F.Q. Pancheri

Bio: F.Q. Pancheri is an academic researcher from Tufts University. The author has contributed to research in topics: Hyperelastic material & Isotropy. The author has an hindex of 1, co-authored 1 publications receiving 39 citations.

A constitutive model for the Mullins effect with changes in material symmetry

Abstract: When an unfilled or particle reinforced rubber is subjected to cyclic loading–unloading with a fixed amplitude from its natural reference configuration, the stress required on reloading is less than on the initial loading for a deformation up to the maximum value of the stretches achieved. The stress differences in successive loading cycles are largest during the first and second cycles and become negligible after about 4–6 cycles. This phenomenon is known as the Mullins effect. In this paper new experimental data are reported showing the change in material symmetry for an initially undamaged and isotropic material subjected to uniaxial and biaxial extension tests. The effect of preconditioning in one direction on the mechanical response when loaded in a perpendicular direction is discussed. A simple phenomenological model is derived to account for stress softening and changes in material symmetry. The formulation is based on the theory of pseudo-elasticity, the basis of which is the inclusion of scalar variables in the energy function. When active, these variables modify the form of the energy function during the deformation process and therefore change the material response. The general formulation is specialized to pure homogeneous deformation in order to fit the new data. The numerical results are in very good agreement with the experimental data.

Chemical Bond Scission and Physical Slippage in the Mullins Effect and Fatigue Behavior of Elastomers

Abstract: Understanding the molecular mechanism of mechanical response under a cyclic loading is of fundamental importance in designing high-performance elastomers with a long service life. Herein, we investigated the mechanical response and accompanying microstructural evolution of elastomers under a long-term cyclic loading by using coarse-grained molecular dynamics simulation. The typical characteristics of the Mullins effect are successfully reproduced to confirm the validation of our simulation. Through a systematical analysis for the evolution of an intrinsic structure for a pure elastomer, we find that the Mullins effect results from chain extension and chain slippage at a low cross-linking density, whereas at a high cross-linking density, it is mainly caused by bond rupture. Particularly, the optimized cross-linking density of a pure system can lead to optimum mechanical strength and fatigue resistance because of much greater energy dissipation by polymer chains with high mobility. The filled elastomer indi...

Novel features of the Mullins effect in filled elastomers revealed by stretching measurements in various geometries.

Abstract: Stretching experiments with various geometries are performed using a custom-built tensile tester to reveal the intriguing features of the mechanical softening phenomena of filled elastomers in loading–unloading cycles, commonly known as the Mullins effect. The dissipated energy (D), residual strain (er), and dissipation factor (Δ; the ratio of D to input strain energy) in the loading–unloading cycles are evaluated as a function of the maximum stretch in cyclic loading (λm) using three types of extension, i.e., uniaxial, planar, and equibiaxial extension, for silica-filled elastomers with various filler contents, and with or without a silane coupling agent. The dissipated energy D and er increase with an increase in λm, and they depend on the type of extension when compared at the same λm: D and er increase in the order of equibiaxial, planar, and uniaxial extension. In contrast, the values of Δ obtained for various degrees and types of extension are collapsed into a single curve when the first invariant of the deformation tensor (I1,m) corresponding to λm is employed as a variable: Δ steeply increases with an increase in I1,m in the small deformation regime of I1,m 3.5. The plateau values of Δ increase with an increase in filler content. The characteristic dependence of Δ on I1,m in each of the small and large deformation regimes is expected to reflect the destruction process of the inherent structures, including filler networks and the filler–polymer interface, and the friction between the fillers and the rubber matrix, respectively.

Permanent set and stress-softening constitutive equation applied to rubber-like materials and soft tissues

Abstract: Many rubber like materials present a phenomenon known as Mullins effect. It is characterized by a difference of behavior between the first and second loadings and by a permanent set after a first loading. Moreover, this phenomenon induces anisotropy in an intially isotropic material. A new constitutive equation is proposed in this paper. It relies on the decomposition of the macromolecular network into two parts: chains related together and chains related to fillers. The first part is modeled by a simple hyperelastic constitutive equation whereas the second one is described by an evolution function introduced in the hyperelastic strain energy. It contributes to describe both the anisotropic stress softening and the permanent set. The model is finally extended to soft tissues mechanical behavior that present also stress softening but with an initially anisotropic behavior. The two models are successfully fitted and compared to experimental data.

Theory and identification of a constitutive model of induced anisotropy by the Mullins effect

Abstract: Rubber-like materials present a stress softening phenomenon after a first loading known as the Mullins effect. Some recent experimental data on filled silicone rubber is presented in literature, using uniaxial and biaxial tests to precondition samples thus induce some primary stress softening. A generic modeling based on the polymer network decomposition into an isotropic hyperelastic one, and a stress-softening evolution one, is proposed taking into account the contribution of many spatial directions. A new stress softening criterion tensor is built by means of a tensor that measures the repartition of energy in space. A general form of the stress softening function associated to a spatial direction is written by the way of two variables: one, the maximal eigenvalue of the energy tensor; the other, the energy in the considered direction. Finally, a particular form of constitutive equation is proposed. The model is fitted and compared to experimental data. The capacities of such modeling are finally discussed.