Craze yielding

The yield criteria of polymers have been reviewed by Ward (7) and more recently by Raghava et al. (8). Except for the craze yield criteria of Sternstein and Ongchin (9) and Bowden and Oxborough (10), most of the yield data can be described by a pressure-modified, von Mises-yield criterion. The corresponding yield surface is everywhere convex. A typical yield locus on the [Pg.103]

The tensile stress-strain response of the homopolymer, and of two rubber modified grades of polystyrene, is shown in Fig. 1. The principal mode of deformation is crazing and all three materials exhibit a craze yield stress. However, there is no evidence of localized necking in any of the three materials. The craze yield stress decreases and the elongation to fracture, and the toughness, increase significantly with increase in rubber content. 

The variation in loop width (at a nominal stress of 10 MPa) for both the tension and compression side of an ABS sample, tested at a stress amplitude of 31 MPa, is shown in Fig. 43. In this case strain softening initiates early and develops rapidly within the first 100 cycles or so. It then continues to increase with increased cycling, at essentially a steady rate on both the tension and compression side of the hysteresis loops. As crazing becomes a significant mode of deformation, which here occurs at about 650 cycles, the loop width increases more rapidly on the tensile side. From this type of graph, we can estimate, for various stress amplitudes, the number of cycles required to initiate shear type softening, N, and the number of cycles required to initiate craze yielding, N. . 

Yield stress in general. With subscript cr, craze-yield stress... 

Tills falls somewhat short of the measured value of about 30 MPa. We attribute this discrepancy to the difficulty of obtaining accurate elastic constants for the particle by the method of Chow, where we used a random spherical block componen t morphology that differs significantly from the randomly wavy rod morphology of the KRO-1 particles. We conclude nevertheless that the craze yield stress is high due to the minimal elastic and thermal expansion mismatch of the particle with its... 

This compares well with the upper craze yield stress of 10.5 MPa shown in Fig. 31 c for the modified spherical shell particle that approximates to the solid PB particle. 

From measures of band width and knowledge of the prevailing stress intensity factor, K, one can compute the craze yield stress, S, from the fracture mechanics relation... 

Ki = 0.23 MPaVm according to equation 2 for a = 2.2MPa and notch size = a = 2.5 mm. This results in a yielded strip size of 0.22 mm according to equation 3. This value, of namely 0.14 mm, is much higher than the measured band size. Possible explanations for this discrepancy are the fact that the material is not behaving in a linear elastic manner and/or that only a part of the craze/yielded zone breaks at once. 

LeGrand, D.G. Crazing, yielding, and fracture of polymers 1. Ductile-brittle transition in polycarbonate. J. Appl. Polym. Sci. 1969, 13, 2129-2147. 

Measurements of the critical angle for reflection at a craze yield a value for the refractive index of the craze and show that it must consist of approximately 50% polymer and 50% void. Investigations by electron microscopy, electron diffraction and small-angle X-ray scattering show... 

Thus, starting with the cases for T = 293 K, first an important modification is introduced in the local craze-initiation statement by normalizing the mean normal-stress term by the more appropriate intrinsic tensile craze yield stress Yq rather than the distortional tensile yield strength Tq by taking the experimental value Yc = 55.8 MPa = 0.542To for it. This modification results in the following mechanistic initiation statement of... 

Bucknall, C. B. (1977). Tm iaud Plaaics. Applied Science, London. A specialized text on rubber toughened plastics, but induding more general discussions of crazing, yielding, and fracture in plastics. (Relevant to Chapter 5.)... 

Molecular Criterion for Craze/Yield Behavior from Chain Structure Parameters... 

N2, Ar, O2, and CO2 usually cause craze yielding whereas in He or in vacuo the polymers undergo brittle fracture without crazing... 

Kinetics of craze growth in methanol explained by liquid flow through the porous crazed material (to which a void spacing of 0.25 juni, a void size of 72 nm and a craze yield stress of 9 MN m are assigned). 

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