Cohesive zone models and the plastically deforming peel test

Abstract

The peel test is a popular test method for measuring the peeling energy between flexible laminates. However, when plastic deformation occurs in the peel arm(s) the determination of the true adhesive fracture energy, G c , from the measured peel load is far from straightforward.Two different methods of approaching this problem have been reported in recently published papers, namely: (a) a simple linear-elastic stiffness approach, and (b) a critical, limiting maximum stress, σ max , approach. In the present paper, these approaches will be explored and contrasted. Our aims include trying to identify the physical meaning, if any, of the parameter σ max and deciding which is the better approach for defining fracture, when suitable definitive experiments are undertaken.Keywords: cohesive zone models; fracture mechanics; laminates; peel tests; plastic deformation. 2 INTRODUCTIONThe peel test is a popular test method for measuring the peeling energy between flexible laminates [e.g. [1][2][3]. The simple single-arm form is shown in Figure 1a and the 'T-peel' variant is illustrated in Figure 1b. For the former test method, the total energy input, G, is related to the applied steady-state peel load, P, the width, b, of the specimen and the peel angle, θ, by:(1 cos )and for the 'T-peel' we essentially have two such specimens 'back-to-back', each with 2 π θ = , such that:This value of G includes the adhesive fracture energy, G c , and any plastic work done in bending the peeling arm(s). The value of the adhesive fracture energy, G c , is assumed to be a 'characteristic' property of the adhesive, or interface, and ideally independent of geometrical details of the peel test such as the thickness, h, of the peel arm and the peel angle, θ.However, the value of G c would, of course, be expected to typically be dependent upon the test rate and temperature, since we are dealing with viscoelastic materials.When only elastic deformation occurs in the peeling arm there is no energy dissipation, so that c G G = . However, in many cases, there is a rather complex bending and unbending process, as shown, for 2 π θ = in Figure 2a where the peeling arm is initially bent and then gradually straightened as the peeling proceeds. A schematic diagram of the bending moment, M/b, per unit width in the peel arm and the inverse of the local radius, 1/R, of curvature at the peel front is shown in Figure 2b and the area under the curve is the plasticenergy dissipated in bending. When a non-work hardening material is used for the peel arm, the moments tend to the plastic limit:3 where M p is the fully plastic moment, y σ is the yield stress and h is the thickness of the peel arm, and for large values of the plastic-energy dissipation:A crucial factor in the analysis is the root rotation, o θ , illustrated in Figure 3. This arises from stretching of the substrate peeling arm before it debonds and reduces the plastic work done such that the proportion of G going into plastic work,Considering now only the 90° peel test, and assuming o θ to be small, then...

Adhesion of polymer coatings studied by laser-induced delamination

“…Another source of plastic deformations not considered so far is the process of delamination described as crack propagation along the interface of two media. 10,11 The effect of the plastic deformation at the crack tip will be included in a future model description.…”

“…A typical example of such energy dissipation is the plastic deformation of a film during a peel test, as extensively discussed elsewhere. 10,11 In the proposed method the contribution of the plastic deformations denoted by is expected to be rather limited for the following reasons. First, the stress components in the bulk of the polymer are below the yield stress of the polymer under study.…”

“…Therefore, the first term on the right-hand side of Eq. ͑1͒ is also called the true adhesive fracture energy, 11 emphasizing that delamination is essentially a fracture process. This definition allows some dissipation of energy due to plastic deformation around the crack tip, crazing or other processes, localized at the interface and accompanying the propagation of the crack.…”

Design of gecko-inspired fibrillar surfaces with strong attachment and easy-removal properties: a numerical analysis of peel-zone

“…The peel angle, the length and shape of the peel region of the adhesive tape can significantly influence its mechanical peeling properties [6,17,[30][31][32]. It is practicable to fabricate adhesive surfaces with specially designed micro-fibrillar structures to modulate their reversible peeling and mechanical properties.…”

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“…Therefore, one usually distinguishes the thermodynamic work of adhesion and the socalled practical work of adhesion. [2][3][4] The thermodynamic work of adhesion is defined in wetting experiments by the Young-Dupré equation. This definition requires that the system is in the thermodynamic equilibrium and that the process of delamination is reversible.…”