Jean-Benoit Le Cam

Failure analysis of carbon black filled styrene butadiene rubber under fatigue loading conditions

Abstract The present paper deals with the fatigue crack growth in a carbon black-filled styrene butadiene rubber (CB-SBR) under fully relaxing loading conditions. More precisely, it is devoted to the determination of the scenario of crack growth. For that purpose, an original ‘microcutting’ technique, previously applied by the authors on natural rubber (NR), is used to observe microscopic phenomena involved in fatigue crack growth thanks to scanning electron microscopy (SEM). Results show that the crack tip grows following a tearing line by generating ligaments; it explains the differences between fatigue responses of crystallisable and non-crystallisable rubbers during crack propagation. So, contrary to crystallisable elastomers such as NR, the microstructure of SBR is similar at crack tip and in the bulk material, and the crack tip does not resist crack propagation. Moreover, the morphology of fracture surfaces only depends on particles encountered by the fatigue crack during its propagation.

Experimental study of the viscoelastic properties of green poplar wood during maturation

During the radial growth of trees, internal stresses are created because of wood-cell maturation. Biomechanical models can compute these stresses, but they are currently limited by a lack of knowledge about the viscoelastic behavior of green wood. The aim of the present paper is to study the viscoelastic behavior of green wood and to obtain measurements of viscoelastic parameters. In order to accomplish this, the effect of internal maturation stresses on the studied samples was first eliminated by studying viscoelastic phenomena. Creep tests were then performed on small slats of wood using a cantilever bending test. It is shown that green wood viscoelastic behavior can be modeled with Burgers’ model. The corresponding parameters are estimated with respect to the wood-cells age. No significant difference between tension wood and normal wood is revealed, but it appears that green wood tends to lose its viscoelastic behavior with maturation.

Quantification of the Compressibility of Elastomers Using DIC

Both filled and unfilled elastomers are generally modeled as incompressible solids. However studies on the compressibility of elastomers have indicated that volume expansion is observed at large stretches coinciding with the onset of cavitation. Varying methods such as dilatometry and hydrostatic weighting have been used to calculate volume changes that elastomers undergo during stretching. However, these techniques cannot map volume variations for a heterogeneous state of stress, motivating the present work. In this study, carbon black filled elastomer samples were subjected to a uniaxial stretch and the deformation was recorded using two back-to-back stereo-correlation systems. The back-to-back stereo-correlation systems allow the in-plane strains on the front and back face of the specimen to be calculated along with the normal strain through the thickness. Using the assumption of plane stress, the volume variation of the elastomer as a function of the applied longitudinal strain was determined.

Application of Full-Field Measurements to Analyse the Thermo-Mechanical Response of a Three-Branch Rubber Specimen

The study deals with the characterization of the thermomechanical behavior of rubber. A test performed on a three-branch-shaped rubber specimen is used for this purpose. This heterogeneous test simultaneously induces the three types of stretch states classically considered to identify mechanical properties of rubber (uniaxial and equibiaxial tension, and pure shear), as well as the intermediary states. Recent works in which such heterogeneous tests are studied only consider the deformation field, but neither the corresponding thermal field nor the heat sources field are taken into account. The aim of the present study is to push forward the idea of heterogeneous tests by measuring both the displacement and thermal fields on the specimen. During the experiments, the displacement and thermal fields are measured using cameras. Measurements are then processed to associate a temperature and a strain level to each material point using a motion compensation procedure. The heat source fields are then derived from the temperature maps. Indeed heat source appears to be more relevant than temperature for characterizing the thermomechanical response of materials. Results obtained during the experiments will be presented in this paper. A discussion will also be initiated on the influence of the loading conditions on the heat source maps.

Mechanical Response and Energy Stored During Deformation of Crystallizing TPU

The present study investigates the thermomechanical behavior of closed-cell TPU foams. The effects of the density and the loading conditions on the softening, the residual strain and the hysteresis have first been characterized. The thermal responses exhibit numerous particularities. First, a threshold effect in terms of the density on the self-heating has been highlighted. Second, entropic effects are strongly weighted by energetic effects (internal energy variations) during the deformation. Typical changes in the thermal response highlight that SIC and crystallite melting occur during the deformation. The characteristic stretches of this phenomenon evolve with the maximum stretch applied. The lower the density, the lower the crystallinity. In the second part of this study, a complete energy balance is carried out during cyclic deformation of compact and foamed crystallizing TPUs. Results show that viscosity is not the only phenomenon involved in the hysteresis loop formation: a significant part of the mechanical energy brought is not dissipated into heat and is stored by the material when the material changes its microstructure, typically when it is crystallizing. Some of this energy is released during unloading, when melting occurs, but with a different rate, which contributes to the hysteresis loop. The part of the mechanical energy stored by the material has been quantified to investigate the effects of the loading rate and the void volume fraction on the energetic response of TPU. These effects cannot be predicted from the mechanical responses and the present study provides therefore information of importance to better understand and model the effects of the density and the loading conditions on the thermomechanical behavior of closed-cell TPU foams.

Contribution to Fatigue Striation Phenomenon Analysis by Using Image Processing

Since the use of energetic approaches for the prediction of the number at macro-crack initiation in elastomers, a special attention is paid on fatigue crack growth at the microscopic scale. In filled natural rubber, failure surfaces exhibit wrenchings and striations (Le Cam et al., Int J Fatigue 52:82–94, 2013). Both are assumed to be due to strain-induced crystallization (SIC). Only four studies address fatigue striations (Le Cam et al., Int J Fatigue 52:82–94, 2013; Le Cam and Toussaint, Macromolecules 43:4708–4714, 2010; Flamm et al., Int J Fatigue 33:1189–1198, 2011; Munoz-Mejia, Dissertation, Universite Claude Bernard, Lyon I, 2011), while they could provide information of importance to better understand how SIC enables natural rubber to resist the crack growth. As striations are similar to fringe patterns, this study aims at using a phase extraction algorithm from a single fringe pattern to analyse the striation morphology (Robin et al., Appl Opt 44:7261–7269, 2005; Takeda et al., J Opt Sot Am 72:156–160, 1982; Servin et al., Appl Opt 36(19):4540–4548, 1997; Robin et Valle, Appl Opt 43(22):4355–4361, 2004; Valle et al., Strain 46(2):175–183, 2008). This phase extraction methodology is split into three steps. The first one consists in extracting the wrapped phase without orientation. The second step is devoted to the determination of the fringe pattern orientation from a classic unwrapping algorithm. The third and last step consists in using an unwrapping algorithm (Zuo et al., Opt Lasers Eng 85:84–103, 2016; Menese et al., Appl Opt 44(7):1207–1215, 2005) and to compute the difference between the unwrapped phase processed and a plane in order to analyse the evolution of the striation morphology. This methodology has been applied to characterize the striation morphology observed at the failure surface of specimen tested under different fatigue loading conditions.

Mechanical and Thermomechanical Characterization of Different Leathers

Leather materials are able to undergo various strain and stress states during their elaboration process and their use in numerous applications. Although the experimental mechanical response in tension of leathers has been studied in the literature for decades, scarce information is available on the nature of their elasticity and more generally on their thermo-mechanical behaviors. In the present study, two leathers were tested under uniaxial cyclic loading while temperature changes were measured at the specimens’ surface by infrared thermography. The heat power at the origin of the temperature changes was then determined by using an adequate version of heat diffusion equation which is applicable to homogeneous tests. Results enabled us to discuss on the physical nature of the thermoelastic coupling in leathers. Intrinsic dissipation caused by the mechanical irreversibility was also detected. Distinct behaviors are evidenced as a function of the type of leathers.

Analysis of the Thermomechanical Response of Granular Materials by Infrared Thermography

Granular materials are defined as a collection of solid particles whose macroscopic mechanical behavior is governed by the interaction forces between the particles. Full-field experimental data on these materials remain few compared to numerical results, even though a wide literature deals with optical imaging (combined with digital image correlation) and photoelasticimetry (to measure shear stresses in particles made of birefringent materials). We applied infrared thermography to analyze two-dimensional granular media composed of cylinders and subjected to confined compression. We analyzed the calorific signature of the contact forces, especially by revealing mechanical dissipation in the interparticle friction zones. Moreover, two constitutive materials featuring entropic and isentropic elasticity were employed to compare distinct types of thermoelastic couplings. Couplings and mechanical dissipation were separately identified at two observation scales. The perspective of this work is the experimental analysis of soft granular media.

Calorific Signature of PLC Bands Under Biaxial Loading Conditions in Al-Mg Alloys

This paper investigates the thermomechanical behavior of Al-Mg alloys submitted to biaxial loading until fracture. The study aims to characterize calorimetric signature accompanying the formation and propagation of Portevin-Le Chatelier (PLC) bands induced by such a loading condition. Full kinematic and thermal fields on the specimen surface were characterized by using Digital Image Correlation (DIC) and infrared thermography (IRT). Heat source field was reconstructed from the temperature field and the heat diffusion equation. The heat source map enables us to visualize spatio-temporal gradients in the calorimetric response of the material and to investigate the kinematics of PLC bands induced by equibiaxial tensile loading. Under certain conditions, heat source maps can be seen as mechanical dissipation maps. At the specimen centre, the heat source exhibits jumps that fit with jumps of temperature and equivalent deformation rate.

How Does Cristallizable Rubber Use Mechanical Energy to Deform

Strain-induced crystallization (SIC) is responsible for the hysteresis loop observed in the mechanical response of Natural Rubber (NR). The present paper aims at determining the physical origin of such mechanical energy dissipation. For that purpose, temperature variations are measured by using infrared thermography during cyclic uniaxial tensile tests at ambient temperature. Heat sources (heat power densities) produced or absorbed by the material due to deformation processes are deduced from temperature fields by using the heat diffusion equation. Energy balance performed for each deformation cycle shows that crystallization/melting process does not produce intrinsic dissipation. The crystallization/melting process dissipates mechanical energy without converting it into heat. Hence, the whole dissipated mechanical energy corresponds to energy used by the material to change its microstructure. The demonstration that NR is able to dissipate mechanical energy without converting it into heat explains its ability to resist the crack growth and the fatigue loading.