Cleavage fracture examples

When grain boundaries are embrittled (for example, by precipitates, see section 6.4.4), cleavage fracture may be intercrystalline. In this case, the grain structure can be clearly seen in a scanning electron microscope picture (see figure 1.10(b)). 

In many cases, the cleavage strength ac cannot be measured experimentally. For example, body-centred cubic metals only fail by cleavage fracture even at temperatures below the ductile-brittle-transition temperature if the stress state is triaxial. For this reason, it is impossible to measure ac in a tensile test. Ceramics usually fail because a microcrack, already present in the material, propagates and causes failure at a stress below ac (see section 7.3). The tensile strength of a ceramic is thus smaller than its cleavage strength, nn... 

A crystalline solid with cleavage in one direction yields platy fragments on fracture. Mica is a well-known example of a mineral with one direction of perfect cleavage. Two directions of cleavage yield prismatic or acicular... 

Spectra for most thietanes contain a strong parent ion peak. The four-membered ring is usually fractured into the four possible halves. For example, the cleavage of thietane derivative (43) produces peaks at mje 240, 230, 84, and 74. The parent ion peak is absent at 35°. Similarly, spirothietane 44 splits to give fragments with m/e 162 (77%), 152 (12%), 84 (100%), and 74 (0.6%). Decomposition of the thietane nucleus is also characterized by desulfurization and retro 2-1-2 cycloaddition fragments. 

Figure 2.36. Example of cleavage and fracture at the atomic (a, b) and macroscopic (c, d) levels. In images (c, d), a crystal of NaQ is exposed to a stress along an oblique angle to the cleavage plane, resulting in both cleavage and fracturing. Images taken with permission from the Journal of Chemical Education online ...

The purpose of surface preparation is to remove contamination and weak surface layers, to change the substrate surface geometry, and/or introduce new chemical groups to provide, at least in the case of metals, an oxide layer more receptive to the adhesive. An appreciation of the effects of pretreatments may be gained from surface analytical or mechanical test techniques. Experimental assessments of the effects of surface pretreatment, even when using appropriate mechanical tests, are of limited value unless environmental exposure is included. Self-stressed fracture mechanical cleavage specimens, as discussed in Chapter 4 and in the texts edited by Kinloch(2,5) for example, are therefore referred to wherever possible. 

Joint fracture toughness. The initial mode I fracture toughness K 1C, obtained from the mean of several tests using wedge cleavage specimens, an example of which is depicted in Fig. A.5, should not be less than 0.5 MN The tests should be carried out at 20 °C... 

The last important test for evaluating adhesive properties is a cleavage or fracture test. Figure 27.3 shows an example of such a test, the double cantilever beam test. The adherends are usually 1 in. (2.54 cm) wide and the thickness depends on the adherend modulus. Described in Standard Practice ASTM D3433 [9], the test is meant to measure fracture resistance (strain energy release rate) under conditions in which the adherends do not plastically deform. The units of fracture resistance (strain energy release rate) are joules per square meter and this quantity is given the symbol... 

Hardness must not be confused with toughness, which relates to the ability of the mineral to resist fracture. Hard minerals can often fracture very easily, for example, by cleavage along crystal planes. 

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