Interface Pressure Distribution Research Articles

High-resolution measurement of contact stresses and friction in indentation and machining by full-field photoelasticity

An experimental study has been made of the sliding plastic contact in wedge indentation and machining of ductile metals. The use of full-field photoelasticity has allowed direct determination of interface pressure and shear stress distribution along the tool (indenter) and workpiece sliding contact at micron-scale resolution. Sapphire, a transparent photoelastic material that possesses sufficiently high hardness to indent and machine metals, is used as the tool material. It is shown that the Coulomb friction model applies only at the edges of the contact where the interface pressure falls below a certain value associated with the material's flow stress. Within the plastic contact zone, the mode of friction as well as the relationship between interface pressure and shear stress are very complex. In both cases, it is shown that shear stress neither remains constant nor varies in proportion to the pressure. It is proposed that the complex nature of friction and stress field at the sliding contact can be understood through simultaneous observations of the metal plastic flow at and near the interface. Using in situ measurements of the velocity field near the interface and local stress distribution at the contact from photoelasticity, friction contribution to the overall work is calculated and shown to be substantial (greater than one-third) in the case of machining.

Modeling and vibration analysis of bolted composite conical-conical shells with flanges

This paper proposes a general semi-analytical modeling method of bolted composite conical-conical shells with flanges based on the first-order shear deformation theory. The coupling connections between conical shells and flanges are achieved by using the penalty function method. A bolted joint model that considers the non-uniform distribution of interface pressure is proposed, which can efficiently simulate any number of bolt connections between flanges and boundaries and between flanges and flanges. The correctness and effectiveness of the proposed modeling method are confirmed through step-by-step comparisons with finite element analysis and experimental testing. Then, the influence of various parameters on the frequency trajectories and modal shapes of bolted composite conical-conical shells with flanges is analyzed, with a particular focus on frequency veering and modal coupling vibration behaviors in these structures. The analyses indicate that the proposed modeling method can flexibly adjust structural geometric parameters, material parameters, bolt parameters, fiber layer parameters, and so on, which can efficiently guide the dynamic design of bolted composite conical shell structures with flanges.

Contact interaction model of liquid and solid phases

The work implements the approach formulated by B.V. Deryagin, which states for replacing the Coulomb and molecular (dispersion) forces with a system of tensions obtained on the basis of the mechanics of continuum equations using an incompressible liquid model. Interphasal activity modeling is considered from the standpoint of the equilibrium equations of mechanics of continuum using an incompressible liquid model and representing the interfacial tension tensor as a combination of spherical and deviatoric components. Analytical expressions are obtained for the components of the tension tensor depending on the wetting conditions of a solid surface with a liquid. Intermolecular interaction is determined by the value of the internal pressure of the liquid, which is variable over the interfacial layer thickness. The anisotropy of the interfacial tension tensor in the liquid-solid interfacial layer is expressed by the representation of the spherical and deviatoric components combination, which provides the equilibrium conditions for the interfacial layer. The authors propose to consider the adhesive contact as an external force effect of a solid surface on the volume phase of a liquid, leading to the formation of an interfacial tension tensor with components that are variable over the layer thickness. The authors established the nonlinear nature of the interfacial pressure distribution over the layer thickness, due to the difference in interfacial pressures in the liquid and solid phases in the direct contact zone. It is shown that the surface phenomena specificity is due to the deformation of the liquid volume in the interfacial layer, which leads to a change in intermolecular distances in different directions, which occurs with any kind of force acting on the volume phase of the liquid. The law of intermolecular forces distribution of a liquid depending on the distance is established based on the study results of a sitting drop. Expressions are obtained for determining the long-range interaction radius of liquid molecules and the thickness of the liquid-solid interfacial layer, which give a result that fits into the known estimates of the values of these quantities.

Research on Seat Static Comfort Evaluation Based on Objective Interface Pressure

<div>The interface pressure distribution is an important indicator for evaluating the static comfort of the seat. At present, the interface pressure of a real person is acquired through testing and used in assessing the seat comfort, which has poor consistency and accuracy, and it is difficult to combine the test results of different persons to make an objective evaluation. The idea is presented to evaluate the seat static comfort by the objective interface pressure. The H-point machine (HPM) is the standard loading device which can be used to acquire the objective interface pressure. The test criterion was made, and the measurement consistency of the standard loading device passed the experimental verification. As a case study, five seats were selected to obtain the subjective rating of static comfort and the objective interface pressure data. Based on these data, the evaluation model of seat static comfort was established. The above standard loading device, the objective interface pressure test criteria, and the seat static comfort evaluation model form a comprehensive system for assessing seat static comfort. By using the system in evaluating and optimizing the studied seat and by the afterward validation, the method of evaluating the seat static comfort by the objective interface pressure illustrated reliable and accurate performance, which is easy to use and showed the good value of engineering application.</div>

A triboelectric nanogenerator-based tactile sensor array system for monitoring pressure distribution inside prosthetic limb

In the prosthetic field, quantifying interfacial pressure distribution between the amputee's residual limb and the socket’s internal environment is imperative for developing new technologies. However, the commercially available pressure sensors have several disadvantages, namely external power supply requirements, low stability, high cost, and complexities related to integrating the sensor system into prosthetic devices. Herein, we propose a self-powered triboelectric nanogenerator (TENG) based flexible tactile sensor array system to monitor prosthetic socket internal pressure distribution in real-time. The proposed sensor consists of biocompatible polydimethylsiloxane (PDMS) polymer and polycaprolactone (PCL) nanofiber membranes as triboelectric materials. The biodegradable PCL nanofiber membrane, fabricated via a scalable and cost-effective electrospinning process, possesses a high surface area-to-volume ratio, facilitating enhanced charge generation during triboelectrification. The as-fabricated TENG-based tactile sensor is highly stable up to 10,000 cycles. Furthermore, the voltage of the tactile sensor is independent of different temperature and humidity values, demonstrating the stability of the sensor under various environmental conditions. Finally, we successfully integrated the tactile sensor array within a prosthetic device to monitor real-time pressure distribution inside the prosthetic socket during gait simulation. We believe that the proposed novel design provides a new dimension for the rapid development of self-powered, low-cost, and highly stable pressure monitoring systems with considerable potential for commercialization.

Nonlinear vibration analysis for bolted CFRC plates based on displacement-dependent surface spring-damping model of bolted joint

Under the effect of external excitation, bolted structures are prone to stiffness weakening and damping energy dissipation at the bolted joint, which makes the structures show nonlinear vibration behavior. In this paper, the displacement-dependent surface springs and tangential surface damping expressed by the S-curve model are proposed to simulate the nonlinear mechanical properties of the bolted joint. The non-uniform distribution of the mechanical parameters for the bolted joint is given to simulate the actual joint interface pressure distribution. Based on these, a nonlinear semi-analytical model for bolted carbon fiber reinforced composites (CFRC) plates considering the nonlinear mechanical characteristics of the bolted joint is established. Combined with the experimental results for a bolted CFRC beam, the specific S-curve expressions for the relevant mechanical parameters of the bolted joint are obtained by the inverse identification technology and numerical fitting method. After that, the bolted joint model is transplanted to the dynamic model of a bolted CFRC plate for numerical calculation. The comparison with the corresponding measured results shows that the proposed modeling method can well reproduce the soft nonlinear vibration of the actual bolted CFRC plate, which verifies the correctness and applicability of the proposed modeling method.

Quantification of Comfort for the Development of Binding Parts in a Standing Rehabilitation Robot.

Human-machine interfaces (HMI) refer to the physical interaction between a user and rehabilitation robots. A persisting excessive load leads to soft tissue damage, such as pressure ulcers. Therefore, it is necessary to define a comfortable binding part for a rehabilitation robot with the subject in a standing posture. The purpose of this study was to quantify the comfort at the binding parts of the standing rehabilitation robot. In Experiment 1, cuff pressures of 10-40 kPa were applied to the thigh, shank, and knee of standing subjects, and the interface pressure and pain scale were obtained. In Experiment 2, cuff pressures of 10-20 kPa were applied to the thigh, and the tissue oxygen saturation and the skin temperature were measured. Questionnaire responses regarding comfort during compression were obtained from the subjects using the visual analog scale and the Likert scale. The greatest pain was perceived in the thigh. The musculoskeletal configuration affected the pressure distribution. The interface pressure distribution by the binding part showed higher pressure at the intermuscular septum. Tissue oxygen saturation (StO2) increased to 111.9 ± 6.7% when a cuff pressure of 10 kPa was applied and decreased to 92.2 ± 16.9% for a cuff pressure of 20 kPa. A skin temperature variation greater than 0.2 °C occurred in the compressed leg. These findings would help evaluate and improve the comfort of rehabilitation robots.

Effect of mattress bedding layer structure on pressure relief performance and subjective lying comfort

Mattresses' pressure relief performance and comfort largely affect sleep quality. Mattress filling materials have been proven to affect the interface pressure distribution and comfort, but the effect of mattress structure is unclear. In this paper, the interface pressure distribution and subjective comfort of 10 subjects were assessed in the different bedding layer structures of mattresses, after mattress support performance was tested. The results show that the mattresses with bedding material hardness gradually increasing from the top layer to the bottom layer (BMH-ITTB) structure have a softer surface layer, a better support core layer, and higher fitness. This enables the mattress to achieve a better decompression effect. The low-pressure area (PAI≤0.67kPa) increased, while the high-pressure area (PAI≥2.67kPa and PAI≥4.00kPa), maximum pressure (P95), average pressure (P50), and pressure index (PI) decreased. This also enables the mattress to achieve higher subjective comfort scores.

Comfort Assessment and Optimization Based on FE Simulation for High-Speed Train Seats: Comparison with Different Design Parameters

Nowadays, riding comfort is more significant than before for evaluating the quality of high–speed railways and sitting is the most common posture for its passengers. This study aimed to analyze and optimize the pressure distribution and sitting comfort of second–class seats with different design parameters. Firstly, 21 pressure features were calculated after the field sitting tests conducted on a CRH Train. The subjective comfort was quantified as a linear combination of 6 pressure features in 21, which were selected using stepwise regression analysis (R2 = 0.684). A seat-human finite element model was established using THUMS for a human body and MAT_57 for the seat foam. Finally, this study analyzed the effects of foam and seat angles on interface pressure distribution and comfort ratings. The set of design parameters with the highest comfort was selected from 12 free combinations. The results show that the seat foam with less stiffness may not improve sitting comfort due to the asymmetry of the seat frame. Moreover, appropriately increasing the stiffness of the cushion and backrest will not lead to a decrease in subjective feelings and the pressure distribution becomes more reasonable as the inclination angle increases within 10 degrees. The final optimization increases the computational comfort of the seat-human model by 6.5 in a −50 to 50 scale.

An Investigation into Design of Blood Pressure Cuffs Using the Tekscan Pressure Sensing System

Cuff design is the most important aspect to work on for the accurate measurement of blood pressure. For the very first time, pressure distribution underneath different parts/components of the fabric cuffs was analysed in detail using an advanced interface pressure sensing system. The pressure distribution at the interface of the selected cuffs and an arm simulator were measured experimentally while simulating blood pressure measurement. Fabrics are the main cuff constructing material and a relationship was found between cuff fabric properties and pressure at the interface of the cuff and the arm simulator. It was also found that the pressure distribution under different parts of the same cuff varied due to the uneven contact of the cuff fabric with the arm simulator. Components attached to cuff fabric which facilitate blood pressure measurement either confine or drive fabric according to their size and placement; they may cause non-uniform interface pressure distribution on the human arm too. It was also examined whether the selected cuffs were applying higher or lower average pressure over the surface of the simulator and the range varied from 8 mmHg to 25 mmHg. This indicates that the subsequent pressure distribution inside the arm tissues and over an artery would also vary further due to the cuffs being of different designs. This study found that cuffs would be unable to transfer the required pressure over the artery to block blood flow as and when needed.

Thermo-mechanical and degradation properties of naturally derived biocomposites for prosthesis applications: Analysis of the interface pressure and stress distribution on the developed socket

Nowadays, the reduction of petroleum products has been aggressively studied due to their environmental concerns and for finding alternative synthetic materials for sustainable development. Thus, naturally derived biocomposites have great potential for creating a sustainable economy with reduced environmental pollution. The present study deals in two major parts; first, the fabrication of the knee prosthetic socket from different laminate biocomposites (FFF, KKK, and RRR) and examined their mechanical performance, thermal stability and degradation behaviour. The interface pressure between the developed socket and residual limb has been examined using the F-socket sensor. Second, the output data of experiential tests and measured interface pressure have been used for numerical analysis to examine the equivalent stress, deformation and safety factor of the sockets at different loading conditions (500 N, 700 N and 900 N). The results observed that FFF biocomposites have maximum compressive strength (121.0 6MPa), impact strength (36.86 kJ/m2), and shear strength (28.51 MPa) which is 8.15%, 49.53%, and 58.88% higher than RRR biocomposites and 2.74%, 34.62%, and 39.28% higher compared to KKK biocomposites, respectively. RRR biocomposites recorded higher hardness (68.42) and fatigue endurance stress (82.86 MPa) due to higher brittleness properties. The thermogravimetric analysis and degradation behaviour have been analyzed to examine thermal stability, degradation temperature, and degradation weight loss behaviour of biocomposites. Finally, the stress generation on the developed socket during the gait cycle is much lower than its calculated strength that did not affect the performance of the socket. Therefore, laminated biocomposites can be an alternative biomaterial to plastics and conventional materials due to their lighter weight, higher flexibility, and economical to provide greater comfort to the users for knee prosthesis applications.

Shape optimization of a snowboard sidecut geometry

The distribution of the contact pressure occurring under the edge of a snowboard during a carved turn is a key factor influencing the riding behaviour. These interface loads are determined by the structural design, and in particular by the sidecut geometry of the edge line. In this study, the following inverse problem was set under investigation: if a certain interface pressure distribution is wished, how can the corresponding initial geometry of the snowboard be determined? A structural optimization strategy was presented, involving parametric B-spline representation of the sidecut geometry, finite element modeling of the composite structure deforming against a rigid surface, frictional contact formulation and derivative-free algorithm. A sidecut geometry leading to a contact pressure uniformly distributed along the sidecut line was sought, to illustrate the capabilities of the method. An actual snowboard prototype was made according to the outcome of the optimization, and an experimental validation was conducted to measure the physical pressure distribution and assess the accuracy of the numerical predictions. With the proposed method, the design can be controlled by the state variables of the deformed structure rather than the initial design variables, thus providing an alternative shortcut to the classical trial and error development strategies.

Interfacial pressure distribution and bonding characteristics in twin-roll casting of Cu/Al clad strip

The mechanical properties and product thickness specifications of bimetallic clad strip prepared by twin-roll casting are tightly related to the mechanical behavior of bonding interface interaction. The thermal−flow coupled simulation and the interface pressure calculation models are established with the cast-rolling velocity as the variable. The results show that the interface temperature decreases, the interface pressure and the proportion of the thickness of the Al side increase with the decrease in cast-rolling velocity. The thinning of Cu strip mainly occurs in the backward slip zone. The higher pressure and longer solid/semi-solid contact time make the interface bonded fully, which provides favorable conditions for atomic diffusion. The inter-diffusion zone with a width of 4.9 μm is attained at a cast-rolling velocity of 2.4 m/min, and the Cu side surface is nearly completely covered by aluminum. Therefore, the ductile fracture occurs on the Al side, which prevents the propagation of interface delamination cracks effectively. Meanwhile, shear effect becomes more significant at high interfacial pressure and large plastic strain, and the microstructure on Al side is composed of slender columnar crystals. Thus, the metallurgical bonding and refinement of grains on the Al side can result in higher bonding strength and tensile properties of the clad strip.

A hybrid method for bolted joint modeling considering multi-scale contact mechanics

Significant effects of the bolt arrangement on the contact characteristics of bolted joints have been demonstrated, which are strongly associated with the dynamic performance of mechanical structures. However, the complexity of interfacial pressure distribution and the randomness of surface microtopography bring challenges to the accurate characterization of the multi-scale contact mechanics. Herein, a hybrid method combining fractal theory with the finite element method (FT-FEM) is proposed, which formulates the interfacial contact evolution in the elastic, elastic-plastic, and fully plastic stages. The complementarity of these two methods makes it possible to reveal the contact mechanism at both micro and macro scales and overcomes difficulties in reflecting the detailed joint contact characteristics in the overall dynamic model. To explore broader implications, FT-FEM is applied to the optimal design of bolt arrangement in an ultra-precision machine tool. Good agreements in calculation results and experimental data validate the accuracy of the proposed FT-FEM and the dynamic simulation. Replacing 15-M10 bolts with 19-M8 bolts results in a 2.9% enhancement in the contact stiffness and a 2.283% attenuation in the vibration response of the workpiece. The results also show that adjusting the bolt arrangement according to the working condition can strengthen the bolted joint effectively. FT-FEM contributes to solving contact problems in various mechanical structures, and the findings have wide practical applications in bolt arrangement optimization.

Relationship between mattress internal air pressure and interface pressure distribution in the lateral position.

Previous research shows that maximum interface pressure increases when the patient is lying in the lateral position. However, it was unclear whether it was better to increase or decrease the internal air pressure to reduce the maximum interface pressure in the lateral position; thus, this study investigated this issue. In this study, we investigated the change in pressure redistribution because of the difference in internal air pressure between the supine and lateral positions on an active air mattress for pressure ulcer prevention management. Each participant's five internal air pressure values served as the independent variables. The interface pressure on the active air mattress was measured for 20 minutes. The sacral left iliac crest and left greater trochanteric interface pressures were measured using a portable pressure-measuring device. When seven of the 10 participants switched from the supine position to the left lateral position, there was a decrease in the maximum interface pressure as the internal air pressure increased. The maximum interface pressure in the greater trochanter in the lateral position was twice that in the sacral region in the supine position. These results show that increasing the internal air pressure in the lateral position might help reduce the maximum interface pressure.

Redundancy Reduction for Sensor Deployment in Prosthetic Socket: A Case Study.

The irregular pressure exerted by a prosthetic socket over the residual limb is one of the major factors that cause the discomfort of amputees using artificial limbs. By deploying the wearable sensors inside the socket, the interfacial pressure distribution can be studied to find the active regions and rectify the socket design. In this case study, a clustering-based analysis method is presented to evaluate the density and layout of these sensors, which aims to reduce the local redundancy of the sensor deployment. In particular, a Self-Organizing Map (SOM) and K-means algorithm are employed to find the clustering results of the sensor data, taking the pressure measurement of a predefined sensor placement as the input. Then, one suitable clustering result is selected to detect the layout redundancy from the input area. After that, the Pearson correlation coefficient (PCC) is used as a similarity metric to guide the removal of redundant sensors and generate a new sparser layout. The Jenson–Shannon Divergence (JSD) and the mean pressure are applied as posterior validation metrics that compare the pressure features before and after sensor removal. A case study of a clinical trial with two sensor strips is used to prove the utility of the clustering-based analysis method. The sensors on the posterior and medial regions are suggested to be reduced, and the main pressure features are kept. The proposed method can help sensor designers optimize sensor configurations for intra-socket measurements and thus assist the prosthetists in improving the socket fitting.

Electrical stimulation to prevent recurring pressure ulcers in individuals with a spinal cord injury compared to usual care: the Spinal Cord Injury PREssure VOLTage (SCI PREVOLT) study protocol

BackgroundPressure ulcers (PUs) on the buttocks are among the most common secondary complications in individuals with chronic spinal cord injury (SCI). PUs can result from sitting for extended periods, disuse atrophy, increased sitting pressure and reduced circulation. Compared with usual care, activation of paralysed muscles using electrical stimulation (ES) has been shown to markedly increase paralysed muscle mass, improve circulation of skin and muscle and improve sitting pressure distribution. ES might therefore be a useful method to reduce PU incidence.MethodsA multicentre randomized controlled trial (SCI PREVOLT) will be conducted with an economic and process evaluation alongside. One hundred participants with a SCI in the chronic phase and a minimal incidence of 1 PU in the last 5 years will be recruited from rehabilitation centres across the Netherlands. Participants will be stratified by centre and age and randomized to the intervention or control group. The intervention group will use ES at least 1 h/day during at least 4 times a week for 1 year next to usual care. The control group will only receive usual care. The primary outcome is the incidence of PUs, measured by a blinded person assessing the presence or absence of a PU on the buttocks on a photo made by the participant or his/her caregiver. The incidence of a PU will be evaluated every 2 weeks. Secondary outcomes include interface pressure distribution, blood flow in the profunda femoris artery, muscle thickness of the hamstrings and gluteal muscles and questionnaires about different dimensions of life, e.g. participation and quality of life. Secondary outcomes will be measured at baseline and 3, 6, 9 and 12 months after randomization.DiscussionThis study will assess if electrical stimulation is a (cost-)effective method to prevent PUs and reduce the risk factors of getting PUs. If ES is effective and cost-effective compared with usual care, ES could be implemented in daily treatment of individuals with a SCI.Trial registrationNetherlands Trials Register NTR NL9469. Registered on 26 May 2021.

Optimizing Backrest Geometry to Minimize Interfacial Pressure Concentrations in the Mid-to-Lumbar Region During Leg Press Resistance Training.

The leg press is a resistance training (RT) exercise common to both weight- and powerlifting, where spine-related injuries remain prevalent. Here, the elevated loading has the potential to result in increased pressure on vertebral bodies and introduce the risk of spinal injury. This study, therefore, investigates back interfacial pressure under leg press loading conditions and offers design recommendations to minimize spatial pressure concentrations. A pressure mat was used to assess the back-backrest interfacial pressure distribution of 15 subjects executing RT leg-presses at 50% body weight, over 16 different back-support geometries. Real-time forces, knee angles, and pressures were captured. The resulting data show that more prominent (≥2.1 cm) back-supports, positioned 19 cm above the seat pan typically produced greater peak pressures (41.8 ± 7.2 kPa). Conversely, less prominent supports (∼0.7 cm) generally achieved lower peak pressures (with greater distribution). Our data suggest that the most prudent choice for fixed-shape backrests to best distribute interfacial pressure on leg-press devices is to incorporate shallow convex supports (∼0.7 cm) and locate them away from P = 19 cm. The result is surprising as this prominence location is a common ergonomic feature. If an adjustable backrest is considered, peak pressures may be reduced by up to 26 ± 8% (9.7 ± 3.1 kPa) compared to flat geometries.

A Hybrid Model for Predicting Steering Brake Squeal Based on Multibody Dynamics and Finite Element Methods

In recent years, the problem of automotive brake squeal during steering braking has attracted attention. Under the conditions of squealing, the loading of sprung mass is transferred, and lateral force is generated on the tire, resulting in stress and deformation of the suspension system. To predict the steering brake squeal propensity and explore its mechanism, we established a hybrid model of multibody dynamics and finite element methods to transfer the displacement values of each suspension connection point between two models. We successfully predicted the occurrence of steering brake squeal using the complex eigenvalue analysis method. Thereafter, we analyzed the interface pressure distribution between the pads and disc, and the results showed that the distribution grew uneven with an increase in the steering wheel angle. In addition, changes in the contact and restraint conditions between the pads and disc are the key mechanisms for steering brake squeal.

Subcutaneous Fat Thickness Remarkably Influences Contact Pressure and Load Distribution of Buttock in Seated Posture.

Spinal cord injury patients are prone to develop deep tissue injury (DTI) as they may spend half their time per day in sitting postures, which produce excessive load in their buttocks. However, the impact of fat thickness on the biomechanical response of buttock in sitting posture remained unclear. This study aimed to investigate the influence of subcutaneous fat thickness on the interface pressure and load distribution of buttock of seated humans. To achieve this goal, a 3-dimensional finite element model of male buttock was constructed and the contact pressure on a rigid cushion was evaluated against experimental results. The modified models, which had various fat thicknesses under ischial tuberosity, were built and used to simulate the sitting conditions with different cushion stiffnesses. In the models simulating sitting on the rigid cushion, the peak contact pressure ranges from 0.052 MPa to 0.149 MPa. In the simulation of sitting on the soft cushion, the peak stress of muscle underneath ischial tuberosity in the model with the thickest fat tissue was slightly higher than that of the other models. The results demonstrate that the fat tissue in the buttock could reduce the contact pressure when sitting on the rigid seat. However, contact pressure solely could not be used to estimate the internal tissue stress of seated buttock, especially in subjects with thicker fat tissue.