Strength of Joints

6.2.3.1 Strength of Adhesive Joints

Adhesion Strength of Loaded Joints

The model of adhesion strength of loaded glued joint was presented by Godzimirski (2002) (Fig. 6.30). In furniture, this type of joint does not, however, have any great significance, aside from few cases of front longitudinal joints. Normal stresses in the glue-line of such a connection can be calculated in a simplified manner from the equation:

1

■.

r

J

l

<Ch

t>

Fig. 6.30 Adhesion load of glued joints

where

F normal force to the surface of the glue-line,

A area of the glue-line.

However, it should be noted that the narrowing occurring in the glue-line proves the presence of tangential stresses. Therefore, when making precise calculations of the strength of such joints, it is better to use numerical methods.

Peel Strength of Loaded Joints

When using narrow edges made of plastic and veneers for finishing narrow surfaces of furniture elements, these glued joints need to ensure a high resistance to peeling (Fig. 6.31).

Peeling of veneers from a massive board element is associated with the for­mation of large normal stresses in the glue-line of irregular distribution and bulges at the edge of the joint. In analysing the calculation model of a joint loaded for peeling, it is assumed that a thinner element is subjected to bending, and the deformations of the board, due to its great stiffness, can be omitted (Misztal 1956; Godzimirski 2002) (Fig. 6.32).

Fig. 6.33 Internal forces acting on the elementary section of a beam on an elastic base

Usually, to describe deformations of such a thin element, calculation methods are used which are appropriate to describe deflections of beams working on an elastic base (Nowacki 1970; Godzimirski 2002). By extracting an elementary section of such a beam, the state of internal forces is illustrated in Fig. 6.33.

For the considered model of a glued joint, the maximum normal stresses appearing on its edges are as follows:

where

E linear elasticity module of the veneer, Ek linear elasticity module of the glue,

S1 thickness of the veneer,

Sk thickness of the glue-line,

b width of the glue-line, M bending moment,

F normal force.