A test using a laboratory model is designed to study the quantitative relationship between the temperature difference and the support force. Through the laboratory model test, the strain and stress variation characteristics of the supporting structure of a water diversion tunnel under different surrounding rock temperatures and different water temperatures were simulated. The variation characteristics of the supporting structure under various working conditions, such as a different initial temperature field, different crossing water temperature of the diversion tunnel during runtime, and repairing period after the water is emptied, were analyzed. The relationship between the high-temperature difference between the inner and outer walls of the tunnel lining support structure and the internal temperature stress of the supporting structure was obtained and compared with the results from numerical experiments. The test results showed that a high circumferential tensile stress is created in the support structure of the high-temperature diversion tunnel due to the temperature difference between the inner and outer walls of the support structure caused by water going through the high-temperature diversion tunnel. The radial compressive stress increases by 45–50%, and the circumferential tensile stress increases by 40–60%. The results provide references for the design of the support structure in a high-temperature tunnel.

Experimental Technique for Dynamic Fragmentation of Liquid-Driving Expanding Ring

Expanding ring experiment is an important method for dynamic fragmentation of solid under 1D tensile loading. Based on the split Hokinson pressure bar (SHPB), a liquid-driving experimental technology was developed for conducting expanding ring tests. The loading fixture includes a hydraulic cylinder filled with water, which is pushed by a piston connected to the input bar. As the water is driven, it expands the metallic ring specimen in the radial direction. The approximately incompressible property of the water makes it possible to drive the specimen in very high radial velocity by low velocity movement of piston, according to the large sectional area ratio of the cylinder to specimen. Using liquid-driving expanding ring device, 1060 aluminum rings (ductile materials)/PMMA rings (brittle materials) were fragmented and the fragments were recovered. Impact deformation of free-flying fragments was avoided through the use of “sample soft-capture” technology. The fragmentation process was observable by high speed camera through modifying the driving direction of the water. From the observations of the fracture morphology and the residual internal cracks of the recovered fragments, it is concluded that the fracture of the rings is caused by the circumferential tensile stress.

Modified Approach for Reorientation Fracturing Propagation Trajectory Prediction

Reorientation fracturing of post-phase development of conventional low productive formations and pro-phase development of unconventional reservoirs such as shale gas plays and coalbed methane formations has been and will always be of considerable significance for oil/gas development. Conventional reorientation trajectory prediction results cannot exactly match the field actual fracture mapping propagation ones. Based on the discrepancy, this research focuses on the dominant factors influencing re-fracture propagation path and re-recognizes the disturbed stress field. Taking advantage of the proposed Principle of Maximum Circumferential Tensile Stress, the fracture propagation path is numerically modeled, which is more in conformity with the fracture mapping compared with the conventional one. Finally, the importance of proper estimation of reorientation fracturing propagation trajectory is analyzed.

Prediction of Ductile Fracture in Cold Forging of Aluminum Alloy

In this paper, the limitation and applicability of the ductile fracture criteria based on a work hypothesis and Cockcroft and Latham were investigated. For this purpose, experimental and numerical investigations for simple upsetting were conducted for aluminum alloys Al1100-O, Al2024-T3, Al6061-T4, and Al7075-T4. As a result, the fracture mode of each alloy was observed. The study was extended for pin-shape cold forging of Al1100-O and Al6061-T4 to compare the likeliness of fracturing according to two criteria. Based on experimental data of simple upsetting, the damage factors for the same two criteria were calculated by adopting rigid-viscoplastic finite element analysis. With this approach, the prediction of surface cracking was attempted by comparing the calculated limiting damage factors between simple upsetting and pin-shape forging. It was observed in simple upsetting that Cockcroft and Latham’s criterion gave a more reasonable prediction for crack initiation site than work hypothesis, but the limiting damage factors differ depending on the process. In spite of the differences, however, Cockcroft and Latham’s criterion might be useful in designing upsetting-like cold forging processes in which the influence of the induced circumferential tensile stress on failure is dominant.

Effect of Interphases on Micro and Macromechanical Behavior of Hexagonal-Array Fiber Composites

The effect of interphase stiffness on microstresses and macromechanical behavior has been investigated for transverse loading of an hexagonal-array unidirectional fiber composite. The interphase is modeled by a layer which resists radial extension and circumferential shear deformation. Taking advantage of the periodicity of the medium, the states of stress, and deformation in a basic cell have been analyzed numerically by the use of the boundary element method. The circumferential tensile stress along the matrix side of the interphase and the radial stress in the interphase have been analyzed for various values of the interphase parameters and the fiber volume ratio. The micromechanical results have also been used to determine the effect of interphase stiffness on the effective moduli. The calculated values have been compared with analytical results that were adjusted for interphase stiffness.