Craze growth in air

In this section the kinetics of craze growth in air will be considered in unnotched specimens as well as at crack tips. We shall not be concerned with the initiation phase and any micromechanism (e.g. leading to craze initiation. 

Fig. 23a and b. Growth of a primary craze at a stationary crack tip in PMMA in air, K, = 19 N/mm a craze length s as a function of loading time t b maximum craze width 2v as a function of loading time t... 

We will only consider craze growth and breakdown in air. Environmental crazing has the extra complication of the sorption and diffusion of the environment the reader interested in comparing the mechanisms discussed here with those for environmental crazes are referred to an earlier review paper as well as more recent papers by Brown and others In addition, we will make no attempt to cover all of the older work on craze growth and breakdown in air. Excellent reviews of this work are available This chapter will concentrate on the more recent developments... 

The in situ deformation of amorphous polymers by shear deformation and craze growth has been observed in optical microscope studies by Donald and Kramer [381]. Grids with thin films of various polymers and polymer blends were prepared on copper grids which were strained in air on a strain frame held in an optical microscope. The films were precracked in an electron microscope by a method more fully described by Lauterwasser and Kramer [382]. Many crazing studies are evaluated by in situ methods, and optical microscopy plays a major role in providing an overview of the deformation structure. Crazing studies will be more fully explored in the next section. 

Fig. 8.26. Argon s model of lateral craze growth based on the phenomenon known as meniscus instability . Side view on the corrugated polymer-air interface (left). Advances of the craze front by a repeated break-up of the interface (view in fibril direction, right) [95]...

Analysis of craze initiation in tubular specimens under combined tension and torsion considering the mechanism of craze growth as one in which craze tufts are produced by repeated break-up of concave air-polymer interfaces at the craze tip (meniscus instability). 

There is, however, a theory for the growth of crazes that is consistent with all the experimental evidence. Argon, Hannoosh and Salama [52] have proposed that the craze front advances by a meniscus instability mechanism in which craze tufts are produced by the repeated break-up of the concave air/polymer interface at the crack tip, as illustrated in Figure 12.15. A theoretical treatment of this model predicted that the steady-state craze velocity would relate to the five-sixths power of the maximum principal tensile stress, and support for this result was obtained from experimental results on polystyrene and PMMA [52]. 

Once the criterion for craze initiation is satisfied, localized plastic flow produces microcavities whose rate of formation at a given temperature depends on the value of the applied stress and increases with increasing stress. This is a slow, time-dependent process that results in an interconnected void network. The subsequent growth of the craze is thought to occur by the repeated breakup of the concave air-polymer interface at the craze tip [39], as shown schematically in Figure 12.23 [40]. The process is similar to what happens when two flat plates... 

---