2 Forces exerted on tape or attachments, Shear forces and Weak points.
The way forces act on the tapes or tape-less devices dictates how much hold they will have on the skin. Understanding how the forces translate into components that have a direct bearing on grip will help overcome or avoid problems. In page 1 the principals of adhesives and friction were explained so this section will endeavour to explain how the shear forces act on the adhesive or friction device that result in the failure to hold the skin. Weak points in any system of skin grip can to a great extent be worked out by understanding the principals involved.
Forces
Two forces can be exactly opposite and nothing moves until one exceeds the other.
1kg force<------------0------------>1kg force (nothing moves)
1kg force<------------0------------>1.5kg force (movement>>>>)
When you have a force acting at an angle then you can translate this into two components as shown below, you can even represent the forces involved by the length of the lines drawn if it is to scale:
The above gives a simple example of opposing forces but it is not normal for situations to be so conveniently so. The diagrames below show how forces can be resolved by 'vectors' and as you can see forces can be conveniently resolved into two component parts at right angles to each other.
Tape
The following show the adhesive layer of tape and how the three types of forces act upon them to separate tape from substrate.
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Pull Apart |
In this mode the forces trying to separate the tape from the substrate is at right angles. The force must overcome the elastic limit of the adhesive layer, suction and surface grip. |
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Peel |
Here the forces are at an angle and the very edge of the adhesive layer takes most of the force. It is the weakest of all the configurations with adhesive tape. |
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Shear |
The strongest of all ways that adhesive tape can resist forces. As the force is parallel to the surfaces it has to overcome the resistance of the adhesive over the entire surface of adhesion. In fact it has to tear the adhesive layer or rip it from from the surfaces. |
From the above take the last example (shear) and imaging a force put in this case, downwards onto the tape at right angles. This will force the tape onto the adhesive layer and the substrate to keep it in intimate contact. it would only need a small additional force in this way to increase the grip and strength of the adhesion by a substantial amount. An ideal way of ensuring maximum grip and avoid failure. This is why the 'Tube' method may be one of the most secure tape methods available. It is easy to apply this extra pressure at right angles to the fixing tape, simply by applying an extra strip of tape on top of the fixing tape circumference. This will, if a slight stretch is introduced whilst applying, create an even inward pressure always at right angles.
Šelks2002 Page last updated 19 October 2003