PSI - Issue 33

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

492

2

To quantify damage accumulation, the stress intensity factors (SIF) of created notches have been involved as current damage indicator. Numerical integration of normalized dependencies of SIF values over lifetime provides the damage accumulation function in an explicit form. Later it has been shown that non-singular fracture mechanics parameters, such as the notch mouth opening displacement and the T-stress can be implemented by the same way to quantify damage accumulation under low-cycle fatigue conditions in stress concentration zone (Matvienko et al., (2021)). Nomenclature � � � , �� , � � damage accumulation function � current number of loading cycles � �� � �,�� stress ratio R 0 the secondary hole radius �� � � , �� normalizing coefficient ∆ stress range ��� , ��� maximum, minimum stress of fatigue cycle ∆ � � � �, ∆� � � � � increments of the secondary hole diameter ∆ �� , ∆ �� fringe order differences �� ( k =1,2) principal residual strain components referred to the secondary hole edge Σ �� � � � square under normalized residual strain curves Direct use of fracture mechanics parameters evolution for quantitative description of damage accumulation employs preliminary tension of the specimen before notch inserting by means of testing machine assembled into interferometer optical system. It is necessary to reach optimal fringe density along the notch borders thus ensuring appropriate accuracy of fracture mechanics parameters determination. This means that measurements of deformation response to local material removing can be performed on only one from two external faces of the specimen with the cold-expanded hole. Usually this surface is mandrel entrance side. But a significant difference in the magnitude of compressive residual stress has been established between the entry side and the exit side. The stress value has a lower value at the entry face compared to the exit face. Thus, simultaneous evaluation of damage accumulation inherent in opposite sides of the specimen with the cold-expanded hole is of considerable interest. The present paper concerns another kind of the destructive method for quantitative description of low-cycle fatigue damage accumulation based on the same principals. The key point of the proposed approach consists of using increments of secondary hole diameters in principal stress directions as the current damage parameter. Secondary hole drilling means enlarging of initial cold-expanded hole diameter. In-plane displacement components measured by electronic speckle-pattern interferometry (ESPI) are served as initial experimental information. The secondary hole drilling technique provides a means for simultaneous two-side measurements of deformation response to local material removing thus quantifying damage accumulation on opposite specimen’s faces. This follows from an absence of the need of tensile load applying to reliably measure a local deformation response as it was shown by Matvienko Y.G. et al. (2019). 2. Extraction of current damage parameters Two sets of rectangular plates measuring 150×30×5 mm with centred cold-expanded holes of nominal 4.0 mm diameter made from 2024 aluminium alloy are the objects of investigation. Pilot holes of 3.84 mm in diameter were drilled, followed by a 4.0 mm final reaming. All coupons are manufactured from a single material bar by the same milling technology. Mechanical properties, obtained by standard tensile test, are the following: Young’s modulus E = 74,000 MPa, yield stress � = 330 MPa and Poisson’s ratio µ = 0.33. Whole set of specimens consists of 13 units. Absence of technological residual stresses in all coupons follows from data of probe hole drilling and further optical interferometric measurements of deformation response to local material removing as it was proposed by