Tensile detachment

In peel separation, the adhesive simply peels away from the surface. Lap shear occurs when the adhered material is subjected to a force that is applied parallel to the bonding plane. Here, the bond becomes deformed and stretched after initial rupture of some portion of the bond. It is a sliding type of failure. In tensile detachment, bond disruption occurs as force is applied at right angles to the bonding surface. Tensile detachment is a ripping type of bond disruption. 

FIGURE 13.9 Descriptions of simple tensile detachment (a) and simple lap shear (b) assemblies for testing adhesion. 

Tensile Detachment from a Rigid Plane. For a circular debonded patch at the interface between a half-space of an elastic material and a rigid substrate (Fig. 21), the applied stress ab sufficient to cause growth of the debond is (158)... 

Tensile Detachment. We consider first the detachment of an elastic half-space from a rigid plane. Fig. 17. If a circular debond of radius a is present at the interface, the tensile stress that will cause it to grow is given by... 

Fig. 17. Tensile detachment of a thick elastic layer with a circular interfacial debond, radius a.

Fig. 18. Tensile detachment of a thin elastic (adhesive) layer.

The drop formation is considered to proceed exactly in the same fashion as the bubble formation under constant flow conditions, viz. the two step (the expansion and detachment) mechanism. The tensile force does not arise in the expansion stage because there is no neck formation. 

The fracture theory is the most widely applied theory in studying mucoadhesion mechanisms. It accormts for the forces required to separate two sttrfaces after adhesion. The maximttm tensile stress (a) produced dttring detachment can be determined by Eq. (6) by dividing the maximiun force of detachment by the total surface area A ) involved in the adhesive interaction ... 

Various mechanical testing methods have been used to assess the bioadhesive properties of materials and formulations. Review of the literature reveals that the technique most commonly used is the tensile test [82,85]. This test provides the measure of the force needed to detach a layer of the tested material or formulation from a mucosal substrate as a function of the displacement occurring at the bioadhesive interface. Besides maximum force of detachment, another parameter provided by tensile test is the work of adhesion calculated as the area under the force versus displacement curve. Such a parameter gives more complete... 

With P=0 we get a30=6jiwR2K x. Hence a is finite even for zero external load P, owing to the adhesion forces. The detachment of tip and sample, the so-called pull-off, requires a tensile force Pa(i which characterises the adhesion force Pad=-(3l2)nwR. With 8 denoting the deformation of the tip-sample contact as measured in the vertical direction, the corresponding stiffness kts=dPld8 of the contact is given by... 

Fig. 3 Schematic representation of various types of failure associated with the mechanical instability of a film deposited on a substrate, (a) cracking of a thin film subjected to residual tensile stress, (b) plastic deformation of the substrate at the end of the crack, (c) deviation of the crack at the interface, (d) cracking of the substrate, (e) detachment and buckling (formation of a blister from an interface defect) of a film subjected to residual compressive stress, and (f) deviation of the crack through the thickness of the film (flaking).

Figure 13-12. Surface energy effects in the detachment of wear particles. (a) Adherent particle is compressed between two surfaces. (b) Elastic compression strain has been relaxed but volume tensile strain has been locked in by adhesion. (c) Direct detachment of wear particle by influence of surface pressure on zone of potential detachment (dashed line).

E, Rabinowicz has proposed a surface energy criterion for the size of detached wear particles [45, 46]. Figure 13-12a and 13-12b illustrates the situation when a rider passes over a fragment adhering to the countersurface. In Fig. 13-12b the elastic compressive strain has been relaxed, but the horizontal tensile strain ve remains locked... 

It follows from Eq. (11.17) that upon cooling, if a,- < large tangential tensile stresses develop that, in turn, could result in the formation of radial matrix cracks. Conversely, if at > a, the inclusion will tend to detach itself from the matrix and produce a porelike flaw. 

The other extreme of behavior is represented by shales that are highly cemented and unable to swell (awsh constant) and the response to the imbalance of chemical potential is an increase in Psh. The shales will tend to exhibit tensile failure by fracturing and large angular fragments may detach from the borehole wall. These shales, which are usually geologically older, contain mostly illite and kaolinite clays. Intermediate behavior of shale hydration has been observed (157). 

SEM examination shows continuous britfle cracks on the surface of iPP irradiated to 1000 kGy these cracks are attributable to chain scission and oxidative degradation of the iPP (see Section 11.3.2). In comparison, the surface morphology of the irradiated blend (1000 kGy) containing 50% iPP is quite different it shows mainly holes and cavities associated with the detachment of the dispersed mbber particles from the continuous iPP matrix, along with minute discontinuous cracks. The irradiated surface of EVAc (1000 kGy) shows brittle cracks, which are more intense and wider than those in iPP. The tensile failure surface of non-irradiated and irradiated iPP exhibits... 

As already mentioned, the stresses in thin films may present problems in many industrial applications of the films. Early observations of thin films in optical applications showed that particularly when the film thickness was large, cracks arose, which had cloudy marks and sometimes the films even became detached from the substrate. Exact measurements of evaporated single films and film systems indicated a partial stress compensation, especially in dielectric multilayers, since low refractive films often have tensile stresses, whilst high refractive films have compressive stresses [157,158]. 

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