Free Inflation Research Articles

Analysis of Vibration Characteristics and Influencing Factors of Complex Tread Pattern Tires Based on Finite Element Method

The vibration of the tires significantly impacts a vehicle’s ride comfort and noise level; however, the current analysis of tire vibration characteristics often involves excessive simplification in their models, leading to a reduction in model accuracy. To analyze the tire vibrational properties and the influence of its design and service conditions, a combined modeling technology was developed to construct a three-dimensional (3D) finite element model of a 205/55R16 specification radial tire with intricate tread patterns. The accuracy and reliability of the simulation model was verified through vibration modal tests. Based on the vibration mode theory, the Lanczos method provided by ABAQUS was adopted to analyze the modal characteristics of the tire under free inflation and grounded conditions, and the effects of different inflation pressures, loads, operating conditions, and belt cord angles on the tire vibration characteristics were analyzed. The results indicate that grounding constraints will suppress the low order radial modal frequency of the tire and enhance the lateral modal frequency. The higher the order of the tire vibration mode, the greater the impact of inflation pressure. As the operating conditions change, the modal frequencies of all directions have the same trend of change, and as the ground load increases, the tire is prone to misalignment at lower lateral frequencies. The radial and lateral grounding modes of the tire are slightly affected by the change of the cord angle in the belt layer, but the circumferential grounding frequency decreases as the belt layer angle increases. These research findings offer a crucial foundation for the structural design of complex tread pattern tires, and also serve as a reference for addressing vibration and comfort issues encountered in the tire matching process.

Analytical Modeling of the Interaction Between Soft Balloon-Like Actuators and Soft Tubular Environment for Gastrointestinal Inspection.

Accessing tubular environment is critical in medicine. For example, gastrointestinal tract related cancers are the leading causes of cancer deaths globally. To diagnose and treat these cancers, clinicians need accessing the gastrointestinal tract, for example, colon and small intestine, which are soft biological tubes. Soft balloon assisted locomotion is one of the promising methods for accessing bio-duct. It has been widely used in enteroscopy and other medical devices. However, the interaction between the balloon and the soft tube is seldom studied, such as the interaction pressure and the anchoring force. In this work, we present the first modeling of the interaction between soft balloon actuators and soft tubular environment. The free inflation model of soft balloon actuators was first presented. Then a constrained inflation model of the soft balloon in a soft tube was established. Finally, the anchoring force model between the soft balloon and the soft tube was developed. On average, the mean error of the predictions in these three models is 0.228 kPa (or 3.14%), 0.56 kPa (or 7.8%), and 0.22 N (or 14.7%), respectively. In the future, these models could be used for guiding balloon-actuator designs by minimizing the interaction pressure while maintaining sufficient anchoring force during the locomotion in soft tubes.

A method for estimating superplastic material parameters via free–inflation tests

In this work, a new methodology for evaluating the constitutive parameters of superplastic materials is presented. The proposed methodology provides the characterization of the material by means of a variable called apparent viscosity. This variable is calculated for three different materials through data collected by free inflation tests made at different temperatures and pressure values. The apparent viscosity was then used to calculate some material parameters by which the experimental tests were reproduced numerically in a finite element commercial code. The results obtained by numerical simulations were compared both with the experimental ones and with ones deriving by simulations run with material parameters calculated by a commonly used methodology. The proposed approach revealed to have a good prediction capability with deviations lower than the one found by the approach taken as reference. A second validation step was then performed by comparing the stress and strain-rate values found through the proposed methodology with the curves constructed by applying uniaxial tests results from literature. This latter comparison showed that results fit well with the behaviour shown using the standardised uniaxial tests.

Small free field inflation in higher curvature gravity

Within General Relativity, a minimally coupled scalar field governed by a quadratic potential is able to produce an accelerated expansion of the universe provided its value and excursion are larger than the Planck scale. This is an archetypical example of the so called large field inflation models. We show that by including higher curvature corrections to the gravitational action in the form of the Geometric Inflation models, it is possible to obtain accelerated expansion with a free scalar field whose values are well below the Planck scale, thereby turning a traditional large field model into a small field one. We provide the conditions the theory has to satisfy in order for this mechanism to operate, and we present two explicit models illustrating it. Finally, we present some open questions raised by this scenario in which inflation takes place completely in a higher curvature dominated regime, such as those concerning the study of perturbations.

Reliability of Free Inflation and Dynamic Mechanics Tests on the Prediction of the Behavior of the Polymethylsilsesquioxane-High-Density Polyethylene Nanocomposite for Thermoforming Applications.

Numerical modeling of the thermoforming process of polymeric sheets requires precise knowledge of the viscoelastic behavior under conjugate effect pressure and temperature. Using two different experiments, bubble inflation and dynamic mechanical testing on a high-density polyethylene (HDPE) nanocomposite reinforced with polymethylsilsesquioxane HDPE (PMSQ–HDPE) nanoparticles, material constants for Christensen’s model were determined by the least squares optimization. The viscoelastic identification relative to the inflation test seemed to be the most appropriate for the numerical study of thermoforming of a thin PMSQ–HDPE part. For this purpose, the finite element method was considered.

Denoised Inflation: A New Measure of Core Inflation

Abstract Existing measures of core inflation ignore a part of ‘should be’ the core inflation. Exclusion based measures ‘exclude’ a part of persistent inflation inherently existing in the excluded part whereas filter based measures ‘filter-out’ the cyclical part also rather than the irregular component only. This study proposes a new idea to define and measure core inflation – noise free inflation or denoised inflation. As against considering only trend to define core inflation, this study proposes using cyclical component also to be part of core inflation. If core inflation is to be useful, for monetary policy making, as an indicator of underlying inflation, it has to include demand related component of inflation associated with current economic cycle. By using wavelet analysis approach to decompose seasonally adjusted price index into noise, cyclical component and trend, we estimate a denoised inflation series for Pakistan for the period July 1992 to June 2017. Since denoised inflation passes ‘statistical’ as well as ‘theoretical’ tests necessary for a series to be core inflation, we think it can be used as a new core inflation measure for Pakistan. This can also be estimated and tested for any country.

Instabilities in the free inflation of a nonlinear hyperelastic toroidal membrane

Study on an incompressible nonlinear hyperelastic thin-walled toroidal mem- brane of circular cross-section subjected to inflation due to a uniform pressure is conducted in this work. Comparisons are made for three elastic constitutive mod- els (neo-Hookean, Mooney–Rivlin, and Ogden) and for different geometric aspect ratios (ratio of the radius of cross-section to the radius of revolution). A variational approach is used to derive the equations of equilibrium and bifurcation. An analysis of the pressure–deformation plots shows occurrence of the well-known limit point (snap through) instabilities in membrane. Calculations are performed to study the elastic buckling point to predict bifurcation of solution corresponding to loss of symmetry. Tension field theory is employed to study the wrinkling instability that, in this case, typically occurs near the inner regions of tori with large aspect ratios.

Deformation Capacity of a Ternary Magnesium Alloy in a Gas-Forming Process at Elevated Temperatures

An experimental ternary magnesium alloy, Mg-2Zn-2Ce, has been investigated. The attention was focused on the possibility of using this alloy for an unconventional sheet-forming process based on a flexible medium (gas-forming) for applications where weight-saving and complex shapes are required. Free inflation tests were performed at temperatures ranging from 350°C to 450°C, employing either a constant forming gas pressure or pressure jumps during the same test in order to evaluate the strain rate sensitivity, m, of the material. Interrupted free inflation tests at known dome heights were also conducted to investigate both the strain and the microstructural evolution. In addition, the texture behavior of the alloy was studied after hot rolling, revealing a weak deformation texture. Even though the material was in the as-cast condition, the alloy showed a good deformation capacity and contribution due to grain boundary sliding, indicating a potential for superplastic deformation.

Effects of Superplastic Forming on Modification of Surface Properties of Ti Alloys for Biomedical Applications

In the present work, both the surface chemical contamination and the mechanical alteration of Ti–6Al–4V (Ti64) and Ti–6Al–4V extra low interstitial (Ti64ELI) titanium alloys subjected to superplastic forming (SPF) for the manufacturing of highly customized biomedical prostheses have been investigated. As case study, a cranial implant was considered. The design of the manufacturing process was assisted by a numerical model calibrated on free inflation experimental tests. Glow discharge optical emission spectrometry (GDOES) analyses, nanoindentation tests, and metallographic analyses allowed to relate the mechanical alteration to the oxygen enrichment due to the environmental exposition during processing. While similar diffusion kinetics were found, different oxidation rates were measured in the two investigated alloys. The hardness variation was strictly related to the oxygen content. In order to verify the material biocompatibility, cytotoxicity tests were conducted on the most oxidized part. Results highlighted that the oxygen enrichment due to the manufacturing process did not significantly affect the cells viability.

Genetic algorithm based inverse analysis for the superplastic characterization of a Ti-6Al-4V biomedical grade

The inverse analysis approach is recently spreading as a valuable methodology to calibrate constitutive models describing the superplastic behaviour of light alloys. In this work, the superplastic behaviour of a titanium grade for biomedical applications was characterized by means of a Genetic Algorithm based inverse analysis. Free inflation tests at 850 °C were carried out on the Ti-6Al-4V Extra Low Interstitial alloy at two different levels of constant pressure; in addition, a jump pressure test in which the gas pressure was changed during the same test between the same levels was performed. The dome height curve was acquired during each test and subsequently adopted as target data: a simple 2D Finite Element model, in which the material behaviour was implemented using the Backofen power law, was then created to calibrate the material constitutive equation by means of an inverse analysis approach. A set of constants, able to minimize the error between numerical and experimental data, was determined for each investigated load condition. Subsequent numerical simulations, run for validation purposes, demonstrated that the sets determined using as target the data from the free inflation tests under a constant level of pressure were capable of effectively describing the material behaviour only for load conditions close to the ones from which they had been determined. On the contrary, the set of material constants obtained using the same inverse approach but adopting as target data the acquisitions from the jump pressure test, revealed to describe the material behaviour over a wider span of operative conditions.

Investigation on the thickness distribution of highly customized titanium biomedical implants manufactured by superplastic forming

Mechanical performances of titanium biomedical implants manufactured by superplastic forming are strongly related to the process parameters: the thickness distribution along the formed sheet has a key role in the evaluation of post-forming characteristics of the prosthesis. In this work, a finite element model able to reliably predict the thickness distribution after the superplastic forming operation was developed and validated in a case study. The material model was built for the investigated titanium alloy (Ti6Al4V-ELI) upon results achieved through free inflation tests in different pressure regimes. Thus, a strain and strain rate dependent material behaviour was implemented in the numerical model. It was found that, especially for relatively low strain rates, the strain rate sensitivity index of the investigated titanium alloy significantly decreases during the deformation process. Results on the case study highlighted that the strain rate has a strong influence on the thickness profile, both on its minimum value and on the position in which such a minimum is found.

Numerical analysis of crimping and inflation process of balloon-expandable coronary stent using implicit solution.

The paper presents an applied methodology for numerical finite element analysis of coronary stent crimping and the free inflation process with the use of a folded noncompliant angioplasty balloon. The use of an implicit scheme is considered as the most original part of the work, as an explicit finite element procedure is very often preferred. Hitherto, when the implicit solution was used for the finite element solution, the simulated issue was largely simplified. Therefore, the authors focused on the modelling methodology with minimum possible simplification, ie, a full load path (compression and inflation in single analysis), solid element discretization, and sophisticated contact models (bodies with highly different stiffness). The obtained results are partially compared with experimental data (radial force during the crimping procedure) and present satisfactory compliance. The authors believe that presented methodology allow for significant improvement of the obtained results, as well as potential extension of the research scope, compared to previous efforts performed using the explicit integration scheme. Moreover, the presented methodology is believed to be suitable for sensitivity and optimization studies.

Modelling the superplastic behaviour of the Ti6Al4V-ELI by means of a numerical/experimental approach

In the present work, the superplastic behaviour of a Ti6Al4V-ELI titanium alloy at the temperature of 850 °C is assessed combining experiments and numerical simulations managed by a genetic algorithm-based optimization loop. The experiments consisted of free inflation tests characterized by either a constant gas pressure or several pressure jumps during the same test. Dome height evolutions from tests setting a constant gas pressure were used to evaluate the parameters of the classical strain rate power law material model using an analytical approach from literature. An alternative set of material constants was then evaluated using the inverse analysis based on a genetic algorithm coupled to dome height data from jump pressure tests. Numerical results, in terms of thickness distribution and dome height evolution, obtained from simulations implementing material constants from the inverse analysis fit experimental data in a wider range of strain rates than the ones implementing material constants from the analytical approach.

Strain behaviour of a friction stir processed superplastic aluminium alloy sheet during free inflation tests

In this work, the strain behaviour of friction stir processed aluminium superplastic sheet was investigated. The aim was to study the feasibility of the combination of the friction stir processing technique with the superplastic forming process. Main friction stir process parameters (tool rotational speed and transverse speed) were varied in order to study the effect of these changes on the strain behaviour and on the formability of the processed sheet. For this purpose, free inflation tests were carried out on the processed sheet at a temperature at which the base material exhibits an optimal superplastic behaviour. A strong influence of the stirring parameters on the strain behaviour of the processed sheet was found denoting only a specified set of friction stir parameters made the processed sheet behave like the base material. The microstructure evolution of the stirred zone played an important role in this direction. Higher tool rotational speeds led to abnormal grain growth and, consequently, to a slower deformation in the processed material. Thickness measurements made after free inflation tests also confirmed this behaviour: the material processed with a higher tool rotational speed had a thickness that is twice the unprocessed one.

Characterization of a Superplastic Titanium Alloy with an Experimental and Numerical Approach Based on Free-Inflation Tests

It’s well known that the microstructure dramatically affects the strain behaviour of superplastic materials. Virtually, each batch should be characterized ex novo: optimal ranges of temperature and strain rate as well as material constants have to be defined. An accurate and simple characterization methodology based on a strain condition close enough to the real forming process is of great industrial interest. In this work, a characterization methodology based on an experimental and numerical approach is proposed. Experimental free inflation tests with a pressure jump were carried out on a titanium alloy. Results were used as reference data for an inverse analysis based on the height evolution of the dome. Material constants were calculated by means of a genetic algorithm. The approach was verified with further experimental results and a good correlation was found.

Approximated Solutions for Axisymmetric Wrinkled Inflated Membranes

The axisymmetric inflation problem for a wrinkled membrane is solved by means of a simple nonlinear ordinary differential equation. The solution is illustrated in full details. Both the free and constrained cases are addressed, in the limit case where the membrane is fully wrinkled. In the constrained inflation problem, no slippage is allowed between the membrane and the constraining surfaces. It is shown that an actual membrane can in no way reach the fully wrinkled configuration during free inflation, regardless of the membrane's initial configuration and constituent material. The fully wrinkled solution is compared to some finite element results obtained by means of an expressly developed iterative–incremental procedure. When the values of the inflating pressure and length of the meridian lie within a suitable applicability range, the fully wrinkled solution may represent a reasonable approximation of the actual solution. A comparison with some numerical and experimental results available in the literature is illustrated.

Gas forming of an AZ31 magnesium alloy at elevated strain rates

In this work, the gas forming of AZ31 magnesium alloy 0.75-mm-thick sheets at elevated strain rates (fast gas forming) is investigated through an experimental-numerical approach. First, free inflation tests were carried out to find the conditions, in terms of temperature and forming pressure, able to give the best compromise between the alloy formability and the forming time. The analysis was successively moved to a closed die forming application with a stepped geometry case study in order to analyse the real forming process. Both an axisymmetric model of the free inflation test and a 3D model of the closed die forming process were built to correlate the results from free inflation tests (in terms of optimal strain rate values) to the closed die forming test: Numerical simulations were run to find the pressure value to be applied in gas forming tests. Experimental gas forming trials were finally conducted in order to support the approach and to analyse post-forming characteristics of the formed parts. Results showed that very small fillet radii can be reached on a commercial Mg alloy sheet setting very short forming times (few seconds). The choice of the forming temperature and of the corresponding optimal strain rate strongly affects the grain growth and the cavitation phenomena. Even if the alloy is prone to a strong static and dynamic grain growth at elevated temperatures, a small mean grain size value can be reached in the formed component due to the short forming times.

Instability investigation on fluid-loaded pre-stretched cylindrical membranes

This paper discusses the evaluation of instabilities on the quasi-static equilibrium path of fluid-loaded pre-stretched cylindrical membranes and the switching to a secondary branch at a bifurcation point. The membrane is represented by only the in-plane stress components, for which an incompressible, isotropic hyperelastic material model is assumed. The free inflation problem yields governing equations and boundary conditions, which are discretized by finite differences and solved by a Newton–Raphson method. An incremental arclength-cubic extrapolation method is used to find generalized equilibrium paths, with different parametrizations. Limit points and bifurcation points are observed on the equilibrium path when fluid level is seen as the controlling parameter. An eigen-mode injection method is employed to switch to a secondary equilibrium branch at the bifurcation point. A limit point with respect to fluid level is observed for a partially filled membrane when the aspect ratio (length/radius) is high, whereas for smaller aspect ratios, the limit point with respect to fluid level is observed at over-filling. Pre-stretch is observed to have a stiffening effect in the pre-limit zone and a softening effect in the post-limit zone.

Analysis of the strain behaviour of a friction stir processed superplastic aluminium alloy

Superplastic forming is a well-established process by which very large, very complex shaped and also multi-sheets components can be manufactured in a single step. Combining this process with a suitable joining technique is of great industrial interest. In this work the strain behaviour of a friction stir processed aluminium alloy was investigated through free inflation tests. Principal parameters of the friction stir process were changed and free inflation tests were performed to assess the formability of the processed sheet. A strong influence of the friction stir process parameters was recorded on the formability of the processed material. Only a specified set of parameters assured a strain behaviour close to the one of the base material.

Preliminary Study on the Formability of a Laser-Welded Superplastic Aluminum Alloy

In this work, the effect of the laser-material interaction on the formability of a superplastic aluminum alloy was investigated. In applications such as Tailor-Welded Blanks and in the manufacturing of very large components with a complex shape, laser welding combined with superplastic forming may be a very fitting industrial tool. Bead on plate tests were carried out in order to simulate the laser-welding process and then, free inflation tests were performed to evaluate the compatibility of these two processing techniques. The Al-Mg alloy used in this work has a very small grain size which ensures the superplastic behavior. With the aim of preserving this peculiarity, the following aspects on the formability were investigated: (i) the surface condition of the bead before the forming test (with and without the removal of the excess of metal); (ii) the effect of the travel speed of the laser source on the mean grain size; (iii) the introduction of a refiner, commonly used in aluminum casts, in the molten pool in order to further reduce the mean grain size.