The different stages of an impact on laminated glass are difficult to observe and quantify but are essential for numerical model validation. To assess the performance of this composite to low-velocity ( ∼ 10 m/s) high-mass impacts, dynamical impact tests, inspired by ball-drop tests, are performed, equipped with multiple high-speed cameras. From the set of images, a continuous 3D shape reconstruction of the deformed laminated glass over time is performed using Stereo-Digital Image Correlation (S-DIC). Significant brightness and contrast variations captured by two cameras due to large rotations of glass fragments and large strain of the polymer interlayer between fragments render the use of S-DIC difficult as the impactor penetrates deeper into the plate. To circumvent this problem, an axisymmetric regularization is introduced which has the advantage of not requiring prior knowledge of the mechanical behavior for the tested specimen. It is shown that it offers a more extended period of time (as compared to regular S-DIC) over which a trustful analysis can be performed. The time evolution of the fragmented laminated glass and the density of cracks, both available from our analysis, are key ingredients to elaborate quantitative modeling of the dynamic performance of the laminate.

Describing quantitatively the response of laminated glass to low-velocity (~5 m/s) impacts by rigid bodies is an important issue because of its significance in terms of structural degradation and integrity, key parameters for people safety and anti-intrusion performances. This study aims to address the formation of cracks during graveling and steel ball drop tests, so, two well-instrumented experimental set-ups are proposed to study cracking in reproducible conditions. The first device can be seen as a mini-Hopkinson bar system, which from two strain gauges, allows to estimate force and velocity at a sharp indent tip. The second device, reproducing a blunt impact, exploits stereo-Digital Image Correlation (D.I.C.) measurements of the laminated glass surface.

The performances of Pressure Sensitive Adhesives (PSA) are generally evaluated using different loading geometries such as tack, peel and shear tests. It is difficult to link the behaviors of PSAs in these different geometries, and to predict the result of one test from another, because the confinement of a soft and dissipative material prevents the use of standard fracture mechanics, which separates the interface debonding behavior from the dissipation associated with the bulk deformation. We present here an original experimental investigation based on the modeling strategy proposed by Creton and Ciccotti[1]. Using instrumented versions of both peel and tack measurements, we compared the adherence performances of a series of model PSAs based on styrene-isoprene block copolymers, while identifying the mesoscale mechanisms at play during debonding. This analysis method allows us to model the contribution of the large strain rheology of the PSAs in the total work of debonding. We clearly show that both the adherence performances and local mechanisms can be closely related between peel and tack when considering both similar confinement and a similar strain rate of the fibrils that are spontaneously formed during debonding. While the overall adherence properties change by a factor of 3 between the different samples, the peel tests only present a minor +20% bias in adherence, which can be attributed to the combination of a 10% increase in the average stress and a 10% increase in the maximum strain of the fibrils. This improvement in the understanding of the PSA performances opens the way to a more sound mechanical design of PSA based joints.

We investigate experimentally the adherence energy Γ of model polyacrylate pressure-sensitive adhesives (PSAs) with combined large strain rheological measurements in uniaxial extension and an instrumented peel test. We develop a nonlinear model for such a peel test which captures the dependence of Γ(V) with peeling rate V, revealing the key role played by the extensional rheology. Our model explains in particular why traditional linear viscoelastic approaches correctly predict the slope of Γ(V) curves for sufficiently elastic PSAs characterized by a simple rate-independent debonding criterion. However, for more viscoelastic adhesives, we identified a more complex rate-dependent debonding criterion yielding a significant modification of the Γ(V) curves, an effect that has been largely overlooked so far. This investigation opens the way toward the understanding of fibrils debonding, which is the main missing block to predict the adherence of PSAs.

During the peeling of a Pressure Sensitive Adhesive (PSA), the adherence energy Γ is several orders of magnitude above the thermodynamic Dupré energy w between the adhesive and the underlying substrate, demonstrating the dominant role of energy dissipation. Moreover, Γ has a strong dependency on the peeling velocity V, and the characteristic Γ vs. V curves obey to a time-temperature superposition principle with a similar scaling to that of the linear rheology of adhesives. This has suggested for a long time that small strain viscoelasticity can be used to predict Γ, leading to two main modeling strategies. The first approach relates back to the sixties and uses the strong lateral confinement of the adhesive in a thin layer to treat it as a (visco)elastic foundation, constituted by a parallel array of springs (and dashpots) linking the flexible tape backing to the underlying substrate. Energy dissipation occurs in the whole thickness of the adhesive and it affects a stress concentration region beyond the peeling front (inside the bonded joint). The link with rheology is made through the time scale associated to the strain of this region caused by the propagation of the peeling front at velocity V. The characteristic peak of dissipation, which is responsible for the stick-slip instability, happens at the peeling velocity corresponding to the crossing of the glass transition. In the second approach, dissipation is obtained by a viscoelastic perturbation of the inverse square root stress singularity of LEFM (Linear Elasticity Fracture Mechanics). Energy dissipation happens in a region neighboring the crack front where the local strain rate (associated with the crack front propagation velocity V) corresponds to the relaxation time of the adhesive. In these models, the peak in Γ(V) is obtained when the size of this dissipative region becomes comparable with the adhesive thickness. In this second approach, the adherence energy Γ(V) is interpreted as an interfacial fracture energy amplified by viscoelasticity and should therefore be independent of the geometry and loading conditions of the adhesive joint. On the contrary, Γ(V) in the first approach is associated to the deformation of the whole adhesive joint, where the crack tip singularity plays a minor role. For this reason, the measured adherence energy Γ should be more properly interpreted as a work of debonding, which is rather an apparent fracture energy since it is not a fundamental property of the interface between the adhesive and the substrate. Phenomenologically, the Γ(V) curves of soft confined adhesives were shown to be dependent on the adhesive thickness a and on the peeling angle θ, which tends to be in favor of the first approach. However, data in the literature are related to very different types of adhesives, especially concerning their liquid/solid behavior. When considering soft solids only (such as most commercial PSA), which typically debond in an interfacial failure mode, this first approach, especially Kaelble’s model, seems to describe quite well the peeling experiments. This essentially linear elastic model (where the storage modulus might possibly be modulated by viscoelasticity) has however an unclear mechanical foundation. Even in a purely Hookean material, it would lead to a large and geometry dependent energy dissipation. This is in contradiction with the energy analysis of Griffith, which can also be applied to soft elastic solids, as shown by Rivlin and Thomas. Moreover, while most authors acknowledge the presence of long fibrils in the debonding region, these are not explicitly included in the modeling. This observation however clearly suggests an important role for the large strain mechanics and non-linear rheology of the adhesive in the work of debonding, as suggested by Gent and Petrich. The aim of this work is to reexamine the physics of these different modelings in light of the recent developments in the mechanics of soft materials and large strain rheology. 1. Angular dependency

The debonding of pressure sensitive adhesives (PSA) is a classical example of the difficult and unsolved issue of fracture in soft viscoelastic confined materials. The presence of a complex debonding region where the adhesive undergoes cavitation and the very large strain of a spontaneously formed fibrillar network has defied many modeling attempts over the past 70 years. We present here a novel technique to provide an accurate measurement of the local large strain response of the fibrillar debonding region during the steady-state peeling of a well known commercial adhesive over a wide range of peeling velocity and angle. The technique is based on high resolution imaging of the debonding region during peeling and is coupled to a cohesive zone modeling of the adhesive interaction between the flexible tape backing and the rigid substrate. The resulting database provides a strong ground for validating and further developing models (Villey et al. in Soft Matter 11:3480–3491, 2015) aiming to capture the effects of both geometry and non-linear adhesive rheology on the exceptional adherence energy of PSAs.

We present an experimental characterization of the detachment front unstable dynamics observed during the peeling of pressure sensitive adhesives. We use an experimental set-up specifically designed to control the peeling angle θ and the peeled tape length L, while peeling an adhesive tape from a flat substrate at a constant driving velocity V. High-speed imaging allows us to report the evolution of the period and amplitude of the front oscillations, as well as the relative durations of their fast and slow phases, as a function of the control parameters V, L and θ. Our study shows that, as the driving velocity or the peeling angle increases, the oscillations of the peeling front progressively evolve from genuine "stick-slip" oscillations, made of alternating long stick phases and very brief slip phases, to sinusoidal oscillations of amplitude twice the peeling velocity. We propose a model which, taking into account the peeling angle-dependent kinetic energy cost to accelerate and decelerate the peeled tape, explains the transition from the "stick-slip" to the "inertial" regime of the dynamical instability. Using independent direct measurements of the effective fracture energy of the adhesive-substrate joint, we show that our model quantitatively accounts for the two regimes of the unstable dynamics.

The modelling of the adherence energy during peeling of Pressure Sensitive Adhesives (PSA) has received much attention since the 1950's, uncovering several factors that aim at explaining their high adherence on most substrates, such as the softness and strong viscoelastic behaviour of the adhesive, the low thickness of the adhesive layer and its confinement by a rigid backing. The more recent investigation of adhesives by probe-tack methods also revealed the importance of cavitation and stringing mechanisms during debonding, underlining the influence of large deformations and of the related non-linear response of the material, which also intervenes during peeling. Although a global modelling of the complex coupling of all these ingredients remains a formidable issue, we report here some key experiments and modelling arguments that should constitute an important step forward. We first measure a non-trivial dependence of the adherence energy on the loading geometry, namely through the influence of the peeling angle, which is found to be separable from the peeling velocity dependence. This is the first time to our knowledge that such adherence energy dependence on the peeling angle is systematically investigated and unambiguously demonstrated. Secondly, we reveal an independent strong influence of the large strain rheology of the adhesives on the adherence energy. We complete both measurements with a microscopic investigation of the debonding region. We discuss existing modellings in light of these measurements and of recent soft material mechanics arguments, to show that the adherence energy during peeling of PSA should not be associated to the propagation of an interfacial stress singularity. The relevant deformation mechanisms are actually located over the whole adhesive thickness, and the adherence energy during peeling of PSA should rather be associated to the energy loss by viscous friction and by rate-dependent elastic hysteresis.

The rheological properties of liquids confined to nanometer scales are important in many physical situations. In this Letter, we demonstrate that the long-range elastic deformation of the confining surfaces must be taken into account when considering the rheology of nanometric liquids. In the case of a squeeze-flow geometry, we show that below a critical distance D(c), the liquid is clamped by its viscosity and its intrinsic properties cannot be disentangled from the global system response. Using nanorheology experiments, we demonstrate that picometer elastic deflections of the rigid confining surfaces dominate the overall mechanical response of nanometric liquids confined between solid walls.

Pour décrire les écoulements à des échelles nanométriques, on postule souvent que les fluides conservent leur comportement macroscopique à ces échelles et peuvent être décrits par des approches de type « milieu continu ». La question suivante se pose alors naturellement : jusqu’à quelle taille de confinement cette hypothèse reste-t-elle valable? Dans cet article, nous montrons qu’en réalité des liquides newtoniens conservent leurs propriétés mécaniques ordinaires à ces échelles, et que l’élasticité mesurée est en fait celle des parois qui confinent. Il faudra en tenir compte à l’avenir. Cela ouvre une piste nouvelle : utiliser un liquide pour sonder une paroi solide.

We present here a new type of distance sensor mounted on a Surface Force Apparatus (SFA), able to detect the position of a buried interface and therefore the thickness of a thin solid or soft matter film coating the SFA surface(s). This sensor relies on the capacitance created by the two metallized surfaces of the SFA. An harmonic oscillation of these polarized surfaces creates a pico- to femto-amps current indicating their relative position. One of the specificities of this sensor is the relatively weak polarization used for the measurements, minimizing the electrical forces and their impact on other interactions, hydrodynamical and mechanical forces measured by the SFA. This original and simple design is of high interest for studying the viscoelastic properties of thin films, to detect adsorbed liquid layers or slippage at liquid-solid interfaces, or even to study complex fluids such as ionic liquids under polarization. We demonstrate the use of this sensor to study the flow boundary condition of silicon oil on a metal surface, and the elastic modulus of a thin elastomer layer.