In view of the inability of velocity field and displacement field in partical flow code (PFC) simulation in showing the characteristics of localization during loading and subsequent failure intuitionisticly , the definition of effective strain in macro‐scope is introduced into the granular materials ,w hich is easy to be understood and accepted . T he numerical models including different confining pressures , granular diameters , frictional coefficients and stiffness coefficients are established ,and then programmed with Fish language for granular materials under certain initial and boundary conditions .Combining the numerical tests of PFC with expression of the effective strain in macro‐scope ,the rules and influencing factors for the formation of failure bands during biaxial tests are analyzed .The numerical results indicate that the effective strain in macro‐scope is able to exhibit the development process and relevant characteristics of deformation and failure for granular materials :the entire failure bands are formed after the peak value and during the softening stage of the stress‐strain curve ,and bigger voids and more obvious rotation of granules appear in failure bands than in other parts . It is also show n that the normal and tangent stiffness coefficients of granules , the frictional coefficients between granules ,the confining pressures around the two sides of the sample , and the diameters of granules affect the formation ,types and thickness of failure bands for granular materials to some degree .% 考虑到颗粒流（PFC ）表示的颗粒速度场和位移场并不能很直观地表示出颗粒材料在加载破坏中的变形特征，从宏观的角度引入易于理解和接受的颗粒集合体的有效应变定义，建立了考虑不同周围压力、颗粒粒径、粗糙程度、接触刚度等因素在内的颗粒材料数值模型，并结合一定的初边值条件，编制了相应的 Fish语言程序。将细观数值分析与宏观表示相结合，分析了双轴压缩试验中颗粒材料变形破坏的规律和各种基本的细、宏观影响因素。数值结果表明，基于宏观连续体概念的有效应变充分显示了颗粒材料变形破坏的发展形成过程及相关特征：破坏区域完全形成于应力‐应变曲线峰值之后的软化阶段，域内有较大的孔隙，域中的颗粒显示出明显的颗粒旋转；颗粒的法向与切向刚度、粗糙程度、颗粒集合体试样所受的压力水平，以及颗粒粒径等对颗粒材料的承载能力，破坏区域的形成、型式与厚度等有一定的影响。
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