PSI- Issue 9

Yu. Matvienko et al. / Procedia Structural Integrity 9 (2018) 16–21 Author name / Structural Integrity Procedia 00 (2018) 000–000

18

3

A sequence of narrow notches of 0.2 mm width is used for crack modeling at different stages of cyclic loading. These notches are performed under the constant external loading. The original points of each symmetrical notch are located at the intersection of the hole boundary and the short symmetry axis of the specimen as it shown in Figure 1. The experimental approach employs optical interferometric measurements of the local deformation response to small notch length increment. Initial experimental data represent in-plane displacement component u and v measured by electronic speckle-pattern interferometry in the vicinity of the crack tip. Thus, the CMOD values are derived directly. The transition from measured in-plane displacement components to required SIF and T-stress values follows from the relationships of modified version of the crack compliance method proposed by Pisarev et al. (2017). The first specimen is tested without influence of cyclic loading. Other specimens both with plane and cold expanded holes are subjected to low-cyclic fatigue loading with the parameters   = 350 MPa, R = –0.4. Maximum remote tensile stress is equal to max  = 250 MPa. This value corresponds to maximum circumferential strain at the open hole edge max   = 0.01, as it follows from performed finite element simulation by MSC/NASTRAN program product. An electro-mechanical testing machine walter + bai ag , Type LFM-Z 200, with loading range 0–200 kN is used for cyclic loading. The number of loading cycles for each investigated specimen is listed in Table 1. The second specimen serves for life-time estimation. The fracture occurred after CF N = 6300 cycles. The cycle number from Table 1 indicates the stage of cyclic loading, at which CMOD, SIF and T-stress values for cracks of different lengths are derived from initial experimental data. Three consecutive notches after specimen’s cycling are performed under the constant external loading. An electro-mechanical testing machine walter + bai ag , Type LFM-L 25, with loading range 0–25 kN serves for applying remote tensile stress during the measurement procedure.

Table 1. Nomenclature of the specimens and the cycle number.

Specimen

T5_20H

T5_13H

T5_16H

T5_12H

T5_18H

T5_19H

T5_17H

T5_00H

N, cycles

0 0

1000

2000

3000

4000

5000

6000

6300

Life-time, %

16

32

48

63

79

95

100

b

a

Fig. 2. Specimen T5_20H. Interference fringe patterns obtained in terms of in-plane displacement component u (a) and v (b); initial crack length 1  a = 1.97 mm with the increment 2   a = 1.87 mm (left) and initial crack length 1  a = 2.18 mm with the increment 2   a = 1.70 mm (right). The details of the experimental procedure are presented by Pisarev et al. (2017). In short, the scheme of the experiment involved resides in the following. External tensile load P is applied to the specimen. The first exposure is made for a crack of current length 1  n a ( n = 1, 2, 3). Then the crack length is increased by a small increment  n a and the second exposure is made for a crack of the final length 1     n n n a a a . During the process of crack length increase the constancy of acting force lies in the interval P –0.01 P with a warranty. Initial experimental information for each measurement step has a form of two interference fringe patterns related to in-plane displacement components u and v as it shown in Figure 2. High quality of presented interferograms is quite evident. Interference fringe patterns of the type shown in Figure 2 are obtained for all specimens in the case of three crack