Crack closure

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

Enhanced crack closure performance of microbial mortar through nitrate reduction

Abstract: Microbial self-healing of concrete has been widely investigated, yet the suggested microbial pathways are limited to ureolysis and the aerobic oxidation of carbon sources. Each of these pathways has certain environment and process related drawbacks which arise a need for an alternative pathway to proceed further. This study presents the NO 3 − reduction as an alternative microbial self-healing strategy. In the tests, we used previously described NO 3 − reducing bacteria, and two different porous protective carriers. The highest crack width healed by the bacteria was 370 ± 20 μm in 28 days and 480 ± 16 μm in 56 days. Water tightness regain up to 85% was achieved at the end of 56 days for 465 ± 21 μm crack width. Precipitates were identified as forms of CaCO 3 and were abundant in microbial specimens particularly on the inner crack surface. The findings evidence the potential of the NO 3 − reduction pathway for development of microbial self-healing concrete.

Factors Affecting Crack Initiation in Low Porosity Crystalline Rocks

Abstract: Crack initiation in uniaxial compressive loading of rocks occurs well before the peak strength is reached. The factors that may influence the onset of cracking and possible initiating mechanisms were explored using a discrete element numerical approach. The numerical approach was based on grain-based model that utilized the Voronoi tessellation scheme to represent low porosity crystalline rocks such as granite. The effect of grain size distribution (sorting coefficient ranging from 1.5 to 1.03), grain size (average grain size ranging from 0.75 to 2.25 mm), and the heterogeneities of different mineral grains (quartz, K-feldspar, plagioclase) on the onset of cracking were examined. The modelling revealed that crack initiation appears to be a tensile mechanism in low porosity rocks, and that shear cracking along grain boundaries is only a prominent mechanism near the peak strength. It was also shown that the heterogeneity introduced by the grain size distribution had the most significant effect on peak strength and crack initiation stress. The peak strength ranges from 140 to 208 MPa as the grain size distribution varies from heterogeneous to uniform, respectively. However, the ratio of crack initiation to peak stress showed only minor variation, as the heterogeneity decreases. The other factors investigated had only minor effects on crack initiation and peak strength, and crack initiation ratio.

Surface roughening and branching instabilities in dynamic fracture

Abstract: W hy are most experimentally observed terminal fracture speeds only around half of the theoretically predicted value, i.e. the Rayleigh wave speed c R ? A wavy-crack model to be discussed in this paper gives a short answer to the above question: at high speeds, cracks tend to propagate along a wavy fracture path so that the apparent crack velocity can be maintained at 0.5 c R in order to maximize the time-rate of energy being absorbed into the fracture process. The wavy-crack model is motivated by experimental observations that rapidly moving cracks develop roughened fracture surfaces. The essence of that model is to separate the microscopic crack-tip motion with local velocity v c from the macroscopically observable crack motion with apparent velocity v a . From the macroscopic point of view, the energy going into the fracture process per unit time per unit length of the crack front is approximately Γ a = (1- v a / c R ) v a G * a , where G * a denotes the quasi-static energy release rate. By propagating along a wavy path, a crack is able to maintain its apparent velocity v a at 0.5 c R to maximize Γ a , while it may locally propagate at a significantly higher velocity in accordance with the local energy balance between the crack driving force and the material resistance to fracture. Much theoretical investigation on the wavy-crack problem and applications remains to be done in future work. For present purposes, a perturbation analysis is used to gain some preliminary insights, particularly on issues regarding the stability of a mode I fracture path during dynamic crack propagation. The perturbation results are supportive of the notion that wavy fracture paths become favorable at high crack speeds, that the apparent crack which moves only at around 0.5 c R favors a mode I path and tends to suppress a branching tendency, and that the local crack tip motion with significantly higher velocity promotes crack branching. Discussions on various aspects of dynamic fracture indicate that the wavy-crack model is capable of explaining important discrepancies currently existing between theory and experiments. In particular, analyses indicate that the basic mechanism of dynamic crack branching is somewhat like a thermally activated kinetic process. The fracture energy supplied from the applied loads acts as the driving force, the high inertia, branch-promoting local crack tip field acts as a nucleation source of microbranches and the relatively low inertia, branch-suppressing apparent crack sets an energetic barrier for macroscopic branching. This energy barrier is controlled by the macroscopic non-singular T a stress, in that it may be increased by a more compressive T a and decreased by a less compressive T a .

Mode II stress intensity factors for a crack parallel to the surface of an elastic half-space subjected to a moving point load

Abstract: The direction of propagation of rolling contact fatigue cracks is observed to depend upon the direction of motion of the load. In this paper approximate calculations are described of the variation of Mode II stress intensity factors at each tip of a subsurface crack, which lies parallel to the surface of an elastic half-space, due to a load moving over the surface. In particular the effect of frictional locking of the crack faces under the load is investigated. In consequence of frictional locking the range of SIF at the trailing tip ΔKT is found to be about 30% greater than that of the leading tip ΔKL, which is consistent with observations that subsurface cracks propagate predominantly in the direction of motion of the load over the surface. The effects on kt and klof crack length, crack face friction, traction forces at the surface and residual shear stresses are also investigated.