Bonded joints shear behaviour

The effect of adhesive non-linearity on the behaviour of FRP adhesively bonded joints was adequately addressed in Section 10.3. However, it is useful to recall that considering adhesive non-linear representation in FEM has a negligible effect on the predicted shear and peel stresses at service load levels of the FRP composite joint, because their distribution along the whole bondlength is mainly linear-elastic. Interfacial adhesive non-linearity has a substantial effect on the accuracy of quantitative FEM predictions of both stresses, and thus joint capacity, only at loads nearing joint failure where all practical adhesives, even brittle ones, exhibit non-linear behaviour. 

Because of the high scattering of experimental results and the great difficulty in reaching the fully cohesive failure of wooden adhesive Joints, a numerical analysis has been made to give a better knowledge of their mechanical behaviour for various parameters (adhesive used. Joint thickness, loading mode, etc...). For the PU resin tested previously in shear, such an analysis has been made on two steps first simulations have been made on bulk adhesive specimen to determine the mechanical behaviour of the resin and the numerical results obtained have been implanted in the FE code CASTEM 2000 [21] for the mTENF bonded specimen loaded by shear. 

In instrumented creep tests taken to failure, one learns not only how long specimens last but also how deformation increases throughout the creep process. For lap joints, delay times have been seen in creep tests, probably due to the increasing uniformity of the shear stress state, as predicted by the shear lag model as the creep compliance of the adhesive increases with time. In other situations, no such delay time is seen. A schematic illustration of a creep curve for an adhesive bond consisting of a butt joint bonded with a pressure sensitive foam tape is shown in Fig. 2, exhibiting classical primary, secondary and tertiary regions of creep behaviour. 

The lap-shear stress distribution, the failure pattern and ultimately the bond strength of FRP joints are also functions of the mechanical properties of the FRP reinforcing fibres. This behavioural dependency is depicted in Fig. 10.3 where lap-shear stress distributions along the bondlength for two identical double-strap CFRP/steel specimens, with different elastic moduli of their reinforcing CF (carbon fibres), are presented. 

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The three-dimensional models predict that the stress level can be reduced if the bond line thickness is increased from 25 to 75 or even 175 xm. However, the curves of Figure 50 show, e.g. that the maximum shearing stress decreases by a factor of two, from 33 to 17 MPa, when the thickness of the adhesive layer increases from 25 to 100 xm. A bond line thickness of 50-75 xm is generally recommended for the die attachment because of the negligible thermal impedance penalty. The experimental results indicate that, between 20 and 80 xm, the thickness of the adhesive joint does not greatly affect the thermal transfer capability. This behaviour has been explained by the fact that the interfacial thermal resistances between the adhesive and both the die and the substrate are much higher than that contributed by the bulk thermal conductivity of the adhesive materials. 

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