Mechanism fracture

The manner in which a particle fractures depends on (i) the nature of the particle and (ii) the manner in which the fracture force is applied. A number of terms have been used to describe the different mechanisms of single particle fracture. The different terms considered here are abrasion, cleavage, shatter, and chipping. It may be pointed out that in practice these events do not occur in isolation. Real breakage involves a combination of these processes, with the proportions changing, depending on the equipment, and on the manner each particle is stressed within it. 

Abrasion fracture occurs when sufficient energy is not supplied to cause significant fracture of the particle. In such circumstances only a localized stressing occurs and a small area is fractured to result in a distribution to be essentially the original particle, together with a small amount of very fine particles. 

Cleavage fracture is said to take place when the applied energy is just sufficient to load a few regions of the particle to the point of fracture. Only a few particles result, and their size is comparatively close to that of the original particle. This situation typically arises under conditions of slow compression where fracture immediately relieves the loading on the particle. 

The term shatter fracture applies to a situation where the applied energy is well in excess of that necessary for fracture. Many regions of the particle are overloaded under these conditions, yielding a comparatively large number of particles with a wide distribution of sizes. This type of fracture occurs under conditions of rapid loading such as those obtained in a high-velocity impact. 

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Using flaw visuahzation system data the strength and fracture mechanics estimations are carried out in accordance with defect assessment regulatory procedure M-02-91 [5]. Recently, the additions had been included in the procedure, concerning interpretation of expert flaw visualization sysf em data, computer modelling, residual stresses, in-site properties of metal, methods of fracture analysis. 

The sensitivity tests are carried out on artificial defects (nickel-chromium specimens of NFA 09.520,see figure 3 of annex 1) and natural defects (one part in "light" alloy, one part in stellite grade 1 containing micropores, 2 specimens of fracture mechanical type CT20 in Z2 CN 12.10 (NFA 03.180). 

In an ideal situation we would like to have a balanced combination between evaluation of the welder s performance on one hand, and a fracture mechanics basis on the other hand in tenns of "being certain that a defect with dimensions exceeding a certain critical value is not present". The second aspect could be regarded as a safety net with a balanced conservatism. 

The Institute has many-year experience of investigations and developments in the field of NDT. These are, mainly, developments which allowed creation of a series of eddy current flaw detectors for various applications. The Institute has traditionally studied the physico-mechanical properties of materials, their stressed-strained state, fracture mechanics and developed on this basis the procedures and instruments which measure the properties and predict the behaviour of materials. Quite important are also developments of technologies and equipment for control of thickness and adhesion of thin protective coatings on various bases, corrosion control of underground pipelines by indirect method, acoustic emission control of hydrogen and corrosion cracking in structural materials, etc. 

Ghassemieli, E. and Nassehi, V., 2001b. rediction of failure and fracture mechanisms of polymeric composites using finite clement analysis. Part 1 particulate filled composites. Poly- Compos. 22, 528-541. 

Bui H.D., Ehrlacher A. (1997) Developments of fracture mechanics in France in the last decades. In Fracture Research in Retrospect. H.P. Rossmanith (Ed.), A.A.Balkema, Rotterdam, 369-387. 

Cherepanov G.P. (1983) Fracture mechanics of composite materials. Nauka, Moscow (in Russian). 

Ohtsuka K. (1986) Generalized G-integral and three-dimensional fracture mechanics. Surface crack problems. Hirosima Math. J. 16 (2), 327-352. 

Ohtsuka K. (1994) Mathematical aspects of fracture mechanics. Lect. Notes in Num. Appl. Anal. 13, 39-59. 

Fracture mechanics (qv) tests are typically used for stmctural adhesives. Thus, tests such as the double cantilever beam test (Fig. 2c), in which two thick adherends joined by an adhesive are broken by cleavage, provide information relating to stmctural flaws. Results can be reported in a number of ways. The most typical uses a quantity known as the strain energy release rate, given in energy per unit area. 

Fracture mechanics is now quite weU estabHshed for metals, and a number of ASTM standards have been defined (4—6). For other materials, standardization efforts are underway (7,8). The techniques and procedures are being adapted from the metals Hterature. The concepts are appHcable to any material, provided the stmcture of the material can be treated as a continuum relative to the size-scale of the primary crack. There are many textbooks on the subject covering the appHcation of fracture mechanics to metals, polymers, and composites (9—15) (see Composite materials). 

Crack Tip Stresses. The simplest case for fracture mechanics analysis is a linear elastic material where stress. O, is proportional to strain, S,... 

In moie ductile materials the assumptions of linear elastic fracture mechanics (LEFM) are not vahd because the material yields more at the crack tip, so that... 

The use of fatigue data and crack length measurements to predict the remaining service life of a stmcture under cyclic loading is possibly the most common application of fracture mechanics for performance prediction. In complex stmctures the growth of cracks is routinely monitored at intervals, and from data about crack growth rates and the applied loadings at that point in the stmcture, a decision is made about whether the stmcmre can continue to operate safely until the next scheduled inspection. 

J. F. Knott, fundamentals of fracture Mechanics, Butterworths, London, 1979. 

D. Broek, E/ementa Engineering fracture Mechanics, Martinus Nijhoff, Dordrecht, the Netherlands, 1987. 

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