E. R. de los Rios

Analysis of the effects of controlled shot peening on fatigue damage of high strength aluminium alloys

The use of two micro-mechanical models for notch sensitivity and fatigue life allowed the development of boundary conditions that would evaluate potential life improvement after controlled shot peening (CSP) in high strength aluminium alloys. The boundary conditions describe the state of equal weight between surface roughening and residual stresses and the implication of material and loading parameters. From the boundary conditions, the performance of CSP on crack arrest and fatigue life can be investigated.

Fatigue Crack Growth Modelling by Successive Blocking of Dislocations

Work hardening and the study of instability is incorporated into the description of the growth of a crack in terms of the successive blocking of the plastic zone by slip barriers, such as grain boundaries, and the subsequent initiation of the slip in neighbouring grains. A simple equation is derived to determine the critical position of the crack tip in relation to the grain boundary where the plastic zone is blocked at the moment of slip transmission. The intermittent pattern of decelerating and accelerating behaviour of short cracks and the existence of non-propagating cracks is explained. Instability in crack growth is seen to occur when the rate of hardening is insufficient to compensate for the increase in crack driving force in relation to the increase in crack length. This is associated with fracture toughness. The transition point between the short and long crack régimes is seen to occur when the size of the plastic zone is of the order of the microstructural parameter.

An alternative model of the blocking of dislocations at grain boundaries

Abstract The blocking of dislocations at a slip barrier, such as a grain boundary, is modelled by considering, in the domain of distribution of dislocations ahead of the slip band, a small zone representing a locked source or the width of the boundary itself. The ‘friction stress’ in this small zone is calculated so that no infinite stresses appear in the analysis. Hall–Petch type relationships are derived for the yield stress and the fatigue limit. When the length of the additional zone is sufficiently small in comparison to the total length of the array, the solution is very close to the one obtained by the common modelling of the block by an infinite delta-type function.

Effect of controlled shot peening and laser shock peening on the fatigue performance of 2024-T351 aluminum alloy

The influence of shot peening, laser shock peening, and dual (shot and laser peening) treatment on the fatigue behavior of 2024-T351 was investigated. Tests showed a fatigue life improvement in all three cases with laser shock peening and dual treatment displaying fatigue performance superior to shot peening. Fractographic analysis showed that the relatively poor performance of the shot peening is caused by ductility loss.

A two-stage micromechanics model for short fatigue cracks

Abstract The influence of microstructure on the propagation behavior of short surface cracks is examined in a simulated HAZ microstructure in C Mn steel. The detection of crack initiation and crack growth measurement is carried out by the replication of hourglass specimens under torsional and push-pull loading. The length of ferrite at the prior austenite grain boundaries is characterized as the effective microstructural parameter. Propagation rate is affected by the microstructure; it decreases when the crack approaches the grain boundary and increases again when it crosses the grain boundary. The short crack growth models proposed by other researchers are critically examined and a modified model based on micromechanics is presented to incorporate the effect of microstructure on the rate of crack growth. The crack growth behavior predicted by the model matches closely with the experimental results.

Compact solution for a multizone BCS crack model with bounded or unbounded end conditions

Abstract Solutions for the distribution of dislocations representing a crack have previously been obtained both for the instances in which the stress field is totally bounded (Bilby-Cottrell-Swinden), and also where the leading dislocations are blocked (Taira-Tanaka-Nakai). This work evaluates a compact formula incorporating both solutions by considering a rectilinear crack transversing an arbitrary number of zones with different friction stresses. Symmetry is not required. The expressions for the dislocation density and the plastic displacement, as well as the stress distribution in the plane of the crack, are obtained.

Experimental study and modelling of short fatigue crack growth in aluminium alloy Al7010-T7451 under random loading

Abstract This paper presents work on short crack initiation and growth behaviour under random loading conditions. Newmans crack closure model was extended to include the influence of grain boundary on crack opening level by introducing the concept of constrained crack tip plasticity by grain boundary blocking. A dislocation-based crack growth model (the Navarro–Rios model) was used in conjunction with a crack closure model for lifetime prediction under random loading conditions. Crack growth tests were performed using hourglass shaped specimens of Al7010-T7451 under constant amplitude loading as well as a variety of random loading cases representing the load spectra encountered in various flight missions. A good agreement (within 14% difference) was achieved between the model results and the experimental data. Crack initiation and growth behaviour for constant-amplitude and random loading conditions was also examined by use of plastic replicas and SEM observations.

Predicting the interfaces between fatigue crack growth regimes in 7150-T651 aluminium alloy using the fatigue damage map

Abstract The fatigue damage map (FDM) is used to establish the domains of different crack growth regimes including microstructural dependent (short), microstructural independent (long) and non-propagating cracks. The FDM is applied over a wide range of applied stresses to determine the extent of each domain from crack instability to crack arrest. Two important boundaries of the FDM are examined in this paper—the crack arrest curve and the transition from short to long crack growth. The accuracy of the model predictions are evaluated through crack arrest experiments and fractographic examination of failure cracks.

Theoretical analysis on the behaviour of short fatigue cracks

Abstract Based on the Navarro–Rios model, the boundaries of short crack growth are predicted. Theoretical analysis reveals that the extent of short crack growth regime is principally governed by the relation between the fatigue limit and the cyclic yield stress of the material. Materials with low values of σ FL / σ cy show extensive short crack behaviour, while in material with high values of σ FL / σ cy short crack behaviour is very limited. The results of the analysis are validated with experimental data.

MODELLING MICROSTRUCTURALLY SENSITIVE FATIGUE SHORT CRACK GROWTH

Microstructurally sensitive fatigue short crack growth can occur in many engineering components devoid of large defects. Continuum mechanics principles, including linear elastic fracture mechanics, used in damage tolerance design and life prediction methods are not applicable in these situations and therefore new concepts need to be developed to characterize this type of growth. A microstructurally sensitive model of fatigue crack growth is presented in which the effect of microstructure is dominant in the early stage of growth but plays a negligible role after the crack has gone through the transition from structure-sensitive to structure-insensitive growth. The effect of both microstructure and structure sensitive variables on the transition from short cracks to continuum mechanics and the conditions for crack instability leading to final failure are examined. The microstructural variables incorporated in the equations that describe the model are those controlling the extent and intensity of crack tip plasticity such as grain size, precipitation and dispersion hardening, strain hardening and misorientation between grains. It is expected that the concepts developed within the model will form the basis for the design of new crack-resistant materials.