Issue 47

Yu. G. Matvienko et alii, Frattura ed Integrità Strutturale, 47 (2019) 303-320; DOI: 10.3221/IGF-ESIS.47.23 318 loading with negative stress ration despite of traditional approaches that operates with positive stress ratio values [2, 26, 29– 31, 39]. The third remarkable capability resides in quantitative determination of fracture mechanics parameters for cracks of different lengths at different stages of cyclic loading under combined influence of remote and residual stresses. These data provide the construction of the dependencies of the fracture mechanics parameters for cracks of fixed lengths against of the loading cycle number, which reveal a process of fatigue damage accumulation. Fracture mechanics parameters can be obtained for both the mandrel entry face and the mandrel exit face as well as for different stress range and stress ratio. Experimental information, if necessary, might be supplemented by data obtained for different remote stress levels to reveal a possible influence of non-linear effects. C ONCLUSIONS he effect of low-cycle fatigue on evolution of fracture mechanics parameters for cracks in residual stress field caused by cold expansion of holes has been analysed. To estimate the crack mouth opening displacement (CMOD), the stress intensity factor (SIF) and the T-stress during narrow notch increment, modelling a crack emanating from cold-expanded holes under constant external load, localized displacement measurements by electronic speckle-pattern interferometry are employed. Initial experimental data represent in-plane displacement component values measured in the vicinity of the crack tip. The transition from in-plane displacement components to SIF and T-stress values follows from the relationships of modified version of the crack compliance method. In general case, the preliminary low-cycle fatigue, namely, stress range and stress ratio, as well as crack length has strong effect on fracture mechanics parameters. The crack length curves of CMOD, SIF and T-stress profiles are obtained for cracks emanating from cold-expanded holes in plane specimens at different stages of cyclic loading with parameters   = 350 MPa, R = –0.4 and   = 350 MPa, R = –1.0. These data provide the construction of the dependencies of the fracture mechanics parameters for cracks of fixed lengths against the loading cycle number, which reveal a process of fatigue damage accumulation. Information is obtained in the mandrel inlet face. The results demonstrate considerable increase of fatigue damage accumulation process when 60% of the lifetime is reached. It is also shown that 60% lifetime indicator does not depend on the value of stress ratio R . A CKNOWLEDGEMENTS he authors acknowledge the support of the Russian Science Foundation (project N 18-19-00351). R EFERENCES [1] Reid, L. (2014). Hole Cold Expansion – The Fatigue Mitigation Game Changer of the Past 50 Years, Advanced Materials Research, 891–892, pp. 679-684. DOI:10.4028 /www.scientific.net/AMR.891-892.679. [2] Stefanescu, D. (2004). Measurement and prediction of fatigue crack growth from cold expanded holes, part 1: the effect of fatigue crack growth on cold expansion residual stresses, J. of Str. Analysis, 39(1), pp. 25-39. DOI:10.1177/030932470403900103. [3] Gopalakrishna, H.D., Narasimha Murthy, H.N., Krishna, M., Vinod, M.S., Suresh, A.V. (2010). Cold expansion of holes and resulting fatigue life enhancement and residual stresses in Al 2024T3 alloy – An experimental study, Eng. Failure Analysis, 17(2), pp. 361-368. DOI:10.1016/j.engfailanal.2009.08.002. [4] Wanlin, G. (1993). Elastic-plastic analysis of a finite sheet with a cold worked hole, Eng. Fract. Mechanics, 46(3), pp. 857-864. DOI:10.1016/0013-7944(93)90238-n. [5] Ball, D.L. (1995). Elastic-plastic stress analysis of cold expanded fastener holes, Fatigue and Fract.of Eng. Mat. and Struct., 18(1), pp. 47-63. [6] Zhang, Y., Fitzpatrick, M.E., Edwards, L. (2005). Analysis of the residual stress around a cold-expanded fastener hole in a finite plate, Strain, 41(2), pp. 59-70. DOI:10.1111/j.1475-1305.2005.00181.x. [7] Pavier, M.J., Poussard, C.G.C., Smith, D.J. (1997). A finite element simulation of the cold working process for fastener holes, J. of Str. Analysis, 32, pp. 287-300. T T