An Experimental Study On Ratcheting Of NYLON6 Polymer With Different Humidity

As a kind of thermoplastic materials, nylon 6 (hereinafter referred to as PA6) polymer has been widely used in automotive, electronics, micro-electric areas due to its comprehensive performances. In its normal working environment, PA6 inevitably bears a cyclic loading. The ratchetting effect is produced by the accumulation of cyclic deformation which will lead to undesired damage of the structure. Especially, nylon-type polymers are apt to the effect of humidity because of their hydrophilic molecular chain acylamino. So it is of great importance to investigate the effect of hygroscopicity on the cyclic deformation of PA6 polymer.In recent years, many scholars have studied the ratchetting of polymers, but there are still some problems:(1)The researches on the cyclic deformation behavior of PA6 materials are still not so systematical, especially related to the ratcheting and cyclic softening/hardening properties. (2)Few researches studied the non-proportional multiaxial cyclic deformation behavior.In this work, a series of tests of PA6 materials have been conducted at room temperature, including the monotonic tension, creep, uniaxial stress-controlled cyclic test, uniaxial strain-controlled cyclic test and multiaxial non-proportional stress-controlled cyclic test.The phenomenon of hygroscopicity -dependence, rate-dependence, the dependence of non-proportional loading paths mainly are revealed. Additionally, the strain cyclic softening/hardening properties and the surface temperature rise with different moisture absorption were additionally discussed.The results are given as follows:when the relative hygroscopicity of the PA6 is 0%, the axial creep deformation is obvious and closely related to the stress level; the higher the stress level, the greater the corresponding creeping deformation is, and then the more unrecoverable residual strain is produced. It is also shown that significant ratchetting occurs in the uniaxial stress-controlled cyclic tests of the PA6, and the ratchetting strain remarkable increases when the stress level is higher, and the peak stress holding time is longer, as well as loading rate is slower. It is also shown that the temperature rise of the PA6 is more remarkable and the cyclic softening is also more obvious when the increases of loading rate and stress amplitude in the symmetrical strain-controlled cyclic tests; the cyclic softening is also more obvious with the increase of mean strain in the asymmetrical strain-controlled cyclic tests. In the non-proportional multiaxial stress-controlled cyclic loading, the ratchetting is also dependent on the non-proportionally multiaxial loading paths, axial valley stress level and loading rate; the ratchetting strain in different non-proportional paths are all higher than that of uniaxial ones with the same loading condition. When the relative hygroscopicity is not 0%, the PA6 shows a significant hygroscopicity -dependence in the monotonic tension, creep, uniaxial and non-proportionally multiaxial cyclic deformation; with the increase of relative hygroscopicity, the responding stress decreases, the hardening behavior is enhanced and the creeping deformation increases. The temperature rise and the stress amplitude are more insignificant in the symmetrical strain-controlled cyclic test, but the temperature rise and the stress relaxation are more insignificant in the asymmetrical strain-controlled cyclic test. The dependence of non-proportional loading path might be changed by the hygroscopicity in the multiaxial experiments.The proportion of recoverable strain increases with the increasing relative hygroscopicity which means that the viscoelastic effect of the material is more obvious.