Modes of fracture

Fig. I. Three modes of fracture where Pis load (a) Mode I, (b) Mode II, and (c) Mode III.

Step 3. The set of fracture properties G(t) are related to the interfaee structure H(t) through suitable deformation mechanisms deduced from the micromechanics of fracture. This is the most difficult part of the problem but the analysis of the fracture process in situ can lead to valuable information on the microscopic deformation mechanisms. SEM, optical and XPS analysis of the fractured interface usually determine the mode of fracture (cohesive, adhesive or mixed) and details of the fracture micromechanics. However, considerable modeling may be required with entanglement and chain fracture mechanisms to realize useful solutions since most of the important events occur within the deformation zone before new fracture surfaces are created. We then obtain a solution to the problem. 

Tests in a Clj + Oj mixture at 427°C have shown that the worst elements for promoting susceptibility are Al, Sn, Cu, V, Cr, Mn, Fe and Ni, while the least harmful are Zr, Ta and Mo. a-phase alloys are generally more susceptible than )3-phase alloys. Heat treatment has not been examined extensively, but some heat treatments render some a-alloys more susceptible or change the mode of fracture. The general effect will depend upon the alloy and the heat-treatment cycle. Subsequent cold work can sometimes considerably lower susceptibility. Failure times decrease as either the testing temperature or initial stress value is raised. 

Materials subjected to high temperatures during their service life are susceptible to another form of fracture which can occur at very low stress levels. This is known as creep failure and is a time dependent mode of fracture and can take many hours to become apparent (Fig. 8.88). 

All the above modes of fracture are affected by the environment around the crack tip. This behaviour is typified by the phenomenon of stress-corrosion cracking where a crack, which is subjected to a subcritical stress concentration, will grow in a corrosive environment when /f, the critical stress concentration for stress-corrosion cracking). Therefore, to predict accurately the occurrence of cracking and crack growth rate, not only the materials properties are required but also information on the immediate environmental conditions. 

The fracture strength and mode of fracture of a material have been found to be related to a number of characteristics of the polymer molecules of which it is made up. These include, among others, constitution, molar mass, polydispersity, crystallinity, and degree of crosslinking. Other factors which also affect fracture strength and mode of fracture are temperature, strain rate, and geometry of the specimen, all of which are decided upon prior to testing the material. 

Fracto-emission (FE) is the emission of particles (electrons, positive ions, and neutral species) and photons, when a material is stressed to failure. In this paper, we examine various FE signals accompanying the deformation and fracture of fiber-reinforced and alumina-filled epoxy, and relate them to the locus and mode of fracture. The intensities are orders of magnitude greater than those observed from the fracture of neat fibers and resins. This difference is attributed to the intense charge separation that accompanies the separation of dissimilar materials (interfacial failure) when a composite fractures. 

The final mechanism of stress relief is thermomechanically activated chain scission. Primary bond breakage can be homolytic, ionic or by a degrading chemical reaction. It is worthwhile to note that the relative slippage of chains, microfibrils and fibrils reduces or prevents the mechanical scission of chains in quasi-isotropic polymeric solids. In other words, chain scission is an important mode of fracture only in highly oriented thermoplastic fibers or in thermosets. 

The index I in Gic refers to the mode of fracture deformation. Mode I is deformation due to tensile stresses, mode II is deformation in sliding and mode III in tearing. 

In the second series of experiments (6>) brittle samples were fractured under heavy impact in the presence or absence of stress orthogonally applied onto the propagating crack tip. Three modes of fracture can exist. In the first, when no lateral stress exists, the normal component of... 

Scanning Electron Photomicroscopy. At 190 and 160 °C, samples aged in nitrogen were significantly less yellowed and stronger than those aged in air. The distinctively different modes. of fractures in fibers tested in the different environments are visually apparent in Figures 19-21 (40). 

One of the most curious aspects of crack growth in most epoxies is the apparently unstable manner by which propagation occurs, even over wide ranges of temperature and test rate. This behavior is commonly referred to as stick-slip , and is characterized by the crack growing in a series of discrete, unstable jumps. Even some of the earliest works on epoxy fracture report this mode of crack growth. The suspected origins of stick-slip fracture behavior in epoxies is discussed in a subsequent section. Unhke epoxies, thermoplastic polymers, such as poly(methyl methacrylate) and polystyrene, are characterized by stable, continuous crack growth. This mode of fracture sometimes can be observed in epoxies, in particular, when they are tested at fast rates and/or low temperatures. 

Figure 8.32 shows the effect of filler type and concentration on the mode of fracture. Several factors are responsible for the behavior. CaT (calcium terephthalate) fillers have good adhesion to the matrix and have an elongated shape. CaCOs (1) and (2) are both untreated fillers of smaller particle sizes (2.2 and 4.1 (dm, respectively). CaCO (3) is a stearate coated filler (better dispersion, but poor adhesion to matrix) with a particle size of 6.1 im. The mode of fracture depends on filler concentration, the degree of adhesion to the matrix, and particle size. 

For a very thin specimen i.e., with B (Kjc/ays) ), the influence of plastic deformation at the surfaces will relieve crack-tip constraint through the entire thickness of the specimen before Kj reaches Kjc. As such, the opening mode of fracture is suppressed in favor of local deformation and a tearing mode of fracture. The behavior is reflected in the load-displacement record by a gradual change in slope and final fracture, which could still be abrupt (see Fig. 4.7b), but the conditions of plane strain would not be achieved. 

Additional test specimens were cut from the smooth and textured areas of each plot and evaluated for penetration depth, flexural strength, bonding, and mode of fracture. 

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