The Influence of Intrinsic Strain Softening on Strain Localization in Polycarbonate: Modeling and Experimental Validation

Abstract

Intrinsic strain softening appears to be the main cause for the occurrence of plastic localization phenomena in deformation of glassy polymers. This is supported by the homogeneous plastic deformation behavior that is observed in polycarbonate samples that have been mechanically pretreated to remove (saturate) the strain softening effect. In this study, some experimental results are presented and a numerical analysis is performed simulating the effect of mechanical conditioning by cyclic torsion on the subsequent deformation of polycarbonate. To facilitate the numerical analysis of the “mechanical rejuvenation” effect, a previously developed model, the “compressible Leonov model,” is extended to describe the phenomenological aspects of the large strain mechanical behavior of glassy polymers. The model covers common observable features, like strain rate, temperature and pressure dependent yield, and the subsequent strain softening and strain-hardening phenomena. The model, as presented in this study, is purely “single mode” (i.e., only one relaxation time is involved), and therefore it is not possible to capture the nonlinear viscoelastic pre-yield behavior accurately. The attention is particularly focused on the large strain phenomena. From the simulations it becomes clear that the preconditioning treatment removes the intrinsic softening effect, which leads to a more stable mode of deformation. [S0094-4289(00)01002-1]

Quantitative Prediction of Long-Term Failure of Polycarbonate

“…From numerical investigations it has become evident that the occurrence of plastic instabilities such as neck formation, but also crazing, is closely related to the intrinsic deformation behavior of polymer materials determined from constant true strain-rate compression experiments; [30][31][32] see Figure 2b. At relatively small deformations, the yield stress of the material is reached.…”

“…Initiation of the instability is triggered by strain softening, whereas strain hardening stabilizes the localization allowing it to grow. [29][30][31][32] Now we can of course also compare this intrinsic behavior in a constant true strain-rate compression test (Figure 2b), with that in a constant true-stress compression experiment, Figure 2a. Upon loading, the strain increases at a decreasing rate, reaching a constant creep rate.…”

“…Using three-dimensional constitutive models to describe the intrinsic deformation behavior, it was shown that the postyield behavior plays an essential role in the initiation and stabilization of strain localization in polymers. [30][31][32] To account for the influence of thermal history, a modification and extension of these models was presented and verified for different molecular weights. 33 This modification is based on the fully rejuvenated state that can be attained through application of a large plastic deformation erasing the initial state of the material.…”

Modeling of the Postyield Response of Glassy Polymers: Influence of Thermomechanical History

“…14 The resulting elastoviscoplastic model, with two basic elementssa linear elastic spring and a 3-D Eyring dashpotswas further refined by Govaert et al 5 by adding pressure dependence and (phenomenological) softening based on the concept of Hasan et al, 15 and a neo-Hookean rubber spring representing strain hardening, as suggested by Tervoort and Govaert. 16 These models were among others used in the investigation of strain localization phenomena in polymers such as shear banding, necking, and crazing.…”

“…[1][2][3][4][5] These efforts were motivated by the desire to predict mechanical properties and performance and to develop (numerical) tools for better understanding and, finally, improving properties of homogeneous, as well as heterogeneous, polymer systems. 6 A landmark in this area of research is the original paper of Haward and Thackray from 1968, 1 where they proposed to govern the large strain deformation of polymers below their glass transition temperature by two separate processes.…”