Thermal-aging constitutive model for a silicone rubber foam under compression

Thermo-Oxidative Aging Effects on Hyperelastic Behavior of EPDM Rubber: A Constitutive Modeling Approach

The effect of thermo-oxidative aging on the hyperelastic behavior of ethylene propylene diene monomer (EPDM) rubber was investigated by a combined experimental and theoretical modeling approach. Firstly, the uniaxial tensile test of aged and unaged EPDM rubber was carried out. The test results show that the unaged EPDM rubber had the nonlinear large deformation characteristic of a "S" shape. The stiffness of the EPDM rubber was found to increase with the aging time and aging temperature. Then, in order to quantitatively characterize the hyperelastic behavior of unaged EPDM rubber, the fitting performances of the Mooney-Rivlin, Arruda-Boyce, and Ogden models were compared based on a uniaxial tensile stress-strain curve. The results show that the Ogden model provided a more accurate representation of the hyperelastic behavior of unaged EPDM rubber. Subsequently, the Dakin dynamic equation was adopted to associate the parameters of the Ogden model with the aging time, and the Arrhenius relationship was utilized to introduce the aging temperature into the rate term of the Dakin dynamic equation, thereby establishing an improved Ogden constitutive model. This improved model expanded the Ogden model's ability to explain aging time and aging temperature. Finally, the improved model prediction results and the test results were compared, and they indicate that the proposed improved Ogden constitutive model can accurately describe the hyperelastic behavior of aged and unaged EPDM rubber.

Investigation of Constitutive Models for Pressure Monitoring of Viscoelastic-Hyperelastic Composite Structures

To ensure ammunition safety, a protective structure and pressure detection system are essential; however, there is a lack of an accurate constitutive model to describe the mechanical response characteristics of protective structures composed of various polymer materials. In this work, a constitutive model for the composite structure based on the superposition principle is successfully constructed derived from the quasi-static compression behavior of rigid polyurethane foam (RPUF), silicone rubber foam (SRF), and flexible pressure sensors (FPSs) through experimental investigations. The constitutive model accurately reflects the influence of each type of polymer foam on the mechanical performance of composite structures, underscoring the significance of thickness ratios. Test results within the temperature range of 25 °C to 55 °C validate the model's accuracy, with an average fitting error of 8.6%. Furthermore, a multi-channel pressure detection system has been integrated into the composite structure. Under conditions of out-of-plane loads ranging from 0 to 10 kilonewtons, the accuracy of the pressure monitoring system, adjusted using the constructed model, has improved by 16%. The constitutive model and the pressure sensing system effectively predict the mechanical properties of the protective structure and enable real-time force state monitoring, which is crucial for ammunition safety and has broader applications for safeguarding other objects.

Constitutive Model for Thermal-Oxygen-Aged EPDM Rubber Based on the Arrhenius Law

Ethylene-propylene-diene monomer (EPDM) is a key engineering material; its mechanical characterization is important for the safe use of the material. In this paper, the coupled effects of thermal degradation temperature and time on the tensile mechanical behavior of EPDM rubber were investigated. The tensile stress-strain curves of the aged and unaged EPDM rubber show strong nonlinearity, demonstrating especially rapid stiffening as the strain increases under small deformation. The popular Mooney-Rivlin and Ogden (N = 3) models were chosen to fit the test data, and the results indicate that neither of the classical models can accurately describe the tensile mechanical behavior of this rubber. Six hyperelastic constitutive models, which are excellent for rubber with highly nonlinearity, were employed, and their abilities to reproduce the stress-strain curve of the unaged EPDM were assessed. Finally, the Davis-De-Thomas model was found to be an appropriate hyperelastic model for EPDM rubber. A Dakin-type kinetic relationship was employed to describe the relationships between the model parameters and aging temperature and time, and, combined with the Arrhenius law, a thermal aging constitutive model for EPDM rubber was established. The ability of the proposed model was checked by independent testing data. In the moderate strain range of 200%, the errors remained below 10%. The maximum errors of the prediction results at 85 °C for 4 days and 100 °C for 2 and 4 days were computed to be 17.06%, 17.51% and 19.77%, respectively. This work develops a theoretical approach to predicting the mechanical behavior of rubber material that has suffered thermal aging; this approach is helpful in determining the safe long-term use of the material.

The Effects of a Crosslinking Agent on the Microrheological Properties and Cellular Structure of Silicone Rubber Foam Prepared via a Green Process

Chemical foaming technology is widely used in the preparation of silicone rubber foam and is attributable to its one-step molding capability and eco-friendly production processes. The microrheological properties of silicone rubber play a pivotal role during the foaming process. In this study, Rheolaser Lab (Formulaction, Toulouse, France) was used to conduct in situ examinations for the influence of a crosslinking agent on the microrheological properties of silicone rubber foam for the first time. This study monitors the entire reaction process of silicone rubber foam from liquid to solid, as well as the matching of crosslinking and foaming reactions. Various parameters, including solid-liquid balance, elasticity index, and macroscopic viscosity index, are measured to analyze the microrheological properties of silicone rubber foam. The results show that the silicone rubber foam exhibits good microrheological properties, thereby demonstrating excellent performance at a crosslinking agent content of 2%. Through adjusting the experimental conditions, a sustainable and efficient approach was proposed for better cellular structure control in the industrial preparation of silicone rubber foam.

Simulation Analysis of Equibiaxial Tension Tests for Rubber-like Materials

For rubber-like materials, there are three popular methods of equibiaxial tension available: inflation tension, equibiaxial planar tension, and radial tension. However, no studies have addressed the accuracy and comparability of these tests. In this work, we model the tension tests for a hyperelastic electroactive polymer (EAP) membrane material using finite element method (FEM) and investigate their experimental accuracy. This study also analyzes the impact of apparatus structure parameters and specimen dimensions on experimental performances. Additionally, a tensile efficiency is proposed to assess non-uniform deformation in equibiaxial planar tension and radial tension tests. The sample points for calculating deformation in inflation tensions should be taken near the top of the inflated balloon to obtain a more accurate characteristic curve; the deformation simulation range will be constrained by the material model and its parameters within a specific limit (λ ≈ 1.9); if the inflation hole size is halved, the required air pressure must be doubled to maintain equivalent stress and strain values, resulting in a reduction in half in inflation height and decreased accuracy. The equibiaxial planar tension test can enhance uniform deformation and reduce stress errors to as low as 2.1% (at λ = 4) with single-corner-point tension. For circular diaphragm specimens in radial tension tests, increasing the number of cuts and using larger punched holes results in more uniform deformation and less stress error, with a minimum value of 3.83% achieved for a specimen with 24 cuts and a 5 mm punched hole. In terms of tensile efficiency, increasing the number of tensile points in the equibiaxial planar tension test can improve it; under radial tension, increasing the number of cuts and decreasing the diameter of the punched hole on the specimen has a hedging effect. The findings of this study are valuable for accurately evaluating various equibiaxial tension methods and analyzing their precision, as well as providing sound guidance for the effective design of testing apparatus and test plans.

The Variety of the Stress–strain Response of Silicone Foam after Aging

The mechanical properties of silicone foam will degrade when exposed to environmental loads such as temperature and pressure for a long time. In recent years, the variation law of the stress–strain response of silicone foam during the aging process has received more and more attention, but there are few works that quantitatively analyze the variation of the stress–strain response. In this work, we quantitatively analyzed the variation law of the stress–strain response of silicone foam during aging by the constitutive model. Firstly, the accelerated aging test of silicone rubber foam under long-term compressive strain was carried out, and its compression set, stress relaxation and strain stress curves of different aging degrees were obtained. Further, degenerate trajectory equations for the compression set and stress–relaxation were obtained. In addition, the hyper-foam constitutive model was obtained by fitting stress–strain curves, and the changes in the model parameters after aging were studied. The results show that the compressed set and stress–relaxation are exponential functions of time, while different to existing research findings, we found that the stress–strain curves do not change monotonically with increasing time, which first softens, then hardens, and finally softens. Additionally, to better understand the changing trend of the stress–strain response, the correlation between the stress–strain curve and the compression set and stress–relaxation was discussed qualitatively. Finally, in the stage of monotonic change of the stress–strain curve, the exponential function of the model parameters with the increase of aging time was obtained.