**1. Introduction**

406 Recent Trends in Processing and Degradation of Aluminium Alloys

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When structures are loaded in catastrophic mode of operation, localization of irreversible strains occurs in regions of stress concentration. This is caused by the geometry of a structure or by the presence of fetaures (dints, holes, cracks, inclusions with mechanical characteristics different from properties of original material). If loading is in progressing, repeated loads cause gradual degradation of the material in regions of the localization of inelastic strain. As a result, this leads to generation and extension of cracks and to loss of a load capacity of the structure. Study of regularities of material degradation in regions of strain localization will permit one to appreciate possible structure resources in a catastrophic situation or consequence of failure, which affects subsequent behaviour of a structure in a common regime.

The distinctive features of fracture surface microrelief of metallic components in fatigue are fatigue striations oriented normally to the crack extension direction. It is appropriate to relate formation of fatigue striations to stepwise crack tip advance, and to record residual deflection of a beam under three-point bending when the loading corresponds to the lowcycle fatigue. A current striation may be formed after several loading cycles due to arrest of a fatigue crack after each advance of its tip in the Laird- Smith model (Laird & Smith, 1962), the material being embrittled in a pre-fracture zone at each loading type. Damage accumulation in the pre-fracture zone is associated with accumulation of inelastic strains in this zone.

The aim of the present work was to study damage accumulation in the regions of inelastic strains near the notch tip having a finite width. Two cases are considered: i) symmetric three-point bending of a beam (the edge notch is made on the underside of a specimen in the transverse symmetry plane, damage accumulation is estimeted by the increment of a residual deflection); ii) tension of a plain specimen with a narrow edge notch (direct viewing of fatigue crack propogation was performed using digitized microscope with resolution of about 22500 pxel/mm2). Mechanisms of deformation, damage accumulation and failure of material under fatigue conditions have been proposed.

In the first case, the choice of geometry of a specimen and the loading type are governed by the following considerations.

Interrelation Between Failure and

is the distance between adjacent curves of group 2.

observed with increase of *N*. At this stage,

δ

for the given value of *P*max after which the

**2.1 Stationary loading** 

the increase in the

Damage Accumulation in the Pre-Fracture Zone Under Low-Cycle Loading 409

Fig. 1 demonstrates, as an example, the experimental diagram represented by curves of beam deflection *w* versus applied force *P* in stationary low-cycle testing a specimen made from D16T. The *P* value is sufficiently large in order that fracture to happen after the limited number of cycles (in this case, \* 390 *N* = ). This allows one to visualize all distinctive features of such diagrams obtained also for test materials for different *P*max values. Curve 1 corresponds to single loading of a specimen until fracture occurs; group of curves 2 corresponds to cyclic loading up to the instant when a crack starts to extend for *P P* < max . Each curve of group 2 corresponds to loading branch of one cycle. All the curves of group 2, except the first one, have the initial horizontal section 0 *P* = . The length of this section is equal to the value of residual deflection accumulated at previous cycles. In this figure,

Damage accumulation can be divided into two specific stages. The first stage (Fig. 1,

subgroup *А* of curves 2) is a stage of cyclic strengthening at which decrease in

between the number of cycles in sub-groups *А* and *В* and the law of

deflection as a function of force for every loading cycle for D16T alloy

saw-like profile. Here the saw-like profiles represent experimental

Regularity in the residual deflection can be visualized as

on material characteristics (Karpov, 2009; Kornev et al., 2010).

δ

δ

measurement accuracy. The second stage (Fig.1, sub-group *В* of curves 2) is characterized by

crack when *P P* < max . Accumulation of micro-defects during the first stage is likely to lead to formation of macroscopic defect, which then progresses during the second stage. Therefore, we call the second stage as a stage of development of a macro-defect. The ratio

Fig. 1. Scheme of low-cycle test of specimen loaded in tree-point bending and plots of beam

such diagrams is given in Fig. 2. Here pairs of curves are shown with numerals 1–5 for curves, one of the pairs being given by the analytical function and the second one being a

D16T alloy. The analysis of experimental diagrams shows that curves can be approximated by plots of some power functions. These functions are to have asymptotes corresponding to the limits beyond which the process described by the diagrams can not take place. That is, the inverse power function with some scaling coefficient can be taken as approximating one.

δ

*w* value as *N* increases, and this stage is accomplished by growth of a

δ*w*

δ*w* is

*w* variation depend

*w* achieves some minimum that is characteristic

*w* value becomes constant within the limits of

δ

*w N*( ) diagrams. An example of

*w N*( ) curves for the

δ

Under symmetric three-point bending of a beam with the transverse notch, main inelastic strains are concentrated ahead of the notch tip where stress concentration occurs. Change of mechanical characteristics of material in this region under repeated loading conditions causes the increase in residual deflection of the specimen. Thus, the amplitude of the residual deflection can be used as a measure of damage accumulation in the zone of localization of inelastic strains. This provides, from macroscopic phenomena, a possibility for qualitative and quantitative estimating the changes directly exhibited by the material due to processes of fatigue fracture. Moreover, in this case, the notch can be considered as a model of an edge crack with the blunted tip, and the region of localization of inelastic strains can be considered as a pre-fracture zone ahead of the tip of this crack.

In the second case, consider a plain specimen with one edge notch since two symmetrical notches lead to uncertainty in a choice of the point of crack initiation. Besides, after a crack initiates, the symmetry of a specimen is broken in one of paired notches and its initial symmetry loses significance.
