Plastics,Rubber and Composites: Macromolecular Engineering

We present fatigue experiments performed on filled natural rubber and study the correlations between crack growth dynamics and fracture morphologies imprinted by an irregular crack path. Slow crack growth dynamics is obtained by cyclic fatigue in a pure shear test. We will show that an unstable crack growth regime exists for high loads. We will also discuss the appearance of sawtooth striations which follow a scenario that significantly differs from previous results reported in the literature.

Since the pioneer works of Gough and Joule, the thermal characterisation of elastomers under mechanical loading has been investigated by numerous research teams. This is not surprising as the thermal signature of rubber is very useful data to investigate the dissipation mechanisms as well as the thermodynamical variables and couplings. In former recent studies dealing with fatigue investigations, an experimental protocol was developed. This protocol imposes cyclic loading to hourglass shaped samples, takes into account the large displacements and permits dissociation between the intrinsic dissipation, responsible for the mean temperature variation (called heat build-up in the literature) and thermomechanical couplings responsible for the temperature variation around this mean value during one cycle. Up to now, the mean temperature has been investigated in order to feed an energetic fatigue criterion. The aim of the present study is to investigate the thermomechanical couplings and the ability of thermal measurements to exhibit some specific thermomechanical properties observed for rubberlike materials. The materials studied are natural rubber and styrene butabiène rubber compounds filled with several amounts of carbons blacks. The experimental data clearly exhibit interesting features such as the thermoelastic inversion point and difference in the temperature signal between mechanical loading and unloading. This rich database is analysed and correlated to other results from the literature. The main results obtained are dealing with the ability of accurate measurements to characterise the thermodynamic couplings and to detect the stress induced crystallisation.

The present work concerns three-dimensional modelling of a filler network microstructure in rubber compounds. The model represents the carbon black filler in three states: primary fractal aggregates consisting of spherical over-lapped particles; secondary structures or agglomerates; and partially broken fragments of micropellets. The information about the structure hierarchy of the filler and its distribution in the matrix was obtained from the analysis of atomic force microscopy images of the material surface.

A homemade stretching machine has been developed to perform fatigue tests on natural rubber in the synchrotron facility Soleil. Strain induced crystallisation is investigated by wide angle X-ray diffraction during in situ fatigue tests of different minimum and maximum strain levels. The index of crystallinity χ decreases with the number of cycles when the minimum strain level reached during the fatigue test is lower than the critical stretch ratio for melting λ_M. On the contrary, when the stretch ratio is maintained higher than the critical stretch ratios for melting λ_M and crystallisation λ_C, χ increases with the number of cycles. The size of the crystallites has the same evolution during fatigue testing as χ. On the contrary, the misorientation of the crystallites does not depend on the minimum strain level reached and decreases with the number of cycles for all the fatigue tests.

A fatigue peeling test has been developed to evaluate the failure of rubber to rubber interfaces under cyclic loading. Results obtained through this method have been compared to those of a typical fatigue crack growth experiment. The results show that the trends between these two failure modes are similar with the peeling necessary to drive the crack being slightly higher than the strain energy release rate at the same crack growth rate. Cyclic and time dependent contributions to the fatigue crack growth behaviour have been calculated using this test for an styrene–butadiene rubber compound and the results appear to be consistent with previous work although the origin of the cyclic contribution remains uncertain. The influence of pressure at the interface during vulcanisation has also been investigated and it has been observed that the fatigue peel behaviour is proportional to the surface area of contact developed during the curing cycle.

Simultaneous experimental measurements of stress, strain and electrical resistivity were carried out by exposing two carbon black filled cross-linked elastomers and two nanocomposites of thermoplastic polyurethane and multiwalled carbon nanotubes to a series of cyclic strain histories. It was found that the resistivity–strain relationships of the materials exhibited different hysteresis and dependence on prestrain. The resistivity of the nanotube filled elastomers changed dramatically with prestrain, making them suitable for memory sensor applications. During cyclic loading, the carbon black filled elastomers exhibit a lower resistivity during the loading part of the cycles than during the unloading part; the opposite effect was seen in the nanotube filled elastomers. The phenomena can be explained in terms of plastic flow processes in the thermoplastics, and of cohesive forces between carbon black particles in the cross-linked elastomers. Bending and buckling of the nanotubes give rise to a region of strain at constant resistance, making them unsuitable for real time sensing.

The dynamic shear modulus of magnetosensitive (MS) natural rubber composites is experimentally studied, where influences of carbon black, plasticiser and iron particle concentrations are investigated at various dynamic shear strain amplitudes and external magnetic fields within the lower structure borne frequency range. The iron particles embedded in natural rubber are irregularly shaped and randomly distributed; the plasticisers simplify the iron particle blending process, while carbon black reduces the production costs and improves the mechanical properties. The results show that the relative MS effect on the shear modulus magnitude increases with increased plasticiser and iron particle concentration and decreases with increased carbon black concentration. Furthermore, their relative contributions are quantified. Consequently, the study provides a basis for optimising the composition of MS natural rubber to meet a variety of requirements, including those of vibration isolation, a promising application area for MS materials.