PSI - Issue 17

Ludvík Kunz et al. / Procedia Structural Integrity 17 (2019) 222–229

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Ludvík Kunz et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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the specimen orientation and no influence of the laser power on the crack growth behaviour. The threshold value  K th determined by the load shedding procedure is 2.7 MPam 1/2 . Based on this value, the equation da/dN = 3.0 × 10 -8 (  K 3.0 -  K th 3.0 ) characterizes well the crack growth in the whole interval from the threshold up to the highest rates of micrometers per cycle, see the full line in Fig. 4.

Fig. 4. Crack growth curve for Ti6Al4V heat treated at 740 °C.

Fig. 3. Crack growth curve for Ti6Al4V heat treated at 380 °C.

Fig. 6. Crack growth curve for Ti6Al4V heat treated at 900 °C, R = 0.8.

Fig. 5. Crack growth curve for Ti6Al4V heat treated at 900 °C.

Fig. 5 shows the result of the crack growth measurement in material heat treated at 900 °C. Also in this case no influence of the specimen orientation was found. The threshold value determined for this heat treatment is higher than that for 380 °C and makes 3.5 MPam 1/2 . The crack growth rate can be well described by the equation da/dN = 2.5 × 10 -8 (  K 3.0 -  K th 3.0 ), which is shown by the full line. With the aim to determine the influence of the mean stress on the crack growth the measurement under loading with very high stress ratio R = 0.8 was done on the material heat treated at 900 °C. The results are shown in Fig. 6. Crack rate was measured on specimens with orientations a and c . The value of  K at which the crack practically stopped at the load shedding method was 2.0 MPam 1/2 . The curve in Fig. 6 represents the equation da/dN = 4.4 × 10 -8 (  K 3.0 -  K th 3.0 ).