Adhesive bonding failure mechanisms

The extent of adhesive bond failure under corrosive environments is greatly accelerated when cyclic mechanical stresses are imposed on the adhesive bond during exposure. Three to four orders of magnitude reduction in fatigue life of adhesive bonds is observed for bonds exposed to environment prior to fatigue testing. 

Joints are potential failure sites. This applies whether they are adhesively bonded or mechanically fastened, and whether they join two reinforced plastics sections, or one reinforced plastics component and one constructed from another material. 

In adhesive bonding, failure analysis is a critically important aspect of both manufacturing and scientific investigations. Identification of the locus of failure of a manufactured product or a test structure is necessary to establish the cause of the failure and either to recommend a remedy to the problem or to understand the mechanisms of crack initiation and propagation and identify the weakest link in the structure. 

Finally, reference should be made to the extensive and recent durability evaluations of rubber-to-steel bonded joints in seawater. These kinds of bonds have been of especial interest to the fabricators of the deep ocean oil rigs. Stevenson ) has shown that the rate of adhesive bond failure between two metals of different electrochemical potential have special qualities that need to be understood by the designer. First, however, he demonstrated that mechanical strain in the elastomer layer did not seem to have any effect on bond durability. Furthermore, if the two adherends were electrochemically inert then the bonds were completely stable after periods as long as three years in saltwater. There was, however, an expected result when one adherend was more noble. While that adherend would be less corroded because of the electrochemical protection by the more anodic adherend, there was also a distinct increase in the rate of adhesive bond failure at that interface. 

One should always strive for stiffness balance between the members being joined together, whether by adhesive bonding or mechanical fastening. (The designs in Fig. 2 deviated slightly from this goal but only because there was yet another failure mode, in the adherends rather than the adhesive, which, left... 

The most commonly used methods for evaluation of adhesive bonds are mechanical tests such as tensile shear and peel tests that determine the weakest link in a bonded assembly. Although these tests are useful in the development and quality control of adhesives, they are destructive and cannot offer failure prediction for in-service components. Ultrasonic inspection is the most commonly used non-destructive test method and can accurately assess de bonding in single adhesive bonds, providing that the sensor is perpendicular to the defect plane. However, ultrasound has some limitations in... 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

Fitzpatrick and Watts [57] also applied imaging TOF-SIMS to deteiTnine the failure mechanisms of adhesively bonded, phosphated hot-dipped galvanized steel... 

A.H. Muhr, A.G. Thomas, and J.K. Varkey, A fracture mechanics study of natural rubber-to-metal bond failure, J. Adhesion Set TechnoL, 10, 593-616, 1996. 

Other explanations of the nature of the polymer to metal bond include mechanical adhesion due to microscopic physical interlocking of the two faces, chemical bonding due to acid/base reactions occuring at the interface, hydrogen bonding at the interface, and electrostatic forces built up between the metal face and the dielectric polymer. It is reasonable to assume that all of these kinds of interactions, to one degree or another, are needed to explain the failure of adhesion in the cathodic delamination process. 

The two predominant mechanisms of failure in adhesively bonded joints are adhesive failure or cohesive failure. Adhesive failure is the interfacial failure between the adhesive and one of the adherends. It indicates a weak boundary layer, often caused by improper surface preparation or adhesive choice. Cohesive failure is the internal failure of either the adhesive or, rarely, one of the adherends. 

With time (under increased temperature and humidity) the crack tip continues to a weaker region which for this surface treatment appears to be near the oxide/alloy interface. Figure 11 summarizes the analysis of the bond failure for this particular surface treatment. The important aspect here is that under identical conditions, different surface preparations show different modes of failure. Weak boundary layers are not developed using some treatment/bonding combinations. Processes have been developed in which the locus of failure remains in the adhesive ("a cohesive failure") and it is necessary to use a mechanical test in which even more stress is placed on the interfacial region (19). 

Certain adhesive systems are more resistant to interfacial degradation by moist environments than are other adhesives. Table 15.16 illustrates that a nitrile-phenolic adhesive does not succumb to failure through the mechanism of preferential displacement at the interface. Failures occurred cohesively within the adhesive even when tested after 24 months of immersion in water. A nylon-epoxy adhesive bond, however, degraded rapidly under the same conditioning owing to its permeability and preferential displacement by moisture. 

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