Subcritical crack growth

The WOL and the double-cantilever beam (DCB) specimens are suitable to determine the arrest of an initially unstable crack the double-torsion (DT) specimen permits the investigation of crack propagation under constant Kj [7—15]. 

Values of various types of energy release rates G and of fracture toughness values K of polymeric materials have been gathered from the literature [12—75] and are compiled in Table 9.1. Although the experimental method and the principal parameters and observations have also been indicated in Table 9.1, a discussion of some molecular aspects will be attempted in the following. 

In many polymeric materials three zones of crack propagation behavior can be distinguished  

Polymer (standard abbrev.) Method and Specimen Gic J/m Kic MN m Parameters of Test, Observations Ref. 

The objective of this section is twofold to describe the phenomenon of SCG and to relate it to what is occurring at the atomic level at the crack tip. Before one proceeds much further, however, it is important to briefly outline how this effect is quantified. 

Experimental Details Measuring Subcritical Crack Growth 

The techniques and test geometries that have been used to measure subcritical crack growth in ceramics are several, but they share a common principle, namely, the subjection of a well-defined crack to a well-defined stress intensity Ki, and a measurement of its velocity v. The technique considered here, the advantages of which are elaborated upon below, is the double torsion geometry shown in Fig. 12.8. For the double torsion specimen, K is given by 

A starter crack is introduced in a specimen, and a load is applied, as shown in Fig. 12.8. As a result, the starter crack will grow with time. 

A good example of this phenomenon that should be familiar to many is the slow crack growth over time in a car s windshield after it has been damaged. 

In the preceding description of the fracture process, it was assumed that the only source of energy to break the structural bonds was the strain energy provided by the loading system. It was also assumed that no crack growth was possible unless the stress intensity at the crack tip exceeded That assumption implies that, for a crack of a given size, no crack growth should occur until the stress reaches the critical value and, once that critical value is obtained, fracture should be immediate. 

Michalske and Freiman introduced a model for subcritical crack growth in silicate glasses in which an originally unreactive Si-O bond becomes more reactive as it is deformed in the stress field at the crack tip. At some level of stress intensity, a water molecule adjacent to the bond is able to react with it and produce two silanols that is, it breaks the bond. At low K, the reactivity of the bond is low but it increases approximately logarithmically as K increases. The higher the reactivity, the more rapidly bonds break and the faster the crack grows. 

Michalske and Freiman pointed out that the water molecule is particularly suited to react with the Si—O bond because it has a labile proton and a lone electron pair 

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Elastic energy release due to subcritical crack growth is one recognized source of structure-related AE within its acknowledged lunitations, AEBIL provides a viable means of early on-line deteetion and localization of stable crack propagation. 

The mechanism of subcritical crack growth is the reaction of the corrosive medium with highly stressed bonds at the crack tip. In siUca, in the absence of stressed bonds, the rate of the reaction between the bonds and corrosive media such as water is very low. The introduction of strain energy into crack tip bonds increases the activity of the bond. For siUca glass in water, attack and bond breakage occurs by the following reaction (47) ... 

Ceramic-matrix fiber composites, 26 775 Ceramics mechanical properties, 5 613-638 cyclic fatigue, 5 633-634 elastic behavior, 5 613-615 fracture analysis, 5 634-635 fracture toughness, 5 619-623 hardness, 5 626-628 impact and erosion, 5 630 plasticity, 5 623-626 strength, 5 615-619 subcritical crack growth, 5 628—630 thermal stress and thermal shock, 5 632-633... 

The reduction of strength after short time corrosion in acids (pit formation) cannot be correlated with the weight loss [511], whereas after intensive corrosion a correlation exists between thickness of the corroded layer and strength [507]. Acid corrosion appears to influence subcritical crack growth in Si3N4 ceramics [507, 523]. 

Ohji T (1994) Tensile Creep Rupture And Subcritical Crack Growth of Silicon Nitride. In Hoffmann MJ, Petzow G (eds) Tailoring of Mechanical Properties of Si3N4 Ceramics. Kluwer Academic Publishers, Netherlands, p 339... 

Hoffman, M., Rodel, J., Stemitzke, M. et al., Fracture toughness and subcritical crack growth in alumina/silicon carbide nanocomposites , Fracture Mechanics of Ceramics, 1996, 12 179. 

Structural failures in general occur by a combination of crack initiation followed by subcritical crack growth mechanism such as stress corrosion, fatigue, and creep until a critical crack size is reached. Thus, fracture mechanics is useful in evaluating ac that leads to failure by fracture. The application of fracture mechanics in failure analysis throws light on the progress from subcritical crack growth to fracture, the amount of load... 

Both AC and DC potential-drop methods are well-established techniques for monitoring subcritical crack growth. A combined AC/DC potential-drop measuring technique can, in some cases, help in obtaining more information from a single test, in particular for the onset of stable crack growth.172... 

Hancock, G.G. and Johnson, H.H., Hydrogen, oxygen, and subcritical crack growth in a high-strength steel. Transactions of the Metallurgical Society ofAIME, 236, 513-516, 1966. 

Very slow subcritical crack growth can be approximately characterized by an empirical law of the form... 

Fig. 17a—d. Details of the fracture surface on a CT-specimen of crystalline PETP a general view of notch (N), subcritical crack growth through crazes (A), transitional region (B), and brittle fracture part (C) b dimple pattern in the central part of the broken craze (region A) c patchwork structure in the transition zone to crack instability (region B) d brittle fracture part (region C)... 

In addition to their successful applications to fracture stability problems, the stress intensity factors have been widely used as correlation parameters in analyzing the subcritical crack growth rates da/dn (in fatigue) and dafdt (in corrosion), n and t referring to the number of... 

Crack Growth Resistance and Subcritical Crack Growth... 

The phenomenon of subcritical crack growth may be subdivided into four categories according to the type of loading and the nature of the external environment as shown in Table 1.1. 

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