Numerical modelling of fatigue crack propagation

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Publications

A parameter for quantitative analysis of plasticity induced crack closure

Publication . Antunes, F.V.; Correia, Luís M.; Ramalho, Armando

Numerical models have been successfully developed to predict plasticity induced crack closure (PICC). However, despite the large research effort a full understanding of the links between physical parameters, residual plastic wake and PICC has not been achieved yet. The plastic extension of material behind crack tip, Dyp, obtained by the integration of vertical plastic deformation perpendicularly to crack flank, is proposed here to quantify the residual plastic field. The values of Dyp and PICC were obtained numerically in a M(T) specimen using the finite element method. An excellent correlation was found between PICC and Dyp which indicates that this parameter controls the phenomenon, and can be used to quantify the effect of physical parameters. An empirical model was developed to predict PICC assuming that the residual plastic field is a set of vertical plastic wedges, that the linear superposition principle applies and that the influence of a particular wedge exponentially decreases with distance to crack tip. The model was applied successfully to predict PICC for different residual plastic fields which provided an additional validation of Dyp as the parameter controlling PICC.

2015Journal article

Open access

Effect of compressive loads on plasticity induced crack closure

Publication . Antunes, F.V.; Correia, Luís M.; Camas, D.; Branco, R.

Compressive stresses play an important role on tension-compression fatigue which can be attributed to plasticity induced crack closure (PICC). The objective here is to study numerically the effect of compressive stresses on PICC and to discuss the applicability of PICC to explain the effect of negative stress ratios on fatigue crack growth rate. The compression produces reversed plastic deformation at the crack tip, reducing linearly the crack opening level. The incursion to negative stress ratios did not produce sudden changes in the behavior of PICC and no saturation with the decrease of minimum load was observed for δKeff. Crack closure was able to collapse da/dN-δK curves with negative stress ratios, indicating the applicability of the crack closure concept to explain the effect of negative R. The analysis of crack tip plastic strain range with and without contact of crack flanks confirmed the validity of crack closure concept.

2015Journal article

Open access

Effect of crack closure on non-linear crack tip parameters

Publication . Antunes, F.V.; Sousa, T.; Branco, R.; Correia, Luís M.

Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, some authors have questioned the relevance of crack closure and have proposed alternative concepts. The main objective here is to check the effectiveness of crack closure concept by linking the contact of crack flanks with non-linear crack tip parameters. Accordingly, 3D-FE numerical models with and without contact were developed for a wide range of loading scenarios and the crack tip parameters usually linked to fatigue crack growth, namely range of cyclic plastic strain, crack tip opening displacement, size of reversed plastic zone and total plastic dissipation per cycle were investigated. It was demonstrated that: (i) LEFM concepts are applicable to the problem under study; (ii) the crack closure phenomenon has a great influence on crack tip parameters decreasing their values; (iii) the ΔKeff concept is able to explain the variations of crack tip parameters produced by the contact of crack flanks; and (iv) the analysis of remote compliance is the best numerical parameter to quantify the crack opening level. Therefore the crack closure concept seems to be valid. Additionally, the curves of crack tip parameters against stress intensity factor range obtained without contact may be seen as master curves.

2015Journal article

Open access

Plasticity induced crack closure: a sensitivity analysis

Publication . Correia, Luís M.; Antunes, F.V.; Ramalho, Armando

Plasticity induced crack closure (PICC) is closely linked to the monotonic and reversed plastic deformation occurring at the crack tip. The objective of the paper is to identify the different physical and numerical parameters affecting PICC, and develop a sensitivity analysis to quantify their relative importance. The main parameters affecting PICC are the load parameters, the yield stress, the size of finite elements and the numerical parameter considered to quantify PICC. The numerical predictions should be independent of numerical parameters, therefore further work is required to optimize the numerical models

2012Conference object

Open access

Finite element meshes for optimal modelling of plasticity induced crack closure

Publication . Antunes, F.V.; Camas, D.; Correia, Luís M.; Branco, R.

The main goal here is to optimise the finite element mesh used to predict plasticity induced crack closure (PICC). A numerical model was developed for a M(T) specimen made of 6016-T4 aluminium alloy. The parameters studied were the size of most refined region perpendicularly to crack flank (ym) and along propagation direction (xr), the size of finite elements near crack tip (L1) and the vertical size of refinement close to crack flank (yA/B). A maximum size of about 1.3mm was found for ym, but a smaller value has a limited impact on PICC. An analytical expression was proposed for xr, dependent on δK and Kmax. An optimum value seems to exist for L1.

2015Journal article

Open access