Joint behaviour

The time-dependent component of polymer response is of extreme importance to the use of structural adhesives which are required to sustain either permanent or transient loads. At temperatures well below the adhesive s Ig, overloading is far more likely to lead to stress rupture than to creep(5). However, at temperatures close to or at Tg, some creep of loaded joints is to be expected. Recalling Chapter 2, highly cross-linked epoxies which are cured at elevated temperature possess the best resistance to creep. 

It is clear that there are load, and therefore stress, levels below which creep will not occur, but it must be recognised that changes in the adhesive s stiffness due to environmental conditions may well give rise to creep after a delay or induction period. This could, of course, be reversible such that steady state creep is not a necessary 

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The non-linear empirical Barton-Bandis (BB) joint model describes the Joint behaviour. The mean input values are given in Table 2. The initial mechanical aperture (o/ ,) at zero normal stress is calculated with the following empirical relationship (Bandise/a/. 1983) ... 

Fatigue and fracture of adhesively-bonded composite joints Behaviour, simulation and modelling... 

Although improving the joint strength and stiffness, major criticisms to this practice are the irreversibility of the intervention, the prevention of future inspection, and the modification of the joint behaviour. Joints become highly rigid, which may change the overall stress distribution in the structure with possible negative effects. 

In the case of decayed timber (Fig. 7(a)), the whole joint area may be replaced by a new solid structural node made with cast-in epoxy grout, connected with rods to sound timber parts (Fig. 7(b)). A much better alternative, although requiring more time and means and more skilled operators, is the individual repair of the members meeting in the joint, thus maintaining the original joint behaviour (Fig. 7(c)). 

The term adhesion refers to the attraction between substances whereby when they are brought into contact work must be done in order to separate them. Adhesion is an important phenomenon in science as well as in engineering, but it is used in a different sense. The engineer uses experimentally determined values, which describe joint behaviour under specified conditions, in order to classify the... 

It is apparent that the effects of moisture and heat, especially in combination with an applied stress, may have a considerable influence on bonded joint behaviour. This is because of changes in the stiffness of adhesive materials with exposure to environmental conditions, and the effect has been emphasised in previous sections. 

Such tests are used to demonstrate that a particular adhesive and substrate combination will be able to carry adequate mechanical load. However, the mechanics of load transfer even within these joints is complex and in order to gain a detailed understanding of the joint behaviour, it is commonplace to perform some form of stress analysis. 

In all cases, non-linear static analyses were performed with the effects of nonlinear geometry included. Both mechanical and temperature loads were applied incrementally if the adhesive or adherends were seen to yield and stress-strain distributions were reported at each load increment so that the joint behaviour could be monitored throughout the application of load. 

What all of these figures show is the significant influence that the thermal shrinkage can have upon the prediction of the stress distribution within a lap joint. One conclusion which can be drawn from this is that if an accurate estimation of joint performance, be it a strength prediction or understanding of the joint behaviour, is to be achieved then it is imperative that the thermal effects be accounted for. 

Chen and Cheng [31] show a development of their earlier work [32] in which they extend their theory of lap-joint behaviour to include non-identical adherends. In order to allow the resulting system of equations to be solved using closed form methods, they assume a uniform shear stress distribution across the adhesive layer thickness. A complementary energy method was then employed to solve the final equations. This theory is also presented by Wu et al. [33], who also show that it decomposes to Goland and Reissner s solution if further simplifying assumptions are made. 

One important area where durability models have been successfully developed is in the prediction of the Joint behaviour under the combined effects of an aggressive environment and cyclic-fatigue loads. In this approach the fracture-... 

Yildirim et al. used FE analysis to determine and show the contact location in the knee during high flexion [4], Finite element methods applied to analysis menisci and meniscectomy effect, and ligaments biomechanics, in knee joint behaviour [2,6,7]. 3-dimentional (3D) FE was performed to analysis contact pressure and compressive stress in healthy human knee in flexion and gait [1,8]. 

Figure 7.25 Effect of temperature on the fracture energy, Gic, for a toughened epoxy adhesive bonding steel substrates using a TDCB specimen [154]. x, Bulk behaviour of adhesive O, adhesive joint behaviour 3, predicted joint behaviour, from Equation 7.62. The plastic zone size, 2r yc, calculated from Equation 7.61 is given in parentheses. 

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