A STUDY OF PHOTO-ELASTIC EXPERIMENT TO FRACTURE PROCESS OF LIMESTONE SPECIMEN WITH PRE-CRACKS

In this paper, the fracture process of specimen with pre-cracks is studied by using photo-elastic expe' merit.The analysis results of cracks stress field show that because the inter-effection appears between the the two cracks and the tensile fracture occured when the tensile stress concentration reaches to a certain degree, and cracks link up with each other, macroscopic failure will gradually result. Positions of cracks will have different effections to speciment's fracture.     

A FRACTURE MECHANICS ANALYSIS OF FRACTURE PLANE CONTROL IN FRACTURE CONTROL BLASTING

A procedure of fracture control blasting for achieving directional breaking of rocks by blasting is presented. The criterion for initiating of radial cracks in borehole wall under dynamic load and the method for determining the permissible rang of initial gas pressure (p0) in the charge cavity are developed .and the branching of radial cracks and the effeets of the pressure (p0) and blasthole geometry (R0.a.L3) on final dimensions of directed cracks are studied .with Fracture Mechanics analysis in all three aspects of fracture process including crack initiation .crack propagation and crack arrest.     

带裂缝混凝土氯离子扩散作用试验研究及数值模拟

为了准确评估带裂缝工作混凝土结构的耐久性能，针对氯离子在带裂缝混凝土中的扩散过程进行研究。提出了无损制备裂缝的方法，可高效易行地在混凝土侧面及内部产生裂缝。对带裂缝的水泥砂浆试件进行氯盐溶液浸泡试验，深入研究了单缝和双缝试件中氯离子的扩散作用，修正了氯离子的扩散系数，并对带裂缝混凝土的氯离子扩散过程进行了数值模拟。研究结果表明：氯离子会沿裂缝发展方向及垂直于裂缝发展方向扩散，随着水灰比的减小，砂浆试件的抗氯离子扩散性能明显提高，在一定范围内，当裂缝间距增大，双缝间的氯离子扩散交互影响作用明显减小，ANSYS软件的模拟结果与试验数据吻合良好。     

Application of Fracture Mechanics in Explosion of Large Iron and Steel Wastes

The ex plosio n process a n d paramete rs in the blast of large iron a nd steel wastesare analysed by using the theory of fracture mechanice.A three-dimensional model is established us-ing pressurised cylinder with multiple semi-elliptical surface cracks.Reasons are given for the expla-nation that the dynamic fracture toughness for iron and steel is generally lower than the correspond-ing static value during the blast.The importance is emphasized of the critical duration time of the dy-namic stress intensity factor for crack extension.     

3D Meso scale Failure Simulation of Four phase Composite Concrete

Taking into account the influence of the internal micro cracks on the strength and deformation of concrete, a four phase composite material model was proposed, in which it included mortar, aggregate, interface and random defects. Meso scale unit in concrete was subjected to elastic plastic damage constitutive of increments form in strain space. With the simulation of mesh concrete crack growth by damage elimination, the damage processes of concrete under 3 D uniaxial compression and the elasto plastic damage process of concrete under uniaxial tension with heterogeneous specimen were obtained. It was shown that the proposed four phase model added the simulation of random defects, with which it was more appropriate to the actual situation in concrete forming. The percentage of random defects in volume was the key to determine the weakening degree of macro strength of concrete. The cracks occlusion was obvious for the lower homogeneous uniaxial tension specimen and the fracture was clearer for higher homogeneous specimen. The simulation results was in good agreement with test results.     

Analysis of Experimental Performance of Dynamic Mixed-mode Fracture Initiation in Rock

This paper discusses the specimen design in experiments of dynamic mixed-mode fracture initiation in rock, and explains that four-point bending specimen is very adapted;then it discusses the experimental method of four-point bending specimen,and an initial experiment and analysis was carried out. Some meaningful fracture curves were obtained.     

STABILITY FACTORS OF THREE-POINT BEND ROUND BEAM SPECIMENS FOR FRACTURE TESTING

Crack growth stability is very important for fracture toughness testing of brittle materials. The width of crack for a three-point bend round beam specimen changes continuously during the process of crack extension, so that the analysis of crack growth stability becomes complicated for this kind of specimen. This paper presents a method for calculating its stability factors, and the results of the calculation are compared with those of other specimens.     

A CATASTROPHE ANALYSIS OF THREE -POINT BEND ROUND BEAM SPECIMENS OF ROCK

The snap-back instability (also known as catastrophe behavior) .which may happen during the process of fracture testing with three-point bend round beam of rock .concrete and ceramic materials is analysed,an instability criterion for this kind of specimen is propoesd.it is different from that of the three-point bend rectangular beam.     

THE SIZE EFFECT OF THE FRACTURE ENERGY OF CONCRETE TESTED BY THREE POINT BENDING

This paper ainalyses the size effect of the fracture energy of concrete tested bythree point bending. It is shown that positive errors of test results of fracture energy increase,ornegative errors decrease with increasing of the specimen ize. The phenomenon is one of main causesresulting in the size effect of fracture energy.     

AN EXPERIMENTAL STUDY ON CEMENT MORTAR SPBCIMENS WITN A CENTRAL SLITS UNDBR UNIAXIAL COMPRESSION

The cement mortar specimens with a central slits are tested under uniaxial com-pression.The test results show that tensile cracks initiate,as a rule near the tip and may propagatesteady for a certain length.Mean while shear fructures appear and propagate slowly to the failurestate in the zone of compressive stress concentration. So the fracture strength of rock under compres-sion might be differentiated into the initial tensile strength,the peak strength and residual strength.     

Indentation Cracking Mechanical Analysis of ZrO2 Laminated Ceramics

The formation of indentation cracking has some relationship with the transformation stress induced by martensitic transformation, besides the relationship with the residual stress by volume change in the plastic zone in ZrO_2 laminated ceramics. Whereas surface residual compressive stress controls the expansion of surface cracks. Contributions of the transformation driving force and surface compressive stress are not considered in the traditional indentation theory. Therefore, fracture toughness value measured by the traditional method is usually lower than the practical one in ZrO_2 laminated ceramics. When indentation-fracture method is used, the value is more reliable.     

A METHOD FOR CALCULATING STRESS INTENSITY FACTORS OF CHEVRON-NOTCHED THREE-POINT BEND ROUND BAR SPECIMENS

An effective approximate method is proposed to obtain the stress intensity factors (SIF) of the chevron notched three-point bend round bar specimens, the least square principle is applied to fit the results into a polynomial for SIF,which is applied to the fracture toughness specimen suggested by the International Society for Rock Mechanics(ISRM) ,the result is satisfatory.     

AN INVESTIGATION FOR TESTING ROCK FRACTURE TOUGHNESS WTH THREE-POINT BEND ROUND BARS

Chevron-notched and straight-through-notched three-point bend round bars are used to test fracture toughness K of a Chongqing limestone. The results show that the former specimen has some advantages over the latter in the aspects of preventing overload,avoiding deviation of crack growth from notch plane and considering nonlinear correction in calculation,etc. However,the size effects of test values are to be solved.     

ENERGY BALANCE OF HYDRAULIC FRACTURE AND FRACTURE TOUGHNESS

Based on the discussion of the phenomenon that the effect of fracture toughness is neglected in general hydraulic fracturing models, this paper gives the energy balance equation in-colding all parameters in hydraulic fracturing and proposes a method of determining the apparent fracture toughness of rock from hydraulic fracture experimental data. The discussion on the factors affecting apparent toughness indicates the inevitability that the apparent fracture toughness of rock measured by hydraulic fracture experimental data is significantly higher than that measured by the standard three-point bending method.     

Fracture Geometry and Pressure Distribution in Fracture for Hydrofracturing

It is proved first that hydrofracture has a self-similar elliptic extending characteristic which is independent of the praseure distribution in fractures by the theory of fracture me-chanics in this paper.Based on this theory ,the approximative analytic solutions of pressure distribution in fractures have been put forward by fracture mechanics coupled with fluid mechanics.     

Simulation on crack growth of drill pipe with XFEM

Crack growth of drill pipe is a typical discontinuous problem. It is difficult to simulate with conventional finite element method and extended finite element method (XFEM) is developed in recent years for the problem. With the introduction of XFEM, the XFEM model of 5 inch drill pipe was proposed with different depth initial cracks under combined action between torque and tension. Based on the simulation, It is found that the smaller cracks is not easy to grow, whilst the crack of depth more than 2mm could grow under relatively lower external load and larger growth surface, eventually leads to pipe fracture failure. Through this simulation on crack growth of drill pipe, it is showed that the XFEM is appropriate in analysis of fracture failure.     

Research on Bifurcation and Chaos Characteristics of Meso-Scopic Fracture Process of Rock

On the basis of test results of meso-scopic fracture process and evolution of microcracks of rock in uniaxial compression,the number and orientation of microcracks at each stress level are obtained.A mathematical model for describing the germination and evolution of microcracks is proposed.The bifurcation and chaos behaviours of the evolution of microcracks and meso-scopic fracture process are studied for the first time with the bifurcation and chaos theory of modern nonlinear science.When the characteristic values which represent the gernimation and closure effect of microcracks take certain values,the evolution of microcracks can be described by the Logistic equation.Therefore,the evolution of microcracks of rock has the bifurcation and chaos characteristics.[WT5HZ]     

Intensification of an irrigated rice system in Senegal: Crop rotations,climate risks,sowing dates and varietal adaptation options

Feeding the future world population requires increased crop production. Here, we investigate the intensification option of increasing production by increasing cropping intensity and choice of varieties with different crop duration. We developed a model to generate, compare and visualise opportunities for single/double/triple cropping systems consisting of irrigated rice and optionally a vegetable. The model was applied in a case study in the Senegal River valley. Results showed that with appropriate choice of sowing dates, severe cold sterility in rice can be avoided, also in rice–rice crop rotations. At optimal sowing dates, simulated total long term average potential yields of single, double and triple cropping yields were 10.3, 19.0 and 18.9 t/ha respectively (total of 1,2 and 3 yields). With a hypothetical completely cold tolerant variety, yields could increase to 11.2, 20.2 and 20.9 respectively. Simulated Triple crop yields are hardly any higher than those of a double crop with two medium duration varieties. Delay in sowing due to late availability of resources (machinery, irrigation water allocation within a scheme, credits for pump fuel) is a known problem in the region. Therefore we also simulated how much delay was possible (width of the sowing windows) whilst still allowing for double cropping. We found enough delay was possible to allow for a rice–rice or a rice-vegetable crop. A rice-rice-vegetable triple cropping system would only be possible without delays and with a very short duration vegetable of 2 months. Most promising options to increase production are through shifting the sowing date to facilitate double cropping, adoption of medium duration varieties and breeding for cold tolerant varieties.     

Compatibility and incompatibility in witloof chicory (Cichorium intybus L.). 4. Formation of self-seeds on a self-incompatible and a moderately self-compatible genotype after double and triple pollinations

Summary After double or triple pollinations including selfing and the use of one-day-old compatible foreign pollen a severely self-incompatible clone could be forced to develop a reasonable amount of self-seeds per flower head. This number of self-seeds increased when the foreign pollination was delayed for e.g. 240 minutes in the double pollination experiment. Even more self seeds were obtained in the triple pollination experiment with a delayed second selfing.The influence of the old foreign pollen may be caused by a mentor pollen or a pioneer pollen effect.     

Application of Extended Finite Element Method to Fracturing Analysis of Gravity Dam

The extended finite element method(XFEM) presented in recent years is a new and effective method for discontinuity analysis,especially fracture problem analyses.The basic XFEM principle is introduced and the technique for fracture analysis is presented.The fracturing process of a gravity dam is simulated by XFEM,and the rules of stresses and displacements distribution are analyzed.Compared with the classical finite element method,the crack growth in the gravity dam can be modeled by XFEM without making the crack surface associate with the finite element mesh and setting dense mesh near the crack tip as well as remeshing after crack growth.The cumbersome work and disadvantages in conducting fracturing analysis by the finite element method thus can be avoided.The predominance of this method for dam fracturing analysis is displayed.