Designing metainterfaces with specified friction laws.

Many devices, including touchscreens and robotic hands, involve frictional contacts. Optimizing these devices requires fine control of the interface's friction law. We lack systematic methods to create dry contact interfaces whose frictional behavior satisfies preset specifications. We propose a generic surface design strategy to prepare dry rough interfaces that have predefined relationships between normal and friction forces. Such metainterfaces circumvent the usual multiscale challenge of tribology by considering simplified surface topographies as assemblies of spherical asperities. Optimizing the individual asperities' heights enables specific friction laws to be targeted. Through various centimeter-scaled elastomer-glass metainterfaces, we illustrate three types of achievable friction laws, including linear laws with a specified friction coefficient and unusual nonlinear laws. This design strategy represents a scale- and material-independent, chemical-free pathway toward energy-saving and adaptable smart interfaces.

Aftershock sequences and seismic-like organization of acoustic events produced by a single propagating crack.

Brittle fractures of inhomogeneous materials like rocks, concrete, or ceramics are of two types: Nominally brittle and driven by the propagation of a single dominant crack or quasi-brittle and resulting from the accumulation of many microcracks. The latter goes along with acoustic noise, whose analysis has revealed that events form aftershock sequences obeying characteristic laws reminiscent of those in seismology. Yet, their origin lacks explanation. Here we show that such a statistical organization is not only specific to the multi-cracking situations of quasi-brittle failure and seismology, but also rules the acoustic events produced by a propagating crack. This simpler situation has permitted us to relate these laws to the overall scale-free distribution of inter-event time and energy and to uncover their selection by the crack speed. These results provide a comprehensive picture of how acoustic events are organized upon material failure in the most fundamental of fracture states: single propagating cracks.

Self-replicating cracks: a collaborative fracture mode in thin films.

Straight cracks are observed in thin coatings under residual tensile stress, resulting into the classical network pattern observed in china crockery, old paintings, or dry mud. Here, we present a novel fracture mechanism where delamination and propagation occur simultaneously, leading to the spontaneous self-replication of an initial template. Surprisingly, this mechanism is active below the standard critical tensile load for channel cracks and selects a robust interaction length scale on the order of 30 times the film thickness. Depending on triggering mechanisms, crescent alleys, spirals, or long bands are generated over a wide range of experimental parameters. We describe with a simple physical model, the selection of the fracture path and provide a configuration diagram displaying the different failure modes.

Fragmentation and limits to dynamical scaling in viscous coarsening: an interrupted in situ x-ray tomographic study.

X-ray microtomography was used to follow the coarsening of the structure of a ternary silicate glass experiencing phase separation in the liquid state. The volumes, surfaces, mean, and Gaussian curvatures of the domains of minority phase were measured after reconstruction of the 3D images and segmentation. A linear growth law of the characteristic length scale ℓ∼t was observed. A detailed morphological study was performed. While dynamical scaling holds for most of the geometrical observables under study, a progressive departure from scaling invariance of the distributions of local curvatures was evidenced. The latter results from a gradual fragmentation of the structure in the less viscous phase that also leads to a power-law size distribution of isolated domains.

Mode III cleavage of a coin-shaped titanium implant in bone: effect of friction and crack propagation.

Endosseous cementless implants are widely used in orthopaedic, maxillofacial and oral surgery. However, failures are still observed and remain difficult to anticipate as remodelling phenomena at the bone-implant interface are poorly understood. The assessment of the biomechanical strength of the bone-implant interface may improve the understanding of the osseointegration process. An experimental approach based on a mode III cleavage mechanical device aims at understanding the behaviour of a planar bone-implant interface submitted to torsional loading. To do so, coin-shaped titanium implants were inserted on the tibiae of a New Zealand white rabbit for seven weeks. After the sacrifice, mode III cleavage experiments were performed on bone samples. An analytical model was developed to understand the debonding process of the bone-implant interface. The model allowed to assess the values of different parameters related to bone tissue at the vicinity of the implant with the additional assumption that bone adhesion occurs over around 70% of the implant surface, which is confirmed by microscopy images. The approach allows to estimate different quantities related to the bone-implant interface such as: torsional stiffness (around 20.5 N m rad(-1)), shear modulus (around 240 MPa), maximal torsional loading (around 0.056 N.m), mode III fracture energy (around 77.5 N m(-1)) and stress intensity factor (0.27 MPa m(1/2)). This study paves the way for the use of mode III cleavage testing for the investigation of torsional loading strength of the bone-implant interface, which might help for the development and optimization of implant biomaterial, surface treatment and medical treatment investigations.

Understanding fast macroscale fracture from microcrack post mortem patterns.

Dynamic crack propagation drives catastrophic solid failures. In many amorphous brittle materials, sufficiently fast crack growth involves small-scale, high-frequency microcracking damage localized near the crack tip. The ultrafast dynamics of microcrack nucleation, growth, and coalescence is inaccessible experimentally and fast crack propagation was therefore studied only as a macroscale average. Here, we overcome this limitation in polymethylmethacrylate, the archetype of brittle amorphous materials: We reconstruct the complete spatiotemporal microcracking dynamics, with micrometer/nanosecond resolution, through post mortem analysis of the fracture surfaces. We find that all individual microcracks propagate at the same low, load-independent velocity. Collectively, the main effect of microcracks is not to slow down fracture by increasing the energy required for crack propagation, as commonly believed, but on the contrary to boost the macroscale velocity through an acceleration factor selected on geometric grounds. Our results emphasize the key role of damage-related internal variables in the selection of macroscale fracture dynamics.

Crack propagation through phase-separated glasses: effect of the characteristic size of disorder.

We perform fracture experiments on nanoscale phase-separated glasses and measure crack surface roughness by atomic force microscopy. The ability of tuning the phase domain size by thermal treatment allows us to test thoroughly the predictions of crack front depinning models about the scaling properties of crack surface roughness. It appears that, in the range of validity of these depinning models developed for the fracture of brittle materials, our experimental results show a quantitative agreement with theoretical predictions: Beyond the characteristic size of disorder, the roughness of crack surfaces obeys the logarithmic scaling early predicted by Ramanathan, Ertas, and Fisher [Phys. Rev. Lett. 79, 873 (1997)].