Comfort Evaluation of Slow-Recovery Ejection Seat Cushions Based on Sitting Pressure Distribution

Sitting discomfort not only affects the health of pilots carrying out long-endurance missions but also affects operational performance. The experimental objects included four ejection seat cushions: N1 was a fast-recovery foam as the comparison group, and the experimental groups were slow-recovery foams with different indentation force deflection (IFD), named N2 (hard), N3 (mid), and N4 (soft). The sitting comfort of 20 participants was tested on the four cushions by using subjective rating and sitting pressure distribution analysis. The results showed that compared with fast-recovery cushion N3 and N4 slow-recovery cushions have lower contact pressure and more uniform pressure distribution. Slow-recovery cushions that were too soft or too hard would reduce the comfort. No matter from the subjective rating or the analysis of the contact pressure data, the N3 cushion with a thickness of 3 cm and 65% IFD of 280 N had the highest comfort. In addition, the seat pressure distribution (SPD%) has a significant correlation with the subjective rating (p = 0.019, R = −0.98), which is more suitable for evaluating the comfort of the cushions. However, the slow-recovery cushions would show a decrease in support after a period of sitting, while the fast-recovery cushion could always maintain constant support.

New Inverse Method for Determining Uniaxial Flow Properties by Spherical Indentation Test

The spherical indentation test has been successfully applied to inversely derive the tensile properties of small regions in a non-destructive way. Current inverse methods mainly rely on extensive iterative calculations, which yield a considerable computational costs. In this paper, a database method is proposed to determine tensile flow properties from a single indentation force-depth curves to avoid iterative simulations. Firstly, a database that contain numerous indentation force-depth curves is established by inputting varied Ludwic material parameters into the indentation finite elements model. Secondly, for a given experimental indentation curve, a mean square error (MSE) is designated to evaluate the deviation between the experimental curve and each curve in the database. Finally, the true stresses at a series of plastic strain can be acquired by analyzing these deviations. To validate this new method, three different steels, i.e. A508, 2.25Cr1Mo and 316L are selected. Both simulated indentation curves and experimental indentation curves are used as inputs of the database to inversely acquire the flow properties. The result indicates that the proposed approach provides impressive accuracy when simulated indentation curves are used, but is less accurate when experimental curves are used. This new method can derive tensile properties in a much higher efficiency compared with traditional inverse method and are therefore more adaptive to engineering application.

Time-Resolved Force Measurements to Determine Positioning Tolerances for Impulse-Based Indentations

High-throughput experimentation methods determine characteristic values, which are correlated with material properties by means of mathematical models. Here, an indentation method based on laser-induced shock waves is presented, which predicts the material properties, such as hardness and tensile strength, by the induced plastic deformation in the substrate material. The shock wave pushes a spherical indenter inside a substrate material. For reproducible indentations, the applied load is of importance. To compare different processes and process parameters, the measured plastic deformation is normalized by the applied load. However, eccentric irradiation leads to altered beam profiles on the surface of spherical indenters and the angle of incidence is changed. Thus, the influence of eccentric irradiation is studied with an adapted time-resolved force measurement setup to determine the required positioning tolerances. The spherical indenter is placed inside a cylindrical pressure cell to increase the laser-induced shock pressure. From the validated time-resolved force measurement method we derive that deviations from the indentation forces are acceptable, when the lateral deviation of the beam center, which depends only on the alignment of the setup, does not exceed ± 0.4 mm. A vertical displacement from the focus position between -3.0 mm and + 2.0 mm still leads to acceptable deviations from the indentation force.

FOREARM AND BICEPS CIRCUMFERENTIAL VARIATIONS IN SKIN TISSUE DIELECTRIC CONSTANT AND FIRMNESS

Tissue dielectric constant (TDC) and skin firmness assessed via indentation force (FORCE) help quantify lymphedema and track changes. We sought to determine potential differences in these parameters dependent on arm circumferential locations. Thus, TDC and FORCE were measured in 40 healthy women at medial, anterior and lateral locations on forearm and biceps. In five other women with unilateral lymphedema (68.6±7.6 years), TDC was measured at corresponding circumferential forearm positions. Measurements were done in triplicate using compact noninvasive devices. Results for healthy women (23.8±2.7 years) showed forearm medial TDC values (26.7±2.2) were less than anterior (28.0±2.4) or lateral (28.0±2.5) positions (p<0.001). Lymphedema patients had elevated values but similar medial-anterior-lateral patterns (33.7±8.0, 39.8±10.2 and 42.9±10.0). Biceps medial TDC values (24.1±2.2) were also less than either anterior (27.0±2.1) or lateral (28.2±3.3). Contrastingly, medial FORCE values at forearm and biceps were less than at anterior and lateral locations (p<0.001) and increased in the order of medial-anterior-lateral on forearm (p<0.001). The present findings provide reference values for both TDC and FORCE of commonly measured arm sites with specificity as to circumferential variations. This observed variation indicates the need for care in locating measurement positions for tracking patients with lymphedema.

Exploitation of static and dynamic methods for the analysis of the mechanical nanoproperties of polymethylmetacrylate by indentation

The development of instrumented nanoindentation consists of non-destructive tests applied to miniature volumes of material (PMMA). The present research focuses on the factors explaining the variation in the trends of the mechanical properties studied. The evolution of Young's modulus (E) and contact hardness (H) with depth (h) and indentation force (P) shows the existence of an inflection point (2.77 nm) at low penetrations which separates two zones with the first increasing trend and the second decreasing. Explained respectively by the surface hardening induced by the preparation of the material surface and the existence of a surface hardness gradient denoted by the indentation size effect (ISE) observed at very low depths. In addition, on detection of a critical penetration depth below which the effect of the surface on the nanohardness dominates, the variation in the penetration charge is of the order of 9.71 nm. The differences in results of E and H between the dynamic and static modes are of the order of 8.46% and 6.44% inducing an overestimation of 35 MPa in value of E and an underestimation of 1.23 MPa in value of H. They tend to affect the expected nanoscale precision of the indentation to determine the nanomechanical properties of PMMA.

A New Inverse Method for Determining Uniaxial Flow Properties by Spherical Indentation Test

This study presents a new inverse method to determine tensile flow properties from a single indentation force-depth curves. A database is established to replace the iterative FE calculations in conventional inverse methods and therefore can process the indentation data more quickly and easily. An axisymmetric FE model is constructed to simulate the elastic-plastic response of indention. Assuming the materials follow Ludwic constitutive model, by systematically changing the material parameters, numerous indentation force-depth curves are extracted from simulation results to establish the database. For a given experimental indentation curves, a mean square error (MSE) is designated to evaluate the deviations between the experimental curve and each curve in the database. Then, the relation of deviations versus stresses are investigated to acquire the true stresses at a series of plastic strain. To validate the new method, three different steels, i.e. A508, 316L and 2.25Cr1Mo are selected. Both simulated indentation curves and experimental indentation curves are used as inputs of the database to inversely acquire the flow properties. The result indicates that the proposed approach provides impressive accuracy when simulated indentation curves is used, but is less accurate when an experimental curve is used. This new method can quickly derive tensile properties without iterative calculations that yield a considerable computational costs and are therefore adaptive to engineering application.

Using Full Field Data to Produce a Single Indentation Test for Fully Characterising the Mooney-Rivlin Material Model

A theoretical testing method for fully characterising the Mooney-Rivlin hyper-elastic material model is proposed by capturing full-field data, namely displacement field and indentation force data. A finite element model with known parameters will act as the experimental model against which all data will be referenced. This paper proposes a method of inverse finite element analysis operating under the assumption of equally objective function optimal planes or “hyper-planes”. The paper concludes that the Mooney-Rivlin material model can theoretically be fully characterised in a single indentation test by applying methods discussed in the paper when using full-field data operating under the assumption of hyper-planes.

Increasing the laying depth of the foundations fordeepened underground buildingsparts

The article lays down the basic rules for increasing the laying depth of foundations and gives examples of how these works are carried out if the underground part of the building needs to be deepened. With the small size of the deepening of the basement in low-rise buildings, it is possible to deepen the foundations without hanging them at a lower cost. Meanwhile the risk of uneven sedimentary deformations is lower than whilehanging of load-bearing walls with the device of through beams. If it is necessary to deepen the underground part with an increase in the bearing capacity of the foundations, it is most rational to use composite piles which are submerged with pressure and are supported by existing belt foundations. The application of this method requires the mandatory installation of distributing beams under the tape foundation, providing the transfer of the load from the pile indentation on the volume of the structures, significantly exceeding by the weight of the indentation force.

Adhesion of a Thin Soft Matter Layer: The Role of Surface Tension

We consider an adhesive contact between a thin soft layer on a rigid substrate and a rigid cylindrical indenter (“line contact”) taking the surface tension of the layer into account. First, it is shown that the boundary condition for the surface outside the contact area is given by the constant contact angle—as in the case of fluids in contact with solid surfaces. In the approximation of thin layer and under usual assumptions of small indentation and small inclination angles of the surface, the problem is solved analytically. In the case of a non-adhesive contact, surface tension makes the contact stiffer (at the given indentation depth, the contact half-width becomes smaller and the indentation force larger). In the case of adhesive contact, the influence of surface tension seems to be more complicated: For a flat-ended punch, it increases with increasing the surface tension, while for a wedge, it decreases. Thus, the influence of the surface tension on the adhesion force seems to be dependent on the particular geometry of the contacting bodies.

Simplified Analytical Solution of the Contact Problem on Indentation of a Coated Half-Space by a Conical Punch

The contact problem on indentation of an elastic coated half-space by a conical punch is considered. To obtain an explicit analytical solution suitable for applications, the bilateral asymptotic method is used in a simplified form. For that purpose, kernel transform of the integral equation is approximated by a ratio of two quadratic functions containing only one parameter. Such an approach allows us to obtain explicit analytical expressions for the distribution of contact stresses and relations between the indentation force, depth, stiffness and contact radius. The obtained solution is suitable both for homogeneous and functionally graded coatings. The dependence of the characteristics of contact interaction on a relative Young’s modulus of the coating and relative coating thickness is analyzed and illustrated by the numerical examples. Ranges of values of elastic and geometrical parameters are obtained, for which the presence of a coating sufficiently changes the contact characteristics. The accuracy of the obtained simplified expressions is studied in detail. Results of the paper sufficiently simplify engineering calculations and are suitable for inverse analysis, e.g., analysis of indentation experiments of coated materials using either a conical or a pyramidal (Berkovich) indenter.