Stick-slip rupture instability in polymers – StickSlip

Study of the peeling angle effect, at fixed peeling velocity and fixed length of free-standing tape, with the possibility to vary them over wide ranges, was performed using a tailor-made setup. An ultrafast video camera allowed the visualization of the stick-slip instability dynamics, including the microscopic instability which required an acquisition rate up to 500 000 images/s. The averaged applied force during peeling, either stable or unstable, was measured. The rapid temporal force variations are time-resolved using a method we have developed consisting in comparing the tape shape, extracted from fast camera images, with theoretical prediction of a model of elastica with cohesive zone (see figure). Analytical and numerical models have been developed to explain the increase in adherence energy with peeling angle and the inertial effects affecting the unstable peeling dynamics. To study the analogy between adhesive tape peeling and elastomer tearing, we have designed a setup allowing to tear apart symmetrically an elastomer in a wide range of velocities.

To date, the main results are:
1. Analyzing the stick-slip peeling instability of commercial 3M adhesives in roll geometry, we have shown that the stick phase duration was well understood, while the slip duration was abnormaly long, suggesting important dynamical effects are present.
2. Thanks to an experimental setup developed at FAST laboratory, measurements of stick-slip dynamics as a function of peeling angle have been performed. These measurements have demonstrated a strong dependence of the instability with peeling angle. We have argued this dependence is coming from an effective angle-dependent stiffness of the mechanical system constituted by the peeled tape. Furthermore, we have uncovered a very strong impact of the tape inertia at large peeling angle. This was an unexpected effect which has leaded us to rewrite the crack propagation equation for the adhesive peeling tape problem. These experimental results and corresponding analysis are part of a currently submitted paper.
3. We have discovered that the appearance and disappearance of the stick-slip instability as a function of peeling velocity is subcritical. Existing theories are incompatible with these observations since they predict instead a supercritical transition.
4. We have studied in detail the internal dynamic of a slip event and have observed a secondary dynamic linked to the regular propagation of transverse fractures making the peeling front move in a stick-slip fashion at very high frequency. This particular stick-slip dynamics is currently under study to understand how it depends on the control parameter of the peeling.

The studies we propose are important for basic science since they will deepen our knowledge of the dependence of the fracture energy on crack velocity and even extend it to the widely unexplored case of materials with interfacial or bulk heterogeneities. Comparisons between adhesive peeling and elastomer tearing will allow us to validate the generality of our analysis.
We should be able to propose a new model of the stick-slip dynamics of the peeling front based on a system of non-linear equations that we can directly compare to experimental data.
From a practical point of view, new solutions for controlling the instability (control of rupture
geometry, control of polymer rheology, control of heterogeneities) would represent a significant breakthrough for industrial applications.

[1] P.-P. Cortet, M.-J. Dalbe, C. Guerra, C. Cohen, M. Ciccotti, S. Santucci, L. Vanel,
Intermittent stick-slip dynamics during the peeling of an adhesive tape from a roller
Physical Review E, 87 022601 (2013)
(Evidence of instability intermittency suggesting influence of peeling angle).
[2] M.-J. Dalbe, S. Santucci, P.-P. Cortet, L. Vanel,
Strong dynamical effects during stick-slip adhesive peeling
Soft Matter, 10 132 (2014)
(Suggests inertial origin for the surprisingly long duration of «slip« phases)
[3] M.-J. Dalbe, S. Santucci, L. Vanel, P.-P. Cortet,
Peeling-angle dependence of the stick-slip instability during adhesive tape peeling
Soft Matter, 10 9637 (2014), Selected in the collection “2014 SoftMatter Hot Papers”
(Experimental proof of the influence of peeling angle on the instability)
[4] R. Villey, C. Creton, P.-P. Cortet, M.-J. Dalbe, T. Jet, B. Saintyves, S. Santucci, L. Vanel, D.J. Yarusso, M. Ciccotti,
Rate-dependent elastic hysteresis during the peeling of Pressure Sensitive Adhesives
Soft Matter, 11 3480 (2015)
(Experimental proof and modelling of the adherence energy increase with peeling angle)
[5] M.-J. Dalbe, P.-P. Cortet, M. Ciccotti, L. Vanel, S. Santucci,
Multiscale Stick-Slip Dynamics of Adhesive Tape Peeling
Physical Review Letters, 115 128301 (2015)
(A second instability at very different spatio-temporal scales due to bending curvature elasticity)
[6] M.-J. Dalbe, R. Villey, M. Ciccotti, P.-P. Cortet, S. Santucci, L. Vanel
Inertial and stick-slip regimes of unstable adhesive tape peeling
Soft Matter (2016)
(Systematic analysis of the peeling angle influence and modelling of the influence of adhesive tape inertia on the instability)

The dynamical rupture instability studied in this project is the so-called stick-slip instability i. e. the intermittent growth of a crack. Stick-slip rupture instabilities are observed in many different systems, for instance in heterogeneous rubbers used in the automotive tire industry and during the high speed peeling of adhesives in automatic labeling industrial processes, gluing of automotive parts or sealing of electronic housings. Overall, adhesive stick-slip reduces industrial productivity and its hard-to-predict nature hinders the development of new technical applications.
From a fundamental perspective, the non-monotonous dependence of fracture energy on crack velocity that triggers stick-slip is still difficult to understand quantitatively due to the complex interplay between the fracture geometry, the strongly non-linear mechanical response of materials in large strain conditions and the existence of heterogeneities either in the material’s microstructure or induced by the rupture mechanism. Furthermore, the various physical parameters influencing the stick-slip instability have rarely been correlated to the actual characteristics of the stick-slip motion (stick and slip velocities, durations and amplitudes).
The StickSlip project will develop and/or use various original and custom-designed experimental tools that are able to resolve in time and space the motion of the crack front during stick-slip in two situations: (i) the rupture of an adhesive at the interface between a backing tape and a substrate; (ii) the bulk rupture of an elastomer.
The experiments that we propose will shed a new light on stick-slip instabilities in fracture. Measurements of the detailed characteristics of the stick-slip motion will allow us to clarify the key physical parameters controlling the instability (geometry, adhesive or elastomer rheology).
The proposed experiments will also be used to test new ideas related to the effect of interfacial or bulk heterogeneities on stick-slip. The collective intermittent motion of the crack front that occurs during adhesive stick-slip might be disturbed by the presence of heterogeneities. A major goal of the project will be to understand whether heterogeneities might, in certain conditions, suppress the stick-slip instability at the macroscopic scale by destroying large scale correlations, and confine the stick-slip dynamics to smaller scales that would be less harmful for industrial applications. Being able to properly control the heterogeneities will be an important challenge of our project.
On the one hand, the studies we propose are important for basic science since they will deepen our knowledge of the dependence of the fracture energy on crack velocity and even extend it to the widely unexplored case of materials with interfacial or bulk heterogeneities. Comparisons between adhesive peeling and elastomer tearing will allow us to validate the generality of our analysis.
On the other hand, new solutions for controlling the instability (control of rupture geometry, control of polymer rheology, control of heterogeneities) would represent a significant breakthrough for industrial applications. In this context, the technical help offered by Bluestar Silicones is a significant added value to our project.