In reply Johnson-Cook强度模型
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Q 1: Is the Johnson/Cook model is the right choice for my kind of application?
I have seen many people using this model in their publications. But my concern
is that is it good model for Polypropylene as it is thermoplastics? I have
checked the material models from Abacus and in their matreial data base they
say it is good for metals only. I can see in this model that it takes into
consideration effects of strain rate, temperature and hardening.
Q2: What kind of tests I have to perfrom to get material data for my model:
tensile, compression etc and at what temperature these tests have to be
conducted? Is it possible that I perform my test at room temperature and at a
particular strain rate and then enter this data and my model will take into
consideration the variation of temperature and strain rates based upon my
boundary conditions etc. I reallly donot understand how it works?
I would be thankful if anyone could write me about my matter.
Thanks very much
Best Regards
Shahid
你的方法似乎是OK的。恭喜重现原始JC曲线的罕见尝试之一。
我记得在对温度进行时间积分时必须使用非常小的步骤，但对于恒定应变率测试，得到了相当好的结果。小于10^-6的delta值应该能给出非常准确的结果。这种差异令人费解。
——Biswajit
我想做的是从Johnson-Cook 1985年出版的《工程断裂力学》中复制4340钢曲线。对材料定义如下参数:
A=792 MPa, B=510 MPa, n=0.26, C=0.014, m=1.03，参考应变率ε_dot*为1.0 1/s。我使用的室温是293K，熔化温度是1793K。本文假设是绝热压缩，所以我将泰勒-昆尼系数Χ设为1.0。
分别为1.0、10.0和100.0三种应变率绘制曲线。我尝试了两种不同的方法，但都不符合文章中的曲线。在第一种方法中，我试图应用我从你文章的最后一段中理解的东西。这是……计算恒定温度下的应力(即仅使用J-C方程中的前两项)，然后确定相关的温度升高，并使用它来更新下一个塑性应变增量处的应力。这与你的解决方案不同，因为我的计算不像在代码中那样依赖于时间，所以这里有一些方差。 The curve shape looks similar to that in the publication, but it fails to reach the maximum stress values.
In the second approach I rearranged the equation for deltaT in terms of stress, and wrote a minimization script to find the increase in temperature associated with an increase in plastic strain that would give the same stress value (in both the J-C model and the deltaT equation). Again, the curve shape is reproduced, but it fails to reach the maximum stress values that are plotted in the J-C journal article.
l'll try to attach my Excel working sheet.
Thanks again for you help!
在回复绘制约翰逊-库克强度模型
答案取决于你想要实现什么以及你是如何首先得到参数的。J-C参数通常通过拟合许多实验真应力与真应变图来确定。其中一些实验名义上是在等温条件下进行的，而另一些实验是在绝热条件下进行的。在这种条件下测量温度是很困难的，如果不是几乎不可能的话。在不了解温度随时间变化的情况下，热软化效应不能从应力-应变曲线中减去。因此，对于给定的塑性应变、应变速率和温度，J-C参数已经内置了温度效应(包括热软化)。然而，当你试图模拟一些复杂的东西时，比如泰勒冲击测试，材料点的塑性应变和应变率可能与相邻点有很大不同。此外，应变率很少是恒定的，在许多情况下，随着能量通过塑性变形耗散，温度将升高。处理可变应变率的通常方法是求解动量方程，并从局部速度梯度估计应变率。处理可变温度就是从一个参考温度开始，在这个温度下进行所有的计算。 The energy equation is solved next (the one for delta T) and an increment of temperature is calculated keeping stress constant and using the increment of plastic strain. This increment in T is used to update the temperature before going to the next time step. This process is equivalent to splitting the coupled moementum and energy equations into two parts. A significant body of research can be found that discusses numerical issues related to this type of "operator splitting".
-- Biswajit
我有高速拉伸测试的应力Vs. Strain数据
测试在1 m/s,5 m/s进行，10 m/s和15 m/s
(对应的Strain rate为31.25、156.25、312.5和468.75 /s)
我不知道如何确定参数C，从测试结果
你能建议我如何在JC模型中找到参数C
Arun