Crazing and strain localization of polycarbonate glass in creep
Graphical abstract
Introduction
Mechanical behavior of glassy polymers is an important topic in polymer science and engineering since several leading thermoplastic and thermoset polymer materials are used below the glass transition temperature. Typically, deformation of polymer glasses is studied in the laboratory in the displacement-controlled mode, where the corresponding mechanical response is measured in terms of the emerging stresses [1], [2], [3]. Along this line, a large quantity of works have been carried out to probe the mechanical responses and failure behavior of amorphous polymer glasses through experiments [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], computer simulation [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46] as well as theoretical modeling [16], [47], [48], [49], [50], [51], [52], [53], [54], [55]. On the other hand, there have been early theoretical and experimental studies involving constant stress [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], carried out in from 1940s to 1970s in spirit of an Eyring type equation [67]. Formation of crazes and cracks under intermittent stresses [27], [59], [60], [61], [68], [69], [70], [71] and necking failure under high stresses [70], [72] were observed. Local hopping was regarded as the main mechanism for the increase of creep strain according to Eyring's equation [58], [67]. More recently, based on the deformation-dependent elastic modulus and activated hopping process, nonlinear creep behavior of polymer glasses has been studied theoretically [73], [74], [75]. On the experimental side, an optical photo-bleaching technique has been applied under the constant loading condition to delineate the change in segmental mobility during tensile extension of a crosslinked PMMA [76], [77], [78], [79], [80].
Craze and necking are two different types of inhomogeneous deformation observable during uniaxial extension of brittle and ductile polymer glasses respectively. Brittle polymer glasses typically show crazing as an extreme form of tensile strain localization before fracture. A large body of work on crazing has been carried out by Kramer and coworkers since late 1970s [81], [82], especially on polystyrene. According to Kramer, craze formation far below Tg involves chain scission; but close to Tg chain pullout is also a competing mechanism [82]. Ductile polymer glasses such as PC typically do not exhibit visible crazing during displacement-controlled tensile extension except at temperatures as high as 130 °C in the post-yield “strain hardening” regime [83], [84], [85], [86], [87] or at relatively low rate above the shear deformation to craze transition temperature [82], [88], [89].
In the present study, we carry out a series of tensile tests on polycarbonate in creep mode. There is a minimum stress σcraze under which crazes form over time. When the applied stress exceeds a second critical stress σsl, shear yielding takes place after a certain induction time tind, and necking ensues where the subscript “sl” stands for strain localization (necking). Such observations are consistent with the previous report [59], [60], [61]. The induction time tind for the strain localization is found to decrease exponentially with increasing load level. More importantly, using a PC that has been subjected to melt-stretching above Tg, we show that such a pre-deformed PC no longer suffers strain localization of any form.
Section snippets
Materials
The bisphenol A-polycarbonate (PC) under study is Lexan TM 141 111 from Sabic (GE Plastic). It has a weight-averaged molecular weight of 63 kg/mol and polydispersity of 1.58. The glass transition of this material is 145 °C as measured by a TA Q2000 DSC at a ramping rate of 10 °C/min. To prepare specimens, the PC pellets were placed into a 160 mm × 160 mm × 0.6 mm mold between two sheets of Mylar® in a 50-ton Dake hydraulic press with a 5″ diameter ram. The two plates of the press were set to
Three regimes under creep (constant load)
Before describing the mechanical behavior of the PC under creep in the different regimes, we present the stress vs. tensile extension curve for the PC stretched at room temperature at a speed of V = 6 mm/min. Fig. 1 shows the characteristic response of PC under the displacement-controlled uniaxial extension: (a) complete Hookean elastic deformation appears to occur for extension lower than 2%, involving a stress lower than 25 MPa; (b) strain softening shows up when the stress starts to deviate
Discussions and behavior of aged PC
In order to discuss the experimental results, we need to briefly mention our hybrid picture for polymer glasses undergoing large deformation [91]: There is a primary structure due to the short-ranged van der Waals bonds that must break down at large deformation. Separately, there is also a chain network due to the molecular interpenetration and uncrossability. In contrast to the entanglement network found in polymer melts, the chains can hardly move without external deformation owing to the
Summary
In this work, we systematically examined the mechanical responses of bisphenol-A polycarbonate at different engineering (tensile) stresses σengr below its glass transition temperature Tg. In contrast to displacement-controlled uniaxial extension, the PC glass responds to the applied stress over time. Only a sufficiently high stress level σcraze ∼ 25 MPa can cause surface crazing within an hour. Treating the polymer glasses under large stress as a hybrid structure, as described recently [91],
Acknowledgment
This work is supported, in part, by the REU program of the NSF (NSF-DMR-1004747) as well as an NSF research grant (CMMI-0926522). Lilian Johnson was a summer REU undergraduate student from University of Maryland, Baltimore County (UMBC).
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