Fracture resistance concept

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

Polymers which yield extensively under stress exhibit nonlinear stress-strain behavior. This invalidates the application of linear elastic fracture mechanics. It is usually assumed that the LEFM approach can be used if the size of the plastic zone is small compared to the dimensions of the object. Alternative concepts have been proposed for rating the fracture resistance of tougher polymers, like polyolelins, but empirical pendulum impact or dart drop tests are deeply entrenched forjudging such behavior. 

The through-the-thickness reinforcement acts as a crack bridging, which requires new concepts and methods to be used for the fracture resistance calculation. 

The DT test [ ] is similar in concept to the CVN test. The simplicity of a three-point bend specimen has been combined with an energy criterion for fracture resistance. In this test, a 16 mmx38 mmx 181 mm specimen with a sharp notch is fractured on a large pendulum or drop-weight impact machine, and the fracture energy is measured. 

VI.6] ASSOCIATION FRANgAISE POUR LES REGLES DE CONCEPTION ET DE CONSTRUCTION DES MATERIELS DES CHAUDIERES ELECTRO-NUCLEAIRES (AFCEN), French Nuclear Construction Code RCCM Design and Construction Rules For Mechanical Components of PWR Nuclear Facilities, Subsection Z, Appendix ZG, Fast Fracture Resistance, Framatome, Paris (1985). 

The discussion of stress concentration near a film edge in the next section is followed by a brief review of linear elastic fracture mechanics concepts, a prelude to a discussion of delamination and cracking due to film residual stress. A survey of these topics set in the context of fracture mechanics has been presented by Hutchinson and Suo (1992). The chapter also includes descriptions of various experimental techniques for evaluating the fracture resistance of interfaces between films and substrates. In addition, representative experimental results on the interface fracture resistance, as a function of interface chemistry and environment, are presented for a variety of thin film and multilayer systems of scientific and technological interest. 

Emulsions. Emulsion fluids and foams came into routine use in competition with crosslinked fluids during 1970-80. Simple, barely stable emulsions had been used early in fracturing. These were mainly emulsified acids that "broke" when the acid spent on the formation surfaces. In the late 1960 s Kiel became a proponent of very high viscosity oil fluids as a method to place exceptional (at the time) amounts of proppant(337,338). To avoid the frictional resistance typical of gelled oils he advanced the concept of preparing a very viscous oil-external emulsion with one part fresh water, 0.1% sodium tallate surfactant, and two parts oil. The viscous emulsion had to be pumped simultaneously with a water stream to minimize frictional pressure. This process was clumsy and still... 

From the results presented in Table II, the resistance of the material increases as crack growth occurs in the material. It is thus necessary to calculate the material resistance values as a function of crack length if the fracture behavior of the material is to be fully characterized. This type of approach known as the R-curve approach has been used to study the fracture behavior of many metallic materials. The R-curve concept has been completely reviewed (11). 

In keeping with the theoretical concepts of fracture mechanics, the load-bearing capacity of a structure depends on the ability of a material to resist spreading of the main crack. The criteria for the beginning of a crack or fracture are established based on the limit equilibrium of a body with a crack [9,11]. 

Thus, these two concepts are equivalent. In the classical failure context, fracture depends on some critical combination of stress at the crack tip and the tip radius, neither of which are precisely defined (or definable) or accessible to measurement. For experimental accuracy and practical apphcation, it is more appropriate to use the accessible quantities o and a to determine the fracture toughness of the material. It is to be recognized that the quantities involving a a and a represent the crackdriving force, and 2y, in the Griffith sense, represents the material s resistance to crack growth, or its fracture toughness. 

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