Crazing deformation methods used

In the second case, the mechanical deformation method will not be standard, but suitable samples will be designed to fit the microscope. The in situ deformation of polymers is almost always conducted in the SEM at low magnification and is most often used for fibers and fabrics. Other microdeformation methods are used to prepare thin films for TEM observations, to model fiber structure or to investigate crazing. Since the mechanical deformation technique is... 

Above its glass transition temperature, shear processes dominate in sPS as was shown earlier. In Figure 19.15, the TEM images of deformed sPS modified with the core-shell modifier prepared using the microsuspension method are reproduced. At the surface of the specimen highly oriented rubber particle are discernible without voiding. In the inner part of the specimen, however, cavi-tated highly oriented particles have been formed. Crazes are not seen in the... 

Several methods, as described in the preceding section, have been used to acquire information relative to the onset of the initial localized plastic deformation under alternating loading. Figure 5 is a plot of reflected light intensity vs. cycles for a transparent PS sample tested at 21 Hz at a stress amplitude of 17.2 MPa The number of cycles, N., to initiate the craze, as determined from the first jump in intensity over background, is about 5,000 cycles and the cycles to fracture, N, is about 11,000. In later sections, the ratio of to Np which in this example is about 0.45, is shown to be a function of both stress amplitude and frequency. 

For non-transparent specimens, as shown by Bucknall and Stevens useful information relative to the deformation mode can be obtained by recording hysteresis loops as a function of cycles. Figure 6 shows hysteresis loops obtained at 0.2 Hz at various N values for PS tested at a stress amplitude of 24.1 MPa and Fig. 7 for HIPS tested at 17.2 MPa. For PS, with Nf = 1,451 cycles, there is no detectable change in loop area at this stress amplitude up to the final cycle. This illustrates the highly localized nature of the fatigue-induced damage zone in PS and indicates that, for this polymer, hysteresis loop observations are not an effective method for detecting craze... 

A third method which recently provided considerable insight into the role of crazes in deformation and fracture of amorphous polymers is the optical interference measurement of crazes (preceding a crack). Since the pioneer work of Kambour, this method has been widely used to determine characteristic craze dimensions and critical displacements. W. Doll gives an overview on recent results and on their interpretation in terms of fracture mechanics parameters (stress intensity factor, plastic zone sizes, fracture surface morphology, fracture energy). 

A transmission electron micrograph of a craze in a thin film of poly(styrene-acrylo-nitrile), shown in Fig. 1 a, will serve to introduce the principal microstructural features of crazes. The direction of the tensile stress is marked and it can be seen that the craze grows with the primary direction of its fibrils parallel to this tensUe stress and with the interfaces between the craze and the nearly undeformed polymer matrix normal to the stress. Since the overwhelming portion of the experiments to be reviewed here rely on the use of thin film deformation and transmission electron microscopy techniques, a brief review of the general methods of these experiments is in order. 

Ductile behavior can be studied using a modification of Method 2, a bending experiment see Fig. 1.53. A bent specimen is illustrated in Fig. 1.53(a), and a result of deformed rubber-toughened PVC is shown in a TEM micrograph in Fig. 1.53(b). The sample from the loaded area is a chemically stained thin section prepared by ultramicrotomy, showing fibrillated crazes between the acrylic rubber particles (both stained dark). 

Moreover, profuse microvoids were found in the LIB fibers by using a focus ion beam (FIB) method (The images can be found in Ref. [233]). Other than the crazes usually observed in the plastic deformation of glassy polymers, the microvoids in the LIB fibers were empty and aligned in the fiber direction. Such microvoids were similar to those observed during deforming semicrystalline... 

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