Frictionless Conditions Research Articles

Experimental study into the effects of pitch and coning angles on the flight performance of the natural samara.

Using their light wing and through the use of a leading-edge vortex (LEV), autorotating samaras can generate high lift while descending at extremely low speeds. But the flight performance of the samara, with respect to the wide design envelope, is still not well understood. Therefore, this paper aims to experimentally assess how the flight performance of three natural samara wings varies with differing wind speeds and flight conditions. The tests were conducted within a vertical wind tunnel and a novel rig was devised to effectively measure the vertical thrust and rotational rate of the autorotating samara at near frictionless conditions. Furthermore, a bespoke hub was implemented to control the coning and pitch angles of the samara wing. The tests generated a novel and comprehensive set of experimental data of autorotating samaras with changing wind speed, coning and pitch angles. The results also revealed that coning angles between 5 and 15 degrees can increase the vertical thrust produced by the samara by up to a maximum of . Additionally, it was found that samaras operate at extremely low pitch angles between -0.7 and -2.6 degrees to maximise their thrust, even though the conditions are close to the autorotational stability boundary.

Phase Field Fracture Model for Assessing the Load Bearing Capacity of Fractured Glass

The use of glass as structural material has highlighted the need for more reliable numerical approaches to analyze its mechanical behavior, especially in the accidental eventuality of fracture. Modelling the behavior of fractured laminated glass, in fact, is fundamental to assess the Post-Fracture load-bearing capacity. However, this is a highly challenging task because of the many interplaying factors, such as the viscoelastic and thermal-dependent behavior of the interlayer, the presence of a highly complex and variable crack pattern and the interaction among fragments. The objective of the present work is the development and testing of a robust numerical model that can naturally introduce the generated crack pattern into virtual specimens and manage the interaction among many fragments. The phase field fracture model is herein explored, by assigning the damage variable to fit the pre-existing crack pattern. Then, the specimen is loaded letting the phase field managing the fragments interaction. The dependence of the stress tensor with the damage variable is herein defined through the Cleavage-Deviatoric model, since it prevents fully damaged regions from transmitting tensile and shear stresses yet keeping their ability to bear compressive forces. Indeed, this model can asymptotically reproduce unilateral and frictionless contact conditions between the existing crack lips. Preliminary case studies are discussed to check the potentiality of the proposed approach.

Investigation of the impact-sliding fretting behaviour of 690 alloy tube using the finite element method

Abstract Flow induced vibration inevitably leads to fretting damage behavior on the surface of steam generator tubes. Impact-sliding fretting wear indicates that alloy tube surface experiences a dynamic impact and a sliding shear behavior simultaneously. Finite element analysis was conducted to investigate the dynamic mechanical response of the Inconel 690 alloy tube, which is influenced by different impact-sliding fretting parameters under frictionless condition. Results showed that the effects of sliding frequency and amplitude on the contact stress, elastic-plastic strain and energy dissipation of the fretting interface were not directly proportional. Increasing the impact amplitude would enhance this dynamic behavior effect.

A comprehensive deep learning geometric shape optimization framework with field prediction surrogate and reinforcement learning

The optimization of aerodynamic components' geometric shapes demands a novel technical approach for adaptive and efficient exploration and decision-making within the design space. In this study, we introduce an innovative shape optimization framework that leverages deep reinforcement learning with neural network surrogate models. The field prediction surrogate, realized by two distinct U-net architectures, can efficiently generate holistic field solutions based on the transformed mesh coordinates. Subsequently, an inference engine dynamically calculates the key metric of the flow fields, serving as the objective function for the subsequent geometry-aware Deep Q network (DQN)-based optimization. The framework's efficacy is validated using a rocket nozzle as an illustrative example. During surrogate validation, under both friction and frictionless conditions, the l1 errors of the entire flow field of both the U-net vision transformer (ViT) and U-net convolutional neural network (CNN) architectures are less than 0.4%. The proposed U-net ViT consistently outperforms U-net CNN, and the superiority is particularly evident in complex flow areas, outlet sections, and vacuum thrust prediction. Following training, the DQN model is employed to explore the design variable space. The B-spline defining profile successfully is optimized to a final expanding segment shape with improved thrust. Under frictionless conditions, it closely approaches the theoretical optimum. In the practical condition considering friction, the optimized shape gains a 2.96% thrust improvement. The results demonstrate that the proposed framework, especially when coupled with U-net ViT, exhibits enhanced accuracy and adaptability for shape optimization tasks.

Robust macroscale superlubricity on carbon-coated metallic surfaces

This paper discusses experimental and computational results of ultralow (near-zero) friction of carbon-coated metallic depositions on substrates of structural steels, Ti, and Ni alloys. The macroscale superlubricity was demonstrated and sustained over several cycles through structurally misoriented carbon coatings on the metallic surfaces. Carbon nanocrystals with variants of graphene footprints were deposited on these metallic surfaces using a novel high temperature biowaste treatment process. The carbon nanocrystals deform, flatten, and coalesce in the wear tracks to form graphitic films leading to a superlubricious coefficient of friction of ∼0.003. A coating life of ∼150,000 cycles with reduced wear rates was obtained on Ni and steel substrates. The experiments were validated with atomistic simulations providing mechanistic insights into the effects of the graphene variants on the observed frictionless conditions. The underlying mechanisms of the coating/substrate interactions contributing to the macroscale superlubricity are elucidated. The implications of the current results are explored for designing robust and low-cost macroscale superlubricious carbon coatings on metallic substrates. Biowaste is a carbon source within a circular economy that uses material recycling to reduce the global carbon footprint.

Improving the performance of nonlinear isolator through triboelectric nanogenerator damper integrating energy harvesting

In order to solve the problem of integration of low-frequency vibration isolation and vibration energy harvesting, this paper proposes a novel nonlinear oscillator with triboelectric nanogenerator damper (NO-TENGD), which can realize different nonlinear characteristics of quasi-zero stiffness (QZS) and bistable by changing the length of rigid links. The mechanism of improving the isolation performance of NO-TENGD was analyzed and discussed with a dynamic model proposed, and the effects of excitation frequency, friction force, geometric parameters, and spring stiffness were studied. The damping can be increased by adjusting the pressure of TENGD, which can effectively suppress the resonance peak value of response amplitude and force transmissibility. The experimental results show that when the excitation frequency is larger, the amplitude response of NO-TENGD is smaller under frictionless condition, and the amplitude response of NO-TENGD under large friction condition is smaller than that under small friction one. NO-TENGD with QZS has a good vibration isolation effect, and the output power can light up 50 LED lights or electronic watches. Finally, NO-TENGD with QZS can achieve low frequency vibration suppression, and increasing damping can reduce the response amplitude and force transmissibility of the beam bridge in the resonance frequency range.

In-hand manipulation using a 3-PRS-finger-based parallel dexterous hand with bidirectional pinching capability

Considering the defects of the traditional dexterous hand in precise in-hand manipulation and bidirectional pinching capability, this paper proposes a 3-PRS-finger-based parallel dexterous hand (PDH). The PDH structure is composed of three 3-PRS parallel fingers. First, a pinching shape analysis is conducted to demonstrate the PDH's bidirectional pinching capability. Then, by analyzing the workspace of the PDH pinching nine objects, the influence on the in-hand manipulation workspace of different objects is determined. The PDH can perform symmetrical in-hand manipulation to address the issue that almost all humanoid dexterous hands pinch objects in a single direction. The force transmission capability of the PDH is compared with that of the 3R serial dexterous hand (SDH) under the same trajectory. The results indicate that the PDH have the possibility to possess a good pinching force performance through the specific trajectory planning. Bidirectional pinching experiments are conducted, and the experimental results demonstrate that the PDH can realize the accurate in-hand manipulation under almost frictionless conditions. The proposed PDH provides a feasible design for the theoretical development and practical application of dexterous hands.

Lagrange multiplier imposition of non-conforming essential boundary conditions in implicit material point method

The Material Point Method (MPM) is an established and powerful numerical method particularly useful for simulating large-scale, rapid soil deformations. Therefore, it is often used for the numerical investigation of mass movement hazards such as landslides, debris flows, or avalanches. It combines the benefits of both mesh-free and mesh-based continuum-based discretization techniques by discretizing the physical domain with Lagrangian moving particles carrying the history-dependent variables while the governing equations are solved at the Eulerian background grid, which brings many similarities with commonly used finite element methods. However, due to this hybrid nature, the material boundaries do not usually coincide with the nodes of the computational grid, which complicates the imposition of boundary conditions. Furthermore, the position of the boundary may change at each time step and, moreover, may be defined at arbitrary locations within the computational grid that do not necessarily coincide with the body contour, leading to different interactions between the material and the boundary. To cope with these challenges, this paper presents a novel element-wise formulation to weakly impose non-conforming Dirichlet conditions using Lagrange multipliers. The proposed formulation introduces a constant Lagrange multiplier approximation within the constrained elements in combination with a methodology to eliminate superfluous constraints. Therefore, in combination with simple element-wise interpolation functions classically utilized in MPM (and FEM) to approximate the unknown field, a suitable Lagrange multiplier discretization is obtained. In this way, we obtain a robust, efficient, and user-friendly boundary imposition method for immersed methods specified herein for implicit MPM. Furthermore, the extension to frictionless slip conditions is derived. The proposed methodologies are assessed by comparing the numerical results with both analytical and experimental data to demonstrate their accuracy and wide range of applications.

Thermodynamically consistent effective stress formulation for unsaturated soils across a wide range of soil saturation

We outline an extension of Biot’s theory of dynamic wave propagation in fluid-saturated media, which can be used to model dynamic soil-structure interaction in frictionless conditions across a wide range of soil saturation levels. In this regard, we present a comprehensive analysis of experimental evidence, the thermodynamic, and the theoretical basis of using the degree of saturation as Bishop’s parameter in unsaturated soils. The analysis highlights the limitations of using this parameter to accurately model unsaturated soil behaviour, particularly as the soil approaches dryness. Based on the analysis, a new definition of effective stress is proposed, and the associated work-conjugate pairs are identified. Recommendations are made for constitutive modelling using the new definition of effective stress. Finally, we introduce a fully coupled finite element contact model that utilises the new effective stress definition. Through numerical examples, we demonstrate the model’s capability to control the vanishing capillary effect on soil-structure interaction as the soil dries.

Stiffness analysis of rotary elastic joint for a flexible bucket wheel reclaimer via theoretical, numerical and experimental methods

Aiming at reducing remaining materials and improving efficiency during bulk materials clearance, a Flexible Bucket Wheel Reclaimer (FBWR) was designed using a series of compact rotary elastic joints to enable bucket adjustment. The dynamics of the FBWR was analysed to calculate the contact force of the buckets. The stiffness of the elastic joints was investigated theoretically, numerically and experimentally for both frictionless and frictional conditions. From comparisons, good agreements were achieved by the calculations, simulations and tests. Furthermore, sensitivity analysis was conducted for the rotary elastic joints, including the stiffness, the rotation, the friction force and the stiffness ratio, from which the trends of these parameters in relationship to the Coefficient of Friction (CoF) and the Elastic Modulus (EM) were obtained. It is believed that the findings in this study can assist in the design of FBWRs and facilitate their applications in practice.

Numerical and experimental analysis of ultra-thin plies in carbon/epoxy cross-ply laminates subjected to bending after tensile cyclic loading

As a result of a fatigue testing campaign in cross-ply laminates with different thicknesses in the 90° ply block (conventional and ultra-thin), different cracking patterns were observed. While laminates with conventional 90° ply thickness show a damage pattern with transverse cracks in the 90° ply block and delaminations along the 0°/90° interface, laminates with ultra-thin plies show paths of debonds inside the ultra-thin 90° ply block both transverse and longitudinal to the loading direction. In order to see the influence that these different fatigue damage patterns could have on the mechanical behaviour of the laminates, static bending tests have been carried out after cyclic loading, and apparent flexural stiffness loss (concerning the pristine state) was measured. Numerical simulations of the bending test, including the cyclic damage pattern (and detailed non-linear frictionless contact conditions) were also carried out for a deeper understanding of the experimental evidence. Although the morphology of the fatigue damages encountered is different for the two studied 90° ply block thicknesses, their flexural behaviours are quite similar, in terms of apparent flexural stiffness loss, for both cross-ply laminates, independently of its 90° ply block thickness.

Characteristic material behaviors in conical indentation with friction

AbstractCharacteristic material behaviors in obtuse‐angled conical indentation for elasto‐viscoplastic solids are explored and discussed. Finite element calculations are carried out for indentation of a rigid sharp conical indenter into a cylindrical body that models a pseudo‐half space, considering the friction between the indenter and the solid. Under low‐friction conditions, it is assumed that breakage of the material by the sharp tip of the indenter occurs and a new surface emerges along the contact interface during indentation. It is revealed that the load required for indentation does not necessarily reach its maximum at the perfect sticking condition, which shows a nonlinear variation from the frictionless condition to the perfect sticking condition. The increase in strain hardening and the decrease in elastic modulus relative to yield strength both reduce the difference between the required loads for different friction conditions. For perfect sticking, the indenter tip is covered by an elastically deforming material cap during indentation similarly to the case of the plane strain wedge indentation. Similarity and discrepancy between the behavior in the conical indentation and that in the plane strain wedge indentation, which is well described by the classical slip‐line field theory, are clarified in detail.

Structural response of a compliant pipe-in-pipe under frictionless and frictional conditions of the seabed

Pipe-in-Pipe (PIP) technology has been studied significantly owing to its superior performance in deep-water and high-pressure high temperature fields than conventional single pipe. The PIP system has excellent track record of mitigating flow assurance problems from subsea wells through maintenance of the fluid's temperature in the pipe. It has also been applied in marine environment where conventional single pipe cannot perform. However, owing to complex interaction and contact within the PIP system and seabed, the mechanism of load transfer and the stresses that developed due to pressure, temperature and combined loading has not been fully understood and quantified. Therefore, this study examined the effect of pressure, temperature and the combined loading on PIP systems for flat seabed subsea pipeline. Simulations are performed to examined frictional and frictionless conditions of the flat seabed on PIP system and individual results of inner pipe, insulation material and outer pipe are presented for each analysis. The analytical calculations are carried-out for determining the operating stresses in each component of the PIP system in view of its significance for the overall structural behaviour of the system and validation of the numerical model. The impact response of the inner pipe, insulation and the outer pipe based on pressure, temperature and the combination of both (pressure and temperature) and the resulting stress on each component of the PIP system are investigated and the result presented. Furthermore, results of axial, radial and hoop stresses for the individual loading condition and with coupled analysis corresponding to each simulation (Frictional and Frictionless seabed conditions) are found to be closely similar with percentage difference less than 5 except for the von Mises stress which give 5.3%. This interesting finding revealed that the friction force does not affect structural integrity of the PIP system compared to conventional - single pipeline assuming all other parameters remains constant. Moreover, the presence of the outer pipe and the insulation material enhanced the performance of the inner pipe. The numerical simulation predicts closely the impact response for pipe-in-pipe composite specimens under individual and combined loading conditions. Therefore, the results obtained will serve as a reference guide for designing, construction and operating PIP system in the future to develop unconventional challenging energy resources like High Pressure High Temperature fields.

Effect of Friction on Finite Element Contact and Wear Predictions of Metal-on-Plastic Hip Replacements

Finite Element wear predictive modelling is a powerful tool that can replace or support costly and time-consuming wear tests. Despite the lively research in this field, particularly in applications to hip replacements, some issues are still open, such as the role of friction in numerical simulations. Indeed, in most wear models, frictionless contact conditions are assumed and wear routines are independent from f. However, the effect of friction on contact and wear modelling of metal-on-plastic hip replacements has never been fully explored in the literature.In this study we analyse how the friction coefficient affects both the contact features (nominal contact point trajectory, components of the contact force and contact pressure) and the wear parameters, (wear volume and linear wear distribution). A case study of a 32 mm implant was simulated under two different loading and kinematic conditions, considering both vertical and 3D load, and varying the friction coefficient f within the range 0–0.3, for a given k value. Results show that a frictional contact has longer and wider trajectories of the nominal contact point, but slightly lower normal force and contact pressure values, with respect to a frictionless solution. Consequently, contact pressure maps are shifted with respect to the frictionless case but values remain almost the same changing f. The wear volume slightly decreases with f whilst wear maps are very sensitive to it. Results suggest that for f ≤ 0.1 the frictionless hypothesis can be adopted achieving accurate results with the advantage of reduced computational costs.

Generalizations of incompressible and compressible Navier–Stokes equations to fractional time and multi-fractional space

This study develops the governing equations of unsteady multi-dimensional incompressible and compressible flow in fractional time and multi-fractional space. When their fractional powers in time and in multi-fractional space are specified to unit integer values, the developed fractional equations of continuity and momentum for incompressible and compressible fluid flow reduce to the classical Navier–Stokes equations. As such, these fractional governing equations for fluid flow may be interpreted as generalizations of the classical Navier–Stokes equations. The derived governing equations of fluid flow in fractional differentiation framework herein are nonlocal in time and space. Therefore, they can quantify the effects of initial and boundary conditions better than the classical Navier–Stokes equations. For the frictionless flow conditions, the corresponding fractional governing equations were also developed as a special case of the fractional governing equations of incompressible flow. When their derivative fractional powers are specified to unit integers, these equations are shown to reduce to the classical Euler equations. The numerical simulations are also performed to investigate the merits of the proposed fractional governing equations. It is shown that the developed equations are capable of simulating anomalous sub- and super-diffusion due to their nonlocal behavior in time and space.

Isothermal hot tube material characterization – Forming limits and flow curves of stainless steel tubes at elevated temperatures

A new isothermal hot-tube-bulge test as well as a modified pressurized-tube-tensile test are presented to characterize forming limits of tubular materials at elevated temperatures – up to 1000 °C. By controlling temperature, internal pressure, and axial force the complete forming limit curve is determined under isothermal, frictionless conditions and nearly linear strain paths. In the hot-tube-bulge test, a tangential localization forms before failure, marking a forming limit. In the pressurized-tube-tensile test, an axial localization occurs. For the former, a new method for estimating the forming limit was developed whereas the later can be evaluated by standardized methods. The resulting forming limits are at comparable levels to those known from literature, while the new hot-tube-bulge test overcomes the uncertainty which arises from friction in other test setups. The materials used for the investigations are the ferritic and martensitic stainless steels X2CrTiNb18 and X12Cr13, respectively. The hot-tube-bulge test is examined for its suitability in determining flow curves. An empirical-analytical model is introduced to determine the stress components. It is based on tangential and meridional force equilibria, taking the internal pressure and the projected effective areas into account. Flow curves up to effective plastic strains of 0.5 are determined, exceeding the maximum strain of common hot tensile-test by a factor of 2. The strain-rate dependent stress-strain data was used to identify the parameters of a simplified Hensel-Spittel approach. This allows the prediction of isothermal flow curves with varying strains and strain rates at a temperature of 1000 °C.

Adhesive depth-sensing indentation tests: Slopes of the force–displacement curves

Depth-sensing indentation experiments are a very important tool for estimating mechanical properties of modern materials. A review of various aspects of contact problems that used as the theoretical basis for interpretation of depth-sensing nanoindentation experiments is presented. Usually, analytical treatment of the indentation tests is based on analysis of the slopes of the force–displacement curves according to the non-adhesive Hertz contact theory. However, molecular adhesion is crucially important for many physical processes at the micro/nano-scales. Here, depth-sensing indentation techniques are reviewed and analyzed using the recent results obtained for adhesive contact problems. Fundamental relations for adhesive nanoindentation tests are derived for both frictionless and no-slip boundary conditions within the contact region. It is argued that the adhesive effects may be very important for treatment of the nanoindentation tests because the slopes of the force–displacement curves may considerably differ from the slopes derived using the non-adhesive contact theory.

Frictionless vs. Frictional Contact in Numerical Wear Predictions of Conformal and Non-conformal Sliding Couplings

The role of friction on wear evolution is manifold since it interplays with lubrication regime, nominal contact point, and contact pressure distribution. Nevertheless, in the literature many wear models simulate wear assuming frictionless contact conditions to simplify the analyses. That assumption, physically not realistic, often appears as a contradiction, permitted in numerical simulations where friction and wear can be considered independent phenomena.This study aims to validate the frictionless assumption in wear models with steady nominal contact point, such as in many common configurations, e.g. pin on plate/pin on disc. Wear was simulated according to the Archard wear law for both non-conformal and conformal pin-on-plate contact pairs in reciprocating motion, assuming frictionless and frictional contact conditions, varying the coefficient of friction f in the range 0–0.4. Finite Element wear models were developed in Ansys® both with implicit and explicit kinematics. Results demonstrate that the effect of friction on contact pressure distribution and worn profiles and on their evolution is negligible (differences lower than 0.05%). Thus, wear can be predicted using models in frictionless conditions which allow to extremely reduce the computational costs that represent a limit of FE wear simulations. Additionally, a procedure with implicit kinematics was compared to the explicit one resulting valid and computationally convenient, especially in case of non-conformal contact.

An improved upper bound analysis for study of the void closure behavior in the plane strain extrusion

In this paper, a new mathematical model has been developed to predict the void closure behavior in metal billets during the plane strain extrusion process. For this purpose, an improved upper bound analysis has been conducted by applying the power balance between the internal work rate, energy release rate along the void, and the external power. Considering the limiting condition of the derived equations on the verge of the void closure, a mechanical criterion has been obtained for finding the minimum extrusion ratio required for the closure of the elliptical voids. The formulation has been developed for both frictionless and frictional conditions on the die-billet interface. The process has also been modeled by the finite element method (FEM) and good agreement has been observed between the results of the derived analytical criterion and the FEM modeling for the minimum required extrusion ratio. It has been found that the minimum extrusion ratio required for the void closure is only a function of the extrusion die angle, the void aspect ratio, and the frictional condition on the die-billet interface. Furthermore, extruding the material under the frictional condition, and in a die with a smaller cone angle is beneficial for closing the voids.

Design and validation of a finite element model of the aponeurotic and free Achilles tendon.

The Achilles tendon (AT) is a common injury site. Ruptures are usually located in the free tendon but may cross the myotendinous junction into the aponeurotic region. Considering the possibility of aponeurotic region involvement in AT ruptures, a novel three dimensional (3D) finite element (FE) model that includes both the aponeurotic and free AT regions and features subtendon twisting and sliding was developed. It was hypothesized that the model would be able to predict in vivo data collected from the literature, thus being considered valid, and that model outputs would be most sensitive to subtendon twist configurations. The 3D model was constructed using magnetic resonance images. The model was divided into soleus and gastrocnemius subtendons. In addition to a frictionless contact condition, the interaction between subtendons was modeled using two contact formulations: sliding with anisotropic friction and no sliding. Loads were applied on the tendon's most proximal cross-section and anterior surface, with magnitudes estimated from in vivo studies. Model outputs were compared with experimental data regarding 3D deformation, transverse plane rotation, and nodal displacements in the free tendon. The FE model adequately simulated the free tendon behavior regarding longitudinal strain, cross-section area variation, transverse plane rotation, and sagittal nodal displacements, provided that subtendon sliding was allowed. The frictionless model exhibited noticeable medial transverse sliding of the soleus subtendon, which was present to a much lesser degree in the anisotropic friction model. Model outputs were most sensitive to variations in subtendon twist and dispersion of the collagen fiber orientations. Clinical Significance: This Achilles tendon finite element model, validated using in vivo experimental data, may be used to study its mechanical behavior, injury mechanisms, and rupture risk factors.