Structural adhesives fracture

The section on design of structural adhesive joints will describe and cite advantages and disadvantages of joint geometries, such as butt, lap, scarf, strap, and combined versions of these. A general design criterion will also be included. Another section of the chapter will pertain to fracture mechanics. General theories on fracture mechanics and test techniques used to characterize structural adhesives fracture behavior will be discussed. 

Brinson, H. F., Renieri, M. P., and Herakovich, C. T., "Rate and Time Dependent Failure of Structural Adhesives," Fracture Mechanics of Composites, STP 593, ASTM, Phil. PA, 1975, p. 177-199. 

Brussat, T.R., Chiu, S.T. and Mostovoy, S. (1977). Fracture mechanics for structural adhesive bonds. AFML-TR-77-163, Air Force Materials Laboratory, Wright-Patterson Air Force base, Dayton, OH. 

The three principal forces to which adhesive bonds are subjected are a shear force in which one adherend is forced past the other, peeling in which at least one of the adherends is flexible enough to be bent away from the adhesive bond, and cleavage force. The cleavage force is very similar to the peeling force, but the former applies when the adherends are nondeformable and the latter when the adherends are deformable. Appropriate mechanical testing of these forces are used. Fracture mechanics tests are also typically used for structural adhesives. 

Novel, toughened one-component epoxy structural adhesives based on epoxy-terminated polyurethane prepolymer incorporating an oxolidone structure were developed to provide improved toughness, fracture resistance and adhesive properties with good chemical and moisture resistance.21 The hybrid resin cures with a standard latent curing agent/accelerator. 

Fracture mechanics (qv) tests are typically used for structural adhesives. Thus, tests such as the double cantilever beam test (Fig. 2c), in which two thick adlierends joined by an adhesive are broken by cleavage, provide information relating to structural flaws. Results can be reported in a number of ways. The most typical uses a quantity known as the strain energy release rate, given in energy per unit area. 

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittle. Tims, the utility of unmodified common resins as structural adhesives would be very limited For highly cross-linked resin systems to be useful structural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin flexibility7, and thus increases the fracture resistance of the cured adhesive. Recently, structural adhesives have been modified by elastomers which are soluble within the uncured structural adhesive, but then phase separate during the cure to form a two-phase system The matrix properties are mostly retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved... 

Practical uses of epoxies include load bearing applications such as structural adhesives and composite matrices. In these applications, their most detrimental feature is a characteristic low resistance to brittle fracture. The desire to improve this property has motivated studies on thermoset fracture behavior for the last two decades. Of particular interest is the relationship between the molecular structure and the failure properties of thermosetting epoxies, the subject of this chapter. 

Ripling EJ, Mostovoy S, Corten HT, Fracture mechanics a tcx)l for evaluating structural adhesives, J. Adhesion, 3, 1971, 107-123. 

ISO, Standard test method for mode I interlaminar fracture toughness, G/c, of unidirectional fibre-reinforced polymer matrix Composites. ISO 15024 2001. Blackman, B.R.K., H. Hadavinia, A.J. Kinloch, M. Paraschi and J.G, Williams, The calculation of adhesive fracture energies in mode I revisiting the tapered double cantilever beam (TDCB) test. Engineering Fracture Mechanics 2003. 70 p. 233-248. BSI, Determination of the mode I adhesive fracture energy, Gic, of structural adhesives using the double cantilever beam (DCB) and tapered double cantilever beam (TDCB) specimens. 2001. BS 7991. 

Blackman, B. R. K. and Kinloch, A. J., "Fracture Tests for Structural Adhesive Joints, in Fracture Mechanics Testing Methods for Polymers," Adhesives and Composites, A. Pavan, D. R. Moore, and J. G. Williams, Eds., Elsevier, Amsterdam, 2001, pp. 225-267. 

The nitrile-epoxy structural adhesive systems have moved from an aircraft orientation to include industrial, automotive and electronics areas. These include a host of film adhesive products (21, 22) and thixotropic pastes (23-26). Representative adhesive models have also been studied from a fracture toughness polnt-of-view permitting comparisons of bulk fracture to that of fracture in the adhesive joint (27, 28). 

Although important for structural adhesive bonds, fracture mechanics is not as critical for non-structural low load-bearing adhesives as used in most electronic modules. For the most part, passing minimum specification requirements for peel and tensile strengths both at ambient conditions and accelerated test conditions are sufficient. However, computer-simulated modeling and reliability analysis have been used for evaluating electrically conductive adhesives as used in electronics assembly. ... 

Standards such as the British Standards Institute BS 7991, Determination of Mode I Adhesive Fracture Energy, GIC, of Structural Adhesives Using The Double Cantilevered Beam (DCB) and Tapered Double Cantilevered Beam (TDCB) apply to these tests. 

Determination of the mode I adhesive fracture energy, Gjc, of structural adhesives using the double cantilever beam (DCB) and tapered double cantilever beam (TDCB) specimens Essential work of fracture (EWF)... 

In a contribution from B. F. Goodrich, Drake and Siebert extensively review the journal and patent literature since 1975 on reactive butadiene/acrylonitrile liquid and solid elastomers used in formulating epoxy structural adhesives. Areas reviewed include the preparation of elastomer-modified epoxy resins, the characterization of rubber-toughened epoxy resins, fracture mechanics and adhesive formulation and testing. 

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