我是做材料沿晶断裂的，看了不少文献，关于cohesive模拟断裂，其中的主要的断裂能不是很明白，求相关专业的同学，老师给答复，真诚求解
1 断裂参量G的数值的大小，和断裂力学中G有关系，微观中的表面能又是怎么一个关系
材料的表面能G单位J/m2,也就是1000J/m2=1mJ/mm2,按断裂力学理论中G=K2/E ,如普通45刚的断裂韧性Kic=100Mpa.m^0.5,转换成Mpa.mm^0.5，其大概的为：3000MPa.mm^2,假设其弹性模量E为200 000Mpa,则这里的G=45 MPa mm 按单位换算，1MPa mm =1N/mm;也就是说金属材料的断裂能在50左右，是个2位数，下面试材料的表面能Fe的大概在2.5~3J/m2，也就是说模拟材料断裂时cohesive单元的参数输入，按Mpa mm 为准是一个10^-3数量级别的单位，文献中有这样做的，有些的确是这样取的，
这样的带来的另一个问题是内聚力单元的刚度系数K的取值，和基体材料E的关系是什么样，我自己尝试的多次，要不没有损失，要不早早的开始损伤，一直到不到1，然而有也文章如模拟Al G=0.15N/mm,到底这个断裂力学参量G怎么取，宏观和微观的不一样，还是哪里出问题了，
2 就是关于收敛的问题，
ABAQUS中收敛参数的设置下面试一个文献中的一段关于收敛的描述，我不清楚几个参数的怎么弄，*static, stabilize=0.002
The convergence of a basic model, without any viscous regularization, forces or large displacement effects (LDE), performed very poorly. The simulation stopped at the 0.8% of the external load,
Table 3. It was established that to improve the numerical stability one needs to apply a small amount of viscous regularization
μ. A 5% value of the load step time has been used in this work, see
Table 2. In
Table 3 this model is labeled as +Visc.reg. If in addition, a small amount of viscous forces to the global equilibrium equations is added (ABAQUS “*static, stabilize=0.002” option (
Systemes, 2013)), further convergence improvement is obtained (model labeled as ++Visc.forces.). For a given iteration the residual,
R , is then decreased by the viscous force term FvFv: R=P−I−Fv=P−I−cMvR=P−I−Fv=P−I−cMv where
P stands for the external,
I for the internal forces,
c for the damping factor,
M for the artificial mass matrix calculated with unit density and vv for the vector of nodal velocities (
Systemes, 2013). When the damage at the grain boundary develops, the local region becomes unstable and the local velocities increase. Consequently, part of the released strain energy is dissipated by the applied damping which helps with the convergence. While a region is stable, viscous forces and the viscous energy dissipated are very small. Thus, the additional artificial damping has almost no effect. Accounting for large displacement effects on top of the first two options (model labeled as +++LDE) has no significant effect on the convergence, see last column in
Table 3.
【来源;
Computer Methods in Applied Mechanics and Engineering
Cohesive zone modeling of intergranular cracking in polycrystalline aggregates】
下面几幅图是相关的文献中的
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发表于 2017-1-7 18:42:12
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