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Crack closure concept has been widely used to explain different issues of fatigue crack propagation. However, different 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; iv) the analysis of remote compliance is the best numerical parameter to quantify the crack
opening level; v) without contact there is no effect of stress ratio on crack tip parameters. Therefore it is proved that the crack
closure concept is valid.
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.
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.
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.
In this work, the effect of single overloads on plasticity induced crack closure is studied. An elastic-plastic
finite element model was developed and the crack opening level was calculated from the contact forces
along the crack flank. The effects of the loading parameters and stress state are analysed, and the mechanisms
behind crack closure variations are identified. An overload is a traumatic event that eliminates
material’s memory relative to the load history. Crack tip blunting is the mechanism behind this memory
loss, since it eliminates crack closure. Material hardening has a major relevance on the evolution of plastic
blunting, which was evident in the variation of the CTOD parameter. On the other hand, the overload
produces strong plastic deformation ahead of the crack tip, giving rise to conditions for the rapid generation
of crack closure higher than before the event. The peak of crack closure was found to increase linearly
with the load increase above the maximum baseline value. The crack is totally closed for overload
ratios of about 2.5. Empirical models were developed for the peak of crack closure, for the delay of this
peak and for the stabilization distance after the overload. Finally, the stress state was found to have a
major effect on crack closure level after an overload.