Stress adhesive test

Thermal stress adhesion test (adhesion) Subjecting a coating-substrate structure to an elevated temperature to introduce stress due to the differences in the thermal coefficients of expansion of the materials. The stress may cause failure or may introduce flaws that cause failure in subsequent testing. See also Adhesion test. 

Many of the qualitative adhesion tests vary with plate thickness. As indicated above, adhesion is better for thinner deposits. That, it was stated, has to do with the stress present in deposited films. A specified plating thickness should therefore be a given parameter requirement for adhesion testing. 

Mechanical stress accelerates the effect of environment on the adhesive joint. A great amount of data is not available on this phenomenon for specific adhesive systems because of the time and expense associated with stress-aging tests. However, it is known that moisture, as an environmental burden, markedly decreases the ability of an adhesive to bear prolonged stress, especially at slightly elevated temperatures. 

Peel values are recorded in pounds per inch of width of the bonded specimen. They tend to fluctuate more than any other adhesive test result because of the extremely small area at which the stress is localized during loading. Even during the test, the peel strength values tend to fluctuate depending on the type of adhesive, adherend, and condition of the test. In preparing the samples, care must be taken to produce void-free laminated bond lines. 

For obvious reasons, fr ture mechanics studies have concentrated on opening mode (Mode 1) fractures, and only a few studies have been made of other fracture modes. In one such study, Bascom etaL performed mixed-mode fracture tests on epoxy resin adhesives, vnth the epoxy layer at 45° to the applied stress . This test combines the opening Mode I widi die in-idaiK shear Mode II. The authors calculated a mixed-mode fracture etffirgy G/ jjc from the results, and obtained a... 

There is also the possibility of constituents from label adhesives migrating through polyethylene or polypropylene containers. This is something to be aware of when carrying out stressed compatibility testing and long-term stability testing. 

One of the main points of debate w ith the above methods is the stress distribution due to gripping the rubber block. Nicholson ct al. [41.42] used a test with two cords embedded in the block of rubber and avoided holding the block in one grip of the testing machine. Further analysis was made by Brodsky [43], who used three cords. Ellul and Emerson [44.45] used cords embedded in steel enclosed rubber cylinders with alternatively hot and cold bonding. Ridha et al. [46] have calculated the stress fields in tire cord adhesion test pieces, and Mollet [47] has compared the various methods. 

Brittle failure was exhibited by all materials, which indicated that at temperatures down to 76°K the stresses imposed upon these materials by the bending are sufficient to crack them and cause loss of adhesion. The important consideration here, however, is not whether a sealant passes or fails this test, but how it reacts to the stress. The test is most severe and it is doubtful that any existing sealant, without considerable reinforcement, will show enough strength and flexibility to be bent under these conditions and not break. But the mode of failure does suggest that some polymers are not as brittle as others and have promise as a basis for further modifications. Photographs of representative bend specimens, after test, are shown in Fig. 2. 

A function of adhesives that is becoming increasingly important is to absorb and dissipate stresses produced in electronic assemblies due to changes in environmental conditions or accelerated testing. The need for stress-absorbing and dissipating adhesives has increased with advancements in microelectronics. Some advancements requiring low-stress adhesives are ... 

Fig. 32 Stress-strain curves for poly(isobutene) (PIBUT), poly(isobutylene) (PIB85000), and a model acrylic adhesive during adhesive testing on a silicone-coated surface. Adopted with permission from [173]. Copyright 2010 John Wiley Sons, Inc.

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