Crack growth, types

Figure 7.24 Effect of crosshead displacement rate on the measured fracture energy, Gic, and type of crack growth [108]. (a) DGEBA-TEPA adhesive (b) DGEBA-TETA adhesive. O stable brittle crack growth (Type C) A initiation and A arrest of unstable crack growth (Type B). (Test temperature = 22 °C TDCB joints aluminium alloy substrates.)...

Stress corrosion cracking, prevalent where boiling occurs, concentrates corrosion products and impurity chemicals, namely in the deep tubesheet crevices on the hot side of the steam generator and under deposits above the tubesheet. The cracking growth rates increase rapidly at both high and low pH. Either of these environments can exist depending on the type of chemical species present. 

In the case of a pressure vessel subjected to cyclic loading (as here) cracks can grow by fatigue and a vessel initially passed as safe may subsequently become unsafe due to this crack growth. The probable extent of crack growth can be determined by making fatigue tests on pre-cracked pieces of steel of the same type as that used in the pressure vessel, and the safe vessel lifetime can be estimated by the method illustrated in Case Studv 3. 

To discover the effective potential ranges for electrochemical protection, the dependence of the relevant corrosion quantities on the potential is ascertained in the laboratory. These include not only weight loss, but also the number and depth of pits, the penetration rate in selective corrosion, and service life as well as crack growth rate in mechanically stressed specimens, etc. Section 2.4 contains a summarized survey of the potential ranges for different systems and types of corrosion. Four groups can be distinguished ... 

Causes of corrosion are the subject of extensive investigation by industry. Almost any type of corrosion can manifest itself under widely differing operating conditions. Also, different types of corrosion can occur simultaneously. It is not uncommon to see crack growth from stress corrosion to... 

The typical effect of rate on the measured value of K,c and the associated type of crack growth can clearly be seen in Fig. 4 for an epoxy polymer based upon a DGEBA resin and cured with various amounts of TETA. The values of K ci and K ca are those for crack initiation and arrest (Fig. 2b) respectively and the difference between them characterises the amount of crack jumping which has taken place. When the difference... 

Fig. 15. Relation between KIc/Klcs, true tensile yield stress and type of crack growth...

From the above observations it appears that the yield behaviour of the material in the vicinity of the crack tip controls the degree of plastic deformation that occurs locally, and hence the measured toughness and the type of crack growth. This leads to a consideration of the stress distribution around a blunt crack. It has been shown50) that, for a crack of tip radius, e, and length, a, the stress, small distance, r, ahead of the tip (Fig. 16) is given by... 

To summarise, the fracture data and type of crack growth over a wide range of rates and temperatures may all be rationalised by the concept of a critical stress acting over a critical distance and this two-parameter model provides a unique failure criterion for thermosetting polymers. 

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