Fracture mechanics adhesive joints

Fig. 4.14. Some adhesive joint fracture mechanics specimens, (a) Tapered double cantilever beam (TDCB). (b) Thick double cantilever beam (DCB). (c) Thin double cantilever beam or wedge cleavage specimen, (d) Independently loaded mixed-mode specimen (ILMMS). (e) Scarf joint.

Another aspect of oxide stability, and often a more assiduous problem, is the observation that very subtle changes may occur in the nature and stability of the oxide, in situ in the adhesive joint, which mechanically weaken the oxide and lead to premature failure through the oxide layer. This is thought to arise from a subtle hydration, and weakening, of the oxide and does not appear to involve any electrochemical-corrosion mechanism. (Although such corrosion may well occur, post-failure, once fracture through the oxide has resulted in fracture of the joint.)... 

Kinloch, A. J. (1972) Mechanics and Mechanisms of Adhesive Joint Fracture, University of London, Queen Mary College. 

The path of failure of an adhesive joint can give information about the mechanism of failure if analysis of the elemental and chemical composition can be conducted along the path. Several authors have performed such analyses by loading the adhesive joint until it fractures and then using XPS to analyze each side of the fracture. 

Usually tj/ is very much larger than Fq. This is why practical fracture energies for adhesive joints are almost always orders of magnitude greater than works of adhesion or cohesion. However, a modest increase in Fq may result in a large increase in adhesion as and Fo are usually coupled. For some mechanically simple systems where is largely associated with viscoelastic loss, a multiplicative relation has been found ... 

Kinloch, A. J., Interfacial Fracture Mechanical Aspects of Adhesion Bonded Joints, Review Article, Journal of Adhesion, vol. 10, 1979, p. 193. 

Papini M, Femlund G Spelt JK, Effect of crack growth mechanism on the prediction of fracture load of adhesive joints. Comp. Sci. Tech., 52, 1994, 561. 

Paraschi, M., A fracture mechanics approach to the failure of adhesive joints, PhD thesis. Department of Mechanical Engineering. 2002, Imperial College, University of London London. 

Fracture Mechanics, Wood adhesive joints, shear loading, cohesive failure. 

Sheppard, A., Kelly, D., Tong, L., (1998), Int. J. of Adhesion and Adhesives 18, 385. Wemersson, H., (1994), Fracture characterization of wood adhesive joints. Report TVSM-1006, Lund University, Division of Structural Mechanics, Lund, Sweden, Simon, F., Morel, S., Valentin, G. (1997). In Proceedings of the Euromech Colloquium 358, Mechanical behaviour of adhesive joints, analysis, testing and design, Pluralis, Paris, pp. 341-351. 

It is my great pleasure to introduce the proceedings of the ESIS TC4 conference, Fracture of Polymers, Composites and Adhesives , which was held in the mountain resort of Les Diablerets, Switzerland between 15-18 September 2002. This was the third conference organised by TC4 and, as on the two previous occasions, it reflects the main activities of the committee which are focussed on developing fracture mechanics test methods for polymers, adhesive joints and composites. 

Because the fracture toughness depends both on cure time and temperature, the arbitrary selection of time and temperature for accelerated tests may not be appropriate for reliable prediction of longterm service life of joints (J7). In order to reduce test variability and improve the durability prediction of adhesive joints, it would be necessary first to control the cure temperature and time required to produce a level of fracture toughness that does not change further (14). The study is thus an excellent example of a multidisciplinary approach combining chemistry, fracture mechanics, and wood science in the investigation of the adhesive bonding of wood. 

Ebewele, R. O. The Fracture Mechanics Approach to the Assessment of Adhesive Joint Performance in Bonded Wood Products, Ph.D. thesis, Univ. of Wisconsin Madison, WI, 1980. 

Adherend fracture Failure of a bonded joint under mechanical stress in the adherend material, thus, outside the adhesive layer. Indicates that the bond strength is higher than the adherend strength. 

It was mentioned above that the simulation method of Termonia [67-72] can be used to calculate the stress-strain curves of many fiber-reinforced or particulate-filled composites up to fracture, including the effects of fiber-matrix adhesion. Such systems are morphologically far more complex than adhesive joints. Many matrix-filler interfaces are dispersed throughout a composite specimen, while an adhesive joint has only the two interfaces (between each of the bottom and top metal plates and the glue layer). If one considers also the fact that there will often he a distribution of filler-matrix interface strengths in a composite, it can be seen that the failure mechanism can become quite complex. It may even involve a complex superposition of adhesive failure at some filler-matrix interfaces and cohesive failure in the bulk of the matrix. 

The fracture-based approach derives from continuum fracture mechanics theory, which claims the strength of most real solids is governed by flaws within the material [2]. To help predict this type of behavior, many test methods have been developed to determine fracture properties of adhesives. These tests are used to characterize the mode I, II, and III fracture properties of many types of material systems. In this study, the focus will be on the mode I and II characteristics of bonded joints for automotive applications. 

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. 

As previously mentioned, the single lap joint is the most common test used to evaluate adhesives because of its practical resemblance to many real-world joint designs. The adhesive lap joint has proven useful over the years and will likely continue to be widely used in the future. This paper discusses some of the complexities of the lap joint. The discussion will now concentrate on using finite element analysis to aid in a fracture mechanics approach to aid in understanding the mechanics of a lap joint. It will, to some extent, explore the validity of the design rules discussed above and look at the affect of some of the joint features that are not considered in these rules. 

Finally, it will be demonstrated that fracture mechanics provides insight into the preferred path of crack growth, A perplexing problem that, despite considerable speculation over the last 50 years, has remained unsolved is why do cracks follow a particular path For example, for many practical joints, it has been observed that cracks typically proceed in the adhesive rather than along the interface. This phenomenon is so common that, in his classical text on adhesives, Kinloch states [11] ... 

The failure of an adhesive joint can be considered to involve the initiation and propagation of naturally occurring (intrinsic) flaws or defects. Fracture mechanics is the field of mechanics concerned with the study of the formation and propagation of cracks in materials. The objective of using fracture mechanics is to determine the bond durability of a specific adhesive and provide a basis for estimating the fracture, fatigue, and service life of the adhesive joints. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the resistance of a material to fracture. 

The failure of adhesive joints depends on the combination of peel and shear loading. In fracture mechanics, three basic modes of loading (Fig. 6.10) are the following ... 

---