Fracture delayed failure

An additional issue in fiber strength is that of fatigue (22), which can produce delayed failure of a fiber. Fatigue is thought to be caused by a surface reaction of fiber and OH causing the growth of subcritical flaws to the point where fracture occurs. 

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) ... 

Figure 7.93 Overall image of the fracture surface of nano-Ni after delayed failure in the presence of Hg...

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).]...

Another consequence of fatigue (which is of significant technological importance) is that a sample may fail after a long time under constant stress which is much less than the short-term fracture stress. This phenomenon is known as delayed failure or static fatigue . Experimental results show that the time to failure, r(7,X,applied stress, a, with increase in relative humidity of the environment, X, and with increase in temperature, 7. 

Kanninen, M.F, Popelar, C.H. (1985) Advanced Fracture Mechanics (Oxford University Press) Knauss, W.G. (1970a) Delayed failure - the Griffith problem for linearly viscoelastic materials. Int. J. Fracture Mech. 6, 7-20... 

The delayed failure curves for precracked specimens of hydrogenated and Cd plated (a) AISI 4340 steel without rare earths, (b) with 0.03% Ce, (c) with 0.09 and 0.17 w/o Ce, (d) with 0.08 and 0.16 w/o La are shown in fig. 18a-d. The Ce and La additions showed a dramatic improvement in the delayed failure of 4340 steel both in fracture times and lower critical stress intensity. The lower critical stress intensity represents a three-fold... 

Fracture. Most mechanical parts will endure a certain amount of plastic deformation before fracture. However, a sudden fracture can occur in brittle materials without warning. Fracture mechanics has been widely used to analyze brittle fracture problems. Fatigue is a progressive fracture caused by a cyclical load. It is the most concerning fracture mode in machine design and for operational safety. Stress rupture is a delayed failure by fracture, which occurs when a metal is subject to a stahc load, usually much lower than the yield strength, at elevated temperatures. 

Whereas ductile materials, such as iron and mild steel, are often considered not to crack when charged with hydrogen and subjected to a tensile stress below the yield stress, the position is different with high-strength ferrous alloys where, depending on the strength of the steel and the hydrogen content, failure may occur well below the yield stress. However, the fracture process is not instantaneous and there is a time delay before cracks are... 

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. 

Even if the Al Oj interlayer accelerates the activation of the transverse cracking, it seems to have the opposite effect on adhesion failure. Indeed, we observe for both systems with an Al Oj interlayer (B and E) that the debonding and buckling are delayed. Therefore, the adhesion of the films is improved. The presence of this thermally grown Al Oj interlayer increases the interfacial fracture energy values to about 15 J.m in both systems. Two qualitative explanations can be proposed for the adhesion improvement. First, the Al Oj certainly permits an increase in the number of 0-Si bonds between the interlayer and the film. Second, prior to the... 

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. 

---