Issue 33

S. Beretta et alii, Frattura ed Integrità Strutturale, 33 (2015) 174-182; DOI: 10.3221/IGF-ESIS.33.22 174 Focussed on characterization of crack tip fields Multi-scale crack closure measurements with digital image correlation on Haynes 230 Stefano Beretta, Silvio Rabbolini, Angelo Di Bello Department of Mechanical Engineering, Politecnico di Milano stefano.beretta@polimi.it, silvio.rabbolini@polimi.it, angelo1.dibello@mail.polimi.it A BSTRACT . An experimental campaign was developed to study fatigue crack growth in Haynes 230, a Ni-based superalloy. The effects of crack closure were investigated with digital image correlation, by applying two different approaches. Initially, full field regression algorithms were applied to extract the effective stress intensity factor ranges from the singular displacement field measured at crack tips. Local closure measurements were then performed by considering crack flanks relative displacements. Two points virtual extensometers were applied in this phase. Experimental results were then compared to the reference da/dN –∆K eff curve: it was found that the correct estimation of crack opening levels shifts all the experimental points on the reference curve, showing that DIC can be successfully applied to measure crack closure effects. K EYWORDS . Fatigue crack growth; Haynes 230; Crack closure; Effective stress intensity factors; Digital image correlation. I NTRODUCTION he continuously increasing demand of electrical power, together with the necessity to provide energy in a more sustainable way, has led to a renewed interest in nickel-based superalloys. This is mainly related to the fact that these materials exhibit an excellent resistance to high temperature and corrosion, features that make them the preferable choice for components working in harsh environments, like those present in the combustion chambers of gas turbines. In this paper, fatigue crack growth of a commercially available nickel-based superalloy, Haynes 230, is investigated. Room temperature conditions are taken into account, in order to measure fatigue crack growth with optical methods, avoiding the effects of further damage mechanisms, like oxidisation. In 1970, Elber[1] discovered the phenomenon of plasticity induced crack closure and proposed to modify the Paris equation, by replacing ∆K with the effective stress intensity factor range, ∆K eff , computed considering only the portion of the load cycle where the crack stays open. This modification removed the dependency of crack growth rates on R , demonstrating that crack closure plays an important role in fatigue and that only a parameter can be used to describe Mode I propagation. In this work, ∆K eff is extracted from the singular field present at the tip of a fatigue crack. A crack tip can be modelled as singularity point in the stress and strain fields of a cracked body. Williams[2] analytically described the stress field present at the tip of a crack in an elastic and isotropic body with an infinite power series. Williams found the singularity at the tip to be of the r -1/2 order, where r is the radial distance from the crack tip, and that stress intensity factors are a measure of the singularity itself. In this work the analytical solution provided by Williams was fitted with the experimental displacements measured by digital image correlation (DIC). DIC was originally applied to fatigue crack growth to measure crack closure effects [3, 4]. Local measurements of crack tip opening levels were initially obtained with two points digital extensometers, following Elber’s proposal[1, 5]. McNeill et al .[6] were the first to propose to extract stress intensity factors from DIC-measured displacements. The necessity to T