Adhesion dissipation mechanism

As is true for macroscopic adhesion and mechanical testing experiments, nanoscale measurements do not a priori sense the intrinsic properties of surfaces or adhesive junctions. Instead, the measurements reflect a combination of interfacial chemistry (surface energy, covalent bonding), mechanics (elastic modulus, Poisson s ratio), and contact geometry (probe shape, radius). Furthermore, the probe/sample interaction may not only consist of elastic deformations, but may also include energy dissipation at the surface and/or in the bulk of the sample (or even within the measurement apparatus). Study of rate-dependent adhesion and mechanical properties is possible with both nanoindentation and... 

Dissipation phenomena generally occur during measurement of the adherence of polymer materials, leading to an adherence energy function of both the number and nature of interfacial interactions (adhesion) and dissipative properties, mainly due to viscoelastic behavior [1-5]. Friction properties of polymers are also governed by interfacial interactions and dissipation mechanisms. Common phenomena (interfacial interaction and dissipation) therefore control adherence and friction behaviors. However, the relationship between the two phenomena is still vague or undefined. The first objective of this experimental work is then to compare adherence and friction of polydimethylsiloxane (PDMS) networks in order to establish relationships between these two properties. 

It is probable that very high fracture toughness values in wood joints bonded with thicker, lower-modulus adhesive layers may be due to the enhancement of an existing energy-dissipating mechanism such as microcracking of the wood as well as the adhesive. 

Polymers generally exhibit complex tribological behaviors due to different energy dissipation mechanisms, notably those induced by internal friction (chain movement), which is dependent on both time and temperature. Polymer friction is then governed by interfacial interactions and viscoelastic dissipation mechanisms that are operative in the interfacial region and also in the bulk, especially in the case of soft materials. Friction of a polymer can be closely linked to its molecular structure. The role of chain mobility has been studied in the case of elastomers, based on dissipation phenomena during adhesion and friction processes of the elastomer in contact with a silicon wafer covered by a grafted layer [1-5]. 

For both PDMSs, for both substrates, and for all friction speeds, a great effect of normal force is observed. The higher friction coefficient observed at low normal force could be explained by the role of adhesion, which is magnified at low load (where the bulk contribution is lower). The contribution of interfacial interactions (or adhesive contact) is then magnified. These interfacial interactions will activate viscoelastic dissipation mechanisms, increasing the friction resistance. 

Modem finite element analysis or other numerical methods have no problem in treating non-linear behavior. Our physical understanding of material behavior at such levels is lacking, however, and effective numerical analysis depends to a large extent on the experimental determination of these properties. Despite these limitations, many researchers have shown that elastic analyses of many adhesive systems can be very Informative and useful. A number of adhesive systems are sufficiently linear, such that it is adequate to lump the plastic deformation and other dissipative mechanisms at the crack tip into the adhesive fracture energy (critical energy release rate) term. 

Dynamic mechanical thermal analysis (DMTA) has also been used by Brinson, et al.,t l to determine the suitability of the technique for evaluating damage in the adhesive bond from the viscoelastic properties of bonded beams and for evaluating the effects of various environmental conditions and various surface treatments. The authors considered that if the bond becomes damaged (either adhesive and/or interphase) due to excessive load, fatigue, moisture, or corrosion, it would seem likely that dissipation mechanisms or loss modulus and tan 5 would change. Therefore, they used DMTA to measure the viscoelastic properties of beams with simulated flaws and beams taken from lap specimens, which had been exposed to humidity and/or corrosion for extended periods. 

In order to seek a better correlation, we must first attempt to remove all dissipative mechanisms from the system. Otherwise, the bond will appear to be stronger than it really is, because of energy expended in processes other than molecular separation or rupture at the interfacial plane. So, for soft polymeric adhesives, the possibility of viscous flow must be eliminated. One way of doing this is by lightly linking the polymeric molecules together in situ. In this way their characteristic interaction with the substrate can be preserved, while they are prevented from flowing apart on separation. 

The fatigue crack growth behavior of a structural adhesive is very sensitive to the mixed mode I/n conditions. The fractographic analysis revealed that energy dissipation mechanisms due to inelastic phenomena like bulk plastic deformation and crazing are more pronounced in... 

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