Layered fracture morphology

The infrared spectra demonstrate that there is a strong interaction between CR and the PS segment of SBS and between SBS and BR. Scanning electron micrographs of the fracture surfaces further support the conclusion. The fracture surface of 70 30 BR/CR shows layer-shaped morphology and 30 70 blends show hillock-shaped protmsions. Addition of 5% SBS reduces the particle size, makes the particles more spherical, and enhances uniform distribution. This provides further evidence of the compatibilizing effect of SBS. 

The most common morphology observed in current mesophase carbon fibers of moderate modulus (55 to 75 Mpsi, 379 to 517 GPa) is a cylindrical filament with a random-structured core and a radial rim (12) Given the fracture section of Figure 3, with its scroll-like features, the core appears to be an array of +2ir and -ir disclinations. The radial rim of heavily wrinkled layers usually constitutes half or more of the cross section. 

The second condition to validate the scheme B is that embrittlement must correspond to a critical morphological state that is the only approach to explain its sudden character. The extensive and careful work of Kennedy et al. (//) on relationships between fracture behavior, molar mass and lamellar morphology, shows that this condition is fulfilled in the case of PE. Comparing various samples of different molar masses with different thermal histories, they found that the thickness of the amorphous layer (la) separating two adjacent lamellae is the key parameter (Fig. 6). As a matter of fact, there is a critical value lac of the order of 6-7 nm. For la > lac the samples are always ductile whatever their molar mass, whereas for U < laC the samples are consistently brittle. As a result, lac appears to be independent of the molar mass. Indeed, there is a specific molar mass, probably close to 70 kg.mof for PE below which crystallization is so fast that it is impossible to have la values higher than lac whatever the processing conditions. 

To examine craze microstructure, and to study the effect of molecular variables on craze morphology, the method described by Kramer was followed. Samples of polymers were cast in the form of thin films, strained in tension while bonded to carbon-coated grids, and examined in the transmission electron microscope either before or after staining. The TEM observations were made with an Hitachi HU-11 A unit or with a JEOL JEM-IOOCX unit, operating usually at 75-80 kV. Fracture surfaces of many bulk samples were coated with a thin layer of gold-palladium and examined by an Etec scanning electron microscope. 

SEM micrographs and visual appearances of the fracture surfaces revealed the presences of a hierarchical organization and microstructures in the macrolayers. Starting torn the macroscopic level, five macrolayers were observed two outer skins, two mid layers with a core in between. Due to differences in color, the macrolayers were readily visible to the naked eye. This skin-core morphology is a characteristic of many injection-molded LCPs (7-81. 

Lamellae that display a parallel orientation with respect to the drawing direction show morphological changes that are in accordance with the deformation mechanism termed hard layer sliding [160], In this mechanism the PS lamellae are assumed to slightly slip towards each other without noticeable deformation. In addition, PS lamellae may be fractured into smaller domains, which are ordered in series to yield a lamellar type structure again. 

It was mentioned above that the simulation method of Termonia [67-72] can be used to calculate the stress-strain curves of many fiber-reinforced or particulate-filled composites up to fracture, including the effects of fiber-matrix adhesion. Such systems are morphologically far more complex than adhesive joints. Many matrix-filler interfaces are dispersed throughout a composite specimen, while an adhesive joint has only the two interfaces (between each of the bottom and top metal plates and the glue layer). If one considers also the fact that there will often he a distribution of filler-matrix interface strengths in a composite, it can be seen that the failure mechanism can become quite complex. It may even involve a complex superposition of adhesive failure at some filler-matrix interfaces and cohesive failure in the bulk of the matrix. 

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