Izod fracture surface

Fig. 5.48 Dispersed phase particles are observed in a SEM of a notched Izod fracture surface of a poly-acetal/polypropylene copolymer. Dispersed phase particles about 0.5-2 //m across and pullouts, holes where particles were pulled out during fracture, are observed.

Determination of dispersed phase morphology is most often conducted by SEM of fractured specimens. Fractures are prepared by manual methods, after immersion in liquid nitrogen, or by standard physical testing procedures. The microstructure of the homopolymers should be examined for comparison with the multiphase polymer. SEI of an Izod fracture surface of a POM/PP copolymer is shown in Fig. 5.42. The two phases are incompatible, i.e. they are present as two distinct phases. The dispersed phase particles range from less than 0.5 to 2[xm in diameter. The sample fracture path follows the particle-matrix interface and holes remain where particles have pulled out of the matrix, showing there is little adhesion between the phases. 

In the notched Izod impact test, with the exception of some rubber modified plastics, the fracture surface of a specimen that has failed ductilely exhibits significant inward collapsing on the sides of the impact bar near the notch, indicating that a large amount of plastic flow has... 

The mechanical properties of rapidly polymerizing acrylic dispersions, in simulated bioconditions, were directly related to microstructural characteristics. The volume fraction of matrix, the crosslinker volume in the matrix, the particle size distribution of the dispersed phase, and polymeric additives in the matrix or dispersed phase were important microstructural factors. The mechanical properties were most sensitive to volume fraction of crosslinker. Ten percent (vol) of ethylene dimethacrylate produced a significant improvement in flexural strength and impact resistance. Qualitative dynamic impact studies provided some insight into the fracture mechanics of the system. A time scale for the elastic, plastic, and failure phenomena in Izod impact specimens was qualitatively established. The time scale and rate sensitivity of the phenomena were correlated with the fracture surface topography and fracture geometry in impact and flexural samples. 

Block polymer B differs substantially in its failure characteristics from BP A polycarbonate. For the block polymer a mixed failure mode predominates in three-point bend tests of notched specimens from —100°-90°C. In the mixed mode craze breakdown and plane strain fracture occur first inside the specimen subsequently shear failure occurs in the surface regions of the specimen. Shear lips (11) are formed as a result. Shear lips are also found on the notched Izod impact fracture surfaces of block polymer B, implying that the same mixed mode of failure occurs under high speed loading conditions. 

The post mortem observation of fracture surfaces of Izod bars confirms cavitational mode in the modified products. The close observation of the TEM pictures offers a further insight while cavities are sometimes visible inside the reactive modifier particles (R), the cavities in the blends containing non-reactive particles (NR) are always located at the modifier / matrix interface. 

The EPDM particles were found to either initiate crazes or to terminate them, depending on the interfacial bonding, the particle size, concentration and the interparticle distances. The variation of notched Izod impact strength of PPBC blends with different EPDM concentrations is shown in Figure 12.16. The fracture surface of a blend with 10 wt% EPDM, as examined under SEM, is shown in Figure 12.20. The hemispherical embeddings and hollows (representing the removed... 

The Izod test is a variant of the notched bending test. A swinging pendulum hits a clamped bar (Fig. 9.16) and loses kinetic energy. The bar has a width of 12.5 mm and a thickness representative of the plastic product considered. The 2.5 mm deep, 45° notch has a tip radius of 0.25 mm. Section 9.3.2 showed that the tensile stress in the yielded zone at the notch tip can exceed the uniaxial tension yield stress by up to 118%. It is common to use 3.2 mm thick specimens, which restricts the value of the data to products of similar thickness. The lower half of the bar is clamped in vice, and the upper part is struck 22 mm above the notch by the pendulum. The results can be quoted as the absorbed energy divided by the specimen thickness, in Jm , or as the absorbed energy divided by the area of one fracture surface, in J m . ... 

Figure 8. Scanning electron micrograph of an Izod impact test fracture surface of a PA 6/EPM 90/10 binary blend,...

Izod impact tests at room temperature were performed on compression molded samples (50x13.0x3.0 mm ), having a notch of 0.5 mm deep and a tip radius of 0.25 mm. The fractured surfaces of the broken specimens were examined in a scanning electron microscope (SEM) after coating with Au-Pd by means of an evaporator. 

Fig. 13.40 Toughness jumps in Izod fracture energy in CaCOs-particle-modified HOPE blends, at an interparticle ligament thickness A of 0.6 pm, which is again twice the thickness of films crystallized on the surfaces of CaCOs particles (from Bartczak et al. (1999b) courtesy of Elsevier).

Zhou and co-workers [27] studied the effect of surface treatment of calcium carbonate with sulfonated PEEK on the mechanical properties of the polymer. Tests used included tensile tests, flexural tests, notched Izod impact tests, TGA, DSC and SEM. The modulus and yield stress of the composites increased with CaCOs particle loading. This increase was attributed to the bonding between the particles and the PEEK matrix, was proved by the SEM of the tensile fracture surface of the composites. The treated fillers were found to give a better combination of properties, which indicated that the sulfonated PEEK played a constructive role in the calcium carbonate/PEEK composites. 

The plastic zone size can usually be determined from the appearance of the fracture surface and is distinguished by a whitened or very rough area directly ahead of the original notch tip. The Gc, values for some plastics for both the Izod and Charpy geometries (25) are in Table 4. With the same method, the Gic of a number of polymers was determined (3) at different rates (Fig. 18). 

The distinction between brittle and ductile failure is also manifested in two other ways (1) the energy dissipated in fracture and (2) the nature of the fracture surface. The energy dissipated is an important consideration for practical applications and forms the basis of the Charpy and Izod impact tests (discussed in Section 12.8 below). At the testing speeds under which the practical impact tests are conducted it is difficult to determine the stress-strain curve, so impact strengths are customarily quoted in terms of the fracture energy for a standard specimen. 

Figure 12.35 (a) Fracture surfaces of modified polystyrene notched Izod impact specimens ... 

Multiphase polymers are commonly toughened plastics which contain a soft, elastomeric or rubbery component in a hard glassy matrix or in a thermoplastic matrix. An example of the typical brittle fracture morphology of an unmodified thermoplastic is shown by SEM of nylon (Fig. 5.45A). Addition of an elastomeric phase modifies the brittle fracture behavior of the matrix, as shown in a fracture surface of a modified nylon (Fig. 5.45B). The modification depends on the composition and deformation mechanism of the material [204, 215], but normally it increases the fracture toughness and strength from that of the unmodified matrix resin. Impact strength, as measured for instance by an Izod impact testing apparatus, is affected by the dispersed phase... 

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