Experimental Research On The Non-linear Viscoelastic Behavior During Creep Failure And Craze Damage In Polymers

With engineering polymer-PMMA taken as the main researched object and viscoelastic failure considered as the theory basis, the non-linear viscoelasticity and crazing damage phenomena under creep condition in macromolecule material were researched by means of experiment.Above all, this paper gave a qualitative analysis to the macromolecule mechanism of the macroscopical viscoelastic phenomenon of high polymer in exordium and pointed out that both creep and relaxation are the gradual process of response to the external force, which is performed by all scales of molecule units with different deformation time.Basing on experiments, the creep constitutive relations of PMMA were researched. Creep experiments of single dimension were done on three specimens with no defects under constant stress and the Andrade theory ,the accumulation theory, the strain-rigidification theory ,the Norton theory were applied to fit the experimental data. Experimental and calculated results showed that creep proceeded in two phases and Andrade theory could simulate appropriately the creep constitutive relations of PMMA in single dimension. Furthermore, the accumulation theory in low stress and the strain-rigidification theory in the first phase of creep could also describe approximately the constitutive relations of PMMA. At the same time, the standard linearity body model integrated with experimental data was used to simulate the creep constitutive relations of the material. Comparison with the experimental curve revealed that this model could embody properly the creep characters of PMMA. The non-linearity and rate-dependence of the deformation behavior of high polymer were discussed primarily in this section.In the third chapter of the article, the strain field around crack-tip in PMMA and its time dependence were researched experimentally. By using Moire method, three groups of PMMA specimens with single edge crack were experimentally researched under constant stress, especially the strain field around crack-tip. Through measure and analysis, we could see its time dependence clearly. We compared the actual strain distribution with the theoretical linearviscoelastic strain distribution. The time-related expression of stain along the crack-tip extension line was given from the use of the elastic-viscoelastic correspondence principle and standard linearity body model and the non-linear factors, which caused the gradual difference between the theoretical and actual value with the increase of time, were analyzed.From the hypothesis of linear viscoelasticity, we predicted the time of life of the three specimens with edge crack which fractured in OUT experiment at last in view of energy releasing. However, we found that the predicted values were on the low side obviously, so we analyzed the reason of being non-linear. On this basis, we modified the method.At last, the theoretical analysis and experimental observation about the craze damage zone around crack-tip and its evolvement with time, its interaction with crack were carried out in this paper. Introducing the time-dependent damage variable into the constitutive equations of the crack zone, we modified the Dugdale model of crack damage zone around crack-tip. Basing on past achievement, we analyzed and gained the damage expression, the craze stress expression in craze zone and the integral equation that the size of craze zone met, all of which changed with time, distance from crack-tip and crack growth ratio. With the experiments of this article integrated, we gained the distribution figures of stress and damage in the craze zone around crack-tip, and explained why and how they changed from the aspects of material damage, energy dissipation and craze evolvement.