PSI - Issue 28

Y. Matvienko et al. / Procedia Structural Integrity 28 (2020) 584–590 Author name / Structural Integrity Procedia 00 (2019) 000–000

585

2

D S

normalization coefficient

 

stress range remote stress



MAX  , MIN 

maximum, minimum stress of fatigue cycle



damage accumulation rate

1. Introduction Presently, damage accumulation and further crack growth predictions in airplane structures are mainly based on deformation, energetic, phenomenological and micro-mechanical models of processes, which are responsible for damage initiation on different stages of cyclic loading as it was mentioned by Lalanne (2014). The main shortcoming of these approaches resides in the fact that a description of each discrete damage step is founded upon parameters, which cannot be reliably established proceeding from direct physical measurements. Zerbst et al. (2015) showed that a determination of fracture mechanics parameters could be considered as promising way to overcome this problem. The key point of the involved approach consists of the fact that plane specimens with stress concentrators (open holes) have to be undergone to periodic fatigue loading with different number of cycles. Deriving measurable parameters, essential for quantitative description of damage accumulation process, can be achieved by inserting a sequence of narrow notches when specimens are subjected to constant external load. The details of this approach are given by Matvienko et al. (2019) to analyze the effect of low-cycle fatigue on evolution of fracture mechanics parameters in residual stress field. These notches serve to estimate a fatigue damage accumulation level similar to a probe hole which is used for residual stress energy release in the hole-drilling method. 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 measured by electronic speckle-pattern interferometry (ESPI) in the vicinity of the notch tip. Thus, values of crack mouth opening displacement (CMOD) 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). 2. Experimental method Objects of present research consist of 2024 aluminium plates of dimensions 180×30×4 mm with a centred open hole of diameter 3.0 mm. Array of specimens includes 31 units. All coupons are manufactured from a single material bar by the same technology. Mechanical properties (Young’s modulus 74,000 MPa, yield stress 330 MPa and Poisson’s ratio 0.33) are established by tensile tests. The specimens, divided by four groups, are subjected to uniaxial push-pull loading according to the parameters which are shown in Table 1. The value of maximal tensile remote stress MAX  equals to 76, 61, 51 and 53% of the yield stress for group T4_AA, T4_BB, T4_CC and T4_XX, respectively.

Table 1. Specimen classification and loading conditions. Specimen’s group Stress range   , MPa

Minimal stress MIN  , MPa

Maximal stress MAX  , MPa

Stress ratio R

T4_AA T4_BB T4_CC T4_XX

333.3 333.3 333.3 233.3

−0.33 −0.66 −1.00 −0.33

250

–83.3 –132.5 –166.65

200.8 166.65 175.4

–57.9

Experimental procedures include the following steps. One specimen (T4_09), which is common for all groups, is tested before low-cycle fatigue loading. Four specimens (T4_A0, T4_B0, T4_C0 and T4_X0) are used for lifetime estimation. Other specimens are subjected to different stage of low-cycle fatigue with the parameters presented in Table 1. The present approach for quantifying damage accumulation employs a determination of local deformation response to small notch length increments under constant load. This deformation response is expressed by constructing stress