Contact Force Distribution Research Articles

Experimental and Numerical Analysis on Mesoscale Mechanical Behavior of Coarse Aggregates in the Asphalt Mixture during Gyratory Compaction

Compaction is a critical step in asphalt pavement construction. The objective of this study is to analyze the mesoscale mechanical behaviors of coarse aggregates in asphalt mixtures during gyratory compaction through experiments and numerical simulation using the Discrete Element Method (DEM). A novel granular sensor (SmartRock) was embedded in an asphalt mixture specimen to collect compaction response data, including acceleration, stress, rotation angle and temperature. Moreover, the irregularly shaped coarse aggregates were regenerated in the DEM model, and numerical simulations were conducted to analyze the evolution of aggregate interaction characteristics. The findings are as follows: (1) the measured contact stress between particles changes periodically during gyratory compaction, and the amplitude of stress tends to be stable with the increase of compaction cycles; (2) the contact stress of particles is influenced by the shape of aggregates: flat-shaped particles are subjected to greater stress than angular, fractured or elongated particles; (3) the proportion of strong contacts among particles is high in the initial gyratory compaction stage, then decreases as the number of gyratory compactions grows, the contacts among particles tending to homogenize; (4) during initial gyratory compactions, the normal contact forces form a vertical distribution due to the aggregates’ gravity accumulation. The isotropic distribution of contact forces increases locally in the loading direction along the axis with a calibrated internal angle orientation (1.25°) in the earlier cyclic loading stage, then the local strong contacts decrease in the later stage, while the strength of the force chains in other directions increase. The anisotropy of aggregate contact force networks tends to weaken. In other words, kneading and shearing action during gyratory compaction have a positive impact on the homogenization and isotropy of asphalt mixture contact forces.

Efficient Three-Dimensional Model to Predict Time History of Structural Dynamics in Cold Rolling Mills

Abstract Introduced is a new physics-based three-dimensional (3D) mathematical model capable of efficiently predicting time histories of the nonlinear structural dynamics in cold rolling mills used to manufacture metal strips and sheets. The described model allows for the prediction of transient strip thickness profiles, contact force distributions, and roll-stack deformations due to dynamic disturbances. Formulation of the new 3D model is achieved through a combination of the highly efficient simplified-mixed finite element method with a Newmark-beta direct time integration approach to solve the system of differential equations that governs the motion of the roll-stack. In contrast to prior approaches to predict structural dynamics in cold rolling, the presented method abandons several simplifying assumptions and restrictions, including 1D or 2D linear lumped parameter analyses, vertical symmetry, continuous and constant contact between the rolls and strip, as well as the inability to model cluster-type mill configurations and accommodate typical profile/flatness control mechanisms used in industry. Following spatial and temporal convergence studies of the undamped step response, and validation of the damped step response, the new model is demonstrated for a 4-high mill equipped with both work-roll bending and work-roll crown, a 6-high mill with continuously variable crown (CVC) intermediate rolls, and finally a complex 20-high cluster mill. Solution times on a single computing processor for the damped 4-high and 20-high case studies are just 0.37 s and 3.38 s per time-step, respectively.

Effect of Bentonite Content and Hydration Time on Mechanical Properties of Sand–Bentonite Mixture

The bentonite is commonly used mixed with soils for groundwater retention and waste contaminant facilities. The incorporation of bentonite could significantly reduce hydraulic conductivity. In this study, the effects of bentonite content, hydration time and effective confining pressure on the static properties of a sand–bentonite mixture were studied using experimental and numerical methods. Firstly, a large number of drainage static triaxial tests on the sand–bentonite mixture with various bentonite contents were conducted. The test results show that the increase in bentonite content and hydration time leads to a slight decrease in shear strength and initial tangent modulus of the sand–bentonite mixture. The presence of bentonite reduces the shear shrinkage and dilatancy trend of the mixture. The cohesion of the mixture increases with the increase in bentonite content and hydration time, but the internal friction angle decreases correspondingly. The hydration of bentonite on the surface of sand particles changes the contact form between particles. The bentonite slurry between pores of the sand skeleton also affects the mechanical behavior of the sand–bentonite mixture. Then, a series of 3D discrete element models were established for numerical simulations of drainage static triaxial tests. The numerical model parameters were calibrated by experimental results. The meso-mechanism of bentonite content affecting the mechanical behavior was revealed according to the contact force distribution between particles. The research results are helpful to understand further the mechanism of bentonite on the mechanical properties of the sand–bentonite mixture.

Post-buckling analysis of two-layer contact concentric tubular strings in pipe

Buckling of concentric tubular strings in pipe is an essential problem in the fields of medical, biological, and nanomaterials, especially in petroleum engineering. Completely different from the buckling of the classic Euler compression columns, the buckling of concentric tubular strings involves geometric nonlinearity and contact nonlinearity, and has more complicated geometric deformations and buckling configurations. In this paper, a finite element method combining the spatial beam element and the two-layer contact gap element is proposed, and a virtual transient dynamic method is introduced to solve the static buckling of concentric tubular strings. The internal concentric tubular strings are discretized into spatial beam elements, and the contact relationships between tubular strings are described by two-layer contact gap element. Meanwhile, the larger damping is used to attenuate the dynamic response of the system, so as to find the static part of the transient dynamic solution. Appropriate damping and element length are determined by numerical simulation. The structural characteristics and contact states of eight post-buckling configurations are studied. The helix pitch values of specific configurations are in good agreement with the theoretical solutions. In addition, the distribution of bending moment, shear force and contact force in various post-buckling configurations is studied. For the full helical buckling, it is found that there is only one non-contact section in the transition section of inner tube and two non-contact sections in the transition section of middle tube. The research results of this paper provide a theoretical basis and reference for further study on the buckling of multi-layer concentric tubular strings.

NONLINEAR PROBLEM OF FRACTURE MECHANICS OF AN INTERFACE CRACK SUBJECTED TO SHEAR WAVE

Solution for the problem for an interface crack under the action of a harmonic shear wave in normal direction is presented. The contact of the crack faces is put into consideration. The problem is solved by the boundary integral equations method, the vector components in the boundary integral equations are presented by Fourier series. The unilateral Signorini boundary conditions are involved to prevent the interpenetration of the crack faces and the emergence of tensile forces in the contact zone. Amonton-Coulomb Friction Law included allows to put into consideration relative resting of the crack opposite faces or their relative motion when one crack face is slipping or sliding across another face. The contact boundary restrictions are implemented using the iterative correction algorithm. The mathematical model adequacy is checked by comparing with classical model solution for statics problems that takes into account the crack faces contact. Numerical researches of friction influence on the displacement and contact forces distribution, size of contact zone are carried out. Influence of the faces contact and value of the friction coefficient on the distribution of stress intensity coefficients of normal rupture and transverse shear, which are the parameters of the biomaterial fracture, are presented and analyzed. It is shown that the nature of change in the distribution of the stress intensity coefficients for the conditions of tensile and shear waves is fundamentally different. It is concluded that it is possible to extend the approach proposed to the problems of three-dimensional fracture mechanics for composites with interfacial cracks at arbitrary dynamic loading.

Sim-to-Real for High-Resolution Optical Tactile Sensing: From Images to Three-Dimensional Contact Force Distributions.

The images captured by vision-based tactile sensors carry information about high-resolution tactile fields, such as the distribution of the contact forces applied to their soft sensing surface. However, extracting the information encoded in the images is challenging and often addressed with learning-based approaches, which generally require a large amount of training data. This article proposes a strategy to generate tactile images in simulation for a vision-based tactile sensor based on an internal camera that tracks the motion of spherical particles within a soft material. The deformation of the material is simulated in a finite element environment under a diverse set of contact conditions, and spherical particles are projected to a simulated image. Features extracted from the images are mapped to the three-dimensional contact force distribution, with the ground truth also obtained using finite-element simulations, with an artificial neural network that is therefore entirely trained on synthetic data avoiding the need for real-world data collection. The resulting model exhibits high accuracy when evaluated on real-world tactile images, is transferable across multiple tactile sensors without further training, and is suitable for efficient real-time inference.

3D Morphology Distribution Characteristics and Discrete Element Simulation of Sand-Gravel Mixtures

Sand-gravel mixtures are typical binary materials, exhibiting highly heterogeneous, discontinuous, and significant structural effects. The contact state between sand and gravel particles has a significant influence on the mechanical properties of the mixtures. This article focused on the complex internal structure and its mesostructural behavior of the mixtures, and a systematic statistical analysis was carried out to study the shape, size, and angularity of the coarse particles. The three-dimensional (3D) shapes of coarse aggregates were approximated to be hexahedron, pentahedron, and tetrahedron. An indicator called angularity and surface texture (AT) index was developed to characterize the combined effect of the coarse aggregate angularity and surface texture. Based on the screening testing and digital image processing, the particle size and AT index of aggregates were extracted, and their means, standard deviations, and statistical distributions were studied. An algorithm for generating 3D aggregates was developed based on the statistical results of the coarse aggregate 3D morphology. The coarse aggregate generating code was written using the fish language in PFC3D. The numerical model was then applied to conduct three typical monotonic or cyclic triaxial test simulations. Retrospective simulation of the laboratory tests using the proposed model showed good agreement, and the reliability of the model is effectively verified. The results interpreted well the mechanism of particle motion and the distribution of interparticle contact force during shearing from mesoscale of the mixtures, which can give better understanding and modeling of the nonlinear behavior of the sand-gravel mixtures.

SmAvo: Packhouse optimization using smart avocadoes in South Africa

Cultivation of avocadoes serves as a key economic driver for many countries, including South Africa. Nearly all fresh produce produced forms part of a complex, interconnected transportation chain. Packhouses, representing a wide range of capacities and degrees of mechanization, serve to classify, treat and package the fresh produce prior to distribution by road, rail and ocean to its destination. Reducing the damage or postharvest stresses is paramount to ensure consistent quality of the ripened fruit. Packhouse operations represent the phase associated with the greatest degree of mechanical handling. Until now, quantification of packhouse activities exerted on the fruit has not been quantified with sufficient accuracy to identify characteristics of an optimized packhouse. This paper demonstrates the successful application of a novel sensor platform that measures the linear acceleration and rotational velocity of a typical avocado for 24 packhouses spread throughout South Africa. Unique statistics were developed pertaining to the distribution of contact forces, peak accelerations and rotational velocities, freefall events, freefall distance and impact energy experienced by avocadoes. A Damage Index Score (DIS) was developed to classify the relative performance of the sampled packhouses, alongside identification of key areas of concern where the greatest intensity and variation of postharvest stresses are observed. Improvement of avocado quality as a result of improving deficient practices and equipment ultimately reduces waste, prevalence of postharvest diseases and improves the income for farming communities.

A computationally efficient method for the prediction of fretting wear in practical engineering applications

A method for simulating fretting wear using the Modified Simplex Method for a contact solution has been developed. The initial separation between two contacting bodies was used as an input to solve the contact force distribution. An average cycle pressure distribution was calculated for the stationary surface over a displacement cycle. The wear depth was calculated for each body based on the modified Archard’s wear equation using the force distributions and the gross sliding distance. The initial separation was updated and the force distribution was solved for the next iteration. Methods for optimizing computational time are presented using a combination of linear jumping and adaptive cycle jumping for the wear depths, and an interpolation weighting method for reducing the grid size. It was found that computational time can be reduced by at least 98% compared with other simulation methods, making this method a viable tool for design. Fretting wear scars and depths were simulated for a cylinder on flat in contact and were found to agree with experimental results and Finite Element modeling results from previous literature. To show the capability of the fretting wear model, three practical applications were simulated: automotive seat sliding rails, steel wire ropes for industrial applications and steam generator tubes for nuclear power stations.

Size effect on contact behavior in DEM simple shear tests

The 2–2.5 times the simulated sand diameter is widely accepted in giving reasonable DEM simulation results for geotechnical testing. However, it neglects the effect of a specimen height to maximum particle diameter ratio in a specific laboratory test, which may lead to a strong stress concentration and flawed simulations. This study compared laboratory simple shear tests with corresponding DEM simulations with different particle sizes. The DEM model used clump rings to simulate physical rings in the test, and decreased the additional stress applied by the widely used wall-type rings. Results showed that (1) DEM models with tested particle size and twofold sand particle size (1D and 2D tests) can better capture the tested stress–strain behavior, volumetric changes, and noncoaxiality, the 4D model has an asymmetrical distribution of contact force and contact number, indicating the specimen is inhomogeneous and has a strong stress concentration. (2) a specimen height to maximum particle diameter ratio smaller than 10 (it is greater than 10 in the ASTM D6528) could provide reasonable macro-meso mechanical behaviors. Similar studies should be carried out after trial tests on determining a reasonable specimen height to maximum particle diameter ratio under the guidance of ASTM D6528.

Rock breaking mechanism during TBM tunnelling: insight from DEM simulations of rotary cutting tests

Tunnel boring machine (TBM) has been widely applied to various tunnel construction projects in hard rocks due to its high efficiency and safety. A number of theoretical or semi-empirical models have been proposed to predict TBM performance under different geological conditions. Cutter forces in these theoretical models are derived mainly based on idealized assumptions, related to the cutting pattern and rock breaking mechanism. However, there is no consensus on the rock breaking as well as the cutter-rock interaction mechanisms. In this study, DEM simulations of the rotary cutting tests are conducted using PFC3D. The cutter-rock contacting behaviour is monitored and the cracking events during the cutting process are recorded. The variation of cutting forces is found to be linked to both the occurrence rate and the total number of cracks. The spatial orientation of cracks is also examined, which seems to vary from horizontal-dominant to vertical-dominant with an overall isotropic pattern. Lastly, the distribution of contact forces between the disc cutter and the rock sample is analysed and its potential implication on the development of TBM performance model is discussed.

Reducing static friction to improve backdrivability in torque control of traveling wave ultrasonic motor

In this study, we aimed to improve the backdrivability and torque control of a traveling wave ultrasonic motor (TWUSM). Because of its operating principle, a TWUSM has strong holding torque. While this feature is useful for maintaining the rotor in a fixed position in many applications, it can be problematic for applications that require backdrivability, such as assistive exoskeletons or haptic devices. We used the theoretical contact model, which considers tangential elasticity, to calculate the torque–speed characteristics of a TWUSM. To evaluate backdrivability, we simulated not only the case in which the TWUSM drives the output shaft actively, but also the case in which the output torque opposes the rotation direction and the TWUSM is driven by an external torque passively. According to the analysis of the contact force distribution, the holding torque was reduced because the sticking contact area was decreased by stator vibration. To verify the simulation results, we built an experimental setup to measure the actual torque characteristics, and the obtained results agreed with the simulation. Furthermore, using the same experimental setup, we compared two torque control methods, driving frequency control and phase difference control. As the theoretical model predicted, the latter method resulted in higher control accuracy.

Study of grasp-energy based optimal distribution of contact forces on a humanoid robotic hand during object grasp

AbstractA novel grasp optimization algorithm for minimizing the net energy utilized by a five-fingered humanoid robotic hand with twenty degrees of freedom for securing a precise grasp is presented in this study. The algorithm utilizes a compliant contact model with a nonlinear spring and damper system to compute the performance measure, called ‘Grasp Energy’. The measure, subject to constraints, has been minimized to obtain locally optimal cartesian trajectories for securing a grasp. A case study is taken to compare the analytical (applying the optimization algorithm) and the simulated data in MSC.Adams $^{^{\circledR}}$ , to prove the efficacy of the proposed formulation.

Fruit Classification Utilizing a Robotic Gripper with Integrated Sensors and Adaptive Grasping

As the core component of agricultural robots, robotic grippers are widely used for plucking, picking, and harvesting fruits and vegetables. Secure grasping is a severe challenge in agricultural applications because of the variation in the shape and hardness of agricultural products during maturation, as well as their variety and delicacy. In this study, a fruit identification method utilizing an adaptive gripper with tactile sensing and machine learning algorithms is reported. An adaptive robotic gripper is designed and manufactured to perform adaptive grasping. A tactile sensing information acquisition circuit is built, and force and bending sensors are integrated into the robotic gripper to measure the contact force distribution on the contact surface and the deformation of the soft fingers. A robotic manipulator platform is developed to collect the tactile sensing data in the grasping process. The performance of the random forest (RF), k-nearest neighbor (KNN), support vector classification (SVC), naive Bayes (NB), linear discriminant analysis (LDA), and ridge regression (RR) classifiers in identifying and classifying five types of fruits using the adaptive gripper is evaluated and compared. The RF classifier achieves the highest accuracy of 98%, while the accuracies of the other classifiers vary from 74% to 97%. The experiment illustrates that efficient and accurate fruit identification can be realized with the adaptive gripper and machine learning classifiers, and that the proposed method can provide a reference for controlling the grasping force and planning the robotic motion in the plucking, picking, and harvesting of fruits and vegetables.

Discrete Element Modeling of Vibration Compaction Effect of the Vibratory Roller in Roundtrips on Gravels

Abstract This paper aims to study the vibration compaction mechanism of the vibratory roller on gravels using a two-dimensional discrete element method. The roadbed model was established by gravel particles with irregular shapes, which was closer to reality. The performance parameters of the vibratory roller, such as operating frequency and rolling velocity, were investigated to explore their influences on the operating efficiency of the vibratory roller in roundtrips. The frequencies of 15 Hz and 17 Hz were proved to be the optimal frequency and resonance frequency in the current simulations, respectively. The vibratory roller could achieve a better vibration compaction effect with less power consumption at the optimal frequency. In addition, the number of roundtrips and power consumption should be considered in the selection of the optimal rolling velocity. The movement direction and the contact force distribution of gravels were illustrated by the displacement field, velocity field, as well as the contact force chains. Our results provide a better understanding of the mechanical behavior of gravel particles and their interactions with the vibratory roller.

Dysplastic Patellofemoral Joints Lead to a Shift in Contact Forces: A 3D-Printed Cadaveric Model

Background: The distribution of contact forces across the dysplastic patellofemoral joint has not been adequately quantified because models cannot easily mimic the dysplasia of both the trochlea and the patella. Thus, the mechanical consequences of surgical treatments to correct dysplasia cannot be established. Purpose/Hypothesis: The objective of this study was to quantify the contact mechanics and kinematics of normal, mild, and severely dysplastic patellofemoral joints using synthetic mimics of the articulating surfaces on cadavers. We tested the hypothesis that severely dysplastic joints would result in significantly increased patellofemoral contact forces and abnormal kinematics. Study Design: Controlled laboratory study. Method: Patellofemoral dysplasia was simulated in 9 cadaveric knees by replacing the native patellar and trochlear surfaces with synthetic patellar and trochlear implants. For each knee, 3 synthetic surface geometries (normal, showing no signs of dysplasia; mild, exemplifying Dejour type A; and severe, exemplifying Dejour type B) were randomized for implantation and testing. Patellar kinematics and the sum of forces acting on the medial and lateral patellar facets were computed for each knee and for each condition at 10° increments from 0° to 70° of flexion. Results: A pronounced lateral shift in the weighted center of contact of the lateral facet occurred for severely dysplastic knees from 20° to 70° of flexion. Compared with normal geometries, lateral patellar facet forces exhibited a significant increase only with mild dysplasia from 50° to 70° of flexion and with severe dysplasia at 70° of flexion. No measurable differences in medial patellar facet mechanics or joint kinematics occurred. Conclusion: Our hypothesis was rejected: Severely dysplastic joints did not result in significantly increased patellofemoral contact forces and abnormal kinematics in our cadaveric simulation. Rather, severe dysplasia resulted in a pronounced lateral shift in contact forces across the lateral patellar facet, while changes in kinematics and the magnitude of contact forces were not significant. Clinical Relevance: Including dysplasia of both the patella and trochlea is required to fully capture the mechanics of this complex joint. The pronounced lateralization of contact force in severely dysplastic patellofemoral joints should be considered to avoid cartilage overload with surgical manipulation.

Advancements in Geo-Inclusions for Ballasted Track: Constitutive Modelling and Numerical Analysis

This paper reviewed some salient features evolving through mathematical and numerical modelling of ballasted track components incorporating recycled rubber products. Firstly, a constitutive model based on the bounding surface concept was introduced to simulate the shear stress-strain response of waste mixtures (i.e., recycled rubber crumbs, coal wash, and steel furnace slag) used for the capping layer placed below the ballast medium, whereby the energy absorbing property resulting from the inclusion of different amounts of rubber has been captured. Subsequently, key research findings concerning the inclusion of recycled rubber mats on ballasted tracks for reduced particle degradation under cyclic loading were examined and discussed. Discrete element modelling (DEM) coupled with Finite element modelling (FEM) to micro-mechanically characterise ballast behaviour with and without rubber mats offers invaluable insight into real-life track operations. In particular, this coupled DEM-FEM model facilitates the exploration of micromechanical aspects of particle breakage, contact force distributions within the granular assembly, and the orientation of contacts during cyclic loading.

Normal contact forces in jacking of casings under conditions of their floating-up and sticking

Trenchless technologies are commonly used in the collector tunnel construction in the world. The most advanced technology in this regard is microtunneling/casing jacking—when casing pipes are forced in ground using bentonite slurry to reduce friction and to make soil mass stable. Two phenomena are possible in this case: floating-up of the casing and its sticking under differential pressure. Tunneling in water-saturated ground necessitates prediction of jacking forces. Here, it is critical to determine resistance to the casing moving off from the rest. To this effect, tangential forces along the casing–soil contact are analyzed, as a rule. The main characteristics are usually the contact area in the roof of the underground opening and the normal force distribution along the casing. The estimation procedure uses three parameters: flexural stiffness of casing, soil hardness and construction clearance. This article offers a model representation of joint casing–soil deformation under conditions of floating-up and sticking of the casing in the roof of the opening under the action of differential pressure. The system of ordinary differential equations describing the casing behavior in the zones of contact and no-contact with soil is obtained and solved. The analytical solution is used to determine the contact zone size and to estimate the normal contact forces. The contact zone sizes versus reduced floating-up lengths of casing and soil hardness in the actual value range are plotted. It is found that the contact zone sizes grow when casing length is within the typical floating-up inflexion length and drop when casing length is 5–10 typical inflexion lengths with respect to soil hardness. In a general case, the contact zone size increases with decreasing flexural stiffness of casing and hardness of soil. The normal contact force distribution has a shape of a hip roof and features higher concentration at the end points of the contact zone in soil of higher hardness. The research results can be used to improve the model of casing breakaway in order to have more accurate estimates of jacking forces.

Gradation optimization and strength mechanism of aggregate structure considering macroscopic and mesoscopic aggregate mechanical behaviour in porous asphalt mixture

Porous asphalt mixture (PAM) as a special pavement material with discontinuous gradation has been widely used in pavement engineering because of its versatility caused by high air void content. Aggregate structure plays a significant role in the pavement performance of porous asphalt mixture. In this study, the effects of different size aggregates on the air void ratio and macroscopic aggregate bearing capacity are investigated to put forward an aggregate gradation with a strong aggregate structure. Laboratory tests show that the proposed gradation provides an improved pavement performance for porous asphalt mixture. Moreover, strength mechanisms of aggregate structure in porous asphalt mixture are revealed from a view of the mesoscopic aggregate contact force composition using discrete element method (DEM) simulations considering aggregate irregularity. Finally, the mechanical role of each size aggregate on the aggregate structure is analyzed via mesoscopic aggregate contact force distribution.

Effect of particle size on the failure behavior of cemented coral sand under impact loading

The effect of particle size distribution (PSD) of coral sands on the failure behavior of cemented coral sand specimens is investigated in this study. A series of static and impact loading tests on the specimens with various grain grading curves have been carried out. Test results show that the compression strength and impact resistance of the cemented coral sand specimen are increased with the increasing of the uniformity coefficient of coral sand particles. The specimen with a higher value of the uniformity coefficient would have a smaller failure angle under the same impact loads. In addition, the micro mechanism of PSD on the macroscale mechanical behavior is explored by a three-dimensional discrete element model. Four typical realistic particles are scanned and incorporated in the numerical models. The numerical model is established with the same PSD and impact loading conditions. Calibration tests are carried out to verify the numerical model. Then, a series of numerical analysis are conducted to examine the effects of particle shape and PSDs on the failure behavior. Analysis results indicate that the contact force distribution and absorbed energy can be linearly influenced by the PSDs. The realistic particles with a lower shape factor will result in a higher impact resistance. Thus, an optimum PSD can be design to meet the requirement of impact loadings. The outcome of this study could benefit for the foundation design with the consideration of the impact loadings.