Critical Tractions for Initiating Adhesion Failure at Interfaces in Encapsulated Components

Abstract

Determining the initiation of adhesive failure at a surface buried deep within the bulk of an epoxy is qualitatively different from measuring the propagation of an existing surface crack. Most current tests are shown to be unsuitable for assessing the critical traction at initiation. A new test geometry is presented that initiates failure away from an air interface, produces a slowly varying stress distribution near the initiation site and minimal contributions from thermal residual stresses, and enables tests with mixed modes of loading. This new geometry is used to examine temperature-dependent adhesive failure in tensile, shear, and mixed modes of loading for both smooth and rough surfaces. Some of the experimental results are unexpected. As examples, the critical traction at initiation of adhesive failure is apparently insensitive to surface roughness, and the critical normal traction is independent of temperature while the critical tangential traction tracks the shear yield stress.

Mechanisms of degradation in adhesive joint strength: Glassy polymer thermoset bond in a humid environment

“…For the napkinring joint, the independence of virgin joint strength on surface preparation may well be associated with the mechanism of failure in the joint. The temperature dependence of the experimental shear stress at failure for the napkin-ring joint coincides with the temperature dependence of the predicted shear "yield" stress of the adhesive, 6 where polymer adhesive viscoelastic relaxation rates increase to the point that stress decays faster than incremented by the loading ramp in what has been termed "run-away" nonlinear viscoelasticity. 2 Figure 4 further illustrates the correlation, with multiple adhesives and multiple methods of predicting shear yield.…”

“…Two geometries that fit this criterion include the napkinring 2 (NR) and the saucer. 6 In this work, only the napkin-ring joint geometry is used. While the napkin-ring is limited to resolving critical shear tractions, the short diffusion path for moisture through the adhesive makes it attractive for realizing the effects of humid environments on joint strength in relatively short periods of time.…”

“…4 Therefore, it is necessary to clearly define the purpose of the current investigation so that the results can be fit into the context of this history. Attempts have been made to understand and predict both the propagation of existing surface cracks 5 and the critical traction defining the initiation of de-bonding in an as-designed, un-cracked geometry, be it under relatively uniform stress loadings 6 or where severe stress gradients exist within the joint. 7 The current study addresses the problem of assessing the ability of an adhesive bond with no known defects to survive an applied load.…”

Time-Dependence of Epoxy Debonding

“…The dimensions and radii of curvature were designed using finite element analysis [4] to ensure that (1) adhesive failure initiates away from an air interface, (2) the stress distribution near the initiation site is slowly varying, (3) the thermal residual stresses are not dominant, and (4) mixed-mode testing is enabled. Stresses in the strain-controlled, monotonically ramped tests were calculated using an accurate, nonlinear, viscoelastic, constitutive equation developed and validated in previous publications [5,6].…”

“…Examples of such lab tests are the napkin-ring [2] and the cruciform [3] geometries, which enable measurement of the critical shear and tensile surface tractions, respectively. In a previous paper [4], we presented a saucer test geometry that also avoided stress risers and enabled determination of critical surface tractions in shear, tension, and mixed modes of deformation. This sample was used to determine the critical tractions for initiating adhesive failure (failure in adhesion) at interfaces in ramped strain tests as a function of test temperature.…”