PSI - Issue 5

Marcin Wachowski et al. / Procedia Structural Integrity 5 (2017) 422–429 Author name / Procedia - Social and Behavioral Sciences 00 (2016) 000 – 000

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These curves are examples of charts to verify the effect of the heat treatment on the laminate. The most valuable result from the data is that the tensile strength increased from 570 MPa for basic material without heat treatment to about 620 MPa for laminate after the heat treatment. For AA2519/AA1050/TI6AL4V composite laminate, the yield stress was defined as the stress corresponding to 0.2% permanent strain. For this material parameter was also observed a positive effect of heat treatment, which improves the yield stress from 500 MPa to 580 MPa (Tab. 3).

Table 3. Mechanical properties of AA2519/AA1050/TI6AL4V composite laminate. Material AA2519/AA1050/Ti6Al4V Tensile strength MPa Yield stress MPa

Elongation %

basic material

500 580

570 620

14 16

after heat treatment

The last compared parameter shows the 2% increase in the minimum elongation after fracture of 14% to 16% for samples after heat treatment. In the next part of this article will be carried out similar considerations of the impact of heat treatment on fatigue properties of the tested laminate. During high cycle fatigue testing, the test specimens were subjected to variable loads until failure. The initial load levels were determined on the basis of static properties of the material selected for examination and geometry of the specimens, which are shown in Fig. 3. Then the load was gradually reduced until the studies have reached limit of cycles to failure. From these tests, it was possible to determine S – N curves that characterize the fatigue life behavior of the material test specimens. Effect of the heat treatment on the fatigue properties of the examined laminate is shown on graphs obtained for samples with a central hole (Fig. 4) and smooth samples (Fig. 5). Comparison chart of fatigue life of material after the heat treatment (diamonds) in relation to the base material (squares) is presented in Fig. 4. Regression lines obtained by least squares method was carried out by the experimental points. A dashed line represents a base material and a solid line material after the heat treatment. Simple regression equations for these lines and the R-square coefficients on the chart are shown as well. Detailed analysis shown in the Fig. 4 allows to determine that the effect of the heat treatment is visible the most at high stress. With maximum stress level  max =250 MPa fatigue strength of the sample increased from N f =2 ∙ 10 4 to approximately N f =5 ∙ 10 4 cycles for samples after the heat treatment. The number of cycles to failure for samples after the heat treatment is bigger by about at 20 % in relation to basic material at the level of  max =250 MPa. However, this effect is not observed for the maximum stress below 150 MPa. Confirmation of these findings is the intersection of regression lines at level  max =150 MPa. The fatigue limits were estimated in accordance with the staircase method and there are presented by the horizontal lines in Fig. 4 and Fig. 5. Also in this case, slightly higher endurance limit for samples after heat treatment (Sf = 89 MPa) relative to basic material (Sf = 85 MPa) can be noted.

Fig. 4. Fatigue curves obtained for samples with a central hole

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