Debond area

An upper limit to the interface fracture toughness, Gic, can thus be estimated from the work of debonding divided by the cylindrical debond area... 

Attractive and simple as the technique is in theory, in practice there are a number of difficulties which severely limit its value. Only areas of disbond, not a weak bond, can be detected although very weak areas can be made to part by pre-stressing, which is in any case necessary to separate the debonded areas. Notwithstanding these remarks, there have been considerable developments in ultrasonic flaw detection over the years although there has not been any widespread adoption of the technique in the rubber industry generally. 

There is a standard, ASTM D4788 (2003) covering infrared thermography of concrete and asphalt-covered concrete bridge decks. This requires a scanner with a minimum thermal resolution of 0.2°C and says that the temperature difference between a sound area and a delamination or a debonded area should be 0.5°C. The standard is for a vehicle mounted system and claims 80-90% of delaminations can be found in concrete decks with or without an asphalt overlay. 

Bolting may be advantageous in the repair of defective bonded joints and in the in-service repair of damaged structures. In both cases bolts transfer significant loads in the debonded areas and reduce peak stresses in the adhesive, since they enable the adhesive to be stressed to a higher level than before the repair. Bolts can also provide the clamping pressure required in bonding. This is often beneficial in repairs. 

In the debonding area negative tensile (compressive) stress is present in the center of the detachment site and shear stress in the annular region. The maximum diameter of the debonding area can serve as a measure of adhesion at the interface. The diameter and, better yet, the area of the debonding zone can... 

Heat flux passing through a coated substrate will provide a uniform temperature on the surface providing there is no flaw or debonded area of the coating. When a flaw or debonded site is present the flow of heat flux is interrupted and a decrease in temperature is detected on the surface. The actual shape of the area of which the temperature decreases is made visible at the surface by an infrared sensor. This can be seen in Figure 11.13. 

Figure 8.6 Pull-out test and the resulting stress-strain curve showing the difference in magnitude of the energies of debonding (area OAB) and pull-out (area OBCD). (From Anderson et al., Materials Science, (4th edn) Chapman and Hall, London, 1990, Ch. 11)...

As the heat flux proceeds from the substrate toward the surface of a test piece any flaw or debonded area will produce a difference in the otherwise uniform temperature on the surface. This is illustrated in Figure 15.13. An infrared sensor is used to make visible the shape of the area in which the temperature decreases at the surface, which is an almost exact reproduction of the real defect. The thinner the coating the better the agreement between the real shape of the disbonded area and the image on the surface. The process is similar to the formation of a shadow from a distant object. 

A distinct reproduction of the defect on the surface can be expected if the linear dimension of the debonded area is twice the distance from the interface to the surface. 

All the debonded surfaces should be examined using the methods described in the following sections. However, damaged joints should be inspected looking for debonded areas not yet exposed for this purpose, the nondestructive techniques (NDT) are very helpful, provided that the NDT procedure and operational parameters are properly selected. The specimen is then dissected to expose the damaged bonded surfaces, and they are also examined in depth, using optical and electron microscopy, and instrumental techniques such as spectroscopic analysis and thermal methods. 

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