Craze initiation stress

This study (34) implies that a right dispersion of rubber particles may permit optimum stress field overlap that affords lower craze-initiation stresses and therefore can rapidly dissipate the strain energy in the HIPS. A more homogeneous spatial distribution of rubber particles allow for a uniform development of crazes. Prevention of the strain localization phenomenon to avoid the detrimental situation, where crazes prefer to develop in certain areas and quickly lead to a catastrophic crack, could result in a larger total volume of crazed material. Further, Donald and Kramer (22) discovered no crazes nucleating from an isolated rubber particle with diameter smaller than 1 urn because of an insufficient size of stress-enhanced zone. Since Sample-A has a small average particle size it should contain a large number of small rubber particles. Two small rubber... 

Glass transition temperature Secondary transition Extension ratio Maximum extension ratio Craze intensification stress Craze initiation stress Tensile strength Compressive yield stress Drop in after yielding A measure of strain softening Test frequency... 

Fig. 25. Variation of craze initiation-stress, craze intensification stress, and tensile fracture stress, Oj, with AN content...

Fig. 8. Tensile strength, aj, craze initiation stress, and stress at start of rapid craze development, vs average contour length between molecular entanglement points, for PSAN samples of varying AN content...

Fig. 11.1 Dependence of the craze initiation stress Gg on the entanglement density Vg for various polymers and their miscible blends (From Wu (1990) reproduced with permission of Wiley)...

Figure 11.30a shows how competition between the various deformation mechanisms affects the yield stress. The solid line denotes Gic, the critical value of Ci at the onset of shear yield, whether before (the first straight section, calculated with Eq. 11.17) or after cavitation. The cavitation stress curve was calculated with Eq. 11.14 scaled accordingly to fit experimental data of the real PA6/rubber blend. Einally, the craze initiation stress curve, similar to those shown in Eig. 11.23, was calculated with Eq. 11.25. 

The deformation mechanisms taking place during rapid loading can be interpreted as follows The deformation is homogeneous up to a strain level where crazes are initiated. The craze initiation stress depends on molecular weight, sample treatment, time and temperature (cf. Chapter 9). For the above experiments it would be reasonable to assume that crazes are initiated once the load attains between 60 and 70% of the maximum load. The maximum of the load corresponds to the point where rapid crack propagation sets in. 

Region-II fatigue fracture is characterized by the fact that a period of craze nu-cleation precedes craze growth and the formation, slow growth, and catastrophic propagation of. a crack. This type of fatigue fracture is observed at stress levels just below the immediate craze initiation stress The dependency of Np on o is considerably reduced. This accounts for the delay in craze initiation as well as for the reduced rate of slow crack growth at lower stress levels. The slope (— 14 MN m per decade of Np) seems to be characteristic for a variety of polymers [142,153]. 

Analysis of craze initiation stresses in tensile samples containing circular holes, in thin-walled biaxial tube samples, and for rods in combined tension-torsion tests... 

Study of craze initiation stress aj, of location of crazes within bulk samples and of change of birefringence R across sample thickness x... 

Relation between yield stress ap and craze initiation stress aj is investigated ffpCT) e = constant (jj (T) e = constant... 

The objective of the present work is to develop an understanding of the effects of molecular parameters such as length, architecture and length distribution on the solid-state mechanical properties of polymers. In this paper we report measurements of the craze initiation stress in isochronal 3-point bending creep experiments on a wide variety of well-characterized grades of atactic polystyrene (PS) and put forward a simple model that relates molecular parameters to craze initiation. A novel miniature crazing test was employed, to enable study of materials where only small quantities are available. 

For linear polydisperse PS, the two contributions to craze initiation stress, arising from the portions of molar mass distribution above and below the disentanglement -chain scission transition, are weighted by mass and added. This agrees with experiment for the one polymer where it could be tested. 

Table 1 - Polystyrene grades used in this study, with weight-average molecular weight polydispersity index PD I, mean craze initiation stress and its standard deviation.

Figure 3 - 300s craze initiation stress results for linear (Af) and branched (Mj) monodisperse PS. 

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