Craze lifetime

The test arrangement is shown diagramatically in Fig. 3. The upper end of the specimen is screwed into a strain-gauged PMMA Hopkinson bar of 19 mm diameter and 1.5 m length, calibrated for short-time response. Pilot tests had shown that the craze lifetime under impact lay within the expected uniaxial stress wave return time of 1.2 ms. 

Fig. 5. Predicted decohesion time for two craze stresses and two molecular weights (lines), compared to measured craze lifetime (solid points).

Many studies have been devoted to the influence of several factors on FCP and the total lifetime to failure however, little information is available in the literature concerning the effect of material and test variables on craze initiation. 

Primitive and mature fibril diameter Craze thickening rate Fibril lifetime... 

For a HIPS sample tested at a stress amplitude of 17.2 MPa and a frequency of 0.2 Hz, hysteresis loops taken at various cycles (Fig. 7) indicated that craze initiation was first observed for this sample after about 20 cycles, while 283 cycles were required to fracture. For similar fatigue tests carried out at the lower frequency of 0.02 Hz, the cycles to fracture were decreased (from 283 to 64) and loop asymmetry and craze formation began sooner, at about 1-2 cycles. The changes produced in hysteresis loops with cycling are shown in Fig. 19. With decrease of test frequency reduces, the entire S-N curve shifts to the left as shown by Fig. 18, and, because of the increased time for each cycle, fatigue induced craze initiation occurs earlier in the specimen lifetime. 

When ABS is compared to HIPS at comparable stress amplitudes, fatigue lifetimes are several decades higher. Several factors are involved. First, tensile strength and modulus of the SAN matrix are increased compared to values for the PS matrix. Secondly, the SAN matrix is more resistant to crazing and thirdly, the primary mode of deformation for ABS appears to be shear yielding rather than crazing. 

The results obtained on E-PMMA differ somewhat with those previously reported for PS and HIPS. In these two materials it was noted, for the specific test conditions employed, that the fractional time for craze initiation was greatest at low stress amplitude and reduced in value as stress amplitude increased. Another variable, aside from composition, that has an influence on craze initiation and an even stronger effect on total lifetime to fracture is molecular weight test data illustrating these effects will be presented and discussed in Sect. 5. 

Fig. 26. The breaking-time or lifetime Xg of the craze fibrils versus the craze stress CTj calculated from K, S and by means of the Dugdale model. Symbols + —10 °C, 0 20 °C, X 60 °C. From Ref. courtesy of Chapman and Hall, Ltd.

Fig. 31. Life-time of the craze fibrils in PMMA in a 23 °C test at 0,1, 1 and 10 Hz loading frequency (same material as Fig. 29). Lifetime here is frequency independant. From Refs. and courtesy of Society of Plastics Engineers Edn.

Fig. 43. The craze fibrils life-time versus load CTj on fibrils. Symbols as in Fig. 39 the lifetime decreases drastically at low stresses (low crack velocities V, in toluene vapors. From Ref. by permission of the publishers, Butter-worth and Co. Ltd.

The DCG process is of considerable interest, not only because it is unique to polymers, but also because a large percentage of the FCP lifetime is spent in the DCG process. Furthermore, the manner in which the DCG process terminates (vis a vis the craze and shear competition) strongly influences the fatigue crack growth resistance (or fatigue toughness). 

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