相对吸湿度对尼龙类材料单轴棘轮行为的影响

Effect of Relative Hygroscopicity on Uniaxial Ratchetting of Nylon-type Polymer

由于尼龙类材料易于吸收环境中的水分,进而在不同湿度条件下体现出不同的力学特性,因此,论文研究了以尼龙6为典型对象,通过两种相对吸湿度(RHs=0、1.0和2.12%)下的应力控制循环变形实验,研究了相对吸湿度对尼龙类材料的单轴棘轮行为的影响,讨论了不同相对吸湿度和不同峰值应力保持时间(即0、5和10s)下该类材料的棘轮变形特征.实验结果表明:尼龙类材料在非对称应力循环加载过程中产生了明显的棘轮行为,其棘轮变形包括可恢复的粘弹性和不可恢复的粘塑性变形两部分;棘轮应变随着峰值保持时间的增加而增加,体现出明显的时间相关性;相对吸湿度越高,棘轮效应越明显,不可恢复的粘塑性变形所占份额越大.

As a kind of thermoplastic materials,nylon-type polymers have been widely used in automotive,electronics,micro-electronics areas due to their excellent mechanical properties.However,such polymers are apt to absorb moisture from ambient air,leading to expansions in volume and variations in mechanical properties.Therefore,it is of great importance to investigate the effect of hygroscopicity on the cyclic deformation of nylon-type polymers,which are often subjected to certain types of cyclic loading in their engineering applications.In this work,the uniaxial ratchetting behaviors of nylon-type polymers（typically for nylon-6,i.e.the PA6polymer）are investigated by performing a series of uniaxial stress-controlled cyclic tests at different relative hygroscopicities（e.g.0,1.0% and 2.12%）to evaluate the effect of relative hygroscopicity on uniaxial ratchetting.Tests are also conducted with different durations of peak stress hold time（e.g.0s,5sand 10s）to address the time-dependent ratchetting of nylon-type polymers.After the ratchetting tests,the specimens are held for a certain period of time at zero stress to investigate the strain recovery denoted by the ratio of the viscoelastic stain to the total stain of the polymer.The obtained results show that significant ratchetting occurs in the uniaxial asymmetrical stress-controlled cyclic tests of the nylon-6polymer;and the ratchetting strain of solid-state nylon-type polymer consists of the recoverable viscoelastic and irrecoverable viscoplastic parts.It is also shown that the ratchetting is time-dependent;and the ratchetting strain increases remarkably with the increase of peak stress hold time.More importantly,the ratchetting of nylon-type polymer shows obvious dependence on the relative hygroscopicity;and the ratchetting becomes more remarkable and the irrecoverable viscoplastic strain increases as the relative hygroscopicity increases.These conclusions provide guidance for the applications of nylon-type polymer materials in engineering,and are very important to the construction of corresponding cyclic constitutive models describing the hygroscopicity-and time-dependent ratchetting of nylon-type polymers.