Creep craze

However, at lower constant loads the rate of crystal plastic deformation decreases and (at 80 °C) disentanglement becomes competitive leading to the development of isolated planar craze-like defects extending perpendicular to the tensile axis (Fig. 15). The ensuing concentration of stress will further localize most of the sample deformation in such creep crazes and lead to a macroscopic ductile-brittle transition—in this material observed at 20 MPa (Fig. 14 [67]). 

The secondary transition has long been recognized as controlling creep. Craze fibril growth is some kind of creep phenomenon, and therefore the influence of the secondary peak seems quite reasonable. Conclusion The phenomenology of the craze at a propagating crack-tip changes fundamentally for a particular temperature which seems to be the secondary relaxation p peak temperature. 

Fig. 29. Times to failure of HD PE water pipes under internal pressure p at different stresses and temperatures A, 20°C B, 40°C C, 60°C D, 80°C. 1 = ductile failure (see Fig. 2) 2 = creep crazing (see Fig. 28). Circumferential stress a = d 2s, where = average diameter and s = wall thickness. From Ref 19.

Fig. 30. (a) Surface of a creep craze formed in HDPE under conditions shown as point 2 in Figure 27 o-y = 6 MN/m, T = 80°C (b) Detail of the fracture surface close to the upper center of the zone which was apparently the point of creep craze initiation. From Ref. 19. 

Fig. 1.3. Long-time failure of a PVC pipe through development of a creep craze (from [131).

Fig. 1.5. Times-to-failure of high-density polyethylene (HDPE) water pipes under internal pressure p at different stresses and temperatures, d average diameter, s wall thickness A ductile failure, B creep crazing (specimens are shown in Figs. 1.6—1.8 after 1141).

Figure 1 - The miniature 3-point bending creep crazing test.

Creep rupture. Creep-rupture data are obtained in the same way as creep data except that higher stresses are used and the time is measured to failure (Figs. 2-28 and 29). The strains are sometimes recorded, but this is not necessary for creep rupture. The results are generally plotted as the log stress versus log time to failure (110). In creep-rupture tests it is the material s behavior just prior to the rupture that is of primary interest. In these tests a number of samples are subjected to different levels of constant stress, with the time to failure being determined for each stress level. General technical literature and product data sheets seldom provide a complete description of a material s behavior prior to rupture. It should include the development of any crazing and stress whitening, its strain-time... 

Crazing. This develops in such amorphous plastics as acrylics, PVCs, PS, and PCs as creep deformation enters the rupture phase. Crazes start sooner under high stress levels. Crazing occurs in crystalline plastics, but in those its onset is not readily visible. It also occurs in most fiber-reinforced plastics, at the time-dependent knee in the stress-strain curve. 

Biaxially oriented films, made by stretching in two mutually perpendicular directions, have reduced creep and stress relaxation compared to unoriented materials. Part ot the effect is due to the increased modulus, but for brittle polymers, the improved behavior can be due to reduced crazing. Biaxial orientation generally makes crazing much more difficult in all directions parallel to the plane of the film. 

Even in cases where the rigid polymer forms the continuous phase, the elastic modulus is less than that of the pure matrix material. Thus two-phase systems have a greater creep compliance than does the pure rigid phase. Many of these materials craze badly near their yield points. When crazing occurs, the creep rate becomes much greater, and stress relaxes rapidly if the deformation is held constant. 

ESC, described in Section 4.9.4, is a physical phenomenon and is the acceleration of stress cracking by contact with a fluid, i.e., stress cracking will occur without the fluid at sufficiently long times. Ultimately, the slow crack growth that follows crazing reaches a critical point when fast crack growth and failure occurs. This failure, with or without the accelerating effect of a fluid, is a creep rupture effect (see Section 8.12). 

If Vt 1240 meters/sec in the matrix and branching will occur in the rubber at 29 meters/sec, we calculate A/Co = 0.047. Thus, branching can occur after a matrix crack acceleration distance of only 2 to 5/x (assuming a Griffith crack length of 50-100fi) hence, ample room for the development of fast cracks or fast crazes exists in the ABS structure. Note that the expressions for craze instability, acceleration, and speed (Equations 1, 6, 7) show that the macro strain rate of the specimen is irrelevant— fast cracks and crazes propagate in specimens strained even at slow creep rates. 

Keywords Crazing Creep Molecular deformation mechanisms Disentanglement Time-dependent strength Toughness... 

Formation of Spatially Distributed Crazes in Amorphous Polymers under Creep Loading... 

Section 3.3 is mainly concerned with the molecular mechanisms effective during creep and the competition between crazing and flow. In Sect. 3.2.2 we indicated that during the phase of secondary creep molecular rearrangements... 

For the studied materials D0 amounts typically to 28 to 150 nm. A plot of smax as a function of creep stress a at a given temperature reveals three distinct regimes each with a straight slope (Fig. 12) the slope is related to the energy r necessary to create the craze fibril surface [60,61] ... 

Table 1 Important parameters of materials used to study craze formation in creep (from [54])...

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