Adhesive bond failure modes

Figure 1.3 Schematics of adhesive bond failure modes (a) adhesive failure (b) cohesive failure in the adhesive layer (c) cohesive failure in the adherend.

Micrographs of the failure surfaces show good adhesion between the adhesive and the primed surface. Fig. 15.2 shows the failure surfaces of Al adherend bonded with the epoxy adhesive. The failure mode is cohesive in the epoxy adhesive. The improved interlocking of the adhesive in the primed surface topography is presented in Fig. 15.3. 

It is seen in Table 1 that SIMS Is applicable to several areas of investigation in adhesive bonding. SIMS may be used in a variety of ways including species imaging of the surface (SUMS) which may be especially useful for clarifying mixed mode failure surfaces. 

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). 

DeLollis, N. J., and Montoya, O., Mode of Failure in Structural Adhesive Bonds, Journal of Applied Polymer Science, vol. 11, 1967, pp. 983-989. 

DeLollis and Montoya, Mode of Failure in Structural Adhesive Bonds. ... 

The paper is presented in three parts. First, the tests employed to determine the mixed mode fracture envelope of a glass fibre reinforced epoxy composite adhesively bonded with either a brittle or a ductile adhesive are briefly described. These include mode I (DCB), and mixed mode (MMB) with various mixed mode (I/II) ratios. In the second part of the paper different structural joints will be discussed. These include single and double lap shear and L-specimens. In a recent European thematic network lap shear and double lap shear composite joints were tested, and predictions of failure load were made by different academic and industrial partners [9,10]. It was apparent that considerable differences existed between different analytical predictions and FE analyses, and correlation with tests proved complex. In particular, the progressive damage development in assemblies bonded with a ductile adhesive was not treated adequately. A more detailed study of damage mechanisms was therefore undertaken, using image analysis combined with microscopy to examine the crack tip strain fields and measure adherend displacements. This is described below and correlation is made between predicted displacements and failure loads, based on the mixed mode envelope determined previously, and measured values. 

Ducept F, Davies P, Gamby D Mixed mode failure criteria for a glass/epoxy composite and an adhesively bonded composite/composite joint, Int. Journal of Adhesion Adhesives, 20, 3, 2000 p233-244. 

The mode of bond failure in the unexposed films was mainly cohesive. However, when a cross-linked polymer matrix was formed with UV exposure, the mode of bond failure was interfacial at all testing temperatures. We believe that by cross-linking the polymer with UV radiation, the modulus of the polyester was increased. This in turn caused the cohesive strength to be much greater than the interfacial bond strength. As a result, the mode of bond failure became interfacial, or adhesive (10) (Figure 8). 

Blackman, B. R. K., et al., "The Failure of Fibre Composites and Adhesively Bonded Fibre Composites under High Rates of Test. 1. Mode 1 Loading - Experimental Studies," Journal of Materials Science, Vol. 30, No. 23, 1995, pp. 5885-5900. 

Table 15.7 presents the single lap shear strength of Al adherends primed with G3 and G4 dendrimers bonded with epoxy adhesive. On anodized Al, results show improvement of 33-53% in lap shear strength and in failure mode using dendrimers as a primer. 

Material Plasma Chemistry Adhesive Bond Strength (psi) Failure Mode Ref. 

Thus, the first questions posed in studying the strength of elastomers (and other materials as well) are where and under what conditions does fracture begin Also, what laws govern the growth of a crack once it has been initiated This chapter seeks to answer such questions, first in a general way and then with particular reference to important modes of failure of elastomers in service. It does not deal with the rather complex problem of the strength of composite structures, such as a pneumatic tire, which involves failure of adhesive bonds at interfaces between the components as well as fracture of the components themselves. 

Blackman BRK, Dear JP, Kinloch AJ, MacGiUivray H, Wang Y, WiUiams JG. The failure of fibre composites aud adhesively bonded fibre composites under high rates of test. 3. Mixed mode I/n and mode II loadings. J Mater Sci 1996 31 4467-77. http // dx.doi.org/10.1007/BF00366342. 

A larger number of failure modes is thought to be possible with composite materials bonded by adhesive joints than with equivalent metal joints. Failure in the adherends can be tensile, interlaminar or transverse. The last two can be either in the resin or in the interface. Finally, cohesive failure can occur in the adhesive. 

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