Craze stiffness

As an example, Fig. 8 shows the fracture toughness for PMMA and Fig. 9 the fringe pattern transition at the critical temperature, whereas Fig. 10 shows the lateral face of the sample with the crack-tip above and below the critical temperature. It has also been shown that neither the bulk modulus nor the craze stress varies near the critical temperature (Fig. 11 and 12). It seems that the local material property varying near that particular temperature is the craze stiffness, as shown in Fig. 13. 

Fig. 13. Craze stiffness above and below the critical temperature in PMMA. Note the transition near T., indicating a change in the craze structure. From Ref. courtesy of Chapman and Hall, Ltd.

Fig. 38. Overall craze stiffness as a function of temperature at which the craze was produced and relaxed the relaxed craze is stiffer than the not relaxed one, when the relaxation temperature is above —25 °C for PMMA. Note that —25 °C is nearly the secondary p relaxation peak for the time scale of that experiment. From Ref. , courtesy of Chapman and Hall, Ltd.

Molecular orientation results in increased stiffness, strength, and toughness (Table 8-12) as well as resistance to liquid and gas permeation, crazing, microcracks, and others in the direction or plane of the orientation. The orientation of fibers in reinforced plastics causes similar positive influences. Orientation in effect provides a means of tailoring and improving the properties of plastics. 

In an RP construction both plastics and fibers influence orientation properties. For example, with certain TPs, the plastic s molecular orientation can be used to aid in increasing stiffness, strength, toughness, as well as craze and microcrack resistance in the direction of the plane or the plane of orientation. By far the main source of... 

Fillers offer a variety of benefits increased strength and stiffness, reduced cost, shrinkage reduction, exothermic heat reduction, thermal expansion coefficient reduction, improved heat resistance, slightly improved heat conductivity, improved surface appearance, reduced porosity, improved wet strength, reduced crazing, improved fabrication mobility, increased viscosity, improved abrasion resistance, and/or impact strength. Fillers also can have disadvantages. They may limit the method of fabrication, inhibit cure of certain resins, and shorten pot life of the resin. 

The craze is modeled by an elastic anisotropic medium with Young s moduli i and 2 corresponding to the stiffness of the cross-tie and the main fibrils ( 1 2). As the strip thickens along the direction 2 (Fig. 4b), the elastic energy density W is stored so that the energy release rate at failure... 

Prior to craze initiation, an < cr r (crm) in Eq. 21 and the stiffness has to be infinitely large to ensure that no separation occurs across the cohesive surface. We propose to use... 

Optical interferometry k basically inadequate for invratigating the craze structure directly. Nevertheless, that structure may be investigate through its mechanical properties, for example, the stiffness. Early work on crazes used that approach to evaluate the craze fibrils structure... 

In the experiment described below, small changes in the craze structure due to low scale molecular motion between craze fibrik will be detected by means of optical interferometry measuring stiffness of the craze material. The experimental procedure is a follows ... 

Wu [1985, 1990] postulated that the brittle/ ductile behavior of a neat amorphous polymer is controlled by two intrinsic molecular parameters the entanglement density, v., and the chain stiffness (given by the characteristic chain constant C. ). Assuming that crazing involves chain scission, the stress, o, should be proportional to and the yield stress, proportional to C . In consequence c,/c, where... 

---