论文标题
聚合物薄膜的外在塑料硬化在平坦的冲头凹痕中
Extrinsic plastic hardening of polymer thin films in flat punch indentation
论文作者
论文摘要
狭窄的几何形状提供了有用的和实验性的机械测试布置，其中研究分子和微观结构过程，这些过程控制着由剪切静水压力主导的压力环境中塑性产量的。但是，由于从不受限制的模式（例如单轴压缩到受限的模式）而导致的宏观应力应变行为的变化通常被忽略并显示出令人惊讶的复杂程度，即使对于简单的弹性塑料构成模型也是如此。在这里，我们报告了通过重复的塑料载荷用圆柱扁平的打孔来实现的聚苯乙烯薄膜中的限制诱导的应变硬化效果，其直径是初始膜厚度的很多倍。这种高纵横比与围绕接触的薄膜材料提供的约束结合，以产生凹痕区域中狭窄的单轴应变状态，从而使变形一维。通过重复加载到塑料结构域中，我们的限制应力从0.3 GPa增加到66％，至0.5 GPa。通过对主要应力和应变的有限元仿真和分析建模，我们表明，这种效果不是源于材料结构的内在变化，而是在塑料加载过程中赋予的残留应力。我们将这种效果与固有的变化与玻璃状薄膜（例如物理衰老和热交联）形成对比。
Confined geometries offer useful and experimentally amenable mechanical testing arrangements in which to study the molecular and micro-structural processes which govern plastic yield in stress environments dominated by hydrostatic pressure over shear. However, the changes to macroscopic stress strain behaviour that result from switching from an unconfined mode such as uniaxial compression to a confined one are often overlooked and display a surprising level of complexity, even for simple elastic plastic constitutive models. Here we report a confinement induced strain hardening effect in polystyrene thin films achieved through repeated plastic loading with a cylindrical flat punch whose diameter is many times the initial film thickness. This high aspect ratio combines with constraint provided by film material surrounding the contact to generate a state of confined uniaxial strain in the indented region, rendering the deformation one dimensional. By repeated loading into the plastic domain, we achieve a 66% increase in the confined yield stress, from 0.3 GPa to 0.5 GPa. Through finite element simulation and analytic modelling of the principal stresses and strains, we show that this effect arises not from intrinsic changes to the structure of the material, but rather residual stresses imparted during plastic loading. We contrast this effect with intrinsic changes to glassy thin films such as physical ageing and thermal cross-linking.
