Crack front waves in dynamic fracture

A rapidly moving tensile crack is often idealized as a one-dimensional object moving through an ideal two-dimensional material, where the crack tip is a singular point. When a material is translationally invariant in the direction normal to the crack's propagation direction, this idealization is justified. A real tensile crack, however, is a planar object whose leading edge forms a propagating one-dimensional singular front (a `crack front'). We consider the interaction of a crack front with localized material inhomogeneities (asperities), in otherwise ideal brittle amorphous materials. We review experiments in these materials which indicate that this interaction excites a new type of elastic wave, a front wave, which propagates along the crack front. We will show that front waves (FW) are highly localized nonlinear entities that propagate along the front at approximately the Rayleigh wave speed, relative to the material. We will first review some of their characteristics. We then show that by breaking the translational invariance of the material, FW effectively act as a mechanism by which initially `massless' cracks acquire inertia.     

Crack front trapping in transformation-toughened ceramics

A theoretical study is made of the trapping of a crack front in brittle matrix ceramic reinforced by dispersed transformable zirconia particles. The trapped discontinuous crack front is modelled by a collinear array of microcracks interspersed by transformable particles. The effect of trapping of the macrocrack front is approximated by a closing pressure on a bridged zone. The maximum closing pressure and the critical opening of the macrocrack are related to the volume (area) fraction and size of the transformable particles. It is found that the maximum toughening due to trapping occurs at moderate volume (area) fractions of large particles. This is in agreement with available theoretical and experimental results.     

Crack border stress and displacement equations revisited

It is more or less accepted in fracture mechanics that the elastic stress and displacements very near to the tip of a plane line crack can be approximated with sufficient accuracy, for all geometries and outer boundary loading conditions, by. a one-parameter representation, i.e. strictly in terms of the stress intensity factors K_I and/or K_II. It is shown here that this presumption which appears to be reasonable on face value, quantitatively speaking, is nevertheless unacceptable as a general proposition. The reason lies with the quite arbitrary practice of omitting the second term of the series representation for the stresses, a contribution which is independent of distance from the crack tip. It is not difficult to show by way of specific examples how such omission can lead to error of serious qualitative nature in the prediction of stress and displacement related quantities of interest.     

Crack front elastic stress state for three-dimensional crack problems

The problems inherent in the estimation of fracture mechanics parameters for three-dimensional crack problems are reviewed. A novel parameter is introduced to quantify the loss of constraint along that portion of a crack front adjacent to a free surface. Finite element analyses of a compact tension specimen with a straight crack front indicate the usefulness of this parameter in studying the influence of Poisson's ratio and plate thickness on the extent of the free surface boundary layer effect. A method for predicting the shape of a natural crack in this specimen is also presented.

---

Résumé On passe en revue les problèmes inhérents à l'estimation des paramètres de mécanique de rupture dans le cas des fissures tridimensionnelles. Afin de quantifier l'absence de bridage qui caractérise la portion du front de fissure qui est adjacente à une surface libre, on a introduit un nouveau paramè00E8;tre. L'analyse par éléments finis d'une éprouvette de traction compacte présentant un front de fissure droit indique l'utilité de ce paramètre pour l'étude de l'influence du module de Poisson et de l'épaisseur d'une tôle sur l'effet de couche de bord caractérisant la surface libre. On présente également une méthode de prédiction de la forme d'une fissure naturelle dans ce type d'éprouvette.

     

Crack front curvature: Influence and effects on the crack tip fields in bi-dimensional specimens

Crack front curvature is evidence in most experimental crack advance test. When classical linear elastic fracture mechanic theory deals with bi-dimensional crack configurations, it ignores the three-dimensional effects of crack propagation. Issues as the influence of the specimen thickness and the crack front curvature are not considered. Previous numerical studies have shed light on out-of-plane plastic zone development or stress state. Nevertheless, these numerical studies are based on the assumption that the crack front is ideally straight; despite it is well known that the crack front has some kind of curvature. In the present work, a CT aluminium specimen has been modelled three-dimensionally and several calculations have been made considering a huge combination of different single load levels, specimen thicknesses and crack front curvatures. Due to the abrupt transition from plane strain to plane stress, an ultrafine mesh along the thickness has been applied. The analysis of the evolution of the plastic zone and the stress state along the thickness provides information about the combined influence of these parameters on fracture mechanics.     

On the influence of diffusion-thermal effects on the development of perturbations of the filtration combustion front of gases

It has been shown that the filtration combustion front of gases is resistant to small perturbations at any asymmetry between the diffusion and thermal transport. A qualitative analysis of the perturbation development with time confirming that the diffusion-thermal asymmetry does not influence the front stability has been carried out. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 2, pp. 48–55, March–April, 2006.     

Calibration of the potential drop method for monitoring small crack growth from surface anomalies – Crack front marking technique and finite element simulations

The Direct Current Potential Drop method is one of the possibilities to measure the crack growth during fatigue tests without optical access to the sample. The accuracy of this technique applied to short cracks mainly depends on the calibration curve. In the present work experimental and numerical approaches are proposed to calibrate the potential drop measurement. An optimization of the calibration procedure is supported by finite element calculations. The crack front shape and the location of the potential probes are found to be of great influence. An extensive study is conducted to identify all the mechanisms controlling the potential drop measurement. Plastic deformation remains the last parameter which is not directly considered in this study; therefore a calibration strategy is suggested to take into account this effect and achieve a calibration curve with high accuracy. This strategy shows a very good agreement between the obtained calibration curve and the experimental calibration.     

Platform perturbations in above-water radiometry

A comparison of above- and in-water spectral measurements in coastal (but predominantly Case-1) conditions has shown that the uncertainty in above-water determinations of water-leaving radiances made from an offshore tower depends on the proximity of the above-water measurement with respect to the side of the platform. For purposes of this study the proximity of the sampling platform is parameterized as the perpendicular distance (denoted x) from the side of the sampling platform to the center of the area on the sea surface observed by the sea-viewing sensor, the so-called surface spot, which is set by the field of view of the radiometer (or the overlapping fields of view of a multiaperture sensor). Two above-water data processing methods were used to create a diagnostic variable (formulated for Case-1 waters only but also applicable to Case-2 conditions over short time scales) to quantify the presence of superstructure reflections. Based on the height of the tower, H, the analyses were partitioned into near- and far-field data sets (x < H and x > H, respectively). The primary conclusions of the radiometric intercomparisons are as follows: (a) the maximum perturbations occur very close to the tower ( x/H < 1), and, as x/H increases and approaches 1 (i.e., as the surface spot becomes as far away as the platform is high), the platform perturbations converge toward smaller and smaller values, and (b) within the far field (x > H) the platform perturbation is negligible, and a remote sensing 5% absolute accuracy objective can be satisfied.