基于颗粒物质力学的粉末高速压制过程中应力传递分布分析

Investigation of the stress transmission characterization in high velocity powder compaction based on mechanics of granular materials

为探究粉末高速压制过程中应力变化规律,结合颗粒物质力学理论与粉末高速压制高速加载特点,采用离散元法研究粉末高速压制过程,获取了应力整体传递变化规律,应力大小和方向性变化规律及应力传递与力链间关系。研究结果表明:在粉末高速压制中应力逐层传递并在边界反射,其沿高度方向在不同时刻展现出峰值流动特性,随压制进行峰值分别处在0.014m、0.0075m、0.0025m、0.011m、0.014m高度位置,并随压制进行沿宽度方向在模壁处出现逐渐降低现象;应力大小分布概率密度基本呈现单峰特性,在0.5ms、0.8ms时刻峰值分别出现在归一化应力值为3和1.5附近,在1.3ms、1.7ms、4ms时刻峰值出现在归一化应力值为1附近;主应力方向经历无规律分布逐渐向各向异性即90°方向转变,最上层粉末主方向角变化最为剧烈,变化率峰值达到61.5?10(o)/s与6-2.5?10(o)/s,最下层粉末主方向角变化最为舒缓,变化率峰值仅为60.3?10(o)/s;同时,归一化力链强度值与应力值绝对差值保持在0.1下,1.3ms^3.3ms阶段平均增长斜率分别为0.31与0.32,应力传递与力链延展保持较为一致趋势。本研究有助于更深入地了解粉末颗粒物质在高速压制过程中受力变化规律,为进一步提高粉末压制件致密化程度与密度均匀性提供指导。

In order to investigate the evolution principle of stress in high velocity powder compaction, Discrete Element Method(DEM) simulations of high velocity powder compaction process based on mechanics of granular materials and high-speed loading characteristics are conducted. Various results are obtained, such as the law of whole stress transmission, magnitude and orientation change of stress, the relation between stress transition and force chains. This investigation shows that stress transmits in a way of layer by layer and reflects at the border. Stress magnitude presents a peak flow along the height direction at different times and the peak values appear at the height of 0.014 m, 0.0075 m, 0.0025 m, 0.011 m and 0.014 m, respectively. Moreover, stress magnitude reduces at the die wall along the width direction at different times. The probability density distribution of stress value shows a single peak characteristic. The normalized peak value is around 3, 1.5 at 0.5, 0.8 ms respectively and is around 1 at 1.3 ms, 1.7 ms, 4 ms. Direction of principal stress gradually changes from random distribution to anisotropy(changes to 90o). The change of principal stress angle is sharpest(change rates reach to 61.5 ?10(o)/s and 6-2.5 ?10(o)/s) in top layer of powder and is slowest(change rate reaches to 60.3 ?10(o)/s) in bottom layer of powder. The trend of stress distribution is almost consistent with the extension of force chains. The absolute difference between normalized strength of force chains and stress is under 0.1 and the growth slopes of them are 0.31 and 0.32 in the interval of 1.3 ms to 3.3 ms, respectively. This investigation may help better understand the variation of force in high velocity powder compaction process and provide guidance to improve densification degree and density uniformity.