2 APPROACHES TO TESTING

Marble test coupons were prepared using a range of adhesives and the bonds evaluated by both shear and tensile tests. Shear testing (tests designed to place opposing loads along opposing faces of a bond) is the most common evaluation of adhesives, since the test coupons are more easily produced and since most demands on an adhesive bond in commercial applications are defined as shear. However, evaluations of the shear test results and the experience of preparing the test samples led the authors to also pursue tensile load evaluations and to increase the number of samples and range of adhesive mixtures tested. Tensile tests impose a force along the axial direction of a bond perpendicular to the bond plane. This kind of load is often prevalent in joints of reassembled largescale stone sculptures. An example of a common tensile load is the outstretched arm of the sculpture in figure 1. If we assume that a shear pin is not part of the joint in this particular example and that the assembly looks like figure 3a, then the forces acting upon the arm might at first appear to be only shear, that is, directly downward due to gravity and roughly parallel to the joint surface. However, should this joint derive from a fracture, the substrates will have a complex surface morphology, and the two surfaces will, to some degree, physically “lock” into place. On closer examination, it is easy to see that the direct downward movement of one surface relative to the other is blocked by the interlocking roughness of the two substrates (the coefficient of friction is high). If the arm is to move in a direction parallel to the joint surface (shear movement), one substrate must move away from the other in a perpendicular direction (fig. 3b) to overcome the “interlocking surfaces” of both substrates. The adhesive layer, as a result, experiences tensile load. Even if we were to introduce a shear pin, which is common in joints such as the one we are discussing, the forces in the adhesive remain predominantly in tensile. Approximately one-half of the joint (that part below midpoint of the joint surface) will be placed in compression, while the other half will be placed in tension as the arm attempts to drop away. With the exception of relatively flat fragments that are adhered to a vertical or nearly vertical surface and have smooth substrates, the predominant force acting upon most joints will be a combination of tension and compression. Nonetheless, shear load evaluations can be significant, particularly when the joint surfaces are smooth, and a proper evaluation of an adhesive system will include both shear and tensile data.

Fig. 3.

A. A joint of two fractured surfaces showing the complex morphology that provides physical “key locking” of the two substrates, increasing the coefficient of friction and the resistance to shear load. B. Diagram as in A, illustrating the necessity of outward movement normal to the two interfaces before downward displacement (shear slip) can occur. This type of movement places the bond in tensile load and is typical of joints made in the reassembly of sculpture.