**5. Conclusion**

420 Recent Trends in Processing and Degradation of Aluminium Alloys

Consider a typical situation. Let failure occur at the fourth loading cycle. During the strain process, the material elongations at the real crack tip coincide with the limit material

Recall that the crack length in the model in (Kornev, 2004, 2010) changes by the pre-fracture

materials in the pre-fracture zone become brittle (Romaniv et al., 1990; Laird & Smith, 1962; Kornev, 2004, 2010). Each such advance is associated with the certain number of cycles when linear and non-linear damages are summed. The performed tests show that the initial state of material, which falls into the pre-fracture zone, influences the process of the step-wise

pronounced. This corresponds to the passage of the second fracture process to the third one

crack tip advance. The magnitude of inelastic strain under stretching

<sup>+</sup> depend on: *i*) load amplitude

preliminary inelastic strain of material. If the basic parameter of inelastic strain

As explained the step-wise advance of the real crack tip by the length 1 1

have been made, embrittlement of layer materials occurs, and the ratio

χ> . χ

= 1 holds. After preliminary inelastic strain of a material from which specimens

The most important question is as follows: what is a distance from the crack onset where the crack will arrest? In the model in (Kornev, 2004, 2010), the case of original material loading is considered when the layers possess the quasi-ductile fracture type. Because of this, the

step-wise crack tip advance in composite materials with quasi-ductile and embrittled layers elucidates the schematic drawing in Fig. 13. At the scheme top, the right-hand tip of a blunted crack is shown before its start. In the middle part of the scheme, the crack tip

In essence, just as in the model in (Kornev, 2004, 2010), so in foregoing experimental results, the distinct influence of the single parameter on the process of damage accumulation under cyclic pulse loading is traced. In the model in (Kornev, 2004, 2010), it is the parameter that

after the third inelastic strain of the fiber nearest to the real crack tip (Fig.

<sup>+</sup> after the step-wise real crack tip advance, and under the repeated loading,

σ

σ

σ ε

*<sup>a</sup>* ; *ii*) initial crack length *2l*; and *iii*)

<sup>+</sup> , the analogous dependence is

σ

χ

= 1 , and for quasi-brittle layers

<sup>+</sup> , then the crack is blunted at the mesoscale, see the ratio 1

χΔ1σ

> σ ε

χ σ

<sup>∗</sup> Δ =Δ <sup>+</sup> <sup>+</sup> , where

> 1 can hold. The

<sup>+</sup> , and then the crack is

<sup>+</sup> , and in experiments, it

χ= .

χ> 1 .

<sup>+</sup> and the pre-

σ ε

<sup>+</sup> slightly

elongation (3)

zone length Δ1

χ

ratio χ

advances by the segment Δ<sup>1</sup>

1 ε

fracture zone length Δ1

σ

12), and the crack tip being advanced.

σ

Fig. 13. Schematic drawing of material damage

is such a coefficient that for quasi-ductile layers

σ

blunted at the mesoscale, see the ratio 1

At the scheme bottom, the crack tip advances by the segment

characterizes inelastic strain of material in the pre-fracture zone

depends on *2l*, then for the pre-fracture length Δ<sup>1</sup>

on the Paris curve (Romaniv et al., 1990; Shaniavski, 2003).

Recording the loading diagrams for beams with the edge crack allows description of both damage accumulation at the macroscale and failure of constructions at every loading cycle. The damage accumulation in the pre-fracture zone is associated with the residual deflection of beams after unloading.

Preliminary plastic deformation of aluminum alloy leads to decrease in its durability. However, here the intensity of damage accumulation also reduces under low-cycle threepoint bending of beams. This results in increase in the number of loads, which structure may withstand.

In the chapter, loading regimes with gradual overloading and with increased loading have been considered. These regimes model a common situation and catastrophic overloading under cyclic loading. The results obtained may be useful for prediction of fatigue life of a structure with the crack and in analyzing situations after overloading.

Direct observation in the vicinity of the notch tip during the process of low-cycle tension of a plain specimen allows one to trace the behavior of a fatigue crack at various stages of its

**Part 6** 

**Microstructures, Nanostructures** 

**and Image Analysis** 

propagation: *i*) crack initiation in the vicinity of stress concentration; *ii*) crack propagation within the plasticity zone; *iii*) onset of generation of a narrow pre-fracture zone formed by the crack itself; *iiii*) fast crack propagation beyond the plasticity zone.

Schemes allowing description of deformation, damage accumulation, and failure of material under fatigue with account of the preliminary inelastic deformation of the material and effect of stress concentration on crack initiation have been proposed.
