Adherends stiffness

Thus decreasing the overlap length or shear modulus of the adhesive, or increasing the adherend stiffness or adhesive layer thickness, will decrease the shear stress concentration in the adhesive layer. 

Shear tests are very common because samples are simple to construct and closely dupUcate the geometry and service conditions for many structural adhesives. As with tensile tests, the stress distribution is not uniform and, while it is often conventional to give the failure shear stress as the load divided by the bonding area (Table 11.1), the maximum stress at the bond line may be considerably higher than the average stress. The stress in the adhesive may also differ from pure shear. Depending on such factors as adhesive thickness and adherend stiffness, the failure of the adhesive shear joint can be dominated by either shear or tensioa ... 

According to Hart Smith the adherend stiffnesses should be balanced as much as the relative strengths of the adherends permit (i.e. product Et should be kept constant). This needs to be considered only when two dissimilar materials are joined together. 

In case of bonded composite joints, the non-uniform stress distribution along the bonding surface should always be accounted for. The peak stress is mainly dependent on the bonding pattern of the joint, adhesive thickness, bonded length, joint geometry, adherend stiffness imbalance, ductile adhesive response, and the composite adherends. 

The adherend stiffness in the terminology of adhesive joints is defined as the elastic modulus ( ) times the adherend thickness (t). It is a parameter... 

Adherend stiffness imbalance and thermal mismatch have a substantial effect on the lap-shear stress distribntion along the joint s bondlength. 

Fig. 8. Loss of joint strength resulting from adherend stiffness imbalance.

It is always possible to design around potential adherend stiffness imbalances, by correcting the design to eliminate them, but there is no such cure available for the reductions in strength that arise when thermally dissimilar materials are... 

The titanium tries to shrink after bonding, but the composite laminate resists this because of its much lower coefficient of thermal expansion. The net result is to preload the adhesive in opposite directions at each end of the overlap. Consequently, when mechanical loads are applied, the residual thermal stresses in the adhesive will increase the tensile strength and decrease the compressive strength of the joint. It is, of course, possible to deliberately incorporate a determinable amount of adherend stiffness imbalance to take advantage of this phenomenon whenever the design tension and compression loads differ, but an ideal result will exist for only one temperature. This, of course, should be for the most critical temperature/load combination but it would also be necessary... 

The formula governing this bond strength limit P, per adhesive layer, for long-overlap joints, has four possible values whenever both adherend stiffness imbalance and thermal mismatch are present, with different tensile and compressive strengths. For tensile loads. 

Fig. 12 depicts the special case of Eqs. 8 and 9 in which there is neither adherend stiffness imbalance nor thermal mismatch. It therefore indicates the strong effect of the toughening of typical structural adhesives, via the elastic-plastic model. Eq. 6 is plotted in Fig. 12, being expressed in nondimensionalized form by... 

Matthews et al (1982) have recently published an up-to-date review on the strength of joints in fibre reinforced plastics. They considered both theoretical and experimental results, although mainly the former. The reader is referred to this excellent article. In essence, they concluded that it is necessary to consider non-linear adhesive behaviour if joint strength is to be predicted. Joint strength is improved as the adherend stiffness increases and the adhesive stiffness decreases, and a ductile adhesive is always preferable to a brittle one. 

Based on -50°F properties giving iowest joint strength and assuming taper of outer splice straps thicker than 0.050 in. Strength vaiues corrected tor adherend stiffness imbalance. For nominal adhesive thickness jj = 0.005 in. For other thicknesses, modify strengths in ratio Vjj/0.005. 

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