Adhesion in service

Frequently, the final composites are subjected to various severe environments during service. These can include heat, flexing and moisture. 

---

Stress analysis of a bonded joint produces the information needed to predict the service performance of the bond. To achieve this, two sets of test data are needed (1) the adhesive properties necessary to calculate the adhesive stress in the joint (2) test data from bonded specimens which provide the allowable stresses on the adhesive. Allowable means stresses that must not be exceeded in service. With these two sets of data in hand, the analyst compares the calculated stress to the allowable stress in order to predict the performance of the adhesive in service. 

Metal-to-metal peel tests quickly became popular, even mandatory, as a demonstration of toughness, yet the data had no use in direct stress analysis. Successful adhesives in service exhibit a range of peel strengths as high as nine to one. This strongly suggests that adhesive peel strength is not wholly relevant to the performance of bonded structure. 

This problem, namely the formaldehyde emission of the wood panel in service, after its manufacture, can be fortunately regarded to day as more or less solved, due to unequivocal and stringent regulations in many European countries, and successful long-term joint R D in the adhesives and wood-working industries. 

Designers must keep in mind that modem aircraft will likely be in service for thirty years and that repair of adhesive bonded structure will be inevitable. They must allow for reasonable disassembly of bonded stmcture for repair access as well as anticipate repair procedures that will not be unduly difficult for airline repair stations. 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

In-service performance. Service performance of bonded composite hardware has been good, at least from the standpoint of adhesive bonding. Like all aerospace hardware there have been occasional in-service problems with materials, design and manufacture of composite parts, but few have been related to adhesive bonding. 

The coating is applied to protect the steel from corrosion due to the acid or alkaline condition of the soil surrounding the pipe in service. Usually, the process requires three layers. First, an epoxy powder is applied to achieve adhesion to the pretreated metal and therefore resistance to cathodic disbondment. Second, a tie layer of polyolefin copolymer is applied and third a thick layer of polyethylene is cascaded, which in effect protects the epoxy from physical damage. 

Ion beams provide useful information either as a diagnostic tool or as a precision etching method in. adhesive bonding research. The combination of SIMS with complementary methods such as ISS or AF.S provides a powerful tool for elemental end limited structural characterization of metals, alloys and adhesives. The results shown here indicate that surface chemistry (and interface chemistry) can be decidedly different from bulk chemistry. Often it is this chemistry which governs the quality and durability of an adhesive bond. These same surface techniques also allow an analysis of the locus of failure of bonded materials which fail in service or test. 

The electroplating of reflective surfaces for exterior mirrors is an important and growing application. Moulding is to a high standard of precision and the electroplated surface creates a mirror with excellent reflection. It will resist impact much better than glass so a longer life in service can be expected— hence, such mirrors are fitted extensively on commercial vehicles, buses, and lorries. ABS is moulded and electroplated also into small mirrors that can be fixed by adhesive on vehicle wing mirrors to cover blind spots . 

Another megatrend in this competitive business environment is that all production processes need to be world-class and lean. This means that processes must be automated lead times shortened rework, scrap, and waste minimized or eliminated and less product must appear in inventory. This places a burden on the adhesive formulator in that the products not only must perform in service (e.g., creep resistance, peel strength) but also must perform in the assembly process (e.g., fast cure time, minimal scrap, low energy cure). 

One of the most common causes of internal stress is due to the difference in the thermal expansion coefficients of the adhesive and the adherend. These stresses must especially be considered when the adhesive solidifies at a temperature that is different from the normal temperature that it will be exposed to in service. 

Predicting the service life of adhesives is a risky business. The most difficult question ever put to an adhesive consultant is, How long will the adhesive joint last in service The problem is that an adhesive joint is not made up of just one element. It contains several elements, and some of them interact. In fact, in most adhesive joints at least five elements must be considered substrate A, interface A, the adhesive, interface B, and substrate B. To understand and predict the rate of degradation of each of these elements is challenging, but it can be done. The most difficult failure situations to predict are those that result from interactive effects. 

Life prediction of adhesive joints in service is difficult because several degradation mechanisms may be operating simultaneously. However, a process sequence to use in estimating the service life of adhesive joints is as follows ... 

---