PSI - Issue 33

Yu. Matvienko et al. / Procedia Structural Integrity 33 (2021) 491–497 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Pisarev et al. (2018). All specimens have been tested after cold hole expansion. The technology of the involved procedure is described by Matvienko et al. (2019). Expansion level is 5% of nominal interference, defined as the ratio of the interference value to the hole diameter. The specimens, divided by two groups, are tested under uniaxial push-pull loading with the same stress range Δ = 320 MPa, but different stress ratio: R 1 = −0.40 (SRS-A group) and R 2 = −0.60 (SRS-B group). Remote cyclic load direction coincides with the long symmetry axis of the rectangular coupon. Stress range ∆ and stress ratio R are defined as: Δ � ��� � ��� and R = ��� ��� ⁄ , where ��� and ��� are maximal and minimal level of applied remote stresses, respectively. The value of maximal tensile stress ��� equals to 228.6 and 200 MPa that corresponds to 69 and 61% of the yield stress for SRS-A and SRS-B group, respectively. One coupon (SRS-0), common for both groups, is the reference specimen and tested before low-cycle fatigue. Hole diameter increments in principal strain directions, obtained for this coupon, give coefficients, which are essential for further constructing normalized distributions of principal residual strain values as a functions of lifetime percentage. Two specimens are used for lifetime estimation (SRS-A5: � �� � � = 10440 and SRS-B5: � �� � � = 19980 cycles). Other specimens are tested under low-cycle fatigue conditions with prefixed number of cycles to reach different fatigue damage levels. To perform the measurement procedure after preliminary low-cycle fatigue, all specimens are subjected to the secondary hole drilling. The number of loading cycles for each investigated specimen is listed in Table 1. Table 1. Nomenclature of specimens and fatigue loading programs

Specimen

SRS-0

SRS-B1

SRS-B2

SRS-B3

SRS-B4

SRS-B5

SRS-A1

SRS-A2

SRS-A3

SRS-A4

SRS-A5

Number of cycles, N

0

2100

4200

6250

8400

4000

8000

10440

12000

16000

19979

Lifetime, %

0

20

40

60

80

100

20

40

60

80

100

Experimental procedure of simultaneous measurements of deformation response to local material removing is described in details by Eleonsky et al. (2021). It was shown in many published papers that maximum compressive residual stress is located at the distance close to 0.5 mm from the edge of the cold-expanded hole. This fact is valid for a wide range of hole diameter values and interference levels. That is why the secondary hole diameter 2 R 0 = 4.9 mm was chosen to derive the current damage parameter. The face of drill entrance and exit, denoted as Side A and Side B, are the mandrel inlet and outlet surface, respectively. Interference fringe patters, obtained on Side A and Side B of the specimen SRS-0 at initial state ( N = 0), are shown in Fig. 1. Interference fringe patters, obtained on Side A and Side B of the specimen SRS-A4, ( N = 8400), are shown in Fig. 2. The displacement components u and v coincide with the short and long symmetry axis of rectangular coupon, respectively. Whole sets of required interferograms (shown in Fig. 1 and Fig. 2) are obtained for all specimens from Tab. 1.