Crack closure

About: Crack closure is a research topic. Over the lifetime, 28157 publications have been published within this topic receiving 588158 citations.

Depth and spacing of tension cracks

Abstract: Contraction cracks in basalt, permafrost, and mud, and crevasses in glaciers are examples of geological phenomena that might be studied by reference to a theoretical model of tension cracks in a semi-infinite solid. The effect of the crack in relieving stress at the ground surface bears on the problem of crack spacing, and the rate of energy dissipation at the advancing crack tip bears on the problem of crack depth. Even though the stresses that cause cracking develop slowly, an elastic model of the stress near a crack can be useful as long as the cracks, once initiated, propagate rapidly. Results are presented for the elastic stress perturbation caused by a crack in an infinite or semi-infinite medium in which the initial stress is a step function or a linear function of depth. Tables and graphs are presented which can be applied directly to problems in which the variation of stress with depth is arbitrary. These results, used with the modified Griffith theory of macroscopic fracture introduced by Irwin [1948] and Orowan [1950], suggest a means of predicting depth and spacing of tension cracks in terms of the stress field and measurable properties. The method is illustrated with a discussion of cooling joints in basalt, and other problems of tension fracture in geology.

The activation energy for dislocation nucleation at a crack

Abstract: THE ACTIVATION energy for dislocation nucleation from a stressed crack tip is calculated within the Peierls framework, in which a periodic shear stress vs displacement relation is assumed to hold on a slip plane emanating from the crack tip. Previous results have revealed that the critical G (energy release rate corresponding to the “screened” crack tip stress field) for dislocation nucleation scales with y., (the unstable stacking energy), in an analysis which neglects any coupling between tension and shear along the slip plane. That analysis represents instantaneous nucleation and takes thermal effects into account only via the weak temperature dependence of the elastic constants. In this work, the energy required to thermally activate a stable, incipient dislocation into its unstable “saddle-point” configuration is directly calculated for loads less than that critical value. We do so only with the simplest case, for which the slip plane is a prolongation of the crack plane. A first calculation reported is 2D in nature, and hence reveals an activation energy per unit length. A more realistic scheme for thermal activation involves the emission of a dislocation loop, an inherently 3D phenomenon. Asymptotic calculations of the activation energy for loads close to the critical load are performed in 2D and in 3D. It is found that the 3D activation energy generally corresponds to the 2D activation energy per unit length multiplied by about 5 -10 Burgers vectors (but by as many as 17 very near to the critical loading). Implications for the emission of dislocations in copper. K-iron, and silicon at elevated temperature are discussed. The effects of thermal activation are very significant in lowering the load for emission. Also, the appropriate activation energy to correspond to molecular dynamics simulations of crack tips is discussed. Such simulations, as typically carried out with only a few atomic planes in a periodic repeat direction parallel to the crack tip. are shown to greatly exaggerate the (aheddy large) effects of temperature on dislocation nucleation. WE BUILD on recent advances in the modeling of dislocation nucleation at a crack tip

Evaluation of Methods for Determining Crack Initiation in Compression Tests on Low-Porosity Rocks

Abstract: Laboratory testing of rocks is traditionally carried out to determine the peak strength using the ISRM Suggested Methods or other suitable standards. However, it is well known that in low-porosity crystalline rocks there are at least three distinct stages of compressive loading that can be readily identified if the stress–strain response is monitored during the loading process: (1) crack initiation, (2) unstable crack growth, i.e., crack coalescence and (3) peak strength. Crack initiation is noted as the first stage of stress-induced damage in low-porosity rocks, yet the suggested guidelines of the ISRM for compression tests make no mention of crack initiation. In addition, recent research suggests that crack initiation can be used as an estimate for the in situ spalling strength, commonly observed around underground excavations in massive to moderately jointed brittle rocks. Various methods have been proposed for identifying crack initiation in laboratory tests. These methods are evaluated using ten samples of Aspo Diorite and the results are compared with a simplified method, lateral strain response. Statistically, all methods give acceptable crack-initiation values. It is proposed that the ISRM Suggested Methods be revised to include procedures suitable for establishing the crack-initiation stress.

Fatigue crack propagation behaviour of short cracks; the effect of microstructure

Abstract: — –Fatigue cracks shorter than some critical length tend to propagate anomalously quickly. This paper examines the concept of a ‘critical length’, identifying three regimes of behaviour for different crack lengths. Some published work is examined, covering a wide range of different materials. It is concluded that there is an approximate correlation between the critical length for short crack behaviour and the scale of the microstructure. LEFM is difficult, if not impossible, to apply to cracks shorter than this critical length because the material surrounding a crack cannot be assumed to approximate to a homogeneous continuum. Suggestions are made for a fatigue design philosophy which incorporates short crack behaviour.