Fatigue crack growth in thin notched woven glass composites under tensile loading. Part II: modelling

Fatigue propagation of a through-the-thickness crack in thin woven glass laminates is difficult to model when using homogeneous material assumption. Crack growth depends on both the fatigue behaviour of the fibres and of the matrix, these two phenomena occurring at different time and space scales. The developed finite element model is based on the architecture of the fabric and on the fatigue behaviours of the matrix and the fibre, even if the pure resin and fibre behaviours are not used. That thus limits the physical meaning of this model. Basically, the objective of this simulation is to illustrate and to confirm proposed crack growth mechanism. The fatigue damage matrix is introduced with user spring elements that link the two fibre directions of the fabric. Fibre fatigue behaviour is based on the S-N curves. Numerical results are compared to experimental crack growth rates and observed damage in the crack tip. Relatively good agreement between predictions and experiments was found

Correlation between the pivot node concept and fatigue crack closure

The study of fatigue crack growth has been commonly done by means of bi-dimensional models and assuming a homogeneous behaviour through the thickness. According to the specimen thickness, a state of plane stress or plane strain is presumed. However, recently, it has been shown that thickness effects influence the crack tip behaviour. These works have revealed a series of effects along the thickness with a strong influence on the crack front growth. One of the experimental evidences that can be explained as a direct consequence of these effects is the curvature of the crack. It is observed that when the crack advance, the crack front changes adopting a curved shape, growing faster at the interior than at the exterior. Two mechanisms can explain this effect: the first one is related to the crack closure effect near the surface. The second one, related to the plastic zone size decrement observed in a small region close to the surface, is due to ΔK being smaller near the surface than in the interior. Both mechanisms are difficult to evaluate separately. A series of works were devoted to study these effects. A research line has been focused in the analysis of the stress intensity factor distribution. These works evaluate the finite element model of an Al 2024-T35 compact tension specimen with no plastic wake effect introduced, according to the methodology developed by the authors. The three-dimensional behaviour in the vicinity of the crack front is simulated through numerical analysis with ANSYS code and J-integral method is used to determinate the curves of K evolution along the thickness. The main finding of these studies is that the distribution of K is not homogeneous. The overall values for the whole model accurately agree with the nominal K applied. The K profiles along the thickness are characterized by a series of parameters that allow us to analyze the distribution of K in terms of the expected Knom against variations of geometrical and external factors.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Fractoluminescence characterization of the energy dissipated during fast fracture of glass

Fractoluminescence experiments are performed on two kinds of silicate glasses. All the light spectra collected during dynamic fracture reveal a black body radiator behaviour, which is interpreted as a crack velocity-dependent temperature rise close to the crack tip. Crack velocities are estimated to be of the order of 1300 m.s$^{-1}$ and fracture process zones are shown to extend over a few nanometers.Comment: Accepted for publication in Europhysics Letters; 5 pages; 4 figure

Simulation of Near-Tip Crack Behaviour and Its Correlation to Fatigue Crack Growth with a Modified Strip-Yield Model

A modified strip-yield model has been developed to simulate the plasticity-induced crack closure under the constant amplitude (CA) and a single overload loading conditions. The paper focuses on the simulation of the near tip crack profiles and stress distributions during the fatigue process. Detailed information on near-tip stress and displacement fields at the maximum load (Pmax), the minimum load (Pmin), and the crack opening load (Pop) of a fatigue load cycle have been presented. The correlation of the crack closure to the near-tip material fatigue damage has been investigated and used to rationalize the crack growth behaviour under the CA and a single overload loading conditions.Peer reviewedSubmitted Versio

Atomistic Representation of Anomalies in the Failure Behaviour of Nanocrystalline Silicene

Silicene, a 2D analogue of graphene, has spurred a tremendous research interest in the scientific community for its unique properties essential for next generation electronic devices. In this work, for the first time, we present a molecular dynamics (MD) investigation to determine the fracture strength and toughness of nanocrystalline silicene (nc silicene) sheet of varied grain size and pre existing crack length at room temperature. Our results suggest that the transition from an inverse pseudo Hall Petch to a pseudo Hall Petch behavior in nc silicene occurs at a critical grain size of 17.32 nm. This phenomenon is also prevalent in nanocrystalline graphene. However, nc silicene with pre existing cracks exhibits anomalous crack propagation and fracture toughness behaviour. We have observed two distinct types of failure mechanisms (crack sensitive and insensitive failure) and devised the mechanophysical conditions under which they occur. Fracture toughness calculated from both Griffiths theory and MD simulations indicate that the former overpredicts the fracture toughness of nc silicene. The most striking outcome, however, is that despite the presence of a pre existing crack, the crack sensitivity of nc silicene is found to be dependent on the grain size and their orientations. This study is the first direct comparison of atomistic simulations to the continuum theories to predict the anomalous behaviour in deformation and failure mechanisms of nc silicene

Quasicontinuum simulation of fracture at the atomic scale

We study the problem of atomic scale fracture using the recently developed quasicontinuum method in which there is a systematic thinning of the atomic-level degrees of freedom in regions where they are not needed. Fracture is considered in two distinct settings. First, a study is made of cracks in single crystals, and second, we consider a crack advancing towards a grain boundary (GB) in its path. In the investigation of single crystal fracture, we evaluate the competition between simple cleavage and crack-tip dislocation emission. In addition, we examine the ability of analytic models to correctly predict fracture behaviour, and find that the existing analytical treatments are too restrictive in their treatment of nonlinearity near the crack tip. In the study of GB-crack interactions, we have found a number of interesting deformation mechanisms which attend the advance of the crack. These include the migration of the GB, the emission of dislocations from the GB, and deflection of the crack front along the GB itself. In each case, these mechanisms are rationalized on the basis of continuum mechanics arguments

Lifetime evaluation of concrete structures under sustained post-peak loading

Experimental tests on crack propagation in concrete under constant post-peak loading are simulated using the finite element method and the cohesive crack model, in both Mode I and Mixed-mode conditions. The time-dependent behaviour of concrete in the process zone is due to the interaction and growth of microcracks, a phenomenon which, for high constant load levels, turns out to be predominant over linear viscoelastic creep in the bulk material. In mechanical systems based on this type of material behaviour (creep and strain-softening taking place simultaneously), the initial value problem is non-parabolic, i.e., the error at one time level is affected by the accumulation of errors introduced at earlier time levels. Despite these difficulties, the scatter in numerical failure lifetime vs. load level turns out to be negligible in Mode I conditions and practically acceptable in Mixed-mode conditions. Therefore the time-dependent behaviour of the process zone can be inferred solely from the results of direct tensile tests

Development and validation of a high constraint modified boundary layer finite element model

When a notched structure is loaded, its behaviour is not only affected by the material properties but also by the geometry (of both the structure and the defect) and loading condition, alternatively termed as constraint condition. Therefore, the relation between the failure behaviour of a small scale fracture mechanics test and a full scale structure needs to be elucidated. In an attempt to understand and describe such relationships, the crack tip stress fields are analysed by means of finite element simulations and compared for several test specimen geometries. A reference for comparison is the crack tip stress field obtained from a high constraint reference geometry, further called a modified boundary layer model. First, this article provides some theoretical background on the modified boundary layer model. Second, the development of a 2D model is outlined in detail, focussing on the mesh design in the vicinity of the crack tip and the applied boundary conditions. Afterwards, an analytical and numerical validation is provided, based on the level of the applied load and, on the other hand, on the magnitude of the crack tip stress fields. Finally, this validated model is used for the comparison of several constraint parameters. This comparison indicates a weak influence of the T-stress on the Q-parameter for positive T-stresses. In contrast, negative T-stresses result in more pronounced negative Q-values

Comparison of the crack pattern in accelerated corrosion tests and in finite elements simulations

In this work, the crack pattern obtained in accelerated corrosion tests is compared to the one obtained in numerical simulations for reinforced steel concrete samples. In the simulations, an expansive joint element is used to simulate the oxide layer behaviour together with finite elements with embedded adaptable cohesive crack to simulate the concrete fracture. In parallel, some samples are artificially corroded imposing constant current and after corrosion they are impregnated with resin containing fluorescein to improve the detection of the cracks. In the paper, the main features of the model and the experimental procedure are described and the crack pattern is analysed. A main crack across the concrete cover is easily seen in both cases, but also secondary cracks are observed after treating the concrete surface, in accordance with the model predictions, which gives further support to the ability of the numerical approach to simulate the real cracking processes