An acoustic signature of extreme failure on model granular materials.

Unexpectedly, granular materials can fail, the structure even destroyed, spontaneously in simple isotropic compression with stick-slip-like frictional behaviour. This extreme behaviour is conceptually impossible for saturated two-phase assembly in classical granular physics. Furthermore, the triggering mechanisms of these laboratory events remain mysterious, as in natural earthquakes. Here, we report a new interpretation of these failures in under-explored isotropic compression using the time-frequency analysis of Cauchy continuous wavelet transform of acoustic emissions and multiphysics numerical simulations. Wavelet transformation techniques can give insights into the temporal evolution of the state of granular materials en route to failure and offer a plausible explanation of the distinctive hearing sound of the stick-slip phenomenon. We also extend the traditional statistical seismic Gutenberg-Richter power-law behaviour for hypothetical biggest earthquakes based on the mechanisms of stick-slip frictional instability, using very large artificial isotropic labquakes and the ultimate unpredictable liquefaction failure.

A versatile flexure-based six-axis force/torque sensor and its application to tribology.

Six-axis force/torque sensors are increasingly needed in mechanical engineering. Here, we introduce a flexure-based design for such sensors, which solves some of the drawbacks of the existing designs. In particular, it is backlash-free, it can be wirelessly monitored, it exactly enforces 90° angles between axes, and it enables visual inspection of the monitored system, thanks to its hollow structure. We first describe the generic design, implementation, and calibration procedure. We then demonstrate its capabilities through three illustration examples relevant to the field of tribology: low friction measurements under ultra-high vacuum, multi-directional friction measurements of elastomer contacts, and force/torque-based contact position monitoring.

High-temporal-resolution quasideterministic dynamics of granular stick-slip.

We report high-temporal-resolution observations of the spontaneous instability of model granular materials under isotropic and triaxial compression in fully drained conditions during laboratory tests representative of earthquakes. Unlike in natural granular materials, in the model granular materials, during the first stage of the tests, i.e., isotropic compression, a series of local collapses of various amplitudes occurs under random triggering cell pressures. During the second stage, i.e., shearing under triaxial compression, the model granular samples exhibit very large quasiperiodic stick-slip motions at random deviatoric triggering stresses. These motions are responsible for very large stress drops that are described by power laws and are accurate over more than 3 decades in logarithmic space. Then, we identify the quasideterministic nature of these stick-slip events, assuming that they are fully controlled by the cell pressure and solid fraction. Finally, we discuss the potential mechanisms that could explain these intriguing behaviors and the possible links with natural earthquakes.

Fluctuations of global energy release and crackling in nominally brittle heterogeneous fracture.

The temporal evolution of mechanical energy and spatially averaged crack speed are both monitored in slowly fracturing artificial rocks. Both signals display an irregular burstlike dynamics, with power-law distributed fluctuations spanning a broad range of scales. Yet, the elastic power released at each time step is proportional to the global velocity all along the process, which enables defining a material-constant fracture energy. We characterize the intermittent dynamics by computing the burst statistics. This latter displays the scale-free features signature of crackling dynamics, in qualitative but not quantitative agreement with the depinning interface models derived for fracture problems. The possible sources of discrepancies are pointed out and discussed.

Brittle-quasibrittle transition in dynamic fracture: an energetic signature.

Dynamic fracture experiments were performed in polymethylmethacrylate over a wide range of velocities and reveal that the fracture energy exhibits an abrupt threefold increase from its value at crack initiation at a well-defined critical velocity, below the one associated with the onset of microbranching instability. This transition is associated with the appearance of conics patterns on fracture surfaces that, in many materials, are the signature of damage spreading through the nucleation and growth of microcracks. A simple model allows us to relate both the energetic and fractographic measurements. These results suggest that dynamic fracture at low velocities in amorphous materials is controlled by the brittle-quasibrittle transition studied here.

Cancer mortality among workers in abattoirs and meatpacking plants: an update.

Workers in abattoirs and meatpacking plants have potential for exposure to bovine leukemia virus (BLV) and bovine papilloma viruses (BPV), which are oncogenic in cattle. These workers also have increased exposure to human papilloma viruses (HPV) and certain chemical carcinogens. We investigated whether such a group showed increased risk of cancers. We report mortality results after an additional 9-year follow-up of a previously studied group of 5,522 workers in abattoirs and 4,589 workers in meatpacking plants. Excess risk of all cancers combined, cancers of the lung, buccal cavity and pharynx, esophagus, colon, bladder, kidney, and bone was observed. Since factors such as tobacco smoking, alcohol, and diet, which have known associations with some of these cancers, were not taken into account, the significance of these findings is not known, except for lung cancer, for which occupational factors are probably involved. Because some of these findings have been consistently reported before, studies that will control for confounding factors as well are now urgently needed.