Objectives of the project
The objective of the project CASCADE is to advance knowledge in the field of friction, by providing a comprehensive picture of the onset of sliding and in particular, of the dynamics of shear cracks along heterogeneous frictional interfaces. The strategy is based on a tight, quantitative dialogue between experiments and numerical/theoretical approaches.
Summary of actions
Experiments
The project started with the development, test and calibration of an original opto-mechanical device to monitor the frictional contact between an elastomer and a rigid substrate. First, the dynamics of the slip field are measured optically using high speed camera acquisition and digital image correlation. This capability of the device was successfully used to provide the first experimental test of classical models for the incipient tangential loading of the contact between a smooth rigid sphere and a rough elastomeric plane.
Second, the dynamics of the area of real contact along the interface are obtained through thresholding of the images. This capability of the device was used to study the influence of a variety of system parameters on the static friction strength of a rough interface. In particular, they first shed light on a poorly recognized dependence of the area of real contact on the tangential force applied to a contact interface. Second, they systematically studied the influence of the thickness of a soft solid coating deposited on the rigid substrate and concluded that, contrary to common belief, shear strength is not a material’s constant. Third, they calibrated the effect of various grafted molecular coatings on shear strength. Analogies were drawn with adhesion and de-wetting properties of such textured surfaces.
Modeling
In parallel with these experimental activities, in collaboration with his numerical and theoretical colleagues, Dr. Scheibert developed a multi-scale model for the rupture dynamics of a multi-contact interface. It has been used to unravel the physical mechanisms behind a series of unexplained observations from the literature. In particular, they reproduced for the first time the transition from fast to abnormally slow fronts propagating along the interface. They established the link between the existence of a slow slip mechanism in the system and the possibility of such slow fronts. They also proposed a physically-based classification of the various types of possible shear crack fronts and provided important insight into the mechanisms behind the selection of front speed.
Studying in details the microscopic part of the multiscale model, Dr. Scheibert and collaborators proposed a generic, statistical framework which includes most of the junction-based friction models available in the literature.
Impact of the project
Overall, Dr. Scheibert’s results provide a comprehensive picture of the dynamics underlying the transition from static to kinetic friction at various scales. They can now be used to address important practical problems in a variety of fields including mechanical engineering, geology, biology and robotics.
Dr. Scheibert has been hired as a permanent researcher by the french CNRS (National Center for Scientific Research). The EU support allowed him to successfully re-integrate the french research system. He now manages his own research group independently. He obtained the French Habilitation to officially supervise his PhD students.