Theoretical analysis of crack patching

Abstract

The development of high-strength fibres and adhesives has made it possible to repair cracked metallic plates (for example, cracked portions of an aircraft’s fuselage) by bonding reinforcing patches to the plate over the crack. The process is referred to as crack patching, and several repairs using this procedure are documented in Chapter 6, Section 7, where the practical advantages over the traditional methods of riveting or welding metallic reinforcements are discussed. The aim of this chapter is to discuss some basic theoretical aspects of the mechanics of bonded reinforcements. To that end, we shall consider a relatively simple repair configuration, namely a centre-cracked plate which is repaired by an elliptical patch and which is subjected to a remote biaxial stress, as illustrated in Figure 5.1. It will be shown that by using simplifying assumptions which are based on an analysis of the load-transfer characteristics of bonded joints, one can derive analytical estimates for the following quantities which are of primary interest in assessing the efficiency and the viability of the repair:

• (i) the reduction in the stress intensity factor,

• (ii) the maximum shear strain in the adhesive,

• (iii) the maximum tensile stress in the reinforcing patch,

• (iv) the change in overall stiffness due to the crack and the bonded reinforcement.