Joint Tip Research Articles

Investigation on uniaxial compression and fracture damage mode of prefabricated parallel double-jointed red sandstone.

As a special type of joint fracture, the fracture evolution characteristics of parallel double joints have important engineering significance for the stability analysis of fractured rock mass. In this work, a new method for calculating stress intensity factor of parallel double-jointed fractures was importantly proposed. Physical uniaxial compression tests were carried out on parallel double jointed red sandstone filled with cement mortar under different geometric parameters, and the macroscopic mechanical properties and failure characteristics of red sandstone are deeply analyzed. The results show that the larger the connectivity rate is, the smaller the peak stress and strain are. The increase of connectivity rate will affect the change rate of transverse strain in the center of rock bridge. The closer the dip angle of the joint is, the lower the peak stress is and the shorter the failure time is. The damage mode of joint tip encroachment affects the lateral displacement of the rock bridge center, and the displacement is always close to the first damage section. The closer the joint tip is to the load, the easier the end-face penetrating cracks occur. The research content can provide basic support for guaranteeing the stability of underground engineering rock mass.

Damage constitutive model and mechanical properties of jointed rock mass under hydro-mechanical coupling

The damage evolution law and mechanical property weakening mechanism of jointed rock mass under hydro-mechanical coupling are important scientific problems in deep rock mass excavation. In this paper, the mineral composition of rock specimens was analyzed, and the distribution law of nuclear magnetic resonance T2 spectrum of granite under different axial stress was studied. Then the mechanical properties of jointed rock samples under triaxial stress and seepage pressure were studied. Based on the analysis of the stress field at the joint tip, the damage model of the jointed rock mass was obtained. Furthermore, the tensor of damage variables was analyzed, and the stress intensity factor under seepage pressure was calculated. The seepage pressure promotes the damage evolution process of jointed rock mass specimens and accelerates the strength failure of jointed rock mass specimens. The damage constitutive model of jointed rock mass under seepage pressure was established and verified. Based on the weakening law of seepage pressure, the developed hydro-mechanical coupling constitutive model was customized and imported into FLAC3D with the calculation function of the Mohr-Coulomb constitutive model as the compilation template. A three-dimensional numerical simulation model of underground rock mass engineering was established to analyze the stability of rock mass with different joint dip angles. For the surrounding rock with the dominant angle of 85° in the mine, its stability is poor, and the support needs to be strengthened.

Fracture evolution and failure behavior around an opening in brittle jointed rocks subjected to uniaxial compression

To investigate the fracture evolution and failure behavior around an opening in brittle jointed rocks, an experimental study was carried out in which cross-jointed samples with granite cavities were uniaxially loaded while the mechanics, acoustics and optics were simultaneously observed. These results were compared and analyzed to better understand the deformation and cracking process. The results show that the presence of cross joints and the change in included angles play a crucial role in influencing the strength and deformation properties of the samples. The peak strength and elastic modulus depend on the included angle and first decrease and then increase with increasing included angle, reaching the minimum values when β equals 45°. A stress fluctuation point generally corresponds to a large acoustic emission (AE) event and a turning point in the accumulated AE count curve. The good correlation of the three observations provided a reasonable holistic picture of the process zone nucleation and the subsequent cracking processes. The cracking behavior around openings and cross joints is significantly affected by the included angle because of drastic changes in the stress distribution surrounding the defects. The highlighted strain localization zones formed near the joint tip and around the chamber interact with each other and jointly dominate the initiation and propagation mode of new cracks, eventually leading to more complex failure patterns and severe fracture degrees. Moreover, there will also be an interaction between the same set of cross joints, and the tips can be connected, showing the characteristics of intensified deformation in local areas. The unstable failure of a sample is primarily dominated by tensile cracks and locally by shear cracks around the cavern and the joint tips. Microcrack zones are regarded as white patches macroscopically, and microcrack nucleation is considered to be the microformation mechanism of a white patch.

Hands to Hexapods, Wearable User Interface Design for Specifying Leg Placement for Legged Robots.

Specifying leg placement is a key element for legged robot control, however current methods for specifying individual leg motions with human-robot interfaces require mental concentration and the use of both arm muscles. In this paper, a new control interface is discussed to specify leg placement for hexapod robot by using finger motions. Two mapping methods are proposed and tested with lab staff, Joint Angle Mapping (JAM) and Tip Position Mapping (TPM). The TPM method was shown to be more efficient. Then a manual controlled gait based on TPM is compared with fixed gait and camera-based autonomous gait in a Webots simulation to test the obstacle avoidance performance on 2D terrain. Number of Contacts (NOC) for each gait are recorded during the tests. The results show that both the camera-based autonomous gait and the TPM are effective methods in adjusting step size to avoid obstacles. In high obstacle density environments, TPM reduces the number of contacts to 25% of the fixed gaits, which is even better than some of the autonomous gaits with longer step size. This shows that TPM has potential in environments and situations where autonomous footfall planning fails or is unavailable. In future work, this approach can be improved by combining with haptic feedback, additional degrees of freedom and artificial intelligence.

Experimental and Numerical Study on the Anchorage Effect of Bolted Jointed Rock Masses

Anchor technology has become an irreplaceable geotechnical engineering reinforcement measure. To clarify anchorage effects and investigate the three-dimensional (3D) crack propagation process of bolted jointed rock masses, a series of physical model tests and 3D numerical simulations were performed, and optimal anchoring conditions of jointed rock masses are found. The results showed that a bolted jointed rock mass had stronger compressive performance and deformation capability, with crack propagation controlled, especially in the anchorage zone, and the formation and slip of shear zones also restrained. Meanwhile, the fractured location is transferred from the joint tip to the interface between the bolt and surrounding rock. The numerical simulation based on the damage model of rocks at the mesoscale and a nonlinear shearing–sliding model for anchoring interfaces were conducted with the FLAC3D code to reproduce the 3D crack propagation and the gradual damage of bolted jointed rock masses. The anchorage effect increased the crack initiation stress of jointed rock masses, but the zone where the bolt passed through the joint cracked more easily. Once onset of the instability stage of the bolted jointed rock mass, cracks began to propagate and penetrate gradually to the anchorage zone. In addition, under uniaxial compression, a “Z”-shaped shear stress concentration zone is observed in the bolt, which is mainly attributed to the role of the bolt on controlling shear failure along the joint plane and transverse dilatancy of the specimen. Better anchorage effects were achieved by installing bolts after deformation of the jointed rock mass had developed to a certain extent. The optimal anchor opportunity for a jointed rock mass varied with the joint angle. More specifically, for the rock mass with a joint angle of 75°, the anchorage effect was best when the bolt was installed at 40% peak strain of the jointed rock mass, while 10% peak strain was perfect for the bolted rock mass with a 45° joint angle.

Simulation of Deformation Process Failure of Jointed Rock Masses Based on the Numerical Manifold Method

On the basis of the numerical manifold method, this work introduces the concept of stress intensity factor at the crack tip in fracture mechanics and proposes the utilisation of artificial joint technology to ensure the accuracy of joint geometric dimensions in the element generation of the numerical manifold method. The contour integral method is used to solve the stress intensity factor at the joint tip, and the failure criterion and direction of crack propagation at the joint tip are determined. Element reconstruction and crack tracking are implemented in crack propagation, and a simulation programme of the entire process of deformation, failure, propagation and coalescence of jointed rock masses is developed. The rationality of the proposed method is verified by performing the typical uniaxial compression test and direct shear test.

Study on Influence of Joint Distribution on Surrounding Rock Failure of an Underground Tunnel

Due to geological structure and artificial disturbance, a large number of joints and fissures are formed in the surrounding rock of an underground tunnel. In order to study the influence of joints on the failure characteristics of tunnels, three test schemes with different joint lengths, joint spacing, and joint positions are designed. The results show that the bearing capacity of the tunnel decreases with the increase in the joint length. With the increase in joint spacing, the bearing capacity of the tunnel decreases first and then increases. The crack propagation law of the three test schemes has experienced four stages: no crack, crack initiation, crack rapid development, and crack gradual reduction. The location of joints has the greatest influence on the failure mode of the tunnel. The crack is most likely to appear at the top of the tunnel and expand along the joint, mainly because it is easy to form tensile stress at the top of the tunnel and compressive stress concentration at the joint tip. Therefore, when excavating the tunnel in the underground space, the influence of joints on the tunnel should be considered. Analyzing the relationship between the tunnel and joints has important practical guiding significance for the control of the surrounding rock of the tunnel. Finally, the failure results of the indoor physical model and numerical model are compared and analyzed. They are in good agreement, which also reflects the rationality of numerical simulation.

Failure Characteristics of Complicated Random Jointed Rock Mass Under Compressive-Shear Loading

Most natural rock masses contain a large number of random joints and fissures, and most of the rock masses at the rock engineering are commonly in both compression and shear stress environment. However, the research on the failure characteristics of complex random jointed rock mass under compressive-shear loading is still limited. To address this gap, this paper uses the particle flow code 2D to establish a discrete fractured rock mass model and carry out a series of numerical tests with different compressive-shear angles (α) and different joint geometric parameters. The effects of compressive-shear angle and joint geometric parameters on the strength and failure characteristics of fractured rock masses are studied. The results indicate that with the increase of α, the peak strength of the specimen decreases gradually, and the failure mode changes from the composite shear failure mode (Mode-I) to a plane shear failure mode (Mode-II) and then to intact shear failure mode (Mode-III). Specifically, the three failure modes occur in the specimens with α = 15°, 30° or 45°, 60°, respectively. The existence of joints affects stress distribution on rock mass during the loading process. Furthermore, the stress at the joint tip is relatively concentrated, while on both sides of the joint is smaller. Three kinds of crack coalescence patterns are observed: tensile, shear, and tensile-shear mixed coalescence. The inclination angle of the rock bridge between adjacent joints affects the specific type of coalescence.

Experimental Study on Shear Failure Characteristics of Jointed Rock Mass Based on Direct Shear Tests and Digital Image Correction Techniques

The instability of rock engineering is normally dominated by the shear failure of rock mass. The dip angle of discontinuous planes widely existing in rock mass is a key parameter affecting the shear strength and failure mode of jointed rock. This paper aims to investigate the influence of discontinuous joints on the shear failure of rock. Direct shear tests are carried out on rock-like specimens with discontinuous joints in different dip angles. During the shear tests, the strain field is monitored in real-time by digital image correction (DIC) technology. Experimental results show that the shear strength, shear strain evolution, and failure mode for the jointed specimens are affected by the dip angles of the discontinuous joints. The maximum shear strain of specimens with joint angles of 45° and 75° increases gradually with the increase of shear loading. The maximum shear strain for the specimens with joint angles of 0°, 15°, 30°, 60°, and 90° increases sharply after the shear load reaches 80% of the peak load. When the joint inclination angle is less than 45°, the crack begins to expand from the joint tip and is interconnected to form a penetrating fracture. When the joint dip angle is greater than 45°, the cracks initiate at the joint tip and then propagate at different paths resulting in multistage shearing and crushing failure.

Numerical Investigation of Effect of Rock Bolt Angle on Shear Behavior of Rock Bridges

In this work, the effect of rock bolt angle on the shear behavior of Rock Bridges is investigated using the particle flow code in two dimensions (PFC2D) for three different Rock Bridge lengths. Firstly, the calibration of PF2D is performed to reproduce the gypsum sample. Then the numerical models with the dimensions of 100 mm * 100 mm are prepared. The Rock Bridge is created in the middle of the model by removal of the narrow bands of discs from it. The uniaxial compressive strength of the Rock Bridge is 7.4 MPa. The Rock Bridge lengths are 30 mm, 50 mm, and 70 mm. The rock bolt is calibrated by a parallel bond. The tensile strength of the simulated rock bolt is 360 MPa.One rock bolt is implemented in the Rock Bridge. The rock bolt angles related to the horizontal axis are the changes from 0 to 75 degrees. Totally, 18 models are prepared. The shear test condition is added to the models. The normal stress is fixed at 2 MPa, and the shear load is added to the model till failure occurs. The results obtained show that in a fixed rock bolt angle, the tensile crack initiates from the joint tip and propagates parallel to the shear loading axis till coalescence to rock bolt. In a constant Rock Bridge length, the shear strength decreases with increase in the rock bolt angle. The highest shear strength occurs when the rock bolt angle is 0°.

Behavior of a Tunnel and its Neighboring Joint with and without Presence of Rock Bolt under Biaxial Loads; Particle Flow Code Approach

In this work, the interaction between the semi-circular space and the neighboring joint with and without the presence of rock bolts was investigated using the particle flow code (PFC) approach. For this purpose, firstly, the calibration of PFC was performed using both the Brazilian experimental test and the uniaxial compression test. Secondly, a numerical model with the dimension of 100 mm * 100 mm was prepared. A semi-circular space with a radius of 25 mm was situated below the model. A joint with a length of 40 mm was situated above the space. The joint opening was 2 mm. The joint angles related to the horizontal direction were 0°, 15°, 30°, 45°, 60°, and 75°. Totally, 6 different configurations of the semi-circular space and neighboring joint were prepared. These models were tested with and without the presence of vertical rock bolts by the biaxial test. The rock bolt length was 50 mm. The value of the lateral force was fixed at 2 MPa. An axial force was applied to the model till the final failure occurred. The results obtained showed that the presence of rock bolts changed the failure pattern of the numerical model. In the absence of rock bolts, two tensile wing cracks initiated from the joint tip and propagated diagonally till coalescence from the model boundary. Also several shear bands were initiated in the left and right sides of the tunnel. In the presence of rock bolts, several shear bands were initiated in the left and right sides of the tunnel. The compressive strength with the presence of rock bolts was more than that without the presence of rock bolts. The failure stress had a minimum value when the joint angle was 45°.

Analytical and numerical analysis for frost heaving stress distribution within rock joints under freezing and thawing cycles

Water-bearing joints within rock engineering in cold areas are often subjected to frost heaving force in cold season due to water–ice phase transition. To evaluate the damage and stability of rock mass in cold regions, a 3D model that considers moisture migration loss during freezing and thawing was established to study the characteristics of frost heaving force within joints. Then, the numerical simulation of cyclic freeze-thawing of water-bearing joints was carried out through equivalent expansion coefficient and particle flow calculation methods. The distribution of frost heaving force in and around the joints was obtained. According to the results of the numerical tests and theoretical calculations, the frost heaving force in joints is basically stable, the tensile stress concentration area appears at the joint tip, and the frost heaving force decreases gradually away from the jointed rock mass area. The frost heaving force decreases considerably with increasing cycle number and moisture migration loss but it increases with increasing mechanical strength and joint geometric size of rock and ice. The comparison between the numerical solution of the equivalent expansion coefficient method and the theoretical solution shows that the force size and distribution law of frost heaving for the two methods are consistent.

Fatigue and damage properties of non-consecutive jointed mudstone samples subjected to cyclic triaxial loading

Non-consecutive jointed mudstone samples with dip angles (0° is the horizontal axis) of 15°, 30°, and 45° were subjected to a series of cyclic triaxial loading tests at various frequencies, with the aim of evaluating the fatigue and damage properties of jointed mudstone samples, including the ultimate cycle number, maximum axial strain, damage evolution, and failure behavior. The results from the tests indicate the following: (i) the higher the frequency, the greater the maximum axial strain difference and the higher the value of Eavl (average Young’s modulus during loading) and Eavu (average Young’s modulus during unloading), and (ii) compared with triaxial compressive loading, additional cracks are found in the samples subjected to cyclic loading, and the propagation of cracks initiates from the joint tip toward the two ends of the sample. Meanwhile, under the condition of a lower frequency, additional cracks occur after failure. Based on the experimental data, a new indicator (RS), considering the effect of the amplitude, peak deviatoric stress, and valley deviatoric stress, was proposed to indicate the stress level. At the same RS and dip angle, there is a positive correlation between frequency and lgNf (Nf is the number of cycles before the failure of the rock samples, and lgNf is the logarithmic value of Nf). Furthermore, a newly proposed damage variable based on shear strain is used to describe the damage evolution of jointed mudstone subjected to cyclic triaxial loading.

Tunable elastic metamaterials using rotatable coupled dual-beam resonators

We present the theoretical background, finite element and spectral element analyses, and experimental validation of a new class of tunable elastic metamaterials which leverage coupled dual-beam resonators that cancel in-phase bending vibration of each beam section. For a metamaterial with an array of rotatable single-beam resonators, we first show that the orthogonal bending modes of each resonator merely cause the shrinkage of one bandgap and the expansion of the other with changing resonator angle. Then, by simply rotating the coupled dual beams while keeping the joint tip mass stationary, we demonstrate that the bandgap of the host elastic metamaterial with an array of coupled dual-beam resonators can be continuously tuned over a wide range of frequencies. While canceling the undesired lateral bending motions, we enable tunable elastic metamaterials through altering the moment of inertia of the beam-type resonator attachments. Continuous bandgap tuning over a broad frequency range is validated experimentally, yielding a 42% change in the starting frequency of the bandgap as the coupled dual-beam resonators are rotated from 0° to 90°. Although passive tuning is considered in our work, active components can be incorporated in the proposed design to enable adaptive tuning as well as time-varying behavior.

The Joint Tip Graft: A Joint Support for Rim, Facet and Infratip Lobule in Rhinoplasty.

The authors of this study have developed a novel graft called the 'The Joint Tip Graft' which adds support to the lateral crus, camouflages the tip grafts, supports the facet and adds volume to the nasal tip as a single graft. The aim of this study was to define and introduce the tripod graft. Thirty patients who underwent primary rhinoplasty at a tertiary academic center were included. Patients were randomly assigned into two groups according to the grafts used: group 1: Joint tip graft was additionally used, and group 2: Joint tip graft was not used. All patients were photographed by a photographer who is familiar with medical photography. Preoperative and postoperative sixth month photographs were taken for analysis. The brightest point of the infratip lobule and the darkest point of the facets were selected and analyzed with computer software. The luminance ratio of facet to infratip lobule was calculated to overcome light and head position differences. Mean infratip luminance scores were 112.20 ± 5.72 and 109.73 ± 7.13 in groups 1 and 2, respectively (p > 0.05). Mean facet luminance scores were 101.33 ± 4.91 and 89.27 ± 5.11 in groups 1 and 2, respectively (p < 0.05). Facet/infratip luminance ratios were calculated for each group. Mean facet/infratip luminance ratios were 0.90 ± 0.01 and 0.82 ± 0.16 for groups 1 and 2, respectively (p < 0.05). The joint tip graft is a novel graft that is easy to harvest and apply. It spans both of the alar cartilages, supports the facet area and creates a smooth gradual light shadow transition. Additionally, it acts as a camouflage over the tip grafts. This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

Aerodynamics and Vortex Flowfield of a Slender Delta Wing With Apex Flap and Tip Flap

The effect of apex flap and tip flap, deflected both independently and jointly, on the vortex flow and lift generation of a 65 deg-sweep delta wing was investigated experimentally. The drooped apex flap produced a higher lift at medium-to-high angle of attack regime and also a delayed stall. The anhedral (introduced by the downward tip flap) generally promoted lift increment, whereas dihedral had the opposite effect. Meanwhile, the joint apex and tip flap deflection gave a delayed leading-edge vortex (LEV) breakdown and an enhanced lift. The LEVs were generally drawn closer to the wing upper surface, while being pushed further away from the wing centerline by the application of apex flap and tip flap. The flap also modified the vorticity distribution in the LEV; the bursting behavior was, however, not affected. Dye-injection flow visualization and particle image velocimetry (PIV) measurements of the vortex flow were also discussed.

Numerical simulation of hydraulic fracturing in circular holes

For investigating the effect of the pre-existing joints on the initiation pattern of hydraulic fractures, the numerical simulation of circular holes under internal hydraulic pressure with a different pattern of the joint distributions are conducted by using a finite element code, FRANC2D. The pattern of hydraulic fracturing initiation are scrutinized with changing the values of the joint length, joint offset angle. The hydraulic pressures with 70% of the peak value of borehole wall breakout pressure are applied at the similar models. The simulation results suggest that the opening-mode fracture initiated from the joint tip and propagated toward the borehole for critical values of ligament angle and joint offset angle. At these critical values, the crack grow length is influenced by joint ligament length. When the ligament length is less than 3 times the borehole diameter the crack growth length increases monotonically with increasing joint length. The opening-mode fracture disappears at the joint tip as the ligament length increases.

Numerical Study of Effect of Joint Tip Distance from Sample End on Rock Failure

In this paper,the mechanical properties of rock experiencing the variation of joint tip distance from sample end under uniaxial compression condition were simulated.Numerical simulation for the different rock sample in the uniaxial compression have been conducted to evaluate the effects of joint tip distance from sample end on the overall mechanical behaviour of jointed rock masses. It was done using the Rock Failure Process Analysis program RFPA2D. Numerically simulated stress-strain curve, peak stress, peak strain and failure patterns were compared with the corresponding physical tests. We found that specimen joint tip distance from sample end corresponding value (distance from the crack tip to the compression surface) linear relationship with the compressive strength values .Numerical simulations agree well with physical results, it is shown that RFPA2Dis suitable for the analysis of joint tip distance from sample end effect on rock fracture.

Failure Mechanism and Mechanical Characteristics of Rockmass Containing Close Intermittent Joints under Plane Stress Condition

Biaxial compression model tests on rockmass containing intermittent joints under plane stress condition were carried out in this study, where the laser speckle photo technique was used for measuring the displacement field. The results obtained have indicated that the laser speckle method was effective and the measuring value was about 14% less than the measuring value of dial gauges. Based on the displacement field and the principal stress field which were measured by laser speckle photo technique, dial gauges and strain gauges, the angle of wing incipient crack, the evolution of stress field at the joint tip and the path of crack extension were deeply studied. The deformation and failure characteristics, including the fracture mechanism of rockmass, were also analyzed in this study.

Thermoelastic Stress Analysis of Composite Finger Joints

The marine industry at present employs the use of aluminium extrusions in the construction of decks and superstructures that could be replaced with pultruded glass reinforced plastic (GRP) profiles. The length of the pultruded section is limited and therefore, efficient and economic jointing techniques must be developed that can withstand the loads applied to ship structures. This paper evaluates a novel means of adhesively bonded connection for GRP structures known as a finger joint. Various joint geometries are examined, load displacement behaviour is established and Thermoelastic Stress Analysis (TSA) is used to provide the full field stress distribution over the joint. The results of the TSA are compared with the load displacement behaviour. It is shown that by increasing fingertip angle there is a decrease in load carrying capacity, a decrease in shear stress and an increase in stress concentration factor at the finger joint tip.