Experimental study of the effect of disorder on subcritical crack growth dynamics

The growth dynamics of a single crack in a heterogeneous material under subcritical loading is an intermittent process; and many features of this dynamics have been shown to agree with simple models of thermally activated rupture. In order to better understand the role of material heterogeneities in this process, we study the subcritical propagation of a crack in a sheet of paper in the presence of a distribution of small defects such as holes. The experimental data obtained for two different distributions of holes are discussed in the light of models that predict the slowing down of crack growth when the disorder in the material is increased; however, in contradiction with these theoretical predictions, the experiments result in longer lasting cracks in a more ordered scenario. We argue that this effect is specific to subcritical crack dynamics and that the weakest zones between holes at close distance to each other are responsible both for the acceleration of the crack dynamics and the slightly different roughness of the crack path.Comment: 4 pages, 5 figures, accepted in Physical Review Letters (http://prl.aps.org

The effects of time correlations in subcritical fracture. An acoustic analysis

The fracture dynamics of heterogeneous materials is a rich subject with obvious practical interests, especially the subcritical fracture, where a material breaks through a series of successive, non-correlated and localized fracture events until the arriving to a critical situation where the whole material fails. Paper has been a common model material to study this phenomenon, and high-resolution and high-speed visualization are the usual ways to follow the dynamics of the process. However, visualization presents many limitations, especially for long experiences. That is one of the reasons why we are coupling acoustics to the measurements in an attempt to establish it as the main source of information. Acoustics presents a much better temporal resolution and captures a higher number of events than visualization. By thresholding the amplitude of the acoustic signal, it is possible to get similar activities in both measurements. The waiting times between events and the energy of the events are both distributed in power laws with exponents which are similar for the two different kind of measurements (visualization and acoustics), corroborating that the recorded acoustic data corresponds indeed to the fracture process

Repulsion and Attraction between a Pair of Cracks in a Plastic Sheet

We study the interaction of two collinear cracks in polymer sheets slowly growing towards each other, when submitted to uniaxial stress at a constant loading velocity. Depending on the sample’s geometry—specifically, the initial distances d between the two cracks’ axes and L between the cracks’ tips—we observe different crack paths with, in particular, a regime where the cracks repel each other prior to being attracted. We show that the angle θ characterizing the amplitude of the repulsion—and specifically its evolution with d—depends strongly on the microscopic behavior of the material. Our results highlight the crucial role of the fracture process zone. At interaction distances larger than the process zone size, crack repulsion is controlled by the microscopic shape of the process zone tip, while at shorter distances, the overall plastic process zone screens the repulsion interaction.Peer reviewe

Repulsion and attraction in a couple of cracks

We have performed an experimental study of the interaction of two collinear cracks in different polymer sheets submitted to uniaxial stress at a constant imposed velocity. Depending on the samples geometry and the material used, we could observe that the two cracks interact in different ways. More specifically, we could observe different crack trajectories, with in particular, a repulsive regime, which evolves systematically with the initial vertical crack separation d. We show that the angle characterizing the amplitude of the repulsion - and specifically its evolution with d - depends strongly on the microscopic behavior of the material. We provide a physical interpretation of our results, based on the observation of different shape and size of the fracture process zone in the different samples studied. At interaction distances larger than the process zone size, the microscopic shape of the process zone tip controls the amplitude of the crack repulsion, contrary to the macroscopic mechanical behavior of the material

Debonding energy of PDMS

International audienceWe investigated the debonding energy between confined layers of a soft elastic solid (PDMS) and a circular steel indenter in a flat punch geometry. PDMS is extensively used in applications, but also a widespread model system for fundamental research. Varying systematically the pulling speed and the viscoelastic properties, notably the modulus, we determined scaling laws for the debonding energy. We showed that the debonding energy is independent of the sample thickness. Applying a new approach and separating the crack initiation and the propagation part of the force curves, we analyzed the thickness dependence more precisely and we demonstrated that the energy to propagate the crack at given average speed does not only depend on the modulus, but also on the sample thickness

Debonding energy of PDMS

We investigated the debonding energy between confined layers of a soft elastic solid (PDMS) and a circular steel indenter in a flat punch geometry. PDMS is extensively used in applications, but also a widespread model system for fundamental research. Varying systematically the pulling speed and the viscoelastic properties, notably the modulus, we determined scaling laws for the debonding energy. We showed that the debonding energy is independent of the sample thickness. Applying a new approach and separating the crack initiation and the propagation part of the force curves, we analyzed the thickness dependence more precisely and we demonstrated that the energy to propagate the crack at given average speed does not only depend on the modulus, but also on the sample thickness

Acoustic localisation in a two-dimensional granular medium

We focus on localizing the source of acoustic emissions within a compressed two-dimensional granular ensemble of photoelastic disks, having as main information the arrival times of the acoustic signal to 6 sensors located in the boundaries of the system. By estimating, thanks to the photoelasticity of the grains, the wave speed at every point of the structure, we are able to compute the arrival times from every point of the system to the sensors. A comparison between the arrival time differences between every set of computed values to those from the actual measurements allows finding the source of the acoustic emissions

Acoustic localisation in a two-dimensional granular medium

We focus on localizing the source of acoustic emissions within a compressed two-dimensional granular ensemble of photoelastic disks, having as main information the arrival times of the acoustic signal to 6 sensors located in the boundaries of the system. By estimating, thanks to the photoelasticity of the grains, the wave speed at every point of the structure, we are able to compute the arrival times from every point of the system to the sensors. A comparison between the arrival time differences between every set of computed values to those from the actual measurements allows finding the source of the acoustic emissions