Bone tissue engineering has been continuously developing since the concept of "tissue engineering" has been proposed. Biomaterials that are used as the basic material for the fabrication of scaffolds ...play a vital role in bone tissue engineering. This paper first introduces a strategy for literature search. Then, it describes the structure, mechanical properties and materials of natural bone and the strategies of bone tissue engineering. Particularly, it focuses on the current knowledge about biomaterials used in the fabrication of bone tissue engineering scaffolds, which includes the history, types, properties and applications of biomaterials. The effects of additives such as signaling molecules, stem cells, and functional materials on the performance of the scaffolds are also discussed.
Bone tissue engineering has been continuously developing since the concept of "tissue engineering" has been proposed. Biomaterials, as the basic material for the fabrication of scaffolds, play a vital role in bone tissue engineering.
Coupled static-dynamic tests were carried out on notched semi-circular bend (NSCB) granitic specimens using a modified split Hopkinson pressure bar (SHPB) device, to investigate the effects of axial ...pre-force and loading rate on dynamic fracture behavior of rocks. Digital image correlation (DIC) method was adopted to obtain the deformation field and fracturing process. The results indicate that the pre-force (up to 50% static peak force) enhances the fracture parameters overall, including fracture initiation toughness, fracture energy, crack propagation velocity (CPV) and fracture propagation toughness, but the effect of loading rate is more significant than that of the pre-force. Digital images show that the increase in pre-force would cause the dominant crack to symmetrically bifurcate since the pre-force strengthened a fan-shaped area for the specimen near the incident bar. The loading rate larger than 124 GPa·m1/2/s could lead to the secondly fracture of the specimen and thereby more fragments. Compared with previous studies, the present investigation deepens the understanding of fracture characteristics of rocks under coupled static and dynamic loads by further reducing the pre-force interval (0.25 kN). In addition, a new method based on the DIC technique is proposed to calculate crack opening distance (COD) and CPV, which can improve the efficiency and accuracy compared to the strain gauge method. Moreover, fracture propagation toughness is also emphasized apart from the initiation toughness. The findings can provide some enlightenments for rock fracture dynamics and crack propagation of highly stressed rocks.
To investigate the effects of inclusions filled in holes, including inclusion strength, stiffness and shape, on the mechanical properties and fracture evolution of rock, a series of uniaxial ...compression tests were conducted on prismatic sandstone containing a prefabricated hole filled with different types of inclusions using a servo-hydraulic machine. Digital image correlation (DIC) and acoustic emission (AE) techniques were applied jointly to record and analyze the deformation and crack process of sandstone. The results indicate that both the inclusion type and shape are important factors affecting the strength and deformation properties of sandstone. The relatively rigid inclusion not only significantly improves the mechanical properties of pre-holed specimens, but also can provide a better support for structural integrity of rock after failure. Observations show that both DIC and AE techniques are capable of revealing the fracture evolution of specimens, and there is a good consistency between the mechanical behavior and surface strain fields and internal AE signals. Based on the strain localization characteristics, the crack sequences and relative stress levels for each group of specimens are summarized, especially the crack development inside the inclusion is first observed. The crack behavior and crack type around inclusions are greatly influenced by the inclusion type and shape due to dramatic changes in stress distribution around inclusions. After rock failure, the crack extent around inclusions can be characterized by the cumulative AE counts, and the final failure modes of specimens under different filling conditions can be categorized as tensile failure, shear failure and mixed tensile/shear failure.
This article evaluates the performance of 20 Coupled Model Intercomparison Project phase 6 (CMIP6) models in simulating temperature and precipitation over China through comparisons with gridded ...observation data for the period of 1995–2014, with a focus on spatial patterns and interannual variability. The evaluations show that the CMIP6 models perform well in reproducing the climatological spatial distribution of temperature and precipitation, with better performance for temperature than for precipitation. Their interannual variability can also be reasonably captured by most models, however, poor performance is noted regarding the interannual variability of winter precipitation. Based on the comprehensive performance for the above two factors, the “highest-ranked” models are selected as an ensemble (BMME). The BMME outperforms the ensemble of all models (AMME) in simulating annual and winter temperature and precipitation, particularly for those subregions with complex terrain but it shows little improvement for summer temperature and precipitation. The AMME and BMME projections indicate annual increases for both temperature and precipitation across China by the end of the 21st century, with larger increases under the scenario of the Shared Socioeconomic Pathway 5/Representative Concentration Pathway 8.5 (SSP585) than under scenario of the Shared Socioeconomic Pathway 2/Representative Concentration Pathway 4.5 (SSP245). The greatest increases of annual temperature are projected for higher latitudes and higher elevations and the largest percentage-based increases in annual precipitation are projected to occur in northern and western China, especially under SSP585. However, the BMME, which generally performs better in these regions, projects lower changes in annual temperature and larger variations in annual precipitation when compared to the AMME projections.
Filled inclusions in rock discontinuities play a key role in the mechanical characteristics of the rock and thereby influence the stability of rock engineering. In this study, a series of impact ...tests were performed using a split Hopkinson pressure bar system with high-speed photography to investigate the effect of interlayer strength on the wave propagation and fracturing process in composite rock-mortar specimens. The results indicate that the transmission coefficient, nominal dynamic strength, interlayer closure, and specific normal stiffness generally increase linearly with increasing interlayer stiffness. The cement mortar layer can serve as a buffer during the deformation of composite specimens. The digital images show that tensile cracks are typically initiated at the rock-mortar interface, propagate along the loading direction, and eventually result in a tensile failure regardless of the interlayer properties. However, when a relatively weaker layer is sandwiched between the rock matrix, an increasing amount of cement mortar is violently ejected and slight slabbing occurs near the rock-mortar interface.
Extensive true-triaxial unloading tests are conducted on rectangular Miluo granite specimens to investigate their crack propagation behavior and peak unloading strength properties. Experimental ...results demonstrate that their failure mode transforms from shear to slabbing with the increase in intermediate principal stresses σ2. Under a lower value of σ2, the dominant failure mode represented by the macro shear band is composed of several meso-extensional cracks parallel to the maximum principal stress σ1, and the fracturing process indicates intensive dislocation and slide owing to the normal force acting on extensional cracks. However, the increase in σ2 exacerbates the extensional cracks and further propagates them toward the end of the specimen along the direction of σ1, if σ3 is unloaded to 0. This is primarily due to the enhanced ability to resist mutual slide and squeeze among extensional cracks in the shear band, and the promotion of crack initiation and propagation in the unloading direction (σ3) owing to the provided free surface. The transformation of the cracking mode is revealed via three-dimensional combined finite/discrete element modeling, scanning electron microscope observations, and acoustic emission signal feature (average frequency, rise angle, Acoustic emission (AE) counts, and accumulated AE energy) analysis. The peak unloading strength exhibits an initial increase followed by a decrease as a function of σ2. Four rock-strength criteria are employed to evaluate the true-triaxial data with emphasis on predictability in practical applications and accuracy between best-fitting functions (experimental data) and theoretical (predicted) solutions. It is demonstrated that modified the Wiebols–Cook criterion and modified Lade criterion cannot accurately estimate the unloading strength throughout the true-triaxial data. Although the Mogi 1971 criterion fits the experimental data (higher correlation coefficients) well, it cannot be correlated to strength parameters (cohesion and internal frictional angle). Hence, it fails to predict the real strength values if complex true-triaxial unloading tests are not performed. The linear Mogi criterion is recommended for predicting the true-triaxial unloading strength of granite.
Stability and safety of rock structures are easily influenced by the dynamic disturbance, especially when weak joint planes exist. In order to investigate the filling joint effect on the dynamic ...response of rock specimens, a series of impact dynamic tests were conducted by a modified split Hopkinson pressure bar (SHPB) system. A sandwich type of sandstone specimens filling with different thicknesses layer of cemented mortar (filling joint) were tested in the study. The results show that the transmission coefficient, dynamic strength and energy absorption all decrease with increasing joint thickness. However, the reflection coefficient, peak strain and joint closure show an opposite variation trend. In addition, the deformation of the cemented mortar joint is the main reason to cause the deformation and final failure of jointed rock specimens. Tensile cracks dominate the fracturing behavior during the dynamic loading, but they have a slight influence on the final failure mode which transforms from localized slabbing to axial splitting failure with the filling joint becoming thicker.
Landslides are one of the most common geological disasters with the complicated spatiotemporal processes, which can be partially reproduced in slip shear failure along the discontinuity in laboratory ...tests. However, the assessment and evaluation of shear behavior along the discontinuity is time-consuming and high-cost. To solve this problem, a new constitutive model is proposed to predict the shear deformation behavior of the discontinuity. In this model, the discontinuity is assumed to be composed of intact units and failed units. In the shearing process, intact units and failed units jointly bear the external loads, and intact units are gradually transformed into failed units. A damage control variable is introduced to link the mechanical response of units with the macroscopic discontinuous deformation, thereby obtaining the final expression of the new model. The model parameters can be determined directly from several standard laboratory shear tests. Subsequently, the comparison analysis among the predictive results of the new model, previous models and engineering projects is conducted. Finally, the sensitivity of model parameters is systematically discussed. A hardening index HI is proposed to characterize the shear deformation characteristics in the hardening stage, and its relationship with model parameters C and r is in a power form. The results are expected to provide theoretical guidance for landslide prediction and thus reduce the occurrence frequency of geohazards.
•The geomechanical connotation of landslides is theoretically linked up with the shear behavior of discontinuities.•Theoretical constitutive modeling of shear behaviors is proposed based on the mechanical response of rock units.•A hardening index is proposed to characterize the ability of rock materials to resist inelastic deformation before failure.
Flaws in different geometries are widely existed in rock masses and play a significant role in the stability of rock engineering structures. To investigate the failure mechanisms of rock block with ...combined flaws under coupled static and dynamic loads, a series of experiments were conducted on granite containing pre-existing fissures in the vicinity of a circular opening by using a modified split Hopkinson pressure bar (SHPB) device. Digital image correlation (DIC) method was applied to record the real-time cracking processes. The results show that the fissure inclination angle and the number of fissure are important factors affecting the mechanical properties and cracking processes. The strength of specimens shows a decrease trend with fissure inclination angle increasing, and the localized strain of fissure tip can be used to identify and reveal the crack initiation in the experiments. Based on the temporal and spatial evolution rules of principal strain field obtained by DIC technique, the crack sequences and crack paths of typical specimens are summarized in the paper, especially the crack initiation position. The roof, floor of the opening and the fissure tip are the critical zones where the new cracks usually initiate from. The crack types of the specimens can be categorized as wing crack, coplanar secondary crack, oblique secondary crack, tensile crack and shear crack based on the observation of the failure processes.
•Failure mechanism of rock with fissure-opening flaws was studied under coupled loads.•Crack process of rock under coupled loads was observed in real time by DIC system.•Localized strain around pre-existing fissures was analyzed in a quantitative way.•Crack initiation positions and failure modes of granite specimens were summarized.
Surface defects are prevalent in natural rock masses, and the newborn cracks that initiate from these defects and significantly affect structural stability usually extend in a three-dimensional (3D) ...manner. Hence, in this research, a series of uniaxial compression tests were conducted on sandstone specimens containing a surface flaw to examine the effect of nondimensional flaw depth (0, 0.3, 0.5 and 0.75) on the fracture behavior. The crack propagation process was monitored by digital image correlation (DIC) technique, and postmortem microscopic fracture morphology was measured using scanning electron microscope (SEM) method. In addition, the fracture energy evolution and stress intensity factors (SIFs) along the semi-elliptical flaw perimeter were discussed in detail to theoretically interpret the experimental phenomena. The results indicate that the strength decreased by 2.70%, 10.90% and 11.91% for nondimensional surface flaw depths of 0.3, 0.5 and 0.75, respectively, and elastic modulus exhibited a reduction of 3.71%, 10.69% and 3.21%, respectively. Full-field deformation contours demonstrate that crack paths in the front surface of the specimen were totally different from those in the back surface, but the failure pattern tended to be similar to that of specimens containing a penetrated flaw with the increasing flaw depth. The micro appearance of a typical fracture further reveals the complicated stress state (mixed I-II-III mode) under uniaxial compression. For a larger flaw depth, the proportion of releasable elastic strain energy in total input strain energy was improved, while the percentage of dissipated energy was reduced. Based on fracture mechanics, SIFs along the flaw periphery uncover the magnitude of KI, KII and KIII and further the type of initiation crack type. Finally, parametric analysis was carried out to investigate the effects of flaw length and specimen thickness. The difference in crack pattern from surface and penetrated flaw was also discussed in detail.
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