Methods adhesion-simulating

Hydrodynamic Factor. In order to discover the reason for the dependence of the adhesive forces on contact time, let us consider the hydrodynamic phenomena taking place when the bodies approach or recede from each other. For this purpose it is customary to employ adhesion-simulating methods (see 8), in particular the method of plane-parallel discs. The hydrodynamic factor due [88] to the motion of the liquid in the gap between the contiguous surfaces, determining the change in adhesion with contact time for the interaction of plane-parallel discs, may be represented by the Stefan—Reynolds equation... 

A dependence of t on i] was observed when measuring the forces of interaction by particle-adhesion simulation methods. 

Molecular dynamics (MD) permits the nature of contact formation, indentation, and adhesion to be examined on the nanometer scale. These are computer experiments in which the equations of motion of each constituent particle are considered. The evolution of the system of interacting particles can thus be tracked with high spatial and temporal resolution. As computer speeds increase, so do the number of constituent particles that can be considered within realistic time frames. To enable experimental comparison, many MD simulations take the form of a tip-substrate geometry correspoudiug to scauniug probe methods of iuvestigatiug siugle-asperity coutacts (see Sectiou III.A). 

In applications where possible degrading elements exist, candidate adhesives must be tested under simulated service conditions. Standard lap shear tests, such as ASTM D1002, which use a single rate of loading and a standard laboratory environment, do not yield optimal information on the service life of the joint. Important information such as the maximum load that the adhesive joint will withstand for extended periods and the degrading effects of various chemical environments are addressed by several test methods. Table 15.2 lists common ASTM environmental tests that are often reported in the literature. 

There are several analytical tools that provide methods of extrapolating test data. One of these tools is the Williams, Landel, Ferry (WLF) transformation.14 This method uses the principle that the work expended in deforming a flexible adhesive is a major component of the overall practical work of adhesion. The materials used as flexible adhesives are usually viscoelastic polymers. As such, the force of separation is highly dependent on their viscoelastic nature and is, therefore, rate- and temperature-dependent. Test data, taken as a function of rate and temperature, can be expressed in the form of master curves obtained by WLF transformation. This offers the possibility of studying adhesive behavior over a sufficient range of temperatures and rates for most practical applications. Fligh rates of strain may be simulated by testing at lower rates of strain and lower temperatures. 

The surface tension, interfacial tension and adhesion phenomena will be discussed, and a new correlation for the molar parachor will be presented, in Chapter 7. The calculation of the interfacial tension from the surface tensions of the components will also be discussed, and shown to be in need for significant improvements. In this context, an introduction will also be provided to advanced numerical simulation methods that are becoming increasingly useful in modeling the interfacial phenomena and phasic behavior of polymer-containing systems. 

Figure 11.20. Evolution of the morphology of a thermoset adhesive layer between two metal plates during uniaxial tensile deformation, as calculated by using the simulation method of Stevens [176,177]. Dr. Mark Stevens from Sandia National Laboratories kindly provided this figure. See the insert showing the colored figures for a better view.

It was mentioned above that the simulation method of Termonia [67-72] can be used to calculate the stress-strain curves of many fiber-reinforced or particulate-filled composites up to fracture, including the effects of fiber-matrix adhesion. Such systems are morphologically far more complex than adhesive joints. Many matrix-filler interfaces are dispersed throughout a composite specimen, while an adhesive joint has only the two interfaces (between each of the bottom and top metal plates and the glue layer). If one considers also the fact that there will often he a distribution of filler-matrix interface strengths in a composite, it can be seen that the failure mechanism can become quite complex. It may even involve a complex superposition of adhesive failure at some filler-matrix interfaces and cohesive failure in the bulk of the matrix. 

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