PSI - Issue 5

A. Eremin et al. / Procedia Structural Integrity 5 (2017) 889–895 A. Eremin et al / Structural Integrity Procedia 00 (2017) 000 – 000

891

3

ε max parameter possessing higher dynamic range was employed to characterize the maximum value of the COD for each loop.

Fig. 1. Scheme of the tested specimen. Thickness is 3 mm.

The tests were conducted under two various loading conditions, which are described in details in forthcoming sections 2.1 and 2.2. 2.1. Single Underload: measurement and loading procedure The underload parameters. “ Basic ” cycles: asymmetry R =0.65, P max =2.8 kN, crack length a ≈ 13.2 mm, K max =13.15 MPa √ m, The lowest load at the underload cycle P UnL =0 kN.

(b)

(a)

Fig. 2. Loading and measuring sequence for (a) the test 1 with the single underload (b) test 2 with the single overload

The loading sequence for the test 1 with the single underload and measurement cycles are schematically presented in fig. 2a. Actual steps are: 1. Precrack growth (at the basic load) was carried out before starting the measurement cycles till a crack increment reaches 1.2 mm; 2. Application of 20 000 basic cycles to stabilize the crack growth rate and ensure linear direction of its propagation; 3. Application of two “slow cycles” at 0.03 Hz ( P =1.82-2.8-1.82 kN) and step-by-step image capturing at 0.3 fps. These reference measurements denote initial stage of the crack growth before running the underload cycle; 4. Application of the underload cycle with the load drop from 1.82 kN down to 0 kN; 5. After passing the underload cycle image capturing is carried out at various time steps: immediately after the underload cycle, and then after 5, 10, 20, 30, 50 and 100 basic cycles. We call them “ measurement cycles ” . Measurements of COD were performed by software extensometer placed in two locations – i) at the crack tip and ii) at the distance of 630 µm behind the crack tip (i.e. in the region where the crack wake has been already formed).