PSI - Issue 13

634 4

Przemysław Strzelecki / Procedia Structural Integrity 13 (2018) 631 – 635 Author name / Structural Integrity Procedia 00 (2018) 000–000

a

b

Fig. 3. (a) Microstructure and ; (b) fatigue fracture of AW 6063 T6 aluminum alloy.

3. Comparison of the experimental results To compare each S-N curve the probability density function for 10 5 cycles was estimated for every geometry of the specimens. Fig. 1 a) shows the graph of the estimated probability density functions. It can be seen that for notched specimens the range of the stress amplitude is smaller than for smooth specimens. The expected value of the stress amplitude for 10 5 cycles is equal 161.8 MPa, 132.2 MPa, 129.2 MPa and 115.2 MPa for specimens with K t 1, 1.6, 2 and 2.6, respectively. While, the stress amplitude for 10 5 cycles for 0.1 % probability is equal to 108 MPa, 96.3 MPa, 86.3 MPa and 73.1 MPa for specimens with K t 1, 1.6, 2 and 2.6, respectively. Differences between the expected value for smooth specimens and each notched specimens are equal to 29.7 MPa, 32.6 MPa and 46.7 MPa. However, a differences between the stress amplitude for 10 5 cycles for 0.1 % probability for smooth specimens and each notched specimens are equal to 11.7 MPa, 21.7 MPa and 35 MPa.

a

b

35 40 45 50

Unnotched specimen Notched specimen r=1.5 Notched specimen r=0.5 Notched specimen r=0.25

Densities 0 0.5  10  5 1  10  5

Coefficient of variation V [%]

60

80

100

120

140

160

180

200

1.0

1.5

2.0

2.5

Stress amplitude [MPa]

Stress concentration factor K t

Fig. 4. (a) The density function for 10 5 cycle; (b) relationship of variation to stress concentration factor.

To verify the scatter of fatigue life the coefficient of variation was calculated and expressed following Rinne (2008): � � � � � � ∙ 100% (2) � � ����� � ∙ �� �1 � � � � � � � �1 � � � � � � � (3) In equation (2) expected value μ x is equal 10 5 . Fig 4 b) shows relation relationship of variation to stress concentration factor.