Issue 35

A. Kowalski et alii, Frattura ed Integrità Strutturale, 35 (2015) 449-455; DOI: 10.3221/IGF-ESIS.35.51 451  cold-rolled with a degree of deformation of 10%.  ageing of the material for 12 hours at a temperature of 150 o C, then cooled down in the open air. Figure 1 : Scheme of low-temperature thermomechanical treatment of AlZn6Mg0.8Zr alloy. The fatigue strength was tested on a test-stand MZGS-100 [6] (Fig. 2a) applying smooth standard samples (Fig. 2b) and a simple state of loading of the type oscillatory bending. Besides the possibility of cycling bending at a constant value of the bending moment and fatigue torsion, the test stand permits also provide the sample with complex state of bending stresses connected with its torsion. Basing on the performed fatigue tests the fatigue characteristics of Basquin could be determined in the system: amplitude of stresses ( σ a ) – number of cycle up to the fatigue (N f ). a) b) Figure 2 : Test stand MZGS-100 (a) and specimen of fatigue strength test (b) . Metallographic tests were carried out on microsections of longitudinal samples stretched statically and tested concerning their fatigue. The preparation of the microsections comprised standard operations of submerging the samples in chemohardened resign, grinding and mechanical polishing on s Struers Labo Pol – 21 machine, as well as etching in a 2% solution of hydrofluoric acid in water. The structure was observed making use of the light microscope OLYMPUS GX71F with a magnifying power of 200 to 1000 times. For fractographic investigations fractures of samples after the decohesion in fatigue bending tests were used. For this purpose an electron scanning microscope (SEM) of the ZEISS SUPRA 25 type was applied with an electron part GEMINI with a voltage of 20 kV and a magnifying power of 140 to 1500 times. The chemical composition of the precipitations was analyzed on the fractures applying the method of X-ray microanalysis and probe EDAX. R ESULTS he results of mechanical fatigue tests of samples of the AlZn6Mg0.8Zr alloy have been gathered in the diagram (Fig. 3). It has been found that the standard characteristics of fatigue according to Basquin, determined in the range fatigue strength in time is typically for metallic materials and can be described statistically by a straight line T