Fracture of bulk

Fracture of metallic filaments differs in many respects from fracture of bulk samples. Particular fabrication processes that are needed to obtain the small lateral dimensions, may introduce specific defects and textures. Their influence on the fracture behavior is discussed. The intrinsic strength and fracture behavior is mainly dependent on the microstructure. The absence of crystalline defects as in whiskers, as well as the presence... 

These extracted chains, when bundled in fibrils, form the crazes observed in fracture of bulk polymers.(81) As the fibrils in the craze sustain an almost constant stress (Jq, the craze zone is the most perfect example of the Dugdale plastic zone at a crack tip, as first suggested by Marshall et and the term v/L can be seen as the term of the Dugdale model. 

All the studies conducted on fracture of bulk polymers are certainly relevant to the adherence of polymers, the mechanisms of losses at a crack tip being the same viscoelastic losses due to moving stresses, work to extract chains or fibrils, and viscous drag in the presence of a liquid. It is probable that the various theories of adhesion, namely, theory of wetting, theory of the rheological factor, theory of the chemical bond, theory of the weak boundary layer, and theory of interdiffusion, are all valid, each corresponding to an emphasis on a dominant mechanism. 

A major problem with nitric acid etching is that it is too strong, etching not only the amorphous surface regions but also the bulk. Chromic acid etching was employed to alleviate this problem. Armond and Atkinson [319] treated PP with fuming nitric acid and then with chromic acid to reveal the cracks and fractures of bulk annealed PP. Bucknall et al. [320] studied... 

Evaluation of bulk industrial material is best done by microtomy for OM, TEM, and SPM and by fracture of bulk molded or extruded samples for SEM and FESEM for determining microstructure. A brief literature review with examples of microscopy characterization of copolymers follows, but this review is not intended to reflect the thousands of studies and references on this important topic. Transmission electron microscopy is by far the most widely used characterization tool for the assessment of copolymers, and it has been used for several decades to uncover and provide understanding of copolymer microstructure. 

The term aK2v", derived from reptation theory, describes the velocity-dependent energy necessary to fracture the bulk adhesive. K2 is the consistency which relates the viscosity to the shear rate for a non-newtonian fluid. a = TtraL fh", with r being the chain radius, L the chain length, a the density of chains crossing over the fracture plane, and h is the distance between the chain and reptation tube. 

In addition to the fact that carbon formed by gas-phase pyrolysis is chemically different from that which forms catalytically on Ni, it is important to recognize that carbon formed by pyrolysis forms on the surface rather than in the bulk of the material. Because of this, pyrolysis does not result in pitting of the surfaces to which the hydrocarbon is exposed. Furthermore, on porous Ni cermets, carbon fiber formation can lead to fracture of the electrode caused by the stresses induced by the carbon fibers. Such stresses do not occur upon deposition of pyrolytic carbon. 

Composite interfaces exist in a variety of forms of differing materials. A convenient way to characterize composite interfaces embedded within the bulk material is to analyze the surfaces of the composite constituents before they are combined together, or the surfaces created by fracture. Surface layers represent only a small portion of the total volume of bulk material. The structure and composition of the local surface often differ from the bulk material, yet they can provide critical information in predicting the overall properties and performance. The basic unknown parameters in physico-chemical surface analysis are the chemical composition, depth, purity and the distribution of specific constituents and their atomic/microscopic structures, which constitute the interfaces. Many factors such as process variables, contaminants, surface treatments and exposure to environmental conditions must be considered in the analysis. 

Bascom, W.D., Cottington, R.L., Jones, R.L. Peyser, P. (1975). The fracture of epoxy and elastomer modified epoxy polymers in bulk and as adhesives. J. Appl. Polym. Sci. 19 2545-2562. 

Before discussing specific aspects of micro deformation and fracture in bulk polyolefins, some basic notions of microdeformation and the micromechanics of fracture mediated by generation and breakdown of cavitated or fibrillar deformation zones or crazes are introduced. SCG in PE and rate-depen-dent fracture in iPP are then considered in more detail. 

There are two main techniques used to measure the fracture toughness of ceramics fracture stress and hardness indentation. The former measures the load to fracture of a pre-cracked specimen using a single edge notched beam (SENB) or a chevron notched beam (CNB) sample. The main drawback of this technique is ensuring that the crack tip is atomically sharp. The second method uses the crack formed at the corners of the indentation produced during a Vickers indentation hardness test. This technique is rapid and relatively inexpensive. However, the toughness values measured are those of the surface, unlike the values obtained by fracture of the pre-cracked beams which are a measure of the bulk material properties. 

The term melt fracture has been applied from the outset [9,13] to refer to various types of visible extrudate distortion. The origin of sharkskin (often called surface melt fracture ) has been shown in Sect. 10 to be related to a local interfacial instability in the die exit region. The alternating quasi-periodic, sometimes bamboo-like, extrudate distortion associated with the flow oscillation is a result of oscillation in extrudate swell under controlled piston speed due to unstable boundary condition, as discussed in Sect. 8. A third type, spiral like, distortion is associated with an entry flow instability. The latter two kinds have often been referred to as gross melt fracture. It is clearly misleading and inaccurate to call these three major types of extrudate distortion melt fracture since they do not arise from a true melt fracture or bulk failure. Unfortunately, for historical reasons, this terminology will stay with us and be used interchangeably with the phase extrudate distortion. ... 

A true melt fracture by definition must involve some kind of bulk failure in melt flow. Either massive chain disentanglement or chain scission or both must occur in the bulk. Such a real cohesive breakdown away from the surface may also produce extrudate distortion. In general, other forms of irregular extrudate distortion do occur at high stresses. Only direct flow visualization may reveal the origin of such extrudate distortions. 

The relevance of bulk fracture properties has therefore been considered essentially within the context of cohesive wear modes such as abrasive and fatigue wear. During abrasive wear, the initial stage is considered to be the process of contact and scratch between the polymer surface and a sharp asperity. The accumulation of the associated microscopic failure events eventually generates wear particles and gives rise to weight loss. Early approaches initiated by Ratner and co-workers [15] and Lancaster [16] attempted to correlate the abrasive wear rate with some estimate of the work to failure of the... 

Figure 3. Fracture of the craze wedge along one craze-bulk boundary (schematic)...

The theory indicates that the mechanical properties of the foam are dependent on the properties of the cell wall materials and their size and shape. By relating the density of the foam to its bulk mechanical properties, the slope of the fitted line (n) can give us information about the type of failure mechanism (Figure 20.19). This also indicates that the size and shape of the bubbles in a foam will have a predictable effect on the strength and fracture of the foam. Bread and extruded cereal foams have been considered as cellular solids using the Gibson and Ashby analysis, and have been shown to follow the Gibson and Ashby prediction (Keetels et al. 1996 Hayter etal. 1986). 

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