Craze criterion structure

To obtain an upper limit for Q0 Oprii is set equal to the failure stress Ojlhrij = Z-efffl, of the continuum fibril structure which gives a failure criterion for the critical crack opening displacement <5 = Sc at the crack tip. Here Z(, is the number of effective connector strands per nominal unit craze area can be less than Z due to the scission of chains during formation of the craze fibrils. 

As glass fibre composites have a relatively low Young s modulus, axial strain can be significant. Hence it may be neccesary to apply a serviceability limit state to this criterion to ensure that if for instance the tension member is used as a brace, it adequately prevents excessive sway in the structure. In any event there is a hmiting allowable strain to ensure that the resin does not craze. 

Molecular Criterion for Craze/Yield Behavior from Chain Structure Parameters... 

The precise function of microfibrils in hard elastic polymers is not well understood. Their importance in various models of crystalline polymers has ranged from mere tie points [4,5] to the fundamental element [9]. The recent disclosure of hard elastic behavior in crazed HIPS [11] clearly revealed the connection between microfibrillar superstructure and hard elastic behavior, since no lamellae exists in this material. Polymers with a fibrillar structure are not necessarily hard elastic,however. Gore-Tex has an extensive fibrillar domain as shown in Figure 2c. Its loading cycle (Figure 5), however, clearly reveals an inelastic material. Hence, the necessary criterion of microfibrillar structure for a hard elastic polymer must be established. 

The criterion (9.6) shows that amorphous glassy polymers yielding process is controlled by cluster structure stability loss (for more details see chapter four) and the condition of transition from shear to crazing can be written as follows [20] ... 

In an attempt to reconcile the various partially insufficient craze initiation criteria with the experimental data Argon [165-167] and Kausch [11] have proposed models of the craze initiation process which take molecular structure, chain rigidity, conformational changes, and intermolecular interaction into consideration. Argon bases his criterion of the transition from A to B on the break-up of concave air-polymer interfaces (cf. Table 9.4). Kausch has described a three-step craze-nucleation mechanism ... 

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