Polymer craze

Xiao, F. and Curtin, W.A., Numerical investigation of polymer craze growth and fracture. Macromolecules, 28, 1654-1660 (1995). 

The loss of entanglements (and the decrease in molecular weight due to chain scission) adversely impacts fibril stability. Fibril breakdown by localized creep should occur more rapidly in polymer crazes with low entanglement densities and small diameter fibrils. 

Since crazing represents a cavitational form of plasticity, it is clear that crazes play an important role in the fracture of polymers. Crazing is, generally, involved when PC fails under plane strain conditions, e.g. in fracture mechanics tests on thick samples with sharp notches where high triaxial stresses are built... 

Athermal tensile yield stress of a glassy polymer Craze half length... 

In all block copolymers investigated, the principal morphological wave length of the rubbery domains, i.e. sizes or mean spacings, is too short to concentrate the stress in a large enough volume element to nucleate crazes in the majority phase of PS. Furthermore, with the exception of perhaps the pure KRO-3 Resin with lamellar morphology, the combination of thermal stresses and the levels of applied stress are also insufficient to cavitate the rubbery domains without any further assist from more macro stress concentrations. Hence the available evidence indicates that in these polymers, crazes initiate entirely from surface stress concentrations. 

Analogous results have been found for other polymers. Crazing at crack tips exhibits exactly the same behavior. In order to evaluate the acceleration of crazing by action of a solvent, the interaction between the polymer and the solvent can be quantified by means of the solubility parameter, 5, defined as the cohesive energy density where AHy is the vaporization... 

COHESIVE PROPERTIES of a CRYSTALLINE POLYMER CRAZE under IMPACT EXTENSION... 

Cohesive Properties of a Crystalline Polymer Craze under Impact Extension... 

The objective of this test method is to measure the cohesive stress and the time to failure of a crystalline polymer craze layer under rapid, uniform extension. The method is an impact variant of the Full Notch Creep test used by Fleissner [12], Duan and Williams [13], Pandya and Williams [14] and others. The specimen (Fig. 2), a square-section tensile bar, is injection moulded. At the mid-plane of the gauge length a sharp, deep circumferential notch reduces the cross-section to about one fifth of its original area. This notch plane is formed by a moulded-in, hardened steel washer. Specimens were injection moulded at 210°C into a warm (100°C) mould and air cooled to 40 C using a hold pressure of 45-50 bar. 

The load-displacement curves for C(T) tests of the neat EpoxyH were almost linear until the final unstable fracture. The fracture toughness value in 77K-LNj was 210 J/m and that in RT-air was 120 J/m. Thus the toughness increased by 1.8 times by changing the test environment from RT-air to 77K-LN. Brown and co-workers have found that amorphous polymers crazed in 77K-LNj, but not in a helium or vacuum at about 78K [20-22]. They have also reported that the stress-strain behavior of all polymers, amorphous and crystalline, is affected by at low temperatures [22]. Kneifel has reported that the fracture toughness of epoxy in 77K-LNj is higher than that in RT-air and 5K, and that the reason for this is the reduced notch effect by plastic deformation [23]. Then, the increase of the fracture toughness of the neat EpoxyH in this study is probably caused by the similar effect. 

J A Sauer and M Hara, Effect of molecular variables on crazing and fatigue of polymers . Crazing in Polymers, Berlin, Springer Verlag, 1990, Volume 2, pp 69-118. 

Crazing requires a stress field that must have at least one tensile component where in unoriented homo-polymers crazes form and grow normal to the maximum principal tensile stress. Craze microstructures in homo-polymers that have been widely studied consist of drawn polymer fibrils a few nanometers in diameter and have extension ratios of 2-4 that bridge the two faces of a craze and result in a density reduction of up to 0.75 locally. 

Sorption is permeation by liquid or gas from the environment, resulting in swelling of the material. Swelling may be followed by dissolution into the solvent. This is generally not observed with cross-linked polymers. Crazing appears on the surface of polymer extrusions, caused mainly by internal stresses. Crazing may extend to cracking. 

Line Zone or Dugdale Model, in many polymers, crazes form at stress concentrations such as crack tips (12). A craze is a planar structure, which can be realistically modeled by a line zone, as shown in Figure 14. Here, microyielding at the craze boundaries is modeled by a line of elastic tractions as in the Dugdale model. There is mechanical equilibrinm if the zone length is... 

Brown has pointed out that gases at sufficiently low temperatures make almost all linear polymers craze [59-61, 63]. Parameters such as the density of the crazes and the craze velocity increase with the pressure of the gas and decrease with... 

R. Schirrer Damage mechanisms in amorphous glassy polymers Crazing, pages 488-499. In J. Lemaitre [92], 2001. 

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