Bond Failure Types

A rubber to metal bonded part can be thought of as a chain which holds rubber and metal together. Any chain is only as strong as its weakest link. So it follows that when a rubber to metal part fails, it will fail in the weakest section of the part. A test method exists which specifically covers adhesion of rubber to metal, ASTM D429-2002 [21]. The vast majority of bond failures can be attributed to one or a combination of the following ASTM designations  

RC - Rubber/Cement failure failure at the rubber to cement (adhesive) interface. 

CP - Cement/Primer failure failure at the cement (adhesive) to primer interface. 

CM - Cement/Metal failure failure at the primer to metal interface. For one coat adhesives, this is failure at the adhesive to metal interface. 

Rubber failure (R) is the type of failure to strive for. It indicates cohesive failure of the rubber. This means that the bond between the rubber and the adhesive is stronger than the tear strength of the rubber. 

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The failure of PC in compression was violent. Compression cylinders would shatter violently and the remaining core of the cylinders had either a cone shape or a near vertical failure surface. Flexural beam specimens also failed in a violent manner as a tensile crack developed in the zone of maximum moment near mid-depth. The specimens were broken into almost two identical pieces and the failure surface was near vertical. The tensile bond strength between the PC overlays and the portland cement concrete substrate was found to be strongly dependent on the type of resin used. In overlay or repair applications, it is usually desirable to have tensile bond failures occurring in the portland cement concrete substrate rather than at the interface between the two materials. 

The reason for the lack of follow-up on this synthetic route may be attributed to literature reports of failures to isolate or even observe the expected dihydropyrimidines, particularly in the case of the simple a,/ -ethylenic aldehydes.143,146 Dihydropyrimidine formation from a,/ -unsat-urated carbonyl compounds and amidines occurs via nucleophilic attack by amidine at the activated double bond (Michael-type addition), followed by ring closure and dehydration (see Scheme 4). In the course of confirming this reaction scheme, the intermediacy of tetrahydropyrimidines and dihydropyrimidines was demonstrated. 

FIG. 5—Ratio of steel bond strength of RPC in comparison with RC and HSM. TAB LE 6—Steel bond strength and failure type of RPC in comparison with RC and HSM. 

Failure Type Bond Failure Steel Break off Steel Break off Bond Failure... 

There were three different failure modes in this slant shear test. The failure type for RC/RC specimens was interface failure. The failure modes for RPC/RPC specimens included repair material failure and substratum failure. The failure modes for HSM/RC and RPC/RC specimens were interface failure or substratum failure. RC and HSM specimens have relatively lower concrete bond strength, while RPC specimens still have relatively higher values after 1000 freeze-thaw cycles. 

Figure 4 shows failure types of the adhesive bonded joint under a pull strength test. The cohesive failures in Fig. 4(a) and (b) occur when fracture is developed either within the adhesive and substrate, while the adhesive failure in Fig. 4(c) separates the substrate and adhesive at the interface. 

In the presence of weak fiber/coating bonds, the matrix cracks generate a single long debond at the surface of fibers (adhesive failure type, figure 10). The associated interface shear stresses are low, and load transfers through the debonded interfaces are poor. The matrix... 

Tables 1.3-1.5 show the result of analyses of several bonds between a substrate and a polyvinyl fluoride him using an acrylic adhesive. All surfaces were analyzed by electron spectroscopy for chemical analysis (ESC A). ESC A yields chemical analysis of organic surfaces in atomic percentage, with the exclusion of hydrogen, which is undetectable by this technique. To determine the type of bond failure, ESCA results for the failed surfaces are compared with those of the adhesive and the polyvinyl fluoride him.

Adhesive joints may fail adhesively or cohesively. Adhesive failure is an interfacial bond failure between the adhesive and the adherend. Cohesive failure occurs when a fracture allows a layer of adhesive to remain on both surfaces. When the adherend fails before the adhesive, it is known as a cohesive failure of the substrate. Various modes of failure are shown in Figure 1.3. Cohesive failure within the adhesive or one of the adherends is the ideal type of failure because with this type of failure the maximum strength of the materials in the joint has been reached. In analyzing an adhesive joint that has been tested to destruction, the mode of failure is often expressed as a percentage cohesive or adhesive failure, as shown in Figure 1.3. The ideal failure is a 100% cohesive failure in the adhesion layer. 

The advent of high resolution scanning electron microscopy (STEM) represents a tremendous advancement in the microscopic examination of adherend and failure surfaces. Venables et al. (15) have demonstrated using STEM that the degradation mechanism causing bond failure of aluminum alloys in a humid environment is the conversion of the surface oxide with a cellular and whisker structure to a surface hydroxide with a "com flake" type structure. 

The adhesives will bond almost all materials (though a primer may be needed with some), except polyolefin plastics (eg Polythene) and other low surface-energy types such as fluoropolymers (eg Teflon) and silicone-based rubbers. Alkaline glass may cause premature bond failure and all glasses should be silane primed if at all possible, as this considerably improves the joint s humidity resistance. May stress crack stressed mouldings or susceptible plastics - polycarbonate, for example. 

Figure 3.21 Troubleshooting flow chart to determine type of bond failure...

Bonds between rubber and substrates can fail for a number of reasons. Section 12.1 deals with some of the causes of rubber to metal bond failures. Section 12.2 examines the type of failures which are adhesion related, in fabric or cord reinforced power transmission belts. Section 12.3 discusses a phenomenon which causes service failures of rubber components, mainly in sealing applications. This phenomenon arises through a bond which is formed between the rubber (nitrile) and the metal mating surface of a valve or similar, which is of sufficient strength to rupture the rubber surface when the valve is opened. 

This term is normally associated with shorter deeper beams. Failure takes place when the compression zone, after its reduetion in size due to cracking, fails under combined aetion of triaxial eompression and shear stresses. This failure type can happen at principal tensile eraeks. The limitation to this shear compression is that the average compressive stress in the concrete has only been found at failure in pure flexure. As it is based on extreme concrete fibre reaching its limiting strain it is possible that shear failure ean oecur on an inclined plane near the centre of a eompression zone. In the vessel the haunch hinge is under a multiaxial state of stress due to prestressing bonded steel, liner and penetrations. This zone will in that case resist shear compression failure. 

In many applications rubber is bonded to metal for fixing purposes or in order to alter the stiffness, and the integrity of the bond is often vital for maintenance of the required stiffness characteristics and to ensure adequate life. The mechanics of bond failure were studied for various types of deformation. Provided that the tests were carried out under suitable loading conditions, time-dependent failure with a similar locus was observed in peeling at 90 or 180 degrees, pure shear and various combinations of simple shear and compression. There were indications that an energetics approach could enable results from different geometries to be quantitatively interrelated. Cavitation-... 

O Fig. 8show. The total oxide thickness is 400 nm. This type of nanoroughness can provide more mechanical interlocking than most other treatments for improved bond strength and durability, but this increase occurs in the interlocking only if the polymeric polymer or adhesive completely wets and penetrates the pores of the oxide. The nanoscale pores create capillary forces that help this penetration. Cross-sectional SEM views of PAA coated with typical epoxy primers show such penetration (Venables 1984). In such cases, strictly interfacial bond failure is very unlikely. 

Failure Types According to the Loading of the Bonded Joint... 

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