Showing papers on "Crack closure published in 2018"

Mechanics Of Fatigue

Abstract: Mechanics of Fatigue addresses the range of topics concerning damage, fatigue, and fracture of engineering materials and structures. The core of this resource builds upon the synthesis of micro- and macro-mechanics of fracture. In micromechanics, both the modeling of mechanical phenomena on the level of material structure and the continuous approach are based on the use of certain internal field parameters characterizing the dispersed micro-damage. This is referred to as continuum damage mechanics.The author develops his own theory for macromechanics, called analytical fracture mechanics. This term means the system cracked body - loading or loading device - is considered as a mechanical system and the tools of analytical (rational) mechanics are applied thoroughly to describe crack propagation until the final failure.Chapter discuss:preliminary information on fatigue and engineering methods for design of machines and structures against failures caused by fatiguefatigue crack nucleation, including microstructural and continuous modelstheory of fatigue crack propagationfatigue crack growth in linear elastic materials subject to dispersed damagefatigue cracks in elasto-plastic material, including crack growth retardation due to overloading as well as quasistationary approximationfatigue and related phenomena in hereditary solidsapplication of the theory fatigue crack growth considering environmental factorsunidirectional fiber composites with ductile matrix and brittle, initially continuous fiberslaminate compositesMechanics of Fatigue serves students dealing with mechanical aspects of fatigue, conducting research in fracture mechanics, structural safety, mechanics of composites, as well as modern branches of mechanics of solids and structures.

Estimation of Crack Initiation and Propagation Thresholds of Confined Brittle Coal Specimens Based on Energy Dissipation Theory

Abstract: A new energy-dissipation method to identify crack initiation and propagation thresholds is introduced. Conventional and cyclic loading–unloading triaxial compression tests and acoustic emission experiments were performed for coal specimens from a 980-m deep mine with different confining pressures of 10, 15, 20, 25, 30, and 35 MPa. Stress–strain relations, acoustic emission patterns, and energy evolution characteristics obtained during the triaxial compression tests were analyzed. The majority of the input energy stored in the coal specimens took the form of elastic strain energy. After the elastic-deformation stage, part of the input energy was consumed by stable crack propagation. However, with an increase in stress levels, unstable crack propagation commenced, and the energy dissipation and coal damage were accelerated. The variation in the pre-peak energy-dissipation ratio was consistent with the coal damage. This new method demonstrates that the crack initiation threshold was proportional to the peak stress (σ p) for ratios that ranged from 0.4351 to 0.4753σ p, and the crack damage threshold ranged from 0.8087 to 0.8677σ p.

Fracture Evolution Around a Cavity in Brittle Rock Under Uniaxial Compression and Coupled Static–Dynamic Loads

Abstract: To experimentally investigate the stability of underground excavations under high in situ stress conditions, several rock samples with a mini-tunnel were prepared and subjected to monotonic axial and coupled static–dynamic loading until failure. Mini-tunnels were generated by drilling circular or cubic cavities in the centre of granite rock blocks. Strain gauges were used to monitor the deformation of the mini-tunnels at different locations, and a high-speed camera system was used to capture the cracking and failure process. We found that the dynamic crack initiation stress, failure mode and dynamic crack velocity of the specimen all depend on the pre-stress level when the sample is under otherwise similar dynamic disturbance conditions. The crack initiation stress threshold first increased slightly and then decreased dramatically with the increase in the pre-stress value. The specimens were mainly fractured by tensile cracks parallel to the compression line under lower pre-stress, while they were severely damaged with additional shear cracks under higher pre-stress. Furthermore, the propagation velocity of the primary crack was significantly larger than that of the subsequent cracks. The effect of applying different amounts of static pre-stresses on the velocity of the primary tensile crack was similar to that observed for the crack initiation stress threshold; however, it did not affect the velocity of the secondary and subsequent tensile cracks.

Anisotropy of fatigue crack growth in wire arc additive manufactured Ti-6Al-4V

Abstract: The effects of crack growth direction on the fatigue crack growth of wire arc additive manufactured Ti-6Al-4V were studied. The fatigue crack growth rate (FCGR) of horizontal and vertical samples has different stress intensity factor transition point, ΔKT, which is 11.3 MPa m1/2 and 10.3 MPa m1/2, respectively. The FCGR of vertical sample is 5% higher than that of horizontal sample when the stress intensity factor, ΔK is smaller than ΔKT. The difference in FCGR is resulted from the microstructure characterization and the fatigue crack growth direction.

Experimental Study on the Damage Evolution of Brittle Rock Under Triaxial Confinement with Full Circumferential Strain Control

Abstract: The identification of crack stress thresholds and damage evolution from circumferential strain control triaxial tests are presented in this paper. As underground excavations become deeper to exploit mineral resources or construct civil projects, it has become increasingly important to determine the full stress–strain and damage evolution behaviours of brittle rock. Therefore, post-peak reaction of Class II rock or ‘snap-back’ behaviour must be captured to show the response of the material under self-sustaining failure. To investigate this, a series of triaxial compression tests were carried out for a granite sourced from over 1000 m depth. The tests were controlled using the feedback of lateral strain gauges attached to the Hoek cell membrane, to allow for constant, slow dilation of the specimen. The test results were then input to existing methods along with two new techniques, to identify the crack stress thresholds of crack closure, crack initiation and damage. It was found that although the crack closure threshold is comparable for axial and lateral control testing, the crack initiation and damage thresholds are significantly higher for the tests conducted in this study compared to most existing research. This result highlights the importance of the circumferential strain control method in triaxial tests when determining the post-peak behaviour and damage evolution of brittle rock. This was made easier with the strain gauged membrane proposed in this study, which provides reliable measurements throughout the duration of rock testing. Therefore, full stress–strain and damage evolution data can be obtained for use in damage-plasticity constitutive models.

Relation between crack initiation-damage stress thresholds and failure strength of intact rock

Abstract: The analysis of a wide literature dataset of mechanical parameters related to intact rocks from more than 480 unconfined compression tests, coupled with new laboratory tests on 132 specimens, is proposed herein with the aim of analyzing the mechanical behavior of a great variety of rock types, mainly focusing on their crack initiation (σci) and crack damage (σcd) stress levels. These thresholds can be employed as warning indicators for rock mass damage and breakouts and represent important input parameters for numerical models. International literature lacks in a detailed analysis on the mutual dependence existing between the main mechanical properties of intact rocks and their crack stress thresholds. In this paper, the study of the correlation between crack initiation-crack damage stress levels and the failure strength of sedimentary, metamorphic and igneous rocks is carried out through single and multiple regression approaches aimed at finding reliable prediction models, which can be useful when time-consuming laboratory experimental procedures need to be avoided. The correlation between predicted and measured values demonstrates that defined models represent a good tool for the empirical estimation of σci and σcd, and can be useful for preliminary engineering design dealing with stress-induced brittle fracturing, especially when the definition of warning indicators for rock mass damage and breakouts is needed. In fact, it is known that underground instability mainly depends on the redistribution of stresses around the excavation, which can produce induced stress concentrations, resulting in sudden release of stored energy and causing stress-induced brittle failure phenomena.

Cyclic deformation and microcrack initiation during stress controlled high cycle fatigue of a titanium alloy

Abstract: Cyclic plastic deformation, slip characteristics and crack nucleation in Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si (TC21) with different morphologies of equiaxed and lamellar α phase were systematically analyzed during high-cycle fatigue. The heterogeneous plastic deformation could take place within different α morphologies during high-cycle fatigue even though the cyclic stress amplitude is much less than yield strength. Slip is the dominant deformation mode in the equiaxed primary α, while the slip and ( 10 1 ¯ 1 ) deformation twin are prevalent in the primary α lath. Interactions between slip, twin and interface result in ledges at the primary α lath interface. The relationship between cyclic slip irreversibility, accumulated irreversible strain, and fatigue life is established. A critical parameter, accumulated irreversible strain per area in the crack initiation region (region I), was calculated to be (8.1 ± 2) × 10−4 μm−2 for initiating fatigue crack. Fatigue cracks will nucleate when the accumulated irreversible strain exceeds the critical value. The primary α lath is the dominant site for crack initiation. The cracks initiate and propagate in interface and slip band, and easily connect each other in the primary α lath. In comparison, most cracks lie within an individual or occupy several equiaxed α phases and often cease in front of the phase boundary, which delays the connection of microcracks. It indicates that the primary α lath is more detrimental than the equiaxed primary α phase during high cycle fatigue.

X-FEM Modeling of Multizone Hydraulic Fracturing Treatments Within Saturated Porous Media

Abstract: In this paper, a fully coupled hydro-mechanical model is presented for the study of multizone hydraulic fracturing The momentum balance equation of the bulk together with the mass balance and momentum balance equation of the fluid phase are employed in order to derive the hydro-mechanical coupled system of governing equations of the porous media known as the $$({\mathbf{u}} - p)$$ formulation The hydro-fracture inflow is modeled based on the Darcy law, where the fracture permeability is determined by using the cubic law Provisions are made for the plausible closure as well as the frictional resistance of the fracture edges in the solid phase by means of Kuhn–Tucker inequalities embedded in an X-FEM penalty method In addition, for the fluid phase, the zero leak-off constrain is imposed through the application of the large time increment-based contact algorithm in the case of crack closure The cohesive crack model is employed to account for the nonlinear fracturing process at the hydro-fracture tips Multiple crack growth patterns are determined by means of energy based cohesive stress functions Based on the X-FEM, the strong discontinuities in the displacement field due to fracture opening as well as the weak discontinuities within the pressure field due to leak-off flow are incorporated by using the Heaviside and modified level-set enrichment functions, respectively A consistent computational algorithm is proposed for the determination of the fracturing fluid flow distribution across the existing perforations Finally, several numerical examples are presented to demonstrate the robustness of the proposed X-FEM framework in the study of multizone hydraulic fracturing treatments through saturated porous media The results appear to accord with the field observations reporting numerous failed attempts of multistage multizone fracturing treatments, which provide a great insight into the complexities encountered in practice

A shape memory polymer concrete crack closure system activated by electrical current

Abstract: The presence of cracks has a negative impact on the durability of concrete by providing paths for corrosive materials to the embedded steel reinforcement. Cracks in concrete can be closed using shape memory polymers (SMP) which produce a compressive stress across the crack faces. This stress has been previously found to enhance the load recovery associated with autogenous self-healing. This paper details the experiments undertaken to incorporate SMP tendons containing polyethylene terephthalate (PET) filaments into reinforced and unreinforced 500 × 100 × 100 mm structural concrete beam samples. These tendons are activated via an electrical supply using a nickel-chrome resistance wire heating system. The set-up, methodology and results of restrained shrinkage stress and crack closure experiments are explained. Crack closure of up to 85% in unreinforced beams and 26%–39% in reinforced beams is measured using crack-mouth opening displacement, microscope and digital image correlation equipment. Conclusions are made as to the effectiveness of the system and its potential for application within industry.