Fracture delayed

In the last section, attention was focused on fast, brittle fracture, in which a specimen under the action of a given load either failed immediately after the application of the load, or did not fail at all. However, it is well known that components also fail after some time in service. Of course, under a varying load, this may happen due to the incidental occurrence of an extra high peak in the load spectrum, but a delayed 

For simplicity, in the following discussions a general uniaxial stress state is assumed. Of course, generalizations as made in the section above (equivalent stress, effective volume, etc.) are also possible in the case of delayed fracture. 

The SCCG data shown in this section have been determined experimentally. Static and constant stress rate strength tests have been performed on standard bend specimens (3 x 4 x 45 mm ) of a commercial 99.7% AI2O3 (Frialit-Degussit F99,7 Friatec AG, Mannheim, Germany) in four-point flexure [99]. All experiments were conducted at room temperature in laboratory air. 

The growth of cracks under load caused a finite lifetime of loaded specimens or components the crack grew from its initial length a to the critical (final) length Uc = Uf, after which fast failure occurred. 

The lifetime can be determined in the following way. If the crack growth rate is the change of the crack length with time v = da/dt, then the differential of the time is dt = (1 /v). Integration is possible by separation of variables. Using Eqs (21) and (3) gives  

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Oral corrosion of metallic restorations does not, per se, generally result in serious damage to the structure. Corrosion can result, however, in various local and systemic effects, notably the hypersensitivity and allergic reactions reported by many workers. Galvanic cells created by mixed metal couples can delay fracture healing and induce oral lesions and cancer. 

In the majority of cases, the tests are conducted using a dead-weight lever-arm stress-rupture rig with an electric timer to determine the moment of fracture, but a variety of test rigs similar to those shown in Fig. 8.89g are also used. The evaluation of embrittlement may be based on a delayed-failure diagram in which the applied nominal stress versus time to failure is plotted (Fig. 8.103) or the specimen may be stressed to a predetermined value (say 75% of the ultimate notched tensile strength) and is considered not to be embrittled if it shows no evidence of cracking within a predetermined time (say 500 h). Troiano considers that the nature of delayed fracture failure can be described by four parameters (see Fig. 8.103) ... 

Glass that has been under stress for a period of time may fracture suddenly. Such delayed fracture is not common in metals (except in cases of hydrogen embrittlement of steels) but sometimes does occur in polymers. It is often called static fatigue. The phenomenon is sensitive to temperature and prior abrasion of the surface. Most important, it is very sensitive to environment. Cracking is much more rapid with exposure to water than if the glass is kept dry (Figure 15.11) because water breaks the Si-O-Si bonds by the reaction — Si-O-Si—H H2O -> Si-OH + HO-Si. 

EFFECT OF HYDROGEN ON DELAYED FRACTURE OF MARAGING STEELS... 

Abstract. Influence of thermoplastic treatment conditions in various corrosive media on tendency of maraging steels to delayed fracture has been investigated. Parameters of the treatment that are optimal for the improvement of the structural strength of steels have been determined. 

Keywords maraging steels, hydrogen, delayed fracture... 

Figure 6.61 Delayed fracture times and minimum stress for cracking of 0.4% C steel for various hydrogen concentrations obtained by different baking times at 150°C of cathodically charged specimens (Craig)5...

Nanocrystalline material (which is free of dislocations) such as nickel was tested for delayed fracture in the presence of mercury. The fracture surface shown in Figure 7.93 has three zones. Zone I shows initial microcrack, and zone III shows the final failure stage. Zone II, of the order of 100 pm width, appears different from zones I and III and shows the path of crack growth. This shows that dislocations are not involved in the crack growth in LMIE conditions of dissolution-condensation model of LMIE is favored. 

T. Okada and G. Sines, Prediction of Delayed Fracture from Crack Coalescence-Alumina, in Fracture Mechanics of Ceramics, Vol. 7. eds. R. C. 

Wang, Y., Peng, X., Chu, W.Y., Su, Y.J., Qiao, L.J., and Gao, K.W., Anisotropy of hydrogen fissure and hydrogen-induced delayed fracture of a PZT ferroelectric ceramic, in Proceedings of the 2nd International Conference on Environment Induced Cracking of Metals, Banff, Canada, October 2004, in press. 

Ascorbic acid, a water-soluble vitamin (1(X) to 250 mg p.o. daily), is indicated in the treatment of frank and subclinical scurvy in extensive bums, delayed fracture or wound healing, postoperative wound healing severe febrile or chronic disease states and in prevention of ascorbic acid deficiency in those with poor nutritional habits or increased requirements. In addition, ascorbic acid has been used for potentiation of meth-enamine in urine acidification and as an adjunctive therapy in the treatment of idiopathic methemoglobinemia. 

This type of attack does not require any specific environment to take place, since it can take place simply in neutral or acidic wet environments. Failure due to hydrogen is named hydrogen embrittlement since it leads to a brittle-Uke fracture surface. Indeed, the ductility of the bulk metal does not change, but the propagation of the crack is due to the mechanical stresses induced in the lattice by hydrogen accumulated near the crack tip. If hydrogen is present in the metal lattice before the application of loads a delayed fracture may occur, i. e. the steel does not fail when the load is applied, but after a certain time. 

Hydrogen embrittlement is brought about, when the material absorbs evolved hydrogen during some processes. Some examples are acid pickUng, electrolysis, and uses in various vaporized atmospheres. It sometimes leads to fractures. Usually, the phenomenmi nfight occur after a long time. Therefore, it is often called delayed fracture or delayed failure . 

Figure 8.12. Delayed fracture times and minimum stress for cracking of 0.4% C steel as a function of hydrogen content. Specimen initially charged cathodically, baked at 150°C for varying times to reduce hydrogen content [59]. [Figure 5 from H. Johnson, J. Morlet, and A. Troiano, Hydrogen, crack initiation, and delayed failure in steel, Trans. AIME 212, 531 (1958).]...

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