Illinois 60680, and 'The Dow Chemical Company, 2301 N. Brazosport Blvd., Freeport, Texas 77541 SYNOPSISOne of the key requirements for developing tough multiphase blend systems, for example, selecting the type of discrete phases (hard or soft) in a polymer matrix, is the ability to predict the fracture path. Most of these selections rely heavily on prior experience or on intuitive rationale. There are few mathematical guidelines for the materials scientists who are designing new multiphase systems. This article is designed mainly to provide such insight through the development of a theoretical model and through experimental observation. A finite element model has been used to predict the crack velocity and the crack path for a crack that approaches and penetrates a hard or a soft inclusion. A novel experimental approach is then utilized to verify these predictions by introducing hard and soft circular domains in poly (ethylene-co-carbon monoxide ) specimens by selective photodegradation. 0

The importance of the process zone (also called the damage zone, plastic zone or active zone) has been recognized since the works of Irwin [1], Orowan [2] and the celebrated models of Dugdale [3] and Barenblatt [4]. Recently, a thermodynamic analysis of the crack and process zone growth as an evolution of one system called the Crack Layer has been advanced [5][6][7][8][9]. The Crack Layer analysis is quite tedious, particularly due to the complexity of the crack tip field resulting from the crack-process zone interaction problem [10]. In the present work the effect of the process zone on the fracture path is analyzed experimentally using a new technique based on controlled exposure of a poly(ethylene-co-carbon monoxide), ECO, to ultraviolet radiation, (UV). UV exposure to ECO results in hardening and embrittlement of the sample [11]. A comparison of the findings in this work with previous studies is also presented [ 12].The problem of a crack propagating in the vicinity of a hole is chosen as a benchmark problem for the investigation of the effect of a process zone on the fracture path in a complex stress field. The geometry of the specimen is shown in Fig. 1. After the specimens were prepared with the notch and hole, they were exposed to UV radiation. The Young's modulus for the exposed specimen was obtained from the calibration test reported in our previous work [11]. Depending on the UV exposure time, the specimens exhibited a range of responses from ductile to very brittle. Four types of specimens with variable UV radiation time, 0, (i.e., as received) 3, 6 and 9 hours were prepared. Five specimens of each type (for the reproducibility analysis) were subjected to tension-tension fatigue with a maximum stress of 1/3 the fracture stress in a ramp test, ~/~ of 0.125 and a frequency of 1 Hz. The propagation of the crack and its surrounding process zone were monitored by a video recording system attached to a traveling microscope. The results of the observation have been processed using an image analyzer.Int Journ of .Fracture 67 (1994) RI4 Micrographs of the crack trajectories for the tested specimens of various UV radiation times are shown in Fig. 2. For the original material (i.e, exposure time t=0) a large process zone was observed during the fatigue test, while for the sample irradiated for 9 hours the process zone was so small that the material could be considered as being brittle. The effect of the process zone on the fracture path is very clear from these micrographs. As the crack and process zone approaches the vicinity of the hole there is a Crack Layer-hole interaction which deviates the crack path toward the hole. This phenomena is illustrated in Fig. 3. Figure 4 shows the fatigue lifetime of the specimens of various UV exposure times. It indicates that the average crack growth speed is much slower for the case with a process zone than for that without the process zone. To explain these results, the crack growth process can be discussed in terms of two separate events, crack initiation and quasi-sta...

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