Effect of Thermal Cycles on the Deformation State at the Crack Tip of Crystallisable Natural Rubber

Abstract

This article deals with the effect of temperature variations on crack tip kinematics in rubbers, especially in crystallising rubbers. In such materials, the high deformation level encountered at the crack tip engenders the formation of crystallites. As a consequence, the crack tip is reinforced and resists crack growth. However, this phenomenon is significantly affected by variations in material temperature. This is classically observed at the macroscopic scale in terms of crack propagation rate and path. In this study, the effect of temperature is studied at the local scale, by measuring the change in the kinematic field at the crack tip during thermal cycles. Results show that, in crystallisable natural rubber, the effect of temperature depends on the stretch ratio attained in the zone under consideration. In slightly stretched zones, the stretch ratio increases with the increase in temperature, whereas it decreases in highly stretched zones. This highlights the competition between the effects of the variations in internal energy and in entropy on the thermomechanical response. Moreover, if crystallites form in highly stretched zones, the increase in temperature leads to crystallite melting, which increases the stretch ratio. This is explained by the fact that crystallites act as fillers by concentrating the stress and therefore by increasing the apparent stiffness of the material.

A Review of the Challenges and Limitations of Full‐Field Measurements Applied to Large Heterogeneous Deformations of Rubbers

“…To the knowledge of the author, only that reported in Ref. [60] investigate the effect of temperature on the kinematic field at the crack tip to link it to the evolution of the microstructure. The authors use full-field measurements to characterise the effect of changes in the microstructure on the kinematic field at the crack tip.…”

“…Digital image correlation at smaller scales is difficult, because of the large displacement of the points at the crystallization l l l l Figure 1: Kinematic field measurement at the crack tip of stretched carbon-filled natural rubber [60]. From the top left corner to the top right corner, the rubber specimen was previously cut with a razorblade before being stretched to put a heated metallic cylinder inside the crack.…”

“…If this is not possible, for instance due to the specific device used (see the example Ref. [60]), measurements could be taken on the same surface. In this case, the cameras are positioned perpendicularly and a dichroic filter-mirror allows each camera to observe the same scene [83].…”

J‐Integral from Full Field Kinematic Data for Natural Rubber Compounds

“…For hyperelastic materials, studies of the thermal and deformation behaviour of rubbers near the tip of a crackvia DIC and thermal imagingwere carried out by Le Cam et al [14] and Martinez et al [15,16]. Rublon et al [17] used DIC to investigate the near tip behaviour in a filled natural rubber under fatigue loading, but DIC results were not used to evaluate a fracture mechanics-related quantity.…”

“…However, it will be shown that the proposed elastic approach works well as long as the stress-strain response is adequately described by a phenomenological law such as Ogden's model, which neglects the actual crystallisation physics: the approach, in fact, allows determining the value of the J-integral correctly without having to consider what happens in the fracture process zone. In this work, the focus is on the determination of the J-integral for materials assumed to be hyperelastic; for details on the aspects related to the physics of fracture and to the process zone phenomena in these materials, the reader is referred to [14,15,23,29].…”