Showing papers on "Crack closure published in 2019"

An experimental study on tensile characteristics of granite rocks exposed to different high-temperature treatments

Abstract: Investigation of temperature dependent tensile strength characteristics of rocks provides essential inputs to continue the development of deep geo-engineering applications, including enhanced geothermal systems, deep mining and deep geological disposal of nuclear waste. The aim of this study is, therefore, to identify the influence of temperature (from room temperature to 1000 °C) and different heating and cooling treatments (constant high temperature, slow and rapid cooling) on Australian granite’s tensile strength and to investigate the corresponding microstructural alterations due thermal treatment. Brazilian tensile strength tests were performed on two types of granites collected from Australia, Strathbogie, and Harcourt. ARAMIS photogrammetry, Acoustic Emissions (AE) and Scanning Electron Microscopy (SEM) were employed to examine the crack initiation and development during the experiment. The results concluded that with the increase of temperature for all three thermal conditions, there was a slight increase in tensile strength for Strathbogie granite between room temperature and 200 °C for constant high-temperature condition and slowly cooled condition whilst beyond this there was a clear negative trend. Harcourt granite experienced a negative trend immediately from room temperature conditions. Further, the influence of rapid cooling was much higher than that of slow cooling, due to the intense thermal shock. Beyond 500 °C, there was a significant reduction in the range of strength values which evidence α to β quartz mineral transition. Therefore, the failure mechanism of granite at considered temperature transitioned from a brittle to a quasi-brittle state. This was confirmed with AE testing and ARAMIS photogrammetry such that, higher temperatures resulted in a considerable crack closure and an increase in the unstable crack propagation. This was mainly due to the closure of pre-existing thermally induced cracks followed by crack re-bonding due to the melting followed by re-crystallization of grains, particularly at 1000 °C which was confirmed with SEM.

Specimen shape and cross-section effects on the mechanical properties of rocks under uniaxial compressive stress

Abstract: Uniaxial compressive properties of rocks are very important for designing and constructing engineering projects. Based on the available standards for determining these properties, high quality core specimens with proper geometry are needed. In many cases, the standard specimens, especially in clay-bearing, fractured, and weathered rocks, are always not able to be prepared. On the other hand, in some natural conditions, rocks with different size, shape, and cross-section are undergoing uniaxial compressive loading. Therefore, in order to evaluate the uniaxial compressive strength dependency behaviors of rocks on the shape and cross-section of tested specimens, some marble specimens with three different cross-sections, including circular, square, and rectangular, as well as four different shape ratios (height to diameter/width ratio) of 0.5, 1, 2, and 3 were prepared and tested. Axial and lateral strains, acoustic emission (AE), and camera photographs were recorded during the tests. Rock strength behavior was evaluated based on several stress thresholds, including crack closure stress (σcc), crack initiation stress (σci), damage stress (σcd), and peak stress (σucs). The results indicated that σcc was not dependent on the cross-sectional shape of specimens. With increasing shape ratio, σcc gradually increased, while σcd and σucs greatly decreased, and σci remained at a constant value. The cross-sectional shape effect became operative when r was less than or equal to 1. Moreover, the values of σcd and σucs of rectangular prism specimens and square prism specimens are lower than those of cylindrical specimens, indicating that the unstable crack propagation of prism specimens occurs earlier. The difference gap of σcd and σucs between specimens with different cross-sectional shapes was dramatically decreased with increasing shape ratio. The AE and camera recorded data indicated that the fracture modes of rectangular and square prism specimens are more likely to change from shearing to slabbing fracture when the shape ratio decreased from 3 to 0.5. The main crack developed surface turned from wide surface to narrow surface with the shape ratio of rectangular prism specimens changing from 3 to 1 and 0.5. The research results are of referential meaning to the design of pillars in underground hard rock mines.

Quantifying fatigue overload retardation mechanisms by energy dispersive X-ray diffraction

Abstract: The fatigue crack retardation mechanisms operating after an overload event are investigated for a bainitic steel using high spatial resolution energy dispersive synchrotron X-ray diffraction. The elastic crack-tip strain fields are mapped at mid-thickness of compact tension samples at R-ratios of 0.1 and 0.4. The same overload stress intensity factor (KOL = 60 MPa m1/2) is applied in each case with the cracks then propagating under the same applied stress intensity range, ΔKapp = 27 MPa m1/2. The competing retardation mechanisms are directly quantified and separated, with the associated fatigue crack growth (FCG) rates then being predicted according to a 2-parameter Walker-type assessment and validated against those measured. The stress intensity factor associated with the overload residual stress field is calculated using a weight function approach. For R = 0.1 , shielding from residual stress controls retardation when crack growth through the overload plastic zone, r p O L , is small (specifically 0.6 r p O L ). For more extensive crack growth, discontinuous crack closure controls the retardation behaviour, with significant load transfer across opposing crack faces being observed at minimum load (for R = 0.1 ). These crack face tractions are associated with the plastic asperity created during overload. The traction forces holding the crack faces open at minimum load are, for the first time, used to directly quantify the associated stress intensity factor, K m i n t r a c t as a function of crack growth. While no crack shielding is expected, nor observed, for R = 0.4 , the variation in FCG rate after overload is explained by changes in effective R-ratio.

Delamination R-curve behavior of curved composite laminates

Abstract: The effect of curvature on the delamination R-curve behavior of composite unidirectional laminates is investigated in both the experimental and numerical manners. The flat and curved double cantilever beam specimens with different radii of curvatures are manufactured and tested subjected to mode I loading. A new data reduction method is developed for the curved specimens by adopting the Timoshenko curved beam theory. Experimental R-curves indicate that curvature has no effect on the initiation toughness, while it significantly affects the steady-state toughness and fiber bridging length. Variation of the steady-state toughness and fiber bridging length vs. The different curvatures is formulated for the curved specimens with R/h > 25 (R/h: the ratio of the radius of curvature to thickness). Finally, delamination propagation is simulated in the curved double cantilever beam specimens in commercial finite element software, ABAQUS, by applying both the virtual crack closure technique and cohesive zone model.

Recovery of fatigue life using laser peening on 2024-T351 aluminium sheet containing scratch damage: The role of residual stress

Abstract: The aim of the current work was to study the effect of laser shock peening (LSP) when applied to 2‐mm thick 2024‐T351 aluminium samples containing scratch‐like defects in the form of V‐shaped scribes 50 to 150μm deep. The scribes decreased fatigue life to 5% of that of the pristine material. The effect of laser peening on fatigue life was dependent on the specifics of the peen treatment, ranging from further reductions in life to restoration of the fatigue life to 61% of pristine material. Fatigue life was markedly sensitive to near‐surface tensile residual stress, even if a compressive residual stress field was present at greater depth. Fatigue life after peening was also dependent on sample distortion generated during the peening process. Sample distortion modified local stresses generated by externally applied loads, producing additional life changes. Models based on residual stress intensity and crack closure concepts were successfully applied to predict fatigue life recovery.

Effect of TiB Orientation on Near-Threshold Fatigue Crack Propagation in TiB-Reinforced Ti-3Al-2.5V Matrix Composites Treated with Heat Extrusion.

Abstract: TiB-reinforced Ti-3Al-2.5V matrix composites, in which TiB whiskers are oriented parallel to the direction of heat extrusion, were fabricated via mechanical alloying and hot isostatic pressing (HIP). To investigate the near-threshold fatigue crack propagation in TiB-reinforced Ti-3Al-2.5V matrix composites, stress intensity factor K-decreasing tests were conducted for disk-shaped compact specimens having two different orientations of TiB whiskers at force ratios from 0.1 to 0.8 under ambient conditions. The crack growth rates, da/dN, for the composites incorporating TiB whiskers oriented perpendicular to the direction of crack growth were constantly lower than those obtained in the case where the orientation was parallel at the same stress intensity range ΔK, while the threshold stress intensity range, ΔKth, was higher. This effect can be explained by the increase in the degree of roughness-induced crack closure resulting from the perpendicular TiB, because fatigue cracks preferentially propagated across the boundaries between the matrix and the TiB in certain regions. In contrast, the effective threshold stress intensity range, ΔKeff,th, for composites was unaffected by the TiB orientation at low force ratios.

Stress–Strain Modeling and Brittleness Variations of Low-Clay Shales with CO 2 /CO 2 -Water Imbibition

Abstract: A better understanding of the stress–strain behaviors of shale samples after shale-CO2 or shale-water–CO2 interactions is of great importance to CO2 enhanced shale gas exploitation and CO2 sequestrating in shale reservoirs. In this study, a constitutive model that combines with the modified Duncan–Chang model and Weibull distribution-based model is applied to investigate the stress–strain characteristics of low-clay shale samples treated by sub-/super-critical CO2 and sub-/super-critical CO2 + water for different times (10 days, 20 days, and 30 days). The results show that the model could describe well the crack closure stage, the elastic stage, and the inelastic stage of shale samples. The axial strain at the connection point between the two models varies from 28.51 to 43.36% of the axial strain at the failure point. Shale-CO2 or shale-water–CO2 interactions make shale samples more ductile at the crack closure stage, which can be depicted as the increase of initial elastic modulus during the imbibition process. The brittleness index values (BI) which are calculated based on the combined constitutive model increase with increasing soaking time for shale samples treated by sub-/super-critical CO2, and decrease with increasing soaking time for shale samples treated by sub-/super-critical CO2 + water.