Fracture mechanics structural adhesives

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. 

The Institute has many-year experience of investigations and developments in the field of NDT. These are, mainly, developments which allowed creation of a series of eddy current flaw detectors for various applications. The Institute has traditionally studied the physico-mechanical properties of materials, their stressed-strained state, fracture mechanics and developed on this basis the procedures and instruments which measure the properties and predict the behaviour of materials. Quite important are also developments of technologies and equipment for control of thickness and adhesion of thin protective coatings on various bases, corrosion control of underground pipelines by indirect method, acoustic emission control of hydrogen and corrosion cracking in structural materials, etc. 

Step 3. The set of fracture properties G(t) are related to the interfaee structure H(t) through suitable deformation mechanisms deduced from the micromechanics of fracture. This is the most difficult part of the problem but the analysis of the fracture process in situ can lead to valuable information on the microscopic deformation mechanisms. SEM, optical and XPS analysis of the fractured interface usually determine the mode of fracture (cohesive, adhesive or mixed) and details of the fracture micromechanics. However, considerable modeling may be required with entanglement and chain fracture mechanisms to realize useful solutions since most of the important events occur within the deformation zone before new fracture surfaces are created. We then obtain a solution to the problem. 

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. 

Materials science associated with fracture mechanics has mainly been confined to composite materials such as concrete, ceramics and metals. Much of the emphasis of the research has been on preventing fatigue and failure rather than designing for it to occur. The way a structure deforms and breaks under stress is crucial for properties such as flow and fracture behaviour, sensory perception of structure, water release and the mobility and release of active compounds. In the case of foods, the ability to break down and interact with the mouth surfaces provides texture and taste attributes. The crack propagation in a complex supramolecular structure is highly dependent on the continuous matrix, interfacial properties and defects and the heterogeneity of the structure. Previous structure-fracture work has dealt with cellular plant foods, and it has been demonstrated that the fracture path differs between fresh and boiled carrots due to cellular adhesion and cell wall strength as well as cell wall porosity and fluid transport (Thiel and Donald 1998 Stoke and Donald 2000 Lillford 2000). 

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. 

The results above suggest that it may be possible to apply fracture mechanics data to determine failure loads of more complex structures, provided that (i) the adhesives used are not too ductile, (ii) bondline thickness is known and controlled, (iii) non-linear behaviour due to adherend and interface damage is limited, and (iv) the specimens employed to determine... 

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. 

A comprehensive review of the measurement of mechanical properties such as fracture strength or adhesion strength can be found in the literature [13]. In general, the best way to determine mechanical properties is to test microma-chined structures that are as close to the actual design as possible using, for example, the micromanipulator developed at the University of Uppsala [14]. 

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. 

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. ... 

At beam processing leads to a significant adhesion enhancement for the metal/PI structures. This stron y indicates that interfacial fracture mechanisms can be adjusted by low energy ion beam treatment. 

Since the first symposium on Recent Advances in Adhesion, held September, 1971 in Washington, D.C., this Division of the American Chemical Society has continuously sponsored several symposia on adhesion and adhesives. The chemists have gradually realized the importance of adhesion in various fields of science and technology. During these years, the science of adhesion has steadily grown along with progress in surface science and fracture mechanics. Moreover, new adhesives have been invented and applied in actual structures, for example, structural and aerospace adhesives. 

The function of a structural adhesive joint is to transmit an external load to the structural member. If the joint fails to function as it is intended, it will undergo damage or failure. The damage could be actual fracture of the structure, excessive elastic deformation, or excessive inelastic flow. The criteria for what constitutes structural failure depend on the performance requirements of the joint. The fundamental problem in the mechanics of adhesives and joints is to obtain some relationship between the loads applied to the joint and a parameter that will adequately describe the criteria for strucmral failure. The most common criterion for such failure of lap-type joints is actual fracmre of the joint. For a given combination of adherend and adhesive, the stress analyst must decide what the mode or theory of failure would be if the applied loads become large enough to cause failure. The decision as to which theory would realistically determine the mode of failure is usually based on past experience, or upon some form of experimental evidence. ... 

In the case of structural joints, cohesive failure of the adhesive is often observed. Indeed, in many instances, the requirement will be to engineer failure within the bulk of the adhesive layer. Modelling of the adhesive response as a function of water uptake with temperature is carried out to represent the influence of environmental exposure. With fatigued structural joints, by combining the use of fracture mechanics and appropriate modelling tools, the resultant crack propagation rates and consequently durability levels can be predicted as a function of various environmental and mechanical test parameters, such as frequency. Fatigue threshold values can be determined, which are used to predict durability performance. " ... 

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