Experimental evaluation of plasticity-induced crack shielding from crack tip displacements fields

In this work it is proposed a methodology for the evaluation of plasticity-induced crack shielding from the analysis of the crack tip displacements fields measured by digital image correlation. This methodology is based on the evaluation of the stress intensity factors determined from the displacements fields measured at the vicinity of the tip of a growing fatigue crack. For the characterisation of the crack tip displacements field, CJP model has been implemented. This model considers the shielding effects due to plasticity generated during fatigue crack growth. For the purpose of the current work, several fatigue experiments at different R-ratios have been conducted on Al2024-T3 compact tension specimens. In addition, compliance based methods have been adopted to perform a comparison of the results with those obtained by DIC. Results show a good level of agreement, illustrating the enormous potential of DIC technique for the study of fracture mechanics problems

An improved prediction of the effective range of stress intensity factor in fatigue crack growth

This paper will summarise the results obtained to date and which demonstrate that the mesoscale CJP model of crack tip fields is capable of providing an improved correlation of fatigue crack growth rates across a range of stress ratios and specimen geometries, compared with the standard stress intensity factor calculations

Influence of Second-Order Effects on Thermoelastic Behaviour in the Proximity of Crack Tips on Titanium

The Stress Intensity Factor (SIF) is used to describe the stress state and the mechanical behaviour of a material in the presence of cracks. SIF can be experimentally assessed using contactless techniques such as Thermoelastic Stress Analysis (TSA). The classic TSA theory concerns the relationship between temperature and stress variations and was successfully applied to fracture mechanics for SIF evaluation and crack tip location. This theory is no longer valid for some materials, such as titanium and aluminium, where the temperature variations also depend on the mean stress. The objective of this work was to present a new thermoelastic equation that includes the mean stress dependence to investigate the thermoelastic effect in the proximity of crack tips on titanium. Westergaard’s equations and Williams’s series expansion were employed in order to express the thermoelastic signal, including the second-order effect. Tests have been carried out to investigate the differences in SIF evaluation between the proposed approach and the classical one. A first qualitative evaluation of the importance of considering second-order effects in the thermoelastic signal in proximity of the crack tip in two loading conditions at two different loading ratios, R = 0.1 and R = 0.5, consisted of comparing the experimental signal and synthetic TSA maps. Moreover, the SIF, evaluated with the proposed and classical approaches, was compared with values from the ASTM standard formulas. The new formulation demonstrates its improved capability for describing the stress distribution in the proximity of the crack tip. The effect of the correction cannot be neglected in either Williams’s or Westergaard’s model

Fatigue crack characterisation in 2024-T351 aluminium alloy through SEM observation combined with the CJP model

This work characterises crack growth in AA2024-T315 by combining different methods to further increase the reliability of the results. The Christopher-James-Patterson (CJP) model was fitted to experimental data obtained by digital image correlation (DIC). The effective value of the CJP stress intensity factors were successfully correlated with the ΔK-da/dN curve as obtained with Scanning Electron Microscopy measurements of the depth of striations on the fracture surface. This approach based on fitting the CJP model with DIC data and SEM observations allowed estimation of opening and closure loads and allowed the propagation rate and fracture mode to be effectively characterised.The authors would like to acknowledge the financial support of Programa Operativo FEDER from the Junta de Andalucia (Spain) through grant reference UMA18-FEDERJA-250. Industrial support from Bettergy SL and from Dr N Ordonez and M Carrera is also greatly acknowledged, as well as access to different components and materials in the energy industry. The authors would also like to acknowledge the financial support from Junta de Andalucia through the research project “1380786” funded by the program “Proyectos de I + D + I en el Marco del Programa Operativo FEDER Andalucia 2014-2020. Convocatoria 2020.” We would also like to acknowledge funding for open access charge: Universidad de Malaga / CBU

Experimental evaluation of CTOD in constant amplitude fatigue crack growth from crack tip displacement fields

© 2017, Gruppo Italiano Frattura. All rights reserved. In the current work an experimental study of the crack tip opening displacement (CTOD) is performed to evaluate the ability of this parameter to characterise fatigue crack growth. A methodology is developed to measure and to analyse the CTOD from experimental data. The vertical displacements measured by implementing Digital Image Correlation on growing fatigue cracks are used to measure the CTOD. Fatigue tests at R ratios of 0.1 and 0.6 were conducted on compact-tension specimens manufactured from commercially pure titanium. A sensitivity analysis was performed to explore the effect of the position selected behind the crack tip for the CTOD measurement. The analysis of a full loading cycle allowed identifying the elastic and plastic components of the CTOD. The plastic CTOD was found to be directly related to the plastic deformation at the crack tip. Moreover, a linear relationship between da/dN and the plastic CTOD for both tests was observed. Results show that the CTOD can be used as a viable alternative to ΔK in characterising fatigue crack propagation because the parameter considers fatigue threshold and crack shielding in an intrinsic way. This work is intended to contribute to a better understanding of the different mechanisms driving fatigue crack growth and the address the outstanding controversy associated with plasticity-induced fatigue crack closure

A more effective rationalisation of fatigue crack growth rate data for various specimen geometries and stress ratios using the CJP model

publisher: Elsevier articletitle: A more effective rationalisation of fatigue crack growth rate data for various specimen geometries and stress ratios using the CJP model journaltitle: International Journal of Fatigue articlelink: https://doi.org/10.1016/j.ijfatigue.2018.05.027 content_type: article copyright: © 2018 Elsevier Ltd. All rights reserved

Towards a new methodology for the characterisation of crack tip fields based on a hybrid computational approach

This work presents a hybrid optimisation technique for the simultaneous calculation of crack tip characterising parameters and its spatial location, which can significantly affect the characterising parameters if the position used is inaccurate. The hybrid technique combines initial use of a genetic algorithm to obtain a well-conditioned set of initial parameter values that is then passed to an interior point optimisation algorithm for subsequent fast optimisation. Use of the hybrid technique is also amenable to easy automation. The capability of the technique is demonstrated using the CJP crack tip field model, with digital image correlation (DIC) being used to measure the 2D crack tip displacement field. This model was chosen, not only for its demonstrated sensitivity to accuracy in crack tip location, but also for its proven utility in providing effective crack growth correlation in the presence of plasticity-induced shielding across a wide range of growth rates. The results obtained from the hybrid technique are shown to be reliable through comparison with results obtained using other established techniques

Characterization of non-planar crack tip displacement fields using a differential geometry approach in combination with 3D digital image correlation

This paper describes a novel differential geometry method that is used in combination with 3D digital image correlation (3D-DIC) for crack tip field characterization on non-planar (curved) surfaces. The proposed approach allows any of the two-dimensional crack tip field models currently available in the literature to be extended to the analysis of a 3D developable surface with zero Gaussian curvature. The method was validated by analyzing the crack tip displacement fields on hollow thin-walled cylindrical specimens, manufactured from either 304L or 2024-T3 alloy that contained a central circumferential crack. The proposed approach was checked via a comparison between experimentally measured displacement fields (3D-DIC) and those reconstructed from a modified 2D crack tip model (utilizing either 2, 3, or 4 terms of the William's expansion series) and implementing a 3D geometrical correction. Further validation was provided by comparing model-derived stress intensity factors with values provided by empirical correlations

Investigation of effective stress intensity factors during overload fatigue cycles using photoelastic and DIC techniques

© 2018 Elsevier Ltd This work uses DIC and photoelastic techniques to investigate the effect of single overload cycles applied during constant amplitude fatigue. Effective values of the range of stress intensity factor were calculated using the CJP model of crack tip stress and displacement fields, as this model considers both wake contact and compatibility-induced influences on the applied elastic field arising from the plastic enclave generated around a fatigue crack. Values of the effective stress intensity factor are related to the observed crack growth acceleration and retardation. In addition, the paper compares the CJP results with those obtained using a compliance-based method. The present work demonstrates the utility of the CJP model in characterising fatigue crack growth rates during variable amplitude loading. It is also possible with the CJP model, through changes in the coefficient values and hence, for the first time, to shed explicit light on the contributions made by different mechanisms to the shielding effects of an overload

Experimental methodology for the quantification of crack tip plastic zone and shape from the analysis of displacement fields

The current work presents a novel methodology for the experimental quantification of the crack tip plastic zone during fatigue crack growth. This methodology is based on the application of yield criteria to estimate the area and the shape of the plastic zone at the crack tip. The implementation of the proposed methodology requires the use of strain maps calculated from the differentiation of the displacement fields obtained by digital image correlation (DIC). Stress maps can subsequently be inferred from both von Mises and Tresca yield criteria. Fatigue tests and associated measurements of plastic zone size and shape were conducted on a compact-tension specimen made from commercially pure titanium at R ratio of 0.6. In addition, the ability to predict the shape and size of the experimentally observed crack tip plastic zone has been explored using three different analytical elastic crack tip models [Westergaard, Williams and Christopher-James-Patterson (CJP)]. This analysis indicated that the CJP model provided the most accurate prediction of the plastic zone and shape