Crack Paths 2009

Irregular fatigue striations surrounded by pits with different sizes in a zone of crack stable

growth ls on the fatigue fracture surface of the sample from alloy AK6tested at R = 0,5 are

observed (Fig. 3d). More regular fatigue striations surrounded by pits on flat sites of fracture

and secondary cracks are visible in accelerated crack extension zone lr (Fig. 3e). Traces of

mutual compression of fracture surfaces are not observed.

Thus, it is visible, that at change of a stress cycle of samples made from alloy АК6from

compressing (R = -19) to symmetric (R = -1) and stretching (R = 0,5) the viscous

component increases in both crack stable growth zone ls and accelerated crack extension

zone lr that is caused by change of the relation between compressing and stretching stresses

and also by degree of mutual compression of the interfaced fracture surfaces during test.

Obtained data of micro fractographic investigation is confirmed by X-ray diffraction

analysis of fracture surfaces. Figure 4 presents an effect of stress ratio R on the diffraction

line (311) Кα width in a zone ls and fracture zone for alloy АК6. (фиг.4). It is visible, that

with decrease of compressing stresses under change of stress ratio R from -19 to -1 the

diffraction line (311) Кα width decreases in a zone ls, that point to decrease of a degree of

crystalline structure distortion on surface of the given zone.

Figure 4. Effect of stress ratio R on the diffraction line (311) Кα width in a zone ls (1) and

fracture zone (2) for alloy АК6. β0 - diffraction line (311) Кα width of standard sample

The minimumof diffraction line (311) Кα width is obtained at a symmetric cycle (R =-1),

that corresponds to the loading scheme at which the maximumand minimumstress cycle in

samples have the minimumdeviations from zero. The diffraction line (311) Кα width again

increases at R = 0,5 (Fig. 4). Attracts attention that fact, that crystalline structure distortion

of a material on a surface of a zone ls, estimated on diffraction line (311) Кα width, can be

more then crystalline structure distortion in a fracture zone, that takes place at R =-19 and

0,5 (Fig. 4). Apparently, the increase of crystalline structure distortion of a material in ls

zone at R = 0,5 is connected with a big stretching stress, and, as consequence, with ductile

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