Adherend Surface Research Articles

1449. Interphase fracturing of composite bodies: S Mostovoy and E J Ripling, Rep AD-639,317, Sep 1966 ( Materials Res Lab, Inc Richton Park, Illinois)

Abstract : Cracking location for short time tests in the Dow epoxy resin (DER) 332, TEPA adhesive was found to depend on composition. At specific ratios of resin to hardener the crack path changed from center of bond (CoB) to interface (IF), depending on adherend material and surface treatment. For compositions where CoB fractures predominate, pre-cracked specimens exposed at zero stress, to either atmospheric humidity or liquid water, showed reinitiation toughness values higher than those seen in prior running or subsequent extension. This toughening is probably a result of the compressive stresses developed in the epoxy by water absorption around the crack tip. Static fatigue exposure of normally CoB pre-cracked specimens was found to cause significant slow crack extension at loads well below those required for rapid extension in short time tests. Debonding occurred starting at one IF directly in line but separated from the pre-existing crack. Propagation rates increased as either applied load or humidity was increased. Dynamic fatigue exposure of similarly CoB pre-cracked specimens also showed slow IF crack extension from the initial flaw. Rates of crack extension per cycle for this loading condition were found to be linear, increasing as stress amplitude or humidity was increased. Effects of a given maximum load and humidity were much more severe in the dynamic fatigue case than in the static. (Author)

Mode of failure in structural adhesive bonds

AbstractThis paper attempts to prove the possibility of adhesive failure by preferential adsorption and adhesive displacement at the adherend surface. Data are presented to demonstrate different modes of failure that are possible after initial cohesive failure.

Rheology of Peeling

The dependence of the peeling strength on the peeling rate is treated in this paper rheologically. First we showed experimentally that the time-temperature superposition principle is applicable to the S-type curve of the peeling strength vs, the rate for the amorphous polymer films (Vinylite) of various plasticizer (DBP) contents above glass transition temperature (Tg). The dependence of the shift factor aT on the temperature obeyed the W. L. F. equation giving reference temperature Ts considerably lower than Tg+50°, where Tg is measured dilatometrically. This means that the fractional free volume at Tg is larger than 0.025, in our case showing 0.037. This discrepancy is explained by the theory of Ferry and Stratton on the increase of fractional free volume due to the extension of polymers of Poisson's ratio less than 1/2. The apparent activation energy at Ts evaluated from the dependence of log aT on temperature, are 46.5-33kcal/mol for samples of plasticizer contents 0-40%, which are reasonable values for the viscous flow of the polymer segments. From these experimental results we conclude that the dependence of the peeling strength on the rate is substantially rheological in character, and that the course of peeling is not governed by the electrostatic mechanism as proposed by Deryagin et al.Secondly, we derived a theoretical formula relating to the peeling strength to the rate of peeling in adhesive tapes, assuming that the adhesive layer is deformed locally only at the peeling end, and its rhelogical behavior is described by the simple Voigt model. It is also assumed for simplicity that the shape of the bend at the peeling end is regarded as a part of a circle of radius R, which is determined by the peeling force P as R=√EoI/P, and that the deformation is vertical to the adherend surface. If the peeling proceeds steadily with velocity v, we may represent the initial and proceeding stage after time t of peeling with two circles whose centers are at (O, R) and (vt, R). Then the elongation of the adhesive layer at the origin, y, is given as a function of v, R, and t, with its time derivative, dy/dt, as a function of v, R, and y. Introducing this into Voigt's equation and integrating the equation dW=fdy from f=0 to f=fb where fb means the critical surface force, we obtain work of deformation, Wd, at the adhesion break. In the work of deformation, elastic energy We must be reserved. Put Wd'=Wd-We, then the sum of Wa, work of adhesion, and Wd' equals to the work done by the applied force, that is Wa+Wd'=P(1-cosθ), where θ is peeling angle. Carrying necessary calculation with some reasonable approximations, we obtain the following equation in the case of L-peeling (θ=π/2), P-C1P1/4v+C2P1/2v2=Wa, C1=2√2h1/2/3(EoI)1/4·(fb/E)3/2η, C2=2/(EoI)1/2(η/E)2fb, where Eo is Young's modulus of polymer film which determines the curvature at the peeling end, I, its moment of inertia, h, the thickness of the adhesive layer, η and E, viscosity coefficient and Young's modulus of the adhesive. This equation represents well the S-type curve of logP vs.

Adherend surface preparation

AbstractIf an adhesive joint is to be strong and permanent its adherend surfaces must be treated at least to remove oil, dirt, and dust. In addition, control of surface roughness, oxide formations, and sometimes adsorbed water is required. Deposition of chromium atoms or TiO2 is indicated for electron donor metals surfaces, and treatments may be useful to protect the adhesive from the adherend or vice versa, or to make their structures more congenial.

The measurement of cell adhesion

Some of the theoretical physical aspects of the measurement of cell adhesion by distractive techniques have been discussed. It is suggested that when adherends are separated by distraction, the tensile strengths of the adherend surfaces may be measured instead of the adhesive forces holding them together. Elementary rheological considerations strongly suggest that the final estimate of adhesive strength may vary not only with the magnitude of the distractive force but also with the rate at which it is applied. An experimental approach to the problem is described, in which accurately determined shearing forces are transmitted through fluid to detach cells adherent to discs of various materials in vitro. The experimental results confirm the predictions that when the strength of adhesion is estimated by distraction, this estimate will always tend to vary considerably with the method used to obtain it.